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Checking references for intended status: Experimental ---------------------------------------------------------------------------- -- Obsolete informational reference (is this intentional?): RFC 6982 (Obsoleted by RFC 7942) Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group P. Wouters 3 Internet-Draft Red Hat 4 Intended status: Experimental January 27, 2016 5 Expires: July 30, 2016 7 Using DANE to Associate OpenPGP public keys with email addresses 8 draft-ietf-dane-openpgpkey-07 10 Abstract 12 OpenPGP is a message format for email (and file) encryption that 13 lacks a standardized lookup mechanism to securely obtain OpenPGP 14 public keys. DNS-Based Authentication of Named Entities ("DANE") is 15 a method for publishing public keys in DNS. This document specifies 16 a DANE method for publishing and locating OpenPGP public keys in DNS 17 for a specific email address using a new OPENPGPKEY DNS Resource 18 Record. Security is provided via Secure DNS, however the OPENPGPKEY 19 record is not a replacement for verification of authenticity via the 20 "Web of Trust" or manual verification. The OPENPGPKEY record can be 21 used to encrypt an email that would otherwise have to be send 22 unencrypted. 24 Status of This Memo 26 This Internet-Draft is submitted in full conformance with the 27 provisions of BCP 78 and BCP 79. 29 Internet-Drafts are working documents of the Internet Engineering 30 Task Force (IETF). Note that other groups may also distribute 31 working documents as Internet-Drafts. The list of current Internet- 32 Drafts is at http://datatracker.ietf.org/drafts/current/. 34 Internet-Drafts are draft documents valid for a maximum of six months 35 and may be updated, replaced, or obsoleted by other documents at any 36 time. It is inappropriate to use Internet-Drafts as reference 37 material or to cite them other than as "work in progress." 39 This Internet-Draft will expire on July 30, 2016. 41 Copyright Notice 43 Copyright (c) 2016 IETF Trust and the persons identified as the 44 document authors. All rights reserved. 46 This document is subject to BCP 78 and the IETF Trust's Legal 47 Provisions Relating to IETF Documents 48 (http://trustee.ietf.org/license-info) in effect on the date of 49 publication of this document. Please review these documents 50 carefully, as they describe your rights and restrictions with respect 51 to this document. Code Components extracted from this document must 52 include Simplified BSD License text as described in Section 4.e of 53 the Trust Legal Provisions and are provided without warranty as 54 described in the Simplified BSD License. 56 Table of Contents 58 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 59 1.1. Experiment goal . . . . . . . . . . . . . . . . . . . . . 3 60 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 61 2. The OPENPGPKEY Resource Record . . . . . . . . . . . . . . . 4 62 2.1. The OPENPGPKEY RDATA component . . . . . . . . . . . . . 4 63 2.1.1. The OPENPGPKEY RDATA content . . . . . . . . . . . . 4 64 2.1.2. Reducing the Transferable Public Key size . . . . . . 5 65 2.2. The OPENPGPKEY RDATA wire format . . . . . . . . . . . . 6 66 2.3. The OPENPGPKEY RDATA presentation format . . . . . . . . 6 67 3. Location of the OPENPGPKEY record . . . . . . . . . . . . . . 6 68 4. Email address variants . . . . . . . . . . . . . . . . . . . 7 69 5. Application use of OPENPGPKEY . . . . . . . . . . . . . . . . 7 70 5.1. Obtaining an OpenPGP key for a specific email address . . 7 71 5.2. Confirming that an OpenPGP key is current . . . . . . . . 8 72 5.3. Public Key UIDs and query names . . . . . . . . . . . . . 8 73 6. OpenPGP Key size and DNS . . . . . . . . . . . . . . . . . . 9 74 7. Security Considerations . . . . . . . . . . . . . . . . . . . 9 75 7.1. MTA behaviour . . . . . . . . . . . . . . . . . . . . . . 10 76 7.2. MUA behaviour . . . . . . . . . . . . . . . . . . . . . . 10 77 7.3. Email client behaviour . . . . . . . . . . . . . . . . . 11 78 7.4. Response size . . . . . . . . . . . . . . . . . . . . . . 11 79 7.5. Email address information leak . . . . . . . . . . . . . 11 80 7.6. Storage of OPENPGPKEY data . . . . . . . . . . . . . . . 12 81 7.7. Security of OpenPGP versus DNSSEC . . . . . . . . . . . . 12 82 8. Implementation Status . . . . . . . . . . . . . . . . . . . . 12 83 8.1. The GNU Privacy Guard (GNUpg) . . . . . . . . . . . . . . 13 84 8.2. hash-slinger . . . . . . . . . . . . . . . . . . . . . . 13 85 8.3. openpgpkey-milter . . . . . . . . . . . . . . . . . . . . 