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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 (September 25, 2004) is 7151 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 1424 looks like a reference -- Missing reference section? 'CMS' on line 1406 looks like a reference -- Missing reference section? 'PKCS-7' on line 1452 looks like a reference -- Missing reference section? 'MIME-SECURE' on line 1430 looks like a reference -- Missing reference section? 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'0' on line 1362 looks like a reference -- Missing reference section? '1' on line 1363 looks like a reference -- Missing reference section? '2' on line 1364 looks like a reference Summary: 6 errors (**), 0 flaws (~~), 2 warnings (==), 22 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-08.txt Sendmail, Inc. 3 March 25, 2004 4 Expires September 25, 2004 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 document [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 name to a public key with a 109 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 3.1 agents should attempt to have the greatest 141 interoperability possible with agents for prior versions of S/MIME. 142 S/MIME version 2 is described in RFC 2311 through RFC 2315, inclusive 143 and S/MIME version 3 is described in RFC 2630 through RFC 2634 144 inclusive. RFC 2311 also has historical information about the 145 development of S/MIME. 147 1.5 Changes Since S/MIME v3.0 149 The RSA public key algorithm was changed to a MUST implement key 150 wrapping algorithm, and the Diffie-Hellman algorithm changed to a 151 SHOULD implement. 153 The AES symmetric encryption algorithm has been included as a SHOULD 154 implement. 156 The RSA public key algorithm was changed to a MUST implement signature 157 algorithm. 159 Ambiguous language about the use of "empty" SignedData messages to 160 transmit certificates was clarified to reflect that transmission of 161 certificate revocation lists is also allowed. 163 The use of binary encoding for some MIME entities is now explicitly 164 discussed. 166 Header protection through the use of the message/rfc822 MIME type has 167 been added. 169 Use of the CompressedData CMS type is allowed, along with required 170 MIME type and file extension additions. 172 2. CMS Options 174 CMS allows for a wide variety of options in content and algorithm 175 support. This section puts forth a number of support requirements and 176 recommendations in order to achieve a base level of interoperability 177 among all S/MIME implementations. [CMSALG] provides additional details 178 regarding the use of the cryptographic algorithms. 180 2.1 DigestAlgorithmIdentifier 182 Sending and receiving agents MUST support SHA-1 [CMSALG]. Receiving 183 agents SHOULD support MD5 [CMSALG] for the purpose of providing 184 backward compatibility with MD5-digested S/MIME v2 SignedData objects. 186 2.2 SignatureAlgorithmIdentifier 188 Receiving agents MUST support id-dsa-with-sha1 defined in [CMSALG]. 189 The algorithm parameters MUST be absent (not encoded as NULL). 190 Receiving agents MUST support rsaEncryption, defined in [CMSALG]. 192 Sending agents MUST support either id-dsa-with-sha1 or rsaEncryption. 194 If using rsaEncryption, sending and receiving agents MUST support the 195 digest algorithms in section 2.1 as specified. 197 Note that S/MIME v3 clients might only implement signing or signature 198 verification using id-dsa-with-sha1, and might also use id-dsa as an 199 AlgorithmIdentifier in this field. Receiving clients SHOULD recognize 200 id-dsa as equivalent to id-dsa-with-sha1, and sending clients MUST use 201 id-dsa-with-sha1 if using that algorithm. Also note that S/MIME v2 202 clients are only required to verify digital signatures using the 203 rsaEncryption algorithm with SHA-1 or MD5, and may not implement 204 id-dsa-with-sha1 or id-dsa at all. 206 2.3 KeyEncryptionAlgorithmIdentifier 208 Sending and receiving agents MUST support rsaEncryption, defined in 209 [CMSALG]. 211 Sending and receiving agents SHOULD support Diffie-Hellman defined in 212 [CMSALG], using the ephemeral-static mode. 214 Note that S/MIME v3 clients might only implement key encryption and 215 decryption using the Diffie-Hellman algorithm. Also note that S/MIME 216 v2 clients are only capable of decrypting content-encryption keys 217 using the rsaEncryption algorithm. 219 2.4 General Syntax 221 There are several CMS content types. Of these, only the Data, 222 SignedData, EnvelopedData and CompressedData content types are 223 currently used for S/MIME. 225 2.4.1 Data Content Type 227 Sending agents MUST use the id-data content type identifier to 228 identify the "inner" MIME message content. For example, when applying 229 a digital signature to MIME data, the CMS SignedData encapContentInfo 230 eContentType MUST include the id-data object identifier and the MIME 231 content MUST be stored in the SignedData encapContentInfo eContent 232 OCTET STRING (unless the sending agent is using multipart/signed, in 233 which case the eContent is absent, per section 3.4.3 of this 234 document). As another example, when applying encryption to MIME data, 235 the CMS EnvelopedData encryptedContentInfo contentType MUST include 236 the id-data object identifier and the encrypted MIME content MUST be 237 stored in the EnvelopedData encryptedContentInfo encryptedContent 238 OCTET STRING. 240 2.4.2 SignedData Content Type 242 Sending agents MUST use the SignedData content type to apply a digital 243 signature to a message or, in a degenerate case where there is no 244 signature information, to convey certificates. Applying a signature to 245 a message provides authentication, message integrity, and 246 non-repudiation of origin. 248 2.4.3 EnvelopedData Content Type 250 This content type is used to apply data confidentiality to a message. 251 A sender needs to have access to a public key for each intended 252 message recipient to use this service. This content type does not 253 provide authentication. 255 2.4.4 CompressedData Content Type 257 This content type is used to apply data compression to a message. This 258 content type does not provide authentication, message integrity, 259 non-repudiation, or data confidentiality, and is only used to reduce 260 message size. 262 See section 3.6 for further guidance on the use of this type in 263 conjunction with other CMS types. 265 2.5 Attributes and the SignerInfo Type 267 The SignerInfo type allows the inclusion of unsigned and signed 268 attributes to be included along with a signature. 270 Receiving agents MUST be able to handle zero or one instance of each 271 of the signed attributes listed here. Sending agents SHOULD generate 272 one instance of each of the following signed attributes in each S/MIME 273 message: 275 - signingTime (section 2.5.1 in this document) 276 - sMIMECapabilities (section 2.5.2 in this document) 277 - sMIMEEncryptionKeyPreference (section 2.5.3 in this document) 278 - id-messageDigest (section 11.2 in [CMS]) 279 - id-contentType (section 11.1 in [CMS]) 281 Further, receiving agents SHOULD be able to handle zero or one 282 instance in the signingCertificate signed attribute, as defined in 283 section 5 of [ESS]. 285 Sending agents SHOULD generate one instance of the signingCertificate 286 signed attribute in each SignerInfo structure. 288 Additional attributes and values for these attributes may be defined 289 in the future. Receiving agents SHOULD handle attributes or values 290 that it does not recognize in a graceful manner. 292 Interactive sending agents that include signed attributes that are not 293 listed here SHOULD display those attributes to the user, so that the 294 user is aware of all of the data being signed. 