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The boilerplate contains a reference [MUSTSHOULD], but that reference does not seem to mention RFC 2119 either. -- The document seems to lack a disclaimer for pre-RFC5378 work, but may have content which was first submitted before 10 November 2008. If you have contacted all the original authors and they are all willing to grant the BCP78 rights to the IETF Trust, then this is fine, and you can ignore this comment. If not, you may need to add the pre-RFC5378 disclaimer. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (April 30, 2003) is 7660 days in the past. Is this intentional? 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? 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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Internet Draft Editor: Blake Ramsdell, 2 draft-ietf-smime-rfc2632bis-02.txt Brute Squad Labs 3 October 30, 2002 4 Expires April 30, 2003 6 S/MIME Version 3.1 Certificate Handling 8 Status of this memo 10 This document is an Internet-Draft and is in full conformance with all 11 provisions of Section 10 of RFC2026. 13 Internet-Drafts are working documents of the Internet Engineering Task 14 Force (IETF), its areas, and its working groups. Note that other 15 groups may also distribute working documents as Internet-Drafts. 17 Internet-Drafts are draft documents valid for a maximum of six months 18 and may be updated, replaced, or obsoleted by other documents at any 19 time. It is inappropriate to use Internet-Drafts as reference material 20 or to cite them other than as "work in progress." 22 The list of current Internet-Drafts can be accessed at 23 http://www.ietf.org/ietf/1id-abstracts.txt 25 The list of Internet-Draft Shadow Directories can be accessed at 26 http://www.ietf.org/shadow.html. 28 1. Overview 30 S/MIME (Secure/Multipurpose Internet Mail Extensions), described in 31 [SMIME-MSG], provides a method to send and receive secure MIME 32 messages. Before using a public key to provide security services, the 33 S/MIME agent MUST certify that the public key is valid. S/MIME agents 34 MUST use PKIX certificates to validate public keys as described in the 35 Internet X.509 Public Key Infrastructure (PKIX) Certificate and CRL 36 Profile [KEYM]. S/MIME agents MUST meet the certificate processing 37 requirements documented in this document in addition to those stated 38 in [KEYM]. 40 This specification is compatible with the Cryptographic Message Syntax 41 [CMS] in that it uses the data types defined by CMS. It also inherits 42 all the varieties of architectures for certificate-based key 43 management supported by CMS. 45 1.1 Definitions 47 For the purposes of this draft, the following definitions apply. 49 ASN.1: Abstract Syntax Notation One, as defined in ITU-T X.680-689. 51 Attribute Certificate (AC): An X.509 AC is a separate structure from a 52 subject's public key X.509 Certificate. A subject may have multiple 53 X.509 ACs associated with each of its public key X.509 Certificates. 54 Each X.509 AC binds one or more Attributes with one of the subject's 55 public key X.509 Certificates. The X.509 AC syntax is defined in 56 [KEYMAC]. 58 BER: Basic Encoding Rules for ASN.1, as defined in ITU-T X.690. 60 Certificate: A type that binds an entity's distinguished name to a 61 public key with a digital signature. This type is defined in the 62 Internet X.509 Public Key Infrastructure (PKIX) Certificate and CRL 63 Profile [KEYM]. This type also contains the distinguished name of the 64 certificate issuer (the signer), an issuer-specific serial number, the 65 issuer's signature algorithm identifier, a validity period, and 66 extensions also defined in that document. 68 Certificate Revocation List (CRL): A type that contains information 69 about certificates whose validity an issuer has prematurely revoked. 70 The information consists of an issuer name, the time of issue, the 71 next scheduled time of issue, a list of certificate serial numbers and 72 their associated revocation times, and extensions as defined in 73 [KEYM]. The CRL is signed by the issuer. The type intended by this 74 specification is the one defined in [KEYM]. 76 DER: Distinguished Encoding Rules for ASN.1, as defined in ITU-T 77 X.690. 79 Receiving agent: software that interprets and processes S/MIME CMS 80 objects, MIME body parts that contain CMS objects, or both. 82 Sending agent: software that creates S/MIME CMS objects, MIME body 83 parts that contain CMS objects, or both. 85 S/MIME agent: user software that is a receiving agent, a sending 86 agent, or both. 88 1.2 Compatibility with Prior Practice of S/MIME 90 S/MIME version 3 agents should attempt to have the greatest 91 interoperability possible with S/MIME version 2 agents. S/MIME version 92 2 is described in RFC 2311 through RFC 2315, inclusive. RFC 2311 also 93 has historical information about the development of S/MIME. 95 1.3 Terminology 97 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 98 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 99 document are to be interpreted as described in [MUSTSHOULD]. 