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Is this intentional? -- Found something which looks like a code comment -- if you have code sections in the document, please surround them with '' and '' lines. 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? 'MUSTSHOULD' on line 635 looks like a reference -- Missing reference section? 'SMIME-MSG' on line 645 looks like a reference -- Missing reference section? 'KEYM' on line 630 looks like a reference -- Missing reference section? 'CMS' on line 618 looks like a reference -- Missing reference section? 'ACAUTH' on line 614 looks like a reference -- Missing reference section? 'PKCS6' on line 653 looks like a reference -- Missing reference section? 'SECLABEL' on line 660 looks like a reference -- Missing reference section? 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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 November 5, 2007 4 Expires: April 5, 2008 6 Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 7 Certificate Handling 8 draft-ietf-smime-3850bis-00.txt 10 Status of this Memo 12 By submitting this Internet-Draft, each author represents that any 13 applicable patent or other IPR claims of which he or she is aware 14 have been or will be disclosed, and any of which he or she becomes 15 aware will be disclosed, in accordance with Section 6 of BCP 79. 17 Internet-Drafts are working documents of the Internet Engineering 18 Task Force (IETF), its areas, and its working groups. Note that 19 other groups may also distribute working documents as Internet- 20 Drafts. 22 Internet-Drafts are draft documents valid for a maximum of six months 23 and may be updated, replaced, or obsoleted by other documents at any 24 time. It is inappropriate to use Internet-Drafts as reference 25 material or to cite them other than as "work in progress." 27 The list of current Internet-Drafts can be accessed at 28 http://www.ietf.org/ietf/1id-abstracts.txt 30 The list of Internet-Draft Shadow Directories can be accessed at 31 http://www.ietf.org/shadow.html 33 This Internet-Draft will expire on April 5, 2007. 35 Copyright Notice 37 Copyright (C) The IETF Trust (2007). 39 Abstract 41 This document specifies conventions for X.509 certificate usage by 42 Secure/Multipurpose Internet Mail Extensions (S/MIME) agents. S/MIME 43 provides a method to send and receive secure MIME messages, and 44 certificates are an integral part of S/MIME agent processing. S/MIME 45 agents validate certificates as described in RFC 3280bis, the 46 Internet X.509 Public Key Infrastructure Certificate and CRL Profile. 47 S/MIME agents must meet the certificate processing requirements in 48 this document as well as those in RFC 3280bis. This document 49 obsoletes RFC 3850. 51 Conventions used in this document 53 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 54 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 55 document are to be interpreted as described in [MUSTSHOULD]. 57 Discussion 59 This draft is being discussed on the 'ietf-smime' mailing list. To 60 subscribe, send a message to ietf-smime-request@imc.org with the 61 single word subscribe in the body of the message. There is a Web site 62 for the mailing list at . 64 Table of Contents 66 1. Introduction...................................................2 67 1.1. Definitions...............................................3 68 1.2. Compatibility with Prior Practice S/MIME..................4 69 1.3. Changes Since S/MIME V3.1 (RFC 3850)......................4 70 2. CMS Options....................................................4 71 2.1. Certificate Revocation Lists..............................4 72 2.2. Certificate Choices.......................................5 73 2.2.1. Historical Note About CMS Certificates...............5 74 2.3. CertificateSet............................................5 75 3. Using Distinguished Names For Internet Mail....................6 76 4. Certificate Processing.........................................7 77 4.1. Certificate Revocation Lists..............................8 78 4.2. Certificate Path Validation...............................9 79 4.3. Certificate and CRL Signing Algorithms...................10 80 4.4. PKIX Certificate Extensions..............................10 81 4.4.1. Basic Constraints...................................11 82 4.4.2. Key Usage Certificate Extension.....................11 83 4.4.3. Subject Alternative Name............................12 84 4.4.4. Extended Key Usage Extension........................12 85 5. IANA Considerations...........................................12 86 6. Security