296 2.5.1 Signing-Time Attribute 298 The signing-time attribute is used to convey the time that a message 299 was signed. The time of signing will most likely be created by a 300 message originator and therefore is only as trustworthy as the 301 originator. 303 Sending agents MUST encode signing time through the year 2049 as 304 UTCTime; signing times in 2050 or later MUST be encoded as 305 GeneralizedTime. When the UTCTime CHOICE is used, S/MIME agents MUST 306 interpret the year field (YY) as follows: 308 if YY is greater than or equal to 50, the year is interpreted as 19YY; 309 if YY is less than 50, the year is interpreted as 20YY. 311 2.5.2 SMIMECapabilities Attribute 313 The SMIMECapabilities attribute includes signature algorithms (such as 314 "sha1WithRSAEncryption"), symmetric algorithms (such as "DES-EDE3- 315 CBC"), and key encipherment algorithms (such as "rsaEncryption"). 316 There are also several identifiers which indicate support for other 317 optional features such as binary encoding and compression. The 318 SMIMECapabilities were designed to be flexible and extensible so that, 319 in the future, a means of identifying other capabilities and 320 preferences such as certificates can be added in a way that will not 321 cause current clients to break. 323 If present, the SMIMECapabilities attribute MUST be a SignedAttribute; 324 it MUST NOT be an UnsignedAttribute. CMS defines SignedAttributes as a 325 SET OF Attribute. The SignedAttributes in a signerInfo MUST NOT 326 include multiple instances of the SMIMECapabilities attribute. CMS 327 defines the ASN.1 syntax for Attribute to include attrValues SET OF 328 AttributeValue. A SMIMECapabilities attribute MUST only include a 329 single instance of AttributeValue. There MUST NOT be zero or multiple 330 instances of AttributeValue present in the attrValues SET OF 331 AttributeValue. 333 The semantics of the SMIMECapabilities attribute specify a partial 334 list as to what the client announcing the SMIMECapabilities can 335 support. A client does not have to list every capability it supports, 336 and probably should not list all its capabilities so that the 337 capabilities list doesn't get too long. In an SMIMECapabilities 338 attribute, the object identifiers (OIDs) are listed in order of their 339 preference, but SHOULD be logically separated along the lines of their 340 categories (signature algorithms, symmetric algorithms, key 341 encipherment algorithms, etc.) 343 The structure of the SMIMECapabilities attribute is to facilitate 344 simple table lookups and binary comparisons in order to determine 345 matches. For instance, the DER-encoding for the SMIMECapability for 346 DES EDE3 CBC MUST be identically encoded regardless of the 347 implementation. Because of the requirement for identical encoding, 348 individuals documenting algorithms to be used in the SMIMECapabilities 349 attribute SHOULD explicitly document the correct byte sequence for the 350 common cases. 352 For any capability, the associated parameters for the OID MUST specify 353 all of the parameters necessary to differentiate between two instances 354 of the same algorithm. For instance, the number of rounds and block 355 size for RC5 must be specified in addition to the key length. 357 The OIDs that correspond to algorithms SHOULD use the same OID as the 358 actual algorithm, except in the case where the algorithm usage is 359 ambiguous from the OID. For instance, in an earlier specification, 360 rsaEncryption was ambiguous because it could refer to either a 361 signature algorithm or a key encipherment algorithm. In the event that 362 an OID is ambiguous, it needs to be arbitrated by the maintainer of 363 the registered SMIMECapabilities list as to which type of algorithm 364 will use the OID, and a new OID MUST be allocated under the 365 smimeCapabilities OID to satisfy the other use of the OID. 367 The registered SMIMECapabilities list specifies the parameters for 368 OIDs that need them, most notably key lengths in the case of variable- 369 length symmetric ciphers. In the event that there are no 370 differentiating parameters for a particular OID, the parameters MUST 371 be omitted, and MUST NOT be encoded as NULL. 373 Additional values for the SMIMECapabilities attribute may be defined 374 in the future. Receiving agents MUST handle a SMIMECapabilities object 375 that has values that it does not recognize in a graceful manner. 377 Section 2.7.1 explains a strategy for caching capabilities. 379 2.5.2.1 SMIMECapability For the RC2 Algorithm 381 For the RC2 algorithm preference SMIMECapability, the capabilityID 382 MUST be set to the value rC2-CBC as defined in [CMSALG]. The 383 parameters field MUST contain SMIMECapabilitiesParametersForRC2CBC 384 (see appendix A). 386 Please note that the SMIMECapabilitiesParametersForRC2CBC is a single 387 INTEGER which contains the effective key length (NOT the corresponding 388 RC2 parameter version value). So, for example, for RC2 with a 128-bit 389 effective key length, the parameter would be encoded as the INTEGER 390 value 128, NOT the corresponding parameter version of 58. 392 2.5.3 Encryption Key Preference Attribute 394 The encryption key preference attribute allows the signer to 395 unambiguously describe which of the signer's certificates has the 396 signer's preferred encryption key. This attribute is designed to 397 enhance behavior for interoperating with those clients which use 398 separate keys for encryption and signing. This attribute is used to 399 convey to anyone viewing the attribute which of the listed 400 certificates should be used for encrypting a session key for future 401 encrypted messages. 403 If present, the SMIMEEncryptionKeyPreference attribute MUST be a 404 SignedAttribute; it MUST NOT be an UnsignedAttribute. CMS defines 405 SignedAttributes as a SET OF Attribute. The SignedAttributes in a 406 signerInfo MUST NOT include multiple instances of the 407 SMIMEEncryptionKeyPreference attribute. CMS defines the ASN.1 syntax 408 for Attribute to include attrValues SET OF AttributeValue. A 409 SMIMEEncryptionKeyPreference attribute MUST only include a single 410 instance of AttributeValue. There MUST NOT be zero or multiple 411 instances of AttributeValue present in the attrValues SET OF 412 AttributeValue. 414 The sending agent SHOULD include the referenced certificate in the set 415 of certificates included in the signed message if this attribute is 416 used. The certificate may be omitted if it has been previously made 417 available to the receiving agent. Sending agents SHOULD use this 418 attribute if the commonly used or preferred encryption certificate is 419 not the same as the certificate used to sign the message. 421 Receiving agents SHOULD store the preference data if the signature on 422 the message is valid and the signing time is greater than the 423 currently stored value. (As with the SMIMECapabilities, the clock skew 424 should be checked and the data not used if the skew is too great.) 425 Receiving agents SHOULD respect the sender's encryption key preference 426 attribute if possible. This however represents only a preference and 427 the receiving agent may use any certificate in replying to the sender 428 that is valid. 430 Section 2.7.1 explains a strategy for caching preference data. 432 2.5.3.1 Selection of Recipient Key Management Certificate 434 In order to determine the key management certificate to be used when 435 sending a future CMS EnvelopedData message for a particular recipient, 436 the following steps SHOULD be followed: 438 - If an SMIMEEncryptionKeyPreference attribute is found in a 439 SignedData object received from the desired recipient, this 440 identifies the X.509 certificate that should be used as the X.509 441 key management certificate for the recipient. 