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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 S/MIME WG Blake Ramsdell, Brute Squad Labs 2 Internet Draft Sean Turner, IECA 3 Intended Status: Standard Track September 18, 2008 4 Obsoletes: 3850 (once approved) 5 Expires: March 18, 2009 7 Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 8 Certificate Handling 9 draft-ietf-smime-3850bis-06.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 March 18, 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 5280, the Internet 47 X.509 Public Key Infrastructure Certificate and CRL Profile. S/MIME 48 agents must meet the certificate processing requirements in this 49 document as well as those in RFC 5280. This document obsoletes RFC 50 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 From S/MIME v3 to S/MIME v3.1.....................4 66 1.5. Changes Since S/MIME v3.1.................................5 67 2. CMS Options....................................................6 68 2.1. Certificate Revocation Lists..............................6 69 2.2. Certificate Choices.......................................6 70 2.2.1. Historical Note About CMS Certificates...............6 71 2.3. CertificateSet............................................7 72 3. Using Distinguished Names For Internet Mail....................8 73 4. Certificate Processing.........................................9 74 4.1. Certificate Revocation Lists.............................10 75 4.2. Certificate Path Validation..............................10 76 4.3. Certificate and CRL Signing Algorithms and Key Sizes.....11 77 4.4. PKIX Certificate Extensions..............................12 78 5. IANA Considerations...........................................14 79 6. Security Considerations.......................................14 80 7. References....................................................16 81 7.1. Normative References.....................................16 82 7.2. Informative References...................................17 83 Appendix A. Moving S/MIME v2 Certificate Handling to Historic 84 Status...............................................20 85 Appendix B. Acknowledgements.....................................20 87 1. Introduction 89 S/MIME (Secure/Multipurpose Internet Mail Extensions), described in 90 [SMIME-MSG], provides a method to send and receive secure MIME 91 messages. Before using a public key to provide security services, 92 the S/MIME agent MUST verify that the public key is valid. S/MIME 93 agents MUST use PKIX certificates to validate public keys as 94 described in the Internet X.509 Public Key Infrastructure (PKIX) 95 Certificate and CRL Profile [KEYM]. S/MIME agents MUST meet the 96 certificate processing requirements documented in this document in 97 addition to those stated in [KEYM]. 99 This specification is compatible with the Cryptographic Message 100 Syntax [CMS] in that it uses the data types defined by CMS. It also 101 inherits all the varieties of architectures for certificate-based key 102 management supported by CMS. 104 1.1. Definitions 106 For the purposes of this document, the following definitions apply. 108 ASN.1: Abstract Syntax Notation One, as defined in ITU-T X.680 109 [X.680]. 111 Attribute Certificate (AC): An X.509 AC is a separate structure from 112 a subject's public key X.509 Certificate. A subject may have 113 multiple X.509 ACs associated with each of its public key X.509 114 Certificates. Each X.509 AC binds one or more Attributes with one of 115 the subject's public key X.509 Certificates. The X.509 AC syntax is 116 defined in [ACAUTH]. 118 Certificate: A type that binds an entity's name to a public key with 119 a digital signature. This type is defined in the Internet X.509 120 Public Key Infrastructure (PKIX) Certificate and CRL Profile [KEYM]. 121 This type also contains the distinguished name of the certificate 122 issuer (the signer), an issuer-specific serial number, the issuer's 123 signature algorithm identifier, a validity period, and extensions 124 also defined in that document. 126 Certificate Revocation List (CRL): A type that contains information 127 about certificates whose validity an issuer has prematurely revoked. 128 The information consists of an issuer name, the time of issue, the 129 next scheduled time of issue, a list of certificate serial numbers 130 and their associated revocation times, and extensions as defined in 131 [KEYM]. The CRL is signed by the issuer. The type intended by this 132 specification is the one defined in [KEYM]. 134 Receiving agent: Software that interprets and processes S/MIME CMS 135 objects, MIME body parts that contain CMS objects, or both. 137 Sending agent: Software that creates S/MIME CMS objects, MIME body 138 parts that contain CMS objects, or both. 140 S/MIME agent: User software that is a receiving agent, a sending 141 agent, or both. 143 1.2. Conventions used in this document 145 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 146 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 147 document are to be interpreted as described in [MUSTSHOULD]. 