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'PKCS9') == Outdated reference: A later version (-11) exists of draft-ietf-smime-3851bis-10 -- Obsolete informational reference (is this intentional?): RFC 2630 (ref. 'SMIMEv3') (Obsoleted by RFC 3369, RFC 3370) Summary: 4 errors (**), 0 flaws (~~), 4 warnings (==), 6 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 S/MIME WG B. Ramsdell 2 Internet Draft Brute Squad Labs 3 Intended Status: Standard Track S. Turner 4 Obsoletes: 3850 (once approved) IECA 5 Expires: October 27, 2009 April 27, 2009 7 Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 8 Certificate Handling 9 draft-ietf-smime-3850bis-10.txt 11 Status of this Memo 13 This Internet-Draft is submitted to IETF in full conformance with the 14 provisions of BCP 78 and BCP 79. This document may contain material 15 from IETF Documents or IETF Contributions published or made publicly 16 available before November 10, 2008. The person(s) controlling the 17 copyright in some of this material may not have granted the IETF 18 Trust the right to allow modifications of such material outside the 19 IETF Standards Process. Without obtaining an adequate license from 20 the person(s) controlling the copyright in such materials, this 21 document may not be modified outside the IETF Standards Process, and 22 derivative works of it may not be created outside the IETF Standards 23 Process, except to format it for publication as an RFC or to 24 translate it into languages other than English. 26 Internet-Drafts are working documents of the Internet Engineering 27 Task Force (IETF), its areas, and its working groups. Note that 28 other groups may also distribute working documents as Internet- 29 Drafts. 31 Internet-Drafts are draft documents valid for a maximum of six months 32 and may be updated, replaced, or obsoleted by other documents at any 33 time. It is inappropriate to use Internet-Drafts as reference 34 material or to cite them other than as "work in progress." 36 The list of current Internet-Drafts can be accessed at 37 http://www.ietf.org/ietf/1id-abstracts.txt. 39 The list of Internet-Draft Shadow Directories can be accessed at 40 http://www.ietf.org/shadow.html. 42 This Internet-Draft will expire on October 27, 2009. 44 Copyright Notice 46 Copyright (c) 2009 IETF Trust and the persons identified as the 47 document authors. All rights reserved. 49 This document is subject to BCP 78 and the IETF Trust's Legal 50 Provisions Relating to IETF Documents in effect on the date of 51 publication of this document (http://trustee.ietf.org/license-info). 52 Please review these documents carefully, as they describe your rights 53 and restrictions with respect to this document. 55 Abstract 57 This document specifies conventions for X.509 certificate usage by 58 Secure/Multipurpose Internet Mail Extensions (S/MIME) v3.2 agents. 59 S/MIME provides a method to send and receive secure MIME messages, 60 and certificates are an integral part of S/MIME agent processing. 61 S/MIME agents validate certificates as described in RFC 5280, the 62 Internet X.509 Public Key Infrastructure Certificate and CRL Profile. 63 S/MIME agents must meet the certificate processing requirements in 64 this document as well as those in RFC 5280. This document obsoletes 65 RFC 3850. 67 Discussion 69 This draft is being discussed on the 'ietf-smime' mailing list. To 70 subscribe, send a message to ietf-smime-request@imc.org with the 71 single word subscribe in the body of the message. There is a Web site 72 for the mailing list at . 74 Table of Contents 76 1. Introduction...................................................3 77 1.1. Definitions...............................................3 78 1.2. Conventions used in this document.........................4 79 1.3. Compatibility with Prior Practice S/MIME..................4 80 1.4. Changes From S/MIME v3 to S/MIME v3.1.....................5 81 1.5. Changes Since S/MIME v3.1.................................5 82 2. CMS Options....................................................6 83 2.1. Certificate Revocation Lists..............................6 84 2.2. Certificate Choices.......................................7 85 2.2.1. Historical Note About CMS Certificates...............7 86 2.3. CertificateSet............................................7 87 3. Using Distinguished Names For Internet Mail....................8 88 4. Certificate Processing.........................................9 89 4.1. Certificate Revocation Lists.............................10 90 4.2. Certificate Path Validation..............................11 91 4.3. Certificate and CRL Signing Algorithms and Key Sizes.....12 92 4.4. PKIX Certificate Extensions..............................12 93 5. IANA Considerations...........................................15 94 6. Security Considerations.......................................15 95 7. References....................................................18 96 7.1. Normative References.....................................18 97 7.2. Informative References...................................19 98 Appendix A. Moving S/MIME v2 Certificate Handling to Historic 99 Status...............................................22 100 Appendix B. Acknowledgements.....................................22 102 1. Introduction 104 S/MIME (Secure/Multipurpose Internet Mail Extensions) v3.2, described 105 in [SMIME-MSG], provides a method to send and receive secure MIME 106 messages. Before using a public key to provide security services, 107 the S/MIME agent MUST verify that the public key is valid. S/MIME 108 agents MUST use PKIX certificates to validate public keys as 109 described in the Internet X.509 Public Key Infrastructure (PKIX) 110 Certificate and CRL Profile [KEYM]. S/MIME agents MUST meet the 111 certificate processing requirements documented in this document in 112 addition to those stated in [KEYM]. 