443 - If an SMIMEEncryptionKeyPreference attribute is not found in a 444 SignedData object received from the desired recipient, the set of 445 X.509 certificates should be searched for a X.509 certificate with 446 the same subject name as the signing X.509 certificate which can be 447 used for key management. 449 - Or use some other method of determining the user's key management 450 key. If a X.509 key management certificate is not found, then 451 encryption cannot be done with the signer of the message. If 452 multiple X.509 key management certificates are found, the S/MIME 453 agent can make an arbitrary choice between them. 455 2.6 SignerIdentifier SignerInfo Type 457 S/MIME v3.1 implementations MUST support both issuerAndSerialNumber as 458 well as subjectKeyIdentifier. Messages that use the 459 subjectKeyIdentifier choice cannot be read by S/MIME v2 clients. 461 It is important to understand that some certificates use a value for 462 subjectKeyIdentifier that is not suitable for uniquely identifying a 463 certificate. Implementations MUST be prepared for multiple 464 certificates for potentially different entities to have the same value 465 for subjectKeyIdentifier, and MUST be prepared to try each matching 466 certificate during signature verification before indicating an error 467 condition. 469 2.7 ContentEncryptionAlgorithmIdentifier 471 Sending and receiving agents MUST support encryption and decryption 472 with DES EDE3 CBC, hereinafter called "tripleDES" [CMSALG]. Receiving 473 agents SHOULD support encryption and decryption using the RC2 [CMSALG] 474 or a compatible algorithm at a key size of 40 bits, hereinafter called 475 "RC2/40". Sending and receiving agents SHOULD support encryption and 476 decryption with AES [CMSAES] at a key size of 128, 192 and 256 bits. 478 2.7.1 Deciding Which Encryption Method To Use 480 When a sending agent creates an encrypted message, it has to decide 481 which type of encryption to use. The decision process involves using 482 information garnered from the capabilities lists included in messages 483 received from the recipient, as well as out-of-band information such 484 as private agreements, user preferences, legal restrictions, and so 485 on. 487 Section 2.5.2 defines a method by which a sending agent can optionally 488 announce, among other things, its decrypting capabilities in its order 489 of preference. The following method for processing and remembering the 490 encryption capabilities attribute in incoming signed messages SHOULD 491 be used. 493 - If the receiving agent has not yet created a list of capabilities 494 for the sender's public key, then, after verifying the signature on 495 the incoming message and checking the timestamp, the receiving agent 496 SHOULD create a new list containing at least the signing time and 497 the symmetric capabilities. 499 - If such a list already exists, the receiving agent SHOULD verify 500 that the signing time in the incoming message is greater than the 501 signing time stored in the list and that the signature is valid. If 502 so, the receiving agent SHOULD update both the signing time and 503 capabilities in the list. Values of the signing time that lie far in 504 the future (that is, a greater discrepancy than any reasonable clock 505 skew), or a capabilities list in messages whose signature could not 506 be verified, MUST NOT be accepted. 508 The list of capabilities SHOULD be stored for future use in creating 509 messages. 511 Before sending a message, the sending agent MUST decide whether it is 512 willing to use weak encryption for the particular data in the message. 513 If the sending agent decides that weak encryption is unacceptable for 514 this data, then the sending agent MUST NOT use a weak algorithm such 515 as RC2/40. The decision to use or not use weak encryption overrides 516 any other decision in this section about which encryption algorithm to 517 use. 519 Sections 2.7.2.1 through 2.7.2.4 describe the decisions a sending 520 agent SHOULD use in deciding which type of encryption should be 521 applied to a message. These rules are ordered, so the sending agent 522 SHOULD make its decision in the order given. 524 2.7.1.1 Rule 1: Known Capabilities 526 If the sending agent has received a set of capabilities from the 527 recipient for the message the agent is about to encrypt, then the 528 sending agent SHOULD use that information by selecting the first 529 capability in the list (that is, the capability most preferred by the 530 intended recipient) for which the sending agent knows how to encrypt. 531 The sending agent SHOULD use one of the capabilities in the list if 532 the agent reasonably expects the recipient to be able to decrypt the 533 message. 535 2.7.1.2 Rule 2: Unknown Capabilities, Unknown Version of S/MIME 537 If the following two conditions are met: 538 - the sending agent has no knowledge of the encryption capabilities 539 of the recipient, 540 - and the sending agent has no knowledge of the version of S/MIME 541 of the recipient, 542 then the sending agent SHOULD use tripleDES because it is a stronger 543 algorithm and is required by S/MIME v3. If the sending agent chooses 544 not to use tripleDES in this step, it SHOULD use RC2/40. 546 2.7.2 Choosing Weak Encryption 548 Like all algorithms that use 40 bit keys, RC2/40 is considered by many 549 to be weak encryption. A sending agent that is controlled by a human 550 SHOULD allow a human sender to determine the risks of sending data 551 using RC2/40 or a similarly weak encryption algorithm before sending 552 the data, and possibly allow the human to use a stronger encryption 553 method such as tripleDES. 555 2.7.3 Multiple Recipients 557 If a sending agent is composing an encrypted message to a group of 558 recipients where the encryption capabilities of some of the recipients 559 do not overlap, the sending agent is forced to send more than one 560 message. It should be noted that if the sending agent chooses to send 561 a message encrypted with a strong algorithm, and then send the same 562 message encrypted with a weak algorithm, someone watching the 563 communications channel may be able to learn the contents of the 564 strongly-encrypted message simply by decrypting the weakly-encrypted 565 message. 567 3. Creating S/MIME Messages 569 This section describes the S/MIME message formats and how they are 570 created. S/MIME messages are a combination of MIME bodies and CMS 571 content types. Several MIME types as well as several CMS content types 572 are used. The data to be secured is always a canonical MIME entity. 573 The MIME entity and other data, such as certificates and algorithm 574 identifiers, are given to CMS processing facilities which produces a 575 CMS object. The CMS object is then finally wrapped in MIME. The 576 Enhanced Security Services for S/MIME [ESS] document provides 577 descriptions of how nested, secured S/MIME messages are formatted. ESS 578 provides a description of how a triple-wrapped S/MIME message is 579 formatted using multipart/signed and application/pkcs7-mime for the 580 signatures. 582 S/MIME provides one format for enveloped-only data, several formats 583 for signed-only data, and several formats for signed and enveloped 584 data. Several formats are required to accommodate several 585 environments, in particular for signed messages. The criteria for 586 choosing among these formats are also described. 588 The reader of this section is expected to understand MIME as described 589 in [MIME-SPEC] and [MIME-SECURE]. 591 3.1 Preparing the MIME Entity for Signing, Enveloping or Compressing 593 S/MIME is used to secure MIME entities. A MIME entity may be a sub- 594 part, sub-parts of a message, or the whole message with all its sub- 595 parts. A MIME entity that is the whole message includes only the MIME 596 headers and MIME body, and does not include the RFC-822 headers. Note 597 that S/MIME can also be used to secure MIME entities used in 598 applications other than Internet mail. If protection of the RFC-822 599 headers is required, the use of the message/rfc822 MIME type is 600 explained later in this section. 