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'PKCS5' ** Downref: Normative reference to an Informational RFC: RFC 1950 ** Downref: Normative reference to an Informational RFC: RFC 1951 ** Downref: Normative reference to an Informational RFC: RFC 2144 ** Obsolete normative reference: RFC 2822 (Obsoleted by RFC 5322) ** Downref: Normative reference to an Informational RFC: RFC 3394 ** Obsolete normative reference: RFC 3447 (Obsoleted by RFC 8017) ** Downref: Normative reference to an Informational RFC: RFC 3713 ** Downref: Normative reference to an Informational RFC: RFC 5639 ** Downref: Normative reference to an Informational RFC: RFC 7253 ** Downref: Normative reference to an Informational RFC: RFC 7748 ** Downref: Normative reference to an Informational RFC: RFC 8032 -- Possible downref: Non-RFC (?) normative reference: ref. 'SCHNEIER' -- Possible downref: Non-RFC (?) normative reference: ref. 'SuiteB' -- Possible downref: Non-RFC (?) normative reference: ref. 'TWOFISH' -- Obsolete informational reference (is this intentional?): RFC 1991 (Obsoleted by RFC 4880) -- Obsolete informational reference (is this intentional?): RFC 2440 (Obsoleted by RFC 4880) Summary: 12 errors (**), 0 flaws (~~), 9 warnings (==), 24 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group W. Koch 3 Internet-Draft GnuPG e.V. 4 Obsoletes4880, 5581, 6637 (if approved) B. Carlson 5 Intended status: Standards Track R.H. Tse 6 Expires: 10 September 2020 Ribose 7 D.A. Atkins 8 D.K. Gillmor 9 9 March 2020 11 OpenPGP Message Format 12 draft-ietf-openpgp-rfc4880bis-09 14 Abstract 16 { Work in progress to update the OpenPGP specification from RFC4880 } 18 This document specifies the message formats used in OpenPGP. OpenPGP 19 provides encryption with public-key or symmetric cryptographic 20 algorithms, digital signatures, compression and key management. 22 This document is maintained in order to publish all necessary 23 information needed to develop interoperable applications based on the 24 OpenPGP format. It is not a step-by-step cookbook for writing an 25 application. It describes only the format and methods needed to 26 read, check, generate, and write conforming packets crossing any 27 network. It does not deal with storage and implementation questions. 28 It does, however, discuss implementation issues necessary to avoid 29 security flaws. 31 Status of This Memo 33 This Internet-Draft is submitted in full conformance with the 34 provisions of BCP 78 and BCP 79. 36 Internet-Drafts are working documents of the Internet Engineering 37 Task Force (IETF). Note that other groups may also distribute 38 working documents as Internet-Drafts. The list of current Internet- 39 Drafts is at https://datatracker.ietf.org/drafts/current/. 41 Internet-Drafts are draft documents valid for a maximum of six months 42 and may be updated, replaced, or obsoleted by other documents at any 43 time. It is inappropriate to use Internet-Drafts as reference 44 material or to cite them other than as "work in progress." 46 This Internet-Draft will expire on 10 September 2020. 48 Copyright Notice 50 Copyright (c) 2020 IETF Trust and the persons identified as the 51 document authors. All rights reserved. 53 This document is subject to BCP 78 and the IETF Trust's Legal 54 Provisions Relating to IETF Documents (http://trustee.ietf.org/ 55 license-info) in effect on the date of publication of this document. 56 Please review these documents carefully, as they describe your rights 57 and restrictions with respect to this document. Code Components 58 extracted from this document must include Simplified BSD License text 59 as described in Section 4.e of the Trust Legal Provisions and are 60 provided without warranty as described in the Simplified BSD License. 62 Table of Contents 64 1. Introduction 65 1.1. Terms 66 2. General functions 67 2.1. Confidentiality via Encryption 68 2.2. Authentication via Digital Signature 69 2.3. Compression 70 2.4. Conversion to Radix-64 71 2.5. Signature-Only Applications 72 3. Data Element Formats 73 3.1. Scalar Numbers 74 3.2. Multiprecision Integers 75 3.3. Key IDs 76 3.4. Text 77 3.5. Time Fields 78 3.6. Keyrings 79 3.7. String-to-Key (S2K) Specifiers 80 3.7.1. String-to-Key (S2K) Specifier Types 81 3.7.2. String-to-Key Usage 82 4. Packet Syntax 83 4.1. Overview 84 4.2. Packet Headers 85 4.2.1. Old Format Packet Lengths 86 4.2.2. New Format Packet Lengths 87 4.2.3. Packet Length Examples 88 4.3. Packet Tags 89 5. Packet Types 90 5.1. Public-Key Encrypted Session Key Packets (Tag 1) 91 5.2. Signature Packet (Tag 2) 92 5.2.1. Signature Types 93 5.2.2. Version 3 Signature Packet Format 94 5.2.3. Version 4 and 5 Signature Packet Formats 95 5.2.4. Computing Signatures 96 5.3. Symmetric-Key Encrypted Session Key Packets (Tag 3) 97 5.4. One-Pass Signature Packets (Tag 4) 98 5.5. Key Material Packet 99 5.5.1. Key Packet Variants 100 5.5.2. Public-Key Packet Formats 101 5.5.3. Secret-Key Packet Formats 102 5.6. Algorithm-specific Parts of Keys 103 5.6.1. Algorithm-Specific Part for RSA Keys 104 5.6.2. Algorithm-Specific Part for DSA Keys 105 5.6.3. Algorithm-Specific Part for Elgamal Keys 106 5.6.4. Algorithm-Specific Part for ECDSA Keys 107 5.6.5. Algorithm-Specific Part for EdDSA Keys 108 5.6.6. Algorithm-Specific Part for ECDH Keys 109 5.7. Compressed Data Packet (Tag 8) 110 5.8. Symmetrically Encrypted Data Packet (Tag 9) 111 5.9. Marker Packet (Obsolete Literal Packet) (Tag 10) 112 5.10. Literal Data Packet (Tag 11) 113 5.11. Trust Packet (Tag 12) 114 5.12. User ID Packet (Tag 13) 115 5.13. User Attribute Packet (Tag 17) 116 5.13.1. The Image Attribute Subpacket 117 5.13.2. User ID Attribute Subpacket 118 5.14. Sym. Encrypted Integrity Protected Data Packet (Tag 119 18) 120 5.15. Modification Detection Code Packet (Tag 19) 121 5.16. AEAD Encrypted Data Packet (Tag 20) 122 5.16.1. EAX Mode 123 5.16.2. OCB Mode 124 6. Radix-64 Conversions 125 6.1. An Implementation of the CRC-24 in "C" 126 6.2. Forming ASCII Armor 127 6.3. Encoding Binary in Radix-64 128 6.4. Decoding Radix-64 129 6.5. Examples of Radix-64 130 6.6. Example of an ASCII Armored Message 131 7. Cleartext Signature Framework 132 7.1. Dash-Escaped Text 133 8. Regular Expressions 134 9. Constants 135 9.1. Public-Key Algorithms 136 9.2. ECC Curve OID 137 9.3. Symmetric-Key Algorithms 138 9.4. Compression Algorithms 139 9.5. Hash Algorithms 140 9.6. AEAD Algorithms 141 10. IANA Considerations 142 10.1. New String-to-Key Specifier Types 143 10.2. New Packets 144 10.2.1. User Attribute Types 145 10.2.2. Image Format Subpacket Types 146 10.2.3. New Signature Subpackets 147 10.2.4. New Packet Versions 148 10.3. New Algorithms 149 10.3.1. Public-Key Algorithms 150 10.3.2. Symmetric-Key Algorithms 151 10.3.3. Hash Algorithms 152 10.3.4. Compression Algorithms 153 11. Packet Composition 154 11.1. Transferable Public Keys 155 11.2. Transferable Secret Keys 156 11.3. OpenPGP Messages 157 11.4. Detached Signatures 158 12. Enhanced Key Formats 159 12.1. Key Structures 160 12.2. Key IDs and Fingerprints 161 13. Elliptic Curve Cryptography 162 13.1. Supported ECC Curves 163 13.2. ECDSA and ECDH Conversion Primitives 164 13.3. EdDSA Point Format 165 13.4. Key Derivation Function 166 13.5. EC DH Algorithm (ECDH) 167 14. Notes on Algorithms 168 14.1. PKCS#1 Encoding in OpenPGP 169 14.1.1. EME-PKCS1-v1_5-ENCODE 170 14.1.2. EME-PKCS1-v1_5-DECODE 171 14.1.3. EMSA-PKCS1-v1_5 172 14.2. Symmetric Algorithm Preferences 173 14.3. Other Algorithm Preferences 174 14.3.1. Compression Preferences 175 14.3.2. Hash Algorithm Preferences 176 14.4. Plaintext 177 14.5. RSA 178 14.6. DSA 179 14.7. Elgamal 180 14.8. EdDSA 181 14.9. Reserved Algorithm Numbers 182 14.10. OpenPGP CFB Mode 183 14.11. Private or Experimental Parameters 184 14.12. Meta-Considerations for Expansion 185 15. Security Considerations 186 16. Compatibility Profiles 187 16.1. OpenPGP ECC Profile 188 16.2. Suite-B Profile 189 16.2.1. Security Strength at 192 Bits 190 16.2.2. Security Strength at 128 Bits 191 17. Implementation Nits 192 18. References 193 18.1. Normative References 194 18.2. Informative References 195 Appendix A. Test vectors 196 A.1. Sample EdDSA key 197 A.2. Sample EdDSA signature 198 A.3. Sample AEAD-EAX encryption and decryption 199 A.3.1. Sample Parameters 200 A.3.2. Sample symmetric-key encrypted session key 201 packet (v5) 202 A.3.3. Starting AEAD-EAX decryption of CEK 203 A.3.4. Sample AEAD encrypted data packet 204 A.3.5. Decryption of data 205 A.3.6. Complete AEAD-EAX encrypted packet sequence 206 A.4. Sample AEAD-OCB encryption and decryption 207 A.4.1. Sample Parameters 208 A.4.2. Sample symmetric-key encrypted session key 209 packet (v5) 210 A.4.3. Starting AEAD-OCB decryption of CEK 211 A.4.4. Sample AEAD encrypted data packet 212 A.4.5. Decryption of data 213 A.4.6. Complete AEAD-OCB encrypted packet sequence 214 Appendix B. ECC Point compression flag bytes 215 Appendix C. Changes since RFC-4880 216 Appendix D. The principal authors of RFC-4880 217 Authors' Addresses 219 1. Introduction 221 { This is work in progress to update OpenPGP. Editorial notes are 222 enclosed in curly braces. } 224 This document provides information on the message-exchange packet 225 formats used by OpenPGP to provide encryption, decryption, signing, 226 and key management functions. It is a revision of RFC 4880, "OpenPGP 227 Message Format", which is a revision of RFC 2440, which itself 228 replaces RFC 1991, "PGP Message Exchange Formats" [RFC1991] [RFC2440] 229 [RFC4880]. 231 This document obsoletes: RFC 4880 (OpenPGP), RFC 5581 (Camellia 232 cipher) and RFC 6637 (ECC for OpenPGP). 234 1.1. Terms 236 * OpenPGP - This is a term for security software that uses PGP 5 as 237 a basis, formalized in this document. 239 * PGP - Pretty Good Privacy. PGP is a family of software systems 240 developed by Philip R. Zimmermann from which OpenPGP is based. 242 * PGP 2 - This version of PGP has many variants; where necessary a 243 more detailed version number is used here. PGP 2 uses only RSA, 244 MD5, and IDEA for its cryptographic transforms. An informational 245 RFC, RFC 1991, was written describing this version of PGP. 247 * PGP 5 - This version of PGP is formerly known as "PGP 3" in the 248 community. It has new formats and corrects a number of problems 249 in the PGP 2 design. It is referred to here as PGP 5 because that 250 software was the first release of the "PGP 3" code base. 252 * GnuPG - GNU Privacy Guard, also called GPG. GnuPG is an OpenPGP 253 implementation that avoids all encumbered algorithms. 254 Consequently, early versions of GnuPG did not include RSA public 255 keys. 257 "PGP", "Pretty Good", and "Pretty Good Privacy" are trademarks of PGP 258 Corporation and are used with permission. The term "OpenPGP" refers 259 to the protocol described in this and related documents. 261 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 262 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 263 document are to be interpreted as described in [RFC2119]. 265 The key words "PRIVATE USE", "EXPERT REVIEW", "SPECIFICATION 266 REQUIRED", "RFC REQUIRED", and "IETF REVIEW" that appear in this 267 document when used to describe namespace allocation are to be 268 interpreted as described in [RFC8126]. 270 2. General functions 272 OpenPGP provides data integrity services for messages and data files 273 by using these core technologies: 275 * digital signatures 277 * encryption 279 * compression 281 * Radix-64 conversion 283 In addition, OpenPGP provides key management and certificate 284 services, but many of these are beyond the scope of this document. 286 2.1. Confidentiality via Encryption 288 OpenPGP combines symmetric-key encryption and public-key encryption 289 to provide confidentiality. When made confidential, first the object 290 is encrypted using a symmetric encryption algorithm. Each symmetric 291 key is used only once, for a single object. A new "session key" is 292 generated as a random number for each object (sometimes referred to 293 as a session). Since it is used only once, the session key is bound 294 to the message and transmitted with it. To protect the key, it is 295 encrypted with the receiver's public key. The sequence is as 296 follows: 298 1. The sender creates a message. 300 2. The sending OpenPGP generates a random number to be used as a 301 session key for this message only. 303 3. The session key is encrypted using each recipient's public key. 304 These "encrypted session keys" start the message. 306 4. The sending OpenPGP encrypts the message using the session key, 307 which forms the remainder of the message. Note that the message 308 is also usually compressed. 310 5. The receiving OpenPGP decrypts the session key using the 311 recipient's private key. 313 6. The receiving OpenPGP decrypts the message using the session key. 314 If the message was compressed, it will be decompressed. 316 With symmetric-key encryption, an object may be encrypted with a 317 symmetric key derived from a passphrase (or other shared secret), or 318 a two-stage mechanism similar to the public-key method described 319 above in which a session key is itself encrypted with a symmetric 320 algorithm keyed from a shared secret. 322 Both digital signature and confidentiality services may be applied to 323 the same message. First, a signature is generated for the message 324 and attached to the message. Then the message plus signature is 325 encrypted using a symmetric session key. Finally, the session key is 326 encrypted using public-key encryption and prefixed to the encrypted 327 block. 329 2.2. Authentication via Digital Signature 331 The digital signature uses a hash code or message digest algorithm, 332 and a public-key signature algorithm. The sequence is as follows: 334 1. The sender creates a message. 336 2. The sending software generates a hash code of the message. 338 3. The sending software generates a signature from the hash code 339 using the sender's private key. 341 4. The binary signature is attached to the message. 343 5. The receiving software keeps a copy of the message signature. 345 6. The receiving software generates a new hash code for the received 346 message and verifies it using the message's signature. If the 347 verification is successful, the message is accepted as authentic. 349 2.3. Compression 351 OpenPGP implementations SHOULD compress the message after applying 352 the signature but before encryption. 354 If an implementation does not implement compression, its authors 355 should be aware that most OpenPGP messages in the world are 356 compressed. Thus, it may even be wise for a space-constrained 357 implementation to implement decompression, but not compression. 359 Furthermore, compression has the added side effect that some types of 360 attacks can be thwarted by the fact that slightly altered, compressed 361 data rarely uncompresses without severe errors. This is hardly 362 rigorous, but it is operationally useful. These attacks can be 363 rigorously prevented by implementing and using Modification Detection 364 Codes as described in sections following. 366 2.4. Conversion to Radix-64 368 OpenPGP's underlying native representation for encrypted messages, 369 signature certificates, and keys is a stream of arbitrary octets. 370 Some systems only permit the use of blocks consisting of seven-bit, 371 printable text. For transporting OpenPGP's native raw binary octets 372 through channels that are not safe to raw binary data, a printable 373 encoding of these binary octets is needed. OpenPGP provides the 374 service of converting the raw 8-bit binary octet stream to a stream 375 of printable ASCII characters, called Radix-64 encoding or ASCII 376 Armor. 378 Implementations SHOULD provide Radix-64 conversions. 380 2.5. Signature-Only Applications 382 OpenPGP is designed for applications that use both encryption and 383 signatures, but there are a number of problems that are solved by a 384 signature-only implementation. Although this specification requires 385 both encryption and signatures, it is reasonable for there to be 386 subset implementations that are non-conformant only in that they omit 387 encryption. 389 3. Data Element Formats 391 This section describes the data elements used by OpenPGP. 393 3.1. Scalar Numbers 395 Scalar numbers are unsigned and are always stored in big-endian 396 format. Using n[k] to refer to the kth octet being interpreted, the 397 value of a two-octet scalar is ((n[0] << 8) + n[1]). The value of a 398 four-octet scalar is ((n[0] << 24) + (n[1] << 16) + (n[2] << 8) + 399 n[3]). 401 3.2. Multiprecision Integers 403 Multiprecision integers (also called MPIs) are unsigned integers used 404 to hold large integers such as the ones used in cryptographic 405 calculations. 407 An MPI consists of two pieces: a two-octet scalar that is the length 408 of the MPI in bits followed by a string of octets that contain the 409 actual integer. 411 These octets form a big-endian number; a big-endian number can be 412 made into an MPI by prefixing it with the appropriate length. 414 Examples: 416 (all numbers are in hexadecimal) 418 The string of octets [00 01 01] forms an MPI with the value 1. The 419 string [00 09 01 FF] forms an MPI with the value of 511. 421 Additional rules: 423 The size of an MPI is ((MPI.length + 7) / 8) + 2 octets. 425 The length field of an MPI describes the length starting from its 426 most significant non-zero bit. Thus, the MPI [00 02 01] is not 427 formed correctly. It should be [00 01 01]. 429 Unused bits of an MPI MUST be zero. 431 Also note that when an MPI is encrypted, the length refers to the 432 plaintext MPI. It may be ill-formed in its ciphertext. 434 3.3. Key IDs 436 A Key ID is an eight-octet scalar that identifies a key. 437 Implementations SHOULD NOT assume that Key IDs are unique. The 438 section "Enhanced Key Formats" below describes how Key IDs are 439 formed. 441 3.4. Text 443 Unless otherwise specified, the character set for text is the UTF-8 444 [RFC3629] encoding of Unicode [ISO10646]. 446 3.5. Time Fields 448 A time field is an unsigned four-octet number containing the number 449 of seconds elapsed since midnight, 1 January 1970 UTC. 451 3.6. Keyrings 453 A keyring is a collection of one or more keys in a file or database. 454 Traditionally, a keyring is simply a sequential list of keys, but may 455 be any suitable database. It is beyond the scope of this standard to 456 discuss the details of keyrings or other databases. 458 3.7. String-to-Key (S2K) Specifiers 460 String-to-key (S2K) specifiers are used to convert passphrase strings 461 into symmetric-key encryption/decryption keys. They are used in two 462 places, currently: to encrypt the secret part of private keys in the 463 private keyring, and to convert passphrases to encryption keys for 464 symmetrically encrypted messages. 466 3.7.1. String-to-Key (S2K) Specifier Types 468 There are three types of S2K specifiers currently supported, and some 469 reserved values: 471 +------------+--------------------------+ 472 | ID | S2K Type | 473 +============+==========================+ 474 | 0 | Simple S2K | 475 +------------+--------------------------+ 476 | 1 | Salted S2K | 477 +------------+--------------------------+ 478 | 2 | Reserved value | 479 +------------+--------------------------+ 480 | 3 | Iterated and Salted S2K | 481 +------------+--------------------------+ 482 | 100 to 110 | Private/Experimental S2K | 483 +------------+--------------------------+ 485 Table 1 487 These are described in the following Sections. 489 3.7.1.1. Simple S2K 491 This directly hashes the string to produce the key data. See below 492 for how this hashing is done. 494 Octet 0: 0x00 495 Octet 1: hash algorithm 497 Simple S2K hashes the passphrase to produce the session key. The 498 manner in which this is done depends on the size of the session key 499 (which will depend on the cipher used) and the size of the hash 500 algorithm's output. If the hash size is greater than the session key 501 size, the high-order (leftmost) octets of the hash are used as the 502 key. 504 If the hash size is less than the key size, multiple instances of the 505 hash context are created -- enough to produce the required key data. 506 These instances are preloaded with 0, 1, 2, ... octets of zeros (that 507 is to say, the first instance has no preloading, the second gets 508 preloaded with 1 octet of zero, the third is preloaded with two 509 octets of zeros, and so forth). 511 As the data is hashed, it is given independently to each hash 512 context. Since the contexts have been initialized differently, they 513 will each produce different hash output. Once the passphrase is 514 hashed, the output data from the multiple hashes is concatenated, 515 first hash leftmost, to produce the key data, with any excess octets 516 on the right discarded. 518 3.7.1.2. Salted S2K 520 This includes a "salt" value in the S2K specifier -- some arbitrary 521 data -- that gets hashed along with the passphrase string, to help 522 prevent dictionary attacks. 524 Octet 0: 0x01 525 Octet 1: hash algorithm 526 Octets 2-9: 8-octet salt value 528 Salted S2K is exactly like Simple S2K, except that the input to the 529 hash function(s) consists of the 8 octets of salt from the S2K 530 specifier, followed by the passphrase. 532 3.7.1.3. Iterated and Salted S2K 534 This includes both a salt and an octet count. The salt is combined 535 with the passphrase and the resulting value is hashed repeatedly. 536 This further increases the amount of work an attacker must do to try 537 dictionary attacks. 539 Octet 0: 0x03 540 Octet 1: hash algorithm 541 Octets 2-9: 8-octet salt value 542 Octet 10: count, a one-octet, coded value 544 The count is coded into a one-octet number using the following 545 formula: 547 #define EXPBIAS 6 548 count = ((Int32)16 + (c & 15)) << ((c >> 4) + EXPBIAS); 550 The above formula is in C, where "Int32" is a type for a 32-bit 551 integer, and the variable "c" is the coded count, Octet 10. 553 Iterated-Salted S2K hashes the passphrase and salt data multiple 554 times. The total number of octets to be hashed is specified in the 555 encoded count in the S2K specifier. Note that the resulting count 556 value is an octet count of how many octets will be hashed, not an 557 iteration count. 559 Initially, one or more hash contexts are set up as with the other S2K 560 algorithms, depending on how many octets of key data are needed. 561 Then the salt, followed by the passphrase data, is repeatedly hashed 562 until the number of octets specified by the octet count has been 563 hashed. The one exception is that if the octet count is less than 564 the size of the salt plus passphrase, the full salt plus passphrase 565 will be hashed even though that is greater than the octet count. 566 After the hashing is done, the data is unloaded from the hash 567 context(s) as with the other S2K algorithms. 569 3.7.2. String-to-Key Usage 571 Implementations SHOULD use salted or iterated-and-salted S2K 572 specifiers, as simple S2K specifiers are more vulnerable to 573 dictionary attacks. 575 3.7.2.1. Secret-Key Encryption 577 An S2K specifier can be stored in the secret keyring to specify how 578 to convert the passphrase to a key that unlocks the secret data. 579 Older versions of PGP just stored a cipher algorithm octet preceding 580 the secret data or a zero to indicate that the secret data was 581 unencrypted. The MD5 hash function was always used to convert the 582 passphrase to a key for the specified cipher algorithm. 584 For compatibility, when an S2K specifier is used, the special value 585 253, 254, or 255 is stored in the position where the hash algorithm 586 octet would have been in the old data structure. This is then 587 followed immediately by a one-octet algorithm identifier, and then by 588 the S2K specifier as encoded above. 590 Therefore, preceding the secret data there will be one of these 591 possibilities: 593 0: secret data is unencrypted (no passphrase) 594 255, 254, or 253: followed by algorithm octet and S2K specifier 595 Cipher alg: use Simple S2K algorithm using MD5 hash 597 This last possibility, the cipher algorithm number with an implicit 598 use of MD5 and IDEA, is provided for backward compatibility; it MAY 599 be understood, but SHOULD NOT be generated, and is deprecated. 601 These are followed by an Initial Vector of the same length as the 602 block size of the cipher for the decryption of the secret values, if 603 they are encrypted, and then the secret-key values themselves. 605 3.7.2.2. Symmetric-Key Message Encryption 607 OpenPGP can create a Symmetric-key Encrypted Session Key (ESK) packet 608 at the front of a message. This is used to allow S2K specifiers to 609 be used for the passphrase conversion or to create messages with a 610 mix of symmetric-key ESKs and public-key ESKs. This allows a message 611 to be decrypted either with a passphrase or a public-key pair. 613 PGP 2 always used IDEA with Simple string-to-key conversion when 614 encrypting a message with a symmetric algorithm. This is deprecated, 615 but MAY be used for backward-compatibility. 617 4. Packet Syntax 619 This section describes the packets used by OpenPGP. 621 4.1. Overview 623 An OpenPGP message is constructed from a number of records that are 624 traditionally called packets. A packet is a chunk of data that has a 625 tag specifying its meaning. An OpenPGP message, keyring, 626 certificate, and so forth consists of a number of packets. Some of 627 those packets may contain other OpenPGP packets (for example, a 628 compressed data packet, when uncompressed, contains OpenPGP packets). 630 Each packet consists of a packet header, followed by the packet body. 631 The packet header is of variable length. 633 4.2. Packet Headers 635 The first octet of the packet header is called the "Packet Tag". It 636 determines the format of the header and denotes the packet contents. 637 The remainder of the packet header is the length of the packet. 639 Note that the most significant bit is the leftmost bit, called bit 7. 640 A mask for this bit is 0x80 in hexadecimal. 642 +---------------+ 643 PTag |7 6 5 4 3 2 1 0| 644 +---------------+ 645 Bit 7 -- Always one 646 Bit 6 -- New packet format if set 648 PGP 2.6.x only uses old format packets. Thus, software that 649 interoperates with those versions of PGP must only use old format 650 packets. If interoperability is not an issue, the new packet format 651 is RECOMMENDED. Note that old format packets have four bits of 652 packet tags, and new format packets have six; some features cannot be 653 used and still be backward-compatible. 655 Also note that packets with a tag greater than or equal to 16 MUST 656 use new format packets. The old format packets can only express tags 657 less than or equal to 15. 659 Old format packets contain: 661 Bits 5-2 -- packet tag 662 Bits 1-0 -- length-type 664 New format packets contain: 666 Bits 5-0 -- packet tag 668 4.2.1. Old Format Packet Lengths 670 The meaning of the length-type in old format packets is: 672 0 The packet has a one-octet length. The header is 2 octets long. 674 1 The packet has a two-octet length. The header is 3 octets long. 676 2 The packet has a four-octet length. The header is 5 octets long. 678 3 The packet is of indeterminate length. The header is 1 octet 679 long, and the implementation must determine how long the packet 680 is. If the packet is in a file, this means that the packet 681 extends until the end of the file. In general, an implementation 682 SHOULD NOT use indeterminate-length packets except where the end 683 of the data will be clear from the context, and even then it is 684 better to use a definite length, or a new format header. The new 685 format headers described below have a mechanism for precisely 686 encoding data of indeterminate length. 688 4.2.2. New Format Packet Lengths 690 New format packets have four possible ways of encoding length: 692 1. A one-octet Body Length header encodes packet lengths of up to 693 191 octets. 695 2. A two-octet Body Length header encodes packet lengths of 192 to 696 8383 octets. 698 3. A five-octet Body Length header encodes packet lengths of up to 699 4,294,967,295 (0xFFFFFFFF) octets in length. (This actually 700 encodes a four-octet scalar number.) 702 4. When the length of the packet body is not known in advance by the 703 issuer, Partial Body Length headers encode a packet of 704 indeterminate length, effectively making it a stream. 706 4.2.2.1. One-Octet Lengths 708 A one-octet Body Length header encodes a length of 0 to 191 octets. 709 This type of length header is recognized because the one octet value 710 is less than 192. The body length is equal to: 712 bodyLen = 1st_octet; 714 4.2.2.2. Two-Octet Lengths 716 A two-octet Body Length header encodes a length of 192 to 8383 717 octets. It is recognized because its first octet is in the range 192 718 to 223. The body length is equal to: 720 bodyLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192 722 4.2.2.3. Five-Octet Lengths 724 A five-octet Body Length header consists of a single octet holding 725 the value 255, followed by a four-octet scalar. The body length is 726 equal to: 728 bodyLen = (2nd_octet << 24) | (3rd_octet << 16) | 729 (4th_octet << 8) | 5th_octet 731 This basic set of one, two, and five-octet lengths is also used 732 internally to some packets. 734 4.2.2.4. Partial Body Lengths 736 A Partial Body Length header is one octet long and encodes the length 737 of only part of the data packet. This length is a power of 2, from 1 738 to 1,073,741,824 (2 to the 30th power). It is recognized by its one 739 octet value that is greater than or equal to 224, and less than 255. 740 The Partial Body Length is equal to: 742 partialBodyLen = 1 << (1st_octet & 0x1F); 744 Each Partial Body Length header is followed by a portion of the 745 packet body data. The Partial Body Length header specifies this 746 portion's length. Another length header (one octet, two-octet, five- 747 octet, or partial) follows that portion. The last length header in 748 the packet MUST NOT be a Partial Body Length header. Partial Body 749 Length headers may only be used for the non-final parts of the 750 packet. 752 Note also that the last Body Length header can be a zero-length 753 header. 755 An implementation MAY use Partial Body Lengths for data packets, be 756 they literal, compressed, or encrypted. The first partial length 757 MUST be at least 512 octets long. Partial Body Lengths MUST NOT be 758 used for any other packet types. 760 4.2.3. Packet Length Examples 762 These examples show ways that new format packets might encode the 763 packet lengths. 765 A packet with length 100 may have its length encoded in one octet: 766 0x64. This is followed by 100 octets of data. 768 A packet with length 1723 may have its length encoded in two octets: 769 0xC5, 0xFB. This header is followed by the 1723 octets of data. 771 A packet with length 100000 may have its length encoded in five 772 octets: 0xFF, 0x00, 0x01, 0x86, 0xA0. 