114 This specification is compatible with the Cryptographic Message 115 Syntax (CMS) RFC 3852 and RFC 4853 [CMS] in that it uses the data 116 types defined by CMS. It also inherits all the varieties of 117 architectures for certificate-based key management supported by CMS. 119 1.1. Definitions 121 For the purposes of this document, the following definitions apply. 123 ASN.1: Abstract Syntax Notation One, as defined in ITU-T X.680 124 [X.680]. 126 Attribute Certificate (AC): An X.509 AC is a separate structure from 127 a subject's public key X.509 Certificate. A subject may have 128 multiple X.509 ACs associated with each of its public key X.509 129 Certificates. Each X.509 AC binds one or more Attributes with one of 130 the subject's public key X.509 Certificates. The X.509 AC syntax is 131 defined in [ACAUTH]. 133 Certificate: A type that binds an entity's name to a public key with 134 a digital signature. This type is defined in the Internet X.509 135 Public Key Infrastructure (PKIX) Certificate and CRL Profile [KEYM]. 136 This type also contains the distinguished name of the certificate 137 issuer (the signer), an issuer-specific serial number, the issuer's 138 signature algorithm identifier, a validity period, and extensions 139 also defined in that document. 141 Certificate Revocation List (CRL): A type that contains information 142 about certificates whose validity an issuer has prematurely revoked. 143 The information consists of an issuer name, the time of issue, the 144 next scheduled time of issue, a list of certificate serial numbers 145 and their associated revocation times, and extensions as defined in 146 [KEYM]. The CRL is signed by the issuer. The type intended by this 147 specification is the one defined in [KEYM]. 149 Receiving agent: Software that interprets and processes S/MIME CMS 150 objects, MIME body parts that contain CMS objects, or both. 152 Sending agent: Software that creates S/MIME CMS objects, MIME body 153 parts that contain CMS objects, or both. 155 S/MIME agent: User software that is a receiving agent, a sending 156 agent, or both. 158 1.2. Conventions used in this document 160 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 161 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 162 document are to be interpreted as described in [MUSTSHOULD]. 164 We define some additional terms here: 166 SHOULD+ This term means the same as SHOULD. However, the authors 167 expect that a requirement marked as SHOULD+ will be promoted at 168 some future time to be a MUST. 170 SHOULD- This term means the same as SHOULD. However, the authors 171 expect that a requirement marked as SHOULD- will be demoted to a 172 MAY in a future version of this document. 174 MUST- This term means the same as MUST. However, the authors 175 expect that this requirement will no longer be a MUST in a future 176 document. Although its status will be determined at a later 177 time, it is reasonable to expect that if a future revision of a 178 document alters the status of a MUST- requirement, it will remain 179 at least a SHOULD or a SHOULD-. 181 1.3. Compatibility with Prior Practice S/MIME 183 S/MIME version 3.2 agents ought to attempt to have the greatest 184 interoperability possible with agents for prior versions of S/MIME. 186 S/MIME version 2 is described in RFC 2311 through RFC 2315 inclusive 187 [SMIMEv2], S/MIME version 3 is described in RFC 2630 through RFC 2634 188 inclusive and RFC 5035 [SMIMEv3], and S/MIME version 3.1 is described 189 in RFC 3850, RFC 3851, RFC 3852, RFC 2634, RFC4853, and RFC 5035 190 [SMIMEv3.1]. RFC 2311 also has historical information about the 191 development of S/MIME. 193 1.4. Changes From S/MIME v3 To S/MIME v3.1 195 Version 1 and Version 2 CRLs MUST be supported. 197 Multiple CA certificates with the same subject and public key, but 198 with overlapping validity periods, MUST be supported. 200 Version 2 attribute certificates SHOULD be supported, and version 1 201 attributes certificates MUST NOT be used. 203 The use of the MD2 digest algorithm for certificate signatures is 204 discouraged and security language added. 206 Clarified use of email address use in certificates. Certificates 207 that do not contain an email address have no requirements for 208 verifying the email address associated with the certificate. 210 Receiving agents SHOULD display certificate information when 211 displaying the results of signature verification. 213 Receiving agents MUST NOT accept a signature made with a certificate 214 that does not have the digitalSignature or nonRepudiation bit set. 216 Clarifications for the interpretation of the key usage and extended 217 key usage extensions. 219 1.5. Changes Since S/MIME v3.1 221 Conventions Used in This Document: Moved to section 1.2. Added 222 definitions for SHOULD+, SHOULD-, and MUST-. 224 Sec 1.1: Updated ASN.1 definition and reference. 226 Sec 1.3: Added text about v3.1 RFCs. 228 Sec 3: Aligned email address text with RFC 5280. Updated note to 229 indicate emailAddress IA5String upper bound is 255 characters. Added 230 text about matching email addresses. 232 Sec 4.2: Added text to indicate how S/MIME agents locate the correct 233 user certificate. 235 Sec 4.3: RSA with SHA-256 (PKCS #1 v1.5) added as MUST, DSA with SHA- 236 256 added as SHOULD+, RSA with SHA-1, DSA with SHA-1, and RSA with 237 MD5 changed to SHOULD-, and RSASSA-PSS with SHA-256 added as SHOULD+. 