149 We define some additional terms here: 151 SHOULD+ This term means the same as SHOULD. However, the authors 152 expect that a requirement marked as SHOULD+ will be promoted at 153 some future time to be a MUST. 155 SHOULD- This term means the same as SHOULD. However, the authors 156 expect a requirement marked as SHOULD- will be demoted to a MAY 157 in a future version of this document. 159 MUST- This term means the same as MUST. However, the authors 160 expect that this requirement will no longer be a MUST in a future 161 document. Although its status will be determined at a later 162 time, it is reasonable to expect that if a future revision of a 163 document alters the status of a MUST- requirement, it will remain 164 at least a SHOULD or a SHOULD-. 166 1.3. Compatibility with Prior Practice S/MIME 168 S/MIME version 3.2 agents should attempt to have the greatest 169 interoperability possible with agents for prior versions of S/MIME. 170 S/MIME version 2 is described in RFC 2311 through RFC 2315 inclusive 171 [SMIMEv2], S/MIME version 3 is described in RFC 2630 through RFC 2634 172 inclusive and RFC 5035 [SMIMEv3], and S/MIME version 3.1 is described 173 in RFC 3850, RFC 3851, RFC 3852, RFC 2634, RFC4853, and RFC 5035 174 [SMIMEv3.1]. RFC 2311 also has historical information about the 175 development of S/MIME. 177 1.4. Changes From S/MIME v3 To S/MIME v3.1 179 Version 1 and Version 2 CRLs MUST be supported. 181 Multiple CA certificates with the same subject and public key, but 182 with overlapping validity periods, MUST be supported. 184 Version 2 attribute certificates SHOULD be supported, and version 1 185 attributes certificates MUST NOT be used. 187 The use of the MD2 digest algorithm for certificate signatures is 188 discouraged and security language added. 190 Clarified use of email address use in certificates. Certificates 191 that do not contain an email address have no requirements for 192 verifying the email address associated with the certificate. 194 Receiving agents SHOULD display certificate information when 195 displaying the results of signature verification. 197 Receiving agents MUST NOT accept a signature made with a certificate 198 that does not have the digitalSignature or nonRepudiation bit set. 200 Clarifications for the interpretation of the key usage and extended 201 key usage extensions. 203 1.5. Changes Since S/MIME v3.1 205 Conventions Used in This Document: Moved to section 1.2. Added 206 definitions for SHOULD+, SHOULD-, and MUST-. 208 Sec 1.2: Updated ASN.1 definition and reference. 210 Sec 1.3: Added text about v3.1 RFCs. 212 Sec 3: Updated note to indicate emailAddress IA5String upper bound is 213 255 characters. 215 Sec 4.2: Added text to indicate how S/MIME agents locate the correct 216 user certificate. 218 Sec 4.3: RSA with SHA-256 (PKCS #1 v1.5) added as MUST, DSA with SHA- 219 256 added as SHOULD+, RSA with SHA-1 changed to SHOULD-, DSA with 220 SHA-1, and RSA with MD5 changed to SHOULD-, and RSA-PSS with SHA-256 221 added as SHOULD+. Updated key sizes and changed pointer from [KEYM] 222 to [KEYMALG]. 224 Sec 4.4.1: Aligned with PKIX on use of basic constraints extension in 225 CA certificates. Clarified which extension is used to constrain EEs 226 from using their keys to perform issuing authority operations. 228 Sec 6: Updated security considerations. 230 Sec 7: Moved references from Appendix B to section 7. Updated the 231 references. 233 Appendix A: Moved Appendix A to Appendix B. Added Appendix A to move 234 S/MIME v2 Certificate Handling to Historic Status. 236 2. CMS Options 238 The CMS message format allows for a wide variety of options in 239 content and algorithm support. This section puts forth a number of 240 support requirements and recommendations in order to achieve a base 241 level of interoperability among all S/MIME implementations. Most of 242 the CMS format for S/MIME messages is defined in [SMIME-MSG]. 244 2.1. Certificate Revocation Lists 246 Receiving agents MUST support the Certificate Revocation List (CRL) 247 format defined in [KEYM]. If sending agents include CRLs in outgoing 248 messages, the CRL format defined in [KEYM] MUST be used. In all 249 cases, both v1 and v2 CRLs MUST be supported. 251 All agents MUST be capable of performing revocation checks using CRLs 252 as specified in [KEYM]. All agents MUST perform revocation status 253 checking in accordance with [KEYM]. Receiving agents MUST recognize 254 CRLs in received S/MIME messages. 256 Agents SHOULD store CRLs received in messages for use in processing 257 later messages. 259 2.2. Certificate Choices 261 Receiving agents MUST support v1 X.509 and v3 X.509 certificates as 262 profiled in [KEYM]. End entity certificates MAY include an Internet 263 mail address, as described in section 3. 