774 It might also be encoded in the following octet stream: 0xEF, first 775 32768 octets of data; 0xE1, next two octets of data; 0xE0, next one 776 octet of data; 0xF0, next 65536 octets of data; 0xC5, 0xDD, last 1693 777 octets of data. This is just one possible encoding, and many 778 variations are possible on the size of the Partial Body Length 779 headers, as long as a regular Body Length header encodes the last 780 portion of the data. 782 Please note that in all of these explanations, the total length of 783 the packet is the length of the header(s) plus the length of the 784 body. 786 4.3. Packet Tags 788 The packet tag denotes what type of packet the body holds. Note that 789 old format headers can only have tags less than 16, whereas new 790 format headers can have tags as great as 63. The defined tags (in 791 decimal) are as follows: 793 +----------+----------------------------------------------------+ 794 | Tag | Packet Type | 795 +==========+====================================================+ 796 | 0 | Reserved - a packet tag MUST NOT have this value | 797 +----------+----------------------------------------------------+ 798 | 1 | Public-Key Encrypted Session Key Packet | 799 +----------+----------------------------------------------------+ 800 | 2 | Signature Packet | 801 +----------+----------------------------------------------------+ 802 | 3 | Symmetric-Key Encrypted Session Key Packet | 803 +----------+----------------------------------------------------+ 804 | 4 | One-Pass Signature Packet | 805 +----------+----------------------------------------------------+ 806 | 5 | Secret-Key Packet | 807 +----------+----------------------------------------------------+ 808 | 6 | Public-Key Packet | 809 +----------+----------------------------------------------------+ 810 | 7 | Secret-Subkey Packet | 811 +----------+----------------------------------------------------+ 812 | 8 | Compressed Data Packet | 813 +----------+----------------------------------------------------+ 814 | 9 | Symmetrically Encrypted Data Packet | 815 +----------+----------------------------------------------------+ 816 | 10 | Marker Packet | 817 +----------+----------------------------------------------------+ 818 | 11 | Literal Data Packet | 819 +----------+----------------------------------------------------+ 820 | 12 | Trust Packet | 821 +----------+----------------------------------------------------+ 822 | 13 | User ID Packet | 823 +----------+----------------------------------------------------+ 824 | 14 | Public-Subkey Packet | 825 +----------+----------------------------------------------------+ 826 | 17 | User Attribute Packet | 827 +----------+----------------------------------------------------+ 828 | 18 | Sym. Encrypted and Integrity Protected Data Packet | 829 +----------+----------------------------------------------------+ 830 | 19 | Modification Detection Code Packet | 831 +----------+----------------------------------------------------+ 832 | 20 | AEAD Encrypted Data Packet | 833 +----------+----------------------------------------------------+ 834 | 60 to 63 | Private or Experimental Values | 835 +----------+----------------------------------------------------+ 837 Table 2 839 5. Packet Types 841 5.1. Public-Key Encrypted Session Key Packets (Tag 1) 843 A Public-Key Encrypted Session Key packet holds the session key used 844 to encrypt a message. Zero or more Public-Key Encrypted Session Key 845 packets and/or Symmetric-Key Encrypted Session Key packets may 846 precede a Symmetrically Encrypted Data Packet, which holds an 847 encrypted message. The message is encrypted with the session key, 848 and the session key is itself encrypted and stored in the Encrypted 849 Session Key packet(s). The Symmetrically Encrypted Data Packet is 850 preceded by one Public-Key Encrypted Session Key packet for each 851 OpenPGP key to which the message is encrypted. The recipient of the 852 message finds a session key that is encrypted to their public key, 853 decrypts the session key, and then uses the session key to decrypt 854 the message. 856 The body of this packet consists of: 858 * A one-octet number giving the version number of the packet type. 859 The currently defined value for packet version is 3. 861 * An eight-octet number that gives the Key ID of the public key to 862 which the session key is encrypted. If the session key is 863 encrypted to a subkey, then the Key ID of this subkey is used here 864 instead of the Key ID of the primary key. 866 * A one-octet number giving the public-key algorithm used. 868 * A string of octets that is the encrypted session key. This string 869 takes up the remainder of the packet, and its contents are 870 dependent on the public-key algorithm used. 872 Algorithm Specific Fields for RSA encryption: 874 - Multiprecision integer (MPI) of RSA encrypted value m**e mod n. 876 Algorithm Specific Fields for Elgamal encryption: 878 - MPI of Elgamal (Diffie-Hellman) value g**k mod p. 880 - MPI of Elgamal (Diffie-Hellman) value m * y**k mod p. 882 Algorithm-Specific Fields for ECDH encryption: 884 - MPI of an EC point representing an ephemeral public key. 886 - a one-octet size, followed by a symmetric key encoded using the 887 method described in Section 13.5. 889 The value "m" in the above formulas is derived from the session key 890 as follows. First, the session key is prefixed with a one-octet 891 algorithm identifier that specifies the symmetric encryption 892 algorithm used to encrypt the following Symmetrically Encrypted Data 893 Packet. Then a two-octet checksum is appended, which is equal to the 894 sum of the preceding session key octets, not including the algorithm 895 identifier, modulo 65536. This value is then encoded as described in 896 PKCS#1 block encoding EME-PKCS1-v1_5 in Section 7.2.1 of [RFC3447] to 897 form the "m" value used in the formulas above. See Section 14.1 of 898 this document for notes on OpenPGP's use of PKCS#1. 900 Note that when an implementation forms several PKESKs with one 901 session key, forming a message that can be decrypted by several keys, 902 the implementation MUST make a new PKCS#1 encoding for each key. 904 An implementation MAY accept or use a Key ID of zero as a "wild card" 905 or "speculative" Key ID. In this case, the receiving implementation 906 would try all available private keys, checking for a valid decrypted 907 session key. This format helps reduce traffic analysis of messages. 909 5.2. Signature Packet (Tag 2) 911 A Signature packet describes a binding between some public key and 912 some data. The most common signatures are a signature of a file or a 913 block of text, and a signature that is a certification of a User ID. 915 Three versions of Signature packets are defined. Version 3 provides 916 basic signature information, while versions 4 and 5 provide an 917 expandable format with subpackets that can specify more information 918 about the signature. PGP 2.6.x only accepts version 3 signatures. 920 Implementations MUST generate version 5 signatures when using a 921 version 5 key. Implementations SHOULD generate V4 signatures with 922 version 4 keys. Implementations MUST NOT create version 3 923 signatures; they MAY accept version 3 signatures. 925 5.2.1. Signature Types 927 There are a number of possible meanings for a signature, which are 928 indicated in a signature type octet in any given signature. Please 929 note that the vagueness of these meanings is not a flaw, but a 930 feature of the system. Because OpenPGP places final authority for 931 validity upon the receiver of a signature, it may be that one 932 signer's casual act might be more rigorous than some other 933 authority's positive act. See Section 5.2.4, "Computing Signatures", 934 for detailed information on how to compute and verify signatures of 935 each type. 937 These meanings are as follows: 939 0x00 Signature of a binary document. This means the signer owns it, 940 created it, or certifies that it has not been modified. 942 0x01 Signature of a canonical text document. This means the signer 943 owns it, created it, or certifies that it has not been 944 modified. The signature is calculated over the text data with 945 its line endings converted to . 947 0x02 Standalone signature. This signature is a signature of only 948 its own subpacket contents. It is calculated identically to a 949 signature over a zero-length binary document. Note that it 950 doesn't make sense to have a V3 standalone signature. 952 0x10 Generic certification of a User ID and Public-Key packet. The 953 issuer of this certification does not make any particular 954 assertion as to how well the certifier has checked that the 955 owner of the key is in fact the person described by the User 956 ID. 958 0x11 Persona certification of a User ID and Public-Key packet. The 959 issuer of this certification has not done any verification of 960 the claim that the owner of this key is the User ID specified. 962 0x12 Casual certification of a User ID and Public-Key packet. The 963 issuer of this certification has done some casual verification 964 of the claim of identity. 966 0x13 Positive certification of a User ID and Public-Key packet. The 967 issuer of this certification has done substantial verification 968 of the claim of identity. Most OpenPGP implementations make 969 their "key signatures" as 0x10 certifications. Some 970 implementations can issue 0x11-0x13 certifications, but few 971 differentiate between the types. 973 0x16 Attestion Key Signature. This signature is issued by the 974 primary key over itself and its user ID (or user attribute). 975 It MUST contain an "Attested Certifications" subpacket and a 976 "Signature Creation Time" subpacket. This type of key 977 signature does not replace or override any standard 978 certification (0x10-0x13). Only the most recent Attestation 979 Key Signature is valid for any given pair. If 980 more than one Certification Attestation Key Signature is 981 present with the same Signature Creation Time, the set of 982 attestations should be treated as the union of all "Attested 983 Certifications" subpackets from all such signatures with the 984 same timestamp. 986 0x18 Subkey Binding Signature. This signature is a statement by the 987 top-level signing key that indicates that it owns the subkey. 988 This signature is calculated directly on the primary key and 989 subkey, and not on any User ID or other packets. A signature 990 that binds a signing subkey MUST have an Embedded Signature 991 subpacket in this binding signature that contains a 0x19 992 signature made by the signing subkey on the primary key and 993 subkey. 995 0x19 Primary Key Binding Signature. This signature is a statement 996 by a signing subkey, indicating that it is owned by the primary 997 key and subkey. This signature is calculated the same way as a 998 0x18 signature: directly on the primary key and subkey, and not 999 on any User ID or other packets. 1001 0x1F Signature directly on a key. This signature is calculated 1002 directly on a key. It binds the information in the Signature 1003 subpackets to the key, and is appropriate to be used for 1004 subpackets that provide information about the key, such as the 1005 Revocation Key subpacket. It is also appropriate for 1006 statements that non-self certifiers want to make about the key 1007 itself, rather than the binding between a key and a name. 1009 0x20 Key revocation signature. The signature is calculated directly 1010 on the key being revoked. A revoked key is not to be used. 1011 Only revocation signatures by the key being revoked, or by an 1012 authorized revocation key, should be considered valid 1013 revocation signatures. 1015 0x28 Subkey revocation signature. The signature is calculated 1016 directly on the subkey being revoked. A revoked subkey is not 1017 to be used. Only revocation signatures by the top-level 1018 signature key that is bound to this subkey, or by an authorized 1019 revocation key, should be considered valid revocation 1020 signatures. 1022 0x30 Certification revocation signature. This signature revokes an 1023 earlier User ID certification signature (signature class 0x10 1024 through 0x13) or direct-key signature (0x1F). It should be 1025 issued by the same key that issued the revoked signature or an 1026 authorized revocation key. The signature is computed over the 1027 same data as the certificate that it revokes, and should have a 1028 later creation date than that certificate. 1030 0x40 Timestamp signature. This signature is only meaningful for the 1031 timestamp contained in it. 1033 0x50 Third-Party Confirmation signature. This signature is a 1034 signature over some other OpenPGP Signature packet(s). It is 1035 analogous to a notary seal on the signed data. A third-party 1036 signature SHOULD include Signature Target subpacket(s) to give 1037 easy identification. Note that we really do mean SHOULD. 1038 There are plausible uses for this (such as a blind party that 1039 only sees the signature, not the key or source document) that 1040 cannot include a target subpacket. 1042 5.2.2. Version 3 Signature Packet Format 1044 The body of a version 3 Signature Packet contains: 1046 * One-octet version number (3). 1048 * One-octet length of following hashed material. MUST be 5. 1050 * One-octet signature type. 1052 * Four-octet creation time. 1054 * Eight-octet Key ID of signer. 1056 * One-octet public-key algorithm. 1058 * One-octet hash algorithm. 1060 * Two-octet field holding left 16 bits of signed hash value. 1062 * One or more multiprecision integers comprising the signature. 1063 This portion is algorithm specific, as described below. 1065 The concatenation of the data to be signed, the signature type, 1066 and creation time from the Signature packet (5 additional octets) 1067 is hashed. The resulting hash value is used in the signature 1068 algorithm. The high 16 bits (first two octets) of the hash are 1069 included in the Signature packet to provide a quick test to reject 1070 some invalid signatures. 1072 Algorithm-Specific Fields for RSA signatures: 1074 - Multiprecision integer (MPI) of RSA signature value m**d mod n. 1076 Algorithm-Specific Fields for DSA and ECDSA signatures: 1078 - MPI of DSA or ECDSA value r. 1080 - MPI of DSA or ECDSA value s. 1082 The signature calculation is based on a hash of the signed data, as 1083 described above. The details of the calculation are different for 1084 DSA signatures than for RSA signatures. 1086 With RSA signatures, the hash value is encoded using PKCS#1 encoding 1087 type EMSA-PKCS1-v1_5 as described in Section 9.2 of RFC 3447. This 1088 requires inserting the hash value as an octet string into an ASN.1 1089 structure. The object identifier for the type of hash being used is 1090 included in the structure. The hexadecimal representations for the 1091 currently defined hash algorithms are as follows: 1093 - MD5: 0x2A, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05 1095 - RIPEMD-160: 0x2B, 0x24, 0x03, 0x02, 0x01 1097 - SHA-1: 0x2B, 0x0E, 0x03, 0x02, 0x1A 1099 - SHA2-224: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x04 1101 - SHA2-256: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01 1103 - SHA2-384: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02 1105 - SHA2-512: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03 1107 The ASN.1 Object Identifiers (OIDs) are as follows: 1109 - MD5: 1.2.840.113549.2.5 1111 - RIPEMD-160: 1.3.36.3.2.1 1113 - SHA-1: 1.3.14.3.2.26 1115 - SHA2-224: 2.16.840.1.101.3.4.2.4 1117 - SHA2-256: 2.16.840.1.101.3.4.2.1 1119 - SHA2-384: 2.16.840.1.101.3.4.2.2 1121 - SHA2-512: 2.16.840.1.101.3.4.2.3 1123 The full hash prefixes for these are as follows: 1125 - MD5: 0x30, 0x20, 0x30, 0x0C, 0x06, 0x08, 0x2A, 0x86, 1126 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05, 0x05, 0x00, 1127 0x04, 0x10 1129 - RIPEMD-160: 0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2B, 0x24, 1130 0x03, 0x02, 0x01, 0x05, 0x00, 0x04, 0x14 1132 - SHA-1: 0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2b, 0x0E, 1133 0x03, 0x02, 0x1A, 0x05, 0x00, 0x04, 0x14 1135 - SHA2-224: 0x30, 0x2D, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, 1136 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x04, 0x05, 1137 0x00, 0x04, 0x1C 1139 - SHA2-256: 0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, 1140 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01, 0x05, 1141 0x00, 0x04, 0x20 1143 - SHA2-384: 0x30, 0x41, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, 1144 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02, 0x05, 1145 0x00, 0x04, 0x30 1147 - SHA2-512: 0x30, 0x51, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, 1148 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03, 0x05, 1149 0x00, 0x04, 0x40 1151 DSA signatures MUST use hashes that are equal in size to the number 1152 of bits of q, the group generated by the DSA key's generator value. 1154 If the output size of the chosen hash is larger than the number of 1155 bits of q, the hash result is truncated to fit by taking the number 1156 of leftmost bits equal to the number of bits of q. This (possibly 1157 truncated) hash function result is treated as a number and used 1158 directly in the DSA signature algorithm. 1160 5.2.3. Version 4 and 5 Signature Packet Formats 1162 The body of a V4 or V5 Signature packet contains: 1164 * One-octet version number. This is 4 for V4 signatures and 5 for 1165 V5 signatures. 1167 * One-octet signature type. 1169 * One-octet public-key algorithm. 1171 * One-octet hash algorithm. 1173 * Two-octet scalar octet count for following hashed subpacket data. 1174 Note that this is the length in octets of all of the hashed 1175 subpackets; a pointer incremented by this number will skip over 1176 the hashed subpackets. 1178 * Hashed subpacket data set (zero or more subpackets). 1180 * Two-octet scalar octet count for the following unhashed subpacket 1181 data. Note that this is the length in octets of all of the 1182 unhashed subpackets; a pointer incremented by this number will 1183 skip over the unhashed subpackets. 1185 * Unhashed subpacket data set (zero or more subpackets). 1187 * Two-octet field holding the left 16 bits of the signed hash value. 1189 * One or more multiprecision integers comprising the signature. 1190 This portion is algorithm specific: 1192 Algorithm-Specific Fields for RSA signatures: 1194 - Multiprecision integer (MPI) of RSA signature value m**d mod n. 1196 Algorithm-Specific Fields for DSA or ECDSA signatures: 1198 - MPI of DSA or ECDSA value r. 1200 - MPI of DSA or ECDSA value s. 1202 Algorithm-Specific Fields for EdDSA signatures: 1204 - MPI of EdDSA compressed value r. 1206 - MPI of EdDSA compressed value s. 1208 The compressed version of R and S for use with EdDSA is described in 1209 [RFC8032]. A version 3 signature MUST NOT be created and MUST NOT be 1210 used with EdDSA. 1212 The concatenation of the data being signed and the signature data 1213 from the version number through the hashed subpacket data (inclusive) 1214 is hashed. The resulting hash value is what is signed. The left 16 1215 bits of the hash are included in the Signature packet to provide a 1216 quick test to reject some invalid signatures. 1218 There are two fields consisting of Signature subpackets. The first 1219 field is hashed with the rest of the signature data, while the second 1220 is unhashed. The second set of subpackets is not cryptographically 1221 protected by the signature and should include only advisory 1222 information. 1224 The difference between a V4 and V5 signature is that the latter 1225 includes additional meta data. 1227 The algorithms for converting the hash function result to a signature 1228 are described in a section below. 1230 5.2.3.1. Signature Subpacket Specification 1232 A subpacket data set consists of zero or more Signature subpackets. 1233 In Signature packets, the subpacket data set is preceded by a two- 1234 octet scalar count of the length in octets of all the subpackets. A 1235 pointer incremented by this number will skip over the subpacket data 1236 set. 1238 Each subpacket consists of a subpacket header and a body. The header 1239 consists of: 1241 * the subpacket length (1, 2, or 5 octets), 1243 * the subpacket type (1 octet), 1245 and is followed by the subpacket-specific data. 1247 The length includes the type octet but not this length. Its format 1248 is similar to the "new" format packet header lengths, but cannot have 1249 Partial Body Lengths. That is: 1251 if the 1st octet < 192, then 1252 lengthOfLength = 1 1253 subpacketLen = 1st_octet 1255 if the 1st octet >= 192 and < 255, then 1256 lengthOfLength = 2 1257 subpacketLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192 1259 if the 1st octet = 255, then 1260 lengthOfLength = 5 1261 subpacket length = [four-octet scalar starting at 2nd_octet] 1263 The value of the subpacket type octet may be: 1265 +------------+----------------------------------------+ 1266 | Type | Description | 1267 +============+========================================+ 1268 | 0 | Reserved | 1269 +------------+----------------------------------------+ 1270 | 1 | Reserved | 1271 +------------+----------------------------------------+ 1272 | 2 | Signature Creation Time | 1273 +------------+----------------------------------------+ 1274 | 3 | Signature Expiration Time | 1275 +------------+----------------------------------------+ 1276 | 4 | Exportable Certification | 1277 +------------+----------------------------------------+ 1278 | 5 | Trust Signature | 1279 +------------+----------------------------------------+ 1280 | 6 | Regular Expression | 1281 +------------+----------------------------------------+ 1282 | 7 | Revocable | 1283 +------------+----------------------------------------+ 1284 | 8 | Reserved | 1285 +------------+----------------------------------------+ 1286 | 9 | Key Expiration Time | 1287 +------------+----------------------------------------+ 1288 | 10 | Placeholder for backward compatibility | 1289 +------------+----------------------------------------+ 1290 | 11 | Preferred Symmetric Algorithms | 1291 +------------+----------------------------------------+ 1292 | 12 | Revocation Key | 1293 +------------+----------------------------------------+ 1294 | 13 to 15 | Reserved | 1295 +------------+----------------------------------------+ 1296 | 16 | Issuer | 1297 +------------+----------------------------------------+ 1298 | 17 to 19 | Reserved | 1299 +------------+----------------------------------------+ 1300 | 20 | Notation Data | 1301 +------------+----------------------------------------+ 1302 | 21 | Preferred Hash Algorithms | 1303 +------------+----------------------------------------+ 1304 | 22 | Preferred Compression Algorithms | 1305 +------------+----------------------------------------+ 1306 | 23 | Key Server Preferences | 1307 +------------+----------------------------------------+ 1308 | 24 | Preferred Key Server | 1309 +------------+----------------------------------------+ 1310 | 25 | Primary User ID | 1311 +------------+----------------------------------------+ 1312 | 26 | Policy URI | 1313 +------------+----------------------------------------+ 1314 | 27 | Key Flags | 1315 +------------+----------------------------------------+ 1316 | 28 | Signer's User ID | 1317 +------------+----------------------------------------+ 1318 | 29 | Reason for Revocation | 1319 +------------+----------------------------------------+ 1320 | 30 | Features | 1321 +------------+----------------------------------------+ 1322 | 31 | Signature Target | 1323 +------------+----------------------------------------+ 1324 | 32 | Embedded Signature | 1325 +------------+----------------------------------------+ 1326 | 33 | Issuer Fingerprint | 1327 +------------+----------------------------------------+ 1328 | 34 | Preferred AEAD Algorithms | 1329 +------------+----------------------------------------+ 1330 | 35 | Intended Recipient Fingerprint | 1331 +------------+----------------------------------------+ 1332 | 37 | Attested Certifications | 1333 +------------+----------------------------------------+ 1334 | 100 to 110 | Private or experimental | 1335 +------------+----------------------------------------+ 1337 Table 3 1339 An implementation SHOULD ignore any subpacket of a type that it does 1340 not recognize. 1342 Bit 7 of the subpacket type is the "critical" bit. If set, it 1343 denotes that the subpacket is one that is critical for the evaluator 1344 of the signature to recognize. If a subpacket is encountered that is 1345 marked critical but is unknown to the evaluating software, the 1346 evaluator SHOULD consider the signature to be in error. 1348 An evaluator may "recognize" a subpacket, but not implement it. The 1349 purpose of the critical bit is to allow the signer to tell an 1350 evaluator that it would prefer a new, unknown feature to generate an 1351 error than be ignored. 1353 Implementations SHOULD implement the four preferred algorithm 1354 subpackets (11, 21, 22, and 34), as well as the "Reason for 1355 Revocation" subpacket. Note, however, that if an implementation 1356 chooses not to implement some of the preferences, it is required to 1357 behave in a polite manner to respect the wishes of those users who do 1358 implement these preferences. 1360 5.2.3.2. Signature Subpacket Types 1362 A number of subpackets are currently defined. Some subpackets apply 1363 to the signature itself and some are attributes of the key. 1364 Subpackets that are found on a self-signature are placed on a 1365 certification made by the key itself. Note that a key may have more 1366 than one User ID, and thus may have more than one self-signature, and 1367 differing subpackets. 1369 A subpacket may be found either in the hashed or unhashed subpacket 1370 sections of a signature. If a subpacket is not hashed, then the 1371 information in it cannot be considered definitive because it is not 1372 part of the signature proper. 1374 5.2.3.3. Notes on Self-Signatures 1376 A self-signature is a binding signature made by the key to which the 1377 signature refers. There are three types of self-signatures, the 1378 certification signatures (types 0x10-0x13), the direct-key signature 1379 (type 0x1F), and the subkey binding signature (type 0x18). For 1380 certification self-signatures, each User ID may have a self- 1381 signature, and thus different subpackets in those self-signatures. 1382 For subkey binding signatures, each subkey in fact has a self- 1383 signature. Subpackets that appear in a certification self-signature 1384 apply to the user name, and subpackets that appear in the subkey 1385 self-signature apply to the subkey. Lastly, subpackets on the 1386 direct-key signature apply to the entire key. 1388 Implementing software should interpret a self-signature's preference 1389 subpackets as narrowly as possible. For example, suppose a key has 1390 two user names, Alice and Bob. Suppose that Alice prefers the 1391 symmetric algorithm AES-256, and Bob prefers Camellia-256 or AES-128. 1392 If the software locates this key via Alice's name, then the preferred 1393 algorithm is AES-256; if software locates the key via Bob's name, 1394 then the preferred algorithm is Camellia-256. If the key is located 1395 by Key ID, the algorithm of the primary User ID of the key provides 1396 the preferred symmetric algorithm. 1398 Revoking a self-signature or allowing it to expire has a semantic 1399 meaning that varies with the signature type. Revoking the self- 1400 signature on a User ID effectively retires that user name. The self- 1401 signature is a statement, "My name X is tied to my signing key K" and 1402 is corroborated by other users' certifications. If another user 1403 revokes their certification, they are effectively saying that they no 1404 longer believe that name and that key are tied together. Similarly, 1405 if the users themselves revoke their self-signature, then the users 1406 no longer go by that name, no longer have that email address, etc. 1407 Revoking a binding signature effectively retires that subkey. 1408 Revoking a direct-key signature cancels that signature. Please see 1409 the "Reason for Revocation" subpacket (Section 5.2.3.24) for more 1410 relevant detail. 1412 Since a self-signature contains important information about the key's 1413 use, an implementation SHOULD allow the user to rewrite the self- 1414 signature, and important information in it, such as preferences and 1415 key expiration. 1417 It is good practice to verify that a self-signature imported into an 1418 implementation doesn't advertise features that the implementation 1419 doesn't support, rewriting the signature as appropriate. 1421 An implementation that encounters multiple self-signatures on the 1422 same object may resolve the ambiguity in any way it sees fit, but it 1423 is RECOMMENDED that priority be given to the most recent self- 1424 signature. 1426 5.2.3.4. Signature Creation Time 1428 (4-octet time field) 1430 The time the signature was made. 1432 MUST be present in the hashed area. 1434 5.2.3.5. Issuer 1436 (8-octet Key ID) 1438 The OpenPGP Key ID of the key issuing the signature. If the version 1439 of that key is greater than 4, this subpacket MUST NOT be included in 1440 the signature. 1442 5.2.3.6. Key Expiration Time 1444 (4-octet time field) 1446 The validity period of the key. This is the number of seconds after 1447 the key creation time that the key expires. If this is not present 1448 or has a value of zero, the key never expires. This is found only on 1449 a self-signature. 1451 5.2.3.7. Preferred Symmetric Algorithms 1453 (array of one-octet values) 1455 Symmetric algorithm numbers that indicate which algorithms the key 1456 holder prefers to use. The subpacket body is an ordered list of 1457 octets with the most preferred listed first. It is assumed that only 1458 algorithms listed are supported by the recipient's software. 1459 Algorithm numbers are in Section 9. This is only found on a self- 1460 signature. 1462 5.2.3.8. Preferred AEAD Algorithms 1464 (array of one-octet values) 1466 AEAD algorithm numbers that indicate which AEAD algorithms the key 1467 holder prefers to use. The subpacket body is an ordered list of 1468 octets with the most preferred listed first. It is assumed that only 1469 algorithms listed are supported by the recipient's software. 1470 Algorithm numbers are in Section 9.6. This is only found on a self- 1471 signature. Note that support for the AEAD Encrypted Data packet in 1472 the general is indicated by a Feature Flag. 1474 5.2.3.9. Preferred Hash Algorithms 1476 (array of one-octet values) 1478 Message digest algorithm numbers that indicate which algorithms the 1479 key holder prefers to receive. Like the preferred symmetric 1480 algorithms, the list is ordered. Algorithm numbers are in 1481 Section 9.5. This is only found on a self-signature. 1483 5.2.3.10. Preferred Compression Algorithms 1485 (array of one-octet values) 1487 Compression algorithm numbers that indicate which algorithms the key 1488 holder prefers to use. Like the preferred symmetric algorithms, the 1489 list is ordered. Algorithm numbers are in Section 9.4. If this 1490 subpacket is not included, ZIP is preferred. A zero denotes that 1491 uncompressed data is preferred; the key holder's software might have 1492 no compression software in that implementation. This is only found 1493 on a self-signature. 1495 5.2.3.11. Signature Expiration Time 1497 (4-octet time field) 1499 The validity period of the signature. This is the number of seconds 1500 after the signature creation time that the signature expires. If 1501 this is not present or has a value of zero, it never expires. 1503 5.2.3.12. Exportable Certification 1505 (1 octet of exportability, 0 for not, 1 for exportable) 1507 This subpacket denotes whether a certification signature is 1508 "exportable", to be used by other users than the signature's issuer. 1509 The packet body contains a Boolean flag indicating whether the 1510 signature is exportable. If this packet is not present, the 1511 certification is exportable; it is equivalent to a flag containing a 1512 1. 1514 Non-exportable, or "local", certifications are signatures made by a 1515 user to mark a key as valid within that user's implementation only. 1517 Thus, when an implementation prepares a user's copy of a key for 1518 transport to another user (this is the process of "exporting" the 1519 key), any local certification signatures are deleted from the key. 1521 The receiver of a transported key "imports" it, and likewise trims 1522 any local certifications. In normal operation, there won't be any, 1523 assuming the import is performed on an exported key. However, there 1524 are instances where this can reasonably happen. For example, if an 1525 implementation allows keys to be imported from a key database in 1526 addition to an exported key, then this situation can arise. 1528 Some implementations do not represent the interest of a single user 1529 (for example, a key server). Such implementations always trim local 1530 certifications from any key they handle. 1532 5.2.3.13. Revocable 1534 (1 octet of revocability, 0 for not, 1 for revocable) 1536 Signature's revocability status. The packet body contains a Boolean 1537 flag indicating whether the signature is revocable. Signatures that 1538 are not revocable have any later revocation signatures ignored. They 1539 represent a commitment by the signer that he cannot revoke his 1540 signature for the life of his key. If this packet is not present, 1541 the signature is revocable. 