238 Updated key sizes and changed pointer to PKIX RFCs. 240 Sec 4.4.1: Aligned with PKIX on use of basic constraints extension in 241 CA certificates. Clarified which extension is used to constrain EEs 242 from using their keys to perform issuing authority operations. 244 Sec 6: Updated security considerations. 246 Sec 7: Moved references from Appendix B to section 7. Updated the 247 references. 249 Appendix A: Moved Appendix A to Appendix B. Added Appendix A to move 250 S/MIME v2 Certificate Handling to Historic Status. 252 2. CMS Options 254 The CMS message format allows for a wide variety of options in 255 content and algorithm support. This section puts forth a number of 256 support requirements and recommendations in order to achieve a base 257 level of interoperability among all S/MIME implementations. Most of 258 the CMS format for S/MIME messages is defined in [SMIME-MSG]. 260 2.1. Certificate Revocation Lists 262 Receiving agents MUST support the Certificate Revocation List (CRL) 263 format defined in [KEYM]. If sending agents include CRLs in outgoing 264 messages, the CRL format defined in [KEYM] MUST be used. In all 265 cases, both v1 and v2 CRLs MUST be supported. 267 All agents MUST be capable of performing revocation checks using CRLs 268 as specified in [KEYM]. All agents MUST perform revocation status 269 checking in accordance with [KEYM]. Receiving agents MUST recognize 270 CRLs in received S/MIME messages. 272 Agents SHOULD store CRLs received in messages for use in processing 273 later messages. 275 2.2. Certificate Choices 277 Receiving agents MUST support v1 X.509 and v3 X.509 certificates as 278 profiled in [KEYM]. End entity certificates MAY include an Internet 279 mail address, as described in section 3. 281 Receiving agents SHOULD support X.509 version 2 attribute 282 certificates. See [ACAUTH] for details about the profile for 283 attribute certificates. 285 2.2.1. Historical Note About CMS Certificates 287 The CMS message format supports a choice of certificate formats for 288 public key content types: PKIX, PKCS #6 Extended Certificates [PKCS6] 289 and PKIX Attribute Certificates. 291 The PKCS #6 format is not in widespread use. In addition, PKIX 292 certificate extensions address much of the same functionality and 293 flexibility as was intended in the PKCS #6. Thus, sending and 294 receiving agents MUST NOT use PKCS #6 extended certificates. 296 X.509 version 1 attribute certificates are also not widely 297 implemented, and have been superseded with version 2 attribute 298 certificates. Sending agents MUST NOT send version 1 attribute 299 certificates. 301 2.3. CertificateSet 303 Receiving agents MUST be able to handle an arbitrary number of 304 certificates of arbitrary relationship to the message sender and to 305 each other in arbitrary order. In many cases, the certificates 306 included in a signed message may represent a chain of certification 307 from the sender to a particular root. There may be, however, 308 situations where the certificates in a signed message may be 309 unrelated and included for convenience. 311 Sending agents SHOULD include any certificates for the user's public 312 key(s) and associated issuer certificates. This increases the 313 likelihood that the intended recipient can establish trust in the 314 originator's public key(s). This is especially important when 315 sending a message to recipients that may not have access to the 316 sender's public key through any other means or when sending a signed 317 message to a new recipient. The inclusion of certificates in 318 outgoing messages can be omitted if S/MIME objects are sent within a 319 group of correspondents that has established access to each other's 320 certificates by some other means such as a shared directory or manual 321 certificate distribution. Receiving S/MIME agents SHOULD be able to 322 handle messages without certificates using a database or directory 323 lookup scheme. 325 A sending agent SHOULD include at least one chain of certificates up 326 to, but not including, a Certificate Authority (CA) that it believes 327 that the recipient may trust as authoritative. A receiving agent 328 MUST be able to handle an arbitrarily large number of certificates 329 and chains. 331 Agents MAY send CA certificates, that is, cross-certificates, self- 332 issued certificates, and self-signed certificates. Note that 333 receiving agents SHOULD NOT simply trust any self-signed certificates 334 as valid CAs, but SHOULD use some other mechanism to determine if 335 this is a CA that should be trusted. Also note that when 336 certificates contain DSA public keys the parameters may be located in 337 the root certificate. This would require that the recipient possess 338 both the end-entity certificate as well as the root certificate to 339 perform a signature verification, and is a valid example of a case 340 where transmitting the root certificate may be required. 342 Receiving agents MUST support chaining based on the distinguished 343 name fields. Other methods of building certificate chains MAY be 344 supported. 346 Receiving agents SHOULD support the decoding of X.509 attribute 347 certificates included in CMS objects. All other issues regarding the 348 generation and use of X.509 attribute certificates are outside of the 349 scope of this specification. One specification that addresses 350 attribute certificate use is defined in [SECLABEL]. 