14 86 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 87 9.1. OPENPGPKEY RRtype . . . . . . . . . . . . . . . . . . . . 14 88 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15 89 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 15 90 11.1. Normative References . . . . . . . . . . . . . . . . . . 15 91 11.2. Informative References . . . . . . . . . . . . . . . . . 16 92 Appendix A. Generating OPENPGPKEY records . . . . . . . . . . . 17 93 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 18 95 1. Introduction 96 OpenPGP [RFC4880] public keys are used to encrypt or sign email 97 messages and files. To encrypt an email message, or verify a 98 sender's OpenPGP signature, the email client or MTA needs to locate 99 the recipient's OpenPGP public key. 101 OpenPGP clients have relied on centralized "well-known" key servers 102 that are accessed using either the HTTP Keyserver Protocol [HKP] 103 Alternatively, users need to manually browse a variety of different 104 front-end websites. These key servers do not require a confirmation 105 of the email address used in the User ID of the uploaded OpenPGP 106 public key. Attackers can - and have - uploaded rogue public keys 107 with other people's email addresses to these key servers. 109 Once uploaded, public keys cannot be deleted. People who did not 110 pre-sign a key revocation can never remove their OpenPGP public key 111 from these key servers once they have lost access to their private 112 key. This results in receiving encrypted email that cannot be 113 decrypted. 115 Therefor, these keyservers are not well suited to support email 116 clients and MTA's to automatically encrypt email - especially in the 117 absence of an interactive user. 119 This document describes a mechanism to associate a user's OpenPGP 120 public key with their email address, using the OPENPGPKEY DNS RRtype. 121 These records are published in the DNS zone of the user's email 122 address. If the user loses their private key, the OPENPGPKEY DNS 123 record can simply be updated or removed from the zone. 125 The OPENPGPKEY data is secured using Secure DNS [RFC4035] 127 The main goal of the OPENPGPKEY resource record is to stop passive 128 attacks against plaintext emails. While it can also thwart some 129 active attacks (such as people uploading rogue keys to keyservers in 130 the hopes that others will encrypt to these rogue keys), this 131 resource record is not a replacement for verifying OpenPGP public 132 keys via the web of trust signatures, or manually via a fingerprint 133 verification. 135 1.1. Experiment goal 137 This document defines an RRtype whose use is Experimental. The goal 138 of the experiment is to see whether encrypted email usage will 139 increase if an automated discovery method is available to MTA's and 140 MUA's to help the enduser with email encryption key management. 142 It is unclear if this RRtype will scale to some of the larger email 143 service deployments. Concerns have been raised about the size of the 144 OPENPGPKEY record and the size of the resulting DNS zone files. This 145 experiment hopefully will give the working group some insight into 146 whether this is a problem or not. 148 If the experiment is successful, it is expected that the findings of 149 the experiment will result in an updated document for standards track 150 approval. 152 The OPENPGPKEY RRtype somewhat resembles the generic CERT record 153 defined in [RFC4398]. However, the CERT record uses sub-typing with 154 many different types of keys and certificates. It is suspected that 155 its general application of very different protocols (PKIX versus 156 OpenPGP) has been the cause for lack of implementation and 157 deployment. Furthermore, the CERT record uses sub-typing, which is 158 now considered to be a bad idea for DNS. 160 1.2. Terminology 162 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 163 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 164 document are to be interpreted as described in RFC 2119 [RFC2119]. 166 This document also makes use of standard DNSSEC and DANE terminology. 167 See DNSSEC [RFC4033], [RFC4034], [RFC4035], and DANE [RFC6698] for 168 these terms. 170 2. The OPENPGPKEY Resource Record 172 The OPENPGPKEY DNS resource record (RR) is used to associate an end 173 entity OpenPGP Transferable Public Key (see Section 11.1 of [RFC4880] 174 with an email address, thus forming a "OpenPGP public key 175 association". A user that wishes to specify more than one OpenPGP 176 key, for example because they are transitioning to a newer stronger 177 key, can do so by adding multiple OPENPGPKEY records. A single 178 OPENPGPKEY DNS record MUST only contain one OpenPGP key. 180 The type value allocated for the OPENPGPKEY RR type is 61. The 181 OPENPGPKEY RR is class independent. 