Considerations.......................................12 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. Compatibility with Prior Practice S/MIME 146 S/MIME version 3.2 agents should attempt to have the greatest 147 interoperability possible with agents for prior versions of S/MIME. 148 S/MIME version 2 is described in RFC 2311 through RFC 2315, inclusive 149 S/MIME version 3 is described in RFC 2630 through RFC 2634 inclusive, 150 and S/MIME version 3.1 is described in RFC 3850 through 3851 151 inclusive and RFC 2634. RFC 2311 also has historical information 152 about the development of S/MIME. 154 1.3. Changes Since S/MIME V3.1 (RFC 3850) 156 Sec 1.2: Added text about v3.1 RFCs. 158 Sec 3: Updated note to indicate emailAddress IA5String upper bound is 159 255 characters. 161 Sec 4.3: RSA with SHA-256 (PKCS #1 v1.5) added as MUST, RSA with SHA- 162 1 changed to MUST-, DSA with SHA-1, and RSA with MD5 changed to 163 SHOULD-, and RSA-PS with SHA-256, ECDSA with SHA-256 added as 164 SHOULD+. 166 Sec A.1: Updated references to latest versions of PKIX profile and 167 S/MIME Message Specification. 169 Sec A.1: Changed reference from KEYMALG to KEYM. 171 2. CMS Options 173 The CMS message format allows for a wide variety of options in 174 content and algorithm support. This section puts forth a number of 175 support requirements and recommendations in order to achieve a base 176 level of interoperability among all S/MIME implementations. Most of 177 the CMS format for S/MIME messages is defined in [SMIME-MSG]. 179 2.1. Certificate Revocation Lists 181 Receiving agents MUST support the Certificate Revocation List (CRL) 182 format defined in [KEYM]. If sending agents include CRLs in outgoing 183 messages, the CRL format defined in [KEYM] MUST be used. In all 184 cases, both v1 and v2 CRLs MUST be supported. 186 All agents MUST be capable of performing revocation checks using CRLs 187 as specified in [KEYM]. All agents MUST perform revocation status 188 checking in accordance with [KEYM]. Receiving agents MUST recognize 189 CRLs in received S/MIME messages. 191 Agents SHOULD store CRLs received in messages for use in processing 192 later messages. 194 2.2. Certificate Choices 196 Receiving agents MUST support v1 X.509 and v3 X.509 identity 197 certificates as profiled in [KEYM]. End entity certificates MAY 198 include an Internet mail address, as described in section 3. 200 Receiving agents SHOULD support X.509 version 2 attribute 201 certificates. See [ACAUTH] for details about the profile for 202 attribute certificates. 204 2.2.1. Historical Note About CMS Certificates 206 The CMS message format supports a choice of certificate formats for 207 public key content types: PKIX, PKCS #6 Extended Certificates [PKCS6] 208 and PKIX Attribute Certificates. 210 The PKCS #6 format is not in widespread use. In addition, PKIX 211 certificate extensions address much of the same functionality and 212 flexibility as was intended in the PKCS #6. Thus, sending and 213 receiving agents MUST NOT use PKCS #6 extended certificates. 215 X.509 version 1 attribute certificates are also not widely 216 implemented, and have been superseded with version 2 attribute 217 certificates. Sending agents MUST NOT send version 1 attribute 218 certificates. 220 2.3. CertificateSet 222 Receiving agents MUST be able to handle an arbitrary number of 223 certificates of arbitrary relationship to the message sender and to 224 each other in arbitrary order. In many cases, the certificates 225 included in a signed message may represent a chain of certification 226 from the sender to a particular root. There may be, however, 227 situations where the certificates in a signed message may be 228 unrelated and included for convenience. 230 Sending agents SHOULD include any certificates for the user's public 231 key(s) and associated issuer certificates. This increases the 232 likelihood that the intended recipient can establish trust in the 233 originator's public key(s). This is especially important when 234 sending a message to recipients that may not have access to the 235 sender's public key through any other means or when sending a signed 236 message to a new recipient. The inclusion of certificates in 237 outgoing messages can be omitted if S/MIME objects are sent within a 238 group of correspondents that has established access to each other's 239 certificates by some other means such as a shared directory or manual 240 certificate distribution. Receiving S/MIME agents SHOULD be able to 241 handle messages without certificates using a database or directory 242 lookup scheme. 