1543 5.2.3.14. Trust Signature 1545 (1 octet "level" (depth), 1 octet of trust amount) 1547 Signer asserts that the key is not only valid but also trustworthy at 1548 the specified level. Level 0 has the same meaning as an ordinary 1549 validity signature. Level 1 means that the signed key is asserted to 1550 be a valid trusted introducer, with the 2nd octet of the body 1551 specifying the degree of trust. Level 2 means that the signed key is 1552 asserted to be trusted to issue level 1 trust signatures, i.e., that 1553 it is a "meta introducer". Generally, a level n trust signature 1554 asserts that a key is trusted to issue level n-1 trust signatures. 1555 The trust amount is in a range from 0-255, interpreted such that 1556 values less than 120 indicate partial trust and values of 120 or 1557 greater indicate complete trust. Implementations SHOULD emit values 1558 of 60 for partial trust and 120 for complete trust. 1560 5.2.3.15. Regular Expression 1562 (null-terminated regular expression) 1564 Used in conjunction with trust Signature packets (of level > 0) to 1565 limit the scope of trust that is extended. Only signatures by the 1566 target key on User IDs that match the regular expression in the body 1567 of this packet have trust extended by the trust Signature subpacket. 1568 The regular expression uses the same syntax as the Henry Spencer's 1569 "almost public domain" regular expression [REGEX] package. A 1570 description of the syntax is found in Section 8 below. 1572 5.2.3.16. Revocation Key 1574 (1 octet of class, 1 octet of public-key algorithm ID, 20 or 32 1575 octets of fingerprint) 1577 V4 keys use the full 20 octet fingerprint; V5 keys use the full 32 1578 octet fingerprint 1580 Authorizes the specified key to issue revocation signatures for this 1581 key. Class octet must have bit 0x80 set. If the bit 0x40 is set, 1582 then this means that the revocation information is sensitive. Other 1583 bits are for future expansion to other kinds of authorizations. This 1584 is only found on a direct-key self-signature (type 0x1f). The use on 1585 other types of self-signatures is unspecified. 1587 If the "sensitive" flag is set, the keyholder feels this subpacket 1588 contains private trust information that describes a real-world 1589 sensitive relationship. If this flag is set, implementations SHOULD 1590 NOT export this signature to other users except in cases where the 1591 data needs to be available: when the signature is being sent to the 1592 designated revoker, or when it is accompanied by a revocation 1593 signature from that revoker. Note that it may be appropriate to 1594 isolate this subpacket within a separate signature so that it is not 1595 combined with other subpackets that need to be exported. 1597 5.2.3.17. Notation Data 1599 (4 octets of flags, 2 octets of name length (M), 1600 2 octets of value length (N), 1601 M octets of name data, 1602 N octets of value data) 1604 This subpacket describes a "notation" on the signature that the 1605 issuer wishes to make. The notation has a name and a value, each of 1606 which are strings of octets. There may be more than one notation in 1607 a signature. Notations can be used for any extension the issuer of 1608 the signature cares to make. The "flags" field holds four octets of 1609 flags. 1611 All undefined flags MUST be zero. Defined flags are as follows: 1613 First octet: 0x80 = human-readable. This note value is text. 1614 Other octets: none. 1616 Notation names are arbitrary strings encoded in UTF-8. They reside 1617 in two namespaces: The IETF namespace and the user namespace. 1619 The IETF namespace is registered with IANA. These names MUST NOT 1620 contain the "@" character (0x40). This is a tag for the user 1621 namespace. 1623 Names in the user namespace consist of a UTF-8 string tag followed by 1624 "@" followed by a DNS domain name. Note that the tag MUST NOT 1625 contain an "@" character. For example, the "sample" tag used by 1626 Example Corporation could be "sample@example.com". 1628 Names in a user space are owned and controlled by the owners of that 1629 domain. Obviously, it's bad form to create a new name in a DNS space 1630 that you don't own. 1632 Since the user namespace is in the form of an email address, 1633 implementers MAY wish to arrange for that address to reach a person 1634 who can be consulted about the use of the named tag. Note that due 1635 to UTF-8 encoding, not all valid user space name tags are valid email 1636 addresses. 1638 If there is a critical notation, the criticality applies to that 1639 specific notation and not to notations in general. 1641 The following subsections define a set of standard notations. 1643 5.2.3.17.1. The 'charset' Notation 1645 The "charset" notation is a description of the character set used to 1646 encode the signed plaintext. The default value is "UTF-8". If used, 1647 the value MUST be encoded as human readable and MUST be present in 1648 the hashed subpacket section of the signature. This notation is 1649 useful for cleartext signatures in cases where it is not possible to 1650 encode the text in UTF-8. By having the used character set a part of 1651 the signed data, attacks exploiting different representation of code 1652 points will be mitigated. 1654 5.2.3.17.2. The 'manu' Notation 1656 The "manu" notation is a string that declares the device 1657 manufacturer's name. The certifier key is asserting this string 1658 (which may or may not be related to the User ID of the certifier's 1659 key). 1661 5.2.3.17.3. The 'make' Notation 1663 This notation defines the product make. It is a free form string. 1665 5.2.3.17.4. The 'model' Notation 1667 This notation defines the product model name/number. It is a free 1668 form string. 1670 5.2.3.17.5. The 'prodid' Notation 1672 This notation contains the product identifier. It is a free form 1673 string. 1675 5.2.3.17.6. The 'pvers' Notation 1677 This notation defines the product version number (which could be a 1678 release number, year, or some other identifier to differentiate 1679 different versions of the same make/model). It is a free form 1680 string. 1682 5.2.3.17.7. The 'lot' Notation 1684 This notation defines the product lot number (which is an indicator 1685 of the batch of product). It is a free form string. 1687 5.2.3.17.8. The 'qty' Notation 1689 This notation defines the quantity of items in this package. It is a 1690 decimal integer representation with no punctuation, e.g. "10", 1691 "1000", "10000", etc. 1693 5.2.3.17.9. The 'loc' and 'dest' Notations 1695 The "loc" and 'dest' notations declare a GeoLocation as defined by 1696 RFC 5870 [RFC5870] but without the leading "geo:" header. For 1697 example, if you had a GeoLocation URI of "geo:13.4125,103.8667" you 1698 would encode that in these notations as "13.4125,103.8667". 1700 The 'loc' notation is meant to encode the geo location where the 1701 signature was made. The 'dest' notation is meant to encode the geo 1702 location where the device is "destined" (i.e., a "destination" for 1703 the device). 1705 5.2.3.17.10. The 'hash' Notation 1707 A 'hash' notation is a means to include external data in the contents 1708 of a signature without including the data itself. This is done by 1709 hashing the external data separately and then including the data's 1710 name and hash in the signature via this notation. This is useful, 1711 for example, to have an external "manifest," "image," or other data 1712 that might not be vital to the signature itself but still needs to be 1713 protected and authenticated without requiring a second signature. 1715 The 'hash' notation has the following structure: * A single byte 1716 specifying the length of the name of the hashed data * A UTF-8 string 1717 of the name of the hashed data * A single byte specifying the hash 1718 algorithm (see section 9.4) * The binary hash output of the hashed 1719 data using the specified algorithm. (The length of this data is 1720 implicit based on the algorithm specified). 1722 Due to its nature a 'hash' notation is not human readable and MUST 1723 NOT be marked as such when used. 1725 5.2.3.18. Key Server Preferences 1727 (N octets of flags) 1729 This is a list of one-bit flags that indicate preferences that the 1730 key holder has about how the key is handled on a key server. All 1731 undefined flags MUST be zero. 1733 First octet: 0x80 = No-modify 1735 The key holder requests that this key only be modified or updated 1736 by the key holder or an administrator of the key server. 1738 If No-modify is set on the most recent self-sig over a user ID, 1739 then a keyserver should only redistribute those third-party 1740 certifications over that user ID that have been attested to in the 1741 most recent Attestation Key Signature packet (see "Attested 1742 Certifications" below). 1744 This is found only on a self-signature. 1746 5.2.3.19. Preferred Key Server 1748 (String) 1750 This is a URI of a key server that the key holder prefers be used for 1751 updates. Note that keys with multiple User IDs can have a preferred 1752 key server for each User ID. Note also that since this is a URI, the 1753 key server can actually be a copy of the key retrieved by ftp, http, 1754 finger, etc. 1756 5.2.3.20. Primary User ID 1758 (1 octet, Boolean) 1760 This is a flag in a User ID's self-signature that states whether this 1761 User ID is the main User ID for this key. It is reasonable for an 1762 implementation to resolve ambiguities in preferences, etc. by 1763 referring to the primary User ID. If this flag is absent, its value 1764 is zero. If more than one User ID in a key is marked as primary, the 1765 implementation may resolve the ambiguity in any way it sees fit, but 1766 it is RECOMMENDED that priority be given to the User ID with the most 1767 recent self-signature. 1769 When appearing on a self-signature on a User ID packet, this 1770 subpacket applies only to User ID packets. When appearing on a self- 1771 signature on a User Attribute packet, this subpacket applies only to 1772 User Attribute packets. That is to say, there are two different and 1773 independent "primaries" -- one for User IDs, and one for User 1774 Attributes. 1776 5.2.3.21. Policy URI 1778 (String) 1780 This subpacket contains a URI of a document that describes the policy 1781 under which the signature was issued. 1783 5.2.3.22. Key Flags 1785 (N octets of flags) 1787 This subpacket contains a list of binary flags that hold information 1788 about a key. It is a string of octets, and an implementation MUST 1789 NOT assume a fixed size. This is so it can grow over time. If a 1790 list is shorter than an implementation expects, the unstated flags 1791 are considered to be zero. The defined flags are as follows: 1793 First octet: 1795 0x01 - This key may be used to certify other keys. 1797 0x02 - This key may be used to sign data. 1799 0x04 - This key may be used to encrypt communications. 1801 0x08 - This key may be used to encrypt storage. 1803 0x10 - The private component of this key may have been split by a 1804 secret-sharing mechanism. 1806 0x20 - This key may be used for authentication. 1808 0x80 - The private component of this key may be in the possession 1809 of more than one person. 1811 Second octet: 1813 0x04 - This key may be used as an additional decryption subkey (ADSK). 1815 0x08 - This key may be used for timestamping. 1817 Usage notes: 1819 The flags in this packet may appear in self-signatures or in 1820 certification signatures. They mean different things depending on 1821 who is making the statement -- for example, a certification signature 1822 that has the "sign data" flag is stating that the certification is 1823 for that use. On the other hand, the "communications encryption" 1824 flag in a self-signature is stating a preference that a given key be 1825 used for communications. Note however, that it is a thorny issue to 1826 determine what is "communications" and what is "storage". This 1827 decision is left wholly up to the implementation; the authors of this 1828 document do not claim any special wisdom on the issue and realize 1829 that accepted opinion may change. 1831 The "split key" (0x10) and "group key" (0x80) flags are placed on a 1832 self-signature only; they are meaningless on a certification 1833 signature. They SHOULD be placed only on a direct-key signature 1834 (type 0x1F) or a subkey signature (type 0x18), one that refers to the 1835 key the flag applies to. 1837 The ADSK flag helps to figure out an encryption subkey. 1839 5.2.3.23. Signer's User ID 1841 (String) 1843 This subpacket allows a keyholder to state which User ID is 1844 responsible for the signing. Many keyholders use a single key for 1845 different purposes, such as business communications as well as 1846 personal communications. This subpacket allows such a keyholder to 1847 state which of their roles is making a signature. 1849 This subpacket is not appropriate to use to refer to a User Attribute 1850 packet. 1852 5.2.3.24. Reason for Revocation 1854 (1 octet of revocation code, N octets of reason string) 1856 This subpacket is used only in key revocation and certification 1857 revocation signatures. It describes the reason why the key or 1858 certificate was revoked. 1860 The first octet contains a machine-readable code that denotes the 1861 reason for the revocation: 1863 +---------+----------------------------------+ 1864 | Code | Reason | 1865 +=========+==================================+ 1866 | 0 | No reason specified (key | 1867 | | revocations or cert revocations) | 1868 +---------+----------------------------------+ 1869 | 1 | Key is superseded (key | 1870 | | revocations) | 1871 +---------+----------------------------------+ 1872 | 2 | Key material has been | 1873 | | compromised (key revocations) | 1874 +---------+----------------------------------+ 1875 | 3 | Key is retired and no longer | 1876 | | used (key revocations) | 1877 +---------+----------------------------------+ 1878 | 32 | User ID information is no longer | 1879 | | valid (cert revocations) | 1880 +---------+----------------------------------+ 1881 | 100-110 | Private Use | 1882 +---------+----------------------------------+ 1884 Table 4 1886 Following the revocation code is a string of octets that gives 1887 information about the Reason for Revocation in human-readable form 1888 (UTF-8). The string may be null, that is, of zero length. The 1889 length of the subpacket is the length of the reason string plus one. 1890 An implementation SHOULD implement this subpacket, include it in all 1891 revocation signatures, and interpret revocations appropriately. 1892 There are important semantic differences between the reasons, and 1893 there are thus important reasons for revoking signatures. 1895 If a key has been revoked because of a compromise, all signatures 1896 created by that key are suspect. However, if it was merely 1897 superseded or retired, old signatures are still valid. If the 1898 revoked signature is the self-signature for certifying a User ID, a 1899 revocation denotes that that user name is no longer in use. Such a 1900 revocation SHOULD include a 0x20 code. 1902 Note that any signature may be revoked, including a certification on 1903 some other person's key. There are many good reasons for revoking a 1904 certification signature, such as the case where the keyholder leaves 1905 the employ of a business with an email address. A revoked 1906 certification is no longer a part of validity calculations. 1908 5.2.3.25. Features 1910 (N octets of flags) 1912 The Features subpacket denotes which advanced OpenPGP features a 1913 user's implementation supports. This is so that as features are 1914 added to OpenPGP that cannot be backwards-compatible, a user can 1915 state that they can use that feature. The flags are single bits that 1916 indicate that a given feature is supported. 1918 This subpacket is similar to a preferences subpacket, and only 1919 appears in a self-signature. 1921 An implementation SHOULD NOT use a feature listed when sending to a 1922 user who does not state that they can use it. 1924 Defined features are as follows: 1926 First octet: 1928 0x01 - Modification Detection (packets 18 and 19) 1930 0x02 - AEAD Encrypted Data Packet (packet 20) and version 5 1931 Symmetric-Key Encrypted Session Key Packets (packet 3) 1933 0x04 - Version 5 Public-Key Packet format and corresponding new 1934 fingerprint format 1936 If an implementation implements any of the defined features, it 1937 SHOULD implement the Features subpacket, too. 1939 An implementation may freely infer features from other suitable 1940 implementation-dependent mechanisms. 1942 5.2.3.26. Signature Target 1944 (1 octet public-key algorithm, 1 octet hash algorithm, N octets hash) 1946 This subpacket identifies a specific target signature to which a 1947 signature refers. For revocation signatures, this subpacket provides 1948 explicit designation of which signature is being revoked. For a 1949 third-party or timestamp signature, this designates what signature is 1950 signed. All arguments are an identifier of that target signature. 1952 The N octets of hash data MUST be the size of the hash of the 1953 signature. For example, a target signature with a SHA-1 hash MUST 1954 have 20 octets of hash data. 1956 5.2.3.27. Embedded Signature 1958 (1 signature packet body) 1960 This subpacket contains a complete Signature packet body as specified 1961 in Section 5.2 above. It is useful when one signature needs to refer 1962 to, or be incorporated in, another signature. 1964 5.2.3.28. Issuer Fingerprint 1966 (1 octet key version number, N octets of fingerprint) 1968 The OpenPGP Key fingerprint of the key issuing the signature. This 1969 subpacket SHOULD be included in all signatures. If the version of 1970 the issuing key is 4 and an Issuer subpacket is also included in the 1971 signature, the key ID of the Issuer subpacket MUST match the low 64 1972 bits of the fingerprint. 1974 Note that the length N of the fingerprint for a version 4 key is 20 1975 octets; for a version 5 key N is 32. 1977 5.2.3.29. Intended Recipient Fingerprint 1979 (1 octet key version number, N octets of fingerprint) 1981 The OpenPGP Key fingerprint of the intended recipient primary key. 1982 If one or more subpackets of this type are included in a signature, 1983 it SHOULD be considered valid only in an encrypted context, where the 1984 key it was encrypted to is one of the indicated primary keys, or one 1985 of their subkeys. This can be used to prevent forwarding a signature 1986 outside of its intended, encrypted context. 1988 Note that the length N of the fingerprint for a version 4 key is 20 1989 octets; for a version 5 key N is 32. 1991 5.2.3.30. Attested Certifications 1993 (N octets of certification digests) 1995 This subpacket MUST only appear as a hashed subpacket of an 1996 Attestation Key Signature. It has no meaning in any other signature 1997 type. It is used by the primary key to attest to a set of third- 1998 party certifications over the associated User ID or User Attribute. 1999 This enables the holder of an OpenPGP primary key to mark specific 2000 third-party certifications as re-distributable with the rest of the 2001 Transferable Public Key (see the "No-modify" flag in "Key Server 2002 Preferences", above). Implementations MUST include exactly one 2003 Attested Certification subpacket in any generated Attestation Key 2004 Signature. 2006 The contents of the subpacket consists of a series of digests using 2007 the same hash algorithm used by the signature itself. Each digest is 2008 made over one third-party signature (any Certification, i.e., 2009 signature type 0x10-0x13) that covers the same Primary Key and User 2010 ID (or User Attribute). For example, an Attestation Key Signature 2011 made by key X over user ID U using hash algorithm SHA256 might 2012 contain an Attested Certifications subpacket of 192 octets (6*32 2013 octets) covering six third-party certification Signatures over . 2014 They SHOULD be ordered by binary hash value from low to high (e.g., a 2015 hash with hexadecimal value 037a... precedes a hash with value 2016 0392..., etc). The length of this subpacket MUST be an integer 2017 multiple of the length of the hash algorithm used for the enclosing 2018 Attestation Key Signature. 2020 The listed digests MUST be calculated over the third-party 2021 certification's Signature packet as described in the "Computing 2022 Signatures" section, but without a trailer: the hash data starts with 2023 the octet 0x88, followed by the four-octet length of the Signature, 2024 and then the body of the Signature packet. (Note that this is an 2025 old-style packet header for a Signature packet with the length-of- 2026 length field set to zero.) The unhashed subpacket data of the 2027 Signature packet being hashed is not included in the hash, and the 2028 unhashed subpacket data length value is set to zero. 2030 If an implementation encounters more than one such subpacket in an 2031 Attestation Key Signature, it MUST treat it as a single Attested 2032 Certifications subpacket containing the union of all hashes. 2034 The Attested Certifications subpacket in the most recent Attestation 2035 Key Signature over a given user ID supersedes all Attested 2036 Certifications subpackets from any previous Attestation Key 2037 Signature. However, note that if more than one Attestation Key 2038 Signatures has the same (most recent) Signature Creation Time 2039 subpacket, implementations MUST consider the union of the 2040 attestations of all Attestation Key Signatures (this allows the 2041 keyholder to attest to more third-party certifications than could fit 2042 in a single Attestation Key Signature). 2044 If a keyholder Alice has already attested to third-party 2045 certifications from Bob and Carol and she wants to add an attestation 2046 to a certification from David, she should issue a new Attestation Key 2047 Signature (with a more recent Signature Creation timestamp) that 2048 contains an Attested Certifications subpacket covering all three 2049 third-party certifications. 2051 If she later decides that she does not want Carol's certification to 2052 be redistributed with her certificate, she can issue a new 2053 Attestation Key Signature (again, with a more recent Signature 2054 Creation timestamp) that contains an Attested Certifications 2055 subpacket covering only the certifications from Bob and David. 2057 Note that Certification Revocation Signatures are not relevant for 2058 Attestation Key Signatures. To rescind all attestations, the primary 2059 key holder needs only to publish a more recent Attestation Key 2060 Signature with an empty Attested Certifications subpacket. 2062 5.2.4. Computing Signatures 2064 All signatures are formed by producing a hash over the signature 2065 data, and then using the resulting hash in the signature algorithm. 2067 For binary document signatures (type 0x00), the document data is 2068 hashed directly. For text document signatures (type 0x01), the 2069 document is canonicalized by converting line endings to , and 2070 the resulting data is hashed. 2072 When a V4 signature is made over a key, the hash data starts with the 2073 octet 0x99, followed by a two-octet length of the key, and then body 2074 of the key packet; when a V5 signature is made over a key, the hash 2075 data starts with the octet 0x9a, followed by a four-octet length of 2076 the key, and then body of the key packet. A subkey binding signature 2077 (type 0x18) or primary key binding signature (type 0x19) then hashes 2078 the subkey using the same format as the main key (also using 0x99 or 2079 0x9a as the first octet). Primary key revocation signatures (type 2080 0x20) hash only the key being revoked. Subkey revocation signature 2081 (type 0x28) hash first the primary key and then the subkey being 2082 revoked. 2084 A certification signature (type 0x10 through 0x13) hashes the User ID 2085 being bound to the key into the hash context after the above data. A 2086 V3 certification hashes the contents of the User ID or attribute 2087 packet packet, without any header. A V4 or V5 certification hashes 2088 the constant 0xB4 for User ID certifications or the constant 0xD1 for 2089 User Attribute certifications, followed by a four-octet number giving 2090 the length of the User ID or User Attribute data, and then the User 2091 ID or User Attribute data. 2093 An Attestation Key Signature (0x16) hashes the same data boy that a 2094 standard certification signature does: primary key, followed by User 2095 ID or User Attribute. 2097 When a signature is made over a Signature packet (type 0x50, "Third- 2098 Party Confirmation signature"), the hash data starts with the octet 2099 0x88, followed by the four-octet length of the signature, and then 2100 the body of the Signature packet. (Note that this is an old-style 2101 packet header for a Signature packet with the length-of-length field 2102 set to zero.) The unhashed subpacket data of the Signature packet 2103 being hashed is not included in the hash, and the unhashed subpacket 2104 data length value is set to zero. 2106 Once the data body is hashed, then a trailer is hashed. This trailer 2107 depends on the version of the signature. 2109 * A V3 signature hashes five octets of the packet body, starting 2110 from the signature type field. This data is the signature type, 2111 followed by the four-octet signature time. 2113 * A V4 signature hashes the packet body starting from its first 2114 field, the version number, through the end of the hashed subpacket 2115 data and a final extra trailer. Thus, the hashed fields are: 2117 - the signature version (0x04), 2119 - the signature type, 2121 - the public-key algorithm, 2123 - the hash algorithm, 2125 - the hashed subpacket length, 2127 - the hashed subpacket body, 2129 - the two octets 0x04 and 0xFF, 2131 - a four-octet big-endian number that is the length of the hashed 2132 data from the Signature packet stopping right before the 0x04, 2133 0xff octets. 2135 The four-octet big-endian number is considered to be an 2136 unsigned integer modulo 2^32. 2138 * A V5 signature hashes the packet body starting from its first 2139 field, the version number, through the end of the hashed subpacket 2140 data and a final extra trailer. Thus, the hashed fields are: 2142 - the signature version (0x05), 2144 - the signature type, 2146 - the public-key algorithm, 2148 - the hash algorithm, 2150 - the hashed subpacket length, 2152 - the hashed subpacket body, 2154 - Only for document signatures (type 0x00 or 0x01) the following 2155 three data items are hashed here: 2157 o the one-octet content format, 2159 o the file name as a string (one octet length, followed by the 2160 file name), 2162 o a four-octet number that indicates a date, 2164 - the two octets 0x05 and 0xFF, 2166 - a eight-octet big-endian number that is the length of the 2167 hashed data from the Signature packet stopping right before the 2168 0x05, 0xff octets. 2170 The three data items hashed for document signatures need to 2171 mirror the values of the Literal Data packet. For detached and 2172 cleartext signatures 6 zero bytes are hashed instead. 2174 After all this has been hashed in a single hash context, the 2175 resulting hash field is used in the signature algorithm and placed at 2176 the end of the Signature packet. 2178 5.2.4.1. Subpacket Hints 2180 It is certainly possible for a signature to contain conflicting 2181 information in subpackets. For example, a signature may contain 2182 multiple copies of a preference or multiple expiration times. In 2183 most cases, an implementation SHOULD use the last subpacket in the 2184 signature, but MAY use any conflict resolution scheme that makes more 2185 sense. Please note that we are intentionally leaving conflict 2186 resolution to the implementer; most conflicts are simply syntax 2187 errors, and the wishy-washy language here allows a receiver to be 2188 generous in what they accept, while putting pressure on a creator to 2189 be stingy in what they generate. 2191 Some apparent conflicts may actually make sense -- for example, 2192 suppose a keyholder has a V3 key and a V4 key that share the same RSA 2193 key material. Either of these keys can verify a signature created by 2194 the other, and it may be reasonable for a signature to contain an 2195 issuer subpacket for each key, as a way of explicitly tying those 2196 keys to the signature. 2198 5.3. Symmetric-Key Encrypted Session Key Packets (Tag 3) 2200 The Symmetric-Key Encrypted Session Key packet holds the symmetric- 2201 key encryption of a session key used to encrypt a message. Zero or 2202 more Public-Key Encrypted Session Key packets and/or Symmetric-Key 2203 Encrypted Session Key packets may precede a Symmetrically Encrypted 2204 Data packet that holds an encrypted message. The message is 2205 encrypted with a session key, and the session key is itself encrypted 2206 and stored in the Encrypted Session Key packet or the Symmetric-Key 2207 Encrypted Session Key packet. 2209 If the Symmetrically Encrypted Data packet is preceded by one or more 2210 Symmetric-Key Encrypted Session Key packets, each specifies a 2211 passphrase that may be used to decrypt the message. This allows a 2212 message to be encrypted to a number of public keys, and also to one 2213 or more passphrases. This packet type is new and is not generated by 2214 PGP 2 or PGP version 5.0. 2216 A version 4 Symmetric-Key Encrypted Session Key packet consists of: 2218 * A one-octet version number with value 4. 2220 * A one-octet number describing the symmetric algorithm used. 2222 * A string-to-key (S2K) specifier, length as defined above. 2224 * Optionally, the encrypted session key itself, which is decrypted 2225 with the string-to-key object. 2227 If the encrypted session key is not present (which can be detected on 2228 the basis of packet length and S2K specifier size), then the S2K 2229 algorithm applied to the passphrase produces the session key for 2230 decrypting the message, using the symmetric cipher algorithm from the 2231 Symmetric-Key Encrypted Session Key packet. 2233 If the encrypted session key is present, the result of applying the 2234 S2K algorithm to the passphrase is used to decrypt just that 2235 encrypted session key field, using CFB mode with an IV of all zeros. 2236 The decryption result consists of a one-octet algorithm identifier 2237 that specifies the symmetric-key encryption algorithm used to encrypt 2238 the following Symmetrically Encrypted Data packet, followed by the 2239 session key octets themselves. 2241 Note: because an all-zero IV is used for this decryption, the S2K 2242 specifier MUST use a salt value, either a Salted S2K or an Iterated- 2243 Salted S2K. The salt value will ensure that the decryption key is 2244 not repeated even if the passphrase is reused. 2246 A version 5 Symmetric-Key Encrypted Session Key packet consists of: 2248 * A one-octet version number with value 5. 2250 * A one-octet cipher algorithm. 2252 * A one-octet AEAD algorithm. 2254 * A string-to-key (S2K) specifier, length as defined above. 2256 * A starting initialization vector of size specified by the AEAD 2257 algorithm. 2259 * The encrypted session key itself, which is decrypted with the 2260 string-to-key object using the given cipher and AEAD mode. 2262 * An authentication tag for the AEAD mode. 2264 The encrypted session key is encrypted using one of the AEAD 2265 algorithms specified for the AEAD Encrypted Packet. Note that no 2266 chunks are used and that there is only one authentication tag. The 2267 Packet Tag in new format encoding (bits 7 and 6 set, bits 5-0 carry 2268 the packet tag), the packet version number, the cipher algorithm 2269 octet, and the AEAD algorithm octet are given as additional data. 2270 For example, the additional data used with EAX and AES-128 consists 2271 of the octets 0xC3, 0x05, 0x07, and 0x01. 