183 2.1. The OPENPGPKEY RDATA component 185 The RDATA portion of an OPENPGPKEY Resource Record contains a single 186 value consisting of a [RFC4880] formatted Transferable Public Key. 188 2.1.1. The OPENPGPKEY RDATA content 190 An OpenPGP Transferable Public Key can be arbitrarily large. DNS 191 records are limited in size. When creating OPENPGPKEY DNS records, 192 the OpenPGP Transferable Public Key should be filtered to only 193 contain appropriate and useful data. At a minimum, an OPENPGPKEY 194 Transferable Public Key for the user hugh@example.com should contain: 196 o The primary key X 197 o One User ID Y, which SHOULD match 'hugh@example.com' 198 o self-signature from X, binding X to Y 200 If the primary key is not encryption-capable, a relevant subkey 201 should be included resulting in an OPENPGPKEY Transferable Public Key 202 containing: 204 o The primary key X 205 o One User ID Y, which SHOULD match 'hugh@example.com' 206 o self-signature from X, binding X to Y 207 o encryption-capable subkey Z 208 o self-signature from X, binding Z to X 209 o [ other subkeys if relevant ... ] 211 The user can also elect to add a few third-party certifications which 212 they believe would be helpful for validation in the traditional Web 213 Of Trust. The resulting OPENPGPKEY Transferable Public Key would 214 then look like: 216 o The primary key X 217 o One User ID Y, which SHOULD match 'hugh@example.com' 218 o self-signature from X, binding X to Y 219 o third-party certification from V, binding Y to X 220 o [ other third-party certifications if relevant ... ] 221 o encryption-capable subkey Z 222 o self-signature from X, binding Z to X 223 o [ other subkeys if relevant ... ] 225 2.1.2. Reducing the Transferable Public Key size 227 When preparing a Transferable Public Key for a specific OPENPGPKEY 228 RDATA format with the goal of minimizing certificate size, a user 229 would typically want to: 231 o Where one User ID from the certifications matches the looked-up 232 address, strip away non-matching User IDs and any associated 233 certifications (self-signatures or third-party certifications) 235 o Strip away all User Attribute packets and associated 236 certifications. 238 o Strip away all expired subkeys. The user may want to keep revoked 239 subkeys if these were revoked prior to their preferred expiration 240 time to ensure that correspondents know about these earlier then 241 expected revocations. 243 o Strip away all but the most recent self-sig for the remaining user 244 IDs and subkeys 246 o Optionally strip away any uninteresting or unimportant third-party 247 User ID certifications. This is a value judgment by the user that 248 is difficult to automate. At the very least, expired and 249 superseded third-party certifcations should be stripped out. The 250 user should attempt to keep the most recent and most well 251 connected certifications in the Web Of Trust in their Transferable 252 Public Key. 254 2.2. The OPENPGPKEY RDATA wire format 256 The RDATA Wire Format consists of a single OpenPGP Transferable 257 Public Key as defined in Section 11.1 of [RFC4880]. Note that this 258 format is without ASCII armor or base64 encoding. 260 2.3. The OPENPGPKEY RDATA presentation format 262 The RDATA Presentation Format, as visible in master files [RFC1035], 263 consists of a single OpenPGP Transferable Public Key as defined in 264 Section 11.1 of [RFC4880] encoded in base64 as defined in Section 4 265 of [RFC4648]. 267 3. Location of the OPENPGPKEY record 269 The DNS does not allow the use of all characters that are supported 270 in the "local-part" of email addresses as defined in [RFC5322] and 271 [RFC6530]. Therefore, email addresses are mapped into DNS using the 272 following method: 274 o The user name (the "left-hand side" of the email address, called 275 the "local-part" in the mail message format definition [RFC5322] 276 and the local-part in the specification for internationalized 277 email [RFC6530]) is encoded in UTF-8 (or its subset ASCII). If 278 the local-part is written in another encoding it MUST be converted 279 to UTF-8. 