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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 June 3, 2008 4 Obsoletes: 3850 (once approved) 5 Expires: December 3, 2008 7 Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 8 Certificate Handling 9 draft-ietf-smime-3850bis-03.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 December 3, 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 Discussion 54 This draft is being discussed on the 'ietf-smime' mailing list. To 55 subscribe, send a message to ietf-smime-request@imc.org with the 56 single word subscribe in the body of the message. There is a Web site 57 for the mailing list at . 59 Table of Contents 61 1. Introduction...................................................2 62 1.1. Definitions...............................................3 63 1.2. Conventions used in this document.........................4 64 1.3. Compatibility with Prior Practice S/MIME..................4 65 1.4. Changes Since S/MIME V3.1 (RFC 3850)......................4 66 2. CMS Options....................................................5 67 2.1. Certificate Revocation Lists..............................5 68 2.2. Certificate Choices.......................................5 69 2.2.1. Historical Note About CMS Certificates...............5 70 2.3. CertificateSet............................................6 71 3. Using Distinguished Names For Internet Mail....................7 72 4. Certificate Processing.........................................8 73 4.1. Certificate Revocation Lists..............................9 74 4.2. Certificate Path Validation...............................9 75 4.3. Certificate and CRL Signing Algorithms and Key Sizes.....10 76 4.4. PKIX Certificate Extensions..............................11 77 4.4.1. Basic Constraints...................................11 78 4.4.2. Key Usage Certificate Extension.....................12 79 4.4.3. Subject Alternative Name............................12 80 4.4.4. Extended Key Usage Extension........................12 81 5. IANA Considerations...........................................13 82 6. Security Considerations.......................................13 83 Appendix A. References...........................................15 84 A.1. Normative References.....................................15 85 A.2. Informative References...................................16 86 Appendix B. Acknowledgements.....................................17 88 1. Introduction 90 S/MIME (Secure/Multipurpose Internet Mail Extensions), described in 91 [SMIME-MSG], provides a method to send and receive secure MIME 92 messages. Before using a public key to provide security services, 93 the S/MIME agent MUST verify that the public key is valid. S/MIME 94 agents MUST use PKIX certificates to validate public keys as 95 described in the Internet X.509 Public Key Infrastructure (PKIX) 96 Certificate and CRL Profile [KEYM]. S/MIME agents MUST meet the 97 certificate processing requirements documented in this document in 98 addition to those stated in [KEYM]. 100 This specification is compatible with the Cryptographic Message 101 Syntax [CMS] in that it uses the data types defined by CMS. It also 102 inherits all the varieties of architectures for certificate-based key 103 management supported by CMS. 105 1.1. Definitions 107 For the purposes of this document, the following definitions apply. 109 ASN.1: Abstract Syntax Notation One, as defined in ITU-T X.208 110 [X.208-88]. 112 Attribute Certificate (AC): An X.509 AC is a separate structure from 113 a subject's public key X.509 Certificate. A subject may have 114 multiple X.509 ACs associated with each of its public key X.509 115 Certificates. Each X.509 AC binds one or more Attributes with one of 116 the subject's public key X.509 Certificates. The X.509 AC syntax is 117 defined in [ACAUTH]. 119 Certificate: A type that binds an entity's name to a public key with 120 a digital signature. This type is defined in the Internet X.509 121 Public Key Infrastructure (PKIX) Certificate and CRL Profile [KEYM]. 122 This type also contains the distinguished name of the certificate 123 issuer (the signer), an issuer-specific serial number, the issuer's 124 signature algorithm identifier, a validity period, and extensions 125 also defined in that document. 127 Certificate Revocation List (CRL): A type that contains information 128 about certificates whose validity an issuer has prematurely revoked. 129 The information consists of an issuer name, the time of issue, the 130 next scheduled time of issue, a list of certificate serial numbers 131 and their associated revocation times, and extensions as defined in 132 [KEYM]. The CRL is signed by the issuer. The type intended by this 133 specification is the one defined in [KEYM]. 