265 Receiving agents SHOULD support X.509 version 2 attribute 266 certificates. See [ACAUTH] for details about the profile for 267 attribute certificates. 269 2.2.1. Historical Note About CMS Certificates 271 The CMS message format supports a choice of certificate formats for 272 public key content types: PKIX, PKCS #6 Extended Certificates [PKCS6] 273 and PKIX Attribute Certificates. 275 The PKCS #6 format is not in widespread use. In addition, PKIX 276 certificate extensions address much of the same functionality and 277 flexibility as was intended in the PKCS #6. Thus, sending and 278 receiving agents MUST NOT use PKCS #6 extended certificates. 280 X.509 version 1 attribute certificates are also not widely 281 implemented, and have been superseded with version 2 attribute 282 certificates. Sending agents MUST NOT send version 1 attribute 283 certificates. 285 2.3. CertificateSet 287 Receiving agents MUST be able to handle an arbitrary number of 288 certificates of arbitrary relationship to the message sender and to 289 each other in arbitrary order. In many cases, the certificates 290 included in a signed message may represent a chain of certification 291 from the sender to a particular root. There may be, however, 292 situations where the certificates in a signed message may be 293 unrelated and included for convenience. 295 Sending agents SHOULD include any certificates for the user's public 296 key(s) and associated issuer certificates. This increases the 297 likelihood that the intended recipient can establish trust in the 298 originator's public key(s). This is especially important when 299 sending a message to recipients that may not have access to the 300 sender's public key through any other means or when sending a signed 301 message to a new recipient. The inclusion of certificates in 302 outgoing messages can be omitted if S/MIME objects are sent within a 303 group of correspondents that has established access to each other's 304 certificates by some other means such as a shared directory or manual 305 certificate distribution. Receiving S/MIME agents SHOULD be able to 306 handle messages without certificates using a database or directory 307 lookup scheme. 309 A sending agent SHOULD include at least one chain of certificates up 310 to, but not including, a Certificate Authority (CA) that it believes 311 that the recipient may trust as authoritative. A receiving agent 312 MUST be able to handle an arbitrarily large number of certificates 313 and chains. 315 Agents MAY send CA certificates, that is, cross-certificates, self- 316 issued certificates, and self-signed certificates. Note that 317 receiving agents SHOULD NOT simply trust any self-signed certificates 318 as valid CAs, but SHOULD use some other mechanism to determine if 319 this is a CA that should be trusted. Also note that when 320 certificates contain DSA public keys the parameters may be located in 321 the root certificate. This would require that the recipient possess 322 both the end-entity certificate as well as the root certificate to 323 perform a signature verification, and is a valid example of a case 324 where transmitting the root certificate may be required. 326 Receiving agents MUST support chaining based on the distinguished 327 name fields. Other methods of building certificate chains MAY be 328 supported. 330 Receiving agents SHOULD support the decoding of X.509 attribute 331 certificates included in CMS objects. All other issues regarding the 332 generation and use of X.509 attribute certificates are outside of the 333 scope of this specification. One specification that addresses 334 attribute certificate use is defined in [SECLABEL]. 336 3. Using Distinguished Names For Internet Mail 338 End-entity certificates MAY contain an Internet mail address as 339 described in [IMF]. The address must be an "addr-spec" as defined in 340 Section 3.4.1 of that specification. The email address SHOULD be in 341 the subjectAltName extension, and SHOULD NOT be in the subject 342 distinguished name. 344 Receiving agents MUST recognize and accept certificates that contain 345 no email address. Agents are allowed to provide an alternative 346 mechanism for associating an email address with a certificate that 347 does not contain an email address, such as through the use of the 348 agent's address book, if available. Receiving agents MUST recognize 349 email addresses in the subjectAltName field. Receiving agents MUST 350 recognize email addresses in the Distinguished Name field in the PKCS 351 #9 [PKCS9] emailAddress attribute: 353 pkcs-9-at-emailAddress OBJECT IDENTIFIER ::= 354 { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 1 } 356 Note that this attribute MUST be encoded as IA5String and has an 357 upper bound of 255 characters. 