281 o The local-part is hashed using the SHA2-256 [RFC5754] algorithm, 282 with the hash truncated to 28 octets and represented in its 283 hexadecimal representation, to become the left-most label in the 284 prepared domain name. 286 o The string "_openpgpkey" becomes the second left-most label in the 287 prepared domain name. 289 o The domain name (the "right-hand side" of the email address, 290 called the "domain" in [RFC5322]) is appended to the result of 291 step 2 to complete the prepared domain name. 293 For example, to request an OPENPGPKEY resource record for a user 294 whose email address is "hugh@example.com", an OPENPGPKEY query would 295 be placed for the following QNAME: "c93f1e400f26708f98cb19d936620da35 296 eec8f72e57f9eec01c1afd6._openpgpkey.example.com". The corresponding 297 RR in the example.com zone might look like (key shortened for 298 formatting): 300 c9[..]d6._openpgpkey.example.com. IN OPENPGPKEY 302 4. Email address variants 304 Mail systems usually handle variant forms of local-parts. The most 305 common variants are upper and lower case, often automatically 306 corrected when a name is recognized as such. Other variants include 307 systems that ignore "noise" characters such as dots, so that local 308 parts johnsmith and John.Smith would be equivalent. Many systems 309 allow "extensions" such as john-ext or mary+ext where john or mary is 310 treated as the effective local-part, and the ext is passed to the 311 recipient for further handling. This can complicate finding the 312 OPENPGPKEY record associated with the dynamically created email 313 address. 315 [RFC5321] and its predecessors have always made it clear that only 316 the recipient MTA is allowed to interpret the local-part of an 317 address. A client supporting OPENPGPKEY therefor MUST NOT perform 318 any kind of mapping rules based on the email address. 320 5. Application use of OPENPGPKEY 322 The OPENPGPKEY record allows an application or service to obtain an 323 OpenPGP public key and use it for verifying a digital signature or 324 encrypting a message to the public key. The DNS answer MUST pass 325 DNSSEC validation; if DNSSEC validation reaches any state other than 326 "Secure" (as specified in [RFC4035]), the DNSSEC validation MUST be 327 treated as a failure. 329 5.1. Obtaining an OpenPGP key for a specific email address 331 If no OpenPGP public keys are known for an email address, an 332 OPENPGPKEY DNS lookup MAY be performed to seek the OpenPGP public key 333 that corresponds to that email address. This public key can then be 334 used to verify a received signed message or can be used to send out 335 an encrypted email message. An application whose attempt fails to 336 retrieve a DNSSEC verified OPENPGPKEY RR from the DNS should remember 337 that failure for some time to avoid sending out a DNS request for 338 each email message the application is sending out; such DNS requests 339 constitute a privacy leak 341 5.2. Confirming that an OpenPGP key is current 343 Locally stored OpenPGP public keys are not automatically refreshed. 344 If the owner of that key creates a new OpenPGP public key, that owner 345 is unable to securely notify all users and applications that have its 346 old OpenPGP public key. Applications and users can perform an 347 OPENPGPKEY lookup to confirm the locally stored OpenPGP public key is 348 still the correct key to use. If the locally stored OpenPGP public 349 key is different from the DNSSEC validated OpenPGP public key 350 currently published in DNS, the verification MUST be treated as a 351 failure unless the locally stored OpenPGP key signed the newly 352 published OpenPGP public key found in DNS. An application that can 353 interact with the user MAY ask the user for guidance. For privacy 354 reasons, an application MUST NOT attempt to lookup an OpenPGP key 355 from DNSSEC at every use of that key. 357 5.3. Public Key UIDs and query names 359 An OpenPGP public key can be associated with multiple email addresses 360 by specifying multiple key uids. The OpenPGP public key obtained 361 from a OPENPGPKEY RR can be used as long as the query and resulting 362 data form a proper email to uid identity association. 364 CNAME's (see [RFC2181]) and DNAME's (see [RFC6672]) can be followed 365 to obtain an OPENPGPKEY RR, as long as the original recipient's email 366 address appears as one of the OpenPGP public key uids. For example, 367 if the OPENPGPKEY RR query for hugh@example.com 368 (8d57[...]b7._openpgpkey.example.com) yields a CNAME to 369 8d57[...]b7._openpgpkey.example.net, and an OPENPGPKEY RR for 370 8d57[...]b7._openpgpkey.example.net exists, then this OpenPGP public 371 key can be used, provided one of the key uids contains 372 "hugh@example.com". This public key cannot be used if it would only 373 contain the key uid "hugh@example.net". 