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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 (July 16, 2003) is 7589 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 1371 looks like a reference -- Missing reference section? 'CMS' on line 1356 looks like a reference -- Missing reference section? 'PKCS-7' on line 1385 looks like a reference -- Missing reference section? 'MIME-SECURE' on line 1377 looks like a reference -- Missing reference section? 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'2' on line 1317 looks like a reference -- Missing reference section? 'CERT31' on line 1350 looks like a reference Summary: 6 errors (**), 0 flaws (~~), 3 warnings (==), 21 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-03.txt Brute Squad Labs 3 January 16, 2003 4 Expires July 16, 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 has to 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. 104 BER: Basic Encoding Rules for ASN.1, as defined in CCITT X.209. 106 Certificate: A type that binds an entity's distinguished name to a 107 public key with a digital signature. 109 DER: Distinguished Encoding Rules for ASN.1, as defined in CCITT 110 X.509. 112 7-bit data: Text data with lines less than 998 characters long, where 113 none of the characters have the 8th bit set, and there are no NULL 114 characters. and occur only as part of a end of line 115 delimiter. 117 8-bit data: Text data with lines less than 998 characters, and where 118 none of the characters are NULL characters. and occur only 119 as part of a end of line delimiter. 121 Binary data: Arbitrary data. 123 Transfer Encoding: A reversible transformation made on data so 8-bit 124 or binary data may be sent via a channel that only transmits 7-bit 125 data. 127 Receiving agent: software that interprets and processes S/MIME CMS 128 objects, MIME body parts that contain CMS objects, or both. 130 Sending agent: software that creates S/MIME CMS objects, MIME body 131 parts that contain CMS objects, or both. 133 S/MIME agent: user software that is a receiving agent, a sending 134 agent, or both. 136 1.4 Compatibility with Prior Practice of S/MIME 138 S/MIME version 3 agents should attempt to have the greatest 139 interoperability possible with S/MIME version 2 agents. S/MIME version 140 2 is described in RFC 2311 through RFC 2315, inclusive. RFC 2311 also 141 has historical information about the development of S/MIME. 143 1.5 Changes Since S/MIME v3.0 145 The RSA public key algorithm was changed to a MUST implement key 146 wrapping algorithm, and the Diffie-Hellman algorithm changed to a 147 SHOULD implement. 149 The RSA public key algorithm was changed to a MUST implement signature 150 algorithm. 152 Ambiguous language about the use of "empty" SignedData messages to 153 transmit certificates was clarified to reflect that transmission of 154 certificate revocation lists is also allowed. 156 The use of binary encoding for some MIME entities is now explicitly 157 discussed. 159 Header protection through the use of the message/rfc822 MIME type has 160 been added. 162 Use of the CompressedData CMS type is allowed, along with required 163 MIME type and file extension additions. 165 1.6 Discussion of This Specification 167 This specification is being discussed on the "ietf-smime" mailing 168 list. To subscribe, send a message to: 170 ietf-smime-request@imc.org 172 with the single word 174 subscribe 176 in the body of the message. There is a Web site for the mailing list 177 at . 179 2. CMS Options 181 CMS allows for a wide variety of options in content and algorithm 182 support. This section puts forth a number of support requirements and 183 recommendations in order to achieve a base level of interoperability 184 among all S/MIME implementations. [CMS] provides additional details 185 regarding the use of the cryptographic algorithms. 187 2.1 DigestAlgorithmIdentifier 189 Sending and receiving agents MUST support SHA-1 [CMSALG]. Receiving 190 agents SHOULD support MD5 [CMSALG] for the purpose of providing 191 backward compatibility with MD5-digested S/MIME v2 SignedData objects. 193 2.2 SignatureAlgorithmIdentifier 195 Receiving agents MUST support id-dsa defined in [CMSALG]. The 196 algorithm parameters MUST be absent (not encoded as NULL). Receiving 197 agents MUST support rsaEncryption, defined in [CMSALG]. 199 Sending agents MUST support either id-dsa or rsaEncryption. 201 Note that S/MIME v3 clients might only implement signing or signature 202 verification using id-dsa. Also note that S/MIME v2 clients are only 203 capable of verifying digital signatures using the rsaEncryption 204 algorithm. 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]. 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 CMS defines multiple content types. Of these, only the Data, 222 SignedData, and EnvelopedData content types are currently used for 223 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. 246 2.4.3 EnvelopedData Content Type 248 This content type is used to apply data confidentiality to a message. 249 A sender needs to have access to a public key for each intended 250 message recipient to use this service. This content type does not 251 provide authentication. 253 2.5 Attribute SignerInfo Type 255 The SignerInfo type allows the inclusion of unsigned and signed 256 attributes to be included along with a signature. 258 Receiving agents MUST be able to handle zero or one instance of each 259 of the signed attributes listed here. Sending agents SHOULD generate 260 one instance of each of the following signed attributes in each S/MIME 261 message: 263 - signingTime (section 2.5.1 in this document) 264 - sMIMECapabilities (section 2.5.2 in this document) 265 - sMIMEEncryptionKeyPreference (section 2.5.3 in this document) 267 Further, receiving agents SHOULD be able to handle zero or one 268 instance in the signed attributes of the signingCertificate attribute 269 (section 5 in [ESS]). 271 Sending agents SHOULD generate one instance of the signingCertificate 272 signed attribute in each S/MIME message. 274 Additional attributes and values for these attributes may be defined 275 in the future. Receiving agents SHOULD handle attributes or values 276 that it does not recognize in a graceful manner. 278 Sending agents that include signed attributes that are not listed here 279 SHOULD display those attributes to the user, so that the user is aware 280 of all of the data being signed. 282 2.5.1 Signing-Time Attribute 284 The signing-time attribute is used to convey the time that a message 285 was signed. The time of signing will most likely be created by a 286 message originator and therefore is only as trustworthy as the 287 originator. 289 Sending agents MUST encode signing time through the year 2049 as 290 UTCTime; signing times in 2050 or later MUST be encoded as 291 GeneralizedTime. When the UTCTime CHOICE is used, S/MIME agents MUST 292 interpret the year field (YY) as follows: 294 if YY is greater than or equal to 50, the year is interpreted as 19YY; 295 if YY is less than 50, the year is interpreted as 20YY. 297 2.5.2 SMIMECapabilities Attribute 299 The SMIMECapabilities attribute includes signature algorithms (such as 300 "sha1WithRSAEncryption"), symmetric algorithms (such as "DES-EDE3- 301 CBC"), and key encipherment algorithms (such as "rsaEncryption"). 