2273 5.4. One-Pass Signature Packets (Tag 4) 2275 The One-Pass Signature packet precedes the signed data and contains 2276 enough information to allow the receiver to begin calculating any 2277 hashes needed to verify the signature. It allows the Signature 2278 packet to be placed at the end of the message, so that the signer can 2279 compute the entire signed message in one pass. 2281 A One-Pass Signature does not interoperate with PGP 2.6.x or earlier. 2283 The body of this packet consists of: 2285 * A one-octet version number. The current version is 3. 2287 * A one-octet signature type. Signature types are described in 2288 Section 5.2.1. 2290 * A one-octet number describing the hash algorithm used. 2292 * A one-octet number describing the public-key algorithm used. 2294 * An eight-octet number holding the Key ID of the signing key. 2296 * A one-octet number holding a flag showing whether the signature is 2297 nested. A zero value indicates that the next packet is another 2298 One-Pass Signature packet that describes another signature to be 2299 applied to the same message data. 2301 Note that if a message contains more than one one-pass signature, 2302 then the Signature packets bracket the message; that is, the first 2303 Signature packet after the message corresponds to the last one-pass 2304 packet and the final Signature packet corresponds to the first one- 2305 pass packet. 2307 5.5. Key Material Packet 2309 A key material packet contains all the information about a public or 2310 private key. There are four variants of this packet type, and two 2311 major versions. Consequently, this section is complex. 2313 5.5.1. Key Packet Variants 2315 5.5.1.1. Public-Key Packet (Tag 6) 2317 A Public-Key packet starts a series of packets that forms an OpenPGP 2318 key (sometimes called an OpenPGP certificate). 2320 5.5.1.2. Public-Subkey Packet (Tag 14) 2322 A Public-Subkey packet (tag 14) has exactly the same format as a 2323 Public-Key packet, but denotes a subkey. One or more subkeys may be 2324 associated with a top-level key. By convention, the top-level key 2325 provides signature services, and the subkeys provide encryption 2326 services. 2328 Note: in PGP version 2.6, tag 14 was intended to indicate a comment 2329 packet. This tag was selected for reuse because no previous version 2330 of PGP ever emitted comment packets but they did properly ignore 2331 them. Public-Subkey packets are ignored by PGP version 2.6 and do 2332 not cause it to fail, providing a limited degree of backward 2333 compatibility. 2335 5.5.1.3. Secret-Key Packet (Tag 5) 2337 A Secret-Key packet contains all the information that is found in a 2338 Public-Key packet, including the public-key material, but also 2339 includes the secret-key material after all the public-key fields. 2341 5.5.1.4. Secret-Subkey Packet (Tag 7) 2343 A Secret-Subkey packet (tag 7) is the subkey analog of the Secret Key 2344 packet and has exactly the same format. 2346 5.5.2. Public-Key Packet Formats 2348 There are three versions of key-material packets. Version 3 packets 2349 were first generated by PGP version 2.6. Version 4 keys first 2350 appeared in PGP 5 and are the preferred key version for OpenPGP. 2352 OpenPGP implementations MUST create keys with version 4 format. V3 2353 keys are deprecated; an implementation MUST NOT generate a V3 key, 2354 but MAY accept it. 2356 A version 3 public key or public-subkey packet contains: 2358 * A one-octet version number (3). 2360 * A four-octet number denoting the time that the key was created. 2362 * A two-octet number denoting the time in days that this key is 2363 valid. If this number is zero, then it does not expire. 2365 * A one-octet number denoting the public-key algorithm of this key. 2367 * A series of multiprecision integers comprising the key material: 2369 - a multiprecision integer (MPI) of RSA public modulus n; 2371 - an MPI of RSA public encryption exponent e. 2373 V3 keys are deprecated. They contain three weaknesses. First, it is 2374 relatively easy to construct a V3 key that has the same Key ID as any 2375 other key because the Key ID is simply the low 64 bits of the public 2376 modulus. Secondly, because the fingerprint of a V3 key hashes the 2377 key material, but not its length, there is an increased opportunity 2378 for fingerprint collisions. Third, there are weaknesses in the MD5 2379 hash algorithm that make developers prefer other algorithms. See 2380 below for a fuller discussion of Key IDs and fingerprints. 2382 V2 keys are identical to the deprecated V3 keys except for the 2383 version number. An implementation MUST NOT generate them and MAY 2384 accept or reject them as it sees fit. 2386 The version 4 format is similar to the version 3 format except for 2387 the absence of a validity period. This has been moved to the 2388 Signature packet. In addition, fingerprints of version 4 keys are 2389 calculated differently from version 3 keys, as described in the 2390 section "Enhanced Key Formats". 2392 A version 4 packet contains: 2394 * A one-octet version number (4). 2396 * A four-octet number denoting the time that the key was created. 2398 * A one-octet number denoting the public-key algorithm of this key. 2400 * A series of values comprising the key material. This is 2401 algorithm-specific and described in Section 5.6. 2403 The version 5 format is similar to the version 4 format except for 2404 the addition of a count for the key material. This count helps 2405 parsing secret key packets (which are an extension of the public key 2406 packet format) in the case of an unknown algoritm. In addition, 2407 fingerprints of version 5 keys are calculated differently from 2408 version 4 keys, as described in the section "Enhanced Key Formats". 2410 A version 5 packet contains: 2412 * A one-octet version number (5). 2414 * A four-octet number denoting the time that the key was created. 2416 * A one-octet number denoting the public-key algorithm of this key. 2418 * A four-octet scalar octet count for the following public key 2419 material. 2421 * A series of values comprising the public key material. This is 2422 algorithm-specific and described in Section 5.6. 2424 5.5.3. Secret-Key Packet Formats 2426 The Secret-Key and Secret-Subkey packets contain all the data of the 2427 Public-Key and Public-Subkey packets, with additional algorithm- 2428 specific secret-key data appended, usually in encrypted form. 2430 The packet contains: 2432 * A Public-Key or Public-Subkey packet, as described above. 2434 * One octet indicating string-to-key usage conventions. Zero 2435 indicates that the secret-key data is not encrypted. 255 or 254 2436 indicates that a string-to-key specifier is being given. Any 2437 other value is a symmetric-key encryption algorithm identifier. A 2438 version 5 packet MUST NOT use the value 255. 2440 * Only for a version 5 packet, a one-octet scalar octet count of the 2441 next 4 optional fields. 2443 * [Optional] If string-to-key usage octet was 255, 254, or 253, a 2444 one-octet symmetric encryption algorithm. 2446 * [Optional] If string-to-key usage octet was 253, a one-octet AEAD 2447 algorithm. 2449 * [Optional] If string-to-key usage octet was 255, 254, or 253, a 2450 string-to-key specifier. The length of the string-to-key 2451 specifier is implied by its type, as described above. 2453 * [Optional] If secret data is encrypted (string-to-key usage octet 2454 not zero), an Initial Vector (IV) of the same length as the 2455 cipher's block size. If string-to-key usage octet was 253 the IV 2456 is used as the nonce for the AEAD algorithm. If the AEAD 2457 algorithm requires a shorter nonce, the high-order bits of the IV 2458 are used and the remaining bits MUST be zero. 2460 * Only for a version 5 packet, a four-octet scalar octet count for 2461 the following secret key material. This includes the encrypted 2462 SHA-1 hash or AEAD tag if the string-to-key usage octet is 254 or 2463 253. 2465 * Plain or encrypted series of values comprising the secret key 2466 material. This is algorithm-specific and described in section 2467 Section 5.6. Note that if the string-to-key usage octet is 254, a 2468 20-octet SHA-1 hash of the plaintext of the algorithm-specific 2469 portion is appended to plaintext and encrypted with it. If the 2470 string-to-key usage octet is 253, then an AEAD authentication tag 2471 is part of that data. 2473 * If the string-to-key usage octet is zero or 255, then a two-octet 2474 checksum of the plaintext of the algorithm-specific portion (sum 2475 of all octets, mod 65536). 2477 Note that the version 5 packet format adds two count values to help 2478 parsing packets with unknown S2K or public key algorithms. 2480 Secret MPI values can be encrypted using a passphrase. If a string- 2481 to-key specifier is given, that describes the algorithm for 2482 converting the passphrase to a key, else a simple MD5 hash of the 2483 passphrase is used. Implementations MUST use a string-to-key 2484 specifier; the simple hash is for backward compatibility and is 2485 deprecated, though implementations MAY continue to use existing 2486 private keys in the old format. The cipher for encrypting the MPIs 2487 is specified in the Secret-Key packet. 2489 Encryption/decryption of the secret data is done in CFB mode using 2490 the key created from the passphrase and the Initial Vector from the 2491 packet. A different mode is used with V3 keys (which are only RSA) 2492 than with other key formats. With V3 keys, the MPI bit count prefix 2493 (i.e., the first two octets) is not encrypted. Only the MPI non- 2494 prefix data is encrypted. Furthermore, the CFB state is 2495 resynchronized at the beginning of each new MPI value, so that the 2496 CFB block boundary is aligned with the start of the MPI data. 2498 With V4 and V5 keys, a simpler method is used. All secret MPI values 2499 are encrypted, including the MPI bitcount prefix. 2501 If the string-to-key usage octet is 253, the encrypted MPI values are 2502 encrypted as one combined plaintext using one of the AEAD algorithms 2503 specified for the AEAD Encrypted Packet. Note that no chunks are 2504 used and that there is only one authentication tag. The Packet Tag 2505 in new format encoding (bits 7 and 6 set, bits 5-0 carry the packet 2506 tag), the packet version number, the cipher algorithm octet, and the 2507 AEAD algorithm octet are given as additional data. For example, the 2508 additional data used with EAX and AES-128 in a Secret-Key Packet of 2509 version 4 consists of the octets 0xC5, 0x04, 0x07, and 0x01; in a 2510 Secret-Subkey Packet the first octet would be 0xC7. 2512 The two-octet checksum that follows the algorithm-specific portion is 2513 the algebraic sum, mod 65536, of the plaintext of all the algorithm- 2514 specific octets (including MPI prefix and data). With V3 keys, the 2515 checksum is stored in the clear. With V4 keys, the checksum is 2516 encrypted like the algorithm-specific data. This value is used to 2517 check that the passphrase was correct. However, this checksum is 2518 deprecated; an implementation SHOULD NOT use it, but should rather 2519 use the SHA-1 hash denoted with a usage octet of 254. The reason for 2520 this is that there are some attacks that involve undetectably 2521 modifying the secret key. If the string-to-key usage octet is 253 no 2522 checksum or SHA-1 hash is used but the authentication tag of the AEAD 2523 algorithm follows. 2525 5.6. Algorithm-specific Parts of Keys 2527 The public and secret key format specifies algorithm-specific parts 2528 of a key. The following sections describe them in detail. 2530 5.6.1. Algorithm-Specific Part for RSA Keys 2532 The public key is this series of multiprecision integers: 2534 * MPI of RSA public modulus n; 2536 * MPI of RSA public encryption exponent e. 2538 The secret key is this series of multiprecision integers: 2540 * MPI of RSA secret exponent d; 2542 * MPI of RSA secret prime value p; 2544 * MPI of RSA secret prime value q (p < q); 2546 * MPI of u, the multiplicative inverse of p, mod q. 2548 5.6.2. Algorithm-Specific Part for DSA Keys 2550 The public key is this series of multiprecision integers: 2552 * MPI of DSA prime p; 2554 * MPI of DSA group order q (q is a prime divisor of p-1); 2556 * MPI of DSA group generator g; 2558 * MPI of DSA public-key value y (= g**x mod p where x is secret). 2560 The secret key is this single multiprecision integer: 2562 * MPI of DSA secret exponent x. 2564 5.6.3. Algorithm-Specific Part for Elgamal Keys 2566 The public key is this series of multiprecision integers: 2568 * MPI of Elgamal prime p; 2570 * MPI of Elgamal group generator g; 2572 * MPI of Elgamal public key value y (= g**x mod p where x is 2573 secret). 2575 The secret key is this single multiprecision integer: 2577 * MPI of Elgamal secret exponent x. 2579 5.6.4. Algorithm-Specific Part for ECDSA Keys 2581 The public key is this series of values: 2583 * a variable-length field containing a curve OID, formatted as 2584 follows: 2586 - a one-octet size of the following field; values 0 and 0xFF are 2587 reserved for future extensions, 2589 - the octets representing a curve OID, defined in Section 9.2; 2591 * a MPI of an EC point representing a public key. 2593 The secret key is this single multiprecision integer: 2595 * MPI of an integer representing the secret key, which is a scalar 2596 of the public EC point. 2598 5.6.5. Algorithm-Specific Part for EdDSA Keys 2600 The public key is this series of values: 2602 * a variable-length field containing a curve OID, formatted as 2603 follows: 2605 - a one-octet size of the following field; values 0 and 0xFF are 2606 reserved for future extensions, 2608 - the octets representing a curve OID, defined in Section 9.2; 2610 * a MPI of an EC point representing a public key Q as described 2611 under EdDSA Point Format below. 2613 The secret key is this single multiprecision integer: 2615 * MPI of an integer representing the secret key, which is a scalar 2616 of the public EC point. 2618 5.6.6. Algorithm-Specific Part for ECDH Keys 2620 The public key is this series of values: 2622 * a variable-length field containing a curve OID, formatted as 2623 follows: 2625 - a one-octet size of the following field; values 0 and 0xFF are 2626 reserved for future extensions, 2628 - the octets representing a curve OID, defined in Section 9.2; 2630 * a MPI of an EC point representing a public key; 2632 * a variable-length field containing KDF parameters, formatted as 2633 follows: 2635 - a one-octet size of the following fields; values 0 and 0xff are 2636 reserved for future extensions; 2638 - a one-octet value 1, reserved for future extensions; 2640 - a one-octet hash function ID used with a KDF; 2642 - a one-octet algorithm ID for the symmetric algorithm used to 2643 wrap the symmetric key used for the message encryption; see 2644 Section 13.5 for details. 2646 Observe that an ECDH public key is composed of the same sequence of 2647 fields that define an ECDSA key, plus the KDF parameters field. 2649 The secret key is this single multiprecision integer: 2651 * MPI of an integer representing the secret key, which is a scalar 2652 of the public EC point. 2654 5.7. Compressed Data Packet (Tag 8) 2656 The Compressed Data packet contains compressed data. Typically, this 2657 packet is found as the contents of an encrypted packet, or following 2658 a Signature or One-Pass Signature packet, and contains a literal data 2659 packet. 2661 The body of this packet consists of: 2663 * One octet that gives the algorithm used to compress the packet. 2665 * Compressed data, which makes up the remainder of the packet. 2667 A Compressed Data Packet's body contains an block that compresses 2668 some set of packets. See section "Packet Composition" for details on 2669 how messages are formed. 2671 ZIP-compressed packets are compressed with raw RFC 1951 [RFC1951] 2672 DEFLATE blocks. Note that PGP V2.6 uses 13 bits of compression. If 2673 an implementation uses more bits of compression, PGP V2.6 cannot 2674 decompress it. 2676 ZLIB-compressed packets are compressed with RFC 1950 [RFC1950] ZLIB- 2677 style blocks. 2679 BZip2-compressed packets are compressed using the BZip2 [BZ2] 2680 algorithm. 2682 5.8. Symmetrically Encrypted Data Packet (Tag 9) 2684 The Symmetrically Encrypted Data packet contains data encrypted with 2685 a symmetric-key algorithm. When it has been decrypted, it contains 2686 other packets (usually a literal data packet or compressed data 2687 packet, but in theory other Symmetrically Encrypted Data packets or 2688 sequences of packets that form whole OpenPGP messages). 2690 This packet is obsolete. An implementation MUST NOT create this 2691 packet. An implementation MAY process such a packet but it MUST 2692 return a clear diagnostic that a non-integrity protected packet has 2693 been processed. The implementation SHOULD also return an error in 2694 this case and stop processing. 2696 The body of this packet consists of: 2698 * Encrypted data, the output of the selected symmetric-key cipher 2699 operating in OpenPGP's variant of Cipher Feedback (CFB) mode. 2701 The symmetric cipher used may be specified in a Public-Key or 2702 Symmetric-Key Encrypted Session Key packet that precedes the 2703 Symmetrically Encrypted Data packet. In that case, the cipher 2704 algorithm octet is prefixed to the session key before it is 2705 encrypted. If no packets of these types precede the encrypted data, 2706 the IDEA algorithm is used with the session key calculated as the MD5 2707 hash of the passphrase, though this use is deprecated. 2709 The data is encrypted in CFB mode, with a CFB shift size equal to the 2710 cipher's block size. The Initial Vector (IV) is specified as all 2711 zeros. Instead of using an IV, OpenPGP prefixes a string of length 2712 equal to the block size of the cipher plus two to the data before it 2713 is encrypted. The first block-size octets (for example, 8 octets for 2714 a 64-bit block length) are random, and the following two octets are 2715 copies of the last two octets of the IV. For example, in an 8-octet 2716 block, octet 9 is a repeat of octet 7, and octet 10 is a repeat of 2717 octet 8. In a cipher of length 16, octet 17 is a repeat of octet 15 2718 and octet 18 is a repeat of octet 16. As a pedantic clarification, 2719 in both these examples, we consider the first octet to be numbered 1. 2721 After encrypting the first block-size-plus-two octets, the CFB state 2722 is resynchronized. The last block-size octets of ciphertext are 2723 passed through the cipher and the block boundary is reset. 2725 The repetition of 16 bits in the random data prefixed to the message 2726 allows the receiver to immediately check whether the session key is 2727 incorrect. See the "Security Considerations" section for hints on 2728 the proper use of this "quick check". 2730 5.9. Marker Packet (Obsolete Literal Packet) (Tag 10) 2732 An experimental version of PGP used this packet as the Literal 2733 packet, but no released version of PGP generated Literal packets with 2734 this tag. With PGP 5, this packet has been reassigned and is 2735 reserved for use as the Marker packet. 2737 The body of this packet consists of: 2739 * The three octets 0x50, 0x47, 0x50 (which spell "PGP" in UTF-8). 2741 Such a packet MUST be ignored when received. It may be placed at the 2742 beginning of a message that uses features not available in PGP 2743 version 2.6 in order to cause that version to report that newer 2744 software is necessary to process the message. 2746 5.10. Literal Data Packet (Tag 11) 2748 A Literal Data packet contains the body of a message; data that is 2749 not to be further interpreted. 2751 The body of this packet consists of: 2753 * A one-octet field that describes how the data is formatted. 2755 If it is a 'b' (0x62), then the Literal packet contains binary 2756 data. If it is a 't' (0x74), then it contains text data, and thus 2757 may need line ends converted to local form, or other text-mode 2758 changes. The tag 'u' (0x75) means the same as 't', but also 2759 indicates that implementation believes that the literal data 2760 contains UTF-8 text. If it is a 'm' (0x6d), then it contains a 2761 MIME message body part [RFC2045]. 2763 Early versions of PGP also defined a value of 'l' as a 'local' 2764 mode for machine-local conversions. RFC 1991 [RFC1991] 2765 incorrectly stated this local mode flag as '1' (ASCII numeral 2766 one). Both of these local modes are deprecated. 2768 * File name as a string (one-octet length, followed by a file name). 2769 This may be a zero-length string. Commonly, if the source of the 2770 encrypted data is a file, this will be the name of the encrypted 2771 file. An implementation MAY consider the file name in the Literal 2772 packet to be a more authoritative name than the actual file name. 2774 If the special name "_CONSOLE" is used, the message is considered 2775 to be "for your eyes only". This advises that the message data is 2776 unusually sensitive, and the receiving program should process it 2777 more carefully, perhaps avoiding storing the received data to 2778 disk, for example. 2780 * A four-octet number that indicates a date associated with the 2781 literal data. Commonly, the date might be the modification date 2782 of a file, or the time the packet was created, or a zero that 2783 indicates no specific time. 2785 * The remainder of the packet is literal data. 2787 Text data is stored with text endings (i.e., network- 2788 normal line endings). These should be converted to native line 2789 endings by the receiving software. 2791 Note that V3 and V4 signatures do not include the formatting octet, 2792 the file name, and the date field of the literal packet in a 2793 signature hash and thus are not protected against tampering in a 2794 signed document. In contrast V5 signatures include them. 2796 5.11. Trust Packet (Tag 12) 2798 The Trust packet is used only within keyrings and is not normally 2799 exported. Trust packets contain data that record the user's 2800 specifications of which key holders are trustworthy introducers, 2801 along with other information that implementing software uses for 2802 trust information. The format of Trust packets is defined by a given 2803 implementation. 2805 Trust packets SHOULD NOT be emitted to output streams that are 2806 transferred to other users, and they SHOULD be ignored on any input 2807 other than local keyring files. 2809 5.12. User ID Packet (Tag 13) 2811 A User ID packet consists of UTF-8 text that is intended to represent 2812 the name and email address of the key holder. By convention, it 2813 includes an RFC 2822 [RFC2822] mail name-addr, but there are no 2814 restrictions on its content. The packet length in the header 2815 specifies the length of the User ID. 2817 5.13. User Attribute Packet (Tag 17) 2819 The User Attribute packet is a variation of the User ID packet. It 2820 is capable of storing more types of data than the User ID packet, 2821 which is limited to text. Like the User ID packet, a User Attribute 2822 packet may be certified by the key owner ("self-signed") or any other 2823 key owner who cares to certify it. Except as noted, a User Attribute 2824 packet may be used anywhere that a User ID packet may be used. 2826 While User Attribute packets are not a required part of the OpenPGP 2827 standard, implementations SHOULD provide at least enough 2828 compatibility to properly handle a certification signature on the 2829 User Attribute packet. A simple way to do this is by treating the 2830 User Attribute packet as a User ID packet with opaque contents, but 2831 an implementation may use any method desired. 2833 The User Attribute packet is made up of one or more attribute 2834 subpackets. Each subpacket consists of a subpacket header and a 2835 body. The header consists of: 2837 * the subpacket length (1, 2, or 5 octets) 2839 * the subpacket type (1 octet) 2841 and is followed by the subpacket specific data. 2843 The following table lists the currently known subpackets: 2845 +---------+-----------------------------+ 2846 | Type | Attribute Subpacket | 2847 +=========+=============================+ 2848 | 1 | Image Attribute Subpacket | 2849 +---------+-----------------------------+ 2850 | [TBD1] | User ID Attribute Subpacket | 2851 +---------+-----------------------------+ 2852 | 100-110 | Private/Experimental Use | 2853 +---------+-----------------------------+ 2855 Table 5 2857 An implementation SHOULD ignore any subpacket of a type that it does 2858 not recognize. 2860 5.13.1. The Image Attribute Subpacket 2862 The Image Attribute subpacket is used to encode an image, presumably 2863 (but not required to be) that of the key owner. 2865 The Image Attribute subpacket begins with an image header. The first 2866 two octets of the image header contain the length of the image 2867 header. Note that unlike other multi-octet numerical values in this 2868 document, due to a historical accident this value is encoded as a 2869 little-endian number. The image header length is followed by a 2870 single octet for the image header version. The only currently 2871 defined version of the image header is 1, which is a 16-octet image 2872 header. The first three octets of a version 1 image header are thus 2873 0x10, 0x00, 0x01. 2875 The fourth octet of a version 1 image header designates the encoding 2876 format of the image. The only currently defined encoding format is 2877 the value 1 to indicate JPEG. Image format types 100 through 110 are 2878 reserved for private or experimental use. The rest of the version 1 2879 image header is made up of 12 reserved octets, all of which MUST be 2880 set to 0. 2882 The rest of the image subpacket contains the image itself. As the 2883 only currently defined image type is JPEG, the image is encoded in 2884 the JPEG File Interchange Format (JFIF), a standard file format for 2885 JPEG images [JFIF]. 2887 An implementation MAY try to determine the type of an image by 2888 examination of the image data if it is unable to handle a particular 2889 version of the image header or if a specified encoding format value 2890 is not recognized. 2892 5.13.2. User ID Attribute Subpacket 2894 A User ID Attribute subpacket has type [IANA -- assignment TBD1]. 2896 A User ID Attribute subpacket, just like a User ID packet, consists 2897 of UTF-8 text that is intended to represent the name and email 2898 address of the key holder. By convention, it includes an RFC 2822 2899 [RFC2822] mail name-addr, but there are no restrictions on its 2900 content. For devices using OpenPGP for device certificates, it may 2901 just be the device identifier. The packet length in the header 2902 specifies the length of the User ID. 2904 Because User Attribute subpackets can be used anywhere a User ID 2905 packet can be used, implementations MAY choose to trust a signed User 2906 Attribute subpacket that includes a User ID Attribute subpacket. 2908 5.14. Sym. Encrypted Integrity Protected Data Packet (Tag 18) 2910 The Symmetrically Encrypted Integrity Protected Data packet is a 2911 variant of the Symmetrically Encrypted Data packet. It is a new 2912 feature created for OpenPGP that addresses the problem of detecting a 2913 modification to encrypted data. It is used in combination with a 2914 Modification Detection Code packet. 2916 There is a corresponding feature in the features Signature subpacket 2917 that denotes that an implementation can properly use this packet 2918 type. An implementation MUST support decrypting these packets and 2919 SHOULD prefer generating them to the older Symmetrically Encrypted 2920 Data packet when possible. Since this data packet protects against 2921 modification attacks, this standard encourages its proliferation. 2922 While blanket adoption of this data packet would create 2923 interoperability problems, rapid adoption is nevertheless important. 2924 An implementation SHOULD specifically denote support for this packet, 2925 but it MAY infer it from other mechanisms. 2927 For example, an implementation might infer from the use of a cipher 2928 such as Advanced Encryption Standard (AES) or Twofish that a user 2929 supports this feature. It might place in the unhashed portion of 2930 another user's key signature a Features subpacket. It might also 2931 present a user with an opportunity to regenerate their own self- 2932 signature with a Features subpacket. 2934 This packet contains data encrypted with a symmetric-key algorithm 2935 and protected against modification by the SHA-1 hash algorithm. When 2936 it has been decrypted, it will typically contain other packets (often 2937 a Literal Data packet or Compressed Data packet). The last decrypted 2938 packet in this packet's payload MUST be a Modification Detection Code 2939 packet. 2941 The body of this packet consists of: 2943 * A one-octet version number. The only defined value is 1. There 2944 won't be any future versions of this packet because the MDC system 2945 has been superseded by the AEAD Encrypted Data packet. 2947 * Encrypted data, the output of the selected symmetric-key cipher 2948 operating in Cipher Feedback mode with shift amount equal to the 2949 block size of the cipher (CFB-n where n is the block size). 2951 The symmetric cipher used MUST be specified in a Public-Key or 2952 Symmetric-Key Encrypted Session Key packet that precedes the 2953 Symmetrically Encrypted Data packet. In either case, the cipher 2954 algorithm octet is prefixed to the session key before it is 2955 encrypted. 2957 The data is encrypted in CFB mode, with a CFB shift size equal to the 2958 cipher's block size. The Initial Vector (IV) is specified as all 2959 zeros. Instead of using an IV, OpenPGP prefixes an octet string to 2960 the data before it is encrypted. The length of the octet string 2961 equals the block size of the cipher in octets, plus two. The first 2962 octets in the group, of length equal to the block size of the cipher, 2963 are random; the last two octets are each copies of their 2nd 2964 preceding octet. For example, with a cipher whose block size is 128 2965 bits or 16 octets, the prefix data will contain 16 random octets, 2966 then two more octets, which are copies of the 15th and 16th octets, 2967 respectively. Unlike the Symmetrically Encrypted Data Packet, no 2968 special CFB resynchronization is done after encrypting this prefix 2969 data. See "OpenPGP CFB Mode" below for more details. 2971 The repetition of 16 bits in the random data prefixed to the message 2972 allows the receiver to immediately check whether the session key is 2973 incorrect. 2975 The plaintext of the data to be encrypted is passed through the SHA-1 2976 hash function, and the result of the hash is appended to the 2977 plaintext in a Modification Detection Code packet. The input to the 2978 hash function includes the prefix data described above; it includes 2979 all of the plaintext, and then also includes two octets of values 2980 0xD3, 0x14. These represent the encoding of a Modification Detection 2981 Code packet tag and length field of 20 octets. 2983 The resulting hash value is stored in a Modification Detection Code 2984 (MDC) packet, which MUST use the two octet encoding just given to 2985 represent its tag and length field. The body of the MDC packet is 2986 the 20-octet output of the SHA-1 hash. 2988 The Modification Detection Code packet is appended to the plaintext 2989 and encrypted along with the plaintext using the same CFB context. 2991 During decryption, the plaintext data should be hashed with SHA-1, 2992 including the prefix data as well as the packet tag and length field 2993 of the Modification Detection Code packet. The body of the MDC 2994 packet, upon decryption, is compared with the result of the SHA-1 2995 hash. 2997 Any failure of the MDC indicates that the message has been modified 2998 and MUST be treated as a security problem. Failures include a 2999 difference in the hash values, but also the absence of an MDC packet, 3000 or an MDC packet in any position other than the end of the plaintext. 3001 Any failure SHOULD be reported to the user. 3003 NON-NORMATIVE EXPLANATION 3005 The MDC system, as packets 18 and 19 are called, were created to 3006 provide an integrity mechanism that is less strong than a 3007 signature, yet stronger than bare CFB encryption. 