359 Sending agents SHOULD make the address in the From or Sender header 360 in a mail message match an Internet mail address in the signer's 361 certificate. Receiving agents MUST check that the address in the 362 From or Sender header of a mail message matches an Internet mail 363 address, if present, in the signer's certificate, if mail addresses 364 are present in the certificate. A receiving agent SHOULD provide 365 some explicit alternate processing of the message if this comparison 366 fails, which may be to display a message that shows the recipient the 367 addresses in the certificate or other certificate details. 369 A receiving agent SHOULD display a subject name or other certificate 370 details when displaying an indication of successful or unsuccessful 371 signature verification. 373 All subject and issuer names MUST be populated (i.e., not an empty 374 SEQUENCE) in S/MIME-compliant X.509 certificates, except that the 375 subject DN in a user's (i.e., end-entity) certificate MAY be an empty 376 SEQUENCE in which case the subjectAltName extension will include the 377 subject's identifier and MUST be marked as critical. 379 4. Certificate Processing 381 S/MIME agents need to provide some certificate retrieval mechanism in 382 order to gain access to certificates for recipients of digital 383 envelopes. There are many ways to implement certificate retrieval 384 mechanisms. [X.500] directory service is an excellent example of a 385 certificate retrieval-only mechanism that is compatible with classic 386 X.500 Distinguished Names. Another method under consideration by the 387 IETF is to provide certificate retrieval services as part of the 388 existing Domain Name System (DNS). Until such mechanisms are widely 389 used, their utility may be limited by the small number of the 390 correspondent's certificates that can be retrieved. At a minimum, for 391 initial S/MIME deployment, a user agent could automatically generate 392 a message to an intended recipient requesting the recipient's 393 certificate in a signed return message. 395 Receiving and sending agents SHOULD also provide a mechanism to allow 396 a user to "store and protect" certificates for correspondents in such 397 a way so as to guarantee their later retrieval. In many 398 environments, it may be desirable to link the certificate 399 retrieval/storage mechanisms together in some sort of certificate 400 database. In its simplest form, a certificate database would be 401 local to a particular user and would function in a similar way as an 402 "address book" that stores a user's frequent correspondents. In this 403 way, the certificate retrieval mechanism would be limited to the 404 certificates that a user has stored (presumably from incoming 405 messages). A comprehensive certificate retrieval/storage solution 406 may combine two or more mechanisms to allow the greatest flexibility 407 and utility to the user. For instance, a secure Internet mail agent 408 may resort to checking a centralized certificate retrieval mechanism 409 for a certificate if it can not be found in a user's local 410 certificate storage/retrieval database. 412 Receiving and sending agents SHOULD provide a mechanism for the 413 import and export of certificates, using a CMS certs-only message. 414 This allows for import and export of full certificate chains as 415 opposed to just a single certificate. This is described in [SMIME- 416 MSG]. 418 Agents MUST handle multiple valid Certification Authority (CA) 419 certificates containing the same subject name and the same public 420 keys but with overlapping validity intervals. 422 4.1. Certificate Revocation Lists 424 In general, it is always better to get the latest CRL information 425 from a CA than to get information stored away from incoming messages. 426 A receiving agent SHOULD have access to some certificate revocation 427 list (CRL) retrieval mechanism in order to gain access to certificate 428 revocation information when validating certification paths. A 429 receiving or sending agent SHOULD also provide a mechanism to allow a 430 user to store incoming certificate revocation information for 431 correspondents in such a way so as to guarantee its later retrieval. 433 Receiving and sending agents SHOULD retrieve and utilize CRL 434 information every time a certificate is verified as part of a 435 certification path validation even if the certificate was already 436 verified in the past. However, in many instances (such as off-line 437 verification) access to the latest CRL information may be difficult 438 or impossible. The use of CRL information, therefore, may be 439 dictated by the value of the information that is protected. The 440 value of the CRL information in a particular context is beyond the 441 scope of this specification but may be governed by the policies 442 associated with particular certification paths. 444 All agents MUST be capable of performing revocation checks using CRLs 445 as specified in [KEYM]. All agents MUST perform revocation status 446 checking in accordance with [KEYM]. Receiving agents MUST recognize 447 CRLs in received S/MIME messages. 