302 There are also several identifiers which indicate support for other 303 optional features such as binary encoding and compression. The 304 SMIMECapabilities were designed to be flexible and extensible so that, 305 in the future, a means of identifying other capabilities and 306 preferences such as certificates can be added in a way that will not 307 cause current clients to break. 309 If present, the SMIMECapabilities attribute MUST be a SignedAttribute; 310 it MUST NOT be an UnsignedAttribute. CMS defines SignedAttributes as a 311 SET OF Attribute. The SignedAttributes in a signerInfo MUST NOT 312 include multiple instances of the SMIMECapabilities attribute. CMS 313 defines the ASN.1 syntax for Attribute to include attrValues SET OF 314 AttributeValue. A SMIMECapabilities attribute MUST only include a 315 single instance of AttributeValue. There MUST NOT be zero or multiple 316 instances of AttributeValue present in the attrValues SET OF 317 AttributeValue. 319 The semantics of the SMIMECapabilites attribute specify a partial list 320 as to what the client announcing the SMIMECapabilites can support. A 321 client does not have to list every capability it supports, and 322 probably should not list all its capabilities so that the capabilities 323 list doesn't get too long. In an SMIMECapabilities attribute, the OIDs 324 are listed in order of their preference, but SHOULD be logically 325 separated along the lines of their categories (signature algorithms, 326 symmetric algorithms, key encipherment algorithms, etc.) 328 The structure of the SMIMECapabilities attribute is to facilitate 329 simple table lookups and binary comparisons in order to determine 330 matches. For instance, the DER-encoding for the SMIMECapability for 331 DES EDE3 CBC MUST be identically encoded regardless of the 332 implementation. 334 In the case of symmetric algorithms, the associated parameters for the 335 OID MUST specify all of the parameters necessary to differentiate 336 between two instances of the same algorithm. For instance, the number 337 of rounds and block size for RC5 must be specified in addition to the 338 key length. 340 There is a list of OIDs (OIDs Used with S/MIME) that is centrally 341 maintained and is separate from this specification. The list of OIDs 342 is maintained by the Internet Mail Consortium at 343 . Note that all OIDs 344 associated with the MUST and SHOULD implement algorithms are included 345 in section A of this document. 347 The OIDs that correspond to algorithms SHOULD use the same OID as the 348 actual algorithm, except in the case where the algorithm usage is 349 ambiguous from the OID. For instance, in an earlier specification, 350 rsaEncryption was ambiguous because it could refer to either a 351 signature algorithm or a key encipherment algorithm. In the event that 352 an OID is ambiguous, it needs to be arbitrated by the maintainer of 353 the registered SMIMECapabilities list as to which type of algorithm 354 will use the OID, and a new OID MUST be allocated under the 355 smimeCapabilities OID to satisfy the other use of the OID. 357 The registered SMIMECapabilities list specifies the parameters for 358 OIDs that need them, most notably key lengths in the case of variable- 359 length symmetric ciphers. In the event that there are no 360 differentiating parameters for a particular OID, the parameters MUST 361 be omitted, and MUST NOT be encoded as NULL. 363 Additional values for the SMIMECapabilities attribute may be defined 364 in the future. Receiving agents MUST handle a SMIMECapabilities object 365 that has values that it does not recognize in a graceful manner. 367 2.5.3 Encryption Key Preference Attribute 369 The encryption key preference attribute allows the signer to 370 unambiguously describe which of the signer's certificates has the 371 signer's preferred encryption key. This attribute is designed to 372 enhance behavior for interoperating with those clients which use 373 separate keys for encryption and signing. This attribute is used to 374 convey to anyone viewing the attribute which of the listed 375 certificates should be used for encrypting a session key for future 376 encrypted messages. 378 If present, the SMIMEEncryptionKeyPreference attribute MUST be a 379 SignedAttribute; it MUST NOT be an UnsignedAttribute. CMS defines 380 SignedAttributes as a SET OF Attribute. The SignedAttributes in a 381 signerInfo MUST NOT include multiple instances of the 382 SMIMEEncryptionKeyPreference attribute. CMS defines the ASN.1 syntax 383 for Attribute to include attrValues SET OF AttributeValue. A 384 SMIMEEncryptionKeyPreference attribute MUST only include a single 385 instance of AttributeValue. There MUST NOT be zero or multiple 386 instances of AttributeValue present in the attrValues SET OF 387 AttributeValue. 389 The sending agent SHOULD include the referenced certificate in the set 390 of certificates included in the signed message if this attribute is 391 used. The certificate may be omitted if it has been previously made 392 available to the receiving agent. Sending agents SHOULD use this 393 attribute if the commonly used or preferred encryption certificate is 394 not the same as the certificate used to sign the message. 396 Receiving agents SHOULD store the preference data if the signature on 397 the message is valid and the signing time is greater than the 398 currently stored value. (As with the SMIMECapabilities, the clock skew 399 should be checked and the data not used if the skew is too great.) 400 Receiving agents SHOULD respect the sender's encryption key preference 401 attribute if possible. This however represents only a preference and 402 the receiving agent may use any certificate in replying to the sender 403 that is valid. 405 2.5.3.1 Selection of Recipient Key Management Certificate 407 In order to determine the key management certificate to be used when 408 sending a future CMS envelopedData message for a particular recipient, 409 the following steps SHOULD be followed: 411 - If an SMIMEEncryptionKeyPreference attribute is found in a 412 signedData object received from the desired recipient, this 413 identifies the X.509 certificate that should be used as the X.509 414 key management certificate for the recipient. 416 - If an SMIMEEncryptionKeyPreference attribute is not found in a 417 signedData object received from the desired recipient, the set of 418 X.509 certificates should be searched for a X.509 certificate with 419 the same subject name as the signing X.509 certificate which can be 420 used for key management. 422 - Or use some other method of determining the user's key management 423 key. If a X.509 key management certificate is not found, then 424 encryption cannot be done with the signer of the message. If 425 multiple X.509 key management certificates are found, the S/MIME 426 agent can make an arbitrary choice between them. 428 2.6 SignerIdentifier SignerInfo Type 430 S/MIME v3 requires the use of SignerInfo version 1, that is the 431 issuerAndSerialNumber CHOICE MUST be used for SignerIdentifier. 433 2.7 ContentEncryptionAlgorithmIdentifier 435 Sending and receiving agents MUST support encryption and decryption 436 with DES EDE3 CBC, hereinafter called "tripleDES" [CMSALG]. Receiving 437 agents SHOULD support encryption and decryption using the RC2 [CMSALG] 438 or a compatible algorithm at a key size of 40 bits, hereinafter called 439 "RC2/40". 