602 The MIME entity that is secured and described in this section can be 603 thought of as the "inside" MIME entity. That is, it is the "innermost" 604 object in what is possibly a larger MIME message. Processing "outside" 605 MIME entities into CMS content types is described in Section 3.2, 3.4 606 and elsewhere. 608 The procedure for preparing a MIME entity is given in [MIME-SPEC]. The 609 same procedure is used here with some additional restrictions when 610 signing. Description of the procedures from [MIME-SPEC] are repeated 611 here, but the reader should refer to that document for the exact 612 procedure. This section also describes additional requirements. 614 A single procedure is used for creating MIME entities that are to have 615 any combination of signing, enveloping and compressing applied. Some 616 additional steps are recommended to defend against known corruptions 617 that can occur during mail transport that are of particular importance 618 for clear- signing using the multipart/signed format. It is 619 recommended that these additional steps be performed on enveloped 620 messages, or signed and enveloped messages in order that the message 621 can be forwarded to any environment without modification. 623 These steps are descriptive rather than prescriptive. The implementer 624 is free to use any procedure as long as the result is the same. 626 Step 1. The MIME entity is prepared according to the local 627 conventions. 629 Step 2. The leaf parts of the MIME entity are converted to canonical 630 form. 632 Step 3. Appropriate transfer encoding is applied to the leaves of the 633 MIME entity. 635 When an S/MIME message is received, the security services on the 636 message are processed, and the result is the MIME entity. That MIME 637 entity is typically passed to a MIME-capable user agent where, it is 638 further decoded and presented to the user or receiving application. 640 In order to protect outer, non-content related message headers (for 641 instance, the "Subject", "To", "From" and "CC" fields), the sending 642 client MAY wrap a full MIME message in a message/rfc822 wrapper in 643 order to apply S/MIME security services to these headers. It is up to 644 the receiving client to decide how to present these "inner" headers 645 along with the unprotected "outer" headers. 647 When an S/MIME message is received, if the top-level protected MIME 648 entity has a Content-Type of message/rfc822, it can be assumed that 649 the intent was to provide header protection. This entity should be 650 presented as the top-level message, taking into account header merging 651 issues as previously discussed. 653 3.1.1 Canonicalization 655 Each MIME entity MUST be converted to a canonical form that is 656 uniquely and unambiguously representable in the environment where the 657 signature is created and the environment where the signature will be 658 verified. MIME entities MUST be canonicalized for enveloping and 659 compressing as well as signing. 661 The exact details of canonicalization depend on the actual MIME type 662 and subtype of an entity, and are not described here. Instead, the 663 standard for the particular MIME type should be consulted. For 664 example, canonicalization of type text/plain is different from 665 canonicalization of audio/basic. Other than text types, most types 666 have only one representation regardless of computing platform or 667 environment which can be considered their canonical representation. In 668 general, canonicalization will be performed by the non-security part 669 of the sending agent rather than the S/MIME implementation. 671 The most common and important canonicalization is for text, which is 672 often represented differently in different environments. MIME entities 673 of major type "text" must have both their line endings and character 674 set canonicalized. The line ending must be the pair of characters 675 , and the charset should be a registered charset [CHARSETS]. 676 The details of the canonicalization are specified in [MIME-SPEC]. The 677 chosen charset SHOULD be named in the charset parameter so that the 678 receiving agent can unambiguously determine the charset used. 680 Note that some charsets such as ISO-2022 have multiple representations 681 for the same characters. When preparing such text for signing, the 682 canonical representation specified for the charset MUST be used. 684 3.1.2 Transfer Encoding 686 When generating any of the secured MIME entities below, except the 687 signing using the multipart/signed format, no transfer encoding at all 688 is required. S/MIME implementations MUST be able to deal with binary 689 MIME objects. If no Content-Transfer-Encoding header is present, the 690 transfer encoding should be considered 7BIT. 692 S/MIME implementations SHOULD however use transfer encoding described 693 in section 3.1.3 for all MIME entities they secure. The reason for 694 securing only 7-bit MIME entities, even for enveloped data that are 695 not exposed to the transport, is that it allows the MIME entity to be 696 handled in any environment without changing it. For example, a trusted 697 gateway might remove the envelope, but not the signature, of a 698 message, and then forward the signed message on to the end recipient 699 so that they can verify the signatures directly. If the transport 700 internal to the site is not 8-bit clean, such as on a wide-area 701 network with a single mail gateway, verifying the signature will not 702 be possible unless the original MIME entity was only 7-bit data. 704 S/MIME implementations which "know" that all intended recipient(s) are 705 capable of handling inner (all but the outermost) binary MIME objects 706 SHOULD use binary encoding as opposed to a 7-bit-safe transfer 707 encoding for the inner entities. The use of a 7-bit-safe encoding 708 (such as base64) would unnecessarily expand the message size. 709 Implementations MAY "know" that recipient implementations are capable 710 of handling inner binary MIME entities either by interpreting the 711 id-cap-preferBinaryInside sMIMECapabilities attribute, by prior 712 agreement, or by other means. 714 If one or more intended recipients are unable to handle inner binary 715 MIME objects, or if this capability in unknown for any of the intended 716 recipients, S/MIME implementations SHOULD use transfer encoding 717 described in section 3.1.3 for all MIME entities they secure. 719 3.1.3 Transfer Encoding for Signing Using multipart/signed 721 If a multipart/signed entity is ever to be transmitted over the 722 standard Internet SMTP infrastructure or other transport that is 723 constrained to 7-bit text, it MUST have transfer encoding applied so 724 that it is represented as 7-bit text. MIME entities that are 7-bit 725 data already need no transfer encoding. Entities such as 8-bit text 726 and binary data can be encoded with quoted-printable or base-64 727 transfer encoding. 729 The primary reason for the 7-bit requirement is that the Internet mail 730 transport infrastructure cannot guarantee transport of 8-bit or binary 731 data. Even though many segments of the transport infrastructure now 732 handle 8-bit and even binary data, it is sometimes not possible to 733 know whether the transport path is 8-bit clean. If a mail message with 734 8-bit data were to encounter a message transfer agent that can not 735 transmit 8-bit or binary data, the agent has three options, none of 736 which are acceptable for a clear-signed message: 738 - The agent could change the transfer encoding; this would invalidate 739 the signature. 