449 4.2. Certificate Path Validation 451 In creating a user agent for secure messaging, certificate, CRL, and 452 certification path validation SHOULD be highly automated while still 453 acting in the best interests of the user. Certificate, CRL, and path 454 validation MUST be performed as per [KEYM] when validating a 455 correspondent's public key. This is necessary before using a public 456 key to provide security services such as: verifying a signature; 457 encrypting a content-encryption key (ex: RSA); or forming a pairwise 458 symmetric key (ex: Diffie-Hellman) to be used to encrypt or decrypt a 459 content-encryption key. 461 Certificates and CRLs are made available to the path validation 462 procedure in two ways: a) incoming messages, and b) certificate and 463 CRL retrieval mechanisms. Certificates and CRLs in incoming messages 464 are not required to be in any particular order nor are they required 465 to be in any way related to the sender or recipient of the message 466 (although in most cases they will be related to the sender). Incoming 467 certificates and CRLs SHOULD be cached for use in path validation and 468 optionally stored for later use. This temporary certificate and CRL 469 cache SHOULD be used to augment any other certificate and CRL 470 retrieval mechanisms for path validation on incoming signed messages. 472 When verifying a signature and the certificates are included in the 473 message, if a signingCertificate or a signingCertificateV2 attribute 474 is found in an S/MIME message, it SHALL be used to identify the 475 signer's certificate. Otherwise, the certificate is identified in an 476 S/MIME message, either using the issuerAndSerialNumber which 477 identifies the signer's certificate by the issuer's distinguished 478 name and the certificate serial number, or the subjectKeyIdentifier 479 which identifies the signer's certificate by a key identifier. 481 When decrypting an encrypted message, if a 482 SMIMEEncryptionKeyPreference attribute is found in an encapsulating 483 SignedData, it SHALL be used to identify the originator's certificate 484 found in OriginatorInfo. Otherwise, a) for DH encrypted messages the 485 certificate is identified by the KeyAgreeRecipientInfo originator 486 field using either the issuer and serial number or subject public key 487 identifier choice or the certificate is omitted and the originator's 488 public key is included in originatorKey b) for RSA encrypted messages 489 the originator's certificate is not required for decryption. 491 4.3. Certificate and CRL Signing Algorithms and Key Sizes 493 Certificates and Certificate Revocation Lists (CRLs) are signed by 494 the certificate issuer. Receiving agents: 496 - MUST support RSA with SHA-256, as specified in [CMS-SHA2] 498 - SHOULD+ support DSA with SHA-256, as specified in [CMS-SHA2] 500 - SHOULD+ support RSA-PSS with SHA-256, as specified in [RSAPSS] 502 - SHOULD- support RSA with SHA-1, as specified in [CMSALG] 504 - SHOULD- support DSA with SHA-1, as specified in [CMSALG] 506 - SHOULD- support RSA with MD5, as specified in [CMSALG] 508 The following are the RSA key size requirements for S/MIME receiving 509 agents during certificate and CRL signature verification: 511 0 < key size < 512 : MAY (see Section 6) 512 512 <= key size <= 4096 : MUST (see Section 6) 513 4096 < key size : MAY (see Section 6) 515 The following are the DSA key size requirements for S/MIME receiving 516 agents during certificate and CRL signature verification: 518 512 <= key size <= 1024 : MAY (see Section 6) 520 4.4. PKIX Certificate Extensions 522 PKIX describes an extensible framework in which the basic certificate 523 information can be extended and describes how such extensions can be 524 used to control the process of issuing and validating certificates. 525 The PKIX Working Group has ongoing efforts to identify and create 526 extensions which have value in particular certification environments. 527 Further, there are active efforts underway to issue PKIX certificates 528 for business purposes. This document identifies the minimum required 529 set of certificate extensions which have the greatest value in the 530 S/MIME environment. The syntax and semantics of all the identified 531 extensions are defined in [KEYM]. 533 Sending and receiving agents MUST correctly handle the basic 534 constraints, key usage, authority key identifier, subject key 535 identifier, and subject alternative names certificate extensions when 536 they appear in end-entity and CA certificates. Some mechanism SHOULD 537 exist to gracefully handle other certificate extensions when they 538 appear in end-entity or CA certificates. 540 Certificates issued for the S/MIME environment SHOULD NOT contain any 541 critical extensions (extensions that have the critical field set to 542 TRUE) other than those listed here. These extensions SHOULD be 543 marked as non-critical unless the proper handling of the extension is 544 deemed critical to the correct interpretation of the associated 545 certificate. Other extensions may be included, but those extensions 546 SHOULD NOT be marked as critical. 