441 2.7.1 Deciding Which Encryption Method To Use 443 When a sending agent creates an encrypted message, it has to decide 444 which type of encryption to use. The decision process involves using 445 information garnered from the capabilities lists included in messages 446 received from the recipient, as well as out-of-band information such 447 as private agreements, user preferences, legal restrictions, and so 448 on. 450 Section 2.5 defines a method by which a sending agent can optionally 451 announce, among other things, its decrypting capabilities in its order 452 of preference. The following method for processing and remembering the 453 encryption capabilities attribute in incoming signed messages SHOULD 454 be used. 456 - If the receiving agent has not yet created a list of capabilities 457 for the sender's public key, then, after verifying the signature on 458 the incoming message and checking the timestamp, the receiving agent 459 SHOULD create a new list containing at least the signing time and 460 the symmetric capabilities. 462 - If such a list already exists, the receiving agent SHOULD verify 463 that the signing time in the incoming message is greater than the 464 signing time stored in the list and that the signature is valid. If 465 so, the receiving agent SHOULD update both the signing time and 466 capabilities in the list. Values of the signing time that lie far in 467 the future (that is, a greater discrepancy than any reasonable clock 468 skew), or a capabilities list in messages whose signature could not 469 be verified, MUST NOT be accepted. 471 The list of capabilities SHOULD be stored for future use in creating 472 messages. 474 Before sending a message, the sending agent MUST decide whether it is 475 willing to use weak encryption for the particular data in the message. 476 If the sending agent decides that weak encryption is unacceptable for 477 this data, then the sending agent MUST NOT use a weak algorithm such 478 as RC2/40. The decision to use or not use weak encryption overrides 479 any other decision in this section about which encryption algorithm to 480 use. 482 Sections 2.7.2.1 through 2.7.2.4 describe the decisions a sending 483 agent SHOULD use in deciding which type of encryption should be 484 applied to a message. These rules are ordered, so the sending agent 485 SHOULD make its decision in the order given. 487 2.7.1.1 Rule 1: Known Capabilities 489 If the sending agent has received a set of capabilities from the 490 recipient for the message the agent is about to encrypt, then the 491 sending agent SHOULD use that information by selecting the first 492 capability in the list (that is, the capability most preferred by the 493 intended recipient) for which the sending agent knows how to encrypt. 494 The sending agent SHOULD use one of the capabilities in the list if 495 the agent reasonably expects the recipient to be able to decrypt the 496 message. 498 2.7.1.2 Rule 2: Unknown Capabilities, Known Use of Encryption 500 If: 501 - the sending agent has no knowledge of the encryption capabilities 502 of the recipient, 503 - and the sending agent has received at least one message from the 504 recipient, 505 - and the last encrypted message received from the recipient had a 506 trusted signature on it, 507 then the outgoing message SHOULD use the same encryption algorithm as 508 was used on the last signed and encrypted message received from the 509 recipient. 511 2.7.1.3 Rule 3: Unknown Capabilities, Unknown Version of S/MIME 513 If: 514 - the sending agent has no knowledge of the encryption capabilities 515 of the recipient, 516 - and the sending agent has no knowledge of the version of S/MIME 517 of the recipient, 518 then the sending agent SHOULD use tripleDES because it is a stronger 519 algorithm and is required by S/MIME v3. If the sending agent chooses 520 not to use tripleDES in this step, it SHOULD use RC2/40. 522 2.7.2 Choosing Weak Encryption 524 Like all algorithms that use 40 bit keys, RC2/40 is considered by many 525 to be weak encryption. A sending agent that is controlled by a human 526 SHOULD allow a human sender to determine the risks of sending data 527 using RC2/40 or a similarly weak encryption algorithm before sending 528 the data, and possibly allow the human to use a stronger encryption 529 method such as tripleDES. 531 2.7.3 Multiple Recipients 533 If a sending agent is composing an encrypted message to a group of 534 recipients where the encryption capabilities of some of the recipients 535 do not overlap, the sending agent is forced to send more than one 536 message. It should be noted that if the sending agent chooses to send 537 a message encrypted with a strong algorithm, and then send the same 538 message encrypted with a weak algorithm, someone watching the 539 communications channel may be able to learn the contents of the 540 strongly-encrypted message simply by decrypting the weakly-encrypted 541 message. 543 3. Creating S/MIME Messages 545 This section describes the S/MIME message formats and how they are 546 created. S/MIME messages are a combination of MIME bodies and CMS 547 objects. Several MIME types as well as several CMS objects are used. 548 The data to be secured is always a canonical MIME entity. The MIME 549 entity and other data, such as certificates and algorithm identifiers, 550 are given to CMS processing facilities which produces a CMS object. 551 The CMS object is then finally wrapped in MIME. The Enhanced Security 552 Services for S/MIME [ESS] document provides examples of how nested, 553 secured S/MIME messages are formatted. ESS provides an example of how 554 a triple-wrapped S/MIME message is formatted using multipart/signed 555 and application/pkcs7-mime for the signatures. 557 S/MIME provides one format for enveloped-only data, several formats 558 for signed-only data, and several formats for signed and enveloped 559 data. Several formats are required to accommodate several 560 environments, in particular for signed messages. The criteria for 561 choosing among these formats are also described. 563 The reader of this section is expected to understand MIME as described 564 in [MIME-SPEC] and [MIME-SECURE]. 566 3.1 Preparing the MIME Entity for Signing or Enveloping 568 S/MIME is used to secure MIME entities. A MIME entity may be a sub- 569 part, sub-parts of a message, or the whole message with all its sub- 570 parts. A MIME entity that is the whole message includes only the MIME 571 headers and MIME body, and does not include the RFC-822 headers. Note 572 that S/MIME can also be used to secure MIME entities used in 573 applications other than Internet mail. 575 The MIME entity that is secured and described in this section can be 576 thought of as the "inside" MIME entity. That is, it is the "innermost" 577 object in what is possibly a larger MIME message. Processing "outside" 578 MIME entities into CMS objects is described in Section 3.2, 3.4 and 579 elsewhere. 581 The procedure for preparing a MIME entity is given in [MIME-SPEC]. The 582 same procedure is used here with some additional restrictions when 583 signing. Description of the procedures from [MIME-SPEC] are repeated 584 here, but the reader should refer to that document for the exact 585 procedure. This section also describes additional requirements. 