3009 It is a limitation of CFB encryption that damage to the 3010 ciphertext will corrupt the affected cipher blocks and the block 3011 following. Additionally, if data is removed from the end of a 3012 CFB-encrypted block, that removal is undetectable. (Note also 3013 that CBC mode has a similar limitation, but data removed from 3014 the front of the block is undetectable.) 3016 The obvious way to protect or authenticate an encrypted block is 3017 to digitally sign it. However, many people do not wish to 3018 habitually sign data, for a large number of reasons beyond the 3019 scope of this document. Suffice it to say that many people 3020 consider properties such as deniability to be as valuable as 3021 integrity. 3023 OpenPGP addresses this desire to have more security than raw 3024 encryption and yet preserve deniability with the MDC system. An 3025 MDC is intentionally not a MAC. Its name was not selected by 3026 accident. It is analogous to a checksum. 3028 Despite the fact that it is a relatively modest system, it has 3029 proved itself in the real world. It is an effective defense to 3030 several attacks that have surfaced since it has been created. 3031 It has met its modest goals admirably. 3033 Consequently, because it is a modest security system, it has 3034 modest requirements on the hash function(s) it employs. It does 3035 not rely on a hash function being collision-free, it relies on a 3036 hash function being one-way. If a forger, Frank, wishes to send 3037 Alice a (digitally) unsigned message that says, "I've always 3038 secretly loved you, signed Bob", it is far easier for him to 3039 construct a new message than it is to modify anything 3040 intercepted from Bob. (Note also that if Bob wishes to 3041 communicate secretly with Alice, but without authentication or 3042 identification and with a threat model that includes forgers, he 3043 has a problem that transcends mere cryptography.) 3045 Note also that unlike nearly every other OpenPGP subsystem, 3046 there are no parameters in the MDC system. It hard-defines 3047 SHA-1 as its hash function. This is not an accident. It is an 3048 intentional choice to avoid downgrade and cross-grade attacks 3049 while making a simple, fast system. (A downgrade attack would 3050 be an attack that replaced SHA2-256 with SHA-1, for example. A 3051 cross-grade attack would replace SHA-1 with another 160-bit 3052 hash, such as RIPE-MD/160, for example.) 3054 However, no update will be needed because the MDC will be 3055 replaced by the AEAD encryption described in this document. 3057 5.15. Modification Detection Code Packet (Tag 19) 3059 The Modification Detection Code packet contains a SHA-1 hash of 3060 plaintext data, which is used to detect message modification. It is 3061 only used with a Symmetrically Encrypted Integrity Protected Data 3062 packet. The Modification Detection Code packet MUST be the last 3063 packet in the plaintext data that is encrypted in the Symmetrically 3064 Encrypted Integrity Protected Data packet, and MUST appear in no 3065 other place. 3067 A Modification Detection Code packet MUST have a length of 20 octets. 3069 The body of this packet consists of: 3071 * A 20-octet SHA-1 hash of the preceding plaintext data of the 3072 Symmetrically Encrypted Integrity Protected Data packet, including 3073 prefix data, the tag octet, and length octet of the Modification 3074 Detection Code packet. 3076 Note that the Modification Detection Code packet MUST always use a 3077 new format encoding of the packet tag, and a one-octet encoding of 3078 the packet length. The reason for this is that the hashing rules for 3079 modification detection include a one-octet tag and one-octet length 3080 in the data hash. While this is a bit restrictive, it reduces 3081 complexity. 3083 5.16. AEAD Encrypted Data Packet (Tag 20) 3085 This packet contains data encrypted with an authenticated encryption 3086 and additional data (AEAD) construction. When it has been decrypted, 3087 it will typically contain other packets (often a Literal Data packet 3088 or Compressed Data packet). 3090 The body of this packet consists of: 3092 * A one-octet version number. The only currently defined value is 3093 1. 3095 * A one-octet cipher algorithm. 3097 * A one-octet AEAD algorithm. 3099 * A one-octet chunk size. 3101 * A starting initialization vector of size specified by the AEAD 3102 algorithm. 3104 * Encrypted data, the output of the selected symmetric-key cipher 3105 operating in the given AEAD mode. 3107 * A final, summary authentication tag for the AEAD mode. 3109 An AEAD encrypted data packet consists of one or more chunks of data. 3110 The plaintext of each chunk is of a size specified using the chunk 3111 size octet using the method specified below. 3113 The encrypted data consists of the encryption of each chunk of 3114 plaintext, followed immediately by the relevant authentication tag. 3115 If the last chunk of plaintext is smaller than the chunk size, the 3116 ciphertext for that data may be shorter; it is nevertheless followed 3117 by a full authentication tag. 3119 For each chunk, the AEAD construction is given the Packet Tag in new 3120 format encoding (bits 7 and 6 set, bits 5-0 carry the packet tag), 3121 version number, cipher algorithm octet, AEAD algorithm octet, chunk 3122 size octet, and an eight-octet, big-endian chunk index as additional 3123 data. The index of the first chunk is zero. For example, the 3124 additional data of the first chunk using EAX and AES-128 with a chunk 3125 size of 64 kiByte consists of the octets 0xD4, 0x01, 0x07, 0x01, 3126 0x10, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, and 0x00. 3128 After the final chunk, the AEAD algorithm is used to produce a final 3129 authentication tag encrypting the empty string. This AEAD instance 3130 is given the additional data specified above, plus an eight-octet, 3131 big-endian value specifying the total number of plaintext octets 3132 encrypted. This allows detection of a truncated ciphertext. Please 3133 note that the big-endian number representing the chunk index in the 3134 additional data is increased accordingly, although it's not really a 3135 chunk. 3137 The chunk size octet specifies the size of chunks using the following 3138 formula (in C), where c is the chunk size octet: 3140 chunk_size = ((uint64_t)1 << (c + 6)) 3142 An implementation MUST support chunk size octets with values from 0 3143 to 56. Chunk size octets with other values are reserved for future 3144 extensions. Implementations SHOULD NOT create data with a chunk size 3145 octet value larger than 21 (128 MiB chunks) to facilitate buffering 3146 of not yet authenticated plaintext. 3148 A new random initialization vector MUST be used for each message. 3149 Failure to do so for each message will lead to a catastrophic failure 3150 depending on the used AEAD mode. 3152 5.16.1. EAX Mode 3154 The EAX algorithm can only use block ciphers with 16-octet blocks. 3155 The starting initialization vector and authentication tag are both 16 3156 octets long. 3158 The starting initialization vector for this mode MUST be unique and 3159 unpredictable. 3161 The nonce for EAX mode is computed by treating the starting 3162 initialization vector as a 16-octet, big-endian value and exclusive- 3163 oring the low eight octets of it with the chunk index. 3165 The security of EAX requires that the nonce is never reused, hence 3166 the requirement that the starting initialization vector be unique. 3168 5.16.2. OCB Mode 3170 The OCB Authenticated-Encryption Algorithm used in this document is 3171 defined in [RFC7253]. 3173 OCB usage requires specification of the following parameters: 3175 * a blockcipher that operate on 128-bit (16-octet) blocks 3177 * an authentication tag length of 16 octets 3179 * a nonce of 15 octets long (which is the longest nonce allowed 3180 specified by [RFC7253]) 3182 * an initialization vector of at least 15 octets long 3184 In the case that the initialization vector is longer than 15 octets 3185 (such as in Section 5.5.1.3, only the 15 leftmost octets are used in 3186 calculations; the remaining octets MUST be considered as zero. 3188 The nonce for OCB mode is computed by the exclusive-oring of the 3189 initialization vector as a 15-octet, big endian value, against the 3190 chunk index. 3192 Security of OCB mode depends on the non-repeated nature of nonces 3193 used for the same key on distinct plaintext [RFC7253]. Therefore the 3194 initialization vector per message MUST be distinct, and OCB mode 3195 SHOULD only be used in environments when there is certainty to 3196 fulfilling this requirement. 3198 6. Radix-64 Conversions 3200 As stated in the introduction, OpenPGP's underlying native 3201 representation for objects is a stream of arbitrary octets, and some 3202 systems desire these objects to be immune to damage caused by 3203 character set translation, data conversions, etc. 3205 In principle, any printable encoding scheme that met the requirements 3206 of the unsafe channel would suffice, since it would not change the 3207 underlying binary bit streams of the native OpenPGP data structures. 3208 The OpenPGP standard specifies one such printable encoding scheme to 3209 ensure interoperability. 3211 OpenPGP's Radix-64 encoding is composed of two parts: a base64 3212 encoding of the binary data and a checksum. The base64 encoding is 3213 identical to the MIME base64 content-transfer-encoding [RFC2045]. 3215 The checksum is a 24-bit Cyclic Redundancy Check (CRC) converted to 3216 four characters of radix-64 encoding by the same MIME base64 3217 transformation, preceded by an equal sign (=). The CRC is computed 3218 by using the generator 0x864CFB and an initialization of 0xB704CE. 3219 The accumulation is done on the data before it is converted to radix- 3220 64, rather than on the converted data. A sample implementation of 3221 this algorithm is in the next section. 3223 The checksum with its leading equal sign MAY appear on the first line 3224 after the base64 encoded data. 3226 Rationale for CRC-24: The size of 24 bits fits evenly into printable 3227 base64. The nonzero initialization can detect more errors than a 3228 zero initialization. 3230 6.1. An Implementation of the CRC-24 in "C" 3232 3233 #define CRC24_INIT 0xB704CEL 3234 #define CRC24_POLY 0x864CFBL 3236 typedef long crc24; 3237 crc24 crc_octets(unsigned char *octets, size_t len) 3238 { 3239 crc24 crc = CRC24_INIT; 3240 int i; 3241 while (len--) { 3242 crc ^= (*octets++) << 16; 3243 for (i = 0; i < 8; i++) { 3244 crc <<= 1; 3245 if (crc & 0x1000000) 3246 crc ^= CRC24_POLY; 3247 } 3248 } 3249 return crc & 0xFFFFFFL; 3250 } 3251 3253 6.2. Forming ASCII Armor 3255 When OpenPGP encodes data into ASCII Armor, it puts specific headers 3256 around the Radix-64 encoded data, so OpenPGP can reconstruct the data 3257 later. An OpenPGP implementation MAY use ASCII armor to protect raw 3258 binary data. OpenPGP informs the user what kind of data is encoded 3259 in the ASCII armor through the use of the headers. 3261 Concatenating the following data creates ASCII Armor: 3263 * An Armor Header Line, appropriate for the type of data 3265 * Armor Headers 3267 * A blank line 3269 * The ASCII-Armored data 3271 * An Armor Checksum 3273 * The Armor Tail, which depends on the Armor Header Line 3275 An Armor Header Line consists of the appropriate header line text 3276 surrounded by five (5) dashes ('-', 0x2D) on either side of the 3277 header line text. The header line text is chosen based upon the type 3278 of data that is being encoded in Armor, and how it is being encoded. 3279 Header line texts include the following strings: 3281 BEGIN PGP MESSAGE Used for signed, encrypted, or compressed files. 3283 BEGIN PGP PUBLIC KEY BLOCK Used for armoring public keys. 3285 BEGIN PGP PRIVATE KEY BLOCK Used for armoring private keys. 3287 BEGIN PGP MESSAGE, PART X/Y Used for multi-part messages, where the 3288 armor is split amongst Y parts, and this is the Xth part out of Y. 3290 BEGIN PGP MESSAGE, PART X Used for multi-part messages, where this 3291 is the Xth part of an unspecified number of parts. Requires the 3292 MESSAGE-ID Armor Header to be used. 3294 BEGIN PGP SIGNATURE Used for detached signatures, OpenPGP/MIME 3295 signatures, and cleartext signatures. Note that PGP 2 uses BEGIN 3296 PGP MESSAGE for detached signatures. 3298 Note that all these Armor Header Lines are to consist of a complete 3299 line. That is to say, there is always a line ending preceding the 3300 starting five dashes, and following the ending five dashes. The 3301 header lines, therefore, MUST start at the beginning of a line, and 3302 MUST NOT have text other than whitespace -- space (0x20), tab (0x09) 3303 or carriage return (0x0d) -- following them on the same line. These 3304 line endings are considered a part of the Armor Header Line for the 3305 purposes of determining the content they delimit. This is 3306 particularly important when computing a cleartext signature (see 3307 below). 3309 The Armor Headers are pairs of strings that can give the user or the 3310 receiving OpenPGP implementation some information about how to decode 3311 or use the message. The Armor Headers are a part of the armor, not a 3312 part of the message, and hence are not protected by any signatures 3313 applied to the message. 3315 The format of an Armor Header is that of a key-value pair. A colon 3316 (':' 0x38) and a single space (0x20) separate the key and value. 3317 OpenPGP should consider improperly formatted Armor Headers to be 3318 corruption of the ASCII Armor. Unknown keys should be reported to 3319 the user, but OpenPGP should continue to process the message. 3321 Note that some transport methods are sensitive to line length. While 3322 there is a limit of 76 characters for the Radix-64 data 3323 (Section 6.3), there is no limit to the length of Armor Headers. 3324 Care should be taken that the Armor Headers are short enough to 3325 survive transport. One way to do this is to repeat an Armor Header 3326 Key multiple times with different values for each so that no one line 3327 is overly long. 3329 Currently defined Armor Header Keys are as follows: 3331 * "Version", which states the OpenPGP implementation and version 3332 used to encode the message. 3334 * "Comment", a user-defined comment. OpenPGP defines all text to be 3335 in UTF-8. A comment may be any UTF-8 string. However, the whole 3336 point of armoring is to provide seven-bit-clean data. 3337 Consequently, if a comment has characters that are outside the US- 3338 ASCII range of UTF, they may very well not survive transport. 3340 * "Hash", a comma-separated list of hash algorithms used in this 3341 message. This is used only in cleartext signed messages. 3343 * "MessageID", a 32-character string of printable characters. The 3344 string must be the same for all parts of a multi-part message that 3345 uses the "PART X" Armor Header. MessageID strings should be 3346 unique enough that the recipient of the mail can associate all the 3347 parts of a message with each other. A good checksum or 3348 cryptographic hash function is sufficient. 3350 The MessageID SHOULD NOT appear unless it is in a multi-part 3351 message. If it appears at all, it MUST be computed from the 3352 finished (encrypted, signed, etc.) message in a deterministic 3353 fashion, rather than contain a purely random value. This is to 3354 allow the legitimate recipient to determine that the MessageID 3355 cannot serve as a covert means of leaking cryptographic key 3356 information. 3358 * "Charset", a description of the character set that the plaintext 3359 is in. Please note that OpenPGP defines text to be in UTF-8. An 3360 implementation will get best results by translating into and out 3361 of UTF-8. However, there are many instances where this is easier 3362 said than done. Also, there are communities of users who have no 3363 need for UTF-8 because they are all happy with a character set 3364 like ISO Latin-5 or a Japanese character set. In such instances, 3365 an implementation MAY override the UTF-8 default by using this 3366 header key. An implementation MAY implement this key and any 3367 translations it cares to; an implementation MAY ignore it and 3368 assume all text is UTF-8. 3370 The blank line can either be zero-length or contain only whitespace, 3371 that is spaces (0x20), tabs (0x09) or carriage returns (0x0d). 3373 The Armor Tail Line is composed in the same manner as the Armor 3374 Header Line, except the string "BEGIN" is replaced by the string 3375 "END". 3377 6.3. Encoding Binary in Radix-64 3379 The encoding process represents 24-bit groups of input bits as output 3380 strings of 4 encoded characters. Proceeding from left to right, a 3381 24-bit input group is formed by concatenating three 8-bit input 3382 groups. These 24 bits are then treated as four concatenated 6-bit 3383 groups, each of which is translated into a single digit in the 3384 Radix-64 alphabet. When encoding a bit stream with the Radix-64 3385 encoding, the bit stream must be presumed to be ordered with the most 3386 significant bit first. That is, the first bit in the stream will be 3387 the high-order bit in the first 8-bit octet, and the eighth bit will 3388 be the low-order bit in the first 8-bit octet, and so on. 3390 +--first octet--+-second octet--+--third octet--+ 3391 |7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0| 3392 +-----------+---+-------+-------+---+-----------+ 3393 |5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0| 3394 +--1.index--+--2.index--+--3.index--+--4.index--+ 3396 Each 6-bit group is used as an index into an array of 64 printable 3397 characters from the table below. The character referenced by the 3398 index is placed in the output string. 3400 Value Encoding Value Encoding Value Encoding Value Encoding 3401 0 A 17 R 34 i 51 z 3402 1 B 18 S 35 j 52 0 3403 2 C 19 T 36 k 53 1 3404 3 D 20 U 37 l 54 2 3405 4 E 21 V 38 m 55 3 3406 5 F 22 W 39 n 56 4 3407 6 G 23 X 40 o 57 5 3408 7 H 24 Y 41 p 58 6 3409 8 I 25 Z 42 q 59 7 3410 9 J 26 a 43 r 60 8 3411 10 K 27 b 44 s 61 9 3412 11 L 28 c 45 t 62 + 3413 12 M 29 d 46 u 63 / 3414 13 N 30 e 47 v 3415 14 O 31 f 48 w (pad) = 3416 15 P 32 g 49 x 3417 16 Q 33 h 50 y 3419 The encoded output stream must be represented in lines of no more 3420 than 76 characters each. 3422 Special processing is performed if fewer than 24 bits are available 3423 at the end of the data being encoded. There are three possibilities: 3425 1. The last data group has 24 bits (3 octets). No special 3426 processing is needed. 3428 2. The last data group has 16 bits (2 octets). The first two 3429 6-bit groups are processed as above. The third (incomplete) 3430 data group has two zero-value bits added to it, and is 3431 processed as above. A pad character (=) is added to the 3432 output. 3434 3. The last data group has 8 bits (1 octet). The first 6-bit 3435 group is processed as above. The second (incomplete) data 3436 group has four zero-value bits added to it, and is processed 3437 as above. Two pad characters (=) are added to the output. 3439 6.4. Decoding Radix-64 3441 In Radix-64 data, characters other than those in the table, line 3442 breaks, and other white space probably indicate a transmission error, 3443 about which a warning message or even a message rejection might be 3444 appropriate under some circumstances. Decoding software must ignore 3445 all white space. 3447 Because it is used only for padding at the end of the data, the 3448 occurrence of any "=" characters may be taken as evidence that the 3449 end of the data has been reached (without truncation in transit). No 3450 such assurance is possible, however, when the number of octets 3451 transmitted was a multiple of three and no "=" characters are 3452 present. 3454 6.5. Examples of Radix-64 3456 Input data: 0x14FB9C03D97E 3457 Hex: 1 4 F B 9 C | 0 3 D 9 7 E 3458 8-bit: 00010100 11111011 10011100 | 00000011 11011001 01111110 3459 6-bit: 000101 001111 101110 011100 | 000000 111101 100101 111110 3460 Decimal: 5 15 46 28 0 61 37 62 3461 Output: F P u c A 9 l + 3462 Input data: 0x14FB9C03D9 3463 Hex: 1 4 F B 9 C | 0 3 D 9 3464 8-bit: 00010100 11111011 10011100 | 00000011 11011001 3465 pad with 00 3466 6-bit: 000101 001111 101110 011100 | 000000 111101 100100 3467 Decimal: 5 15 46 28 0 61 36 3468 pad with = 3469 Output: F P u c A 9 k = 3470 Input data: 0x14FB9C03 3471 Hex: 1 4 F B 9 C | 0 3 3472 8-bit: 00010100 11111011 10011100 | 00000011 3473 pad with 0000 3474 6-bit: 000101 001111 101110 011100 | 000000 110000 3475 Decimal: 5 15 46 28 0 48 3476 pad with = = 3477 Output: F P u c A w = = 3479 6.6. Example of an ASCII Armored Message 3481 -----BEGIN PGP MESSAGE----- 3482 Version: OpenPrivacy 0.99 3484 yDgBO22WxBHv7O8X7O/jygAEzol56iUKiXmV+XmpCtmpqQUKiQrFqclFqUDBovzS 3485 vBSFjNSiVHsuAA== 3486 =njUN 3487 -----END PGP MESSAGE----- 3489 Note that this example has extra indenting; an actual armored message 3490 would have no leading whitespace. 3492 7. Cleartext Signature Framework 3494 It is desirable to be able to sign a textual octet stream without 3495 ASCII armoring the stream itself, so the signed text is still 3496 readable without special software. In order to bind a signature to 3497 such a cleartext, this framework is used, which follows the same 3498 basic format and restrictions as the ASCII armoring described above 3499 in "Forming ASCII Armor" (Section 6.2). (Note that this framework is 3500 not intended to be reversible. RFC 3156 [RFC3156] defines another 3501 way to sign cleartext messages for environments that support MIME.) 3503 The cleartext signed message consists of: 3505 * The cleartext header "-----BEGIN PGP SIGNED MESSAGE-----" on a 3506 single line, 3508 * One or more "Hash" Armor Headers, 3510 * Exactly one blank line not included into the message digest, 3512 * The dash-escaped cleartext that is included into the message 3513 digest, 3515 * The ASCII armored signature(s) including the "-----BEGIN PGP 3516 SIGNATURE-----" Armor Header and Armor Tail Lines. 3518 If the "Hash" Armor Header is given, the specified message digest 3519 algorithm(s) are used for the signature. If there are no such 3520 headers, MD5 is used. If MD5 is the only hash used, then an 3521 implementation MAY omit this header for improved V2.x compatibility. 3522 If more than one message digest is used in the signature, the "Hash" 3523 armor header contains a comma-delimited list of used message digests. 3525 Current message digest names are described below with the algorithm 3526 IDs. 3528 An implementation SHOULD add a line break after the cleartext, but 3529 MAY omit it if the cleartext ends with a line break. This is for 3530 visual clarity. 3532 7.1. Dash-Escaped Text 3534 The cleartext content of the message must also be dash-escaped. 3536 Dash-escaped cleartext is the ordinary cleartext where every line 3537 starting with a dash '-' (0x2D) is prefixed by the sequence dash '-' 3538 (0x2D) and space ' ' (0x20). This prevents the parser from 3539 recognizing armor headers of the cleartext itself. An implementation 3540 MAY dash-escape any line, SHOULD dash-escape lines commencing "From" 3541 followed by a space, and MUST dash-escape any line commencing in a 3542 dash. The message digest is computed using the cleartext itself, not 3543 the dash-escaped form. 3545 As with binary signatures on text documents, a cleartext signature is 3546 calculated on the text using canonical line endings. The 3547 line ending (i.e., the ) before the "-----BEGIN PGP 3548 SIGNATURE-----" line that terminates the signed text is not 3549 considered part of the signed text. 3551 When reversing dash-escaping, an implementation MUST strip the string 3552 "- " if it occurs at the beginning of a line, and SHOULD warn on "-" 3553 and any character other than a space at the beginning of a line. 3555 Also, any trailing whitespace -- spaces (0x20), tabs (0x09) or 3556 carriage returns (0x0d) -- at the end of any line is removed when the 3557 cleartext signature is generated and verified. 3559 8. Regular Expressions 3561 A regular expression is zero or more branches, separated by '|'. It 3562 matches anything that matches one of the branches. 3564 A branch is zero or more pieces, concatenated. It matches a match 3565 for the first, followed by a match for the second, etc. 3567 A piece is an atom possibly followed by '_', '+', or '?'. An atom 3568 followed by '_' matches a sequence of 0 or more matches of the atom. 3569 An atom followed by '+' matches a sequence of 1 or more matches of 3570 the atom. An atom followed by '?' matches a match of the atom, or 3571 the null string. 3573 An atom is a regular expression in parentheses (matching a match for 3574 the regular expression), a range (see below), '.' (matching any 3575 single character), '^' (matching the null string at the beginning of 3576 the input string), '$' (matching the null string at the end of the 3577 input string), a '\' followed by a single character (matching that 3578 character), or a single character with no other significance 3579 (matching that character). 3581 A range is a sequence of characters enclosed in "[]". It normally 3582 matches any single character from the sequence. If the sequence 3583 begins with '^', it matches any single character not from the rest of 3584 the sequence. If two characters in the sequence are separated by 3585 '-', this is shorthand for the full list of ASCII characters between 3586 them (e.g., "[0-9]" matches any decimal digit). To include a literal 3587 ']' in the sequence, make it the first character (following a 3588 possible '^'). To include a literal '-', make it the first or last 3589 character. 3591 9. Constants 3593 This section describes the constants used in OpenPGP. 3595 Note that these tables are not exhaustive lists; an implementation 3596 MAY implement an algorithm not on these lists, so long as the 3597 algorithm numbers are chosen from the private or experimental 3598 algorithm range. 3600 See the section "Notes on Algorithms" below for more discussion of 3601 the algorithms. 3603 9.1. Public-Key Algorithms 3605 +---------+---------------------------------------------------+ 3606 | ID | Algorithm | 3607 +=========+===================================================+ 3608 | 1 | RSA (Encrypt or Sign) [HAC] | 3609 +---------+---------------------------------------------------+ 3610 | 2 | RSA Encrypt-Only [HAC] | 3611 +---------+---------------------------------------------------+ 3612 | 3 | RSA Sign-Only [HAC] | 3613 +---------+---------------------------------------------------+ 3614 | 16 | Elgamal (Encrypt-Only) [ELGAMAL] [HAC] | 3615 +---------+---------------------------------------------------+ 3616 | 17 | DSA (Digital Signature Algorithm) [FIPS186] [HAC] | 3617 +---------+---------------------------------------------------+ 3618 | 18 | ECDH public key algorithm | 3619 +---------+---------------------------------------------------+ 3620 | 19 | ECDSA public key algorithm [FIPS186] | 3621 +---------+---------------------------------------------------+ 3622 | 20 | Reserved (formerly Elgamal Encrypt or Sign) | 3623 +---------+---------------------------------------------------+ 3624 | 21 | Reserved for Diffie-Hellman (X9.42, as defined | 3625 | | for IETF-S/MIME) | 3626 +---------+---------------------------------------------------+ 3627 | 22 | EdDSA [RFC8032] | 3628 +---------+---------------------------------------------------+ 3629 | 23 | Reserved for AEDH | 3630 +---------+---------------------------------------------------+ 3631 | 24 | Reserved for AEDSA | 3632 +---------+---------------------------------------------------+ 3633 | 100-110 | Private/Experimental algorithm | 3634 +---------+---------------------------------------------------+ 3636 Table 6 3638 Implementations MUST implement RSA (1) and ECDSA (19) for signatures, 3639 and RSA (1) and ECDH (18) for encryption. Implementations SHOULD 3640 implement EdDSA (22) keys. 3642 RSA Encrypt-Only (2) and RSA Sign-Only (3) are deprecated and SHOULD 3643 NOT be generated, but may be interpreted. See Section 14.5. See 3644 Section 14.9 for notes on Elgamal Encrypt or Sign (20), and X9.42 3645 (21). Implementations MAY implement any other algorithm. 3647 Note that implementations conforming to previous versions of this 3648 standard (RFC-4880) have DSA (17) and Elgamal (16) as its only MUST- 3649 implement algorithm. 3651 A compatible specification of ECDSA is given in [RFC6090] as "KT-I 3652 Signatures" and in [SEC1]; ECDH is defined in Section 13.5 this 3653 document. 3655 9.2. ECC Curve OID 3657 The parameter curve OID is an array of octets that define a named 3658 curve. The table below specifies the exact sequence of bytes for 3659 each named curve referenced in this document: 3661 +------------------------+-----+-----------------+-----------------+ 3662 | ASN.1 Object | OID | Curve OID bytes | Curve name | 3663 | Identifier | len | in hexadecimal | | 3664 | | | representation | | 3665 +========================+=====+=================+=================+ 3666 | 1.2.840.10045.3.1.7 | 8 | 2A 86 48 CE 3D | NIST P-256 | 3667 | | | 03 01 07 | | 3668 +------------------------+-----+-----------------+-----------------+ 3669 | 1.3.132.0.34 | 5 | 2B 81 04 00 22 | NIST P-384 | 3670 +------------------------+-----+-----------------+-----------------+ 3671 | 1.3.132.0.35 | 5 | 2B 81 04 00 23 | NIST P-521 | 3672 +------------------------+-----+-----------------+-----------------+ 3673 | 1.3.36.3.3.2.8.1.1.7 | 9 | 2B 24 03 03 02 | brainpoolP256r1 | 3674 | | | 08 01 01 07 | | 3675 +------------------------+-----+-----------------+-----------------+ 3676 | 1.3.36.3.3.2.8.1.1.13 | 9 | 2B 24 03 03 02 | brainpoolP512r1 | 3677 | | | 08 01 01 0D | | 3678 +------------------------+-----+-----------------+-----------------+ 3679 | 1.3.6.1.4.1.11591.15.1 | 9 | 2B 06 01 04 01 | Ed25519 | 3680 | | | DA 47 0F 01 | | 3681 +------------------------+-----+-----------------+-----------------+ 3682 | 1.3.6.1.4.1.3029.1.5.1 | 10 | 2B 06 01 04 01 | Curve25519 | 3683 | | | 97 55 01 05 01 | | 3684 +------------------------+-----+-----------------+-----------------+ 3686 Table 7 3688 The sequence of octets in the third column is the result of applying 3689 the Distinguished Encoding Rules (DER) to the ASN.1 Object Identifier 3690 with subsequent truncation. The truncation removes the two fields of 3691 encoded Object Identifier. The first omitted field is one octet 3692 representing the Object Identifier tag, and the second omitted field 3693 is the length of the Object Identifier body. For example, the 3694 complete ASN.1 DER encoding for the NIST P-256 curve OID is "06 08 2A 3695 86 48 CE 3D 03 01 07", from which the first entry in the table above 3696 is constructed by omitting the first two octets. Only the truncated 3697 sequence of octets is the valid representation of a curve OID. 3699 9.3. Symmetric-Key Algorithms 3701 +---------+--------------------------------------+ 3702 | ID | Algorithm | 3703 +=========+======================================+ 3704 | 0 | Plaintext or unencrypted data | 3705 +---------+--------------------------------------+ 3706 | 1 | IDEA [IDEA] | 3707 +---------+--------------------------------------+ 3708 | 2 | TripleDES (DES-EDE, [SCHNEIER] [HAC] | 3709 | | - 168 bit key derived from 192) | 3710 +---------+--------------------------------------+ 3711 | 3 | CAST5 (128 bit key, as per | 3712 | | [RFC2144]) | 3713 +---------+--------------------------------------+ 3714 | 4 | Blowfish (128 bit key, 16 rounds) | 3715 | | [BLOWFISH] | 3716 +---------+--------------------------------------+ 3717 | 5 | Reserved | 3718 +---------+--------------------------------------+ 3719 | 6 | Reserved | 3720 +---------+--------------------------------------+ 3721 | 7 | AES with 128-bit key [AES] | 3722 +---------+--------------------------------------+ 3723 | 8 | AES with 192-bit key | 3724 +---------+--------------------------------------+ 3725 | 9 | AES with 256-bit key | 3726 +---------+--------------------------------------+ 3727 | 10 | Twofish with 256-bit key [TWOFISH] | 3728 +---------+--------------------------------------+ 3729 | 11 | Camellia with 128-bit key [RFC3713] | 3730 +---------+--------------------------------------+ 3731 | 12 | Camellia with 192-bit key | 3732 +---------+--------------------------------------+ 3733 | 13 | Camellia with 256-bit key | 3734 +---------+--------------------------------------+ 3735 | 100-110 | Private/Experimental algorithm | 3736 +---------+--------------------------------------+ 3738 Table 8 3740 Implementations MUST implement AES-128. Implementations SHOULD 3741 implement AES-256. Implementations that interoperate with RFC-4880 3742 implementations need to support TripleDES and CAST5. Implementations 3743 that interoperate with PGP 2.6 or earlier need to support IDEA, as 3744 that is the only symmetric cipher those versions use. 3745 Implementations MAY implement any other algorithm. 