352 3. Using Distinguished Names For Internet Mail 354 End-entity certificates MAY contain an Internet mail address as 355 described in [KEYM] Section 4.2.1.6. The email address SHOULD be in 356 the subjectAltName extension, and SHOULD NOT be in the subject 357 distinguished name. 359 Receiving agents MUST recognize and accept certificates that contain 360 no email address. Agents are allowed to provide an alternative 361 mechanism for associating an email address with a certificate that 362 does not contain an email address, such as through the use of the 363 agent's address book, if available. Receiving agents MUST recognize 364 email addresses in the subjectAltName field. Receiving agents MUST 365 recognize email addresses in the Distinguished Name field in the PKCS 366 #9 [PKCS9] emailAddress attribute: 368 pkcs-9-at-emailAddress OBJECT IDENTIFIER ::= 369 { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 1 } 371 Note that this attribute MUST be encoded as IA5String and has an 372 upper bound of 255 characters. The right side of the email address 373 SHOULD be treated as ASCII-case-insensitive. 375 Sending agents SHOULD make the address in the From or Sender header 376 in a mail message match an Internet mail address in the signer's 377 certificate. Receiving agents MUST check that the address in the 378 From or Sender header of a mail message matches an Internet mail 379 address, if present, in the signer's certificate, if mail addresses 380 are present in the certificate. A receiving agent SHOULD provide 381 some explicit alternate processing of the message if this comparison 382 fails, which may be to display a message that shows the recipient the 383 addresses in the certificate or other certificate details. 385 A receiving agent SHOULD display a subject name or other certificate 386 details when displaying an indication of successful or unsuccessful 387 signature verification. 389 All subject and issuer names MUST be populated (i.e., not an empty 390 SEQUENCE) in S/MIME-compliant X.509 certificates, except that the 391 subject DN in a user's (i.e., end-entity) certificate MAY be an empty 392 SEQUENCE in which case the subjectAltName extension will include the 393 subject's identifier and MUST be marked as critical. 395 4. Certificate Processing 397 S/MIME agents need to provide some certificate retrieval mechanism in 398 order to gain access to certificates for recipients of digital 399 envelopes. There are many ways to implement certificate retrieval 400 mechanisms. [X.500] directory service is an excellent example of a 401 certificate retrieval-only mechanism that is compatible with classic 402 X.500 Distinguished Names. Another method under consideration by the 403 IETF is to provide certificate retrieval services as part of the 404 existing Domain Name System (DNS). Until such mechanisms are widely 405 used, their utility may be limited by the small number of the 406 correspondent's certificates that can be retrieved. At a minimum, for 407 initial S/MIME deployment, a user agent could automatically generate 408 a message to an intended recipient requesting the recipient's 409 certificate in a signed return message. 411 Receiving and sending agents SHOULD also provide a mechanism to allow 412 a user to "store and protect" certificates for correspondents in such 413 a way so as to guarantee their later retrieval. In many 414 environments, it may be desirable to link the certificate 415 retrieval/storage mechanisms together in some sort of certificate 416 database. In its simplest form, a certificate database would be 417 local to a particular user and would function in a similar way as an 418 "address book" that stores a user's frequent correspondents. In this 419 way, the certificate retrieval mechanism would be limited to the 420 certificates that a user has stored (presumably from incoming 421 messages). A comprehensive certificate retrieval/storage solution 422 may combine two or more mechanisms to allow the greatest flexibility 423 and utility to the user. For instance, a secure Internet mail agent 424 may resort to checking a centralized certificate retrieval mechanism 425 for a certificate if it can not be found in a user's local 426 certificate storage/retrieval database. 428 Receiving and sending agents SHOULD provide a mechanism for the 429 import and export of certificates, using a CMS certs-only message. 430 This allows for import and export of full certificate chains as 431 opposed to just a single certificate. This is described in [SMIME- 432 MSG]. 434 Agents MUST handle multiple valid Certification Authority (CA) 435 certificates containing the same subject name and the same public 436 keys but with overlapping validity intervals. 438 4.1. Certificate Revocation Lists 440 In general, it is always better to get the latest CRL information 441 from a CA than to get information stored away from incoming messages. 442 A receiving agent SHOULD have access to some certificate revocation 443 list (CRL) retrieval mechanism in order to gain access to certificate 444 revocation information when validating certification paths. A 445 receiving or sending agent SHOULD also provide a mechanism to allow a 446 user to store incoming certificate revocation information for 447 correspondents in such a way so as to guarantee its later retrieval. 449 Receiving and sending agents SHOULD retrieve and utilize CRL 450 information every time a certificate is verified as part of a 451 certification path validation even if the certificate was already 452 verified in the past. However, in many instances (such as off-line 453 verification) access to the latest CRL information may be difficult 454 or impossible. The use of CRL information, therefore, may be 455 dictated by the value of the information that is protected. The 456 value of the CRL information in a particular context is beyond the 457 scope of this specification but may be governed by the policies 458 associated with particular certification paths. 