587 A single procedure is used for creating MIME entities that are to be 588 signed, enveloped, or both signed and enveloped. Some additional steps 589 are recommended to defend against known corruptions that can occur 590 during mail transport that are of particular importance for clear- 591 signing using the multipart/signed format. It is recommended that 592 these additional steps be performed on enveloped messages, or signed 593 and enveloped messages in order that the message can be forwarded to 594 any environment without modification. 596 These steps are descriptive rather than prescriptive. The implementor 597 is free to use any procedure as long as the result is the same. 599 Step 1. The MIME entity is prepared according to the local conventions 601 Step 2. The leaf parts of the MIME entity are converted to canonical 602 form 604 Step 3. Appropriate transfer encoding is applied to the leaves of the 605 MIME entity 607 When an S/MIME message is received, the security services on the 608 message are processed, and the result is the MIME entity. That MIME 609 entity is typically passed to a MIME-capable user agent where, it is 610 further decoded and presented to the user or receiving application. 612 In order to protect outer, non-content related message headers (for 613 instance, the "Subject", "To", "From" and "CC" fields), the sending 614 client MAY wrap a full MIME message in a message/rfc822 wrapper in 615 order to apply S/MIME security services to these headers. It is up to 616 the receiving client to decide how to present these "inner" headers 617 along with the unprotected "outer" headers. 619 3.1.1 Canonicalization 621 Each MIME entity MUST be converted to a canonical form that is 622 uniquely and unambiguously representable in the environment where the 623 signature is created and the environment where the signature will be 624 verified. MIME entities MUST be canonicalized for enveloping as well 625 as signing. 627 The exact details of canonicalization depend on the actual MIME type 628 and subtype of an entity, and are not described here. Instead, the 629 standard for the particular MIME type should be consulted. For 630 example, canonicalization of type text/plain is different from 631 canonicalization of audio/basic. Other than text types, most types 632 have only one representation regardless of computing platform or 633 environment which can be considered their canonical representation. In 634 general, canonicalization will be performed by the non-security part 635 of the sending agent rather than the S/MIME implementation. 637 The most common and important canonicalization is for text, which is 638 often represented differently in different environments. MIME entities 639 of major type "text" must have both their line endings and character 640 set canonicalized. The line ending must be the pair of characters 641 , and the charset should be a registered charset [CHARSETS]. 642 The details of the canonicalization are specified in [MIME-SPEC]. The 643 chosen charset SHOULD be named in the charset parameter so that the 644 receiving agent can unambiguously determine the charset used. 646 Note that some charsets such as ISO-2022 have multiple representations 647 for the same characters. When preparing such text for signing, the 648 canonical representation specified for the charset MUST be used. 650 3.1.2 Transfer Encoding 652 When generating any of the secured MIME entities below, except the 653 signing using the multipart/signed format, no transfer encoding at all 654 is required. S/MIME implementations MUST be able to deal with binary 655 MIME objects. If no Content-Transfer-Encoding header is present, the 656 transfer encoding should be considered 7BIT. 658 S/MIME implementations SHOULD however use transfer encoding described 659 in section 3.1.3 for all MIME entities they secure. The reason for 660 securing only 7-bit MIME entities, even for enveloped data that are 661 not exposed to the transport, is that it allows the MIME entity to be 662 handled in any environment without changing it. For example, a trusted 663 gateway might remove the envelope, but not the signature, of a 664 message, and then forward the signed message on to the end recipient 665 so that they can verify the signatures directly. If the transport 666 internal to the site is not 8-bit clean, such as on a wide-area 667 network with a single mail gateway, verifying the signature will not 668 be possible unless the original MIME entity was only 7-bit data. 670 S/MIME implementations which "know" that all intended recipient(s) are 671 capable of handling inner (all but the outermost) binary MIME objects 672 SHOULD NOT use 7-bit transfer encoding for the inner entities since 673 this would unnecessarily expand the message size. Implementations MAY 674 "know" that recipient implementations are capable of handling inner 675 binary MIME entities either by interpreting the 676 id-cap-preferBinaryInside sMIMECapabilities attribute, by prior 677 agreement, or by other means. 679 If one or more intended recipients are unable to handle inner binary 680 MIME objects, or if this capability in unknown for any of the intended 681 recipients, S/MIME implementations SHOULD use transfer encoding 682 described in section 3.1.3 for all MIME entities they secure. 684 3.1.3 Transfer Encoding for Signing Using multipart/signed 686 If a multipart/signed entity is EVER to be transmitted over the 687 standard Internet SMTP infrastructure or other transport that is 688 constrained to 7-bit text, it MUST have transfer encoding applied so 689 that it is represented as 7-bit text. MIME entities that are 7-bit 690 data already need no transfer encoding. Entities such as 8-bit text 691 and binary data can be encoded with quoted-printable or base-64 692 transfer encoding. 694 The primary reason for the 7-bit requirement is that the Internet mail 695 transport infrastructure cannot guarantee transport of 8-bit or binary 696 data. Even though many segments of the transport infrastructure now 697 handle 8-bit and even binary data, it is sometimes not possible to 698 know whether the transport path is 8-bit clear. If a mail message with 699 8-bit data were to encounter a message transfer agent that can not 700 transmit 8-bit or binary data, the agent has three options, none of 701 which are acceptable for a clear-signed message: 703 - The agent could change the transfer encoding; this would invalidate 704 the signature. 705 - The agent could transmit the data anyway, which would most likely 706 result in the 8th bit being corrupted; this too would invalidate the 707 signature. 708 - The agent could return the message to the sender. 710 [MIME-SECURE] prohibits an agent from changing the transfer encoding 711 of the first part of a multipart/signed message. If a compliant agent 712 that can not transmit 8-bit or binary data encounters a 713 multipart/signed message with 8-bit or binary data in the first part, 714 it would have to return the message to the sender as undeliverable. 716 3.1.4 Sample Canonical MIME Entity 718 This example shows a multipart/mixed message with full transfer 719 encoding. This message contains a text part and an attachment. The 720 sample message text includes characters that are not US-ASCII and thus 721 must be transfer encoded. Though not shown here, the end of each line 722 is . The line ending of the MIME headers, the text, and 723 transfer encoded parts, all must be . 