244 A sending agent SHOULD include at least one chain of certificates up 245 to, but not including, a Certificate Authority (CA) that it believes 246 that the recipient may trust as authoritative. A receiving agent 247 MUST be able to handle an arbitrarily large number of certificates 248 and chains. 250 Agents MAY send CA certificates, that is, certificates which can be 251 considered the "root" of other chains, and which MAY be self-signed. 252 Note that receiving agents SHOULD NOT simply trust any self-signed 253 certificates as valid CAs, but SHOULD use some other mechanism to 254 determine if this is a CA that should be trusted. Also note that 255 when certificates contain DSA public keys the parameters may be 256 located in the root certificate. This would require that the 257 recipient possess both the end-entity certificate as well as the root 258 certificate to perform a signature verification, and is a valid 259 example of a case where transmitting the root certificate may be 260 required. 262 Receiving agents MUST support chaining based on the distinguished 263 name fields. Other methods of building certificate chains MAY be 264 supported. 266 Receiving agents SHOULD support the decoding of X.509 attribute 267 certificates included in CMS objects. All other issues regarding the 268 generation and use of X.509 attribute certificates are outside of the 269 scope of this specification. One specification that addresses 270 attribute certificate use is defined in [SECLABEL]. 272 3. Using Distinguished Names For Internet Mail 274 End-entity certificates MAY contain an Internet mail address as 275 described in [RFC-2822]. The address must be an "addr-spec" as 276 defined in Section 3.4.1 of that specification. The email address 277 SHOULD be in the subjectAltName extension, and SHOULD NOT be in the 278 subject distinguished name. 280 Receiving agents MUST recognize and accept certificates that contain 281 no email address. Agents are allowed to provide an alternative 282 mechanism for associating an email address with a certificate that 283 does not contain an email address, such as through the use of the 284 agent's address book, if available. Receiving agents MUST recognize 285 email addresses in the subjectAltName field. Receiving agents MUST 286 recognize email addresses in the Distinguished Name field in the PKCS 287 #9 [PKCS9] emailAddress attribute: 289 pkcs-9-at-emailAddress OBJECT IDENTIFIER ::= 291 {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 1 } 293 Note that this attribute MUST be encoded as IA5String and has an 294 upper bounds of 255 characters. 296 Sending agents SHOULD make the address in the From or Sender header 297 in a mail message match an Internet mail address in the signer's 298 certificate. Receiving agents MUST check that the address in the 299 From or Sender header of a mail message matches an Internet mail 300 address, if present, in the signer's certificate, if mail addresses 301 are present in the certificate. A receiving agent SHOULD provide 302 some explicit alternate processing of the message if this comparison 303 fails, which may be to display a message that shows the recipient the 304 addresses in the certificate or other certificate details. 306 A receiving agent SHOULD display a subject name or other certificate 307 details when displaying an indication of successful or unsuccessful 308 signature verification. 310 All subject and issuer names MUST be populated (i.e., not an empty 311 SEQUENCE) in S/MIME-compliant X.509 identity certificates, except 312 that the subject DN in a user's (i.e., end-entity) certificate MAY be 313 an empty SEQUENCE in which case the subjectAltName extension will 314 include the subject's identifier and MUST be marked as critical. 316 4. Certificate Processing 318 A receiving agent needs to provide some certificate retrieval 319 mechanism in order to gain access to certificates for recipients of 320 digital envelopes. There are many ways to implement certificate 321 retrieval mechanisms. X.500 directory service is an excellent 322 example of a certificate retrieval-only mechanism that is compatible 323 with classic X.500 Distinguished Names. Another method under 324 consideration by the IETF is to provide certificate retrieval 325 services as part of the existing Domain Name System (DNS). Until 326 such mechanisms are widely used, their utility may be limited by the 327 small number of correspondent's certificates that can be retrieved. 328 At a minimum, for initial S/MIME deployment, a user agent could 329 automatically generate a message to an intended recipient requesting 330 that recipient's certificate in a signed return message. 