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If you have contacted all the original authors and they are all willing to grant the BCP78 rights to the IETF Trust, then this is fine, and you can ignore this comment. If not, you may need to add the pre-RFC5378 disclaimer. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (December 30, 2003) is 7416 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Missing reference section? 'MIME-SPEC' on line 1398 looks like a reference -- Missing reference section? 'CMS' on line 1377 looks like a reference -- Missing reference section? 'PKCS-7' on line 1412 looks like a reference -- Missing reference section? 'MIME-SECURE' on line 1404 looks like a reference -- Missing reference section? 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'0' on line 1336 looks like a reference -- Missing reference section? '1' on line 1337 looks like a reference -- Missing reference section? '2' on line 1338 looks like a reference -- Missing reference section? 'CERT31' on line 1371 looks like a reference Summary: 6 errors (**), 0 flaws (~~), 3 warnings (==), 23 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Internet Draft Editor: Blake Ramsdell, 2 draft-ietf-smime-rfc2633bis-05.txt Brute Squad Labs 3 June 30, 2003 4 Expires December 30, 2003 6 S/MIME Version 3.1 Message Specification 8 Status of this memo 10 This document is an Internet-Draft and is in full conformance with all 11 provisions of Section 10 of RFC2026. 13 Internet-Drafts are working documents of the Internet Engineering Task 14 Force (IETF), its areas, and its working groups. Note that other 15 groups may also distribute working documents as Internet-Drafts. 17 Internet-Drafts are draft documents valid for a maximum of six months 18 and may be updated, replaced, or obsoleted by other documents at any 19 time. It is inappropriate to use Internet-Drafts as reference material 20 or to cite them other than as "work in progress." 22 The list of current Internet-Drafts can be accessed at 23 http://www.ietf.org/ietf/1id-abstracts.txt 25 The list of Internet-Draft Shadow Directories can be accessed at 26 http://www.ietf.org/shadow.html. 28 1. Introduction 30 S/MIME (Secure/Multipurpose Internet Mail Extensions) provides a 31 consistent way to send and receive secure MIME data. Based on the 32 popular Internet MIME standard, S/MIME provides the following 33 cryptographic security services for electronic messaging applications: 34 authentication, message integrity and non-repudiation of origin (using 35 digital signatures) and data confidentiality (using encryption). 37 S/MIME can be used by traditional mail user agents (MUAs) to add 38 cryptographic security services to mail that is sent, and to interpret 39 cryptographic security services in mail that is received. However, 40 S/MIME is not restricted to mail; it can be used with any transport 41 mechanism that transports MIME data, such as HTTP. As such, S/MIME 42 takes advantage of the object-based features of MIME and allows secure 43 messages to be exchanged in mixed-transport systems. 45 Further, S/MIME can be used in automated message transfer agents that 46 use cryptographic security services that do not require any human 47 intervention, such as the signing of software-generated documents and 48 the encryption of FAX messages sent over the Internet. 50 1.1 Specification Overview 52 This document describes a protocol for adding cryptographic signature 53 and encryption services to MIME data. The MIME standard [MIME-SPEC] 54 provides a general structure for the content type of Internet messages 55 and allows extensions for new content type applications. 57 This specification defines how to create a MIME body part that has 58 been cryptographically enhanced according to CMS [CMS], which is 59 derived from PKCS #7 [PKCS-7]. This specification also defines the 60 application/pkcs7-mime MIME type that can be used to transport those 61 body parts. 63 This specification also discusses how to use the multipart/signed MIME 64 type defined in [MIME-SECURE] to transport S/MIME signed messages. 65 multipart/signed is used in conjunction with the 66 application/pkcs7-signature MIME type, which is used to transport 67 a detached S/MIME signature. 69 In order to create S/MIME messages, an S/MIME agent MUST follow the 70 specifications in this document, as well as the specifications 71 listed in the Cryptographic Message Syntax [CMS]. 73 Throughout this specification, there are requirements and 74 recommendations made for how receiving agents handle incoming 75 messages. There are separate requirements and recommendations for how 76 sending agents create outgoing messages. In general, the best strategy 77 is to "be liberal in what you receive and conservative in what you 78 send". Most of the requirements are placed on the handling of incoming 79 messages while the recommendations are mostly on the creation of 80 outgoing messages. 82 The separation for requirements on receiving agents and sending agents 83 also derives from the likelihood that there will be S/MIME systems 84 that involve software other than traditional Internet mail clients. 85 S/MIME can be used with any system that transports MIME data. An 86 automated process that sends an encrypted message might not be able to 87 receive an encrypted message at all, for example. Thus, the 88 requirements and recommendations for the two types of agents are 89 listed separately when appropriate. 91 1.2 Terminology 93 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 94 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 95 document are to be interpreted as described in [MUSTSHOULD]. 97 1.3 Definitions 99 For the purposes of this specification, the following definitions 100 apply. 102 ASN.1: Abstract Syntax Notation One, as defined in CCITT X.208 103 [X.208-88]. 105 BER: Basic Encoding Rules for ASN.1, as defined in CCITT X.209 106 [X.209-88]. 108 Certificate: A type that binds an entity's distinguished name to a 109 public key with a digital signature. 111 DER: Distinguished Encoding Rules for ASN.1, as defined in CCITT 112 X.509 [X.509-88]. 114 7-bit data: Text data with lines less than 998 characters long, where 115 none of the characters have the 8th bit set, and there are no NULL 116 characters. and occur only as part of a end of line 117 delimiter. 119 8-bit data: Text data with lines less than 998 characters, and where 120 none of the characters are NULL characters. and occur only 121 as part of a end of line delimiter. 123 Binary data: Arbitrary data. 125 Transfer Encoding: A reversible transformation made on data so 8-bit 126 or binary data may be sent via a channel that only transmits 7-bit 127 data. 129 Receiving agent: software that interprets and processes S/MIME CMS 130 objects, MIME body parts that contain CMS content types, or both. 132 Sending agent: software that creates S/MIME CMS content types, MIME 133 body parts that contain CMS content types, or both. 135 S/MIME agent: user software that is a receiving agent, a sending 136 agent, or both. 138 1.4 Compatibility with Prior Practice of S/MIME 140 S/MIME version 3 agents should attempt to have the greatest 141 interoperability possible with S/MIME version 2 agents. S/MIME version 142 2 is described in RFC 2311 through RFC 2315, inclusive. RFC 2311 also 143 has historical information about the development of S/MIME. 145 1.5 Changes Since S/MIME v3.0 147 The RSA public key algorithm was changed to a MUST implement key 148 wrapping algorithm, and the Diffie-Hellman algorithm changed to a 149 SHOULD implement. 151 The AES symmetric encryption algorithm has been included as a SHOULD 152 implement. 154 The RSA public key algorithm was changed to a MUST implement signature 155 algorithm. 157 Ambiguous language about the use of "empty" SignedData messages to 158 transmit certificates was clarified to reflect that transmission of 159 certificate revocation lists is also allowed. 161 The use of binary encoding for some MIME entities is now explicitly 162 discussed. 164 Header protection through the use of the message/rfc822 MIME type has 165 been added. 167 Use of the CompressedData CMS type is allowed, along with required 168 MIME type and file extension additions. 