135 Receiving agent: Software that interprets and processes S/MIME CMS 136 objects, MIME body parts that contain CMS objects, or both. 138 Sending agent: Software that creates S/MIME CMS objects, MIME body 139 parts that contain CMS objects, or both. 141 S/MIME agent: User software that is a receiving agent, a sending 142 agent, or both. 144 1.2. Conventions used in this document 146 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 147 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 148 document are to be interpreted as described in [MUSTSHOULD]. 150 We define some additional terms here: 152 SHOULD+ This term means the same as SHOULD. However, the authors 153 expect that a requirement marked as SHOULD+ will be promoted at 154 some future time to be a MUST. 156 SHOULD- This term means the same as SHOULD. However, the authors 157 expect a requirement marked as SHOULD- will be demoted to a MAY 158 in a future version of this document. 160 MUST- This term means the same as MUST. However, the authors 161 expect that this requirement will no longer be a MUST in a future 162 document. Although its status will be determined at a later 163 time, it is reasonable to expect that if a future revision of a 164 document alters the status of a MUST- requirement, it will remain 165 at least a SHOULD or a SHOULD-. 167 1.3. Compatibility with Prior Practice S/MIME 169 S/MIME version 3.2 agents should attempt to have the greatest 170 interoperability possible with agents for prior versions of S/MIME. 171 S/MIME version 2 is described in RFC 2311 through RFC 2315, inclusive 172 S/MIME version 3 is described in RFC 2630 through RFC 2634 inclusive, 173 and S/MIME version 3.1 is described in RFC 3850 through 3851 174 inclusive and RFC 2634. RFC 2311 also has historical information 175 about the development of S/MIME. 177 1.4. Changes Since S/MIME V3.1 (RFC 3850) 179 Conventions Used in This Document: Moved to section 1.2. Added 180 definitions for SHOULD+, SHOULD-, and MUST-. 182 Sec 1.2: Added text about v3.1 RFCs. 184 Sec 3: Updated note to indicate emailAddress IA5String upper bound is 185 255 characters. 187 Sec 4.3: RSA with SHA-256 (PKCS #1 v1.5) added as MUST, RSA with SHA- 188 1 changed to MUST-, DSA with SHA-1, and RSA with MD5 changed to 189 SHOULD-, and RSA-PS with SHA-256. Updated key sizes. 191 Sec A.1: Updated references to latest versions of PKIX profile and 192 S/MIME Message Specification. 194 Sec A.1: Changed reference from KEYMALG to KEYM. 196 2. CMS Options 198 The CMS message format allows for a wide variety of options in 199 content and algorithm support. This section puts forth a number of 200 support requirements and recommendations in order to achieve a base 201 level of interoperability among all S/MIME implementations. Most of 202 the CMS format for S/MIME messages is defined in [SMIME-MSG]. 204 2.1. Certificate Revocation Lists 206 Receiving agents MUST support the Certificate Revocation List (CRL) 207 format defined in [KEYM]. If sending agents include CRLs in outgoing 208 messages, the CRL format defined in [KEYM] MUST be used. In all 209 cases, both v1 and v2 CRLs MUST be supported. 211 All agents MUST be capable of performing revocation checks using CRLs 212 as specified in [KEYM]. All agents MUST perform revocation status 213 checking in accordance with [KEYM]. Receiving agents MUST recognize 214 CRLs in received S/MIME messages. 216 Agents SHOULD store CRLs received in messages for use in processing 217 later messages. 219 2.2. Certificate Choices 221 Receiving agents MUST support v1 X.509 and v3 X.509 identity 222 certificates as profiled in [KEYM]. End entity certificates MAY 223 include an Internet mail address, as described in section 3. 225 Receiving agents SHOULD support X.509 version 2 attribute 226 certificates. See [ACAUTH] for details about the profile for 227 attribute certificates. 229 2.2.1. Historical Note About CMS Certificates 231 The CMS message format supports a choice of certificate formats for 232 public key content types: PKIX, PKCS #6 Extended Certificates [PKCS6] 233 and PKIX Attribute Certificates. 235 The PKCS #6 format is not in widespread use. In addition, PKIX 236 certificate extensions address much of the same functionality and 237 flexibility as was intended in the PKCS #6. Thus, sending and 238 receiving agents MUST NOT use PKCS #6 extended certificates. 240 X.509 version 1 attribute certificates are also not widely 241 implemented, and have been superseded with version 2 attribute 242 certificates. Sending agents MUST NOT send version 1 attribute 243 certificates. 