332 Receiving and sending agents SHOULD also provide a mechanism to allow 333 a user to "store and protect" certificates for correspondents in such 334 a way so as to guarantee their later retrieval. In many 335 environments, it may be desirable to link the certificate 336 retrieval/storage mechanisms together in some sort of certificate 337 database. In its simplest form, a certificate database would be 338 local to a particular user and would function in a similar way as an 339 "address book" that stores a user's frequent correspondents. In this 340 way, the certificate retrieval mechanism would be limited to the 341 certificates that a user has stored (presumably from incoming 342 messages). A comprehensive certificate retrieval/storage solution 343 may combine two or more mechanisms to allow the greatest flexibility 344 and utility to the user. For instance, a secure Internet mail agent 345 may resort to checking a centralized certificate retrieval mechanism 346 for a certificate if it can not be found in a user's local 347 certificate storage/retrieval database. 349 Receiving and sending agents SHOULD provide a mechanism for the 350 import and export of certificates, using a CMS certs-only message. 351 This allows for import and export of full certificate chains as 352 opposed to just a single certificate. This is described in [SMIME- 353 MSG]. 355 Agents MUST handle multiple valid Certification Authority (CA) 356 certificates containing the same subject name and the same public 357 keys but with overlapping validity intervals. 359 4.1. Certificate Revocation Lists 361 In general, it is always better to get the latest CRL information 362 from a CA than to get information stored away from incoming messages. 363 A receiving agent SHOULD have access to some certificate revocation 364 list (CRL) retrieval mechanism in order to gain access to certificate 365 revocation information when validating certification paths. A 366 receiving or sending agent SHOULD also provide a mechanism to allow a 367 user to store incoming certificate revocation information for 368 correspondents in such a way so as to guarantee its later retrieval. 370 Receiving and sending agents SHOULD retrieve and utilize CRL 371 information every time a certificate is verified as part of a 372 certification path validation even if the certificate was already 373 verified in the past. However, in many instances (such as off-line 374 verification) access to the latest CRL information may be difficult 375 or impossible. The use of CRL information, therefore, may be 376 dictated by the value of the information that is protected. The 377 value of the CRL information in a particular context is beyond the 378 scope of this specification but may be governed by the policies 379 associated with particular certification paths. 381 All agents MUST be capable of performing revocation checks using CRLs 382 as specified in [KEYM]. All agents MUST perform revocation status 383 checking in accordance with [KEYM]. Receiving agents MUST recognize 384 CRLs in received S/MIME messages. 386 4.2. Certificate Path Validation 388 In creating a user agent for secure messaging, certificate, CRL, and 389 certification path validation SHOULD be highly automated while still 390 acting in the best interests of the user. Certificate, CRL, and path 391 validation MUST be performed as per [KEYM] when validating a 392 correspondent's public key. This is necessary before using a public 393 key to provide security services such as: verifying a signature; 394 encrypting a content-encryption key (ex: RSA); or forming a pairwise 395 symmetric key (ex: Diffie-Hellman) to be used to encrypt or decrypt a 396 content-encryption key. 398 Certificates and CRLs are made available to the path validation 399 procedure in two ways: a) incoming messages, and b) certificate and 400 CRL retrieval mechanisms. Certificates and CRLs in incoming messages 401 are not required to be in any particular order nor are they required 402 to be in any way related to the sender or recipient of the message 403 (although in most cases they will be related to the sender). Incoming 404 certificates and CRLs SHOULD be cached for use in path validation and 405 optionally stored for later use. This temporary certificate and CRL 406 cache SHOULD be used to augment any other certificate and CRL 407 retrieval mechanisms for path validation on incoming signed messages. 