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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 S/MIME WG Blake Ramsdell, SendMail 2 Internet Draft Sean Turner, IECA 3 Intended Status: Standard Track February 21, 2008 4 Obsoletes: 3850 (once approved) 5 Expires: August 21, 2008 7 Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 8 Certificate Handling 9 draft-ietf-smime-3850bis-01.txt 11 Status of this Memo 13 By submitting this Internet-Draft, each author represents that any 14 applicable patent or other IPR claims of which he or she is aware 15 have been or will be disclosed, and any of which he or she becomes 16 aware will be disclosed, in accordance with Section 6 of BCP 79. 18 Internet-Drafts are working documents of the Internet Engineering 19 Task Force (IETF), its areas, and its working groups. Note that 20 other groups may also distribute working documents as Internet- 21 Drafts. 23 Internet-Drafts are draft documents valid for a maximum of six months 24 and may be updated, replaced, or obsoleted by other documents at any 25 time. It is inappropriate to use Internet-Drafts as reference 26 material or to cite them other than as "work in progress." 28 The list of current Internet-Drafts can be accessed at 29 http://www.ietf.org/ietf/1id-abstracts.txt 31 The list of Internet-Draft Shadow Directories can be accessed at 32 http://www.ietf.org/shadow.html 34 This Internet-Draft will expire on August 21, 2008. 36 Copyright Notice 38 Copyright (C) The IETF Trust (2008). 40 Abstract 42 This document specifies conventions for X.509 certificate usage by 43 Secure/Multipurpose Internet Mail Extensions (S/MIME) agents. S/MIME 44 provides a method to send and receive secure MIME messages, and 45 certificates are an integral part of S/MIME agent processing. S/MIME 46 agents validate certificates as described in RFC 3280bis, the 47 Internet X.509 Public Key Infrastructure Certificate and CRL Profile. 48 S/MIME agents must meet the certificate processing requirements in 49 this document as well as those in RFC 3280bis. This document 50 obsoletes RFC 3850. 52 Conventions used in this document 54 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 55 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 56 document are to be interpreted as described in [MUSTSHOULD]. 58 We define some additional terms here: 60 SHOULD+ This term means the same as SHOULD. However, it is likely 61 that an algorithm marked as SHOULD+ will be promoted at some 62 future time to be a MUST. 64 SHOULD- This term means the same as SHOULD. However, an algorithm 65 marked as SHOULD- may be deprecated to a MAY in a future version 66 of this document. 68 MUST- This term means the same as MUST. However, we expect at some 69 point that this algorithm will no longer be a MUST in a future 70 document. Although its status will be determined at a later 71 time, it is reasonable to expect that if a future revision of a 72 document alters the status of a MUST- algorithm, it will remain 73 at least a SHOULD or a SHOULD-. 75 Discussion 77 This draft is being discussed on the 'ietf-smime' mailing list. To 78 subscribe, send a message to ietf-smime-request@imc.org with the 79 single word subscribe in the body of the message. There is a Web site 80 for the mailing list at . 82 Table of Contents 84 1. Introduction...................................................3 85 1.1. Definitions...............................................3 86 1.2. Compatibility with Prior Practice S/MIME..................4 87 1.3. Changes Since S/MIME V3.1 (RFC 3850)......................4 88 2. CMS Options....................................................5 89 2.1. Certificate Revocation Lists..............................5 90 2.2. Certificate Choices.......................................5 91 2.2.1. Historical Note About CMS Certificates...............5 92 2.3. CertificateSet............................................6 94 3. Using Distinguished Names For Internet Mail....................7 95 4. Certificate Processing.........................................8 96 4.1. Certificate Revocation Lists..............................9 97 4.2. Certificate Path Validation...............................9 98 4.3. Certificate and CRL Signing Algorithms...................10 99 4.4. PKIX Certificate Extensions..............................10 100 4.4.1. Basic Constraints...................................11 101 