245 2.3. CertificateSet 247 Receiving agents MUST be able to handle an arbitrary number of 248 certificates of arbitrary relationship to the message sender and to 249 each other in arbitrary order. In many cases, the certificates 250 included in a signed message may represent a chain of certification 251 from the sender to a particular root. There may be, however, 252 situations where the certificates in a signed message may be 253 unrelated and included for convenience. 255 Sending agents SHOULD include any certificates for the user's public 256 key(s) and associated issuer certificates. This increases the 257 likelihood that the intended recipient can establish trust in the 258 originator's public key(s). This is especially important when 259 sending a message to recipients that may not have access to the 260 sender's public key through any other means or when sending a signed 261 message to a new recipient. The inclusion of certificates in 262 outgoing messages can be omitted if S/MIME objects are sent within a 263 group of correspondents that has established access to each other's 264 certificates by some other means such as a shared directory or manual 265 certificate distribution. Receiving S/MIME agents SHOULD be able to 266 handle messages without certificates using a database or directory 267 lookup scheme. 269 A sending agent SHOULD include at least one chain of certificates up 270 to, but not including, a Certificate Authority (CA) that it believes 271 that the recipient may trust as authoritative. A receiving agent 272 MUST be able to handle an arbitrarily large number of certificates 273 and chains. 275 Agents MAY send CA certificates, that is, certificates which can be 276 considered the "root" of other chains, and which MAY be self-signed. 277 Note that receiving agents SHOULD NOT simply trust any self-signed 278 certificates as valid CAs, but SHOULD use some other mechanism to 279 determine if this is a CA that should be trusted. Also note that 280 when certificates contain DSA public keys the parameters may be 281 located in the root certificate. This would require that the 282 recipient possess both the end-entity certificate as well as the root 283 certificate to perform a signature verification, and is a valid 284 example of a case where transmitting the root certificate may be 285 required. 287 Receiving agents MUST support chaining based on the distinguished 288 name fields. Other methods of building certificate chains MAY be 289 supported. 291 Receiving agents SHOULD support the decoding of X.509 attribute 292 certificates included in CMS objects. All other issues regarding the 293 generation and use of X.509 attribute certificates are outside of the 294 scope of this specification. One specification that addresses 295 attribute certificate use is defined in [SECLABEL]. 297 3. Using Distinguished Names For Internet Mail 299 End-entity certificates MAY contain an Internet mail address as 300 described in [IMF]. The address must be an "addr-spec" as defined in 301 Section 3.4.1 of that specification. The email address SHOULD be in 302 the subjectAltName extension, and SHOULD NOT be in the subject 303 distinguished name. 305 Receiving agents MUST recognize and accept certificates that contain 306 no email address. Agents are allowed to provide an alternative 307 mechanism for associating an email address with a certificate that 308 does not contain an email address, such as through the use of the 309 agent's address book, if available. Receiving agents MUST recognize 310 email addresses in the subjectAltName field. Receiving agents MUST 311 recognize email addresses in the Distinguished Name field in the PKCS 312 #9 [PKCS9] emailAddress attribute: 314 pkcs-9-at-emailAddress OBJECT IDENTIFIER ::= 315 { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 1 } 317 Note that this attribute MUST be encoded as IA5String and has an 318 upper bound of 255 characters. 320 Sending agents SHOULD make the address in the From or Sender header 321 in a mail message match an Internet mail address in the signer's 322 certificate. Receiving agents MUST check that the address in the 323 From or Sender header of a mail message matches an Internet mail 324 address, if present, in the signer's certificate, if mail addresses 325 are present in the certificate. A receiving agent SHOULD provide 326 some explicit alternate processing of the message if this comparison 327 fails, which may be to display a message that shows the recipient the 328 addresses in the certificate or other certificate details. 330 A receiving agent SHOULD display a subject name or other certificate 331 details when displaying an indication of successful or unsuccessful 332 signature verification. 334 All subject and issuer names MUST be populated (i.e., not an empty 335 SEQUENCE) in S/MIME-compliant X.509 identity certificates, except 336 that the subject DN in a user's (i.e., end-entity) certificate MAY be 337 an empty SEQUENCE in which case the subjectAltName extension will 338 include the subject's identifier and MUST be marked as critical. 