548 Interpretation and syntax for all extensions MUST follow [KEYM], 549 unless otherwise specified here. 551 4.4.1. Basic Constraints 553 The basic constraints extension serves to delimit the role and 554 position that an issuing authority or end-entity certificate plays in 555 a certification path. 557 For example, certificates issued to CAs and subordinate CAs contain a 558 basic constraint extension that identifies them as issuing authority 559 certificates. End-entity certificates contain the key usage 560 extension which restrains EEs from using the key to performing 561 issuing authority operations (see Section 4.4.2). 563 As per [KEYM], Certificates MUST contain a basicConstraints extension 564 in CA certificates, and SHOULD NOT contain that extension in end 565 entity certificates. 567 4.4.2. Key Usage Certificate Extension 569 The key usage extension serves to limit the technical purposes for 570 which a public key listed in a valid certificate may be used. Issuing 571 authority certificates may contain a key usage extension that 572 restricts the key to signing certificates, certificate revocation 573 lists and other data. 575 For example, a certification authority may create subordinate issuer 576 certificates which contain a key usage extension which specifies that 577 the corresponding public key can be used to sign end user 578 certificates and sign CRLs. 580 If a key usage extension is included in a PKIX certificate, then it 581 MUST be marked as critical. 583 S/MIME receiving agents MUST NOT accept the signature of a message if 584 it was verified using a certificate which contains the key usage 585 extension without either the digitalSignature or nonRepudiation bit 586 set. Sometimes S/MIME is used as a secure message transport for 587 applications beyond interpersonal messaging. In such cases, the 588 S/MIME-enabled application can specify additional requirements 589 concerning the digitalSignature or nonRepudiation bits within this 590 extension. 592 If the key usage extension is not specified, receiving clients MUST 593 presume that the digitalSignature and nonRepudiation bits are set. 595 4.4.3. Subject Alternative Name 597 The subject alternative name extension is used in S/MIME as the 598 preferred means to convey the RFC-2822 email address(es) that 599 correspond(s) to the entity for this certificate. Any RFC-2822 email 600 addresses present MUST be encoded using the rfc822Name CHOICE of the 601 GeneralName type. Since the SubjectAltName type is a SEQUENCE OF 602 GeneralName, multiple RFC-2822 email addresses MAY be present. 604 4.4.4. Extended Key Usage Extension 606 The extended key usage extension also serves to limit the technical 607 purposes for which a public key listed in a valid certificate may be 608 used. The set of technical purposes for the certificate therefore 609 are the intersection of the uses indicated in the key usage and 610 extended key usage extensions. 612 For example, if the certificate contains a key usage extension 613 indicating digital signature and an extended key usage extension 614 which includes the email protection OID, then the certificate may be 615 used for signing but not encrypting S/MIME messages. If the 616 certificate contains a key usage extension indicating digital 617 signature, but no extended key usage extension then the certificate 618 may also be used to sign but not encrypt S/MIME messages. 620 If the extended key usage extension is present in the certificate 621 then interpersonal message S/MIME receiving agents MUST check that it 622 contains either the emailProtection or the anyExtendedKeyUsage OID as 623 defined in [KEYM]. S/MIME uses other than interpersonal messaging 624 MAY require the explicit presence of the extended key usage extension 625 or other OIDs to be present in the extension or both. 627 5. IANA Considerations 629 None: All identifiers are already registered. Please remove this 630 section prior to publication as an RFC. 632 6. Security Considerations 634 All of the security issues faced by any cryptographic application 635 must be faced by a S/MIME agent. Among these issues are protecting 636 the user's private key, preventing various attacks, and helping the 637 user avoid mistakes such as inadvertently encrypting a message for 638 the wrong recipient. The entire list of security considerations is 639 beyond the scope of this document, but some significant concerns are 640 listed here. 642 When processing certificates, there are many situations where the 643 processing might fail. Because the processing may be done by a user 644 agent, a security gateway, or other program, there is no single way 645 to handle such failures. Just because the methods to handle the 646 failures has not been listed, however, the reader should not assume 647 that they are not important. The opposite is true: if a certificate 648 is not provably valid and associated with the message, the processing 