725 Note that this example is not of an S/MIME message. 727 Content-Type: multipart/mixed; boundary=bar 729 --bar 730 Content-Type: text/plain; charset=iso-8859-1 731 Content-Transfer-Encoding: quoted-printable 733 =A1Hola Michael! 735 How do you like the new S/MIME specification? 737 It's generally a good idea to encode lines that begin with 738 From=20because some mail transport agents will insert a greater- 739 than (>) sign, thus invalidating the signature. 741 Also, in some cases it might be desirable to encode any =20 742 trailing whitespace that occurs on lines in order to ensure =20 743 that the message signature is not invalidated when passing =20 744 a gateway that modifies such whitespace (like BITNET). =20 746 --bar 747 Content-Type: image/jpeg 748 Content-Transfer-Encoding: base64 750 iQCVAwUBMJrRF2N9oWBghPDJAQE9UQQAtl7LuRVndBjrk4EqYBIb3h5QXIX/LC// 751 jJV5bNvkZIGPIcEmI5iFd9boEgvpirHtIREEqLQRkYNoBActFBZmh9GC3C041WGq 752 uMbrbxc+nIs1TIKlA08rVi9ig/2Yh7LFrK5Ein57U/W72vgSxLhe/zhdfolT9Brn 753 HOxEa44b+EI= 755 --bar-- 757 3.2 The application/pkcs7-mime Type 759 The application/pkcs7-mime type is used to carry CMS objects of 760 several types including envelopedData and signedData. The details of 761 constructing these entities is described in subsequent sections. This 762 section describes the general characteristics of the 763 application/pkcs7- mime type. 765 The carried CMS object always contains a MIME entity that is prepared 766 as described in section 3.1 if the eContentType is id-data. Other 767 contents may be carried when the eContentType contains different 768 values. See [ESS] for an example of this with signed receipts. 770 Since CMS objects are binary data, in most cases base-64 transfer 771 encoding is appropriate, in particular when used with SMTP transport. 772 The transfer encoding used depends on the transport through which the 773 object is to be sent, and is not a characteristic of the MIME type. 775 Note that this discussion refers to the transfer encoding of the CMS 776 object or "outside" MIME entity. It is completely distinct from, and 777 unrelated to, the transfer encoding of the MIME entity secured by the 778 CMS object, the "inside" object, which is described in section 3.1. 780 Because there are several types of application/pkcs7-mime objects, a 781 sending agent SHOULD do as much as possible to help a receiving agent 782 know about the contents of the object without forcing the receiving 783 agent to decode the ASN.1 for the object. The MIME headers of all 784 application/pkcs7-mime objects SHOULD include the optional "smime- 785 type" parameter, as described in the following sections. 787 3.2.1 The name and filename Parameters 789 For the application/pkcs7-mime, sending agents SHOULD emit the 790 optional "name" parameter to the Content-Type field for compatibility 791 with older systems. Sending agents SHOULD also emit the optional 792 Content-Disposition field [CONTDISP] with the "filename" parameter. If 793 a sending agent emits the above parameters, the value of the 794 parameters SHOULD be a file name with the appropriate extension: 796 MIME Type File Extension 798 Application/pkcs7-mime (signedData, envelopedData) .p7m 800 Application/pkcs7-mime (degenerate signedData .p7c 801 certificate management message) 803 Application/pkcs7-mime (compressedData) .p7z 805 Application/pkcs7-signature .p7s 807 In addition, the file name SHOULD be limited to eight characters 808 followed by a three letter extension. The eight character filename 809 base can be any distinct name; the use of the filename base "smime" 810 SHOULD be used to indicate that the MIME entity is associated with 811 S/MIME. 813 Including a file name serves two purposes. It facilitates easier use 814 of S/MIME objects as files on disk. It also can convey type 815 information across gateways. When a MIME entity of type 816 application/pkcs7-mime (for example) arrives at a gateway that has no 817 special knowledge of S/MIME, it will default the entity's MIME type to 818 application/octet-stream and treat it as a generic attachment, thus 819 losing the type information. However, the suggested filename for an 820 attachment is often carried across a gateway. This often allows the 821 receiving systems to determine the appropriate application to hand the 822 attachment off to, in this case a stand-alone S/MIME processing 823 application. Note that this mechanism is provided as a convenience for 824 implementations in certain environments. A proper S/MIME 825 implementation MUST use the MIME types and MUST NOT rely on the file 826 extensions. 828 3.2.2 The smime-type parameter 830 The application/pkcs7-mime content type defines the optional "smime- 831 type" parameter. The intent of this parameter is to convey details 832 about the security applied (signed or enveloped) along with infomation 833 about the contained content. This specification defines the following 834 smime- types. 836 Name CMS type Inner Content 838 enveloped-data EnvelopedData id-data 840 signed-data SignedData id-data 842 certs-only SignedData none 844 compressed-data CompressedData id-data 846 In order that consistency can be obtained with future, the following 847 guidelines should be followed when assigning a new smime-type 848 parameter. 850 1. If both signing and encryption can be applied to the content, then 851 two values for smime-type SHOULD be assigned "signed-*" and 852 "encrypted- *". If one operation can be assigned then this may be 853 omitted. Thus since "certs-only" can only be signed, "signed-" is 854 omitted. 856 2. A common string for a content oid should be assigned. We use "data" 857 for the id-data content OID when MIME is the inner content. 859 3. If no common string is assigned. Then the common string of 860 "OID." is recommended (for example, "OID.1.3.6.1.5.5.7.6.1" would 861 be DES40). 863 3.3 Creating an Enveloped-only Message 865 This section describes the format for enveloping a MIME entity without 866 signing it. It is important to note that sending enveloped but not 867 signed messages does not provide for data integrity. It is possible to 868 replace ciphertext in such a way that the processed message will still 869 be valid, but the meaning may be altered. 871 Step 1. The MIME entity to be enveloped is prepared according to 872 section 3.1. 874 Step 2. The MIME entity and other required data is processed into a 875 CMS object of type envelopedData. In addition to encrypting a copy of 876 the content-encryption key for each recipient, a copy of the content- 877 encryption key SHOULD be encrypted for the originator and included in 878 the envelopedData (see CMS Section 6). 880 Step 3. The envelopedData object is wrapped in a CMS ContentInfo 881 object. 883 Step 4. The ContentInfo object is inserted into an 884 application/pkcs7-mime MIME entity. 886 The smime-type parameter for enveloped-only messages is "enveloped- 887 data". The file extension for this type of message is ".p7m". 