3747 9.4. Compression Algorithms 3749 +---------+--------------------------------+ 3750 | ID | Algorithm | 3751 +=========+================================+ 3752 | 0 | Uncompressed | 3753 +---------+--------------------------------+ 3754 | 1 | ZIP [RFC1951] | 3755 +---------+--------------------------------+ 3756 | 2 | ZLIB [RFC1950] | 3757 +---------+--------------------------------+ 3758 | 3 | BZip2 [BZ2] | 3759 +---------+--------------------------------+ 3760 | 100-110 | Private/Experimental algorithm | 3761 +---------+--------------------------------+ 3763 Table 9 3765 Implementations MUST implement uncompressed data. Implementations 3766 SHOULD implement ZLIB. For interoperability reasons implementations 3767 SHOULD be able to decompress using ZIP. Implementations MAY 3768 implement any other algorithm. 3770 9.5. Hash Algorithms 3772 +---------+--------------------------------+-------------+ 3773 | ID | Algorithm | Text Name | 3774 +=========+================================+=============+ 3775 | 1 | MD5 [HAC] | "MD5" | 3776 +---------+--------------------------------+-------------+ 3777 | 2 | SHA-1 [FIPS180] | "SHA1" | 3778 +---------+--------------------------------+-------------+ 3779 | 3 | RIPE-MD/160 [HAC] | "RIPEMD160" | 3780 +---------+--------------------------------+-------------+ 3781 | 4 | Reserved | | 3782 +---------+--------------------------------+-------------+ 3783 | 5 | Reserved | | 3784 +---------+--------------------------------+-------------+ 3785 | 6 | Reserved | | 3786 +---------+--------------------------------+-------------+ 3787 | 7 | Reserved | | 3788 +---------+--------------------------------+-------------+ 3789 | 8 | SHA2-256 [FIPS180] | "SHA256" | 3790 +---------+--------------------------------+-------------+ 3791 | 9 | SHA2-384 [FIPS180] | "SHA384" | 3792 +---------+--------------------------------+-------------+ 3793 | 10 | SHA2-512 [FIPS180] | "SHA512" | 3794 +---------+--------------------------------+-------------+ 3795 | 11 | SHA2-224 [FIPS180] | "SHA224" | 3796 +---------+--------------------------------+-------------+ 3797 | 12 | SHA3-256 [FIPS202] | "SHA3-256" | 3798 +---------+--------------------------------+-------------+ 3799 | 13 | Reserved | | 3800 +---------+--------------------------------+-------------+ 3801 | 14 | SHA3-512 [FIPS202] | "SHA3-512" | 3802 +---------+--------------------------------+-------------+ 3803 | 100-110 | Private/Experimental algorithm | | 3804 +---------+--------------------------------+-------------+ 3806 Table 10 3808 Implementations MUST implement SHA2-256. Implementations MAY 3809 implement other algorithms. Implementations SHOULD NOT create 3810 messages which require the use of SHA-1 with the exception of 3811 computing version 4 key fingerprints and for purposes of the MDC 3812 packet. Implementations SHOULD NOT use MD5 or RIPE-MD/160. 3814 9.6. AEAD Algorithms 3816 +---------+--------------------------------+ 3817 | ID | Algorithm | 3818 +=========+================================+ 3819 | 1 | EAX [EAX] | 3820 +---------+--------------------------------+ 3821 | 2 | OCB [RFC7253] | 3822 +---------+--------------------------------+ 3823 | 100-110 | Private/Experimental algorithm | 3824 +---------+--------------------------------+ 3826 Table 11 3828 Implementations MUST implement EAX. Implementations MAY implement 3829 OCB and other algorithms. 3831 10. IANA Considerations 3833 OpenPGP is highly parameterized, and consequently there are a number 3834 of considerations for allocating parameters for extensions. This 3835 section describes how IANA should look at extensions to the protocol 3836 as described in this document. 3838 { FIXME: Also add forward references, like "The list of S2K specifier 3839 types is maintained by IANA as described in Section 10." } 3841 10.1. New String-to-Key Specifier Types 3843 OpenPGP S2K specifiers contain a mechanism for new algorithms to turn 3844 a string into a key. This specification creates a registry of S2K 3845 specifier types. The registry includes the S2K type, the name of the 3846 S2K, and a reference to the defining specification. The initial 3847 values for this registry can be found in Section 3.7.1. Adding a new 3848 S2K specifier MUST be done through the SPECIFICATION REQUIRED method, 3849 as described in [RFC8126]. 3851 10.2. New Packets 3853 Major new features of OpenPGP are defined through new packet types. 3854 This specification creates a registry of packet types. The registry 3855 includes the packet type, the name of the packet, and a reference to 3856 the defining specification. The initial values for this registry can 3857 be found in Section 4.3. Adding a new packet type MUST be done 3858 through the RFC REQUIRED method, as described in [RFC8126]. 3860 10.2.1. User Attribute Types 3862 The User Attribute packet permits an extensible mechanism for other 3863 types of certificate identification. This specification creates a 3864 registry of User Attribute types. The registry includes the User 3865 Attribute type, the name of the User Attribute, and a reference to 3866 the defining specification. The initial values for this registry can 3867 be found in Section 5.13. Adding a new User Attribute type MUST be 3868 done through the SPECIFICATION REQUIRED method, as described in 3869 [RFC8126]. 3871 This document requests that IANA register the User ID Attribute Type 3872 found in Section 5.13.2: 3874 +-------+-----------+------------------------------+ 3875 | Value | Attribute | Reference | 3876 +=======+===========+==============================+ 3877 | TBD1 | User ID | This Document Section 5.13.2 | 3878 +-------+-----------+------------------------------+ 3880 Table 12 3882 10.2.2. Image Format Subpacket Types 3884 Within User Attribute packets, there is an extensible mechanism for 3885 other types of image-based user attributes. This specification 3886 creates a registry of Image Attribute subpacket types. The registry 3887 includes the Image Attribute subpacket type, the name of the Image 3888 Attribute subpacket, and a reference to the defining specification. 3889 The initial values for this registry can be found in Section 5.13.1. 3890 Adding a new Image Attribute subpacket type MUST be done through the 3891 SPECIFICATION REQUIRED method, as described in [RFC8126]. 3893 10.2.3. New Signature Subpackets 3895 OpenPGP signatures contain a mechanism for signed (or unsigned) data 3896 to be added to them for a variety of purposes in the Signature 3897 subpackets as discussed in Section 5.2.3.1. This specification 3898 creates a registry of Signature subpacket types. The registry 3899 includes the Signature subpacket type, the name of the subpacket, and 3900 a reference to the defining specification. The initial values for 3901 this registry can be found in Section 5.2.3.1. Adding a new 3902 Signature subpacket MUST be done through the SPECIFICATION REQUIRED 3903 method, as described in [RFC8126]. 3905 10.2.3.1. Signature Notation Data Subpackets 3907 OpenPGP signatures further contain a mechanism for extensions in 3908 signatures. These are the Notation Data subpackets, which contain a 3909 key/value pair. Notations contain a user space that is completely 3910 unmanaged and an IETF space. 3912 This specification creates a registry of Signature Notation Data 3913 types. The registry includes the Signature Notation Data type, the 3914 name of the Signature Notation Data, its allowed values, and a 3915 reference to the defining specification. The initial values for this 3916 registry can be found in Section 5.2.3.17. Adding a new Signature 3917 Notation Data subpacket MUST be done through the SPECIFICATION 3918 REQUIRED method, as described in [RFC8126]. 3920 This document requests IANA register the following Signature Notation 3921 Data types: 3923 +----------------+---------+--------------------+----------------+ 3924 | Allowed Values | Name | Type | Reference | 3925 +================+=========+====================+================+ 3926 | A String | charset | Character Set | This Doc Secti | 3927 | | | | on 5.2.3.17.1 | 3928 +----------------+---------+--------------------+----------------+ 3929 | Any String | manu | Manufacturer Name | This Doc Secti | 3930 | | | | on 5.2.3.17.2 | 3931 +----------------+---------+--------------------+----------------+ 3932 | Any String | make | Product Make | This Doc Secti | 3933 | | | | on 5.2.3.17.3 | 3934 +----------------+---------+--------------------+----------------+ 3935 | Any String | model | Product Model | This Doc Secti | 3936 | | | | on 5.2.3.17.4 | 3937 +----------------+---------+--------------------+----------------+ 3938 | Any String | prodid | Product ID | This Doc Secti | 3939 | | | | on 5.2.3.17.5 | 3940 +----------------+---------+--------------------+----------------+ 3941 | Any String | pvers | Product Version | This Doc Secti | 3942 | | | | on 5.2.3.17.6 | 3943 +----------------+---------+--------------------+----------------+ 3944 | Any String | lot | Product Lot Number | This Doc Secti | 3945 | | | | on 5.2.3.17.7 | 3946 +----------------+---------+--------------------+----------------+ 3947 | Decimal | qty | Package Quantity | This Doc Secti | 3948 | Integer String | | | on 5.2.3.17.8 | 3949 +----------------+---------+--------------------+----------------+ 3950 | A geo: URI | loc | Current | This Doc Secti | 3951 | without the | | Geolocation | on 5.2.3.17.9 | 3952 | "geo:" | | Latitude/Longitude | | 3953 +----------------+---------+--------------------+----------------+ 3954 | A geo: URI | dest | Destination | This Doc Secti | 3955 | without the | | Geolocation | on 5.2.3.17.9 | 3956 | "geo:" | | Latitude/Longitude | | 3957 +----------------+---------+--------------------+----------------+ 3958 | Hash Notation | hash | The Hash of | This Doc Secti | 3959 | data | | external data | on 5.2.3.17.10 | 3960 +----------------+---------+--------------------+----------------+ 3962 Table 13 3964 10.2.3.2. Signature Notation Data Subpacket Notation Flags 3966 This specification creates a new registry of Signature Notation Data 3967 Subpacket Notation Flags. The registry includes the columns "Flag", 3968 "Description", "Security Recommended", "Interoperability 3969 Recommended", and "Reference". The initial values for this registry 3970 can be found in Section 5.2.3.17. Adding a new item MUST be done 3971 through the SPECIFICATION REQUIRED method, as described in [RFC8126]. 3973 10.2.3.3. Key Server Preference Extensions 3975 OpenPGP signatures contain a mechanism for preferences to be 3976 specified about key servers. This specification creates a registry 3977 of key server preferences. The registry includes the key server 3978 preference, the name of the preference, and a reference to the 3979 defining specification. The initial values for this registry can be 3980 found in Section 5.2.3.18. Adding a new key server preference MUST 3981 be done through the SPECIFICATION REQUIRED method, as described in 3982 [RFC8126]. 3984 10.2.3.4. Key Flags Extensions 3986 OpenPGP signatures contain a mechanism for flags to be specified 3987 about key usage. This specification creates a registry of key usage 3988 flags. The registry includes the key flags value, the name of the 3989 flag, and a reference to the defining specification. The initial 3990 values for this registry can be found in Section 5.2.3.22. Adding a 3991 new key usage flag MUST be done through the SPECIFICATION REQUIRED 3992 method, as described in [RFC8126]. 3994 10.2.3.5. Reason for Revocation Extensions 3996 OpenPGP signatures contain a mechanism for flags to be specified 3997 about why a key was revoked. This specification creates a registry 3998 of "Reason for Revocation" flags. The registry includes the "Reason 3999 for Revocation" flags value, the name of the flag, and a reference to 4000 the defining specification. The initial values for this registry can 4001 be found in Section 5.2.3.24. Adding a new feature flag MUST be done 4002 through the SPECIFICATION REQUIRED method, as described in [RFC8126]. 4004 10.2.3.6. Implementation Features 4006 OpenPGP signatures contain a mechanism for flags to be specified 4007 stating which optional features an implementation supports. This 4008 specification creates a registry of feature-implementation flags. 4009 The registry includes the feature-implementation flags value, the 4010 name of the flag, and a reference to the defining specification. The 4011 initial values for this registry can be found in Section 5.2.3.25. 4012 Adding a new feature-implementation flag MUST be done through the 4013 SPECIFICATION REQUIRED method, as described in [RFC8126]. 4015 Also see Section 14.12 for more information about when feature flags 4016 are needed. 4018 10.2.4. New Packet Versions 4020 The core OpenPGP packets all have version numbers, and can be revised 4021 by introducing a new version of an existing packet. This 4022 specification creates a registry of packet types. The registry 4023 includes the packet type, the number of the version, and a reference 4024 to the defining specification. The initial values for this registry 4025 can be found in Section 5. Adding a new packet version MUST be done 4026 through the RFC REQUIRED method, as described in [RFC8126]. 4028 10.3. New Algorithms 4030 Section 9 lists the core algorithms that OpenPGP uses. Adding in a 4031 new algorithm is usually simple. For example, adding in a new 4032 symmetric cipher usually would not need anything more than allocating 4033 a constant for that cipher. If that cipher had other than a 64-bit 4034 or 128-bit block size, there might need to be additional 4035 documentation describing how OpenPGP-CFB mode would be adjusted. 4036 Similarly, when DSA was expanded from a maximum of 1024-bit public 4037 keys to 3072-bit public keys, the revision of FIPS 186 contained 4038 enough information itself to allow implementation. Changes to this 4039 document were made mainly for emphasis. 4041 10.3.1. Public-Key Algorithms 4043 OpenPGP specifies a number of public-key algorithms. This 4044 specification creates a registry of public-key algorithm identifiers. 4045 The registry includes the algorithm name, its key sizes and 4046 parameters, and a reference to the defining specification. The 4047 initial values for this registry can be found in Section 9.1. Adding 4048 a new public-key algorithm MUST be done through the SPECIFICATION 4049 REQUIRED method, as described in [RFC8126]. 4051 This document requests IANA register the following new public-key 4052 algorithm: 4054 +----+----------------------------+------------------------+ 4055 | ID | Algorithm | Reference | 4056 +====+============================+========================+ 4057 | 22 | EdDSA public key algorithm | This doc, Section 14.8 | 4058 +----+----------------------------+------------------------+ 4059 | 23 | Reserved for AEDH | This doc | 4060 +----+----------------------------+------------------------+ 4061 | 24 | Reserved for AEDSA | This doc | 4062 +----+----------------------------+------------------------+ 4064 Table 14 4066 [Notes to RFC-Editor: Please remove the table above on publication. 4067 It is desirable not to reuse old or reserved algorithms because some 4068 existing tools might print a wrong description. A higher number is 4069 also an indication for a newer algorithm. As of now 22 is the next 4070 free number.] 4072 10.3.2. Symmetric-Key Algorithms 4074 OpenPGP specifies a number of symmetric-key algorithms. This 4075 specification creates a registry of symmetric-key algorithm 4076 identifiers. The registry includes the algorithm name, its key sizes 4077 and block size, and a reference to the defining specification. The 4078 initial values for this registry can be found in Section 9.3. Adding 4079 a new symmetric-key algorithm MUST be done through the SPECIFICATION 4080 REQUIRED method, as described in [RFC8126]. 4082 10.3.3. Hash Algorithms 4084 OpenPGP specifies a number of hash algorithms. This specification 4085 creates a registry of hash algorithm identifiers. The registry 4086 includes the algorithm name, a text representation of that name, its 4087 block size, an OID hash prefix, and a reference to the defining 4088 specification. The initial values for this registry can be found in 4089 Section 9.5 for the algorithm identifiers and text names, and 4090 Section 9.2 for the OIDs and expanded signature prefixes. Adding a 4091 new hash algorithm MUST be done through the SPECIFICATION REQUIRED 4092 method, as described in [RFC8126]. 4094 This document requests IANA register the following hash algorithms: 4096 +----+-----------+-----------+ 4097 | ID | Algorithm | Reference | 4098 +====+===========+===========+ 4099 | 12 | SHA3-256 | This doc | 4100 +----+-----------+-----------+ 4101 | 13 | Reserved | | 4102 +----+-----------+-----------+ 4103 | 14 | SHA3-512 | This doc | 4104 +----+-----------+-----------+ 4106 Table 15 4108 [Notes to RFC-Editor: Please remove the table above on publication. 4109 It is desirable not to reuse old or reserved algorithms because some 4110 existing tools might print a wrong description. The ID 13 has been 4111 reserved so that the SHA3 algorithm IDs align nicely with their SHA2 4112 counterparts.] 4114 10.3.4. Compression Algorithms 4116 OpenPGP specifies a number of compression algorithms. This 4117 specification creates a registry of compression algorithm 4118 identifiers. The registry includes the algorithm name and a 4119 reference to the defining specification. The initial values for this 4120 registry can be found in Section 9.4. Adding a new compression key 4121 algorithm MUST be done through the SPECIFICATION REQUIRED method, as 4122 described in [RFC8126]. 4124 11. Packet Composition 4126 OpenPGP packets are assembled into sequences in order to create 4127 messages and to transfer keys. Not all possible packet sequences are 4128 meaningful and correct. This section describes the rules for how 4129 packets should be placed into sequences. 4131 11.1. Transferable Public Keys 4133 OpenPGP users may transfer public keys. The essential elements of a 4134 transferable public key are as follows: 4136 * One Public-Key packet 4138 * Zero or more revocation signatures 4140 * Zero or more User ID packets 4142 * After each User ID packet, zero or more Signature packets 4143 (certifications and attestation key signatures) 4145 * Zero or more User Attribute packets 4147 * After each User Attribute packet, zero or more Signature packets 4148 (certifications and attestation key signatures) 4150 * Zero or more Subkey packets 4152 * After each Subkey packet, one Signature packet, plus optionally a 4153 revocation 4155 The Public-Key packet occurs first. Each of the following User ID 4156 packets provides the identity of the owner of this public key. If 4157 there are multiple User ID packets, this corresponds to multiple 4158 means of identifying the same unique individual user; for example, a 4159 user may have more than one email address, and construct a User ID 4160 for each one. 4162 Immediately following each User ID packet, there are zero or more 4163 Signature packets. Each Signature packet is calculated on the 4164 immediately preceding User ID packet and the initial Public-Key 4165 packet. The signature serves to certify the corresponding public key 4166 and User ID. In effect, the signer is testifying to his or her 4167 belief that this public key belongs to the user identified by this 4168 User ID. Intermixed with these certifications may be Attestation Key 4169 Signature packets issued by the primary key over the same User ID and 4170 Public Key packet. The most recent of these is used to attest to 4171 third-party certifications over the associated User ID. 4173 Within the same section as the User ID packets, there are zero or 4174 more User Attribute packets. Like the User ID packets, a User 4175 Attribute packet is followed by zero or more Signature packets 4176 calculated on the immediately preceding User Attribute packet and the 4177 initial Public-Key packet. 4179 User Attribute packets and User ID packets may be freely intermixed 4180 in this section, so long as the signatures that follow them are 4181 maintained on the proper User Attribute or User ID packet. 4183 After the User ID packet or Attribute packet, there may be zero or 4184 more Subkey packets. In general, subkeys are provided in cases where 4185 the top-level public key is a signature-only key. However, any V4 or 4186 V5 key may have subkeys, and the subkeys may be encryption-only keys, 4187 signature-only keys, or general-purpose keys. V3 keys MUST NOT have 4188 subkeys. 4190 Each Subkey packet MUST be followed by one Signature packet, which 4191 should be a subkey binding signature issued by the top-level key. 4192 For subkeys that can issue signatures, the subkey binding signature 4193 MUST contain an Embedded Signature subpacket with a primary key 4194 binding signature (0x19) issued by the subkey on the top-level key. 4196 Subkey and Key packets may each be followed by a revocation Signature 4197 packet to indicate that the key is revoked. Revocation signatures 4198 are only accepted if they are issued by the key itself, or by a key 4199 that is authorized to issue revocations via a Revocation Key 4200 subpacket in a self-signature by the top-level key. 4202 Transferable public-key packet sequences may be concatenated to allow 4203 transferring multiple public keys in one operation. 4205 11.2. Transferable Secret Keys 4207 OpenPGP users may transfer secret keys. The format of a transferable 4208 secret key is the same as a transferable public key except that 4209 secret-key and secret-subkey packets are used instead of the public 4210 key and public-subkey packets. Implementations SHOULD include self- 4211 signatures on any user IDs and subkeys, as this allows for a complete 4212 public key to be automatically extracted from the transferable secret 4213 key. Implementations MAY choose to omit the self-signatures, 4214 especially if a transferable public key accompanies the transferable 4215 secret key. 4217 11.3. OpenPGP Messages 4219 An OpenPGP message is a packet or sequence of packets that 4220 corresponds to the following grammatical rules (comma represents 4221 sequential composition, and vertical bar separates alternatives): 4223 OpenPGP Message :- Encrypted Message | Signed Message | 4224 Compressed Message | Literal Message. 4226 Compressed Message :- Compressed Data Packet. 4228 Literal Message :- Literal Data Packet. 4230 ESK :- Public-Key Encrypted Session Key Packet | 4231 Symmetric-Key Encrypted Session Key Packet. 4233 ESK Sequence :- ESK | ESK Sequence, ESK. 4235 Encrypted Data :- Symmetrically Encrypted Data Packet | 4236 Symmetrically Encrypted Integrity Protected Data Packet 4238 Encrypted Message :- Encrypted Data | ESK Sequence, Encrypted Data. 4240 One-Pass Signed Message :- One-Pass Signature Packet, 4241 OpenPGP Message, Corresponding Signature Packet. 4243 Signed Message :- Signature Packet, OpenPGP Message | 4244 One-Pass Signed Message. 4246 In addition, decrypting a Symmetrically Encrypted Data packet or a 4247 Symmetrically Encrypted Integrity Protected Data packet as well as 4248 decompressing a Compressed Data packet must yield a valid OpenPGP 4249 Message. 4251 11.4. Detached Signatures 4253 Some OpenPGP applications use so-called "detached signatures". For 4254 example, a program bundle may contain a file, and with it a second 4255 file that is a detached signature of the first file. These detached 4256 signatures are simply a Signature packet stored separately from the 4257 data for which they are a signature. 4259 12. Enhanced Key Formats 4261 12.1. Key Structures 4263 The format of an OpenPGP V3 key is as follows. Entries in square 4264 brackets are optional and ellipses indicate repetition. 4266 RSA Public Key 4267 [Revocation Self Signature] 4268 User ID [Signature ...] 4269 [User ID [Signature ...] ...] 4271 Each signature certifies the RSA public key and the preceding User 4272 ID. The RSA public key can have many User IDs and each User ID can 4273 have many signatures. V3 keys are deprecated. Implementations MUST 4274 NOT generate new V3 keys, but MAY continue to use existing ones. 4276 The format of an OpenPGP V4 key that uses multiple public keys is 4277 similar except that the other keys are added to the end as "subkeys" 4278 of the primary key. 4280 Primary-Key 4281 [Revocation Self Signature] 4282 [Direct Key Signature...] 4283 [User ID [Signature ...] ...] 4284 [User Attribute [Signature ...] ...] 4285 [[Subkey [Binding-Signature-Revocation] 4286 Primary-Key-Binding-Signature] ...] 4288 A subkey always has a single signature after it that is issued using 4289 the primary key to tie the two keys together. This binding signature 4290 may be in either V3 or V4 format, but SHOULD be V4. Subkeys that can 4291 issue signatures MUST have a V4 binding signature due to the REQUIRED 4292 embedded primary key binding signature. 4294 In the above diagram, if the binding signature of a subkey has been 4295 revoked, the revoked key may be removed, leaving only one key. 4297 In a V4 key, the primary key SHOULD be a key capable of 4298 certification. There are cases, such as device certificates, where 4299 the primary key may not be capable of certification. A primary key 4300 capable of making signatures SHOULD be accompanied by either a 4301 certification signature (on a User ID or User Attribute) or a 4302 signature directly on the key. 4304 Implementations SHOULD accept encryption-only primary keys without a 4305 signature. It also SHOULD allow importing any key accompanied either 4306 by a certification signature or a signature on itself. It MAY accept 4307 signature-capable primary keys without an accompanying signature. 4309 The subkeys may be keys of any other type. There may be other 4310 constructions of V4 keys, too. For example, there may be a single- 4311 key RSA key in V4 format, a DSA primary key with an RSA encryption 4312 key, or RSA primary key with an Elgamal subkey, etc. 4314 It is also possible to have a signature-only subkey. This permits a 4315 primary key that collects certifications (key signatures), but is 4316 used only for certifying subkeys that are used for encryption and 4317 signatures. 4319 12.2. Key IDs and Fingerprints 4321 For a V3 key, the eight-octet Key ID consists of the low 64 bits of 4322 the public modulus of the RSA key. 4324 The fingerprint of a V3 key is formed by hashing the body (but not 4325 the two-octet length) of the MPIs that form the key material (public 4326 modulus n, followed by exponent e) with MD5. Note that both V3 keys 4327 and MD5 are deprecated. 4329 A V4 fingerprint is the 160-bit SHA-1 hash of the octet 0x99, 4330 followed by the two-octet packet length, followed by the entire 4331 Public-Key packet starting with the version field. The Key ID is the 4332 low-order 64 bits of the fingerprint. Here are the fields of the 4333 hash material, with the example of a DSA key: 4335 a.1) 0x99 (1 octet) 4337 a.2) two-octet scalar octet count of (b)-(e) 4339 b) version number = 4 (1 octet); 4341 c) timestamp of key creation (4 octets); 4343 d) algorithm (1 octet): 17 = DSA (example); 4345 e) Algorithm-specific fields. 4347 Algorithm-Specific Fields for DSA keys (example): 4349 e.1) MPI of DSA prime p; 4351 e.2) MPI of DSA group order q (q is a prime divisor of p-1); 4353 e.3) MPI of DSA group generator g; 4355 e.4) MPI of DSA public-key value y (= g\*\*x mod p where x is secret). 4357 A V5 fingerprint is the 256-bit SHA2-256 hash of the octet 0x9A, 4358 followed by the four-octet packet length, followed by the entire 4359 Public-Key packet starting with the version field. The Key ID is the 4360 high-order 64 bits of the fingerprint. Here are the fields of the 4361 hash material, with the example of a DSA key: 4363 a.1) 0x9A (1 octet) 4365 a.2) four-octet scalar octet count of (b)-(f) 4367 b) version number = 5 (1 octet); 4369 c) timestamp of key creation (4 octets); 4371 d) algorithm (1 octet): 17 = DSA (example); 4373 e) four-octet scalar octet count for the following key material; 4375 f) algorithm-specific fields. 4377 Algorithm-Specific Fields for DSA keys (example): 4379 f.1) MPI of DSA prime p; 4381 f.2) MPI of DSA group order q (q is a prime divisor of p-1); 4383 f.3) MPI of DSA group generator g; 4385 f.4) MPI of DSA public-key value y (= g\*\*x mod p where x is secret). 4387 Note that it is possible for there to be collisions of Key IDs -- two 4388 different keys with the same Key ID. Note that there is a much 4389 smaller, but still non-zero, probability that two different keys have 4390 the same fingerprint. 4392 Also note that if V3, V4, and V5 format keys share the same RSA key 4393 material, they will have different Key IDs as well as different 4394 fingerprints. 4396 Finally, the Key ID and fingerprint of a subkey are calculated in the 4397 same way as for a primary key, including the 0x99 (V3 and V4 key) or 4398 0x9A (V5 key) as the first octet (even though this is not a valid 4399 packet ID for a public subkey). 4401 13. Elliptic Curve Cryptography 4403 This section descripes algorithms and parameters used with Elliptic 4404 Curve Cryptography (ECC) keys. A thorough introduction to ECC can be 4405 found in [KOBLITZ]. 4407 13.1. Supported ECC Curves 4409 This document references five named prime field curves, defined in 4410 [FIPS186] as "Curve P-256", "Curve P-384", and "Curve P-521"; and 4411 defined in [RFC5639] as "brainpoolP256r1", and "brainpoolP512r1". 4412 Further curve "Curve25519", defined in [RFC7748] is referenced for 4413 use with Ed25519 (EdDSA signing) and X25519 (encryption). 4415 The named curves are referenced as a sequence of bytes in this 4416 document, called throughout, curve OID. Section 9.2 describes in 4417 detail how this sequence of bytes is formed. 4419 13.2. ECDSA and ECDH Conversion Primitives 4421 This document defines the uncompressed point format for ECDSA and 4422 ECDH and a custom compression format for certain curves. The point 4423 is encoded in the Multiprecision Integer (MPI) format. 4425 For an uncompressed point the content of the MPI is: 4427 B = 04 || x || y 4429 where x and y are coordinates of the point P = (x, y), each encoded 4430 in the big-endian format and zero-padded to the adjusted underlying 4431 field size. The adjusted underlying field size is the underlying 4432 field size that is rounded up to the nearest 8-bit boundary. This 4433 encoding is compatible with the definition given in [SEC1]. 4435 For a custom compressed point the content of the MPI is: 4437 B = 40 || x 4439 where x is the x coordinate of the point P encoded to the rules 4440 defined for the specified curve. This format is used for ECDH keys 4441 based on curves expressed in Montgomery form. 4443 Therefore, the exact size of the MPI payload is 515 bits for "Curve 4444 P-256", 771 for "Curve P-384", 1059 for "Curve P-521", and 263 for 4445 Curve25519. 4447 Even though the zero point, also called the point at infinity, may 4448 occur as a result of arithmetic operations on points of an elliptic 4449 curve, it SHALL NOT appear in data structures defined in this 4450 document. 4452 If other conversion methods are defined in the future, a compliant 4453 application MUST NOT use a new format when in doubt that any 4454 recipient can support it. Consider, for example, that while both the 4455 public key and the per-recipient ECDH data structure, respectively 4456 defined in Section 5.6.6 and Section 5.1, contain an encoded point 4457 field, the format changes to the field in Section 5.1 only affect a 4458 given recipient of a given message. 