375 If one of the OpenPGP key uids contains only a single wildcard as the 376 LHS of the email address, such as "*@example.com", the OpenPGP public 377 key may be used for any email address within that domain. Wildcards 378 at other locations (eg hugh@*.com) or regular expressions in key uids 379 are not allowed, and any OPENPGPKEY RR containing these MUST be 380 ignored. 382 6. OpenPGP Key size and DNS 384 Due to the expected size of the OPENPGPKEY record, applications 385 SHOULD use TCP - not UDP - to perform queries for the OPENPGPKEY 386 Resource Record. 388 Although the reliability of the transport of large DNS Resource 389 Records has improved in the last years, it is still recommended to 390 keep the DNS records as small as possible without sacrificing the 391 security properties of the public key. The algorithm type and key 392 size of OpenPGP keys should not be modified to accommodate this 393 section. 395 OpenPGP supports various attributes that do not contribute to the 396 security of a key, such as an embedded image file. It is recommended 397 that these properties not be exported to OpenPGP public keyrings that 398 are used to create OPENPGPKEY Resource Records. Some OpenPGP 399 software, for example GnuPG, support a "minimal key export" that is 400 well suited to use as OPENPGPKEY RDATA. See Appendix A. 402 7. Security Considerations 404 DNSSEC is not an alternative for the "web of trust" or for manual 405 fingerprint verification by users. DANE for OpenPGP as specified in 406 this document is a solution aimed to ease obtaining someone's public 407 key. Without manual verification of the OpenPGP key obtained via 408 DANE, this retrieved key should only be used for encryption if the 409 only other alternative is sending the message in plaintext. While 410 this thwarts all passive attacks that simply capture and log all 411 plaintext email content, it is not a security measure against active 412 attacks. A user who publishes an OPENPGPKEY record in DNS still 413 expects senders to perform their due diligence by additional (non- 414 DNSSEC) verification of their public key via other out-of-band 415 methods before sending any confidential or sensitive information. 417 In other words, the OPENPGPKEY record MUST NOT be used to send 418 sensitive information without additional verification or confirmation 419 that the OpenPGP key actually belongs to the target recipient. 421 Various components could be responsible for encrypting an email 422 message to a target recipient. It could be done by the sender's 423 email client or software plugin, the sender's Mail User Agent (MUA) 424 or the sender's Mail Transfer Agent (MTA). Each of these have their 425 own characteristics. An email client can ask the user to make a 426 decision before continuing. The MUA can either accept or refuse a 427 message. The MTA must deliver the message as-is, or encrypt the 428 message before delivering. Each of these programs should attempt to 429 encrypt an unencrypted received message whenever possible. 431 In theory, two different local-parts could hash to the same value. 432 This document assumes that such a hash collision has a negliable 433 chance of happening. 435 Organisations that are required to be able to read everyone's 436 encrypted email should publish the escrow key as the OPENPGPKEY 437 record. Mail servers of such organizations MAY optionally re-encrypt 438 the message to the individual's OpenPGP key. 440 7.1. MTA behaviour 442 An MTA could be operating in a stand-alone mode, without access to 443 the sender's OpenPGP public keyring, or in a way where it can access 444 the user's OpenPGP public keyring. Regardless, the MTA MUST NOT 445 modify the user's OpenPGP keyring. 447 An MTA sending an email MUST NOT add the public key obtained from an 448 OPENPGPKEY resource record to a permanent public keyring for future 449 use beyond the TTL. 451 If the obtained public key is revoked, the MTA MUST NOT use the key 452 for encryption, even if that would result in sending the message in 453 plaintext. 455 If a message is already encrypted, the MTA SHOULD NOT re-encrypt the 456 message, even if different encryption schemes or different encryption 457 keys would be used. 459 If the DNS request for an OPENPGPKEY record returned an Indeterminate 460 or Bogus answer as specified in [RFC4035], the MTA MUST NOT send the 461 message and queue the plaintext message for encrypted delivery at a 462 later time. If the problem persists, the email should be returned 463 via the regular bounce methods. 465 If multiple non-revoked OPENPGPKEY resource records are found, the 466 MTA SHOULD pick the most secure RR based on its local policy. 