4.4.2. Key Usage Certificate Extension.....................11 102 4.4.3. Subject Alternative Name............................12 103 4.4.4. Extended Key Usage Extension........................12 104 5. IANA Considerations...........................................12 105 6. Security Considerations.......................................13 107 1. Introduction 109 S/MIME (Secure/Multipurpose Internet Mail Extensions), described in 110 [SMIME-MSG], provides a method to send and receive secure MIME 111 messages. Before using a public key to provide security services, 112 the S/MIME agent MUST verify that the public key is valid. S/MIME 113 agents MUST use PKIX certificates to validate public keys as 114 described in the Internet X.509 Public Key Infrastructure (PKIX) 115 Certificate and CRL Profile [KEYM]. S/MIME agents MUST meet the 116 certificate processing requirements documented in this document in 117 addition to those stated in [KEYM]. 119 This specification is compatible with the Cryptographic Message 120 Syntax [CMS] in that it uses the data types defined by CMS. It also 121 inherits all the varieties of architectures for certificate-based key 122 management supported by CMS. 124 1.1. Definitions 126 For the purposes of this document, the following definitions apply. 128 ASN.1: Abstract Syntax Notation One, as defined in ITU-T X.208 129 [X.208-88]. 131 Attribute Certificate (AC): An X.509 AC is a separate structure from 132 a subject's public key X.509 Certificate. A subject may have 133 multiple X.509 ACs associated with each of its public key X.509 134 Certificates. Each X.509 AC binds one or more Attributes with one of 135 the subject's public key X.509 Certificates. The X.509 AC syntax is 136 defined in [ACAUTH]. 138 Certificate: A type that binds an entity's name to a public key with 139 a digital signature. This type is defined in the Internet X.509 140 Public Key Infrastructure (PKIX) Certificate and CRL Profile [KEYM]. 142 This type also contains the distinguished name of the certificate 143 issuer (the signer), an issuer-specific serial number, the issuer's 144 signature algorithm identifier, a validity period, and extensions 145 also defined in that document. 147 Certificate Revocation List (CRL): A type that contains information 148 about certificates whose validity an issuer has prematurely revoked. 149 The information consists of an issuer name, the time of issue, the 150 next scheduled time of issue, a list of certificate serial numbers 151 and their associated revocation times, and extensions as defined in 152 [KEYM]. The CRL is signed by the issuer. The type intended by this 153 specification is the one defined in [KEYM]. 155 Receiving agent: Software that interprets and processes S/MIME CMS 156 objects, MIME body parts that contain CMS objects, or both. 158 Sending agent: Software that creates S/MIME CMS objects, MIME body 159 parts that contain CMS objects, or both. 161 S/MIME agent: User software that is a receiving agent, a sending 162 agent, or both. 164 1.2. Compatibility with Prior Practice S/MIME 166 S/MIME version 3.2 agents should attempt to have the greatest 167 interoperability possible with agents for prior versions of S/MIME. 168 S/MIME version 2 is described in RFC 2311 through RFC 2315, inclusive 169 S/MIME version 3 is described in RFC 2630 through RFC 2634 inclusive, 170 and S/MIME version 3.1 is described in RFC 3850 through 3851 171 inclusive and RFC 2634. RFC 2311 also has historical information 172 about the development of S/MIME. 174 1.3. Changes Since S/MIME V3.1 (RFC 3850) 176 Conventions Used in This Document: Added definitions for SHOULD+, 177 SHOULD-, and MUST-. 179 Sec 1.2: Added text about v3.1 RFCs. 181 Sec 3: Updated note to indicate emailAddress IA5String upper bound is 182 255 characters. 184 Sec 4.3: RSA with SHA-256 (PKCS #1 v1.5) added as MUST, RSA with SHA- 185 1 changed to MUST-, DSA with SHA-1, and RSA with MD5 changed to 186 SHOULD-, and RSA-PS with SHA-256. Updated key sizes. 188 Sec A.1: Updated references to latest versions of PKIX profile and 189 S/MIME Message Specification. 191 Sec A.1: Changed reference from KEYMALG to KEYM. 193 2. CMS Options 195 The CMS message format allows for a wide variety of options in 196 content and algorithm support. This section puts forth a number of 197 support requirements and recommendations in order to achieve a base 198 level of interoperability among all S/MIME implementations. Most of 199 the CMS format for S/MIME messages is defined in [SMIME-MSG]. 