340 4. Certificate Processing 342 A receiving agent needs to provide some certificate retrieval 343 mechanism in order to gain access to certificates for recipients of 344 digital envelopes. There are many ways to implement certificate 345 retrieval mechanisms. X.500 directory service is an excellent 346 example of a certificate retrieval-only mechanism that is compatible 347 with classic X.500 Distinguished Names. Another method under 348 consideration by the IETF is to provide certificate retrieval 349 services as part of the existing Domain Name System (DNS). Until 350 such mechanisms are widely used, their utility may be limited by the 351 small number of correspondent's certificates that can be retrieved. 352 At a minimum, for initial S/MIME deployment, a user agent could 353 automatically generate a message to an intended recipient requesting 354 that recipient's certificate in a signed return message. 356 Receiving and sending agents SHOULD also provide a mechanism to allow 357 a user to "store and protect" certificates for correspondents in such 358 a way so as to guarantee their later retrieval. In many 359 environments, it may be desirable to link the certificate 360 retrieval/storage mechanisms together in some sort of certificate 361 database. In its simplest form, a certificate database would be 362 local to a particular user and would function in a similar way as an 363 "address book" that stores a user's frequent correspondents. In this 364 way, the certificate retrieval mechanism would be limited to the 365 certificates that a user has stored (presumably from incoming 366 messages). A comprehensive certificate retrieval/storage solution 367 may combine two or more mechanisms to allow the greatest flexibility 368 and utility to the user. For instance, a secure Internet mail agent 369 may resort to checking a centralized certificate retrieval mechanism 370 for a certificate if it can not be found in a user's local 371 certificate storage/retrieval database. 373 Receiving and sending agents SHOULD provide a mechanism for the 374 import and export of certificates, using a CMS certs-only message. 375 This allows for import and export of full certificate chains as 376 opposed to just a single certificate. This is described in [SMIME- 377 MSG]. 379 Agents MUST handle multiple valid Certification Authority (CA) 380 certificates containing the same subject name and the same public 381 keys but with overlapping validity intervals. 383 4.1. Certificate Revocation Lists 385 In general, it is always better to get the latest CRL information 386 from a CA than to get information stored away from incoming messages. 387 A receiving agent SHOULD have access to some certificate revocation 388 list (CRL) retrieval mechanism in order to gain access to certificate 389 revocation information when validating certification paths. A 390 receiving or sending agent SHOULD also provide a mechanism to allow a 391 user to store incoming certificate revocation information for 392 correspondents in such a way so as to guarantee its later retrieval. 394 Receiving and sending agents SHOULD retrieve and utilize CRL 395 information every time a certificate is verified as part of a 396 certification path validation even if the certificate was already 397 verified in the past. However, in many instances (such as off-line 398 verification) access to the latest CRL information may be difficult 399 or impossible. The use of CRL information, therefore, may be 400 dictated by the value of the information that is protected. The 401 value of the CRL information in a particular context is beyond the 402 scope of this specification but may be governed by the policies 403 associated with particular certification paths. 405 All agents MUST be capable of performing revocation checks using CRLs 406 as specified in [KEYM]. All agents MUST perform revocation status 407 checking in accordance with [KEYM]. Receiving agents MUST recognize 408 CRLs in received S/MIME messages. 410 4.2. Certificate Path Validation 412 In creating a user agent for secure messaging, certificate, CRL, and 413 certification path validation SHOULD be highly automated while still 414 acting in the best interests of the user. Certificate, CRL, and path 415 validation MUST be performed as per [KEYM] when validating a 416 correspondent's public key. This is necessary before using a public 417 key to provide security services such as: verifying a signature; 418 encrypting a content-encryption key (ex: RSA); or forming a pairwise 419 symmetric key (ex: Diffie-Hellman) to be used to encrypt or decrypt a 420 content-encryption key. 422 Certificates and CRLs are made available to the path validation 423 procedure in two ways: a) incoming messages, and b) certificate and 424 CRL retrieval mechanisms. Certificates and CRLs in incoming messages 425 are not required to be in any particular order nor are they required 426 to be in any way related to the sender or recipient of the message 427 (although in most cases they will be related to the sender). Incoming 428 certificates and CRLs SHOULD be cached for use in path validation and 429 optionally stored for later use. This temporary certificate and CRL 430 cache SHOULD be used to augment any other certificate and CRL 431 retrieval mechanisms for path validation on incoming signed messages. 