889 A sample message would be: 891 Content-Type: application/pkcs7-mime; smime-type=enveloped-data; 892 name=smime.p7m 893 Content-Transfer-Encoding: base64 894 Content-Disposition: attachment; filename=smime.p7m 896 rfvbnj756tbBghyHhHUujhJhjH77n8HHGT9HG4VQpfyF467GhIGfHfYT6 897 7n8HHGghyHhHUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H 898 f8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4 899 0GhIGfHfQbnj756YT64V 901 3.4 Creating a Signed-only Message 903 There are two formats for signed messages defined for S/MIME: 904 application/pkcs7-mime with SignedData, and multipart/signed. In 905 general, the multipart/signed form is preferred for sending, and 906 receiving agents SHOULD be able to handle both. 908 3.4.1 Choosing a Format for Signed-only Messages 910 There are no hard-and-fast rules when a particular signed-only format 911 should be chosen because it depends on the capabilities of all the 912 receivers and the relative importance of receivers with S/MIME 913 facilities being able to verify the signature versus the importance of 914 receivers without S/MIME software being able to view the message. 916 Messages signed using the multipart/signed format can always be viewed 917 by the receiver whether they have S/MIME software or not. They can 918 also be viewed whether they are using a MIME-native user agent or they 919 have messages translated by a gateway. In this context, "be viewed" 920 means the ability to process the message essentially as if it were not 921 a signed message, including any other MIME structure the message might 922 have. 924 Messages signed using the signedData format cannot be viewed by a 925 recipient unless they have S/MIME facilities. However, if they have 926 S/MIME facilities, these messages can always be verified if they were 927 not changed in transit. 929 3.4.2 Signing Using application/pkcs7-mime with SignedData 931 This signing format uses the application/pkcs7-mime MIME type. The 932 steps to create this format are: 934 Step 1. The MIME entity is prepared according to section 3.1 936 Step 2. The MIME entity and other required data is processed into a 937 CMS object of type signedData 939 Step 3. The signedData object is wrapped in a CMS ContentInfo 940 object. 942 Step 4. The ContentInfo object is inserted into an 943 application/pkcs7-mime MIME entity. 945 The smime-type parameter for messages using application/pkcs7-mime 946 with SignedData is "signed-data". The file extension for this type of 947 message is ".p7m". 949 A sample message would be: 951 Content-Type: application/pkcs7-mime; smime-type=signed-data; 952 name=smime.p7m 953 Content-Transfer-Encoding: base64 954 Content-Disposition: attachment; filename=smime.p7m 956 567GhIGfHfYT6ghyHhHUujpfyF4f8HHGTrfvhJhjH776tbB9HG4VQbnj7 957 77n8HHGT9HG4VQpfyF467GhIGfHfYT6rfvbnj756tbBghyHhHUujhJhjH 958 HUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H7n8HHGghyHh 959 6YT64V0GhIGfHfQbnj75 961 3.4.3 Signing Using the multipart/signed Format 963 This format is a clear-signing format. Recipients without any S/MIME 964 or CMS processing facilities are able to view the message. It makes 965 use of the multipart/signed MIME type described in [MIME-SECURE]. The 966 multipart/signed MIME type has two parts. The first part contains the 967 MIME entity that is signed; the second part contains the "detached 968 signature" CMS SignedData object in which the encapContentInfo 969 eContent field is absent. 971 3.4.3.1 The application/pkcs7-signature MIME Type 973 This MIME type always contains a CMS ContentInfo containing a single 974 CMS object of type signedData. The signedData encapContentInfo 975 eContent field MUST be absent. The signerInfos field contains the 976 signatures for the MIME entity. 978 The file extension for signed-only messages using application/pkcs7- 979 signature is ".p7s". 981 3.4.3.2 Creating a multipart/signed Message 983 Step 1. The MIME entity to be signed is prepared according to section 984 3.1, taking special care for clear-signing. 986 Step 2. The MIME entity is presented to CMS processing in order to 987 obtain an object of type signedData in which the encapContentInfo 988 eContent field is absent. 990 Step 3. The MIME entity is inserted into the first part of a 991 multipart/signed message with no processing other than that described 992 in section 3.1. 994 Step 4. Transfer encoding is applied to the "detached signature" CMS 995 SignedData object and it is inserted into a MIME entity of type 996 application/pkcs7-signature. 998 Step 5. The MIME entity of the application/pkcs7-signature is inserted 999 into the second part of the multipart/signed entity. 1001 The multipart/signed Content type has two required parameters: the 1002 protocol parameter and the micalg parameter. 1004 The protocol parameter MUST be "application/pkcs7-signature". Note 1005 that quotation marks are required around the protocol parameter 1006 because MIME requires that the "/" character in the parameter value 1007 MUST be quoted. 1009 The micalg parameter allows for one-pass processing when the signature 1010 is being verified. The value of the micalg parameter is dependent on 1011 the message digest algorithm(s) used in the calculation of the Message 1012 Integrity Check. If multiple message digest algorithms are used they 1013 MUST be separated by commas per [MIME-SECURE]. The values to be placed 1014 in the micalg parameter SHOULD be from the following: 1016 Algorithm Value 1017 used 1019 MD5 md5 1020 SHA-1 sha1 1021 Any other unknown 1023 (Historical note: some early implementations of S/MIME emitted and 1024 expected "rsa-md5" and "rsa-sha1" for the micalg parameter.) Receiving 1025 agents SHOULD be able to recover gracefully from a micalg parameter 1026 value that they do not recognize. 1028 3.4.3.3 Sample multipart/signed Message 1030 Content-Type: multipart/signed; 1031 protocol="application/pkcs7-signature"; 1032 micalg=sha1; boundary=boundary42 1034 --boundary42 1035 Content-Type: text/plain 1037 This is a clear-signed message. 1039 --boundary42 1040 Content-Type: application/pkcs7-signature; name=smime.p7s 1041 Content-Transfer-Encoding: base64 1042 Content-Disposition: attachment; filename=smime.p7s 1044 ghyHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6 1045 4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj 1046 n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4 1047 7GhIGfHfYT64VQbnj756 1049 --boundary42-- 1051 3.5 Creating an Compressed-only Message 1053 This section describes the format for compressing a MIME entity. 1054 Please note that versions of S/MIME prior to 3.1 did not specify any 1055 use of compressedData, and will not recognize it. The use of a 1056 capability to indicate the ability to receive compressedData is 1057 described in [CMSCOMPR] and is the preferred method for compatibility. 1059 Step 1. The MIME entity to be enveloped is prepared according to 1060 section 3.1. 1062 Step 2. The MIME entity and other required data is processed into a 1063 CMS object of type compressedData. 1065 Step 3. The compressedData object is wrapped in a CMS ContentInfo 1066 object. 1068 Step 4. The ContentInfo object is inserted into an 1069 application/pkcs7-mime MIME entity. 1071 The smime-type parameter for compressed-only messages is "compressed- 1072 data". The file extension for this type of message is ".p7z". 