740 - The agent could transmit the data anyway, which would most likely 741 result in the 8th bit being corrupted; this too would invalidate the 742 signature. 743 - The agent could return the message to the sender. 745 [MIME-SECURE] prohibits an agent from changing the transfer encoding 746 of the first part of a multipart/signed message. If a compliant agent 747 that can not transmit 8-bit or binary data encounters a 748 multipart/signed message with 8-bit or binary data in the first part, 749 it would have to return the message to the sender as undeliverable. 751 3.1.4 Sample Canonical MIME Entity 753 This example shows a multipart/mixed message with full transfer 754 encoding. This message contains a text part and an attachment. The 755 sample message text includes characters that are not US-ASCII and thus 756 must be transfer encoded. Though not shown here, the end of each line 757 is . The line ending of the MIME headers, the text, and 758 transfer encoded parts, all must be . 760 Note that this example is not of an S/MIME message. 762 Content-Type: multipart/mixed; boundary=bar 764 --bar 765 Content-Type: text/plain; charset=iso-8859-1 766 Content-Transfer-Encoding: quoted-printable 768 =A1Hola Michael! 770 How do you like the new S/MIME specification? 772 It's generally a good idea to encode lines that begin with 773 From=20because some mail transport agents will insert a greater- 774 than (>) sign, thus invalidating the signature. 776 Also, in some cases it might be desirable to encode any =20 777 trailing whitespace that occurs on lines in order to ensure =20 778 that the message signature is not invalidated when passing =20 779 a gateway that modifies such whitespace (like BITNET). =20 781 --bar 782 Content-Type: image/jpeg 783 Content-Transfer-Encoding: base64 785 iQCVAwUBMJrRF2N9oWBghPDJAQE9UQQAtl7LuRVndBjrk4EqYBIb3h5QXIX/LC// 786 jJV5bNvkZIGPIcEmI5iFd9boEgvpirHtIREEqLQRkYNoBActFBZmh9GC3C041WGq 787 uMbrbxc+nIs1TIKlA08rVi9ig/2Yh7LFrK5Ein57U/W72vgSxLhe/zhdfolT9Brn 788 HOxEa44b+EI= 790 --bar-- 792 3.2 The application/pkcs7-mime Type 794 The application/pkcs7-mime type is used to carry CMS content types 795 including EnvelopedData, SignedData and CompressedData. The details of 796 constructing these entities is described in subsequent sections. This 797 section describes the general characteristics of the 798 application/pkcs7-mime type. 800 The carried CMS object always contains a MIME entity that is prepared 801 as described in section 3.1 if the eContentType is id-data. Other 802 contents may be carried when the eContentType contains different 803 values. See [ESS] for an example of this with signed receipts. 805 Since CMS content types are binary data, in most cases base-64 806 transfer encoding is appropriate, in particular when used with SMTP 807 transport. The transfer encoding used depends on the transport through 808 which the object is to be sent, and is not a characteristic of the 809 MIME type. 811 Note that this discussion refers to the transfer encoding of the CMS 812 object or "outside" MIME entity. It is completely distinct from, and 813 unrelated to, the transfer encoding of the MIME entity secured by the 814 CMS object, the "inside" object, which is described in section 3.1. 816 Because there are several types of application/pkcs7-mime objects, a 817 sending agent SHOULD do as much as possible to help a receiving agent 818 know about the contents of the object without forcing the receiving 819 agent to decode the ASN.1 for the object. The MIME headers of all 820 application/pkcs7-mime objects SHOULD include the optional "smime- 821 type" parameter, as described in the following sections. 823 3.2.1 The name and filename Parameters 825 For the application/pkcs7-mime, sending agents SHOULD emit the 826 optional "name" parameter to the Content-Type field for compatibility 827 with older systems. Sending agents SHOULD also emit the optional 828 Content-Disposition field [CONTDISP] with the "filename" parameter. If 829 a sending agent emits the above parameters, the value of the 830 parameters SHOULD be a file name with the appropriate extension: 832 MIME Type File Extension 834 Application/pkcs7-mime (SignedData, EnvelopedData) .p7m 836 Application/pkcs7-mime (degenerate SignedData .p7c 837 certificate management message) 839 Application/pkcs7-mime (CompressedData) .p7z 841 Application/pkcs7-signature (SignedData) .p7s 843 In addition, the file name SHOULD be limited to eight characters 844 followed by a three letter extension. The eight character filename 845 base can be any distinct name; the use of the filename base "smime" 846 SHOULD be used to indicate that the MIME entity is associated with 847 S/MIME. 849 Including a file name serves two purposes. It facilitates easier use 850 of S/MIME objects as files on disk. It also can convey type 851 information across gateways. When a MIME entity of type 852 application/pkcs7-mime (for example) arrives at a gateway that has no 853 special knowledge of S/MIME, it will default the entity's MIME type to 854 application/octet-stream and treat it as a generic attachment, thus 855 losing the type information. However, the suggested filename for an 856 attachment is often carried across a gateway. This often allows the 857 receiving systems to determine the appropriate application to hand the 858 attachment off to, in this case a stand-alone S/MIME processing 859 application. Note that this mechanism is provided as a convenience for 860 implementations in certain environments. A proper S/MIME 861 implementation MUST use the MIME types and MUST NOT rely on the file 862 extensions. 864 3.2.2 The smime-type parameter 866 The application/pkcs7-mime content type defines the optional "smime- 867 type" parameter. The intent of this parameter is to convey details 868 about the security applied (signed or enveloped) along with 869 information about the contained content. This specification defines 870 the following smime-types. 872 Name CMS type Inner Content 874 enveloped-data EnvelopedData id-data 876 signed-data SignedData id-data 878 certs-only SignedData none 880 compressed-data CompressedData id-data 882 In order that consistency can be obtained with future, the following 883 guidelines should be followed when assigning a new smime-type 884 parameter. 886 1. If both signing and encryption can be applied to the content, then 887 two values for smime-type SHOULD be assigned "signed-*" and 888 "encrypted-*". If one operation can be assigned then this may be 889 omitted. Thus since "certs-only" can only be signed, "signed-" is 890 omitted. 892 2. A common string for a content oid should be assigned. We use "data" 893 for the id-data content OID when MIME is the inner content. 895 3. If no common string is assigned. Then the common string of 896 "OID." is recommended (for example, "OID.1.3.6.1.5.5.7.6.1" would 897 be DES40). 899 It is explicitly intended that this field be a suitable hint for mail 900 client applications to indicate whether a message is "signed" or 901 "encrypted" without having to tunnel into the CMS payload. 903 3.3 Creating an Enveloped-only Message 905 This section describes the format for enveloping a MIME entity without 906 signing it. It is important to note that sending enveloped but not 907 signed messages does not provide for data integrity. It is possible to 908 replace ciphertext in such a way that the processed message will still 909 be valid, but the meaning may be altered. 911 Step 1. The MIME entity to be enveloped is prepared according to 912 section 3.1. 914 Step 2. The MIME entity and other required data is processed into a 915 CMS object of type EnvelopedData. In addition to encrypting a copy of 916 the content-encryption key for each recipient, a copy of the content- 917 encryption key SHOULD be encrypted for the originator and included in 918 the EnvelopedData (see [CMS] Section 6). 