433 4.3. Certificate and CRL Signing Algorithms and Key Sizes 435 Certificates and Certificate Revocation Lists (CRLs) are signed by 436 the certificate issuer. Receiving agents: 438 - MUST support RSA with SHA-256, as specified in [CMS-SHA2] 440 - MUST- support RSA with SHA-1, as specified in [CMSALG] 442 - SHOULD+ support RSA-PSS with SHA-256, as specified in [RSAPSS] 444 - SHOULD- support DSA with SHA-1, as specified in [CMSALG]. 446 - SHOULD- support RSA with MD5, as specified in [CMSALG]. 448 The following RSA and DSA key size requirements for S/MIME receiving 449 agents during certificate and CRL signature verification: 451 0 < key size < 512 : MAY (see Section 6) 452 512 <= key size <= 2048 : MUST (see Section 6) 453 2048 < key size <= 4096 : MAY (see Section 6) 455 4.4. PKIX Certificate Extensions 457 PKIX describes an extensible framework in which the basic certificate 458 information can be extended and how such extensions can be used to 459 control the process of issuing and validating certificates. The PKIX 460 Working Group has ongoing efforts to identify and create extensions 461 which have value in particular certification environments. Further, 462 there are active efforts underway to issue PKIX certificates for 463 business purposes. This document identifies the minimum required set 464 of certificate extensions which have the greatest value in the S/MIME 465 environment. The syntax and semantics of all the identified 466 extensions are defined in [KEYM]. 468 Sending and receiving agents MUST correctly handle the basic 469 constraints, key usage, authority key identifier, subject key 470 identifier, and subject alternative names certificate extensions when 471 they appear in end-entity and CA certificates. Some mechanism SHOULD 472 exist to gracefully handle other certificate extensions when they 473 appear in end-entity or CA certificates. 475 Certificates issued for the S/MIME environment SHOULD NOT contain any 476 critical extensions (extensions that have the critical field set to 477 TRUE) other than those listed here. These extensions SHOULD be 478 marked as non-critical unless the proper handling of the extension is 479 deemed critical to the correct interpretation of the associated 480 certificate. Other extensions may be included, but those extensions 481 SHOULD NOT be marked as critical. 483 Interpretation and syntax for all extensions MUST follow [KEYM], 484 unless otherwise specified here. 486 4.4.1. Basic Constraints 488 The basic constraints extension serves to delimit the role and 489 position that an issuing authority or end-entity certificate plays in 490 a certification path. 492 For example, certificates issued to CAs and subordinate CAs contain a 493 basic constraint extension that identifies them as issuing authority 494 certificates. End-entity certificates contain an extension that 495 constrains the certificate from being an issuing authority 496 certificate. 498 Certificates SHOULD contain a basicConstraints extension in CA 499 certificates, and SHOULD NOT contain that extension in end entity 500 certificates. 502 4.4.2. Key Usage Certificate Extension 504 The key usage extension serves to limit the technical purposes for 505 which a public key listed in a valid certificate may be used. Issuing 506 authority certificates may contain a key usage extension that 507 restricts the key to signing certificates, certificate revocation 508 lists and other data. 510 For example, a certification authority may create subordinate issuer 511 certificates which contain a key usage extension which specifies that 512 the corresponding public key can be used to sign end user 513 certificates and sign CRLs. 515 If a key usage extension is included in a PKIX certificate, then it 516 MUST be marked as critical. 518 S/MIME receiving agents MUST NOT accept the signature of a message if 519 it was verified using a certificate which contains the key usage 520 extension without either the digitalSignature or nonRepudiation bit 521 set. Sometimes S/MIME is used as a secure message transport for 522 applications beyond interpersonal messaging. In such cases, the 523 S/MIME-enabled application can specify additional requirements 524 concerning the digitalSignature or nonRepudiation bits within this 525 extension. 