468 7.2. MUA behaviour 470 If the public key for a recipient obtained from the locally stored 471 sender's public keyring differs from the recipient's OPENPGPKEY RR, 472 the MUA MUST NOT accept the message for delivery. 474 If the public key for a recipient obtained from the locally stored 475 sender's public keyring contains contradicting properties for the 476 same key obtained from an OPENPGPKEY RR, the MUA SHOULD NOT accept 477 the message for delivery. 479 If multiple non-revoked OPENPGPKEY resource records are found, the 480 MUA SHOULD pick the most secure OpenPGP public key based on its local 481 policy. 483 7.3. Email client behaviour 485 Email clients should adhere to the above listed MUA behaviour. 486 Additionally, an email client MAY interact with the user to resolve 487 any conflicts between locally stored keyrings and OPENPGPKEY RRdata. 489 An email client that is encrypting a message SHOULD clearly indicate 490 to the user the difference between encrypting to a locally stored and 491 user verified public key and encrypting to an unverified public key 492 obtained via an OPENPGPKEY resource record. 494 7.4. Response size 496 To prevent amplification attacks, an Authoritative DNS server MAY 497 wish to prevent returning OPENPGPKEY records over UDP unless the 498 source IP address has been confirmed with [EDNS-COOKIE]. Such 499 servers MUST NOT return REFUSED, but answer the query with an empty 500 Answer Section and the truncation flag set ("TC=1"). 502 7.5. Email address information leak 504 The hashing of the user name in this document is not a security 505 feature. Publishing OPENPGPKEY records however, will create a list 506 of hashes of valid email addresses, which could simplify obtaining a 507 list of valid email addresses for a particular domain. It is 508 desirable to not ease the harvesting of email addresses where 509 possible. 511 The domain name part of the email address is not used as part of the 512 hash so that hashes can be used in multiple zones deployed using 513 DNAME [RFC6672]. This does makes it slightly easier and cheaper to 514 brute-force the SHA2-256 hashes into common and short user names, as 515 single rainbow tables can be re-used across domains. This can be 516 somewhat countered by using NSEC3. 518 DNS zones that are signed with DNSSEC using NSEC for denial of 519 existence are susceptible to zone-walking, a mechanism that allows 520 someone to enumerate all the OPENPGPKEY hashes in a zone. This can 521 be used in combination with previously hashed common or short user 522 names (in rainbow tables) to deduce valid email addresses. DNSSEC- 523 signed zones using NSEC3 for denial of existence instead of NSEC are 524 significantly harder to brute-force after performing a zone-walk. 526 7.6. Storage of OPENPGPKEY data 528 Users may have a local key store with OpenPGP public keys. An 529 application supporting the use of OPENPGPKEY DNS records MUST NOT 530 modify the local key store without explicit confirmation of the user, 531 as the application is unaware of the user's personal policy for 532 adding, removing or updating their local key store. An application 533 MAY warn the user if an OPENPGPKEY record does not match the OpenPGP 534 public key in the local key store. 536 Applications that cannot interact with users, such as daemon 537 processes, SHOULD store OpenPGP public keys obtained via OPENPGPKEY 538 up to their DNS TTL value. This avoids repeated DNS lookups that 539 third parties could monitor to determine when an email is being sent 540 to a particular user. 542 7.7. Security of OpenPGP versus DNSSEC 544 Anyone who can obtain a DNSSEC private key of a domain name via 545 coercion, theft or brute force calculations, can replace any 546 OPENPGPKEY record in that zone and all of the delegated child zones. 547 Any future messages encrypted with the malicious OpenPGP key could 548 then be read. 550 Therefore, an OpenPGP key obtained via a DNSSEC validated OPENPGPKEY 551 record can only be trusted as much as the DNS domain can be trusted, 552 and is no substitute for in-person OpenPGP key verification or 553 additional Openpgp verification via "Web of Trust" signatures present 554 on the OpenPGP in question. 556 8. Implementation Status 558 [RFC Editor Note: Please remove this entire seciton prior to 559 publication as an RFC.] 561 This section records the status of known implementations of the 562 protocol defined by this specification at the time of posting of this 563 Internet-Draft, and is based on a proposal described in [RFC6982]. 