920 Step 3. The EnvelopedData object is wrapped in a CMS ContentInfo 921 object. 923 Step 4. The ContentInfo object is inserted into an 924 application/pkcs7-mime MIME entity. 926 The smime-type parameter for enveloped-only messages is "enveloped- 927 data". The file extension for this type of message is ".p7m". 929 A sample message would be: 931 Content-Type: application/pkcs7-mime; smime-type=enveloped-data; 932 name=smime.p7m 933 Content-Transfer-Encoding: base64 934 Content-Disposition: attachment; filename=smime.p7m 936 rfvbnj756tbBghyHhHUujhJhjH77n8HHGT9HG4VQpfyF467GhIGfHfYT6 937 7n8HHGghyHhHUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H 938 f8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4 939 0GhIGfHfQbnj756YT64V 941 3.4 Creating a Signed-only Message 943 There are two formats for signed messages defined for S/MIME: 944 application/pkcs7-mime with SignedData, and multipart/signed. In 945 general, the multipart/signed form is preferred for sending, and 946 receiving agents SHOULD be able to handle both. 948 3.4.1 Choosing a Format for Signed-only Messages 950 There are no hard-and-fast rules when a particular signed-only format 951 should be chosen because it depends on the capabilities of all the 952 receivers and the relative importance of receivers with S/MIME 953 facilities being able to verify the signature versus the importance of 954 receivers without S/MIME software being able to view the message. 956 Messages signed using the multipart/signed format can always be viewed 957 by the receiver whether they have S/MIME software or not. They can 958 also be viewed whether they are using a MIME-native user agent or they 959 have messages translated by a gateway. In this context, "be viewed" 960 means the ability to process the message essentially as if it were not 961 a signed message, including any other MIME structure the message might 962 have. 964 Messages signed using the SignedData format cannot be viewed by a 965 recipient unless they have S/MIME facilities. However, the SignedData 966 format protects the message content from being changed by benign 967 intermediate agents. Such agents might do line wrapping or 968 content-transfer encoding changes which would break the signature. 970 3.4.2 Signing Using application/pkcs7-mime with SignedData 972 This signing format uses the application/pkcs7-mime MIME type. The 973 steps to create this format are: 975 Step 1. The MIME entity is prepared according to section 3.1. 977 Step 2. The MIME entity and other required data is processed into a 978 CMS object of type SignedData. 980 Step 3. The SignedData object is wrapped in a CMS ContentInfo 981 object. 983 Step 4. The ContentInfo object is inserted into an 984 application/pkcs7-mime MIME entity. 986 The smime-type parameter for messages using application/pkcs7-mime 987 with SignedData is "signed-data". The file extension for this type of 988 message is ".p7m". 990 A sample message would be: 992 Content-Type: application/pkcs7-mime; smime-type=signed-data; 993 name=smime.p7m 994 Content-Transfer-Encoding: base64 995 Content-Disposition: attachment; filename=smime.p7m 997 567GhIGfHfYT6ghyHhHUujpfyF4f8HHGTrfvhJhjH776tbB9HG4VQbnj7 998 77n8HHGT9HG4VQpfyF467GhIGfHfYT6rfvbnj756tbBghyHhHUujhJhjH 999 HUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H7n8HHGghyHh 1000 6YT64V0GhIGfHfQbnj75 1002 3.4.3 Signing Using the multipart/signed Format 1004 This format is a clear-signing format. Recipients without any S/MIME 1005 or CMS processing facilities are able to view the message. It makes 1006 use of the multipart/signed MIME type described in [MIME-SECURE]. The 1007 multipart/signed MIME type has two parts. The first part contains the 1008 MIME entity that is signed; the second part contains the "detached 1009 signature" CMS SignedData object in which the encapContentInfo 1010 eContent field is absent. 1012 3.4.3.1 The application/pkcs7-signature MIME Type 1014 This MIME type always contains a CMS ContentInfo containing a single 1015 CMS object of type SignedData. The SignedData encapContentInfo 1016 eContent field MUST be absent. The signerInfos field contains the 1017 signatures for the MIME entity. 1019 The file extension for signed-only messages using application/pkcs7- 1020 signature is ".p7s". 1022 3.4.3.2 Creating a multipart/signed Message 1024 Step 1. The MIME entity to be signed is prepared according to section 1025 3.1, taking special care for clear-signing. 1027 Step 2. The MIME entity is presented to CMS processing in order to 1028 obtain an object of type SignedData in which the encapContentInfo 1029 eContent field is absent. 1031 Step 3. The MIME entity is inserted into the first part of a 1032 multipart/signed message with no processing other than that described 1033 in section 3.1. 1035 Step 4. Transfer encoding is applied to the "detached signature" CMS 1036 SignedData object and it is inserted into a MIME entity of type 1037 application/pkcs7-signature. 1039 Step 5. The MIME entity of the application/pkcs7-signature is inserted 1040 into the second part of the multipart/signed entity. 1042 The multipart/signed Content type has two required parameters: the 1043 protocol parameter and the micalg parameter. 1045 The protocol parameter MUST be "application/pkcs7-signature". Note 1046 that quotation marks are required around the protocol parameter 1047 because MIME requires that the "/" character in the parameter value 1048 MUST be quoted. 1050 The micalg parameter allows for one-pass processing when the signature 1051 is being verified. The value of the micalg parameter is dependent on 1052 the message digest algorithm(s) used in the calculation of the Message 1053 Integrity Check. If multiple message digest algorithms are used they 1054 MUST be separated by commas per [MIME-SECURE]. The values to be placed 1055 in the micalg parameter SHOULD be from the following: 1057 Algorithm Value 1058 used 1060 MD5 md5 1061 SHA-1 sha1 1062 SHA-256 sha256 1063 SHA-384 sha384 1064 SHA-512 sha512 1065 Any other (defined separately in algorithm profile or "unknown" 1066 if not defined) 1068 (Historical note: some early implementations of S/MIME emitted and 1069 expected "rsa-md5" and "rsa-sha1" for the micalg parameter.) Receiving 1070 agents SHOULD be able to recover gracefully from a micalg parameter 1071 value that they do not recognize. 1073 The SHA-256, SHA-384 and SHA-512 algorithms [FIPS180-2] are not 1074 currently recommended in S/MIME, and are included here for completeness. 1076 3.4.3.3 Sample multipart/signed Message 1078 Content-Type: multipart/signed; 1079 protocol="application/pkcs7-signature"; 1080 micalg=sha1; boundary=boundary42 1082 --boundary42 1083 Content-Type: text/plain 1085 This is a clear-signed message. 1087 --boundary42 1088 Content-Type: application/pkcs7-signature; name=smime.p7s 1089 Content-Transfer-Encoding: base64 1090 Content-Disposition: attachment; filename=smime.p7s 1092 ghyHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6 1093 4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj 1094 n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4 1095 7GhIGfHfYT64VQbnj756 1097 --boundary42-- 1099 The content that is digested (the first part of the multipart/signed) 1100 are the bytes: 1102 43 6f 6e 74 65 6e 74 2d 54 79 70 65 3a 20 74 65 78 74 2f 70 6c 61 69 1103 6e 0d 0a 0d 0a 54 68 69 73 20 69 73 20 61 20 63 6c 65 61 72 2d 73 69 1104 67 6e 65 64 20 6d 65 73 73 61 67 65 2e 0d 0a 1106 3.5 Creating an Compressed-only Message 1108 This section describes the format for compressing a MIME entity. 