101 1.4 Discussion of This Draft 103 This draft is being discussed on the "ietf-smime" mailing list. 104 To subscribe, send a message to: 106 ietf-smime-request@imc.org 108 with the single word 110 subscribe 112 in the body of the message. There is a Web site for the mailing list 113 at . 115 2. CMS Options 117 The CMS message format allows for a wide variety of options in content 118 and algorithm support. This section puts forth a number of support 119 requirements and recommendations in order to achieve a base level of 120 interoperability among all S/MIME implementations. Most of the CMS 121 format for S/MIME messages is defined in [SMIME-MSG]. 123 2.1 CertificateRevocationLists 125 Receiving agents MUST support the Certificate Revocation List (CRL) 126 format defined in [KEYM]. If sending agents include CRLs in outgoing 127 messages, the CRL format defined in [KEYM] MUST be used. In all cases, 128 both v1 and v2 CRLs MUST be supported. 130 All agents MUST be capable of performing revocation checks using CRLs 131 as specified in [KEYM]. All agents MUST perform revocation status 132 checking in accordance with [KEYM]. Receiving agents MUST recognize 133 CRLs in received S/MIME messages. 135 Agents SHOULD store CRLs received in messages for use in processing 136 later messages. 138 2.2 CertificateChoices 140 Receiving agents MUST support PKIX v1 and PKIX v3 certificates. See 141 [KEYM] for details about the profile for certificate formats. End 142 entity certificates MAY include an Internet mail address, as described 143 in section 3. 145 Receiving agents SHOULD support X.509 version 2 attribute 146 certificates. See [KEYMAC] for details about the profile for attribute 147 certificates. 149 2.2.1 Historical Note About CMS Certificates 151 The CMS message format supports a choice of certificate formats for 152 public key content types: PKIX, PKCS #6 Extended Certificates and 153 X.509 Attribute Certificates. 155 The PKCS #6 format is not in widespread use. In addition, PKIX 156 certificate extensions address much of the same functionality and 157 flexibility as was intended in the PKCS #6. Thus, sending and 158 receiving agents MUST NOT use PKCS #6 extended certificates. 160 X.509 version 1 attribute certificates are also not widely 161 implemented, and have been superceded with version 2 attribute 162 certificates. Sending agents MUST NOT send version 1 attribute 163 certificates. 165 2.3 CertificateSet 167 Receiving agents MUST be able to handle an arbitrary number of 168 certificates of arbitrary relationship to the message sender and to 169 each other in arbitrary order. In many cases, the certificates 170 included in a signed message may represent a chain of certification 171 from the sender to a particular root. There may be, however, 172 situations where the certificates in a signed message may be unrelated 173 and included for convenience. 175 Sending agents SHOULD include any certificates for the user's public 176 key(s) and associated issuer certificates. This increases the 177 likelihood that the intended recipient can establish trust in the 178 originator's public key(s). This is especially important when sending 179 a message to recipients that may not have access to the sender's 180 public key through any other means or when sending a signed message to 181 a new recipient. The inclusion of certificates in outgoing messages 182 can be omitted if S/MIME objects are sent within a group of 183 correspondents that has established access to each other's 184 certificates by some other means such as a shared directory or manual 185 certificate distribution. Receiving S/MIME agents SHOULD be able to 186 handle messages without certificates using a database or directory 187 lookup scheme. 189 A sending agent SHOULD include at least one chain of certificates up 190 to, but not including, a Certificate Authority (CA) that it believes 191 that the recipient may trust as authoritative. A receiving agent 192 SHOULD be able to handle an arbitrarily large number of certificates 193 and chains. 195 Agents MAY send CA certificates, that is, certificates that are self- 196 signed and can be considered the "root" of other chains. Note that 197 receiving agents SHOULD NOT simply trust any self-signed certificates 198 as valid CAs, but SHOULD use some other mechanism to determine if this 199 is a CA that should be trusted. Also note that when certificates 200 contain DSA public keys the parameters may be located in the root 201 certificate. This would require that the recipient possess both the 202 end-entity certificate as well as the root certificate to perform a 203 signature verification, and is a valid example of a case where 204 transmitting the root certificate may be required. 206 Receiving agents MUST support chaining based on the distinguished name 207 fields. Other methods of building certificate chains may be supported. 209 Receiving agents SHOULD support the decoding of X.509 attribute 210 certificates included in CMS objects. All other issues regarding the 211 generation and use of X.509 attribute certificates are outside of the 212 scope of this specification. One specification that addresses 213 attribute certificate use is defined in [SECLABEL]. 