460 All agents MUST be capable of performing revocation checks using CRLs 461 as specified in [KEYM]. All agents MUST perform revocation status 462 checking in accordance with [KEYM]. Receiving agents MUST recognize 463 CRLs in received S/MIME messages. 465 4.2. Certificate Path Validation 467 In creating a user agent for secure messaging, certificate, CRL, and 468 certification path validation SHOULD be highly automated while still 469 acting in the best interests of the user. Certificate, CRL, and path 470 validation MUST be performed as per [KEYM] when validating a 471 correspondent's public key. This is necessary before using a public 472 key to provide security services such as: verifying a signature; 473 encrypting a content-encryption key (e.g., RSA); or forming a 474 pairwise symmetric key (e.g., Diffie-Hellman) to be used to encrypt 475 or decrypt a content-encryption key. 477 Certificates and CRLs are made available to the path validation 478 procedure in two ways: a) incoming messages, and b) certificate and 479 CRL retrieval mechanisms. Certificates and CRLs in incoming messages 480 are not required to be in any particular order nor are they required 481 to be in any way related to the sender or recipient of the message 482 (although in most cases they will be related to the sender). Incoming 483 certificates and CRLs SHOULD be cached for use in path validation and 484 optionally stored for later use. This temporary certificate and CRL 485 cache SHOULD be used to augment any other certificate and CRL 486 retrieval mechanisms for path validation on incoming signed messages. 488 When verifying a signature and the certificates that are included in 489 the message, if a signingCertificate attribute from RFC 2634 [ESS] or 490 a signingCertificateV2 attribute from RFC 5035 [ESS] is found in an 491 S/MIME message, it SHALL be used to identify the signer's 492 certificate. Otherwise, the certificate is identified in an S/MIME 493 message, either using the issuerAndSerialNumber which identifies the 494 signer's certificate by the issuer's distinguished name and the 495 certificate serial number, or the subjectKeyIdentifier which 496 identifies the signer's certificate by a key identifier. 498 When decrypting an encrypted message, if a 499 SMIMEEncryptionKeyPreference attribute is found in an encapsulating 500 SignedData, it SHALL be used to identify the originator's certificate 501 found in OriginatorInfo. See [CMS] for the CMS fields that reference 502 the originator's and recipient's certificates. 504 4.3. Certificate and CRL Signing Algorithms and Key Sizes 506 Certificates and Certificate Revocation Lists (CRLs) are signed by 507 the certificate issuer. Receiving agents: 509 - MUST support RSA with SHA-256 511 - SHOULD+ support DSA with SHA-256 513 - SHOULD+ support RSASSA-PSS with SHA-256 515 - SHOULD- support RSA with SHA-1 517 - SHOULD- support DSA with SHA-1 519 - SHOULD- support RSA with MD5 521 The following are the RSA key size requirements for S/MIME receiving 522 agents during certificate and CRL signature verification: 524 key size <= 1023 : MAY (see Section 6) 525 1024 <= key size <= 4096 : MUST (see Section 6) 526 4096 < key size : MAY (see Section 6) 528 The following are the DSA key size requirements for S/MIME receiving 529 agents during certificate and CRL signature verification: 531 key size <= 1023 : MAY (see Section 6) 532 1024 = key size : SHOULD (see Section 6) 534 For 512-bit RSA with SHA-1 see [KEYMALG] and [FIPS186-2] without 535 Change Notice 1, for 512-bit RSA with SHA-256 see [RSAOAEP] and 536 [FIPS186-2] without Change Notice 1, for 1024-bit through 3072-bit 537 RSA with SHA-256 see [RSAOAEP] and [FIPS186-2] with Change Notice 1, 538 and for 4096-bit RSA with SHA-256 see [RSAOAEP] and [PKCS1]. In 539 either case, the first reference provides the signature algorithm's 540 object identifier and the second provides the signature algorithm's 541 definition. 543 For 512-bit DSA with SHA-1 see [KEYMALG] and [FIPS186-2] without 544 Change Notice 1, for 512-bit DSA with SHA-256 see [KEYMALG2] and 545 [FIPS186-2] without Change Notice 1, for 1024-bit DSA with SHA-1 see 546 [KEYMALG] and [FIPS186-2] with Change Notice 1, for 1024-bit DSA with 547 SHA-256 see [KEYMALG2] and [FIPS186-3]. In either case, the first 548 reference provides the signature algorithm's object identifier and 549 the second provides the signature algorithm's definition. 551 For 512-4096-bit RSASSA-PSS with SHA-256 see [RSAPSS]. 553 4.4. PKIX Certificate Extensions 555 PKIX describes an extensible framework in which the basic certificate 556 information can be extended and describes how such extensions can be 557 used to control the process of issuing and validating certificates. 558 The PKIX Working Group has ongoing efforts to identify and create 559 extensions which have value in particular certification environments. 560 Further, there are active efforts underway to issue PKIX certificates 561 for business purposes. This document identifies the minimum required 562 set of certificate extensions which have the greatest value in the 563 S/MIME environment. The syntax and semantics of all the identified 564 extensions are defined in [KEYM]. 566 Sending and receiving agents MUST correctly handle the basic 567 constraints, key usage, authority key identifier, subject key 568 identifier, and subject alternative names certificate extensions when 569 they appear in end-entity and CA certificates. Some mechanism SHOULD 570 exist to gracefully handle other certificate extensions when they 571 appear in end-entity or CA certificates. 