527 If the key usage extension is not specified, receiving clients MUST 528 presume that the digitalSignature and nonRepudiation bits are set. 530 4.4.3. Subject Alternative Name 532 The subject alternative name extension is used in S/MIME as the 533 preferred means to convey the RFC-2822 email address(es) that 534 correspond(s) to the entity for this certificate. Any RFC-2822 email 535 addresses present MUST be encoded using the rfc822Name CHOICE of the 536 GeneralName type. Since the SubjectAltName type is a SEQUENCE OF 537 GeneralName, multiple RFC-2822 email addresses MAY be present. 539 4.4.4. Extended Key Usage Extension 541 The extended key usage extension also serves to limit the technical 542 purposes for which a public key listed in a valid certificate may be 543 used. The set of technical purposes for the certificate therefore 544 are the intersection of the uses indicated in the key usage and 545 extended key usage extensions. 547 For example, if the certificate contains a key usage extension 548 indicating digital signature and an extended key usage extension 549 which includes the email protection OID, then the certificate may be 550 used for signing but not encrypting S/MIME messages. If the 551 certificate contains a key usage extension indicating digital 552 signature, but no extended key usage extension then the certificate 553 may also be used to sign but not encrypt S/MIME messages. 555 If the extended key usage extension is present in the certificate 556 then interpersonal message S/MIME receiving agents MUST check that it 557 contains either the emailProtection or the anyExtendedKeyUsage OID as 558 defined in [KEYM]. S/MIME uses other than interpersonal messaging 559 MAY require the explicit presence of the extended key usage extension 560 or other OIDs to be present in the extension or both. 562 5. IANA Considerations 564 None: All identifiers are already registered. Please remove this 565 section prior to publication as an RFC. 567 6. Security Considerations 569 All of the security issues faced by any cryptographic application 570 must be faced by a S/MIME agent. Among these issues are protecting 571 the user's private key, preventing various attacks, and helping the 572 user avoid mistakes such as inadvertently encrypting a message for 573 the wrong recipient. The entire list of security considerations is 574 beyond the scope of this document, but some significant concerns are 575 listed here. 577 When processing certificates, there are many situations where the 578 processing might fail. Because the processing may be done by a user 579 agent, a security gateway, or other program, there is no single way 580 to handle such failures. Just because the methods to handle the 581 failures has not been listed, however, the reader should not assume 582 that they are not important. The opposite is true: if a certificate 583 is not provably valid and associated with the message, the processing 584 software should take immediate and noticeable steps to inform the end 585 user about it. 587 Some of the many places where signature and certificate checking 588 might fail include: 590 - no Internet mail addresses in a certificate matches the sender of 591 a message, if the certificate contains at least one mail address 593 - no certificate chain leads to a trusted CA 595 - no ability to check the CRL for a certificate 596 - an invalid CRL was received 598 - the CRL being checked is expired 600 - the certificate is expired 602 - the certificate has been revoked 604 There are certainly other instances where a certificate may be 605 invalid, and it is the responsibility of the processing software to 606 check them all thoroughly, and to decide what to do if the check 607 fails. 609 It is possible for there to be multiple unexpired CRLs for a CA. If 610 an agent is consulting CRLs for certificate validation, it SHOULD 611 make sure that the most recently issued CRL for that CA is consulted, 612 since an S/MIME message sender could deliberately include an older 613 unexpired CRL in an S/MIME message. This older CRL might not include 614 recent revoked certificates, which might lead an agent to accept a 615 certificate that has been revoked in a subsequent CRL. 617 When determining the time for a certificate validity check, agents 618 have to be careful to use a reliable time. Unless it is from a 619 trusted agent, this time MUST NOT be the SigningTime attribute found 620 in an S/MIME message. For most sending agents, the SigningTime 621 attribute could be deliberately set to direct the receiving agent to 622 check a CRL that could have out-of-date revocation status for a 623 certificate, or cause an improper result when checking the Validity 624 field of a certificate. 