409 4.3. Certificate and CRL Signing Algorithms 411 Certificates and Certificate Revocation Lists (CRLs) are signed by 412 the certificate issuer. Receiving agents: 414 - MUST support RSA with SHA-256, as specified in [CMS-SHA2] 416 - MUST- support RSA with SHA-1, as specified in [CMSALG] 418 - SHOULD+ support RSA-PSS with SHA-256, as specified in [RSAPSS] 420 - SHOULD+ support ECDSA with SHA-256, as specified in [CMS-SHA2] 422 - SHOULD- support DSA with SHA-1, as specified in [CMSALG]. 424 - SHOULD- support RSA with MD5, as specified in [CMSALG]. 426 Key sizes from 512 bits to 2048 bits MUST be supported. 428 4.4. PKIX Certificate Extensions 430 PKIX describes an extensible framework in which the basic certificate 431 information can be extended and how such extensions can be used to 432 control the process of issuing and validating certificates. The PKIX 433 Working Group has ongoing efforts to identify and create extensions 434 which have value in particular certification environments. Further, 435 there are active efforts underway to issue PKIX certificates for 436 business purposes. This document identifies the minimum required set 437 of certificate extensions which have the greatest value in the S/MIME 438 environment. The syntax and semantics of all the identified 439 extensions are defined in [KEYM]. 441 Sending and receiving agents MUST correctly handle the basic 442 constraints, key usage, authority key identifier, subject key 443 identifier, and subject alternative names certificate extensions when 444 they appear in end-entity and CA certificates. Some mechanism SHOULD 445 exist to gracefully handle other certificate extensions when they 446 appear in end-entity or CA certificates. 448 Certificates issued for the S/MIME environment SHOULD NOT contain any 449 critical extensions (extensions that have the critical field set to 450 TRUE) other than those listed here. These extensions SHOULD be 451 marked as non-critical unless the proper handling of the extension is 452 deemed critical to the correct interpretation of the associated 453 certificate. Other extensions may be included, but those extensions 454 SHOULD NOT be marked as critical. 456 Interpretation and syntax for all extensions MUST follow [KEYM], 457 unless otherwise specified here. 459 4.4.1. Basic Constraints 461 The basic constraints extension serves to delimit the role and 462 position that an issuing authority or end-entity certificate plays in 463 a certification path. 465 For example, certificates issued to CAs and subordinate CAs contain a 466 basic constraint extension that identifies them as issuing authority 467 certificates. End-entity certificates contain an extension that 468 constrains the certificate from being an issuing authority 469 certificate. 471 Certificates SHOULD contain a basicConstraints extension in CA 472 certificates, and SHOULD NOT contain that extension in end entity 473 certificates. 475 4.4.2. Key Usage Certificate Extension 477 The key usage extension serves to limit the technical purposes for 478 which a public key listed in a valid certificate may be used. Issuing 479 authority certificates may contain a key usage extension that 480 restricts the key to signing certificates, certificate revocation 481 lists and other data. 483 For example, a certification authority may create subordinate issuer 484 certificates which contain a key usage extension which specifies that 485 the corresponding public key can be used to sign end user 486 certificates and sign CRLs. 488 If a key usage extension is included in a PKIX certificate, then it 489 MUST be marked as critical. 491 S/MIME receiving agents MUST NOT accept the signature of a message if 492 it was verified using a certificate which contains the key usage 493 extension without either the digitalSignature or nonRepudiation bit 494 set. Sometimes S/MIME is used as a secure message transport for 495 applications beyond interpersonal messaging. In such cases, the 496 S/MIME-enabled application can specify additional requirements 497 concerning the digitalSignature or nonRepudiation bits within this 498 extension. 500 If the key usage extension is not specified, receiving clients MUST 501 presume that the digitalSignature and nonRepudiation bits are set. 503 4.4.3. Subject Alternative Name 505 The subject alternative name extension is used in S/MIME as the 506 preferred means to convey the RFC-2822 email address(es) that 507 correspond(s) to the entity for this certificate. Any RFC-2822 email 508 addresses present MUST be encoded using the rfc822Name CHOICE of the 509 GeneralName type. Since the SubjectAltName type is a SEQUENCE OF 510 GeneralName, multiple RFC-2822 email addresses MAY be present. 