201 2.1. Certificate Revocation Lists 203 Receiving agents MUST support the Certificate Revocation List (CRL) 204 format defined in [KEYM]. If sending agents include CRLs in outgoing 205 messages, the CRL format defined in [KEYM] MUST be used. In all 206 cases, both v1 and v2 CRLs MUST be supported. 208 All agents MUST be capable of performing revocation checks using CRLs 209 as specified in [KEYM]. All agents MUST perform revocation status 210 checking in accordance with [KEYM]. Receiving agents MUST recognize 211 CRLs in received S/MIME messages. 213 Agents SHOULD store CRLs received in messages for use in processing 214 later messages. 216 2.2. Certificate Choices 218 Receiving agents MUST support v1 X.509 and v3 X.509 identity 219 certificates as profiled in [KEYM]. End entity certificates MAY 220 include an Internet mail address, as described in section 3. 222 Receiving agents SHOULD support X.509 version 2 attribute 223 certificates. See [ACAUTH] for details about the profile for 224 attribute certificates. 226 2.2.1. Historical Note About CMS Certificates 228 The CMS message format supports a choice of certificate formats for 229 public key content types: PKIX, PKCS #6 Extended Certificates [PKCS6] 230 and PKIX Attribute Certificates. 232 The PKCS #6 format is not in widespread use. In addition, PKIX 233 certificate extensions address much of the same functionality and 234 flexibility as was intended in the PKCS #6. Thus, sending and 235 receiving agents MUST NOT use PKCS #6 extended certificates. 237 X.509 version 1 attribute certificates are also not widely 238 implemented, and have been superseded with version 2 attribute 239 certificates. Sending agents MUST NOT send version 1 attribute 240 certificates. 242 2.3. CertificateSet 244 Receiving agents MUST be able to handle an arbitrary number of 245 certificates of arbitrary relationship to the message sender and to 246 each other in arbitrary order. In many cases, the certificates 247 included in a signed message may represent a chain of certification 248 from the sender to a particular root. There may be, however, 249 situations where the certificates in a signed message may be 250 unrelated and included for convenience. 252 Sending agents SHOULD include any certificates for the user's public 253 key(s) and associated issuer certificates. This increases the 254 likelihood that the intended recipient can establish trust in the 255 originator's public key(s). This is especially important when 256 sending a message to recipients that may not have access to the 257 sender's public key through any other means or when sending a signed 258 message to a new recipient. The inclusion of certificates in 259 outgoing messages can be omitted if S/MIME objects are sent within a 260 group of correspondents that has established access to each other's 261 certificates by some other means such as a shared directory or manual 262 certificate distribution. Receiving S/MIME agents SHOULD be able to 263 handle messages without certificates using a database or directory 264 lookup scheme. 266 A sending agent SHOULD include at least one chain of certificates up 267 to, but not including, a Certificate Authority (CA) that it believes 268 that the recipient may trust as authoritative. A receiving agent 269 MUST be able to handle an arbitrarily large number of certificates 270 and chains. 272 Agents MAY send CA certificates, that is, certificates which can be 273 considered the "root" of other chains, and which MAY be self-signed. 274 Note that receiving agents SHOULD NOT simply trust any self-signed 275 certificates as valid CAs, but SHOULD use some other mechanism to 276 determine if this is a CA that should be trusted. Also note that 277 when certificates contain DSA public keys the parameters may be 278 located in the root certificate. This would require that the 279 recipient possess both the end-entity certificate as well as the root 280 certificate to perform a signature verification, and is a valid 281 example of a case where transmitting the root certificate may be 282 required. 284 Receiving agents MUST support chaining based on the distinguished 285 name fields. Other methods of building certificate chains MAY be 286 supported. 