215 3. Using Distinguished Names for Internet Mail 217 End-entity certificates MAY contain an Internet mail address as 218 described in [RFC-2822]. The address must be an "addr-spec" as defined 219 in Section 3.4.1 of that specification. The email address SHOULD be in 220 the subjectAltName extension, and SHOULD NOT be in the subject 221 distinguished name. 223 Receiving agents MUST recognize and accept certificates that contain 224 no email address. Receiving agents MUST recognize email addresses in 225 the subjectAltName field. Receiving agents MUST recognize email 226 addresses in the Distinguished Name field in the PKCS #9 [PKCS9] 227 emailAddress attribute: 229 pkcs-9-at-emailAddress OBJECT IDENTIFIER ::= 230 {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 1 } 232 Sending agents SHOULD make the address in the From or Sender header in 233 a mail message match an Internet mail address in the signer's 234 certificate. Receiving agents MUST check that the address in the From 235 or Sender header of a mail message matches an Internet mail address, 236 if present, in the signer's certificate, if mail addresses are present 237 in the certificate. A receiving agent SHOULD provide some explicit 238 alternate processing of the message if this comparison fails, which 239 may be to display a message that shows the recipient the addresses in 240 the certificate or other certificate details. 242 A receiving agent SHOULD display a subject name or other certificate 243 details when displaying an indication of successful or unsuccessful 244 signature verification. 246 All subject and issuer names MUST be populated (i.e. not an empty 247 SEQUENCE) in S/MIME-compliant PKIX certificates, except that the 248 subject DN in a user's (i.e. end-entity) certificate MAY be an empty 249 SEQUENCE in which case the subjectAltName extension will include the 250 subject's identifier and MUST be marked as critical. 252 4. Certificate Processing 254 A receiving agent needs to provide some certificate retrieval 255 mechanism in order to gain access to certificates for recipients of 256 digital envelopes. There are many ways to implement certificate 257 retrieval mechanisms. X.500 directory service is an excellent example 258 of a certificate retrieval-only mechanism that is compatible with 259 classic X.500 Distinguished Names. The PKIX Working Group is 260 investigating other mechanisms such as directory servers. Another 261 method under consideration by the IETF is to provide certificate 262 retrieval services as part of the existing Domain Name System (DNS). 263 Until such mechanisms are widely used, their utility may be limited by 264 the small number of correspondent's certificates that can be 265 retrieved. At a minimum, for initial S/MIME deployment, a user agent 266 could automatically generate a message to an intended recipient 267 requesting that recipient's certificate in a signed return message. 269 Receiving and sending agents SHOULD also provide a mechanism to allow 270 a user to "store and protect" certificates for correspondents in such 271 a way so as to guarantee their later retrieval. In many environments, 272 it may be desirable to link the certificate retrieval/storage 273 mechanisms together in some sort of certificate database. In its 274 simplest form, a certificate database would be local to a particular 275 user and would function in a similar way as a "address book" that 276 stores a user's frequent correspondents. In this way, the certificate 277 retrieval mechanism would be limited to the certificates that a user 278 has stored (presumably from incoming messages). A comprehensive 279 certificate retrieval/storage solution may combine two or more 280 mechanisms to allow the greatest flexibility and utility to the user. 281 For instance, a secure Internet mail agent may resort to checking a 282 centralized certificate retrieval mechanism for a certificate if it 283 can not be found in a user's local certificate storage/retrieval 284 database. 286 Receiving and sending agents SHOULD provide a mechanism for the import 287 and export of certificates, using a CMS certs-only message. This 288 allows for import and export of full certificate chains as opposed to 289 just a single certificate. This is described in [SMIME-MSG]. 291 Agents MUST handle multiple valid Certificate Authority (CA) 292 certificates containing the same subject name and the same public keys 293 but with overlapping validity intervals. 295 4.1 Certificate Revocation Lists 297 In general, it is always better to get the latest CRL information from 298 a CA than to get information stored away from incoming messages. A 299 receiving agent SHOULD have access to some certificate revocation list 300 (CRL) retrieval mechanism in order to gain access to certificate 301 revocation information when validating certification paths. A 302 receiving or sending agent SHOULD also provide a mechanism to allow a 303 user to store incoming certificate revocation information for 304 correspondents in such a way so as to guarantee its later retrieval. 