573 Certificates issued for the S/MIME environment SHOULD NOT contain any 574 critical extensions (extensions that have the critical field set to 575 TRUE) other than those listed here. These extensions SHOULD be 576 marked as non-critical unless the proper handling of the extension is 577 deemed critical to the correct interpretation of the associated 578 certificate. Other extensions may be included, but those extensions 579 SHOULD NOT be marked as critical. 581 Interpretation and syntax for all extensions MUST follow [KEYM], 582 unless otherwise specified here. 584 4.4.1. Basic Constraints 586 The basic constraints extension serves to delimit the role and 587 position that an issuing authority or end-entity certificate plays in 588 a certification path. 590 For example, certificates issued to CAs and subordinate CAs contain a 591 basic constraint extension that identifies them as issuing authority 592 certificates. End-entity certificates contain the key usage 593 extension which restrains EEs from using the key when performing 594 issuing authority operations (see Section 4.4.2). 596 As per [KEYM], Certificates MUST contain a basicConstraints extension 597 in CA certificates, and SHOULD NOT contain that extension in end 598 entity certificates. 600 4.4.2. Key Usage Certificate Extension 602 The key usage extension serves to limit the technical purposes for 603 which a public key listed in a valid certificate may be used. Issuing 604 authority certificates may contain a key usage extension that 605 restricts the key to signing certificates, certificate revocation 606 lists and other data. 608 For example, a certification authority may create subordinate issuer 609 certificates which contain a key usage extension which specifies that 610 the corresponding public key can be used to sign end user 611 certificates and sign CRLs. 613 If a key usage extension is included in a PKIX certificate, then it 614 MUST be marked as critical. 616 S/MIME receiving agents MUST NOT accept the signature of a message if 617 it was verified using a certificate which contains the key usage 618 extension without either the digitalSignature or nonRepudiation bit 619 set. Sometimes S/MIME is used as a secure message transport for 620 applications beyond interpersonal messaging. In such cases, the 621 S/MIME-enabled application can specify additional requirements 622 concerning the digitalSignature or nonRepudiation bits within this 623 extension. 625 If the key usage extension is not specified, receiving clients MUST 626 presume that the digitalSignature and nonRepudiation bits are set. 628 4.4.3. Subject Alternative Name 630 The subject alternative name extension is used in S/MIME as the 631 preferred means to convey the email address(es) that correspond(s) to 632 the entity for this certificate. Any email addresses present MUST be 633 encoded using the rfc822Name CHOICE of the GeneralName type as 634 described in [KEYM] Section 4.2.1.6. Since the SubjectAltName type 635 is a SEQUENCE OF GeneralName, multiple email addresses MAY be 636 present. 638 4.4.4. Extended Key Usage Extension 640 The extended key usage extension also serves to limit the technical 641 purposes for which a public key listed in a valid certificate may be 642 used. The set of technical purposes for the certificate therefore 643 are the intersection of the uses indicated in the key usage and 644 extended key usage extensions. 646 For example, if the certificate contains a key usage extension 647 indicating digital signature and an extended key usage extension 648 which includes the email protection OID, then the certificate may be 649 used for signing but not encrypting S/MIME messages. If the 650 certificate contains a key usage extension indicating digital 651 signature, but no extended key usage extension then the certificate 652 may also be used to sign but not encrypt S/MIME messages. 654 If the extended key usage extension is present in the certificate 655 then interpersonal message S/MIME receiving agents MUST check that it 656 contains either the emailProtection or the anyExtendedKeyUsage OID as 657 defined in [KEYM]. S/MIME uses other than interpersonal messaging 658 MAY require the explicit presence of the extended key usage extension 659 or other OIDs to be present in the extension or both. 661 5. IANA Considerations 663 None: All identifiers are already registered. Please remove this 664 section prior to publication as an RFC. 666 6. Security Considerations 668 All of the security issues faced by any cryptographic application 669 must be faced by a S/MIME agent. Among these issues are protecting 670 the user's private key, preventing various attacks, and helping the 671 user avoid mistakes such as inadvertently encrypting a message for 672 the wrong recipient. The entire list of security considerations is 673 beyond the scope of this document, but some significant concerns are 674 listed here. 676 When processing certificates, there are many situations where the 677 processing might fail. Because the processing may be done by a user 678 agent, a security gateway, or other program, there is no single way 679 to handle such failures. Just because the methods to handle the 680 failures has not been listed, however, the reader should not assume 681 that they are not important. The opposite is true: if a certificate 682 is not provably valid and associated with the message, the processing 683 software should take immediate and noticeable steps to inform the end 684 user about it. 686 Some of the many places where signature and certificate checking 687 might fail include: 689 - no Internet mail addresses in a certificate matches the sender of 690 a message, if the certificate contains at least one mail address 692 - no certificate chain leads to a trusted CA 694 - no ability to check the CRL for a certificate 696 - an invalid CRL was received 698 - the CRL being checked is expired 700 - the certificate is expired 702 - the certificate has been revoked 704 There are certainly other instances where a certificate may be 705 invalid, and it is the responsibility of the processing software to 706 check them all thoroughly, and to decide what to do if the check 707 fails. 