4460 13.3. EdDSA Point Format 4462 The EdDSA algorithm defines a specific point compression format. To 4463 indicate the use of this compression format and to make sure that the 4464 key can be represented in the Multiprecision Integer (MPI) format the 4465 octet string specifying the point is prefixed with the octet 0x40. 4466 This encoding is an extension of the encoding given in [SEC1] which 4467 uses 0x04 to indicate an uncompressed point. 4469 For example, the length of a public key for the curve Ed25519 is 263 4470 bit: 7 bit to represent the 0x40 prefix octet and 32 octets for the 4471 native value of the public key. 4473 13.4. Key Derivation Function 4475 A key derivation function (KDF) is necessary to implement the EC 4476 encryption. The Concatenation Key Derivation Function (Approved 4477 Alternative 1) [SP800-56A] with the KDF hash function that is 4478 SHA2-256 [FIPS180] or stronger is REQUIRED. See Section 16 for the 4479 details regarding the choice of the hash function. 4481 For convenience, the synopsis of the encoding method is given below 4482 with significant simplifications attributable to the restricted 4483 choice of hash functions in this document. However, [SP800-56A] is 4484 the normative source of the definition. 4486 // Implements KDF( X, oBits, Param ); 4487 // Input: point X = (x,y) 4488 // oBits - the desired size of output 4489 // hBits - the size of output of hash function Hash 4490 // Param - octets representing the parameters 4491 // Assumes that oBits <= hBits 4492 // Convert the point X to the octet string: 4493 // ZB' = 04 || x || y 4494 // and extract the x portion from ZB' 4495 ZB = x; 4496 MB = Hash ( 00 || 00 || 00 || 01 || ZB || Param ); 4497 return oBits leftmost bits of MB. 4499 Note that ZB in the KDF description above is the compact 4500 representation of X, defined in Section 4.2 of [RFC6090]. 4502 13.5. EC DH Algorithm (ECDH) 4504 The method is a combination of an ECC Diffie-Hellman method to 4505 establish a shared secret, a key derivation method to process the 4506 shared secret into a derived key, and a key wrapping method that uses 4507 the derived key to protect a session key used to encrypt a message. 4509 The One-Pass Diffie-Hellman method C(1, 1, ECC CDH) [SP800-56A] MUST 4510 be implemented with the following restrictions: the ECC CDH primitive 4511 employed by this method is modified to always assume the cofactor as 4512 1, the KDF specified in Section 13.4 is used, and the KDF parameters 4513 specified below are used. 4515 The KDF parameters are encoded as a concatenation of the following 5 4516 variable-length and fixed-length fields, compatible with the 4517 definition of the OtherInfo bitstring [SP800-56A]: 4519 * a variable-length field containing a curve OID, formatted as 4520 follows: 4522 - a one-octet size of the following field 4524 - the octets representing a curve OID, defined in Section 9.2 4526 * a one-octet public key algorithm ID defined in Section 9.1 4528 * a variable-length field containing KDF parameters, identical to 4529 the corresponding field in the ECDH public key, formatted as 4530 follows: 4532 - a one-octet size of the following fields; values 0 and 0xff are 4533 reserved for future extensions 4535 - a one-octet value 01, reserved for future extensions 4537 - a one-octet hash function ID used with the KDF 4539 - a one-octet algorithm ID for the symmetric algorithm used to 4540 wrap the symmetric key for message encryption; see Section 13.5 4541 for details 4543 * 20 octets representing the UTF-8 encoding of the string "Anonymous 4544 Sender ", which is the octet sequence 41 6E 6F 6E 79 6D 6F 75 73 4545 20 53 65 6E 64 65 72 20 20 20 20 4547 * 20 octets representing a recipient encryption subkey or a master 4548 key fingerprint, identifying the key material that is needed for 4549 the decryption. For version 5 keys the 20 leftmost octets of the 4550 fingerprint are used. 4552 The size of the KDF parameters sequence, defined above, is either 54 4553 for the NIST curve P-256, 51 for the curves P-384 and P-521, or 56 4554 for Curve25519. 4556 The key wrapping method is described in [RFC3394]. KDF produces a 4557 symmetric key that is used as a key-encryption key (KEK) as specified 4558 in [RFC3394]. Refer to Section 15 for the details regarding the 4559 choice of the KEK algorithm, which SHOULD be one of three AES 4560 algorithms. Key wrapping and unwrapping is performed with the 4561 default initial value of [RFC3394]. 4563 The input to the key wrapping method is the value "m" derived from 4564 the session key, as described in Section 5.1, "Public-Key Encrypted 4565 Session Key Packets (Tag 1)", except that the PKCS #1.5 padding step 4566 is omitted. The result is padded using the method described in 4567 [PKCS5] to the 8-byte granularity. For example, the following 4568 AES-256 session key, in which 32 octets are denoted from k0 to k31, 4569 is composed to form the following 40 octet sequence: 4571 09 k0 k1 ... k31 c0 c1 05 05 05 05 05 4573 The octets c0 and c1 above denote the checksum. This encoding allows 4574 the sender to obfuscate the size of the symmetric encryption key used 4575 to encrypt the data. For example, assuming that an AES algorithm is 4576 used for the session key, the sender MAY use 21, 13, and 5 bytes of 4577 padding for AES-128, AES-192, and AES-256, respectively, to provide 4578 the same number of octets, 40 total, as an input to the key wrapping 4579 method. 4581 The output of the method consists of two fields. The first field is 4582 the MPI containing the ephemeral key used to establish the shared 4583 secret. The second field is composed of the following two fields: 4585 * a one-octet encoding the size in octets of the result of the key 4586 wrapping method; the value 255 is reserved for future extensions; 4588 * up to 254 octets representing the result of the key wrapping 4589 method, applied to the 8-byte padded session key, as described 4590 above. 4592 Note that for session key sizes 128, 192, and 256 bits, the size of 4593 the result of the key wrapping method is, respectively, 32, 40, and 4594 48 octets, unless the size obfuscation is used. 4596 For convenience, the synopsis of the encoding method is given below; 4597 however, this section, [SP800-56A], and [RFC3394] are the normative 4598 sources of the definition. 4600 Obtain the authenticated recipient public key R 4601 Generate an ephemeral key pair {v, V=vG} 4602 Compute the shared point S = vR; 4603 m = symm_alg_ID || session key || checksum || pkcs5_padding; 4604 curve_OID_len = (byte)len(curve_OID); 4605 Param = curve_OID_len || curve_OID || public_key_alg_ID || 03 4606 || 01 || KDF_hash_ID || KEK_alg_ID for AESKeyWrap || "Anonymous 4607 Sender " || recipient_fingerprint; 4608 Z_len = the key size for the KEK_alg_ID used with AESKeyWrap 4609 Compute Z = KDF( S, Z_len, Param ); 4610 Compute C = AESKeyWrap( Z, m ) as per [RFC3394] 4611 VB = convert point V to the octet string 4612 Output (MPI(VB) || len(C) || C). 4614 The decryption is the inverse of the method given. Note that the 4615 recipient obtains the shared secret by calculating 4617 S = rV = rvG, where (r,R) is the recipient's key pair. 4619 Consistent with Section 5.16, "AEAD Encrypted Data Packet (Tag 20)" 4620 and Section 5.14, "Sym. Encrypted Integrity Protected Data Packet 4621 (Tag 18)", AEAD encryption or a Modification Detection Code (MDC) 4622 MUST be used anytime the symmetric key is protected by ECDH. 4624 14. Notes on Algorithms 4626 14.1. PKCS#1 Encoding in OpenPGP 4628 This standard makes use of the PKCS#1 functions EME-PKCS1-v1_5 and 4629 EMSA-PKCS1-v1_5. However, the calling conventions of these functions 4630 has changed in the past. To avoid potential confusion and 4631 interoperability problems, we are including local copies in this 4632 document, adapted from those in PKCS#1 v2.1 [RFC3447]. RFC 3447 4633 should be treated as the ultimate authority on PKCS#1 for OpenPGP. 4634 Nonetheless, we believe that there is value in having a self- 4635 contained document that avoids problems in the future with needed 4636 changes in the conventions. 4638 14.1.1. EME-PKCS1-v1_5-ENCODE 4640 Input: 4642 k = the length in octets of the key modulus. 4644 M = message to be encoded, an octet string of length mLen, 4645 where mLen <= k - 11. 4647 Output: 4649 EM = encoded message, an octet string of length k. 4651 Error: "message too long". 4653 1. Length checking: If mLen > k - 11, output "message too long" 4654 and stop. 4656 2. Generate an octet string PS of length k - mLen - 3 consisting 4657 of pseudo-randomly generated nonzero octets. The length of PS 4658 will be at least eight octets. 4660 3. Concatenate PS, the message M, and other padding to form an 4661 encoded message EM of length k octets as 4663 EM = 0x00 || 0x02 || PS || 0x00 || M. 4665 4. Output EM. 4667 14.1.2. EME-PKCS1-v1_5-DECODE 4669 Input: 4671 EM = encoded message, an octet string 4673 Output: 4675 M = message, an octet string, 4677 Error: "decryption error", 4679 To decode an EME-PKCS1_v1_5 message, separate the encoded message EM 4680 into an octet string PS consisting of nonzero octets and a message M 4681 as follows 4683 EM = 0x00 || 0x02 || PS || 0x00 || M. 4685 If the first octet of EM does not have hexadecimal value 0x00, if the 4686 second octet of EM does not have hexadecimal value 0x02, if there is 4687 no octet with hexadecimal value 0x00 to separate PS from M, or if the 4688 length of PS is less than 8 octets, output "decryption error" and 4689 stop. See also the security note in Section 15 regarding differences 4690 in reporting between a decryption error and a padding error. 4692 14.1.3. EMSA-PKCS1-v1_5 4694 This encoding method is deterministic and only has an encoding 4695 operation. 4697 Option: 4699 Hash - a hash function in which hLen denotes the length in octets 4700 of the hash function output. 4702 Input: 4704 M = message to be encoded. 4706 emLen = intended length in octets of the encoded message, at least 4707 tLen + 11, where tLen is the octet length of the DER encoding 4708 T of a certain value computed during the encoding operation. 4710 Output: 4712 EM = encoded message, an octet string of length emLen. 4714 Errors: "message too long"; 4715 "intended encoded message length too short". 4717 Steps: 4719 1. Apply the hash function to the message M to produce a hash 4720 value H: 4722 H = Hash(M). 4724 If the hash function outputs "message too long," output 4725 "message too long" and stop. 4727 2. Using the list in Section {FIXREF} 5.2.2, "Version 2 Signature Packet 4728 Format", produce an ASN.1 DER value for the hash function 4729 used. Let T be the full hash prefix from the list, and let 4730 tLen be the length in octets of T. 4732 3. If emLen < tLen + 11, output "intended encoded message length 4733 too short" and stop. 4735 4. Generate an octet string PS consisting of emLen - tLen - 3 4736 octets with hexadecimal value 0xFF. The length of PS will be 4737 at least 8 octets. 4739 5. Concatenate PS, the hash prefix T, and other padding to form 4740 the encoded message EM as 4742 EM = 0x00 || 0x01 || PS || 0x00 || T. 4744 6. Output EM. 4746 14.2. Symmetric Algorithm Preferences 4748 The symmetric algorithm preference is an ordered list of algorithms 4749 that the keyholder accepts. Since it is found on a self-signature, 4750 it is possible that a keyholder may have multiple, different 4751 preferences. For example, Alice may have AES-128 only specified for 4752 "alice@work.com" but Camellia-256, Twofish, and AES-128 specified for 4753 "alice@home.org". Note that it is also possible for preferences to 4754 be in a subkey's binding signature. 4756 Since AES-128 is the MUST-implement algorithm, if it is not 4757 explicitly in the list, it is tacitly at the end. However, it is 4758 good form to place it there explicitly. Note also that if an 4759 implementation does not implement the preference, then it is 4760 implicitly an AES-128-only implementation. Note further that 4761 implementations conforming to previous versions of this standard 4762 (RFC-4880) have TripleDES as its only MUST-implement algorithm. 4764 An implementation MUST NOT use a symmetric algorithm that is not in 4765 the recipient's preference list. When encrypting to more than one 4766 recipient, the implementation finds a suitable algorithm by taking 4767 the intersection of the preferences of the recipients. Note that the 4768 MUST-implement algorithm, AES-128, ensures that the intersection is 4769 not null. The implementation may use any mechanism to pick an 4770 algorithm in the intersection. 4772 If an implementation can decrypt a message that a keyholder doesn't 4773 have in their preferences, the implementation SHOULD decrypt the 4774 message anyway, but MUST warn the keyholder that the protocol has 4775 been violated. For example, suppose that Alice, above, has software 4776 that implements all algorithms in this specification. Nonetheless, 4777 she prefers subsets for work or home. If she is sent a message 4778 encrypted with IDEA, which is not in her preferences, the software 4779 warns her that someone sent her an IDEA-encrypted message, but it 4780 would ideally decrypt it anyway. 4782 14.3. Other Algorithm Preferences 4784 Other algorithm preferences work similarly to the symmetric algorithm 4785 preference, in that they specify which algorithms the keyholder 4786 accepts. There are two interesting cases that other comments need to 4787 be made about, though, the compression preferences and the hash 4788 preferences. 4790 14.3.1. Compression Preferences 4792 Compression has been an integral part of PGP since its first days. 4793 OpenPGP and all previous versions of PGP have offered compression. 4794 In this specification, the default is for messages to be compressed, 4795 although an implementation is not required to do so. Consequently, 4796 the compression preference gives a way for a keyholder to request 4797 that messages not be compressed, presumably because they are using a 4798 minimal implementation that does not include compression. 4799 Additionally, this gives a keyholder a way to state that it can 4800 support alternate algorithms. 4802 Like the algorithm preferences, an implementation MUST NOT use an 4803 algorithm that is not in the preference vector. If the preferences 4804 are not present, then they are assumed to be [ZIP(1), 4805 Uncompressed(0)]. 4807 Additionally, an implementation MUST implement this preference to the 4808 degree of recognizing when to send an uncompressed message. A robust 4809 implementation would satisfy this requirement by looking at the 4810 recipient's preference and acting accordingly. A minimal 4811 implementation can satisfy this requirement by never generating a 4812 compressed message, since all implementations can handle messages 4813 that have not been compressed. 4815 14.3.2. Hash Algorithm Preferences 4817 Typically, the choice of a hash algorithm is something the signer 4818 does, rather than the verifier, because a signer rarely knows who is 4819 going to be verifying the signature. This preference, though, allows 4820 a protocol based upon digital signatures ease in negotiation. 4822 Thus, if Alice is authenticating herself to Bob with a signature, it 4823 makes sense for her to use a hash algorithm that Bob's software uses. 4824 This preference allows Bob to state in his key which algorithms Alice 4825 may use. 4827 Since SHA2-256 is the MUST-implement hash algorithm, if it is not 4828 explicitly in the list, it is tacitly at the end. However, it is 4829 good form to place it there explicitly. 4831 14.4. Plaintext 4833 Algorithm 0, "plaintext", may only be used to denote secret keys that 4834 are stored in the clear. Implementations MUST NOT use plaintext in 4835 Symmetrically Encrypted Data packets; they must use Literal Data 4836 packets to encode unencrypted or literal data. 4838 14.5. RSA 4840 There are algorithm types for RSA Sign-Only, and RSA Encrypt-Only 4841 keys. These types are deprecated. The "key flags" subpacket in a 4842 signature is a much better way to express the same idea, and 4843 generalizes it to all algorithms. An implementation SHOULD NOT 4844 create such a key, but MAY interpret it. 4846 An implementation SHOULD NOT implement RSA keys of size less than 4847 1024 bits. 4849 14.6. DSA 4851 An implementation SHOULD NOT implement DSA keys of size less than 4852 1024 bits. It MUST NOT implement a DSA key with a q size of less 4853 than 160 bits. DSA keys MUST also be a multiple of 64 bits, and the 4854 q size MUST be a multiple of 8 bits. The Digital Signature Standard 4855 (DSS) [FIPS186] specifies that DSA be used in one of the following 4856 ways: 4858 * 1024-bit key, 160-bit q, SHA-1, SHA2-224, SHA2-256, SHA2-384, or 4859 SHA2-512 hash 4861 * 2048-bit key, 224-bit q, SHA2-224, SHA2-256, SHA2-384, or SHA2-512 4862 hash 4864 * 2048-bit key, 256-bit q, SHA2-256, SHA2-384, or SHA2-512 hash 4866 * 3072-bit key, 256-bit q, SHA2-256, SHA2-384, or SHA2-512 hash 4868 The above key and q size pairs were chosen to best balance the 4869 strength of the key with the strength of the hash. Implementations 4870 SHOULD use one of the above key and q size pairs when generating DSA 4871 keys. If DSS compliance is desired, one of the specified SHA hashes 4872 must be used as well. [FIPS186] is the ultimate authority on DSS, 4873 and should be consulted for all questions of DSS compliance. 4875 Note that earlier versions of this standard only allowed a 160-bit q 4876 with no truncation allowed, so earlier implementations may not be 4877 able to handle signatures with a different q size or a truncated 4878 hash. 4880 14.7. Elgamal 4882 An implementation SHOULD NOT implement Elgamal keys of size less than 4883 1024 bits. 4885 14.8. EdDSA 4887 Although the EdDSA algorithm allows arbitrary data as input, its use 4888 with OpenPGP requires that a digest of the message is used as input 4889 (pre-hashed). See section Section 5.2.4, "Computing Signatures" for 4890 details. Truncation of the resulting digest is never applied; the 4891 resulting digest value is used verbatim as input to the EdDSA 4892 algorithm. 4894 14.9. Reserved Algorithm Numbers 4896 A number of algorithm IDs have been reserved for algorithms that 4897 would be useful to use in an OpenPGP implementation, yet there are 4898 issues that prevent an implementer from actually implementing the 4899 algorithm. These are marked in Section 9.1, "Public-Key Algorithms", 4900 as "reserved for". 4902 The reserved public-key algorithm X9.42 (21) does not have the 4903 necessary parameters, parameter order, or semantics defined. The 4904 same is currently true for reserved public-key algorithms AEDH (23) 4905 and AEDSA (24). 4907 Previous versions of OpenPGP permitted Elgamal [ELGAMAL] signatures 4908 with a public-key identifier of 20. These are no longer permitted. 4909 An implementation MUST NOT generate such keys. An implementation 4910 MUST NOT generate Elgamal signatures. See [BLEICHENBACHER]. 4912 14.10. OpenPGP CFB Mode 4914 OpenPGP does symmetric encryption using a variant of Cipher Feedback 4915 mode (CFB mode). This section describes the procedure it uses in 4916 detail. This mode is what is used for Symmetrically Encrypted Data 4917 Packets; the mechanism used for encrypting secret-key material is 4918 similar, and is described in the sections above. 4920 In the description below, the value BS is the block size in octets of 4921 the cipher. Most ciphers have a block size of 8 octets. The AES and 4922 Twofish have a block size of 16 octets. Also note that the 4923 description below assumes that the IV and CFB arrays start with an 4924 index of 1 (unlike the C language, which assumes arrays start with a 4925 zero index). 4927 OpenPGP CFB mode uses an initialization vector (IV) of all zeros, and 4928 prefixes the plaintext with BS+2 octets of random data, such that 4929 octets BS+1 and BS+2 match octets BS-1 and BS. It does a CFB 4930 resynchronization after encrypting those BS+2 octets. 4932 Thus, for an algorithm that has a block size of 8 octets (64 bits), 4933 the IV is 10 octets long and octets 7 and 8 of the IV are the same as 4934 octets 9 and 10. For an algorithm with a block size of 16 octets 4935 (128 bits), the IV is 18 octets long, and octets 17 and 18 replicate 4936 octets 15 and 16. Those extra two octets are an easy check for a 4937 correct key. 4939 Step by step, here is the procedure: 4941 1. The feedback register (FR) is set to the IV, which is all zeros. 4943 2. FR is encrypted to produce FRE (FR Encrypted). This is the 4944 encryption of an all-zero value. 4946 3. FRE is xored with the first BS octets of random data prefixed to 4947 the plaintext to produce C[1] through C[BS], the first BS octets 4948 of ciphertext. 4950 4. FR is loaded with C[1] through C[BS]. 4952 5. FR is encrypted to produce FRE, the encryption of the first BS 4953 octets of ciphertext. 4955 6. The left two octets of FRE get xored with the next two octets of 4956 data that were prefixed to the plaintext. This produces C[BS+1] 4957 and C[BS+2], the next two octets of ciphertext. 4959 7. (The resynchronization step) FR is loaded with C[3] through 4960 C[BS+2]. 4962 8. FRE is xored with the first BS octets of the given plaintext, 4963 now that we have finished encrypting the BS+2 octets of prefixed 4964 data. This produces C[BS+3] through C[BS+(BS+2)], the next BS 4965 octets of ciphertext. 4967 9. FR is encrypted to produce FRE. 4969 10. FR is loaded with C[BS+3] to C[BS + (BS+2)] (which is C11-C18 4970 for an 8-octet block). 4972 11. FR is encrypted to produce FRE. 4974 12. FRE is xored with the next BS octets of plaintext, to produce 4975 the next BS octets of ciphertext. These are loaded into FR, and 4976 the process is repeated until the plaintext is used up. 4978 14.11. Private or Experimental Parameters 4980 S2K specifiers, Signature subpacket types, user attribute types, 4981 image format types, and algorithms described in Section 9 all reserve 4982 the range 100 to 110 for private and experimental use. Packet types 4983 reserve the range 60 to 63 for private and experimental use. These 4984 are intentionally managed with the PRIVATE USE method, as described 4985 in [RFC8126]. 4987 However, implementations need to be careful with these and promote 4988 them to full IANA-managed parameters when they grow beyond the 4989 original, limited system. 4991 14.12. Meta-Considerations for Expansion 4993 If OpenPGP is extended in a way that is not backwards-compatible, 4994 meaning that old implementations will not gracefully handle their 4995 absence of a new feature, the extension proposal can be declared in 4996 the key holder's self-signature as part of the Features signature 4997 subpacket. 4999 We cannot state definitively what extensions will not be upwards- 5000 compatible, but typically new algorithms are upwards-compatible, 5001 whereas new packets are not. 5003 If an extension proposal does not update the Features system, it 5004 SHOULD include an explanation of why this is unnecessary. If the 5005 proposal contains neither an extension to the Features system nor an 5006 explanation of why such an extension is unnecessary, the proposal 5007 SHOULD be rejected. 5009 15. Security Considerations 5011 * As with any technology involving cryptography, you should check 5012 the current literature to determine if any algorithms used here 5013 have been found to be vulnerable to attack. 5015 * This specification uses Public-Key Cryptography technologies. It 5016 is assumed that the private key portion of a public-private key 5017 pair is controlled and secured by the proper party or parties. 5019 * Certain operations in this specification involve the use of random 5020 numbers. An appropriate entropy source should be used to generate 5021 these numbers (see [RFC4086]). 5023 * The MD5 hash algorithm has been found to have weaknesses, with 5024 collisions found in a number of cases. MD5 is deprecated for use 5025 in OpenPGP. Implementations MUST NOT generate new signatures 5026 using MD5 as a hash function. They MAY continue to consider old 5027 signatures that used MD5 as valid. 5029 * SHA2-224 and SHA2-384 require the same work as SHA2-256 and 5030 SHA2-512, respectively. In general, there are few reasons to use 5031 them outside of DSS compatibility. You need a situation where one 5032 needs more security than smaller hashes, but does not want to have 5033 the full 256-bit or 512-bit data length. 5035 * Many security protocol designers think that it is a bad idea to 5036 use a single key for both privacy (encryption) and integrity 5037 (signatures). In fact, this was one of the motivating forces 5038 behind the V4 key format with separate signature and encryption 5039 keys. If you as an implementer promote dual-use keys, you should 5040 at least be aware of this controversy. 5042 * The DSA algorithm will work with any hash, but is sensitive to the 5043 quality of the hash algorithm. Verifiers should be aware that 5044 even if the signer used a strong hash, an attacker could have 5045 modified the signature to use a weak one. Only signatures using 5046 acceptably strong hash algorithms should be accepted as valid. 5048 * As OpenPGP combines many different asymmetric, symmetric, and hash 5049 algorithms, each with different measures of strength, care should 5050 be taken that the weakest element of an OpenPGP message is still 5051 sufficiently strong for the purpose at hand. While consensus 5052 about the strength of a given algorithm may evolve, NIST Special 5053 Publication 800-57 [SP800-57] recommends the following list of 5054 equivalent strengths: 5056 +---------------------+-----------+--------------------+ 5057 | Asymmetric key size | Hash size | Symmetric key size | 5058 +=====================+===========+====================+ 5059 | 1024 | 160 | 80 | 5060 +---------------------+-----------+--------------------+ 5061 | 2048 | 224 | 112 | 5062 +---------------------+-----------+--------------------+ 5063 | 3072 | 256 | 128 | 5064 +---------------------+-----------+--------------------+ 5065 | 7680 | 384 | 192 | 5066 +---------------------+-----------+--------------------+ 5067 | 15360 | 512 | 256 | 5068 +---------------------+-----------+--------------------+ 5070 Table 16 5072 * There is a somewhat-related potential security problem in 5073 signatures. If an attacker can find a message that hashes to the 5074 same hash with a different algorithm, a bogus signature structure 5075 can be constructed that evaluates correctly. 5077 For example, suppose Alice DSA signs message M using hash 5078 algorithm H. Suppose that Mallet finds a message M' that has the 5079 same hash value as M with H'. Mallet can then construct a 5080 signature block that verifies as Alice's signature of M' with H'. 5081 However, this would also constitute a weakness in either H or H' 5082 or both. Should this ever occur, a revision will have to be made 5083 to this document to revise the allowed hash algorithms. 5085 * If you are building an authentication system, the recipient may 5086 specify a preferred signing algorithm. However, the signer would 5087 be foolish to use a weak algorithm simply because the recipient 5088 requests it. 5090 * Some of the encryption algorithms mentioned in this document have 5091 been analyzed less than others. For example, although CAST5 is 5092 presently considered strong, it has been analyzed less than 5093 TripleDES. Other algorithms may have other controversies 5094 surrounding them. 5096 * In late summer 2002, Jallad, Katz, and Schneier published an 5097 interesting attack on the OpenPGP protocol and some of its 5098 implementations [JKS02]. In this attack, the attacker modifies a 5099 message and sends it to a user who then returns the erroneously 5100 decrypted message to the attacker. The attacker is thus using the 5101 user as a random oracle, and can often decrypt the message. 5103 Compressing data can ameliorate this attack. The incorrectly 5104 decrypted data nearly always decompresses in ways that defeat the 5105 attack. However, this is not a rigorous fix, and leaves open some 5106 small vulnerabilities. For example, if an implementation does not 5107 compress a message before encryption (perhaps because it knows it 5108 was already compressed), then that message is vulnerable. Because 5109 of this happenstance -- that modification attacks can be thwarted 5110 by decompression errors -- an implementation SHOULD treat a 5111 decompression error as a security problem, not merely a data 5112 problem. 5114 This attack can be defeated by the use of modification detection, 5115 provided that the implementation does not let the user naively 5116 return the data to the attacker. The modification detection is 5117 prefereabble implemented by using the AEAD Encrypted Data Packet 5118 and only if the recipients don't supports this by use of the 5119 Symmmetric Encrypted and Integrity Protected Data Packet. An 5120 implementation MUST treat an authentication or MDC failure as a 5121 security problem, not merely a data problem. 5123 In either case, the implementation SHOULD NOT allow the user 5124 access to the erroneous data, and MUST warn the user as to 5125 potential security problems should that data be returned to the 5126 sender. 5128 While this attack is somewhat obscure, requiring a special set of 5129 circumstances to create it, it is nonetheless quite serious as it 5130 permits someone to trick a user to decrypt a message. 5131 Consequently, it is important that: 5133 1. Implementers treat authentication errors, MDC errors, 5134 decompression failures or no use of MDC or AEAD as security 5135 problems. 5137 2. Implementers implement AEAD with all due speed and encourage 5138 its spread. 5140 3. Users migrate to implementations that support AEAD encryption 5141 with all due speed. 5143 * PKCS#1 has been found to be vulnerable to attacks in which a 5144 system that reports errors in padding differently from errors in 5145 decryption becomes a random oracle that can leak the private key 5146 in mere millions of queries. Implementations must be aware of 5147 this attack and prevent it from happening. The simplest solution 5148 is to report a single error code for all variants of decryption 5149 errors so as not to leak information to an attacker. 5151 * Some technologies mentioned here may be subject to government 5152 control in some countries. 5154 * In winter 2005, Serge Mister and Robert Zuccherato from Entrust 5155 released a paper describing a way that the "quick check" in 5156 OpenPGP CFB mode can be used with a random oracle to decrypt two 5157 octets of every cipher block [MZ05]. They recommend as prevention 5158 not using the quick check at all. 5160 Many implementers have taken this advice to heart for any data 5161 that is symmetrically encrypted and for which the session key is 5162 public-key encrypted. In this case, the quick check is not needed 5163 as the public-key encryption of the session key should guarantee 5164 that it is the right session key. In other cases, the 5165 implementation should use the quick check with care. 5167 On the one hand, there is a danger to using it if there is a 5168 random oracle that can leak information to an attacker. In 5169 plainer language, there is a danger to using the quick check if 5170 timing information about the check can be exposed to an attacker, 5171 particularly via an automated service that allows rapidly repeated 5172 queries. 