288 Receiving agents SHOULD support the decoding of X.509 attribute 289 certificates included in CMS objects. All other issues regarding the 290 generation and use of X.509 attribute certificates are outside of the 291 scope of this specification. One specification that addresses 292 attribute certificate use is defined in [SECLABEL]. 294 3. Using Distinguished Names For Internet Mail 296 End-entity certificates MAY contain an Internet mail address as 297 described in [RFC-2822]. The address must be an "addr-spec" as 298 defined in Section 3.4.1 of that specification. The email address 299 SHOULD be in the subjectAltName extension, and SHOULD NOT be in the 300 subject distinguished name. 302 Receiving agents MUST recognize and accept certificates that contain 303 no email address. Agents are allowed to provide an alternative 304 mechanism for associating an email address with a certificate that 305 does not contain an email address, such as through the use of the 306 agent's address book, if available. Receiving agents MUST recognize 307 email addresses in the subjectAltName field. Receiving agents MUST 308 recognize email addresses in the Distinguished Name field in the PKCS 309 #9 [PKCS9] emailAddress attribute: 311 pkcs-9-at-emailAddress OBJECT IDENTIFIER ::= 313 {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 1 } 315 Note that this attribute MUST be encoded as IA5String and has an 316 upper bounds of 255 characters. 318 Sending agents SHOULD make the address in the From or Sender header 319 in a mail message match an Internet mail address in the signer's 320 certificate. Receiving agents MUST check that the address in the 321 From or Sender header of a mail message matches an Internet mail 322 address, if present, in the signer's certificate, if mail addresses 323 are present in the certificate. A receiving agent SHOULD provide 324 some explicit alternate processing of the message if this comparison 325 fails, which may be to display a message that shows the recipient the 326 addresses in the certificate or other certificate details. 328 A receiving agent SHOULD display a subject name or other certificate 329 details when displaying an indication of successful or unsuccessful 330 signature verification. 332 All subject and issuer names MUST be populated (i.e., not an empty 333 SEQUENCE) in S/MIME-compliant X.509 identity certificates, except 334 that the subject DN in a user's (i.e., end-entity) certificate MAY be 335 an empty SEQUENCE in which case the subjectAltName extension will 336 include the subject's identifier and MUST be marked as critical. 338 4. Certificate Processing 340 A receiving agent needs to provide some certificate retrieval 341 mechanism in order to gain access to certificates for recipients of 342 digital envelopes. There are many ways to implement certificate 343 retrieval mechanisms. X.500 directory service is an excellent 344 example of a certificate retrieval-only mechanism that is compatible 345 with classic X.500 Distinguished Names. Another method under 346 consideration by the IETF is to provide certificate retrieval 347 services as part of the existing Domain Name System (DNS). Until 348 such mechanisms are widely used, their utility may be limited by the 349 small number of correspondent's certificates that can be retrieved. 350 At a minimum, for initial S/MIME deployment, a user agent could 351 automatically generate a message to an intended recipient requesting 352 that recipient's certificate in a signed return message. 354 Receiving and sending agents SHOULD also provide a mechanism to allow 355 a user to "store and protect" certificates for correspondents in such 356 a way so as to guarantee their later retrieval. In many 357 environments, it may be desirable to link the certificate 358 retrieval/storage mechanisms together in some sort of certificate 359 database. In its simplest form, a certificate database would be 360 local to a particular user and would function in a similar way as an 361 "address book" that stores a user's frequent correspondents. In this 362 way, the certificate retrieval mechanism would be limited to the 363 certificates that a user has stored (presumably from incoming 364 messages). A comprehensive certificate retrieval/storage solution 365 may combine two or more mechanisms to allow the greatest flexibility 366 and utility to the user. For instance, a secure Internet mail agent 367 may resort to checking a centralized certificate retrieval mechanism 368 for a certificate if it can not be found in a user's local 369 certificate storage/retrieval database. 