709 It is possible for there to be multiple unexpired CRLs for a CA. If 710 an agent is consulting CRLs for certificate validation, it SHOULD 711 make sure that the most recently issued CRL for that CA is consulted, 712 since an S/MIME message sender could deliberately include an older 713 unexpired CRL in an S/MIME message. This older CRL might not include 714 recently revoked certificates, which might lead an agent to accept a 715 certificate that has been revoked in a subsequent CRL. 717 When determining the time for a certificate validity check, agents 718 have to be careful to use a reliable time. Unless it is from a 719 trusted agent, this time MUST NOT be the SigningTime attribute found 720 in an S/MIME message. For most sending agents, the SigningTime 721 attribute could be deliberately set to direct the receiving agent to 722 check a CRL that could have out-of-date revocation status for a 723 certificate, or cause an improper result when checking the Validity 724 field of a certificate. 726 In addition to the Security Considerations identified in [KEYM], 727 caution should be taken when processing certificates which have not 728 first been validated to a trust anchor. Certificates could be 729 manufactured by untrusted sources for the purpose of mounting denial 730 of service or other attacks. For example, keys selected to require 731 excessive cryptographic processing, or extensive lists of CDP and/or 732 AIA addresses in the certificate, could be used to mount denial of 733 service attacks. Similarly, attacker-specified CRL Distribution 734 Point (CRLDP) and/or Authority Information Access (AIA) addresses 735 could be included in fake certificates to allow the originator to 736 detect receipt of the message even if signature verification fails. 738 The 4096-bit RSA key size requirement for certificate and CRL 739 verification is larger than the 2048-bit RSA key sizes for message 740 signature generation/verification or message encryption/decryption in 741 [SMIME-MSG] because many Root CAs included in certificate stores have 742 already issued Root certificates with 4096-bit key. The standard 743 that defines comparable key sizes for DSA is not yet available. In 744 particular, [FIPS186-2] without Change Notice 1 allowed DSA key sizes 745 between 512 and 1024 bits and [FIPS186-2] with Change Notice 1 only 746 allowed DSA key sizes of 1024 bits. A revision to support larger key 747 sizes is being developed, and once it is available, implementors 748 ought to support DSA key sizes comparable to the RSA key sizes 749 recommended in this specification. Further, 4096-bit keys are 750 normally only used by Root certificates and not by subordinate CA 751 certificates; thereby, lengthening the Root CA certificate's validity 752 period. 754 RSA and DSA keys of less than 1024 bits are now considered by many 755 experts to be cryptographically insecure (due to advances in 756 computing power), and should no longer be used to sign certificates 757 or CRLs. Such keys were previously considered secure, so processing 758 previously received signed and encrypted mail may require processing 759 certificates or CRLs signed with weak keys. Implementations that 760 wish to support previous versions of S/MIME or process old messages 761 need to consider the security risks that result from accepting 762 certificates and CRLs with smaller key sizes (e.g., spoofed 763 certificates) versus the costs of denial of service. If an 764 implementation supports verification of certificates or CRLs 765 generated with RSA and DSA keys of less than 1024 bits, it MUST warn 766 the user. Implementers should consider providing a stronger warning 767 for weak signatures on certificates and CRLs associated with newly 768 received messages than the one provided for certificates and CRLs 769 associated with previously stored messages. Server implementations 770 (e.g., secure mail list servers) where user warnings are not 771 appropriate SHOULD reject messages with weak cryptography. 773 If an implementation is concerned about compliance with NIST key size 774 recommendations, then see [SP800-57]. 776 7. References 778 7.1. Normative References 780 [ACAUTH] Farrell, S., Housley, R., and S. Turner, "An Internet 781 Attribute Certificate Profile for Authorization", 782 draft-ietf-pkix-3281update-04.txt, work-in-progress. 784 [CMS] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 785 3852, July 2004. 787 Housley, R., "Cryptographic Message Syntax (CMS) 788 Multiple Signer Clarification", RFC 4853, April 2007. 790 [ESS] Hoffman, P., "Enhanced Security Services for S/MIME", 791 RFC 2634, June 1999. 793 Schaad, J., "ESS Update: Adding CertID Algorithm 794 Agility", RFC 5035, August 2007. 