1074 A sample message would be: 1076 Content-Type: application/pkcs7-mime; smime-type=compressed-data; 1077 name=smime.p7z 1078 Content-Transfer-Encoding: base64 1079 Content-Disposition: attachment; filename=smime.p7z 1081 rfvbnj756tbBghyHhHUujhJhjH77n8HHGT9HG4VQpfyF467GhIGfHfYT6 1082 7n8HHGghyHhHUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H 1083 f8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4 1084 0GhIGfHfQbnj756YT64V 1086 3.6 Multiple Operations 1088 Any of the signed-only, compressed-only and encrypted-only MIME 1089 formats listed above may be nested. This is allowed because the above 1090 formats are all MIME entities, and because they all encapsulate MIME 1091 entities. 1093 An S/MIME implementation MUST be able to receive and process 1094 arbitrarily nested S/MIME within reasonable resource limits of the 1095 recipient computer. 1097 It is possible to apply any of the signing, encrypting and compressing 1098 operations in any order. It is up to the implementor and the user to 1099 choose. When signing first, the signatories are then securely obscured 1100 by the enveloping. When enveloping first the signatories are exposed, 1101 but it is possible to verify signatures without removing the 1102 enveloping. This may be useful in an environment were automatic 1103 signature verification is desired, as no private key material is 1104 required to verify a signature. 1106 There are security ramifications to choosing whether to sign first or 1107 encrypt first. A recipient of a message that is encrypted and then 1108 signed can validate that the encrypted block was unaltered, but cannot 1109 determine any relationship between the signer and the unencrypted 1110 contents of the message. A recipient of a message that is signed-then- 1111 encrypted can assume that the signed message itself has not been 1112 altered, but that a careful attacker may have changed the 1113 unauthenticated portions of the encrypted message. 1115 When using compression, keep the following guidelines in mind: 1117 - Compression of binary encoded encrypted data is discouraged, since 1118 it will not yield significant compression. Base64 encrypted data 1119 could very well benefit, however. 1120 - If a lossy compression algorithm is used with signing, you will need 1121 to compress first, then sign. 1123 3.7 Creating a Certificate Management Message 1125 The certificate management message or MIME entity is used to transport 1126 certificates and/or certificate revocation lists, such as in response 1127 to a registration request. 1129 Step 1. The certificates and/or certificate revocation lists are made 1130 available to the CMS generating process which creates a CMS object of 1131 type signedData. The signedData encapContentInfo eContent field MUST 1132 be absent and signerInfos field MUST be empty. 1134 Step 2. The signedData object is wrapped in a CMS ContentInfo 1135 object. 1137 Step 3. The ContentInfo object is enclosed in an application/pkcs7- 1138 mime MIME entity 1140 The smime-type parameter for a certificate management message is 1141 "certs-only". The file extension for this type of message is ".p7c". 1143 3.8 Registration Requests 1145 A sending agent that signs messages MUST have a certificate for the 1146 signature so that a receiving agent can verify the signature. There 1147 are many ways of getting certificates, such as through an exchange 1148 with a certificate authority, through a hardware token or diskette, 1149 and so on. 1151 S/MIME v2 [SMIMEV2] specified a method for "registering" public keys 1152 with certificate authorities using an application/pkcs10 body part. 1153 The IETF's PKIX Working Group is preparing another method for 1154 requesting certificates; however, that work was not finished at the 1155 time of this specification. S/MIME v3 does not specify how to request 1156 a certificate, but instead mandates that every sending agent already 1157 has a certificate. Standardization of certificate management is being 1158 pursued separately in the IETF. 1160 3.9 Identifying an S/MIME Message 1162 Because S/MIME takes into account interoperation in non-MIME 1163 environments, several different mechanisms are employed to carry the 1164 type information, and it becomes a bit difficult to identify S/MIME 1165 messages. The following table lists criteria for determining whether 1166 or not a message is an S/MIME message. A message is considered an 1167 S/MIME message if it matches any below. 1169 The file suffix in the table below comes from the "name" parameter in 1170 the content-type header, or the "filename" parameter on the content- 1171 disposition header. These parameters that give the file suffix are not 1172 listed below as part of the parameter section. 1174 MIME type: application/pkcs7-mime 1175 parameters: any 1176 file suffix: any 1178 MIME type: multipart/signed 1179 parameters: protocol="application/pkcs7-signature" 1180 file suffix: any 1182 MIME type: application/octet-stream 1183 parameters: any 1184 file suffix: p7m, p7s, p7c, p7z 1186 4. Certificate Processing 1188 A receiving agent MUST provide some certificate retrieval mechanism in 1189 order to gain access to certificates for recipients of digital 1190 envelopes. This specification does not cover how S/MIME agents handle 1191 certificates, only what they do after a certificate has been validated 1192 or rejected. S/MIME certification issues are covered in [CERT3]. 1194 At a minimum, for initial S/MIME deployment, a user agent could 1195 automatically generate a message to an intended recipient requesting 1196 that recipient's certificate in a signed return message. Receiving and 1197 sending agents SHOULD also provide a mechanism to allow a user to 1198 "store and protect" certificates for correspondents in such a way so 1199 as to guarantee their later retrieval. 1201 4.1 Key Pair Generation 1203 If an S/MIME agent needs to generate a key pair, then the S/MIME agent 1204 or some related administrative utility or function MUST be capable of 1205 generating separate DH and DSS public/private key pairs on behalf of 1206 the user. Each key pair MUST be generated from a good source of non- 1207 deterministic random input [RANDOM] and the private key MUST be 1208 protected in a secure fashion. 1210 If an S/MIME agent needs to generate a key pair, then the S/MIME agent 1211 or some related administrative utility or function SHOULD generate RSA 1212 key pairs. 1214 A user agent SHOULD generate RSA key pairs at a minimum key size of 1215 768 bits. A user agent MUST NOT generate RSA key pairs less than 512 1216 bits long. Creating keys longer than 1024 bits may cause some older 1217 S/MIME receiving agents to not be able to verify signatures, but gives 1218 better security and is therefore valuable. A receiving agent SHOULD be 1219 able to verify signatures with keys of any size over 512 bits. Some 1220 agents created in the United States have chosen to create 512 bit keys 1221 in order to get more advantageous export licenses. However, 512 bit 1222 keys are considered by many to be cryptographically insecure. 1223 Implementors should be aware that multiple (active) key pairs may be 1224 associated with a single individual. For example, one key pair may be 1225 used to support confidentiality, while a different key pair may be 1226 used for authentication. 1228 5. Security 1230 40-bit encryption is considered weak by most cryptographers. Using 1231 weak cryptography in S/MIME offers little actual security over sending 1232 plaintext. However, other features of S/MIME, such as the 1233 specification of tripleDES and the ability to announce stronger 1234 cryptographic capabilities to parties with whom you communicate, allow 1235 senders to create messages that use strong encryption. Using weak 1236 cryptography is never recommended unless the only alternative is no 1237 cryptography. When feasible, sending and receiving agents should 1238 inform senders and recipients the relative cryptographic strength of 1239 messages. 