5174 On the other hand, it is inconvenient to the user to be informed 5175 that they typed in the wrong passphrase only after a petabyte of 5176 data is decrypted. There are many cases in cryptographic 5177 engineering where the implementer must use care and wisdom, and 5178 this is one. 5180 * Refer to [FIPS186], B.4.1, for the method to generate a uniformly 5181 distributed ECC private key. 5183 * The curves proposed in this document correspond to the symmetric 5184 key sizes 128 bits, 192 bits, and 256 bits, as described in the 5185 table below. This allows a compliant application to offer 5186 balanced public key security, which is compatible with the 5187 symmetric key strength for each AES algorithm defined here. 5189 The following table defines the hash and the symmetric encryption 5190 algorithm that SHOULD be used with a given curve for ECDSA or 5191 ECDH. A stronger hash algorithm or a symmetric key algorithm MAY 5192 be used for a given ECC curve. However, note that the increase in 5193 the strength of the hash algorithm or the symmetric key algorithm 5194 may not increase the overall security offered by the given ECC 5195 key. 5197 +------------+-----+--------------+---------------------+-----------+ 5198 | Curve name | ECC | RSA | Hash size strength, | Symmetric | 5199 | | | strength | informative | key size | 5200 +============+=====+==============+=====================+===========+ 5201 | NIST P-256 | 256 | 3072 | 256 | 128 | 5202 +------------+-----+--------------+---------------------+-----------+ 5203 | NIST P-384 | 384 | 7680 | 384 | 192 | 5204 +------------+-----+--------------+---------------------+-----------+ 5205 | NIST P-521 | 521 | 15360 | 512 | 256 | 5206 +------------+-----+--------------+---------------------+-----------+ 5208 Table 17 5210 * Requirement levels indicated elsewhere in this document lead to 5211 the following combinations of algorithms in the OpenPGP profile: 5212 MUST implement NIST curve P-256 / SHA2-256 / AES-128, SHOULD 5213 implement NIST curve P-521 / SHA2-512 / AES-256, MAY implement 5214 NIST curve P-384 / SHA2-384 / AES-256, among other allowed 5215 combinations. 5217 Consistent with the table above, the following table defines the 5218 KDF hash algorithm and the AES KEK encryption algorithm that 5219 SHOULD be used with a given curve for ECDH. A stronger KDF hash 5220 algorithm or AES KEK algorithm MAY be used for a given ECC curve. 5222 +------------+-----------------+----------------------+ 5223 | Curve name | Recommended KDF | Recommended KEK | 5224 | | hash algorithm | encryption algorithm | 5225 +============+=================+======================+ 5226 | NIST P-256 | SHA2-256 | AES-128 | 5227 +------------+-----------------+----------------------+ 5228 | NIST P-384 | SHA2-384 | AES-192 | 5229 +------------+-----------------+----------------------+ 5230 | NIST P-521 | SHA2-512 | AES-256 | 5231 +------------+-----------------+----------------------+ 5233 Table 18 5235 * This document explicitly discourages the use of algorithms other 5236 than AES as a KEK algorithm because backward compatibility of the 5237 ECDH format is not a concern. The KEK algorithm is only used 5238 within the scope of a Public-Key Encrypted Session Key Packet, 5239 which represents an ECDH key recipient of a message. Compare this 5240 with the algorithm used for the session key of the message, which 5241 MAY be different from a KEK algorithm. 5243 Compliant applications SHOULD implement, advertise through key 5244 preferences, and use the strongest algorithms specified in this 5245 document. 5247 Note that the symmetric algorithm preference list may make it 5248 impossible to use the balanced strength of symmetric key 5249 algorithms for a corresponding public key. For example, the 5250 presence of the symmetric key algorithm IDs and their order in the 5251 key preference list affects the algorithm choices available to the 5252 encoding side, which in turn may make the adherence to the table 5253 above infeasible. Therefore, compliance with this specification 5254 is a concern throughout the life of the key, starting immediately 5255 after the key generation when the key preferences are first added 5256 to a key. It is generally advisable to position a symmetric 5257 algorithm ID of strength matching the public key at the head of 5258 the key preference list. 5260 Encryption to multiple recipients often results in an unordered 5261 intersection subset. For example, if the first recipient's set is 5262 {A, B} and the second's is {B, A}, the intersection is an 5263 unordered set of two algorithms, A and B. In this case, a 5264 compliant application SHOULD choose the stronger encryption 5265 algorithm. 5267 Resource constraints, such as limited computational power, is a 5268 likely reason why an application might prefer to use the weakest 5269 algorithm. On the other side of the spectrum are applications 5270 that can implement every algorithm defined in this document. Most 5271 applications are expected to fall into either of two categories. 5272 A compliant application in the second, or strongest, category 5273 SHOULD prefer AES-256 to AES-192. 5275 SHA-1 MUST NOT be used with the ECDSA or the KDF in the ECDH 5276 method. 5278 MDC MUST be used when a symmetric encryption key is protected by 5279 ECDH. None of the ECC methods described in this document are 5280 allowed with deprecated V3 keys. A compliant application MUST 5281 only use iterated and salted S2K to protect private keys, as 5282 defined in Section 3.7.1.3, "Iterated and Salted S2K". 5284 Side channel attacks are a concern when a compliant application's 5285 use of the OpenPGP format can be modeled by a decryption or 5286 signing oracle model, for example, when an application is a 5287 network service performing decryption to unauthenticated remote 5288 users. ECC scalar multiplication operations used in ECDSA and 5289 ECDH are vulnerable to side channel attacks. Countermeasures can 5290 often be taken at the higher protocol level, such as limiting the 5291 number of allowed failures or time-blinding of the operations 5292 associated with each network interface. Mitigations at the scalar 5293 multiplication level seek to eliminate any measurable distinction 5294 between the ECC point addition and doubling operations. 5296 * Although technically possible, the EdDSA algorithm MUST NOT be 5297 used with a digest algorithms weaker than SHA2-256. 5299 OpenPGP was designed with security in mind, with many smart, 5300 intelligent people spending a lot of time thinking about the 5301 ramifications of their decisions. Removing the requirement for self- 5302 certifying User ID (and User Attribute) packets on a key means that 5303 someone could surreptitiously add an unwanted ID to a key and sign 5304 it. If enough "trusted" people sign that surreptitious identity then 5305 other people might believe it. The attack could wind up sending 5306 encrypted mail destined for alice to some other target, bob, because 5307 someone added "alice" to bob's key without bob's consent. 5309 In the case of device certificates the device itself does not have 5310 any consent. It is given an identity by the device manufacturer and 5311 the manufacturer can insert that ID on the device certificate, 5312 signing it with the manufacturer's key. If another people wants to 5313 label the device by another name, they can do so. There is no harm 5314 in multiple IDs, because the verification is all done based on who 5315 has signed those IDs. 5317 When a key can self-sign, it is still suggested to self-certify IDs, 5318 even if it no longer required by this modification to OpenPGP. This 5319 at least signals to recipients of keys that yes, the owner of this 5320 key asserts that this identity belongs to herself. Note, however, 5321 that mallet could still assert that he is 'alice' and could even 5322 self-certify that. So the attack is not truly different. Moreover, 5323 in the case of device certificates, it's more the manufacturer than 5324 the device that wants to assert an identity (even if the device could 5325 self-certify). 5327 There is no signaling whether a key is using this looser-requirement 5328 key format. An attacker could therefore just remove the self- 5329 signature off a published key. However one would hope that wide 5330 publication would result in another copy still having that signature 5331 and it being returned quickly. However, the lack of signaling also 5332 means that a user with an application following RFC 4880 directly 5333 would see a key following this specification as "broken" and may not 5334 accept it. 5336 On a different note, including the "geo" notation could leak 5337 information about where a signer is located. However it is just an 5338 assertion (albeit a signed assertion) so there is no verifiable truth 5339 to the location information released. Similarly, all the rest of the 5340 signature notations are pure assertions, so they should be taken with 5341 the trustworthiness of the signer. 5343 Combining the User ID with the User Attribute means that an ID and 5344 image would not be separable. For a person this is probably not 5345 good, but for a device it's unlikely the image will change so it 5346 makes sense to combine the ID and image into a single signed packet 5347 with a single signature. 5349 16. Compatibility Profiles 5351 16.1. OpenPGP ECC Profile 5353 A compliant application MUST implement NIST curve P-256, SHOULD 5354 implement NIST curve P-521, SHOULD implemend Ed25519, SHOULD 5355 implement Curve25519, MAY implement NIST curve P-384, MAY implement 5356 brainpoolP256r1, and MAY implement brainpoolP512r1, as defined in 5357 Section 9.2. A compliant application MUST implement SHA2-256 and 5358 SHOULD implement SHA2-384 and SHA2-512. A compliant application MUST 5359 implement AES-128 and SHOULD implement AES-256. 5361 A compliant application SHOULD follow Section 15 regarding the choice 5362 of the following algorithms for each curve: 5364 * the KDF hash algorithm, 5366 * the KEK algorithm, 5368 * the message digest algorithm and the hash algorithm used in the 5369 key certifications, 5371 * the symmetric algorithm used for message encryption. 5373 It is recommended that the chosen symmetric algorithm for message 5374 encryption be no less secure than the KEK algorithm. 5376 16.2. Suite-B Profile 5378 A subset of algorithms allowed by this document can be used to 5379 achieve [SuiteB] compatibility. The references to [SuiteB] in this 5380 document are informative. This document is primarily concerned with 5381 format specification, leaving additional security restrictions 5382 unspecified, such as matching the assigned security level of 5383 information to authorized recipients or interoperability concerns 5384 arising from fewer allowed algorithms in [SuiteB] than allowed by 5385 this document. 5387 16.2.1. Security Strength at 192 Bits 5389 To achieve the security strength of 192 bits, [SuiteB] requires NIST 5390 curve P-384, AES-256, and SHA2-384. The symmetric algorithm 5391 restriction means that the algorithm of KEK used for key wrapping in 5392 Section 13.5 and an OpenPGP session key used for message encryption 5393 must be AES-256. The hash algorithm restriction means that the hash 5394 algorithms of KDF and the OpenPGP message digest calculation must be 5395 SHA2-384. 5397 16.2.2. Security Strength at 128 Bits 5399 The set of algorithms in Section 16.2.1 is extended to allow NIST 5400 curve P-256, AES-128, and SHA2-256. 5402 17. Implementation Nits 5404 This section is a collection of comments to help an implementer, 5405 particularly with an eye to backward compatibility. Previous 5406 implementations of PGP are not OpenPGP compliant. Often the 5407 differences are small, but small differences are frequently more 5408 vexing than large differences. Thus, this is a non-comprehensive 5409 list of potential problems and gotchas for a developer who is trying 5410 to be backward-compatible. 5412 * The IDEA algorithm is patented, and yet it is required for PGP 2 5413 interoperability. It is also the de-facto preferred algorithm for 5414 a V3 key with a V3 self-signature (or no self- signature). 5416 * When exporting a private key, PGP 2 generates the header "BEGIN 5417 PGP SECRET KEY BLOCK" instead of "BEGIN PGP PRIVATE KEY BLOCK". 5418 All previous versions ignore the implied data type, and look 5419 directly at the packet data type. 5421 * PGP versions 2.0 through 2.5 generated V2 Public-Key packets. 5422 These are identical to the deprecated V3 keys except for the 5423 version number. An implementation MUST NOT generate them and may 5424 accept or reject them as it sees fit. Some older PGP versions 5425 generated V2 PKESK packets (Tag 1) as well. An implementation may 5426 accept or reject V2 PKESK packets as it sees fit, and MUST NOT 5427 generate them. 5429 * PGP version 2.6 will not accept key-material packets with versions 5430 greater than 3. 5432 * There are many ways possible for two keys to have the same key 5433 material, but different fingerprints (and thus Key IDs). Perhaps 5434 the most interesting is an RSA key that has been "upgraded" to V4 5435 format, but since a V4 fingerprint is constructed by hashing the 5436 key creation time along with other things, two V4 keys created at 5437 different times, yet with the same key material will have 5438 different fingerprints. 5440 * If an implementation is using zlib to interoperate with PGP 2, 5441 then the "windowBits" parameter should be set to -13. 5443 * The 0x19 back signatures were not required for signing subkeys 5444 until relatively recently. Consequently, there may be keys in the 5445 wild that do not have these back signatures. Implementing 5446 software may handle these keys as it sees fit. 5448 * OpenPGP does not put limits on the size of public keys. However, 5449 larger keys are not necessarily better keys. Larger keys take 5450 more computation time to use, and this can quickly become 5451 impractical. Different OpenPGP implementations may also use 5452 different upper bounds for public key sizes, and so care should be 5453 taken when choosing sizes to maintain interoperability. As of 5454 2007 most implementations have an upper bound of 4096 bits. 5456 * ASCII armor is an optional feature of OpenPGP. The OpenPGP 5457 working group strives for a minimal set of mandatory-to-implement 5458 features, and since there could be useful implementations that 5459 only use binary object formats, this is not a "MUST" feature for 5460 an implementation. For example, an implementation that is using 5461 OpenPGP as a mechanism for file signatures may find ASCII armor 5462 unnecessary. OpenPGP permits an implementation to declare what 5463 features it does and does not support, but ASCII armor is not one 5464 of these. Since most implementations allow binary and armored 5465 objects to be used indiscriminately, an implementation that does 5466 not implement ASCII armor may find itself with compatibility 5467 issues with general-purpose implementations. Moreover, 5468 implementations of OpenPGP-MIME [RFC3156] already have a 5469 requirement for ASCII armor so those implementations will 5470 necessarily have support. 5472 * The OCB mode is patented and a debate is still underway on whether 5473 it can be included in RFC4880bis or needs to be moved to a 5474 separate document. For the sole purpose of experimenting with the 5475 Preferred AEAD Algorithms signature subpacket it is has been 5476 included in this I-D. 5478 18. References 5480 18.1. Normative References 5482 [AES] NIST, "FIPS PUB 197, Advanced Encryption Standard (AES)", 5483 November 2001, 5484 . 5487 [BLOWFISH] Schneier, B., "Description of a New Variable-Length Key, 5488 64-Bit Block Cipher (Blowfish)", Fast Software Encryption, 5489 Cambridge Security Workshop Proceedings Springer-Verlag, 5490 1994, pp191-204, December 1993, 5491 . 5493 [BZ2] Seward, J., "The Bzip2 and libbzip2 home page", 2010, 5494 . 5496 [EAX] Bellare, M., Rogaway, P., and D. Wagner, "A Conventional 5497 Authenticated-Encryption Mode", April 2003. 5499 [ELGAMAL] Elgamal, T., "A Public-Key Cryptosystem and a Signature 5500 Scheme Based on Discrete Logarithms", IEEE Transactions on 5501 Information Theory v. IT-31, n. 4, 1985, pp. 469-472, 5502 1985. 5504 [FIPS180] National Institute of Standards and Technology, U.S. 5505 Department of Commerce, "Secure Hash Standard (SHS), FIPS 5506 180-4", August 2015, 5507 . 5509 [FIPS186] National Institute of Standards and Technology, U.S. 5510 Department of Commerce, "Digital Signature Standard (DSS), 5511 FIPS 186-4", July 2013, 5512 . 5514 [FIPS202] National Institute of Standards and Technology, U.S. 5515 Department of Commerce, "SHA-3 Standard: Permutation-Based 5516 Hash and Extendable-Output Functions, FIPS 202", August 5517 2015, . 5519 [HAC] Menezes, A.J., Oorschot, P.v., and S. Vanstone, "Handbook 5520 of Applied Cryptography", 1996. 5522 [IDEA] Lai, X., "On the design and security of block ciphers", 5523 ETH Series in Information Processing, J.L. Massey 5524 (editor) Vol. 1, Hartung-Gorre Verlag Konstanz, Technische 5525 Hochschule (Zurich), 1992. 5527 [ISO10646] International Organization for Standardization, 5528 "Information Technology - Universal Multiple-octet coded 5529 Character Set (UCS) - Part 1: Architecture and Basic 5530 Multilingual Plane", ISO Standard 10646-1, May 1993. 5532 [JFIF] CA, E.H.M., "JPEG File Interchange Format (Version 5533 1.02).", September 1996. 5535 [PKCS5] RSA Laboratories, "PKCS #5 v2.0: Password-Based 5536 Cryptography Standard", 25 March 1999. 5538 [RFC1950] Deutsch, P. and J-L. Gailly, "ZLIB Compressed Data Format 5539 Specification version 3.3", RFC 1950, 5540 DOI 10.17487/RFC1950, May 1996, 5541 . 5543 [RFC1951] Deutsch, P., "DEFLATE Compressed Data Format Specification 5544 version 1.3", RFC 1951, DOI 10.17487/RFC1951, May 1996, 5545 . 5547 [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 5548 Extensions (MIME) Part One: Format of Internet Message 5549 Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996, 5550 . 5552 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 5553 Requirement Levels", BCP 14, RFC 2119, 5554 DOI 10.17487/RFC2119, March 1997, 5555 . 5557 [RFC2144] Adams, C., "The CAST-128 Encryption Algorithm", RFC 2144, 5558 DOI 10.17487/RFC2144, May 1997, 5559 . 5561 [RFC2822] Resnick, P., Ed., "Internet Message Format", RFC 2822, 5562 DOI 10.17487/RFC2822, April 2001, 5563 . 5565 [RFC3156] Elkins, M., Del Torto, D., Levien, R., and T. Roessler, 5566 "MIME Security with OpenPGP", RFC 3156, 5567 DOI 10.17487/RFC3156, August 2001, 5568 . 5570 [RFC3394] Schaad, J. and R. Housley, "Advanced Encryption Standard 5571 (AES) Key Wrap Algorithm", RFC 3394, DOI 10.17487/RFC3394, 5572 September 2002, . 5574 [RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography 5575 Standards (PKCS) #1: RSA Cryptography Specifications 5576 Version 2.1", RFC 3447, DOI 10.17487/RFC3447, February 5577 2003, . 5579 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 5580 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 5581 2003, . 5583 [RFC3713] Matsui, M., Nakajima, J., and S. Moriai, "A Description of 5584 the Camellia Encryption Algorithm", RFC 3713, 5585 DOI 10.17487/RFC3713, April 2004, 5586 . 5588 [RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker, 5589 "Randomness Requirements for Security", BCP 106, RFC 4086, 5590 DOI 10.17487/RFC4086, June 2005, 5591 . 5593 [RFC5639] Lochter, M. and J. Merkle, "Elliptic Curve Cryptography 5594 (ECC) Brainpool Standard Curves and Curve Generation", 5595 RFC 5639, DOI 10.17487/RFC5639, March 2010, 5596 . 5598 [RFC5870] Mayrhofer, A. and C. Spanring, "A Uniform Resource 5599 Identifier for Geographic Locations ('geo' URI)", 5600 RFC 5870, DOI 10.17487/RFC5870, June 2010, 5601 . 5603 [RFC7253] Krovetz, T. and P. Rogaway, "The OCB Authenticated- 5604 Encryption Algorithm", RFC 7253, DOI 10.17487/RFC7253, May 5605 2014, . 5607 [RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves 5608 for Security", RFC 7748, DOI 10.17487/RFC7748, January 5609 2016, . 5611 [RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital 5612 Signature Algorithm (EdDSA)", RFC 8032, 5613 DOI 10.17487/RFC8032, January 2017, 5614 . 5616 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 5617 Writing an IANA Considerations Section in RFCs", BCP 26, 5618 RFC 8126, DOI 10.17487/RFC8126, June 2017, 5619 . 5621 [SCHNEIER] Schneier, B., "Applied Cryptography Second Edition: 5622 protocols, algorithms, and source code in C", 1996. 5624 [SP800-56A]Barker, E., Johnson, D., and M. Smid, "Recommendation for 5625 Pair-Wise Key Establishment Schemes Using Discrete 5626 Logarithm Cryptography", NIST Special Publication 800-56A 5627 Revision 1, March 2007. 5629 [SuiteB] National Security Agency, "NSA Suite B Cryptography", 11 5630 March 2010, 5631 . 5633 [TWOFISH] Schneier, B., Kelsey, J., Whiting, D., Wagner, D., Hall, 5634 C., and N. Ferguson, "The Twofish Encryption Algorithm", 5635 1999. 5637 18.2. Informative References 5639 [BLEICHENBACHER] 5640 Bleichenbacher, D., "Generating ElGamal Signatures Without 5641 Knowing the Secret Key", Lecture Notes in Computer 5642 Science Volume 1070, pp. 10-18, 1996. 5644 [JKS02] Jallad, K., Katz, J., and B. Schneier, "Implementation of 5645 Chosen-Ciphertext Attacks against PGP and GnuPG", 2002, 5646 . 5648 [KOBLITZ] Koblitz, N., "A course in number theory and cryptography, 5649 Chapter VI. Elliptic Curves", ISBN 0-387-96576-9, 1997. 5651 [MZ05] Mister, S. and R. Zuccherato, "An Attack on CFB Mode 5652 Encryption As Used By OpenPGP", IACR ePrint Archive Report 5653 2005/033, 8 February 2005, 5654 . 5656 [REGEX] Friedl, J., "Mastering Regular Expressions", 5657 ISBN 0-596-00289-0, August 2002. 5659 [RFC1423] Balenson, D., "Privacy Enhancement for Internet Electronic 5660 Mail: Part III: Algorithms, Modes, and Identifiers", 5661 RFC 1423, DOI 10.17487/RFC1423, February 1993, 5662 . 5664 [RFC1991] Atkins, D., Stallings, W., and P. Zimmermann, "PGP Message 5665 Exchange Formats", RFC 1991, DOI 10.17487/RFC1991, August 5666 1996, . 5668 [RFC2440] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer, 5669 "OpenPGP Message Format", RFC 2440, DOI 10.17487/RFC2440, 5670 November 1998, . 5672 [RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. 5673 Thayer, "OpenPGP Message Format", RFC 4880, 5674 DOI 10.17487/RFC4880, November 2007, 5675 . 5677 [RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic 5678 Curve Cryptography Algorithms", RFC 6090, 5679 DOI 10.17487/RFC6090, February 2011, 5680 . 5682 [SEC1] Standards for Efficient Cryptography Group, "SEC 1: 5683 Elliptic Curve Cryptography", September 2000. 5685 [SP800-57] NIST, "Recommendation on Key Management", NIST Special 5686 Publication 800-57, March 2007, 5687 . 5690 Appendix A. Test vectors 5692 To help implementing this specification a non-normative example for 5693 the EdDSA algorithm is given. 5695 A.1. Sample EdDSA key 5697 The secret key used for this example is: 5699 D: 1a8b1ff05ded48e18bf50166c664ab023ea70003d78d9e41f5758a91d850f8d2 5701 Note that this is the raw secret key used as input to the EdDSA 5702 signing operation. The key was created on 2014-08-19 14:28:27 and 5703 thus the fingerprint of the OpenPGP key is: 5705 C959 BDBA FA32 A2F8 9A15 3B67 8CFD E121 9796 5A9A 5707 The algorithm specific input parameters without the MPI length 5708 headers are: 5710 oid: 2b06010401da470f01 5712 q: 403f098994bdd916ed4053197934e4a87c80733a1280d62f8010992e43ee3b2406 5714 The entire public key packet is thus: 5716 98 33 04 53 f3 5f 0b 16 09 2b 06 01 04 01 da 47 5717 0f 01 01 07 40 3f 09 89 94 bd d9 16 ed 40 53 19 5718 79 34 e4 a8 7c 80 73 3a 12 80 d6 2f 80 10 99 2e 5719 43 ee 3b 24 06 5721 A.2. Sample EdDSA signature 5723 The signature is created using the sample key over the input data 5724 "OpenPGP" on 2015-09-16 12:24:53 and thus the input to the hash 5725 function is: 5727 m: 4f70656e504750040016080006050255f95f9504ff0000000c 5729 Using the SHA2-256 hash algorithm yields the digest: 5731 d: f6220a3f757814f4c2176ffbb68b00249cd4ccdc059c4b34ad871f30b1740280 5733 Which is fed into the EdDSA signature function and yields this 5734 signature: 5736 r: 56f90cca98e2102637bd983fdb16c131dfd27ed82bf4dde5606e0d756aed3366 5738 s: d09c4fa11527f038e0f57f2201d82f2ea2c9033265fa6ceb489e854bae61b404 5740 The entire signature packet is thus: 5742 88 5e 04 00 16 08 00 06 05 02 55 f9 5f 95 00 0a 5743 09 10 8c fd e1 21 97 96 5a 9a f6 22 01 00 56 f9 5744 0c ca 98 e2 10 26 37 bd 98 3f db 16 c1 31 df d2 5745 7e d8 2b f4 dd e5 60 6e 0d 75 6a ed 33 66 01 00 5746 d0 9c 4f a1 15 27 f0 38 e0 f5 7f 22 01 d8 2f 2e 5747 a2 c9 03 32 65 fa 6c eb 48 9e 85 4b ae 61 b4 04 5749 A.3. Sample AEAD-EAX encryption and decryption 5751 Encryption is performed with the string 'Hello, world!' and password 5752 'password', using AES-128 with AEAD-EAX encryption. 5754 A.3.1. Sample Parameters 5756 S2K: 5758 type 3 5760 Iterations: 5762 524288 (144), SHA-256 5764 Salt: 5766 cd5a9f70fbe0bc65 5768 A.3.2. Sample symmetric-key encrypted session key packet (v5) 5770 Packet header: 5772 c3 3e 5774 Version, algorithms, S2K fields: 5776 05 07 01 03 08 cd 5a 9f 70 fb e0 bc 65 90 5778 AEAD IV: 5780 bc 66 9e 34 e5 00 dc ae dc 5b 32 aa 2d ab 02 35 5782 AEAD encrypted CEK: 5784 9d ee 19 d0 7c 34 46 c4 31 2a 34 ae 19 67 a2 fb 5786 Authentication tag: 5788 7e 92 8e a5 b4 fa 80 12 bd 45 6d 17 38 c6 3c 36 5790 A.3.3. Starting AEAD-EAX decryption of CEK 5792 The derived key is: 5794 b2 55 69 b9 54 32 45 66 45 27 c4 97 6e 7a 5d 6e 5796 Authenticated Data: 5798 c3 05 07 01 5800 Nonce: 5802 bc 66 9e 34 e5 00 dc ae dc 5b 32 aa 2d ab 02 35 5804 Decrypted CEK: 5806 86 f1 ef b8 69 52 32 9f 24 ac d3 bf d0 e5 34 6d 5808 A.3.4. Sample AEAD encrypted data packet 5810 Packet header: 5812 d4 4a 5814 Version, AES-128, EAX, Chunk bits (14): 5816 01 07 01 0e 5818 IV: 5820 b7 32 37 9f 73 c4 92 8d e2 5f ac fe 65 17 ec 10 5822 AEAD-EAX Encrypted data chunk #0: 5824 5d c1 1a 81 dc 0c b8 a2 f6 f3 d9 00 16 38 4a 56 5825 fc 82 1a e1 1a e8 5827 Chunk #0 authentication tag: 5829 db cb 49 86 26 55 de a8 8d 06 a8 14 86 80 1b 0f 5831 Final (zero-size chunk #1) authentication tag: 5833 f3 87 bd 2e ab 01 3d e1 25 95 86 90 6e ab 24 76 5835 A.3.5. Decryption of data 5837 Starting AEAD-EAX decryption of data, using the CEK. 5839 Chunk #0: 5841 Authenticated data: 5843 d4 01 07 01 0e 00 00 00 00 00 00 00 00 5845 Nonce: 5847 b7 32 37 9f 73 c4 92 8d e2 5f ac fe 65 17 ec 10 5849 Decrypted chunk #0. 5851 Literal data packet with the string contents 'Hello, world!\n'. 5853 cb 14 62 00 00 00 00 00 48 65 6c 6c 6f 2c 20 77 5854 6f 72 6c 64 21 0a 5856 Authenticating final tag: 5858 Authenticated data: 5860 d4 01 07 01 0e 00 00 00 00 00 00 00 01 00 00 00 5861 00 00 00 00 16 5863 Nonce: 5865 b7 32 37 9f 73 c4 92 8d e2 5f ac fe 65 17 ec 11 5867 A.3.6. Complete AEAD-EAX encrypted packet sequence 5869 Symmetric-key encrypted session key packet (v5): 5871 c3 3e 05 07 01 03 08 cd 5a 9f 70 fb e0 bc 65 90 5872 bc 66 9e 34 e5 00 dc ae dc 5b 32 aa 2d ab 02 35 5873 9d ee 19 d0 7c 34 46 c4 31 2a 34 ae 19 67 a2 fb 5874 7e 92 8e a5 b4 fa 80 12 bd 45 6d 17 38 c6 3c 36 5876 AEAD encrypted data packet: 5878 d4 4a 01 07 01 0e b7 32 37 9f 73 c4 92 8d e2 5f 5879 ac fe 65 17 ec 10 5d c1 1a 81 dc 0c b8 a2 f6 f3 5880 d9 00 16 38 4a 56 fc 82 1a e1 1a e8 db cb 49 86 5881 26 55 de a8 8d 06 a8 14 86 80 1b 0f f3 87 bd 2e 5882 ab 01 3d e1 25 95 86 90 6e ab 24 76 5884 A.4. Sample AEAD-OCB encryption and decryption 5886 Encryption is performed with the string 'Hello, world!' and password 5887 'password', using AES-128 with AEAD-OCB encryption. 5889 A.4.1. Sample Parameters 5891 S2K: 5893 type 3 5895 Iterations: 5897 524288 (144), SHA-256 5899 Salt: 5901 9f0b7da3e5ea6477 5903 A.4.2. Sample symmetric-key encrypted session key packet (v5) 5905 Packet header: 5907 c3 3d 5909 Version, algorithms, S2K fields: 5911 05 07 02 03 08 9f 0b 7d a3 e5 ea 64 77 90 5913 AEAD IV: 5915 99 e3 26 e5 40 0a 90 93 6c ef b4 e8 eb a0 8c 5917 AEAD encrypted CEK: 5919 67 73 71 6d 1f 27 14 54 0a 38 fc ac 52 99 49 da 5921 Authentication tag: 5923 c5 29 d3 de 31 e1 5b 4a eb 72 9e 33 00 33 db ed 5925 A.4.3. Starting AEAD-OCB decryption of CEK 5927 The derived key is: 5929 eb 9d a7 8a 9d 5d f8 0e c7 02 05 96 39 9b 65 08 5931 Authenticated Data: 5933 c3 05 07 02 5935 Nonce: 5937 99 e3 26 e5 40 0a 90 93 6c ef b4 e8 eb a0 8c 5939 Decrypted CEK: 5941 d1 f0 1b a3 0e 13 0a a7 d2 58 2c 16 e0 50 ae 44 5943 A.4.4. Sample AEAD encrypted data packet 5945 Packet header: 5947 d4 49 5949 Version, AES-128, OCB, Chunk bits (14): 5951 01 07 02 0e 5953 IV: 5955 5e d2 bc 1e 47 0a be 8f 1d 64 4c 7a 6c 8a 56 5957 AEAD-OCB Encrypted data chunk #0: 5959 7b 0f 77 01 19 66 11 a1 54 ba 9c 25 74 cd 05 62 5960 84 a8 ef 68 03 5c 5962 Chunk #0 authentication tag: 5964 62 3d 93 cc 70 8a 43 21 1b b6 ea f2 b2 7f 7c 18 5966 Final (zero-size chunk #1) authentication tag: 5968 d5 71 bc d8 3b 20 ad d3 a0 8b 73 af 15 b9 a0 98 5970 A.4.5. Decryption of data 5972 Starting AEAD-OCB decryption of data, using the CEK. 5974 Chunk #0: 5976 Authenticated data: 5978 r4 01 07 02 0e 00 00 00 00 00 00 00 00 5980 Nonce: 5982 5e d2 bc 1e 47 0a be 8f 1d 64 4c 7a 6c 8a 56 5984 Decrypted chunk #0. 5986 Literal data packet with the string contents 'Hello, world!\n'. 5988 cb 14 62 00 00 00 00 00 48 65 6c 6c 6f 2c 20 77 5989 6f 72 6c 64 21 0a 5991 Authenticating final tag: 5993 Authenticated data: 5995 d4 01 07 02 0e 00 00 00 00 00 00 00 01 00 00 00 5996 00 00 00 00 16 5998 Nonce: 6000 5e d2 bc 1e 47 0a be 8f 1d 64 4c 7a 6c 8a 57 6002 A.4.6. Complete AEAD-OCB encrypted packet sequence 6004 Symmetric-key encrypted session key packet (v5): 6006 c3 3d 05 07 02 03 08 9f 0b 7d a3 e5 ea 64 77 90 6007 99 e3 26 e5 40 0a 90 93 6c ef b4 e8 eb a0 8c 67 6008 73 71 6d 1f 27 14 54 0a 38 fc ac 52 99 49 da c5 6009 29 d3 de 31 e1 5b 4a eb 72 9e 33 00 33 db ed 6011 AEAD encrypted data packet: 6013 d4 49 01 07 02 0e 5e d2 bc 1e 47 0a be 8f 1d 64 6014 4c 7a 6c 8a 56 7b 0f 77 01 19 66 11 a1 54 ba 9c 6015 25 74 cd 05 62 84 a8 ef 68 03 5c 62 3d 93 cc 70 6016 8a 43 21 1b b6 ea f2 b2 7f 7c 18 d5 71 bc d8 3b 6017 20 ad d3 a0 8b 73 af 15 b9 a0 98 6019 Appendix B. ECC Point compression flag bytes 6021 This specification introduces the new flag byte 0x40 to indicate the 6022 point compression format. The value has been chosen so that the high 6023 bit is not cleared and thus to avoid accidental sign extension. Two 6024 other values might also be interesting for other ECC specifications: 6026 Flag Description 6027 ---- ----------- 6028 0x04 Standard flag for uncompressed format 6029 0x40 Native point format of the curve follows 6030 0x41 Only X coordinate follows. 6031 0x42 Only Y coordinate follows. 6033 Appendix C. Changes since RFC-4880 6035 * Applied errata 2270, 2271, 2242, 3298. 6037 * Added Camellia cipher from RFC 5581. 6039 * Incorporated RFC 6637 (ECC for OpenPGP) 6041 * Added draft-atkins-openpgp-device-certificates 6043 * Added draft-koch-eddsa-for-openpgp-04 6045 * Added Issuer Fingerprint signature subpacket. 6047 * Added a v5 key and fingerprint format. 6049 * Added OIDs for brainpool curves and Curve25519. 6051 * Marked SHA2-256 as MUST implement. 6053 * Marked Curve25519 and Ed25519 as SHOULD implement. 6055 * Marked SHA-1 as SHOULD NOT be used to create messages. 6057 * Marked MD5 as SHOULD NOT implement. 6059 * Changed v5 key fingerprint format to full 32 octets. 6061 * Added Literal Data Packet format octet 'm'. 6063 * Added Feature Flag for v5 key support. 6065 * Added AEAD Encrypted Data Packet. 6067 * Removed notes on extending the MDC packet. 6069 * Added v5 Symmetric-Key Encrypted Session Key packet. 6071 * Added AEAD encryption of secret keys. 6073 * Added test vectors for AEAD. 6075 * Added the Additional Encryption Subkey key flag. 6077 * Deprecated the Symmetrically Encrypted Data Packet. 6079 * Suggest limitation of the AEAD chunksize to 128 MiB. 6081 * Specified the V5 signature format. 6083 * Deprectated the creation of V3 signatures. 6085 * Adapted terms from RFC 8126. 6087 * Removed editorial marks and updated cross-references. 6089 * Added the timestamping usage key flag. 6091 * Added Intended Recipient signature subpacket. 6093 * Added Attested Certifications signature subpacket and signature 6094 class. 6096 Appendix D. The principal authors of RFC-4880 6098 Jon Callas 6099 EMail: jon@callas.org 6101 Lutz Donnerhacke 6102 EMail: lutz@iks-jena.de 6104 Hal Finney 6106 David Shaw 6107 EMail: dshaw@jabberwocky.com 6109 Rodney Thayer 6110 EMail: rodney@canola-jones.com 6112 Authors' Addresses 6114 Werner Koch 6115 GnuPG e.V. 6116 Rochusstr. 44 6117 40479 Duesseldorf 6118 Germany 6120 Email: wk@gnupg.org 6121 URI: https://gnupg.org/verein 6123 Email: sandals@crustytoothpaste.net 6125 Hong Kong 6126 Central, Hong Kong 6127 Suite 1111, 1 Pedder Street 6128 Ribose 6129 Ronald Henry Tse 6131 Email: ronald.tse@ribose.com 6132 URI: https://www.ribose.com 6134 Email: derek@ihtfp.com 6136 Email: dkg@fifthhorseman.net