170 1.6 Discussion of This Specification 172 This specification is being discussed on the "ietf-smime" mailing 173 list. To subscribe, send a message to: 175 ietf-smime-request@imc.org 177 with the single word 179 subscribe 181 in the body of the message. There is a Web site for the mailing list 182 at . 184 2. CMS Options 186 CMS allows for a wide variety of options in content and algorithm 187 support. This section puts forth a number of support requirements and 188 recommendations in order to achieve a base level of interoperability 189 among all S/MIME implementations. [CMS] provides additional details 190 regarding the use of the cryptographic algorithms. 192 2.1 DigestAlgorithmIdentifier 194 Sending and receiving agents MUST support SHA-1 [CMSALG]. Receiving 195 agents SHOULD support MD5 [CMSALG] for the purpose of providing 196 backward compatibility with MD5-digested S/MIME v2 SignedData objects. 198 2.2 SignatureAlgorithmIdentifier 200 Receiving agents MUST support id-dsa-with-sha1 defined in [CMSALG]. 201 The algorithm parameters MUST be absent (not encoded as NULL). 202 Receiving agents MUST support rsaEncryption, defined in [CMSALG]. 204 Sending agents MUST support either id-dsa-with-sha1 or rsaEncryption. 206 Note that S/MIME v3 clients might only implement signing or signature 207 verification using id-dsa-with-sha1, and might also use id-dsa as an 208 AlgorithmIdentifier in this field. Receiving clients SHOULD recognize 209 id-dsa as equivalent to id-dsa-with-sha1, and sending clients MUST use 210 id-dsa-with-sha1 if using that algorithm. Also note that S/MIME v2 211 clients are only capable of verifying digital signatures using the 212 rsaEncryption algorithm. 214 2.3 KeyEncryptionAlgorithmIdentifier 216 Sending and receiving agents MUST support rsaEncryption, defined in 217 [CMSALG]. 219 Sending and receiving agents SHOULD support Diffie-Hellman defined in 220 [CMSALG]. 222 Note that S/MIME v3 clients might only implement key encryption and 223 decryption using the Diffie-Hellman algorithm. Also note that S/MIME 224 v2 clients are only capable of decrypting content-encryption keys 225 using the rsaEncryption algorithm. 227 2.4 General Syntax 229 CMS defines multiple content types. Of these, only the Data, 230 SignedData, EnvelopedData and CompressedData content types are 231 currently used for S/MIME. 233 2.4.1 Data Content Type 235 Sending agents MUST use the id-data content type identifier to 236 identify the "inner" MIME message content. For example, when applying 237 a digital signature to MIME data, the CMS signedData encapContentInfo 238 eContentType MUST include the id-data object identifier and the MIME 239 content MUST be stored in the SignedData encapContentInfo eContent 240 OCTET STRING (unless the sending agent is using multipart/signed, in 241 which case the eContent is absent, per section 3.4.3 of this 242 document). As another example, when applying encryption to MIME data, 243 the CMS EnvelopedData encryptedContentInfo ContentType MUST include 244 the id-data object identifier and the encrypted MIME content MUST be 245 stored in the envelopedData encryptedContentInfo encryptedContent 246 OCTET STRING. 248 2.4.2 SignedData Content Type 250 Sending agents MUST use the signedData content type to apply a digital 251 signature to a message or, in a degenerate case where there is no 252 signature information, to convey certificates. 254 2.4.3 EnvelopedData Content Type 256 This content type is used to apply data confidentiality to a message. 257 A sender needs to have access to a public key for each intended 258 message recipient to use this service. This content type does not 259 provide authentication. 261 2.4.4 CompressedData Content Type 263 This content type is used to apply data compression to a message. This 264 content type does not provide authentication or privacy, and is only 265 used to reduce message size. 267 2.5 Attribute SignerInfo Type 269 The SignerInfo type allows the inclusion of unsigned and signed 270 attributes to be included along with a signature. 272 Receiving agents MUST be able to handle zero or one instance of each 273 of the signed attributes listed here. Sending agents SHOULD generate 274 one instance of each of the following signed attributes in each S/MIME 275 message: 277 - signingTime (section 2.5.1 in this document) 278 - sMIMECapabilities (section 2.5.2 in this document) 279 - sMIMEEncryptionKeyPreference (section 2.5.3 in this document) 280 - id-messageDigest (section 11.2 in [CMS]) 281 - id-contentType (section 11.1 in [CMS]) 283 Further, receiving agents SHOULD be able to handle zero or one 284 instance in the signingCertificate signed attribute, as defined in 285 section 5 of [ESS]. 287 Sending agents SHOULD generate one instance of the signingCertificate 288 signed attribute in each S/MIME message. 290 Additional attributes and values for these attributes may be defined 291 in the future. Receiving agents SHOULD handle attributes or values 292 that it does not recognize in a graceful manner. 294 Interactive sending agents that include signed attributes that are not 295 listed here SHOULD display those attributes to the user, so that the 296 user is aware of all of the data being signed. 298 2.5.1 Signing-Time Attribute 300 The signing-time attribute is used to convey the time that a message 301 was signed. The time of signing will most likely be created by a 302 message originator and therefore is only as trustworthy as the 303 originator. 305 Sending agents MUST encode signing time through the year 2049 as 306 UTCTime; signing times in 2050 or later MUST be encoded as 307 GeneralizedTime. When the UTCTime CHOICE is used, S/MIME agents MUST 308 interpret the year field (YY) as follows: 310 if YY is greater than or equal to 50, the year is interpreted as 19YY; 311 if YY is less than 50, the year is interpreted as 20YY. 313 2.5.2 SMIMECapabilities Attribute 315 The SMIMECapabilities attribute includes signature algorithms (such as 316 "sha1WithRSAEncryption"), symmetric algorithms (such as "DES-EDE3- 317 CBC"), and key encipherment algorithms (such as "rsaEncryption"). 318 There are also several identifiers which indicate support for other 319 optional features such as binary encoding and compression. The 320 SMIMECapabilities were designed to be flexible and extensible so that, 321 in the future, a means of identifying other capabilities and 322 preferences such as certificates can be added in a way that will not 323 cause current clients to break. 325 If present, the SMIMECapabilities attribute MUST be a SignedAttribute; 326 it MUST NOT be an UnsignedAttribute. CMS defines SignedAttributes as a 327 SET OF Attribute. The SignedAttributes in a signerInfo MUST NOT 328 include multiple instances of the SMIMECapabilities attribute. CMS 329 defines the ASN.1 syntax for Attribute to include attrValues SET OF 330 AttributeValue. A SMIMECapabilities attribute MUST only include a 331 single instance of AttributeValue. There MUST NOT be zero or multiple 332 instances of AttributeValue present in the attrValues SET OF 333 AttributeValue. 335 The semantics of the SMIMECapabilites attribute specify a partial list 336 as to what the client announcing the SMIMECapabilites can support. A 337 client does not have to list every capability it supports, and 338 probably should not list all its capabilities so that the capabilities 339 list doesn't get too long. In an SMIMECapabilities attribute, the 340 object identifiers (OIDs) are listed in order of their preference, but 341 SHOULD be logically separated along the lines of their categories 342 (signature algorithms, symmetric algorithms, key encipherment 343 algorithms, etc.) 345 The structure of the SMIMECapabilities attribute is to facilitate 346 simple table lookups and binary comparisons in order to determine 347 matches. For instance, the DER-encoding for the SMIMECapability for 348 DES EDE3 CBC MUST be identically encoded regardless of the 349 implementation. 351 In the case of symmetric algorithms, the associated parameters for the 352 OID MUST specify all of the parameters necessary to differentiate 353 between two instances of the same algorithm. For instance, the number 354 of rounds and block size for RC5 must be specified in addition to the 355 key length. 357 The OIDs used with S/MIME are assigned in several different RFCs. Note 358 that all OIDs associated with the MUST and SHOULD implement algorithms 359 are included in section A of this document. 