371 Receiving and sending agents SHOULD provide a mechanism for the 372 import and export of certificates, using a CMS certs-only message. 373 This allows for import and export of full certificate chains as 374 opposed to just a single certificate. This is described in [SMIME- 375 MSG]. 377 Agents MUST handle multiple valid Certification Authority (CA) 378 certificates containing the same subject name and the same public 379 keys but with overlapping validity intervals. 381 4.1. Certificate Revocation Lists 383 In general, it is always better to get the latest CRL information 384 from a CA than to get information stored away from incoming messages. 385 A receiving agent SHOULD have access to some certificate revocation 386 list (CRL) retrieval mechanism in order to gain access to certificate 387 revocation information when validating certification paths. A 388 receiving or sending agent SHOULD also provide a mechanism to allow a 389 user to store incoming certificate revocation information for 390 correspondents in such a way so as to guarantee its later retrieval. 392 Receiving and sending agents SHOULD retrieve and utilize CRL 393 information every time a certificate is verified as part of a 394 certification path validation even if the certificate was already 395 verified in the past. However, in many instances (such as off-line 396 verification) access to the latest CRL information may be difficult 397 or impossible. The use of CRL information, therefore, may be 398 dictated by the value of the information that is protected. The 399 value of the CRL information in a particular context is beyond the 400 scope of this specification but may be governed by the policies 401 associated with particular certification paths. 403 All agents MUST be capable of performing revocation checks using CRLs 404 as specified in [KEYM]. All agents MUST perform revocation status 405 checking in accordance with [KEYM]. Receiving agents MUST recognize 406 CRLs in received S/MIME messages. 408 4.2. Certificate Path Validation 410 In creating a user agent for secure messaging, certificate, CRL, and 411 certification path validation SHOULD be highly automated while still 412 acting in the best interests of the user. Certificate, CRL, and path 413 validation MUST be performed as per [KEYM] when validating a 414 correspondent's public key. This is necessary before using a public 415 key to provide security services such as: verifying a signature; 416 encrypting a content-encryption key (ex: RSA); or forming a pairwise 417 symmetric key (ex: Diffie-Hellman) to be used to encrypt or decrypt a 418 content-encryption key. 420 Certificates and CRLs are made available to the path validation 421 procedure in two ways: a) incoming messages, and b) certificate and 422 CRL retrieval mechanisms. Certificates and CRLs in incoming messages 423 are not required to be in any particular order nor are they required 424 to be in any way related to the sender or recipient of the message 425 (although in most cases they will be related to the sender). Incoming 426 certificates and CRLs SHOULD be cached for use in path validation and 427 optionally stored for later use. This temporary certificate and CRL 428 cache SHOULD be used to augment any other certificate and CRL 429 retrieval mechanisms for path validation on incoming signed messages. 431 4.3. Certificate and CRL Signing Algorithms 433 Certificates and Certificate Revocation Lists (CRLs) are signed by 434 the certificate issuer. Receiving agents: 436 - MUST support RSA with SHA-256, as specified in [CMS-SHA2] 438 - MUST- support RSA with SHA-1, as specified in [CMSALG] 440 - SHOULD+ support RSA-PSS with SHA-256, as specified in [RSAPSS] 442 - SHOULD- support DSA with SHA-1, as specified in [CMSALG]. 444 - SHOULD- support RSA with MD5, as specified in [CMSALG]. 446 Key sizes from 1024 bits to 2048 bits MUST be supported. 448 4.4. PKIX Certificate Extensions 450 PKIX describes an extensible framework in which the basic certificate 451 information can be extended and how such extensions can be used to 452 control the process of issuing and validating certificates. The PKIX 453 Working Group has ongoing efforts to identify and create extensions 454 which have value in particular certification environments. Further, 455 there are active efforts underway to issue PKIX certificates for 456 business purposes. This document identifies the minimum required set 457 of certificate extensions which have the greatest value in the S/MIME 458 environment. The syntax and semantics of all the identified 459 extensions are defined in [KEYM]. 