1241 It is impossible for most software or people to estimate the value of 1242 a message. Further, it is impossible for most software or people to 1243 estimate the actual cost of decrypting a message that is encrypted 1244 with a key of a particular size. Further, it is quite difficult to 1245 determine the cost of a failed decryption if a recipient cannot decode 1246 a message. Thus, choosing between different key sizes (or choosing 1247 whether to just use plaintext) is also impossible. However, decisions 1248 based on these criteria are made all the time, and therefore this 1249 specification gives a framework for using those estimates in choosing 1250 algorithms. 1252 If a sending agent is sending the same message using different 1253 strengths of cryptography, an attacker watching the communications 1254 channel may be able to determine the contents of the strongly- 1255 encrypted message by decrypting the weakly-encrypted version. In other 1256 words, a sender should not send a copy of a message using weaker 1257 cryptography than they would use for the original of the message. 1259 Modification of the ciphertext can go undetected if authentication is 1260 not also used, which is the case when sending EnvelopedData without 1261 wrapping it in SignedData or enclosing SignedData within it. 1263 See RFC 3218 [MMA] for more information about thwarting the adaptive 1264 chosen ciphertext vulnerability in PKCS #1 Version 1.5 1265 implementations. 1267 In some circumstances the use of the Diffie-Hellman key agreement 1268 scheme in a prime order subgroup of a large prime p is vulnerable to 1269 certain attacks known as "small-subgroup" attacks. Methods exist, 1270 however, to prevent these attacks. These methods are described in RFC 1271 2785 [DHSUB]. 1273 A. ASN.1 Module 1275 SecureMimeMessageV3dot1 1276 { iso(1) member-body(2) us(840) rsadsi(113549) 1277 pkcs(1) pkcs-9(9) smime(16) modules(0) msg-v3dot1(21) } 1279 DEFINITIONS IMPLICIT TAGS ::= 1280 BEGIN 1282 IMPORTS 1283 -- Cryptographic Message Syntax 1284 SubjectKeyIdentifier, IssuerAndSerialNumber, 1285 RecipientKeyIdentifier 1286 FROM CryptographicMessageSyntax 1287 { iso(1) member-body(2) us(840) rsadsi(113549) 1288 pkcs(1) pkcs-9(9) smime(16) modules(0) cms-2001(14) }; 1290 -- id-aa is the arc with all new authenticated and unauthenticated 1291 -- attributes produced the by S/MIME Working Group 1293 id-aa OBJECT IDENTIFIER ::= {iso(1) member-body(2) usa(840) 1294 rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) attributes(2)} 1296 -- S/MIME Capabilities provides a method of broadcasting the symetric 1297 -- capabilities understood. Algorithms should be ordered by 1298 -- preference and grouped by type 1300 smimeCapabilities OBJECT IDENTIFIER ::= 1301 {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 15} 1303 SMIMECapability ::= SEQUENCE { 1304 capabilityID OBJECT IDENTIFIER, 1305 parameters ANY DEFINED BY capabilityID OPTIONAL } 1307 SMIMECapabilities ::= SEQUENCE OF SMIMECapability 1309 -- Encryption Key Preference provides a method of broadcasting the 1310 -- prefered encryption certificate. 1312 id-aa-encrypKeyPref OBJECT IDENTIFIER ::= {id-aa 11} 1314 SMIMEEncryptionKeyPreference ::= CHOICE { 1315 issuerAndSerialNumber [0] IssuerAndSerialNumber, 1316 receipentKeyId [1] RecipientKeyIdentifier, 1317 subjectAltKeyIdentifier [2] SubjectKeyIdentifier 1318 } 1320 id-smime OBJECT IDENTIFIER ::= { iso(1) member-body(2) 1321 us(840) rsadsi(113549) pkcs(1) pkcs9(9) 16 } 1323 id-cap OBJECT IDENTIFIER ::= { id-smime 11 } 1325 -- The preferBinaryInside indicates an ability to receive messages 1326 -- with binary encoding inside the CMS wrapper 1328 id-cap-preferBinaryInside OBJECT IDENTIFIER ::= { id-cap 1 } 1330 -- The following list the OIDs to be used with S/MIME V3 1332 -- Signature Algorithms Not Found in [CMSALG] 1333 -- 1334 -- md2WithRSAEncryption OBJECT IDENTIFIER ::= 1335 -- {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 1336 -- 2} 1337 -- 1338 -- Other Signed Attributes 1339 -- 1340 -- signingTime OBJECT IDENTIFIER ::= 1341 -- {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 1342 -- 5} 1343 -- See [CMS] for a description of how to encode the attribute 1344 -- value. 1346 END 1348 B. References 1350 [CERT31] "S/MIME Version 3.1 Certificate Handling", Internet Draft 1351 draft-ietf-smime-rfc2632bis 1353 [CHARSETS] Character sets assigned by IANA. See . 1356 [CMS] "Cryptographic Message Syntax", RFC 3369 1358 [CMSALG] "Cryptographic Message Syntax (CMS) Algorithms", RFC 3370 1360 [CMSCOMPR] "Compressed Data Content Type for Cryptographic Message 1361 Syntax (CMS)", RFC 3274 1363 [CONTDISP] "Communicating Presentation Information in Internet 1364 Messages: The Content-Disposition Header Field", RFC 2183 1366 [DHSUB] "Methods for Avoiding the "Small-Subgroup" Attacks on the 1367 Diffie-Hellman Key Agreement Method for S/MIME", RFC 2785 1369 [ESS] "Enhanced Security Services for S/MIME", RFC 2634 1371 [MIME-SPEC] The primary definition of MIME. "MIME Part 1: Format of 1372 Internet Message Bodies", RFC 2045; "MIME Part 2: Media Types", RFC 1373 2046; "MIME Part 3: Message Header Extensions for Non-ASCII Text", RFC 1374 2047; "MIME Part 4: Registration Procedures", RFC 2048; "MIME Part 5: 1375 Conformance Criteria and Examples", RFC 2049 1377 [MIME-SECURE] "Security Multiparts for MIME: Multipart/Signed and 1378 Multipart/Encrypted", RFC 1847 1380 [MMA] "Preventing the Million Message Attack on CMS", RFC 3218 1382 [MUSTSHOULD] "Key words for use in RFCs to Indicate Requirement 1383 Levels", RFC 2119 1385 [PKCS-7] "PKCS #7: Cryptographic Message Syntax Version 1.5", RFC 2315 1387 [RANDOM] "Randomness Recommendations for Security", RFC 1750 1389 [SMIMEV2] "S/MIME Version 2 Message Specification", RFC 2311 1391 C. Acknowledgements 1393 Many thanks go out to the other authors of the S/MIME Version 2 1394 Message Specification RFC: Steve Dusse, Paul Hoffman, Laurence 1395 Lundblade and Lisa Repka. 1397 A number of the members of the S/MIME Working Group have also worked 1398 very hard and contributed to this document. Any list of people is 1399 doomed to omission, and for that I apologize. In alphabetical order, 1400 the following people stand out in my mind due to the fact that they 1401 made direct contributions to this document. 1403 Tony Capel 1404 Piers Chivers 1405 Dave Crocker 1406 Bill Flanigan 1407 Paul Hoffman 1408 Russ Housley 1409 William Ottaway 1410 John Pawling 1411 Jim Schaad 1413 D. Editor's address 1415 Blake Ramsdell 1416 Brute Squad Labs 1417 Suite 217-C 1418 16451 Redmond Way 1419 Redmond, WA 98052-4482 1421 blake@brutesquadlabs.com 1423 E. Changes from last draft 1425 English fixes (Jim Schaad) 1427 Compression guidance (Jim Schaad) 1429 MMA language taken verbatim from [CMSALG] (Blake Ramsdell, Russ 1430 Housley) 1432 Small subgroup Diffie Hellman taken verbatim from [DHSUB] (Blake 1433 Ramsdell, R. Zuccherato) 1435 Module ID changes (Jim Schaad, Russ Housley) 1437 Acknowlegements (Blake Ramsdell) 1439 Changes since S/MIME v3 (Blake Ramsdell)