361 The OIDs that correspond to algorithms SHOULD use the same OID as the 362 actual algorithm, except in the case where the algorithm usage is 363 ambiguous from the OID. For instance, in an earlier specification, 364 rsaEncryption was ambiguous because it could refer to either a 365 signature algorithm or a key encipherment algorithm. In the event that 366 an OID is ambiguous, it needs to be arbitrated by the maintainer of 367 the registered SMIMECapabilities list as to which type of algorithm 368 will use the OID, and a new OID MUST be allocated under the 369 smimeCapabilities OID to satisfy the other use of the OID. 371 The registered SMIMECapabilities list specifies the parameters for 372 OIDs that need them, most notably key lengths in the case of variable- 373 length symmetric ciphers. In the event that there are no 374 differentiating parameters for a particular OID, the parameters MUST 375 be omitted, and MUST NOT be encoded as NULL. 377 Additional values for the SMIMECapabilities attribute may be defined 378 in the future. Receiving agents MUST handle a SMIMECapabilities object 379 that has values that it does not recognize in a graceful manner. 381 2.5.3 Encryption Key Preference Attribute 383 The encryption key preference attribute allows the signer to 384 unambiguously describe which of the signer's certificates has the 385 signer's preferred encryption key. This attribute is designed to 386 enhance behavior for interoperating with those clients which use 387 separate keys for encryption and signing. This attribute is used to 388 convey to anyone viewing the attribute which of the listed 389 certificates should be used for encrypting a session key for future 390 encrypted messages. 392 If present, the SMIMEEncryptionKeyPreference attribute MUST be a 393 SignedAttribute; it MUST NOT be an UnsignedAttribute. CMS defines 394 SignedAttributes as a SET OF Attribute. The SignedAttributes in a 395 signerInfo MUST NOT include multiple instances of the 396 SMIMEEncryptionKeyPreference attribute. CMS defines the ASN.1 syntax 397 for Attribute to include attrValues SET OF AttributeValue. A 398 SMIMEEncryptionKeyPreference attribute MUST only include a single 399 instance of AttributeValue. There MUST NOT be zero or multiple 400 instances of AttributeValue present in the attrValues SET OF 401 AttributeValue. 403 The sending agent SHOULD include the referenced certificate in the set 404 of certificates included in the signed message if this attribute is 405 used. The certificate may be omitted if it has been previously made 406 available to the receiving agent. Sending agents SHOULD use this 407 attribute if the commonly used or preferred encryption certificate is 408 not the same as the certificate used to sign the message. 410 Receiving agents SHOULD store the preference data if the signature on 411 the message is valid and the signing time is greater than the 412 currently stored value. (As with the SMIMECapabilities, the clock skew 413 should be checked and the data not used if the skew is too great.) 414 Receiving agents SHOULD respect the sender's encryption key preference 415 attribute if possible. This however represents only a preference and 416 the receiving agent may use any certificate in replying to the sender 417 that is valid. 419 2.5.3.1 Selection of Recipient Key Management Certificate 421 In order to determine the key management certificate to be used when 422 sending a future CMS envelopedData message for a particular recipient, 423 the following steps SHOULD be followed: 425 - If an SMIMEEncryptionKeyPreference attribute is found in a 426 signedData object received from the desired recipient, this 427 identifies the X.509 certificate that should be used as the X.509 428 key management certificate for the recipient. 430 - If an SMIMEEncryptionKeyPreference attribute is not found in a 431 signedData object received from the desired recipient, the set of 432 X.509 certificates should be searched for a X.509 certificate with 433 the same subject name as the signing X.509 certificate which can be 434 used for key management. 436 - Or use some other method of determining the user's key management 437 key. If a X.509 key management certificate is not found, then 438 encryption cannot be done with the signer of the message. If 439 multiple X.509 key management certificates are found, the S/MIME 440 agent can make an arbitrary choice between them. 442 2.6 SignerIdentifier SignerInfo Type 444 S/MIME v3.1 and S/MIME v3 requires the use of SignerInfo version 1, 445 that is the issuerAndSerialNumber CHOICE MUST be used for 446 SignerIdentifier. 448 2.7 ContentEncryptionAlgorithmIdentifier 450 Sending and receiving agents MUST support encryption and decryption 451 with DES EDE3 CBC, hereinafter called "tripleDES" [CMSALG]. Receiving 452 agents SHOULD support encryption and decryption using the RC2 [CMSALG] 453 or a compatible algorithm at a key size of 40 bits, hereinafter called 454 "RC2/40". Sending and receiving agents SHOULD support encryption and 455 decryption with AES [CMSAES] at a key size of 128, 192 and 256 bits. 457 2.7.1 Deciding Which Encryption Method To Use 459 When a sending agent creates an encrypted message, it has to decide 460 which type of encryption to use. The decision process involves using 461 information garnered from the capabilities lists included in messages 462 received from the recipient, as well as out-of-band information such 463 as private agreements, user preferences, legal restrictions, and so 464 on. 466 Section 2.5 defines a method by which a sending agent can optionally 467 announce, among other things, its decrypting capabilities in its order 468 of preference. The following method for processing and remembering the 469 encryption capabilities attribute in incoming signed messages SHOULD 470 be used. 472 - If the receiving agent has not yet created a list of capabilities 473 for the sender's public key, then, after verifying the signature on 474 the incoming message and checking the timestamp, the receiving agent 475 SHOULD create a new list containing at least the signing time and 476 the symmetric capabilities. 478 - If such a list already exists, the receiving agent SHOULD verify 479 that the signing time in the incoming message is greater than the 480 signing time stored in the list and that the signature is valid. If 481 so, the receiving agent SHOULD update both the signing time and 482 capabilities in the list. Values of the signing time that lie far in 483 the future (that is, a greater discrepancy than any reasonable clock 484 skew), or a capabilities list in messages whose signature could not 485 be verified, MUST NOT be accepted. 487 The list of capabilities SHOULD be stored for future use in creating 488 messages. 490 Before sending a message, the sending agent MUST decide whether it is 491 willing to use weak encryption for the particular data in the message. 492 If the sending agent decides that weak encryption is unacceptable for 493 this data, then the sending agent MUST NOT use a weak algorithm such 494 as RC2/40. The decision to use or not use weak encryption overrides 495 any other decision in this section about which encryption algorithm to 496 use. 498 Sections 2.7.2.1 through 2.7.2.4 describe the decisions a sending 499 agent SHOULD use in deciding which type of encryption should be 500 applied to a message. These rules are ordered, so the sending agent 501 SHOULD make its decision in the order given. 503 2.7.1.1 Rule 1: Known Capabilities 505 If the sending agent has received a set of capabilities from the 506 recipient for the message the agent is about to encrypt, then the 507 sending agent SHOULD use that information by selecting the first 508 capability in the list (that is, the capability most preferred by the 509 intended recipient) for which the sending agent knows how to encrypt. 510 The sending agent SHOULD use one of the capabilities in the list if 511 the agent reasonably expects the recipient to be able to decrypt the 512 message. 