564 The description of implementations in this section is intended to 565 assist the IETF in its decision processes in progressing drafts to 566 RFCs. Please note that the listing of any individual implementation 567 here does not imply endorsement by the IETF. Furthermore, no effort 568 has been spent to verify the information presented here that was 569 supplied by IETF contributors. This is not intended as, and must not 570 be construed to be, a catalog of available implementations or their 571 features. Readers are advised to note that other implementations may 572 exist. According to RFC 6982, "this will allow reviewers and working 573 groups to assign due consideration to documents that have the benefit 574 of running code, which may serve as evidence of valuable 575 experimentation and feedback that have made the implemented protocols 576 more mature. It is up to the individual working groups to use this 577 information as they see fit." 579 8.1. The GNU Privacy Guard (GNUpg) 581 Implementation Name and Details: The GNUpg software, more commonly 582 known as "gpg", is is available at https://gnupg.org/ 584 Brief Description: Support has been added to gnupg in their git 585 repository. This code is expected to be part of the next official 586 release. 588 Level of Maturity: The implementation has just been added and has 589 not seen widespread deployment. 591 Coverage: The implementation follows the latest draft with the 592 exception that it first performs a lowercase of the local-part 593 before hashing. This is done because other parts in the code that 594 perform a lookup of uid already performed a localcasing to ensure 595 case insensitivity. The implementors are tracking the development 596 of this draft in particular with respect to the lowercase issue. 598 Licensing: All code is covered under the GNU Public License version 599 3 or later. 601 Implementation Experience: Currrent experience limited to small test 602 networks only 604 Contact Information: https://gnupg.org/ 606 Interoperability: No report. 608 8.2. hash-slinger 610 Implementation Name and Details: The hash-slinger software is a 611 collection of tools to generate, download and verify application 612 public keys and application fingerprints. It uses DNSSEC 613 validation. The tool is written by the author of this document. 614 It is available at http://people.redhat.com/pwouters/ 616 Brief Description: Support has been added in the form of an 617 "openpgpkey" command that can generate, fetch, validate the DNSSEC 618 authentication and verify OPENPGPKEY records. 620 Level of Maturity: The implementation has been around for a few 621 months but has not seen widespread deployment. 623 Coverage: The implementation follows the latest draft with the 624 exception that it first performs a lowercase of the local-part 625 before hashing. 627 Licensing: All code is covered under the GNU Public License version 628 3 or later. 630 Implementation Experience: Currrent experience limited to small test 631 networks only 633 Contact Information: pwouters@redhat.com 635 Interoperability: No report. 637 8.3. openpgpkey-milter 639 Implementation Name and Details: The openpgpkey-milter is a Postfix 640 and Sendmail Mail server plugin (milter) that automatically 641 encrypts email before sending further to other SMTP servers. It 642 is written by the author of this document. It is available at 643 http://github.com/letoams/openpgpkey-milter/ 645 Brief Description: Before forwarding an unencrypted email, the 646 plugin looks for the presence of an OPENPGPKEY record. When 647 available, it will encrypt the email message and send out the 648 encrypted email. 650 Level of Maturity: The implementation has been around for a few 651 months but has not seen widespread deployment. 653 Coverage: The implementation follows the latest draft with the 654 exception that it first performs a lowercase of the local-part 655 before hashing. 657 Licensing: All code is covered under the GNU Public License version 658 3 or later. 660 Implementation Experience: Currrent experience limited to small test 661 networks only 663 Contact Information: pwouters@redhat.com 665 Interoperability: No report. 667 9. IANA Considerations 669 9.1. OPENPGPKEY RRtype 670 This document uses a new DNS RR type, OPENPGPKEY, whose value 61 has 671 been allocated by IANA from the Resource Record (RR) TYPEs 672 subregistry of the Domain Name System (DNS) Parameters registry. 674 10. Acknowledgments 676 This document is based on RFC-4255 and draft-ietf-dane-smime whose 677 authors are Paul Hoffman, Jacob Schlyter and W. Griffin. Olafur 678 Gudmundsson provided feedback and suggested various improvements. 679 Willem Toorop contributed the gpg and hexdump command options. 