626 Appendix A. References 628 A.1. Normative References 630 [ACAUTH] Farrell, S. and R. Housley, "An Internet Attribute 631 Certificate Profile for Authorization", RFC 3281, April 632 2002. 634 [CMS] Housely, R., "Cryptographic Message Syntax (CMS)", RFC 635 3852, July 2004. 637 Housley, R., "Cryptographic Message Syntax (CMS) 638 Multiple Signer Clarification", RFC 4852, April 2007. 640 [CMSALG] Housley, R., "Cryptographic Message Syntax (CMS) 641 Algorithms", RFC 3370, August 2002. 643 [CMS-SHA2] Turner. S., "Using SHA2 Algorithms with Cryptographic 644 Message Syntax", work in progress. 646 [KEYM] Cooper, D., Santesson, S., Farrell, S., Boeyen, S. 647 Housley, R., and W. Polk, "Internet X.509 Public Key 648 Infrastructure Certificate and Certificate Revocation 649 List (CRL) Profile", RFC 5280, May 2008. 651 [MUSTSHOULD] Bradner, S., "Key words for use in RFCs to Indicate 652 Requirement Levels", BCP 14, RFC 2119, March 1997. 654 [PKCS9] Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object 655 Classes and Attribute Types Version 2.0", RFC 2985, 656 November 2000. 658 [IMF] Resnick, P., "Internet Message Format", work-in- 659 progress. 661 [RSAPSS] Schaad, J., "Use of RSASA-PSS Signature Algorithm in 662 Cryptographic Message Syntax (CMS)", RFC 4056, June 663 2005. 665 [SMIME-MSG] Ramsdell, B., and S. Turner, "S/MIME Version 3.2 666 Message Specification", work in progress. 668 [X.208-88] ITU-T. Recommendation X.208: Specification of Abstract 669 Syntax Notation One (ASN.1). 1988. 671 A.2. Informative References 673 [PKCS6] RSA Laboratories, "PKCS #6: Extended-Certificate Syntax 674 Standard", November 1993. 676 [RC95] Rogier, N. and Chauvaud, P., "The compression function 677 of MD2 is not collision free," Presented at Selected 678 Areas in Cryptography '95, May 1995. 680 [SECLABEL] Nicolls, W., "Implementing Company Classification 681 Policy with the S/MIME Security Label", RFC 3114, May 682 2002. 684 [X.500] ITU-T Recommendation X.500 (1997) | ISO/IEC 9594- 685 1:1997, Information technology - Open Systems 686 Interconnection - The Directory: Overview of concepts, 687 models and services. 689 [X.501] ITU-T Recommendation X.501 (1997) | ISO/IEC 9594- 690 2:1997, Information technology - Open Systems 691 Interconnection - The Directory: Models. 693 [X.509] ITU-T Recommendation X.509 (1997) | ISO/IEC 9594- 694 8:1997, Information technology - Open Systems 695 Interconnection - The Directory: Authentication 696 framework. 698 [X.520] ITU-T Recommendation X.520 (1997) | ISO/IEC 9594- 699 6:1997, Information technology - Open Systems 700 Interconnection - The Directory: Selected attribute 701 types. 703 Appendix B. Acknowledgements 705 Many thanks go out to the other authors of the S/MIME v2 RFC: Steve 706 Dusse, Paul Hoffman and Jeff Weinstein. Without v2, there wouldn't 707 be a v3. 709 A number of the members of the S/MIME Working Group have also worked 710 very hard and contributed to v3 of this document. Any list of people 711 is doomed to omission and for that I apologize. In alphabetical 712 order, the following people stand out in my mind due to the fact that 713 they made direct contributions to this document. 715 Bill Flanigan, Trevor Freeman, Elliott Ginsburg, Paul Hoffman, Russ 716 Housley, David P. Kemp, Michael Myers, John Pawling, Denis Pinkas, 717 and Jim Schaad. 719 Author's Addresses 721 Blake Ramsdell 722 SendMail 724 Email: blake@sendmail.com 726 Sean Turner 728 IECA, Inc. 729 3057 Nutley Street, Suite 106 730 Fairfax, VA 22031 731 USA 733 Email: turners@ieca.com 735 Full Copyright Statement 737 Copyright (C) The IETF Trust (2008). 739 This document is subject to the rights, licenses and restrictions 740 contained in BCP 78, and except as set forth therein, the authors 741 retain all their rights. 743 This document and the information contained herein are provided on an 744 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 745 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 746 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 747 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 748 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 749 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 751 Intellectual Property 753 The IETF takes no position regarding the validity or scope of any 754 Intellectual Property Rights or other rights that might be claimed to 755 pertain to the implementation or use of the technology described in 756 this document or the extent to which any license under such rights 757 might or might not be available; nor does it represent that it has 758 made any independent effort to identify any such rights. 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