514 2.7.1.2 Rule 2: Unknown Capabilities, Known Use of Encryption 516 If the following three conditions are met: 517 - the sending agent has no knowledge of the encryption capabilities 518 of the recipient, 519 - and the sending agent has received at least one message from the 520 recipient, 521 - and the last encrypted message received from the recipient had a 522 trusted signature on it, 523 then the outgoing message SHOULD use the same encryption algorithm as 524 was used on the last signed and encrypted message received from the 525 recipient. 527 2.7.1.3 Rule 3: Unknown Capabilities, Unknown Version of S/MIME 529 If the following two conditions are met: 530 - the sending agent has no knowledge of the encryption capabilities 531 of the recipient, 532 - and the sending agent has no knowledge of the version of S/MIME 533 of the recipient, 534 then the sending agent SHOULD use tripleDES because it is a stronger 535 algorithm and is required by S/MIME v3. If the sending agent chooses 536 not to use tripleDES in this step, it SHOULD use RC2/40. 538 2.7.2 Choosing Weak Encryption 540 Like all algorithms that use 40 bit keys, RC2/40 is considered by many 541 to be weak encryption. A sending agent that is controlled by a human 542 SHOULD allow a human sender to determine the risks of sending data 543 using RC2/40 or a similarly weak encryption algorithm before sending 544 the data, and possibly allow the human to use a stronger encryption 545 method such as tripleDES. 547 2.7.3 Multiple Recipients 549 If a sending agent is composing an encrypted message to a group of 550 recipients where the encryption capabilities of some of the recipients 551 do not overlap, the sending agent is forced to send more than one 552 message. It should be noted that if the sending agent chooses to send 553 a message encrypted with a strong algorithm, and then send the same 554 message encrypted with a weak algorithm, someone watching the 555 communications channel may be able to learn the contents of the 556 strongly-encrypted message simply by decrypting the weakly-encrypted 557 message. 559 3. Creating S/MIME Messages 561 This section describes the S/MIME message formats and how they are 562 created. S/MIME messages are a combination of MIME bodies and CMS 563 content types. Several MIME types as well as several CMS content types 564 are used. The data to be secured is always a canonical MIME entity. 565 The MIME entity and other data, such as certificates and algorithm 566 identifiers, are given to CMS processing facilities which produces a 567 CMS object. The CMS object is then finally wrapped in MIME. The 568 Enhanced Security Services for S/MIME [ESS] document provides examples 569 of how nested, secured S/MIME messages are formatted. ESS provides an 570 example of how a triple-wrapped S/MIME message is formatted using 571 multipart/signed and application/pkcs7-mime for the signatures. 573 S/MIME provides one format for enveloped-only data, several formats 574 for signed-only data, and several formats for signed and enveloped 575 data. Several formats are required to accommodate several 576 environments, in particular for signed messages. The criteria for 577 choosing among these formats are also described. 579 The reader of this section is expected to understand MIME as described 580 in [MIME-SPEC] and [MIME-SECURE]. 582 3.1 Preparing the MIME Entity for Signing or Enveloping 584 S/MIME is used to secure MIME entities. A MIME entity may be a sub- 585 part, sub-parts of a message, or the whole message with all its sub- 586 parts. A MIME entity that is the whole message includes only the MIME 587 headers and MIME body, and does not include the RFC-822 headers. Note 588 that S/MIME can also be used to secure MIME entities used in 589 applications other than Internet mail. If protection of the RFC-822 590 headers is required, the use of the message/rfc822 MIME type is 591 explained later in this section. 593 The MIME entity that is secured and described in this section can be 594 thought of as the "inside" MIME entity. That is, it is the "innermost" 595 object in what is possibly a larger MIME message. Processing "outside" 596 MIME entities into CMS content types is described in Section 3.2, 3.4 597 and elsewhere. 599 The procedure for preparing a MIME entity is given in [MIME-SPEC]. The 600 same procedure is used here with some additional restrictions when 601 signing. Description of the procedures from [MIME-SPEC] are repeated 602 here, but the reader should refer to that document for the exact 603 procedure. This section also describes additional requirements. 605 A single procedure is used for creating MIME entities that are to be 606 signed, enveloped, or both signed and enveloped. Some additional steps 607 are recommended to defend against known corruptions that can occur 608 during mail transport that are of particular importance for clear- 609 signing using the multipart/signed format. It is recommended that 610 these additional steps be performed on enveloped messages, or signed 611 and enveloped messages in order that the message can be forwarded to 612 any environment without modification. 614 These steps are descriptive rather than prescriptive. The implementor 615 is free to use any procedure as long as the result is the same. 617 Step 1. The MIME entity is prepared according to the local conventions 619 Step 2. The leaf parts of the MIME entity are converted to canonical 620 form 622 Step 3. Appropriate transfer encoding is applied to the leaves of the 623 MIME entity 625 When an S/MIME message is received, the security services on the 626 message are processed, and the result is the MIME entity. That MIME 627 entity is typically passed to a MIME-capable user agent where, it is 628 further decoded and presented to the user or receiving application. 630 In order to protect outer, non-content related message headers (for 631 instance, the "Subject", "To", "From" and "CC" fields), the sending 632 client MAY wrap a full MIME message in a message/rfc822 wrapper in 633 order to apply S/MIME security services to these headers. It is up to 634 the receiving client to decide how to present these "inner" headers 635 along with the unprotected "outer" headers. 637 3.1.1 Canonicalization 639 Each MIME entity MUST be converted to a canonical form that is 640 uniquely and unambiguously representable in the environment where the 641 signature is created and the environment where the signature will be 642 verified. MIME entities MUST be canonicalized for enveloping as well 643 as signing. 645 The exact details of canonicalization depend on the actual MIME type 646 and subtype of an entity, and are not described here. Instead, the 647 standard for the particular MIME type should be consulted. For 648 example, canonicalization of type text/plain is different from 649 canonicalization of audio/basic. Other than text types, most types 650 have only one representation regardless of computing platform or 651 environment which can be considered their canonical representation. In 652 general, canonicalization will be performed by the non-security part 653 of the sending agent rather than the S/MIME implementation. 655 The most common and important canonicalization is for text, which is 656 often represented differently in different environments. MIME entities 657 of major type "text" must have both their line endings and character 658 set canonicalized. The line ending must be the pair of characters 659 , and the charset should be a registered charset [CHARSETS]. 