680 Daniel Kahn Gillmor provided the text describing the OpenPGP packet 681 formats and filtering options. Edwin Taylor contributed language 682 improvements for various iterations of this document. Text regarding 683 email mappings was taken from draft-levine-dns-mailbox whose author 684 is John Levine. 686 11. References 688 11.1. Normative References 690 [RFC1035] Mockapetris, P., "Domain names - implementation and 691 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 692 November 1987, . 694 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 695 Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ 696 RFC2119, March 1997, 697 . 699 [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS 700 Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997, 701 . 703 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 704 Rose, "DNS Security Introduction and Requirements", RFC 705 4033, DOI 10.17487/RFC4033, March 2005, 706 . 708 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 709 Rose, "Resource Records for the DNS Security Extensions", 710 RFC 4034, DOI 10.17487/RFC4034, March 2005, 711 . 713 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 714 Rose, "Protocol Modifications for the DNS Security 715 Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005, 716 . 718 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 719 Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, 720 . 722 [RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. 723 Thayer, "OpenPGP Message Format", RFC 4880, DOI 10.17487/ 724 RFC4880, November 2007, 725 . 727 [RFC5754] Turner, S., "Using SHA2 Algorithms with Cryptographic 728 Message Syntax", RFC 5754, DOI 10.17487/RFC5754, January 729 2010, . 731 11.2. Informative References 733 [EDNS-COOKIE] 734 Eastlake, Donald., "Domain Name System (DNS) Cookies", 735 draft-ietf-dnsop-cookies (work in progress), August 2015. 737 [HKP] Shaw, D., "The OpenPGP HTTP Keyserver Protocol (HKP)", 738 draft-shaw-openpgp-hkp (work in progress), March 2013. 740 [RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record 741 (RR) Types", RFC 3597, DOI 10.17487/RFC3597, September 742 2003, . 744 [RFC4398] Josefsson, S., "Storing Certificates in the Domain Name 745 System (DNS)", RFC 4398, DOI 10.17487/RFC4398, March 2006, 746 . 748 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 749 DOI 10.17487/RFC5321, October 2008, 750 . 752 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, DOI 753 10.17487/RFC5322, October 2008, 754 . 756 [RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for 757 Internationalized Email", RFC 6530, DOI 10.17487/RFC6530, 758 February 2012, . 760 [RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the 761 DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012, 762 . 764 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication 765 of Named Entities (DANE) Transport Layer Security (TLS) 766 Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August 767 2012, . 769 [RFC6982] Sheffer, Y. and A. Farrel, "Improving Awareness of Running 770 Code: The Implementation Status Section", RFC 6982, DOI 771 10.17487/RFC6982, July 2013, 772 . 774 Appendix A. Generating OPENPGPKEY records 776 The commonly available GnuPG software can be used to generate a 777 minimum Transferable Public Key for the RRdata portion of an 778 OPENPGPKEY record: 780 gpg --export --export-options export-minimal,no-export-attributes \ 781 hugh@example.com | base64 783 The --armor or -a option of the gpg command should NOT be used, as it 784 adds additional markers around the armored key. 786 When DNS software reading or signing the zone file does not yet 787 support the OPENPGPKEY RRtype, the Generic Record Syntax of [RFC3597] 788 can be used to generate the RDATA. One needs to calculate the number 789 of octets and the actual data in hexadecimal: 791 gpg --export --export-options export-minimal,no-export-attributes \ 792 hugh@example.com | wc -c 794 gpg --export --export-options export-minimal,no-export-attributes \ 795 hugh@example.com | hexdump -e \ 796 '"\t" /1 "%.2x"' -e '/32 "\n"' 798 These values can then be used to generate a generic record (line 799 break has been added for formatting): 801 ._openpgpkey.example.com. IN TYPE61 \# \ 802 804 The openpgpkey command in the hash-slinger software can be used to 805 generate complete OPENPGPKEY records 807 ~> openpgpkey --output rfc hugh@example.com 808 c9[..]d6._openpgpkey.example.com. IN OPENPGPKEY mQCNAzIG[...] 810 ~> openpgpkey --output generic hugh@example.com 811 c9[..]d6._openpgpkey.example.com. IN TYPE61 \# 2313 99008d03[...] 813 Author's Address 815 Paul Wouters 816 Red Hat 818 Email: pwouters@redhat.com