461 Sending and receiving agents MUST correctly handle the basic 462 constraints, key usage, authority key identifier, subject key 463 identifier, and subject alternative names certificate extensions when 464 they appear in end-entity and CA certificates. Some mechanism SHOULD 465 exist to gracefully handle other certificate extensions when they 466 appear in end-entity or CA certificates. 468 Certificates issued for the S/MIME environment SHOULD NOT contain any 469 critical extensions (extensions that have the critical field set to 470 TRUE) other than those listed here. These extensions SHOULD be 471 marked as non-critical unless the proper handling of the extension is 472 deemed critical to the correct interpretation of the associated 473 certificate. Other extensions may be included, but those extensions 474 SHOULD NOT be marked as critical. 476 Interpretation and syntax for all extensions MUST follow [KEYM], 477 unless otherwise specified here. 479 4.4.1. Basic Constraints 481 The basic constraints extension serves to delimit the role and 482 position that an issuing authority or end-entity certificate plays in 483 a certification path. 485 For example, certificates issued to CAs and subordinate CAs contain a 486 basic constraint extension that identifies them as issuing authority 487 certificates. End-entity certificates contain an extension that 488 constrains the certificate from being an issuing authority 489 certificate. 491 Certificates SHOULD contain a basicConstraints extension in CA 492 certificates, and SHOULD NOT contain that extension in end entity 493 certificates. 495 4.4.2. Key Usage Certificate Extension 497 The key usage extension serves to limit the technical purposes for 498 which a public key listed in a valid certificate may be used. Issuing 499 authority certificates may contain a key usage extension that 500 restricts the key to signing certificates, certificate revocation 501 lists and other data. 503 For example, a certification authority may create subordinate issuer 504 certificates which contain a key usage extension which specifies that 505 the corresponding public key can be used to sign end user 506 certificates and sign CRLs. 508 If a key usage extension is included in a PKIX certificate, then it 509 MUST be marked as critical. 511 S/MIME receiving agents MUST NOT accept the signature of a message if 512 it was verified using a certificate which contains the key usage 513 extension without either the digitalSignature or nonRepudiation bit 514 set. Sometimes S/MIME is used as a secure message transport for 515 applications beyond interpersonal messaging. In such cases, the 516 S/MIME-enabled application can specify additional requirements 517 concerning the digitalSignature or nonRepudiation bits within this 518 extension. 520 If the key usage extension is not specified, receiving clients MUST 521 presume that the digitalSignature and nonRepudiation bits are set. 523 4.4.3. Subject Alternative Name 525 The subject alternative name extension is used in S/MIME as the 526 preferred means to convey the RFC-2822 email address(es) that 527 correspond(s) to the entity for this certificate. Any RFC-2822 email 528 addresses present MUST be encoded using the rfc822Name CHOICE of the 529 GeneralName type. Since the SubjectAltName type is a SEQUENCE OF 530 GeneralName, multiple RFC-2822 email addresses MAY be present. 532 4.4.4. Extended Key Usage Extension 534 The extended key usage extension also serves to limit the technical 535 purposes for which a public key listed in a valid certificate may be 536 used. The set of technical purposes for the certificate therefore 537 are the intersection of the uses indicated in the key usage and 538 extended key usage extensions. 540 For example, if the certificate contains a key usage extension 541 indicating digital signature and an extended key usage extension 542 which includes the email protection OID, then the certificate may be 543 used for signing but not encrypting S/MIME messages. If the 544 certificate contains a key usage extension indicating digital 545 signature, but no extended key usage extension then the certificate 546 may also be used to sign but not encrypt S/MIME messages. 