660 The details of the canonicalization are specified in [MIME-SPEC]. The 661 chosen charset SHOULD be named in the charset parameter so that the 662 receiving agent can unambiguously determine the charset used. 664 Note that some charsets such as ISO-2022 have multiple representations 665 for the same characters. When preparing such text for signing, the 666 canonical representation specified for the charset MUST be used. 668 3.1.2 Transfer Encoding 670 When generating any of the secured MIME entities below, except the 671 signing using the multipart/signed format, no transfer encoding at all 672 is required. S/MIME implementations MUST be able to deal with binary 673 MIME objects. If no Content-Transfer-Encoding header is present, the 674 transfer encoding should be considered 7BIT. 676 S/MIME implementations SHOULD however use transfer encoding described 677 in section 3.1.3 for all MIME entities they secure. The reason for 678 securing only 7-bit MIME entities, even for enveloped data that are 679 not exposed to the transport, is that it allows the MIME entity to be 680 handled in any environment without changing it. For example, a trusted 681 gateway might remove the envelope, but not the signature, of a 682 message, and then forward the signed message on to the end recipient 683 so that they can verify the signatures directly. If the transport 684 internal to the site is not 8-bit clean, such as on a wide-area 685 network with a single mail gateway, verifying the signature will not 686 be possible unless the original MIME entity was only 7-bit data. 688 S/MIME implementations which "know" that all intended recipient(s) are 689 capable of handling inner (all but the outermost) binary MIME objects 690 SHOULD NOT use 7-bit transfer encoding for the inner entities since 691 this would unnecessarily expand the message size. Implementations MAY 692 "know" that recipient implementations are capable of handling inner 693 binary MIME entities either by interpreting the 694 id-cap-preferBinaryInside sMIMECapabilities attribute, by prior 695 agreement, or by other means. 697 If one or more intended recipients are unable to handle inner binary 698 MIME objects, or if this capability in unknown for any of the intended 699 recipients, S/MIME implementations SHOULD use transfer encoding 700 described in section 3.1.3 for all MIME entities they secure. 702 3.1.3 Transfer Encoding for Signing Using multipart/signed 704 If a multipart/signed entity is ever to be transmitted over the 705 standard Internet SMTP infrastructure or other transport that is 706 constrained to 7-bit text, it MUST have transfer encoding applied so 707 that it is represented as 7-bit text. MIME entities that are 7-bit 708 data already need no transfer encoding. Entities such as 8-bit text 709 and binary data can be encoded with quoted-printable or base-64 710 transfer encoding. 712 The primary reason for the 7-bit requirement is that the Internet mail 713 transport infrastructure cannot guarantee transport of 8-bit or binary 714 data. Even though many segments of the transport infrastructure now 715 handle 8-bit and even binary data, it is sometimes not possible to 716 know whether the transport path is 8-bit clear. If a mail message with 717 8-bit data were to encounter a message transfer agent that can not 718 transmit 8-bit or binary data, the agent has three options, none of 719 which are acceptable for a clear-signed message: 721 - The agent could change the transfer encoding; this would invalidate 722 the signature. 723 - The agent could transmit the data anyway, which would most likely 724 result in the 8th bit being corrupted; this too would invalidate the 725 signature. 726 - The agent could return the message to the sender. 728 [MIME-SECURE] prohibits an agent from changing the transfer encoding 729 of the first part of a multipart/signed message. If a compliant agent 730 that can not transmit 8-bit or binary data encounters a 731 multipart/signed message with 8-bit or binary data in the first part, 732 it would have to return the message to the sender as undeliverable. 734 3.1.4 Sample Canonical MIME Entity 736 This example shows a multipart/mixed message with full transfer 737 encoding. This message contains a text part and an attachment. The 738 sample message text includes characters that are not US-ASCII and thus 739 must be transfer encoded. Though not shown here, the end of each line 740 is . The line ending of the MIME headers, the text, and 741 transfer encoded parts, all must be . 743 Note that this example is not of an S/MIME message. 745 Content-Type: multipart/mixed; boundary=bar 747 --bar 748 Content-Type: text/plain; charset=iso-8859-1 749 Content-Transfer-Encoding: quoted-printable 751 =A1Hola Michael! 753 How do you like the new S/MIME specification? 755 It's generally a good idea to encode lines that begin with 756 From=20because some mail transport agents will insert a greater- 757 than (>) sign, thus invalidating the signature. 759 Also, in some cases it might be desirable to encode any =20 760 trailing whitespace that occurs on lines in order to ensure =20 761 that the message signature is not invalidated when passing =20 762 a gateway that modifies such whitespace (like BITNET). =20 764 --bar 765 Content-Type: image/jpeg 766 Content-Transfer-Encoding: base64 768 iQCVAwUBMJrRF2N9oWBghPDJAQE9UQQAtl7LuRVndBjrk4EqYBIb3h5QXIX/LC// 769 jJV5bNvkZIGPIcEmI5iFd9boEgvpirHtIREEqLQRkYNoBActFBZmh9GC3C041WGq 770 uMbrbxc+nIs1TIKlA08rVi9ig/2Yh7LFrK5Ein57U/W72vgSxLhe/zhdfolT9Brn 771 HOxEa44b+EI= 773 --bar-- 775 3.2 The application/pkcs7-mime Type 777 The application/pkcs7-mime type is used to carry CMS content types 778 including envelopedData, signedData and compressedData. The details of 779 constructing these entities is described in subsequent sections. This 780 section describes the general characteristics of the 781 application/pkcs7-mime type. 783 The carried CMS object always contains a MIME entity that is prepared 784 as described in section 3.1 if the eContentType is id-data. Other 785 contents may be carried when the eContentType contains different 786 values. See [ESS] for an example of this with signed receipts. 788 Since CMS content types are binary data, in most cases base-64 789 transfer encoding is appropriate, in particular when used with SMTP 790 transport. The transfer encoding used depends on the transport through 791 which the object is to be sent, and is not a characteristic of the 792 MIME type. 794 Note that this discussion refers to the transfer encoding of the CMS 795 object or "outside" MIME entity. It is completely distinct from, and 796 unrelated to, the transfer encoding of the MIME entity secured by the 797 CMS object, the "inside" object, which is described in section 3.1. 799 Because there are several types of application/pkcs7-mime objects, a 800 sending agent SHOULD do as much as possible to help a receiving agent 801 know about the contents of the object without forcing the receiving 802 agent to decode the ASN.1 for the object. The MIME headers of all 803 application/pkcs7-mime objects SHOULD include the optional "smime- 804 type" parameter, as described in the following sections. 806 3.2.1 The name and filename Parameters 808 For the application/pkcs7-mime, sending agents SHOULD emit the 809 optional "name" parameter to the Content-Type field for compatibility 810 with older systems. Sending agents SHOULD also emit the optional 811 Content-Disposition field [CONTDISP] with the "filename" parameter. If 812 a sending agent emits the above parameters, the value of the 813 parameters SHOULD be a file name with the appropriate extension: 815 MIME Type File Extension 817 Application/pkcs7-mime (signedData, envelopedData) .p7m 819 Application/pkcs7-mime (degenerate signedData .p7c 820 certificate management message) 822 Application/pkcs7-mime (compressedData) .p7z 824 Application/pkcs7-signature .p7s 826 In addition, the file name SHOULD be limited to eight characters 827 followed by a three letter extension. The eight character filename 828 base can be any distinct name; the use of the filename base "smime" 829 SHOULD be used to indicate that the MIME entity is associated with 830 S/MIME. 832 Including a file name serves two purposes. It facilitates easier use 833 of S/MIME objects as files on disk. It also can convey type 834 information across gateways. When a MIME entity of type 835 application/pkcs7-mime (for example) arrives at a gateway that has no 836 special knowledge of S/MIME, it will default the entity's MIME type to 837 application/octet-stream and treat it as a generic attachment, thus 838 losing the type information. However, the suggested filename for an 839 attachment is often carried across a gateway. This often allows the 840 receiving systems to determine the appropriate application to hand the 841 attachment off to, in this case a stand-alone S/MIME processing 842 application. Note that this mechanism is provided as a convenience for 843 implementations in certain environments. A proper S/MIME 844 implementation MUST use the MIME types and MUST NOT rely on the file 845 extensions. 