512 4.4.4. Extended Key Usage Extension 514 The extended key usage extension also serves to limit the technical 515 purposes for which a public key listed in a valid certificate may be 516 used. The set of technical purposes for the certificate therefore 517 are the intersection of the uses indicated in the key usage and 518 extended key usage extensions. 520 For example, if the certificate contains a key usage extension 521 indicating digital signature and an extended key usage extension 522 which includes the email protection OID, then the certificate may be 523 used for signing but not encrypting S/MIME messages. If the 524 certificate contains a key usage extension indicating digital 525 signature, but no extended key usage extension then the certificate 526 may also be used to sign but not encrypt S/MIME messages. 528 If the extended key usage extension is present in the certificate 529 then interpersonal message S/MIME receiving agents MUST check that it 530 contains either the emailProtection or the anyExtendedKeyUsage OID as 531 defined in [KEYM]. S/MIME uses other than interpersonal messaging 532 MAY require the explicit presence of the extended key usage extension 533 or other OIDs to be present in the extension or both. 535 5. IANA Considerations 537 None: All identifiers are already registered. Please remove this 538 section prior to publication as an RFC. 540 6. Security Considerations 542 All of the security issues faced by any cryptographic application 543 must be faced by a S/MIME agent. Among these issues are protecting 544 the user's private key, preventing various attacks, and helping the 545 user avoid mistakes such as inadvertently encrypting a message for 546 the wrong recipient. The entire list of security considerations is 547 beyond the scope of this document, but some significant concerns are 548 listed here. 550 When processing certificates, there are many situations where the 551 processing might fail. Because the processing may be done by a user 552 agent, a security gateway, or other program, there is no single way 553 to handle such failures. Just because the methods to handle the 554 failures has not been listed, however, the reader should not assume 555 that they are not important. The opposite is true: if a certificate 556 is not provably valid and associated with the message, the processing 557 software should take immediate and noticeable steps to inform the end 558 user about it. 560 Some of the many places where signature and certificate checking 561 might fail include: 563 - no Internet mail addresses in a certificate matches the sender of 564 a message, if the certificate contains at least one mail address 566 - no certificate chain leads to a trusted CA 568 - no ability to check the CRL for a certificate 570 - an invalid CRL was received 572 - the CRL being checked is expired 574 - the certificate is expired 576 - the certificate has been revoked 578 There are certainly other instances where a certificate may be 579 invalid, and it is the responsibility of the processing software to 580 check them all thoroughly, and to decide what to do if the check 581 fails. 583 At the Selected Areas in Cryptography '95 conference in May 1995, 584 Rogier and Chauvaud presented an attack on MD2 that can nearly find 585 collisions [RC95]. Collisions occur when one can find two different 586 messages that generate the same message digest. A checksum operation 587 in MD2 is the only remaining obstacle to the success of the attack. 588 For this reason, the use of MD2 for new applications is discouraged. 589 It is still reasonable to use MD2 to verify existing signatures, as 590 the ability to find collisions in MD2 does not enable an attacker to 591 find new messages having a previously computed hash value. 593 It is possible for there to be multiple unexpired CRLs for a CA. If 594 an agent is consulting CRLs for certificate validation, it SHOULD 595 make sure that the most recently issued CRL for that CA is consulted, 596 since an S/MIME message sender could deliberately include an older 597 unexpired CRL in an S/MIME message. This older CRL might not include 598 recent revoked certificates, which might lead an agent to accept a 599 certificate that has been revoked in a subsequent CRL. 601 When determining the time for a certificate validity check, agents 602 have to be careful to use a reliable time. Unless it is from a 603 trusted agent, this time MUST NOT be the SigningTime attribute found 604 in an S/MIME message. For most sending agents, the SigningTime 605 attribute could be deliberately set to direct the receiving agent to 606 check a CRL that could have out-of-date revocation status for a 607 certificate, or cause an improper result when checking the Validity 608 field of a certificate. 