548 If the extended key usage extension is present in the certificate 549 then interpersonal message S/MIME receiving agents MUST check that it 550 contains either the emailProtection or the anyExtendedKeyUsage OID as 551 defined in [KEYM]. S/MIME uses other than interpersonal messaging 552 MAY require the explicit presence of the extended key usage extension 553 or other OIDs to be present in the extension or both. 555 5. IANA Considerations 557 None: All identifiers are already registered. Please remove this 558 section prior to publication as an RFC. 560 6. Security Considerations 562 All of the security issues faced by any cryptographic application 563 must be faced by a S/MIME agent. Among these issues are protecting 564 the user's private key, preventing various attacks, and helping the 565 user avoid mistakes such as inadvertently encrypting a message for 566 the wrong recipient. The entire list of security considerations is 567 beyond the scope of this document, but some significant concerns are 568 listed here. 570 When processing certificates, there are many situations where the 571 processing might fail. Because the processing may be done by a user 572 agent, a security gateway, or other program, there is no single way 573 to handle such failures. Just because the methods to handle the 574 failures has not been listed, however, the reader should not assume 575 that they are not important. The opposite is true: if a certificate 576 is not provably valid and associated with the message, the processing 577 software should take immediate and noticeable steps to inform the end 578 user about it. 580 Some of the many places where signature and certificate checking 581 might fail include: 583 - no Internet mail addresses in a certificate matches the sender of 584 a message, if the certificate contains at least one mail address 586 - no certificate chain leads to a trusted CA 588 - no ability to check the CRL for a certificate 590 - an invalid CRL was received 592 - the CRL being checked is expired 594 - the certificate is expired 596 - the certificate has been revoked 598 There are certainly other instances where a certificate may be 599 invalid, and it is the responsibility of the processing software to 600 check them all thoroughly, and to decide what to do if the check 601 fails. 603 At the Selected Areas in Cryptography '95 conference in May 1995, 604 Rogier and Chauvaud presented an attack on MD2 that can nearly find 605 collisions [RC95]. Collisions occur when one can find two different 606 messages that generate the same message digest. A checksum operation 607 in MD2 is the only remaining obstacle to the success of the attack. 608 For this reason, the use of MD2 for new applications is discouraged. 609 It is still reasonable to use MD2 to verify existing signatures, as 610 the ability to find collisions in MD2 does not enable an attacker to 611 find new messages having a previously computed hash value. 613 It is possible for there to be multiple unexpired CRLs for a CA. If 614 an agent is consulting CRLs for certificate validation, it SHOULD 615 make sure that the most recently issued CRL for that CA is consulted, 616 since an S/MIME message sender could deliberately include an older 617 unexpired CRL in an S/MIME message. This older CRL might not include 618 recent revoked certificates, which might lead an agent to accept a 619 certificate that has been revoked in a subsequent CRL. 621 When determining the time for a certificate validity check, agents 622 have to be careful to use a reliable time. Unless it is from a 623 trusted agent, this time MUST NOT be the SigningTime attribute found 624 in an S/MIME message. For most sending agents, the SigningTime 625 attribute could be deliberately set to direct the receiving agent to 626 check a CRL that could have out-of-date revocation status for a 627 certificate, or cause an improper result when checking the Validity 628 field of a certificate. 630 Appendix A. References 632 A.1. Normative References 634 [ACAUTH] Farrell, S. and R. Housley, "An Internet Attribute 635 Certificate Profile for Authorization", RFC 3281, April 636 2002. 638 [CMS] Housely, R., "Cryptographic Message Syntax (CMS)", RFC 639 3852, July 2004. 