796 [FIPS186-2] National Institute of Standards and Technology (NIST), 797 "Digital Signature Standard (DSS)", FIPS Publication 798 186-3, January 2000. [With Change Notice 1] 800 [FIPS186-3] National Institute of Standards and Technology (NIST), 801 FIPS Publication 186-3: Digital Signature Standard, 802 (draft) March 2006. 804 [KEYM] Cooper, D., Santesson, S., Farrell, S., Boeyen, S. 805 Housley, R., and W. Polk, "Internet X.509 Public Key 806 Infrastructure Certificate and Certificate Revocation 807 List (CRL) Profile", RFC 5280, May 2008. 809 [KEYMALG] Bassham, L., Polk, W., and R. Housley, "Algorithms and 810 Identifiers for the Internet X.509 Public Key 811 Infrastructure Certificate and Certificate Revocation 812 List (CRL) Profile", RFC 3279, April 2002. 814 [KEYMALG2] Dang, Q., Santesson, S., Moriarty, K., Brown, D., and 815 T. Polk, "Internet X.509 Public Key Infrastructure: 816 Additional Algorithms and Identifiers for DSA and 817 ECDSA", draft-ietf-pkix-sha2-dsa-ecdsa-06.txt, work-in- 818 progress. 820 [MUSTSHOULD] Bradner, S., "Key words for use in RFCs to Indicate 821 Requirement Levels", BCP 14, RFC 2119, March 1997. 823 [PKCS1] Jonsson, J. and B. Kaliki, "Public-Key Cryptography 824 Standards (PKCS) #1: RSA Cryptography Specifications 825 Version 2.1", RFC 3447, February 2003. 827 [PKCS9] Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object 828 Classes and Attribute Types Version 2.0", RFC 2985, 829 November 2000. 831 [RSAPSS] Schaad, J., "Use of RSASSA-PSS Signature Algorithm in 832 Cryptographic Message Syntax (CMS)", RFC 4056, June 833 2005. 835 [RSAOAEP] Schaad, J., Kaliski, B., and R. Housley, "Additional 836 Algorithms and Identifiers for RSA Cryptography for use 837 in the Internet X.509 Public Key Infrastructure 838 Certificate and Certificate Revocation List (CRL) 839 Profile", RFC 4055, June 2005. 841 [SMIME-MSG] Ramsdell, B., and S. Turner, "S/MIME Version 3.2 842 Message Specification", draft-ietf-smime-3851bis- 843 10.txt, work-in-progress. 845 [X.680] ITU-T Recommendation X.680 (2002) | ISO/IEC 8824- 846 1:2002. Information Technology - Abstract Syntax 847 Notation One (ASN.1): Specification of basic notation. 849 7.2. Informative References 851 [PKCS6] RSA Laboratories, "PKCS #6: Extended-Certificate Syntax 852 Standard", November 1993. 854 [SECLABEL] Nicolls, W., "Implementing Company Classification 855 Policy with the S/MIME Security Label", RFC 3114, May 856 2002. 858 [SMIMEv2] Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L. and 859 L. Repka, "S/MIME Version 2 Message Specification", RFC 860 2311, March 1998. 862 Dusse, S., Hoffman, P., Ramsdell, B., and J. Weinstein, 863 "S/MIME Version 2 Certificate Handling", RFC 2312, 864 March 1998. 866 Kaliski, B., "PKCS #1: RSA Encryption Version 1.5", RFC 867 2313, March 1998. 869 Kaliski, B., "PKCS #10: Certificate Request Syntax 870 Version 1.5", RFC 2314, March 1998. 872 Kaliski, B., "PKCS #7: Certificate Message Syntax 873 Version 1.5", RFC 2315, March 1998. 875 [SMIMEv3] Housley, R., "Cryptographic Message Syntax", RFC 2630, 876 June 1999. 878 Rescorla, E., "Diffie-Hellman Key Agreement Method", 879 RFC 2631, June 1999. 881 Ramsdell, B., "S/MIME Version 3 Certificate Handling", 882 RFC 2632, June 1999. 884 Ramsdell, B., "S/MIME Version 3 Message Specification", 885 RFC 2633, June 1999. 887 Hoffman, P., "Enhanced Security Services for S/MIME", 888 RFC 2634, June 1999. 890 Schaad, J., "ESS Update: Adding CertID Algorithm 891 Agility", RFC 5035, August 2007. 893 [SMIMEv3.1] Housley, R., "Cryptographic Message Syntax", RFC 3852, 894 July 2004. 896 Housley, R., "Cryptographic Message Syntax (CMS) 897 Multiple Signer Clarification", RFC 4853, April 2007. 899 Ramsdell, B., "S/MIME Version 3.1 Certificate 900 Handling", RFC 3850, July 2004. 902 Ramsdell, B., "S/MIME Version 3.1 Message 903 Specification", RFC 3851, July 2004. 905 Hoffman, P., "Enhanced Security Services for S/MIME", 906 RFC 2634, June 1999. 908 Schaad, J., "ESS Update: Adding CertID Algorithm 909 Agility", RFC 5035, August 2007. 911 [SP800-57] National Institute of Standards and Technology (NIST), 912 Special Publication 800-57: Recommendation for Key 913 Management, August 2005. 915 [X.500] ITU-T Recommendation X.500 (1997) | ISO/IEC 9594- 916 1:1997, Information technology - Open Systems 917 Interconnection - The Directory: Overview of concepts, 918 models and services. 920 Appendix A. Moving S/MIME v2 Certificate Handling to Historic Status 922 The S/MIME v3 [SMIMEv3], v3.1 [SMIMEv3.1], and v3.2 (this document) 923 are backwards compatible with the S/MIME v2 Certificate Handling 924 Specification [SMIMEv2], with the exception of the algorithms 925 (dropped RC2/40 requirement and added DSA and RSASSA-PSS 926 requirements). Therefore, it is recommended that RFC 2312 [SMIMEv2] 927 be moved to Historic status. 929 Appendix B. Acknowledgments 931 Many thanks go out to the other authors of the S/MIME v2 RFC: Steve 932 Dusse, Paul Hoffman and Jeff Weinstein. Without v2, there wouldn't 933 be a v3, v3.1 or v3.2. 935 A number of the members of the S/MIME Working Group have also worked 936 very hard and contributed to this document. Any list of people is 937 doomed to omission and for that I apologize. In alphabetical order, 938 the following people stand out in my mind due to the fact that they 939 made direct contributions to this document. 941 Bill Flanigan, Trevor Freeman, Elliott Ginsburg, Alfred Hoenes, Paul 942 Hoffman, Russ Housley, David P. Kemp, Michael Myers, John Pawling, 943 Denis Pinkas, and Jim Schaad. 945 Authors' Addresses 947 Blake Ramsdell 949 Brute Squad Labs, Inc. 951 EMail: blaker@gmail.com 953 Sean Turner 955 IECA, Inc. 956 3057 Nutley Street, Suite 106 957 Fairfax, VA 22031 958 USA 960 EMail: turners@ieca.com