1109 Please note that versions of S/MIME prior to 3.1 did not specify any 1110 use of CompressedData, and will not recognize it. The use of a 1111 capability to indicate the ability to receive CompressedData is 1112 described in [CMSCOMPR] and is the preferred method for compatibility. 1114 Step 1. The MIME entity to be compressed is prepared according to 1115 section 3.1. 1117 Step 2. The MIME entity and other required data is processed into a 1118 CMS object of type CompressedData. 1120 Step 3. The CompressedData object is wrapped in a CMS ContentInfo 1121 object. 1123 Step 4. The ContentInfo object is inserted into an 1124 application/pkcs7-mime MIME entity. 1126 The smime-type parameter for compressed-only messages is "compressed- 1127 data". The file extension for this type of message is ".p7z". 1129 A sample message would be: 1131 Content-Type: application/pkcs7-mime; smime-type=compressed-data; 1132 name=smime.p7z 1133 Content-Transfer-Encoding: base64 1134 Content-Disposition: attachment; filename=smime.p7z 1136 rfvbnj756tbBghyHhHUujhJhjH77n8HHGT9HG4VQpfyF467GhIGfHfYT6 1137 7n8HHGghyHhHUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H 1138 f8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4 1139 0GhIGfHfQbnj756YT64V 1141 3.6 Multiple Operations 1143 The signed-only, encrypted-only, and compressed-only MIME formats can 1144 be nested. This works because these formats are all MIME entities that 1145 encapsulate other MIME entities. 1147 An S/MIME implementation MUST be able to receive and process 1148 arbitrarily nested S/MIME within reasonable resource limits of the 1149 recipient computer. 1151 It is possible to apply any of the signing, encrypting and compressing 1152 operations in any order. It is up to the implementer and the user to 1153 choose. When signing first, the signatories are then securely obscured 1154 by the enveloping. When enveloping first the signatories are exposed, 1155 but it is possible to verify signatures without removing the 1156 enveloping. This may be useful in an environment were automatic 1157 signature verification is desired, as no private key material is 1158 required to verify a signature. 1160 There are security ramifications to choosing whether to sign first or 1161 encrypt first. A recipient of a message that is encrypted and then 1162 signed can validate that the encrypted block was unaltered, but cannot 1163 determine any relationship between the signer and the unencrypted 1164 contents of the message. A recipient of a message that is signed-then- 1165 encrypted can assume that the signed message itself has not been 1166 altered, but that a careful attacker may have changed the 1167 unauthenticated portions of the encrypted message. 1169 When using compression, keep the following guidelines in mind: 1171 - Compression of binary encoded encrypted data is discouraged, since 1172 it will not yield significant compression. Base64 encrypted data 1173 could very well benefit, however. 1174 - If a lossy compression algorithm is used with signing, you will need 1175 to compress first, then sign. 1177 3.7 Creating a Certificate Management Message 1179 The certificate management message or MIME entity is used to transport 1180 certificates and/or certificate revocation lists, such as in response 1181 to a registration request. 1183 Step 1. The certificates and/or certificate revocation lists are made 1184 available to the CMS generating process which creates a CMS object of 1185 type SignedData. The SignedData encapContentInfo eContent field MUST 1186 be absent and signerInfos field MUST be empty. 1188 Step 2. The SignedData object is wrapped in a CMS ContentInfo 1189 object. 1191 Step 3. The ContentInfo object is enclosed in an application/pkcs7- 1192 mime MIME entity. 1194 The smime-type parameter for a certificate management message is 1195 "certs-only". The file extension for this type of message is ".p7c". 1197 3.8 Registration Requests 1199 A sending agent that signs messages MUST have a certificate for the 1200 signature so that a receiving agent can verify the signature. There 1201 are many ways of getting certificates, such as through an exchange 1202 with a certificate authority, through a hardware token or diskette, 1203 and so on. 1205 S/MIME v2 [SMIMEV2] specified a method for "registering" public keys 1206 with certificate authorities using an application/pkcs10 body part. 1207 Since that time, the IETF PKIX Working Group has developed other 1208 methods for requesting certificates. However, S/MIME v3.1 does not 1209 require a particular certificate request mechanism. 1211 3.9 Identifying an S/MIME Message 1213 Because S/MIME takes into account interoperation in non-MIME 1214 environments, several different mechanisms are employed to carry the 1215 type information, and it becomes a bit difficult to identify S/MIME 1216 messages. The following table lists criteria for determining whether 1217 or not a message is an S/MIME message. A message is considered an 1218 S/MIME message if it matches any of the criteria listed below. 1220 The file suffix in the table below comes from the "name" parameter in 1221 the content-type header, or the "filename" parameter on the content- 1222 disposition header. These parameters that give the file suffix are not 1223 listed below as part of the parameter section. 1225 MIME type: application/pkcs7-mime 1226 parameters: any 1227 file suffix: any 1229 MIME type: multipart/signed 1230 parameters: protocol="application/pkcs7-signature" 1231 file suffix: any 1233 MIME type: application/octet-stream 1234 parameters: any 1235 file suffix: p7m, p7s, p7c, p7z 1237 4. Certificate Processing 1239 A receiving agent MUST provide some certificate retrieval mechanism in 1240 order to gain access to certificates for recipients of digital 1241 envelopes. This specification does not cover how S/MIME agents handle 1242 certificates, only what they do after a certificate has been validated 1243 or rejected. S/MIME certificate issues are covered in [CERT31]. 1245 At a minimum, for initial S/MIME deployment, a user agent could 1246 automatically generate a message to an intended recipient requesting 1247 that recipient's certificate in a signed return message. Receiving and 1248 sending agents SHOULD also provide a mechanism to allow a user to 1249 "store and protect" certificates for correspondents in such a way so 1250 as to guarantee their later retrieval. 1252 4.1 Key Pair Generation 1254 All generated key pairs MUST be generated from a good source of non- 1255 deterministic random input [RANDOM] and the private key MUST be 1256 protected in a secure fashion. 1258 If an S/MIME agent needs to generate an RSA key pair, then the S/MIME 1259 agent or some related administrative utility or function SHOULD 1260 generate RSA key pairs using the following guidelines. A user agent 1261 SHOULD generate RSA key pairs at a minimum key size of 768 bits. A 1262 user agent MUST NOT generate RSA key pairs less than 512 bits long. 1263 Creating keys longer than 1024 bits may cause some older S/MIME 1264 receiving agents to not be able to verify signatures, but gives better 1265 security and is therefore valuable. A receiving agent SHOULD be able 1266 to verify signatures with keys of any size over 512 bits. Some agents 1267 created in the United States have chosen to create 512 bit keys in 1268 order to get more advantageous export licenses. However, 512 bit keys 1269 are considered by many to be cryptographically insecure. Implementers 1270 should be aware that multiple (active) key pairs may be associated 1271 with a single individual. For example, one key pair may be used to 1272 support confidentiality, while a different key pair may be used for 1273 authentication. 