641 Housley, R., "Cryptographic Message Syntax (CMS) 642 Multiple Signer Clarification", RFC 4852, April 2007. 644 [CMSALG] Housley, R., "Cryptographic Message Syntax (CMS) 645 Algorithms", RFC 3370, August 2002. 647 [CMS-SHA2] Turner. S., "Using SHA2 Algorithms with Cryptographic 648 Message Syntax", work in progress. 650 [KEYM] Cooper, D., Santesson, S., Farrell, S., Boeyen, S. 651 Housley, R., and W. Polk, "Internet X.509 Public Key 652 Infrastructure Certificate and Certificate Revocation 653 List (CRL) Profile", work in progress. 655 [MUSTSHOULD] Bradner, S., "Key words for use in RFCs to Indicate 656 Requirement Levels", BCP 14, RFC 2119, March 1997. 658 [PKCS9] Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object 659 Classes and Attribute Types Version 2.0", RFC 2985, 660 November 2000. 662 [RFC-2822] Resnick, P., "Internet Message Format", RFC 2822, April 663 2001. 665 [RSAPSS] Schaad, J., "Use of RSASA-PSS Signature Algorithm in 666 Cryptographic Message Syntax (CMS)", RFC 4056, June 667 2005. 669 [SMIME-MSG] Ramsdell, B., and S. Turner, "S/MIME Version 3.2 670 Message Specification", work in progress. 672 [X.208-88] ITU-T. Recommendation X.208: Specification of Abstract 673 Syntax Notation One (ASN.1). 1988. 675 A.2. Informative References 677 [PKCS6] RSA Laboratories, "PKCS #6: Extended-Certificate Syntax 678 Standard", November 1993. 680 [RC95] Rogier, N. and Chauvaud, P., "The compression function 681 of MD2 is not collision free," Presented at Selected 682 Areas in Cryptography '95, May 1995. 684 [SECLABEL] Nicolls, W., "Implementing Company Classification 685 Policy with the S/MIME Security Label", RFC 3114, May 686 2002. 688 [X.500] ITU-T Recommendation X.500 (1997) | ISO/IEC 9594- 689 1:1997, Information technology - Open Systems 690 Interconnection - The Directory: Overview of concepts, 691 models and services. 693 [X.501] ITU-T Recommendation X.501 (1997) | ISO/IEC 9594- 694 2:1997, Information technology - Open Systems 695 Interconnection - The Directory: Models. 697 [X.509] ITU-T Recommendation X.509 (1997) | ISO/IEC 9594- 698 8:1997, Information technology - Open Systems 699 Interconnection - The Directory: Authentication 700 framework. 702 [X.520] ITU-T Recommendation X.520 (1997) | ISO/IEC 9594- 703 6:1997, Information technology - Open Systems 704 Interconnection - The Directory: Selected attribute 705 types. 707 Appendix B. Acknowledgements 709 Many thanks go out to the other authors of the S/MIME v2 RFC: Steve 710 Dusse, Paul Hoffman and Jeff Weinstein. Without v2, there wouldn't 711 be a v3. 713 A number of the members of the S/MIME Working Group have also worked 714 very hard and contributed to v3 of this document. Any list of people 715 is doomed to omission and for that I apologize. In alphabetical 716 order, the following people stand out in my mind due to the fact that 717 they made direct contributions to this document. 719 Bill Flanigan, Trevor Freeman, Elliott Ginsburg, Paul Hoffman, Russ 720 Housley, David P. Kemp, Michael Myers, John Pawling, Denis Pinkas, 721 Jim Schaad. 723 Author's Addresses 725 Blake Ramsdell 726 SendMail 728 Email: ramsdell@sendmail.com 730 Sean Turner 732 IECA, Inc. 733 3057 Nutley Street, Suite 106 734 Fairfax, VA 22031 735 USA 737 Email: turners@ieca.com 739 Full Copyright Statement 741 Copyright (C) The IETF Trust (2008). 743 This document is subject to the rights, licenses and restrictions 744 contained in BCP 78, and except as set forth therein, the authors 745 retain all their rights. 747 This document and the information contained herein are provided on an 748 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 749 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 750 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 751 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 752 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 753 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 755 Intellectual Property 757 The IETF takes no position regarding the validity or scope of any 758 Intellectual Property Rights or other rights that might be claimed to 759 pertain to the implementation or use of the technology described in 760 this document or the extent to which any license under such rights 761 might or might not be available; nor does it represent that it has 762 made any independent effort to identify any such rights. 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