649 software should take immediate and noticeable steps to inform the end 650 user about it. 652 Some of the many places where signature and certificate checking 653 might fail include: 655 - no Internet mail addresses in a certificate matches the sender of 656 a message, if the certificate contains at least one mail address 658 - no certificate chain leads to a trusted CA 660 - no ability to check the CRL for a certificate 662 - an invalid CRL was received 664 - the CRL being checked is expired 666 - the certificate is expired 668 - the certificate has been revoked 670 There are certainly other instances where a certificate may be 671 invalid, and it is the responsibility of the processing software to 672 check them all thoroughly, and to decide what to do if the check 673 fails. 675 It is possible for there to be multiple unexpired CRLs for a CA. If 676 an agent is consulting CRLs for certificate validation, it SHOULD 677 make sure that the most recently issued CRL for that CA is consulted, 678 since an S/MIME message sender could deliberately include an older 679 unexpired CRL in an S/MIME message. This older CRL might not include 680 recently revoked certificates, which might lead an agent to accept a 681 certificate that has been revoked in a subsequent CRL. 683 When determining the time for a certificate validity check, agents 684 have to be careful to use a reliable time. Unless it is from a 685 trusted agent, this time MUST NOT be the SigningTime attribute found 686 in an S/MIME message. For most sending agents, the SigningTime 687 attribute could be deliberately set to direct the receiving agent to 688 check a CRL that could have out-of-date revocation status for a 689 certificate, or cause an improper result when checking the Validity 690 field of a certificate. 692 In addition to the Security Considerations identified in [KEYM], 693 caution should be taken when processing certificates which have not 694 first been validated to a trust anchor. Certificates could be 695 manufactured by untrusted sources for the purpose of mounting denial 696 of service or other attacks. For example, keys selected to require 697 excessive cryptographic processing, or extensive lists of CDP and/or 698 AIA addresses in the certificate, could be used to mount denial of 699 service attacks. Similarly, attacker-specified CRL Distribution 700 Point (CRLDP) and/or Authority Information Access (AIA) addresses 701 could be included in fake certificates to allow the originator to 702 detect receipt of the message even if signature verification fails. 704 The 4096-bit RSA key size requirement for certificate and CRL 705 verification is larger than the 2048-bit RSA key sizes for message 706 signature generation/verification or message encryption/decryption in 707 [SMIME-MSG] because many Root CAs included in certificate stores have 708 already issued Root certificates with 4096-bit key. The standard 709 that defines comparable key sizes for DSA is not yet available. In 710 particular, [FIPS186-3] only defines DSA key sizes up to 1024 bits. 711 A revision to support larger key sizes is being developed, and once 712 it is available, implementors ought to support DSA key sizes 713 comparable to the RSA key sizes recommended in this specification. 715 Today, 512-bit RSA and DSA keys are considered by many experts to be 716 cryptographically insecure. 718 7. References 720 7.1. Normative References 722 [ACAUTH] Farrell, S. and R. Housley, "An Internet Attribute 723 Certificate Profile for Authorization", RFC 3281, April 724 2002. 726 [CMS] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 727 3852, July 2004. 729 Housley, R., "Cryptographic Message Syntax (CMS) 730 Multiple Signer Clarification", RFC 4852, April 2007. 732 [CMSALG] Housley, R., "Cryptographic Message Syntax (CMS) 733 Algorithms", RFC 3370, August 2002. 735 [CMS-SHA2] Turner. S., "Using SHA2 Algorithms with Cryptographic 736 Message Syntax", draft-ietf-smime-sha2, work-in- 737 progress. 739 [FIPS186-3] National Institute of Standards and Technology (NIST), 740 "Digital Signature Standard (DSS)", FIPS Publication 741 186-3, March 2006. 743 [KEYM] Cooper, D., Santesson, S., Farrell, S., Boeyen, S. 744 Housley, R., and W. Polk, "Internet X.509 Public Key 745 Infrastructure Certificate and Certificate Revocation 746 List (CRL) Profile", RFC 5280, May 2008. 748 [KEYMALG] Bassham, L., Polk, W., and R. Housley, "Algorithms and 749 Identifiers for the Internet X.509 Public Key 750 Infrastructure Certificate and Certificate Revocation 751 List (CRL) Profile", RFC 3279, April 2002. 753 [MUSTSHOULD] Bradner, S., "Key words for use in RFCs to Indicate 754 Requirement Levels", BCP 14, RFC 2119, March 1997. 756 [PKCS9] Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object 757 Classes and Attribute Types Version 2.0", RFC 2985, 758 November 2000. 760 [IMF] Resnick, P., "Internet Message Format", work-in- 761 progress. 