1275 5. Security 1277 40-bit encryption is considered weak by most cryptographers. Using 1278 weak cryptography in S/MIME offers little actual security over sending 1279 plaintext. However, other features of S/MIME, such as the 1280 specification of tripleDES and the ability to announce stronger 1281 cryptographic capabilities to parties with whom you communicate, allow 1282 senders to create messages that use strong encryption. Using weak 1283 cryptography is never recommended unless the only alternative is no 1284 cryptography. When feasible, sending and receiving agents should 1285 inform senders and recipients the relative cryptographic strength of 1286 messages. 1288 It is impossible for most software or people to estimate the value of 1289 a message. Further, it is impossible for most software or people to 1290 estimate the actual cost of decrypting a message that is encrypted 1291 with a key of a particular size. Further, it is quite difficult to 1292 determine the cost of a failed decryption if a recipient cannot decode 1293 a message. Thus, choosing between different key sizes (or choosing 1294 whether to just use plaintext) is also impossible. However, decisions 1295 based on these criteria are made all the time, and therefore this 1296 specification gives a framework for using those estimates in choosing 1297 algorithms. 1299 If a sending agent is sending the same message using different 1300 strengths of cryptography, an attacker watching the communications 1301 channel may be able to determine the contents of the strongly- 1302 encrypted message by decrypting the weakly-encrypted version. In other 1303 words, a sender should not send a copy of a message using weaker 1304 cryptography than they would use for the original of the message. 1306 Modification of the ciphertext can go undetected if authentication is 1307 not also used, which is the case when sending EnvelopedData without 1308 wrapping it in SignedData or enclosing SignedData within it. 1310 See RFC 3218 [MMA] for more information about thwarting the adaptive 1311 chosen ciphertext vulnerability in PKCS #1 Version 1.5 1312 implementations. 1314 In some circumstances the use of the Diffie-Hellman key agreement 1315 scheme in a prime order subgroup of a large prime p is vulnerable to 1316 certain attacks known as "small-subgroup" attacks. Methods exist, 1317 however, to prevent these attacks. These methods are described in RFC 1318 2785 [DHSUB]. 1320 A. ASN.1 Module 1322 SecureMimeMessageV3dot1 1323 { iso(1) member-body(2) us(840) rsadsi(113549) 1324 pkcs(1) pkcs-9(9) smime(16) modules(0) msg-v3dot1(21) } 1326 DEFINITIONS IMPLICIT TAGS ::= 1327 BEGIN 1329 IMPORTS 1330 -- Cryptographic Message Syntax 1331 SubjectKeyIdentifier, IssuerAndSerialNumber, 1332 RecipientKeyIdentifier 1333 FROM CryptographicMessageSyntax 1334 { iso(1) member-body(2) us(840) rsadsi(113549) 1335 pkcs(1) pkcs-9(9) smime(16) modules(0) cms-2001(14) }; 1337 -- id-aa is the arc with all new authenticated and unauthenticated 1338 -- attributes produced the by S/MIME Working Group 1340 id-aa OBJECT IDENTIFIER ::= {iso(1) member-body(2) usa(840) 1341 rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) attributes(2)} 1343 -- S/MIME Capabilities provides a method of broadcasting the symetric 1344 -- capabilities understood. Algorithms should be ordered by 1345 -- preference and grouped by type 1347 smimeCapabilities OBJECT IDENTIFIER ::= 1348 {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 15} 1350 SMIMECapability ::= SEQUENCE { 1351 capabilityID OBJECT IDENTIFIER, 1352 parameters ANY DEFINED BY capabilityID OPTIONAL } 1354 SMIMECapabilities ::= SEQUENCE OF SMIMECapability 1356 -- Encryption Key Preference provides a method of broadcasting the 1357 -- preferred encryption certificate. 1359 id-aa-encrypKeyPref OBJECT IDENTIFIER ::= {id-aa 11} 1361 SMIMEEncryptionKeyPreference ::= CHOICE { 1362 issuerAndSerialNumber [0] IssuerAndSerialNumber, 1363 receipentKeyId [1] RecipientKeyIdentifier, 1364 subjectAltKeyIdentifier [2] SubjectKeyIdentifier 1365 } 1367 id-smime OBJECT IDENTIFIER ::= { iso(1) member-body(2) 1368 us(840) rsadsi(113549) pkcs(1) pkcs9(9) 16 } 1370 id-cap OBJECT IDENTIFIER ::= { id-smime 11 } 1372 -- The preferBinaryInside indicates an ability to receive messages 1373 -- with binary encoding inside the CMS wrapper 1375 id-cap-preferBinaryInside OBJECT IDENTIFIER ::= { id-cap 1 } 1377 -- The following list the OIDs to be used with S/MIME V3 1379 -- Signature Algorithms Not Found in [CMSALG] 1380 -- 1381 -- md2WithRSAEncryption OBJECT IDENTIFIER ::= 1382 -- {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 1383 -- 2} 1384 -- 1385 -- Other Signed Attributes 1386 -- 1387 -- signingTime OBJECT IDENTIFIER ::= 1388 -- {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 1389 -- 5} 1390 -- See [CMS] for a description of how to encode the attribute 1391 -- value. 1393 SMIMECapabilitiesParametersForRC2CBC ::= INTEGER 1394 -- (RC2 Key Length (number of bits)) 1396 END 1398 B. Normative References 1400 [CERT31] "S/MIME Version 3.1 Certificate Handling", Internet Draft 1401 draft-ietf-smime-rfc2632bis 1403 [CHARSETS] Character sets assigned by IANA. See . 1406 [CMS] "Cryptographic Message Syntax", RFC 3369 1408 [CMSAES] "Use of the AES Encryption Algorithm in CMS", 1409 RFC 3565 1411 [CMSALG] "Cryptographic Message Syntax (CMS) Algorithms", RFC 3370 1413 [CMSCOMPR] "Compressed Data Content Type for Cryptographic Message 1414 Syntax (CMS)", RFC 3274 1416 [CONTDISP] "Communicating Presentation Information in Internet 1417 Messages: The Content-Disposition Header Field", RFC 2183 1419 [ESS] "Enhanced Security Services for S/MIME", RFC 2634 1421 [FIPS180-2] "Secure Hash Signature Standard (SHS)", National Institute 1422 of Standards and Technology (NIST). FIPS Publication 180-2 1424 [MIME-SPEC] The primary definition of MIME. "MIME Part 1: Format of 1425 Internet Message Bodies", RFC 2045; "MIME Part 2: Media Types", RFC 1426 2046; "MIME Part 3: Message Header Extensions for Non-ASCII Text", RFC 1427 2047; "MIME Part 4: Registration Procedures", RFC 2048; "MIME Part 5: 1428 Conformance Criteria and Examples", RFC 2049 1430 [MIME-SECURE] "Security Multiparts for MIME: Multipart/Signed and 1431 Multipart/Encrypted", RFC 1847 1433 [MUSTSHOULD] "Key words for use in RFCs to Indicate Requirement 1434 Levels", RFC 2119 1436 [X.208-88] CCITT. Recommendation X.208: Specification of Abstract 1437 Syntax Notation One (ASN.1). 1988. 1439 [X.209-88] CCITT. Recommendation X.209: Specification of Basic 1440 Encoding Rules for Abstract Syntax Notation One (ASN.1). 1988. 1442 [X.509-88] CCITT. Recommendation X.509: The Directory - Authentication 1443 Framework. 1988. 1445 C. Informative References 1447 [DHSUB] "Methods for Avoiding the "Small-Subgroup" Attacks on the 1448 Diffie-Hellman Key Agreement Method for S/MIME", RFC 2785 1450 [MMA] "Preventing the Million Message Attack on CMS", RFC 3218 1452 [PKCS-7] "PKCS #7: Cryptographic Message Syntax Version 1.5", RFC 2315 1454 [RANDOM] "Randomness Recommendations for Security", RFC 1750 1456 [SMIMEV2] "S/MIME Version 2 Message Specification", RFC 2311 1458 D. Acknowledgements 1460 Many thanks go out to the other authors of the S/MIME Version 2 1461 Message Specification RFC: Steve Dusse, Paul Hoffman, Laurence 1462 Lundblade and Lisa Repka. 1464 A number of the members of the S/MIME Working Group have also worked 1465 very hard and contributed to this document. Any list of people is 1466 doomed to omission, and for that I apologize. In alphabetical order, 1467 the following people stand out in my mind due to the fact that they 1468 made direct contributions to this document. 1470 Tony Capel 1471 Piers Chivers 1472 Dave Crocker 1473 Bill Flanigan 1474 Peter Gutmann 1475 Paul Hoffman 1476 Russ Housley 1477 William Ottaway 1478 John Pawling 1479 Jim Schaad 1481 E. Editor's address 1483 Blake Ramsdell 1484 Sendmail, Inc. 1485 704 228th Ave NE #775 1486 Sammamish, WA 98074 1488 blake@sendmail.com