847 3.2.2 The smime-type parameter 849 The application/pkcs7-mime content type defines the optional "smime- 850 type" parameter. The intent of this parameter is to convey details 851 about the security applied (signed or enveloped) along with infomation 852 about the contained content. This specification defines the following 853 smime-types. 855 Name CMS type Inner Content 857 enveloped-data EnvelopedData id-data 859 signed-data SignedData id-data 861 certs-only SignedData none 863 compressed-data CompressedData id-data 865 In order that consistency can be obtained with future, the following 866 guidelines should be followed when assigning a new smime-type 867 parameter. 869 1. If both signing and encryption can be applied to the content, then 870 two values for smime-type SHOULD be assigned "signed-*" and 871 "encrypted-*". If one operation can be assigned then this may be 872 omitted. Thus since "certs-only" can only be signed, "signed-" is 873 omitted. 875 2. A common string for a content oid should be assigned. We use "data" 876 for the id-data content OID when MIME is the inner content. 878 3. If no common string is assigned. Then the common string of 879 "OID." is recommended (for example, "OID.1.3.6.1.5.5.7.6.1" would 880 be DES40). 882 3.3 Creating an Enveloped-only Message 884 This section describes the format for enveloping a MIME entity without 885 signing it. It is important to note that sending enveloped but not 886 signed messages does not provide for data integrity. It is possible to 887 replace ciphertext in such a way that the processed message will still 888 be valid, but the meaning may be altered. 890 Step 1. The MIME entity to be enveloped is prepared according to 891 section 3.1. 893 Step 2. The MIME entity and other required data is processed into a 894 CMS object of type envelopedData. In addition to encrypting a copy of 895 the content-encryption key for each recipient, a copy of the content- 896 encryption key SHOULD be encrypted for the originator and included in 897 the envelopedData (see CMS Section 6). 899 Step 3. The envelopedData object is wrapped in a CMS ContentInfo 900 object. 902 Step 4. The ContentInfo object is inserted into an 903 application/pkcs7-mime MIME entity. 905 The smime-type parameter for enveloped-only messages is "enveloped- 906 data". The file extension for this type of message is ".p7m". 908 A sample message would be: 910 Content-Type: application/pkcs7-mime; smime-type=enveloped-data; 911 name=smime.p7m 912 Content-Transfer-Encoding: base64 913 Content-Disposition: attachment; filename=smime.p7m 915 rfvbnj756tbBghyHhHUujhJhjH77n8HHGT9HG4VQpfyF467GhIGfHfYT6 916 7n8HHGghyHhHUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H 917 f8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4 918 0GhIGfHfQbnj756YT64V 920 3.4 Creating a Signed-only Message 922 There are two formats for signed messages defined for S/MIME: 923 application/pkcs7-mime with SignedData, and multipart/signed. In 924 general, the multipart/signed form is preferred for sending, and 925 receiving agents SHOULD be able to handle both. 927 3.4.1 Choosing a Format for Signed-only Messages 929 There are no hard-and-fast rules when a particular signed-only format 930 should be chosen because it depends on the capabilities of all the 931 receivers and the relative importance of receivers with S/MIME 932 facilities being able to verify the signature versus the importance of 933 receivers without S/MIME software being able to view the message. 935 Messages signed using the multipart/signed format can always be viewed 936 by the receiver whether they have S/MIME software or not. They can 937 also be viewed whether they are using a MIME-native user agent or they 938 have messages translated by a gateway. In this context, "be viewed" 939 means the ability to process the message essentially as if it were not 940 a signed message, including any other MIME structure the message might 941 have. 943 Messages signed using the signedData format cannot be viewed by a 944 recipient unless they have S/MIME facilities. However, if they have 945 S/MIME facilities, these messages can always be verified if they were 946 not changed in transit. 948 3.4.2 Signing Using application/pkcs7-mime with SignedData 950 This signing format uses the application/pkcs7-mime MIME type. The 951 steps to create this format are: 953 Step 1. The MIME entity is prepared according to section 3.1 955 Step 2. The MIME entity and other required data is processed into a 956 CMS object of type signedData 958 Step 3. The signedData object is wrapped in a CMS ContentInfo 959 object. 961 Step 4. The ContentInfo object is inserted into an 962 application/pkcs7-mime MIME entity. 964 The smime-type parameter for messages using application/pkcs7-mime 965 with SignedData is "signed-data". The file extension for this type of 966 message is ".p7m". 968 A sample message would be: 970 Content-Type: application/pkcs7-mime; smime-type=signed-data; 971 name=smime.p7m 972 Content-Transfer-Encoding: base64 973 Content-Disposition: attachment; filename=smime.p7m 975 567GhIGfHfYT6ghyHhHUujpfyF4f8HHGTrfvhJhjH776tbB9HG4VQbnj7 976 77n8HHGT9HG4VQpfyF467GhIGfHfYT6rfvbnj756tbBghyHhHUujhJhjH 977 HUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H7n8HHGghyHh 978 6YT64V0GhIGfHfQbnj75 980 3.4.3 Signing Using the multipart/signed Format 982 This format is a clear-signing format. Recipients without any S/MIME 983 or CMS processing facilities are able to view the message. It makes 984 use of the multipart/signed MIME type described in [MIME-SECURE]. The 985 multipart/signed MIME type has two parts. The first part contains the 986 MIME entity that is signed; the second part contains the "detached 987 signature" CMS SignedData object in which the encapContentInfo 988 eContent field is absent. 990 3.4.3.1 The application/pkcs7-signature MIME Type 992 This MIME type always contains a CMS ContentInfo containing a single 993 CMS object of type signedData. The signedData encapContentInfo 994 eContent field MUST be absent. The signerInfos field contains the 995 signatures for the MIME entity. 997 The file extension for signed-only messages using application/pkcs7- 998 signature is ".p7s". 1000 3.4.3.2 Creating a multipart/signed Message 1002 Step 1. The MIME entity to be signed is prepared according to section 1003 3.1, taking special care for clear-signing. 1005 Step 2. The MIME entity is presented to CMS processing in order to 1006 obtain an object of type signedData in which the encapContentInfo 1007 eContent field is absent. 1009 Step 3. The MIME entity is inserted into the first part of a 1010 multipart/signed message with no processing other than that described 1011 in section 3.1. 1013 Step 4. Transfer encoding is applied to the "detached signature" CMS 1014 SignedData object and it is inserted into a MIME entity of type 1015 application/pkcs7-signature. 1017 Step 5. The MIME entity of the application/pkcs7-signature is inserted 1018 into the second part of the multipart/signed entity. 1020 The multipart/signed Content type has two required parameters: the 1021 protocol parameter and the micalg parameter. 1023 The protocol parameter MUST be "application/pkcs7-signature". Note 1024 that quotation marks are required around the protocol parameter 1025 because MIME requires that the "/" character in the parameter value 1026 MUST be quoted. 1028 The micalg parameter allows for one-pass processing when the signature 1029 is being verified. The value of the micalg parameter is dependent on 1030 the message digest algorithm(s) used in the calculation of the Message 1031 Integrity Check. If multiple message digest algorithms are used they 1032 MUST be separated by commas per [MIME-SECURE]. The values to be placed 1033 in the micalg parameter SHOULD be from the following: 1035 Algorithm Value 1036 used 1038 MD5 md5 1039 SHA-1 sha1 1040 SHA-256 sha256 1041 SHA-384 sha384 1042 SHA-512 sha512 1043 Any other unknown 1045 (Historical note: some early implementations of S/MIME emitted and 1046 expected "rsa-md5" and "rsa-sha1" for the micalg parameter.) Receiving 1047 agents SHOULD be able to recover gracefully from a micalg parameter 1048 value that they do not recognize. 1050 The SHA-256, SHA-384 and SHA-512 algorithms [FIPS180-2] are not 1051 currently supported in S/MIME, and are included here for completeness. 