610 Appendix A. References 612 A.1. Normative References 614 [ACAUTH] Farrell, S. and R. Housley, "An Internet Attribute 615 Certificate Profile for Authorization", RFC 3281, April 616 2002. 618 [CMS] Housely, R., "Cryptographic Message Syntax (CMS)", RFC 619 3852, July 2004. 621 Housley, R., "Cryptographic Message Syntax (CMS) 622 Multiple Signer Clarification", RFC 4852, April 2007. 624 [CMSALG] Housley, R., "Cryptographic Message Syntax (CMS) 625 Algorithms", RFC 3370, August 2002. 627 [CMS-SHA2] Turner. S., "Using SHA2 Algorithms with Cryptographic 628 Message Syntax", work in progress. 630 [KEYM] Cooper, D., Santesson, S., Farrell, S., Boeyen, S. 631 Housley, R., and W. Polk, "Internet X.509 Public Key 632 Infrastructure Certificate and Certificate Revocation 633 List (CRL) Profile", work in progress. 635 [MUSTSHOULD] Bradner, S., "Key words for use in RFCs to Indicate 636 Requirement Levels", BCP 14, RFC 2119, March 1997. 638 [PKCS9] Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object 639 Classes and Attribute Types Version 2.0", RFC 2985, 640 November 2000. 642 [RFC-2822] Resnick, P., "Internet Message Format", RFC 2822, April 643 2001. 645 [SMIME-MSG] Ramsdell, B., and S. Turner, "S/MIME Version 3.2 646 Message Specification", work in progress. 648 [X.208-88] ITU-T. Recommendation X.208: Specification of Abstract 649 Syntax Notation One (ASN.1). 1988. 651 A.2. Informative References 653 [PKCS6] RSA Laboratories, "PKCS #6: Extended-Certificate Syntax 654 Standard", November 1993. 656 [RC95] Rogier, N. and Chauvaud, P., "The compression function 657 of MD2 is not collision free," Presented at Selected 658 Areas in Cryptography '95, May 1995. 660 [SECLABEL] Nicolls, W., "Implementing Company Classification 661 Policy with the S/MIME Security Label", RFC 3114, May 662 2002. 664 [X.500] ITU-T Recommendation X.500 (1997) | ISO/IEC 9594- 665 1:1997, Information technology - Open Systems 666 Interconnection - The Directory: Overview of concepts, 667 models and services. 669 [X.501] ITU-T Recommendation X.501 (1997) | ISO/IEC 9594- 670 2:1997, Information technology - Open Systems 671 Interconnection - The Directory: Models. 673 [X.509] ITU-T Recommendation X.509 (1997) | ISO/IEC 9594- 674 8:1997, Information technology - Open Systems 675 Interconnection - The Directory: Authentication 676 framework. 678 [X.520] ITU-T Recommendation X.520 (1997) | ISO/IEC 9594- 679 6:1997, Information technology - Open Systems 680 Interconnection - The Directory: Selected attribute 681 types. 683 Appendix B. Acknowledgements 685 Many thanks go out to the other authors of the S/MIME v2 RFC: Steve 686 Dusse, Paul Hoffman and Jeff Weinstein. Without v2, there wouldn't 687 be a v3. 689 A number of the members of the S/MIME Working Group have also worked 690 very hard and contributed to v3 of this document. Any list of people 691 is doomed to omission and for that I apologize. In alphabetical 692 order, the following people stand out in my mind due to the fact that 693 they made direct contributions to this document. 695 Bill Flanigan, Trevor Freeman, Elliott Ginsburg, Paul Hoffman, Russ 696 Housley, David P. Kemp, Michael Myers, John Pawling, Denis Pinkas, 697 Jim Schaad. 699 Author's Addresses 701 Blake Ramsdell 702 SendMail 704 Email: ramsdell (at) sendmail (dot) com 706 Sean Turner 707 IECA, Inc. 709 Email: turners (at) ieca (dot) com 711 Full Copyright Statement 713 Copyright (C) The IETF Trust (2007). 715 This document is subject to the rights, licenses and restrictions 716 contained in BCP 78, and except as set forth therein, the authors 717 retain all their rights. 719 This document and the information contained herein are provided on an 720 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 721 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 722 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 723 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 724 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 725 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 727 Intellectual Property 729 The IETF takes no position regarding the validity or scope of any 730 Intellectual Property Rights or other rights that might be claimed to 731 pertain to the implementation or use of the technology described in 732 this document or the extent to which any license under such rights 733 might or might not be available; nor does it represent that it has 734 made any independent effort to identify any such rights. 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