763 [RSAPSS] Schaad, J., "Use of RSASA-PSS Signature Algorithm in 764 Cryptographic Message Syntax (CMS)", RFC 4056, June 765 2005. 767 [SMIME-MSG] Ramsdell, B., and S. Turner, "S/MIME Version 3.2 768 Message Specification", draft-ietf-smime-3851bis, work- 769 in-progress. 771 [X.680] ITU-T Recommendation X.680 (2002) | ISO/IEC 8824- 772 1:2002. Information Technology - Abstract Syntax 773 Notation One (ASN.1): Specification of basic notation. 775 7.2. Informative References 777 [PKCS6] RSA Laboratories, "PKCS #6: Extended-Certificate Syntax 778 Standard", November 1993. 780 [SECLABEL] Nicolls, W., "Implementing Company Classification 781 Policy with the S/MIME Security Label", RFC 3114, May 782 2002. 784 [SMIMEv2] Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L. and 785 L. Repka, "S/MIME Version 2 Message Specification", RFC 786 2311, March 1998. 788 Dusse, S., Hoffman, P., Ramsdell, B., and J. Weinstein, 789 "S/MIME Version 2 Certificate Handling", RFC 2312, 790 March 1998. 792 Kaliski, B., "PKCS #1: RSA Encryption Version 1.5", RFC 793 2313, March 1998. 795 Kaliski, B., "PKCS #10: Certificate Request Syntax 796 Version 1.5", RFC 2314, March 1998. 798 Kaliski, B., "PKCS #7: Certificate Message Syntax 799 Version 1.5", RFC 2315, March 1998. 801 [SMIMEv3] Housley, R., "Cryptographic Message Syntax", RFC 2630, 802 June 1999. 804 Rescorla, E., "Diffie-Hellman Key Agreement Method", 805 RFC 2631, June 1999. 807 Ramsdell, B., "S/MIME Version 3 Certificate Handling", 808 RFC 2632, June 1999. 810 Ramsdell, B., "S/MIME Version 3 Message Specification", 811 RFC 2633, June 1999. 813 Hoffman, P., "Enhanced Security Services for S/MIME", 814 RFC 2634, June 1999. 816 Schaad, J., "ESS Update: Adding CertID Algorithm 817 Agility", RFC 5035, August 2007. 819 [SMIMEv3.1] Housley, R., "Cryptographic Message Syntax", RFC 3852, 820 July 2004. 822 Housley, R., "Cryptographic Message Syntax (CMS) 823 Multiple Signer Clarification", RFC 4853, April 2007. 825 Ramsdell, B., "S/MIME Version 3.1 Certificate 826 Handling", RFC 3850, July 2004. 828 Ramsdell, B., "S/MIME Version 3.1 Message 829 Specification", RFC 3851, July 2004. 831 Hoffman, P., "Enhanced Security Services for S/MIME", 832 RFC 2634, June 1999. 834 Schaad, J., "ESS Update: Adding CertID Algorithm 835 Agility", RFC 5035, August 2007. 837 [X.500] ITU-T Recommendation X.500 (1997) | ISO/IEC 9594- 838 1:1997, Information technology - Open Systems 839 Interconnection - The Directory: Overview of concepts, 840 models and services. 842 Appendix A. Moving S/MIME v2 Certificate Handling to Historic Status 844 The S/MIME v3 [SMIMEv3], v3.1 [SMIMEv3.1], and v3.2 (this document) 845 are backwards compatible with the S/MIME v2 Certificate Handling 846 Specification [SMIMEv2], with the exception of the algorithms 847 (dropped RC2/40 requirement and added DSA and RSA-PSS requirements). 848 Therefore, it is recommended that RFC 2312 [SMIMEv2] be moved to 849 Historic status. 851 Appendix B. Acknowledgements 853 Many thanks go out to the other authors of the S/MIME v2 RFC: Steve 854 Dusse, Paul Hoffman and Jeff Weinstein. Without v2, there wouldn't 855 be a v3, v3.1 or v3.2. 857 A number of the members of the S/MIME Working Group have also worked 858 very hard and contributed to this document. Any list of people is 859 doomed to omission and for that I apologize. In alphabetical order, 860 the following people stand out in my mind due to the fact that they 861 made direct contributions to this document. 863 Bill Flanigan, Trevor Freeman, Elliott Ginsburg, Alfred Hoenes, Paul 864 Hoffman, Russ Housley, David P. Kemp, Michael Myers, John Pawling, 865 Denis Pinkas, and Jim Schaad. 867 Author's Addresses 869 Blake Ramsdell 870 Brute Squad Labs, Inc. 872 Email: blaker@gmail.com 874 Sean Turner 876 IECA, Inc. 877 3057 Nutley Street, Suite 106 878 Fairfax, VA 22031 879 USA 881 Email: turners@ieca.com 883 Full Copyright Statement 885 Copyright (C) The IETF Trust (2008). 887 This document is subject to the rights, licenses and restrictions 888 contained in BCP 78, and except as set forth therein, the authors 889 retain all their rights. 891 This document and the information contained herein are provided on an 892 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 893 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 894 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 895 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 896 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 897 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 899 Intellectual Property 901 The IETF takes no position regarding the validity or scope of any 902 Intellectual Property Rights or other rights that might be claimed to 903 pertain to the implementation or use of the technology described in 904 this document or the extent to which any license under such rights 905 might or might not be available; nor does it represent that it has 906 made any independent effort to identify any such rights. 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