1053 3.4.3.3 Sample multipart/signed Message 1055 Content-Type: multipart/signed; 1056 protocol="application/pkcs7-signature"; 1057 micalg=sha1; boundary=boundary42 1059 --boundary42 1060 Content-Type: text/plain 1062 This is a clear-signed message. 1064 --boundary42 1065 Content-Type: application/pkcs7-signature; name=smime.p7s 1066 Content-Transfer-Encoding: base64 1067 Content-Disposition: attachment; filename=smime.p7s 1069 ghyHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6 1070 4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj 1071 n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4 1072 7GhIGfHfYT64VQbnj756 1074 --boundary42-- 1076 3.5 Creating an Compressed-only Message 1078 This section describes the format for compressing a MIME entity. 1079 Please note that versions of S/MIME prior to 3.1 did not specify any 1080 use of compressedData, and will not recognize it. The use of a 1081 capability to indicate the ability to receive compressedData is 1082 described in [CMSCOMPR] and is the preferred method for compatibility. 1084 Step 1. The MIME entity to be enveloped is prepared according to 1085 section 3.1. 1087 Step 2. The MIME entity and other required data is processed into a 1088 CMS object of type compressedData. 1090 Step 3. The compressedData object is wrapped in a CMS ContentInfo 1091 object. 1093 Step 4. The ContentInfo object is inserted into an 1094 application/pkcs7-mime MIME entity. 1096 The smime-type parameter for compressed-only messages is "compressed- 1097 data". The file extension for this type of message is ".p7z". 1099 A sample message would be: 1101 Content-Type: application/pkcs7-mime; smime-type=compressed-data; 1102 name=smime.p7z 1103 Content-Transfer-Encoding: base64 1104 Content-Disposition: attachment; filename=smime.p7z 1106 rfvbnj756tbBghyHhHUujhJhjH77n8HHGT9HG4VQpfyF467GhIGfHfYT6 1107 7n8HHGghyHhHUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H 1108 f8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4 1109 0GhIGfHfQbnj756YT64V 1111 3.6 Multiple Operations 1113 The signed-only, encrypted-only, and compressed-only MIME formats can 1114 be nested. This works because these formats are all MIME entities that 1115 encapsulate other MIME entities. 1117 An S/MIME implementation MUST be able to receive and process 1118 arbitrarily nested S/MIME within reasonable resource limits of the 1119 recipient computer. 1121 It is possible to apply any of the signing, encrypting and compressing 1122 operations in any order. It is up to the implementor and the user to 1123 choose. When signing first, the signatories are then securely obscured 1124 by the enveloping. When enveloping first the signatories are exposed, 1125 but it is possible to verify signatures without removing the 1126 enveloping. This may be useful in an environment were automatic 1127 signature verification is desired, as no private key material is 1128 required to verify a signature. 1130 There are security ramifications to choosing whether to sign first or 1131 encrypt first. A recipient of a message that is encrypted and then 1132 signed can validate that the encrypted block was unaltered, but cannot 1133 determine any relationship between the signer and the unencrypted 1134 contents of the message. A recipient of a message that is signed-then- 1135 encrypted can assume that the signed message itself has not been 1136 altered, but that a careful attacker may have changed the 1137 unauthenticated portions of the encrypted message. 1139 When using compression, keep the following guidelines in mind: 1141 - Compression of binary encoded encrypted data is discouraged, since 1142 it will not yield significant compression. Base64 encrypted data 1143 could very well benefit, however. 1144 - If a lossy compression algorithm is used with signing, you will need 1145 to compress first, then sign. 1147 3.7 Creating a Certificate Management Message 1149 The certificate management message or MIME entity is used to transport 1150 certificates and/or certificate revocation lists, such as in response 1151 to a registration request. 1153 Step 1. The certificates and/or certificate revocation lists are made 1154 available to the CMS generating process which creates a CMS object of 1155 type signedData. The signedData encapContentInfo eContent field MUST 1156 be absent and signerInfos field MUST be empty. 1158 Step 2. The signedData object is wrapped in a CMS ContentInfo 1159 object. 1161 Step 3. The ContentInfo object is enclosed in an application/pkcs7- 1162 mime MIME entity 1164 The smime-type parameter for a certificate management message is 1165 "certs-only". The file extension for this type of message is ".p7c". 1167 3.8 Registration Requests 1169 A sending agent that signs messages MUST have a certificate for the 1170 signature so that a receiving agent can verify the signature. There 1171 are many ways of getting certificates, such as through an exchange 1172 with a certificate authority, through a hardware token or diskette, 1173 and so on. 1175 S/MIME v2 [SMIMEV2] specified a method for "registering" public keys 1176 with certificate authorities using an application/pkcs10 body part. 1177 Since that time, the IETF PKIX Working Group has developed other 1178 methods for requesting certificates. However, S/MIME v3.1 does not 1179 require a particular certificate request mechanism. 1181 3.9 Identifying an S/MIME Message 1183 Because S/MIME takes into account interoperation in non-MIME 1184 environments, several different mechanisms are employed to carry the 1185 type information, and it becomes a bit difficult to identify S/MIME 1186 messages. The following table lists criteria for determining whether 1187 or not a message is an S/MIME message. A message is considered an 1188 S/MIME message if it matches any of the criteria listed below. 1190 The file suffix in the table below comes from the "name" parameter in 1191 the content-type header, or the "filename" parameter on the content- 1192 disposition header. These parameters that give the file suffix are not 1193 listed below as part of the parameter section. 1195 MIME type: application/pkcs7-mime 1196 parameters: any 1197 file suffix: any 1199 MIME type: multipart/signed 1200 parameters: protocol="application/pkcs7-signature" 1201 file suffix: any 1203 MIME type: application/octet-stream 1204 parameters: any 1205 file suffix: p7m, p7s, p7c, p7z 1207 4. Certificate Processing 1209 A receiving agent MUST provide some certificate retrieval mechanism in 1210 order to gain access to certificates for recipients of digital 1211 envelopes. This specification does not cover how S/MIME agents handle 1212 certificates, only what they do after a certificate has been validated 1213 or rejected. S/MIME certification issues are covered in [CERT3]. 1215 At a minimum, for initial S/MIME deployment, a user agent could 1216 automatically generate a message to an intended recipient requesting 1217 that recipient's certificate in a signed return message. Receiving and 1218 sending agents SHOULD also provide a mechanism to allow a user to 1219 "store and protect" certificates for correspondents in such a way so 1220 as to guarantee their later retrieval. 1222 4.1 Key Pair Generation 1224 If an S/MIME agent needs to generate a key pair, then the S/MIME agent 1225 or some related administrative utility or function MUST be capable of 1226 generating separate DH and DSS public/private key pairs on behalf of 1227 the user. Each key pair MUST be generated from a good source of non- 1228 deterministic random input [RANDOM] and the private key MUST be 1229 protected in a secure fashion. 1231 If an S/MIME agent needs to generate a key pair, then the S/MIME agent 1232 or some related administrative utility or function SHOULD generate RSA 1233 key pairs. 1235 A user agent SHOULD generate RSA key pairs at a minimum key size of 1236 768 bits. A user agent MUST NOT generate RSA key pairs less than 512 1237 bits long. Creating keys longer than 1024 bits may cause some older 1238 S/MIME receiving agents to not be able to verify signatures, but gives 1239 better security and is therefore valuable. A receiving agent SHOULD be 1240 able to verify signatures with keys of any size over 512 bits. Some 1241 agents created in the United States have chosen to create 512 bit keys 1242 in order to get more advantageous export licenses. However, 512 bit 1243 keys are considered by many to be cryptographically insecure. 1244 Implementors should be aware that multiple (active) key pairs may be 1245 associated with a single individual. For example, one key pair may be 1246 used to support confidentiality, while a different key pair may be 1247 used for authentication. 1249 5. Security 1251 40-bit encryption is considered weak by most cryptographers. Using 1252 weak cryptography in S/MIME offers little actual security over sending 1253 plaintext. However, other features of S/MIME, such as the 1254 specification of tripleDES and the ability to announce stronger 1255 cryptographic capabilities to parties with whom you communicate, allow 1256 senders to create messages that use strong encryption. Using weak 1257 cryptography is never recommended unless the only alternative is no 1258 cryptography. When feasible, sending and receiving agents should 1259 inform senders and recipients the relative cryptographic strength of 1260 messages. 1262 It is impossible for most software or people to estimate the value of 1263 a message. Further, it is impossible for most software or people to 1264 estimate the actual cost of decrypting a message that is encrypted 1265 with a key of a particular size. Further, it is quite difficult to 1266 determine the cost of a failed decryption if a recipient cannot decode 1267 a message. Thus, choosing between different key sizes (or choosing 1268 whether to just use plaintext) is also impossible. However, decisions 1269 based on these criteria are made all the time, and therefore this 1270 specification gives a framework for using those estimates in choosing 1271 algorithms. 1273 If a sending agent is sending the same message using different 1274 strengths of cryptography, an attacker watching the communications 1275 channel may be able to determine the contents of the strongly- 1276 encrypted message by decrypting the weakly-encrypted version. In other 1277 words, a sender should not send a copy of a message using weaker 1278 cryptography than they would use for the original of the message. 1280 Modification of the ciphertext can go undetected if authentication is 1281 not also used, which is the case when sending EnvelopedData without 1282 wrapping it in SignedData or enclosing SignedData within it. 1284 See RFC 3218 [MMA] for more information about thwarting the adaptive 1285 chosen ciphertext vulnerability in PKCS #1 Version 1.5 1286 implementations. 1288 In some circumstances the use of the Diffie-Hellman key agreement 1289 scheme in a prime order subgroup of a large prime p is vulnerable to 1290 certain attacks known as "small-subgroup" attacks. Methods exist, 1291 however, to prevent these attacks. These methods are described in RFC 1292 2785 [DHSUB]. 1294 A. ASN.1 Module 1296 SecureMimeMessageV3dot1 1297 { iso(1) member-body(2) us(840) rsadsi(113549) 1298 pkcs(1) pkcs-9(9) smime(16) modules(0) msg-v3dot1(21) } 1300 DEFINITIONS IMPLICIT TAGS ::= 1301 BEGIN 1303 IMPORTS 1304 -- Cryptographic Message Syntax 1305 SubjectKeyIdentifier, IssuerAndSerialNumber, 1306 RecipientKeyIdentifier 1307 FROM CryptographicMessageSyntax 1308 { iso(1) member-body(2) us(840) rsadsi(113549) 1309 pkcs(1) pkcs-9(9) smime(16) modules(0) cms-2001(14) }; 1311 -- id-aa is the arc with all new authenticated and unauthenticated 1312 -- attributes produced the by S/MIME Working Group 1314 id-aa OBJECT IDENTIFIER ::= {iso(1) member-body(2) usa(840) 1315 rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) attributes(2)} 1317 -- S/MIME Capabilities provides a method of broadcasting the symetric 1318 -- capabilities understood. Algorithms should be ordered by 1319 -- preference and grouped by type 1321 smimeCapabilities OBJECT IDENTIFIER ::= 1322 {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 15} 1324 SMIMECapability ::= SEQUENCE { 1325 capabilityID OBJECT IDENTIFIER, 1326 parameters ANY DEFINED BY capabilityID OPTIONAL } 1328 SMIMECapabilities ::= SEQUENCE OF SMIMECapability 1330 -- Encryption Key Preference provides a method of broadcasting the 1331 -- prefered encryption certificate. 1333 id-aa-encrypKeyPref OBJECT IDENTIFIER ::= {id-aa 11} 1335 SMIMEEncryptionKeyPreference ::= CHOICE { 1336 issuerAndSerialNumber [0] IssuerAndSerialNumber, 1337 receipentKeyId [1] RecipientKeyIdentifier, 1338 subjectAltKeyIdentifier [2] SubjectKeyIdentifier 1339 } 1341 id-smime OBJECT IDENTIFIER ::= { iso(1) member-body(2) 1342 us(840) rsadsi(113549) pkcs(1) pkcs9(9) 16 } 1344 id-cap OBJECT IDENTIFIER ::= { id-smime 11 } 1346 -- The preferBinaryInside indicates an ability to receive messages 1347 -- with binary encoding inside the CMS wrapper 1349 id-cap-preferBinaryInside OBJECT IDENTIFIER ::= { id-cap 1 } 1351 -- The following list the OIDs to be used with S/MIME V3 1353 -- Signature Algorithms Not Found in [CMSALG] 1354 -- 1355 -- md2WithRSAEncryption OBJECT IDENTIFIER ::= 1356 -- {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 1357 -- 2} 1358 -- 1359 -- Other Signed Attributes 1360 -- 1361 -- signingTime OBJECT IDENTIFIER ::= 1362 -- {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 1363 -- 5} 1364 -- See [CMS] for a description of how to encode the attribute 1365 -- value. 1367 END 1369 B. References 1371 [CERT31] "S/MIME Version 3.1 Certificate Handling", Internet Draft 1372 draft-ietf-smime-rfc2632bis 1374 [CHARSETS] Character sets assigned by IANA. See . 1377 [CMS] "Cryptographic Message Syntax", RFC 3369 1379 [CMSAES] "Use of the AES Encryption Algorithm in CMS", 1380 draft-ietf-smime-aes-alg-07 1382 [CMSALG] "Cryptographic Message Syntax (CMS) Algorithms", RFC 3370 1384 [CMSCOMPR] "Compressed Data Content Type for Cryptographic Message 1385 Syntax (CMS)", RFC 3274 1387 [CONTDISP] "Communicating Presentation Information in Internet 1388 Messages: The Content-Disposition Header Field", RFC 2183 1390 [DHSUB] "Methods for Avoiding the "Small-Subgroup" Attacks on the 1391 Diffie-Hellman Key Agreement Method for S/MIME", RFC 2785 1393 [ESS] "Enhanced Security Services for S/MIME", RFC 2634 1395 [FIPS180-2] "Secure Hash Signature Standard (SHS)", National Institute 1396 of Standards and Technology (NIST). FIPS Publication 180-2 1398 [MIME-SPEC] The primary definition of MIME. "MIME Part 1: Format of 1399 Internet Message Bodies", RFC 2045; "MIME Part 2: Media Types", RFC 1400 2046; "MIME Part 3: Message Header Extensions for Non-ASCII Text", RFC 1401 2047; "MIME Part 4: Registration Procedures", RFC 2048; "MIME Part 5: 1402 Conformance Criteria and Examples", RFC 2049 1404 [MIME-SECURE] "Security Multiparts for MIME: Multipart/Signed and 1405 Multipart/Encrypted", RFC 1847 1407 [MMA] "Preventing the Million Message Attack on CMS", RFC 3218 1409 [MUSTSHOULD] "Key words for use in RFCs to Indicate Requirement 1410 Levels", RFC 2119 1412 [PKCS-7] "PKCS #7: Cryptographic Message Syntax Version 1.5", RFC 2315 1414 [RANDOM] "Randomness Recommendations for Security", RFC 1750 1416 [SMIMEV2] "S/MIME Version 2 Message Specification", RFC 2311 1418 [X.208-88] CCITT. Recommendation X.208: Specification of Abstract 1419 Syntax Notation One (ASN.1). 1988. 1421 [X.209-88] CCITT. Recommendation X.209: Specification of Basic 1422 Encoding Rules for Abstract Syntax Notation One (ASN.1). 1988. 1424 [X.509-88] CCITT. Recommendation X.509: The Directory - Authentication 1425 Framework. 1988. 1427 C. Acknowledgements 1429 Many thanks go out to the other authors of the S/MIME Version 2 1430 Message Specification RFC: Steve Dusse, Paul Hoffman, Laurence 1431 Lundblade and Lisa Repka. 1433 A number of the members of the S/MIME Working Group have also worked 1434 very hard and contributed to this document. Any list of people is 1435 doomed to omission, and for that I apologize. In alphabetical order, 1436 the following people stand out in my mind due to the fact that they 1437 made direct contributions to this document. 1439 Tony Capel 1440 Piers Chivers 1441 Dave Crocker 1442 Bill Flanigan 1443 Paul Hoffman 1444 Russ Housley 1445 William Ottaway 1446 John Pawling 1447 Jim Schaad 1449 D. Editor's address 1451 Blake Ramsdell 1452 Brute Squad Labs 1453 Suite 217-C 1454 16451 Redmond Way 1455 Redmond, WA 98052-4482 1457 blake@brutesquadlabs.com 1459 E. Changes from last draft 1461 id-dsa change to id-dsa-with-sha1 (Blake Ramsdell) 1463 Added AES as a SHOULD (Paul Hoffman)