306 Receiving and sending agents SHOULD retrieve and utilize CRL 307 information every time a certificate is verified as part of a 308 certification path validation even if the certificate was already 309 verified in the past. However, in many instances (such as off-line 310 verification) access to the latest CRL information may be difficult or 311 impossible. The use of CRL information, therefore, may be dictated by 312 the value of the information that is protected. The value of the CRL 313 information in a particular context is beyond the scope of this 314 specification but may be governed by the policies associated with 315 particular certification paths. 317 All agents MUST be capable of performing revocation checks using CRLs 318 as specified in [KEYM]. All agents MUST perform revocation status 319 checking in accordance with [KEYM]. Receiving agents MUST recognize 320 CRLs in received S/MIME messages. 322 4.2 Certification Path Validation 324 In creating a user agent for secure messaging, certificate, CRL, and 325 certification path validation SHOULD be highly automated while still 326 acting in the best interests of the user. Certificate, CRL, and path 327 validation MUST be performed as per [KEYM] when validating a 328 correspondent's public key. This is necessary before using a public 329 key to provide security services such as: verifying a signature; 330 encrypting a content-encryption key (ex: RSA); or forming a pairwise 331 symmetric key (ex: Diffie-Hellman) to be used to encrypt or decrypt a 332 content-encryption key. 334 Certificates and CRLs are made available to the path validation 335 procedure in two ways: a) incoming messages, and b) certificate and 336 CRL retrieval mechanisms. Certificates and CRLs in incoming messages 337 are not required to be in any particular order nor are they required 338 to be in any way related to the sender or recipient of the message 339 (although in most cases they will be related to the sender). Incoming 340 certificates and CRLs SHOULD be cached for use in path validation and 341 optionally stored for later use. This temporary certificate and CRL 342 cache SHOULD be used to augment any other certificate and CRL 343 retrieval mechanisms for path validation on incoming signed messages. 345 4.3 Certificate and CRL Signing Algorithms 347 Certificates and Certificate Revocation Lists (CRLs) are signed by the 348 certificate issuer. A receiving agent MUST be capable of verifying the 349 signatures on certificates and CRLs made with id-dsa-with-sha1 350 [CMSALG]. 352 A receiving agent MUST be capable of verifying the signatures on 353 certificates and CRLs made with md5WithRSAEncryption and 354 sha1WithRSAEncryption signature algorithms with key sizes from 512 355 bits to 2048 bits described in [CMSALG]. 357 Because of the security issues surrounding MD2 [RC95], and in light of 358 current use, md2WithRSAEncryption MAY be supported. 360 4.4 PKIX Certificate Extensions 362 PKIX describes an extensible framework in which the basic certificate 363 information can be extended and how such extensions can be used to 364 control the process of issuing and validating certificates. The PKIX 365 Working Group has ongoing efforts to identify and create extensions 366 which have value in particular certification environments. Further, 367 there are active efforts underway to issue PKIX certificates for 368 business purposes. This document identifies the minimum required set 369 of certificate extensions which have the greatest value in the S/MIME 370 environment. The syntax and semantics of all the identified extensions 371 are defined in [KEYM]. 373 Sending and receiving agents MUST correctly handle the basic 374 constraints, key usage, authority key identifier, subject key 375 identifier, and subject alternative names certificate extensions when 376 they appear in end-entity certificates. Some mechanism SHOULD exist to 377 gracefully handle other certificate extensions when they appear in 378 end-entity or CA certificates. 380 Certificates issued for the S/MIME environment SHOULD NOT contain any 381 critical extensions (extensions that have the critical field set to 382 TRUE) other than those listed here. These extensions SHOULD be marked 383 as non-critical unless the proper handling of the extension is deemed 384 critical to the correct interpretation of the associated certificate. 385 Other extensions may be included, but those extensions SHOULD NOT be 386 marked as critical. 388 Interpretation and syntax for all extensions MUST follow [KEYM], 389 unless otherwise specified here. 391 4.4.1 Basic Constraints Certificate Extension 393 The basic constraints extension serves to delimit the role and 394 position of an issuing authority or end-entity certificate plays in a 395 certification path. 397 For example, certificates issued to CAs and subordinate CAs contain a 398 basic constraint extension that identifies them as issuing authority 399 certificates. End-entity certificates contain an extension that 400 constrains the certificate from being an issuing authority 401 certificate. 403 Certificates SHOULD contain a basicConstraints extension in CA 404 certificates, and SHOULD NOT contain that extension in end entity 405 certificates. 407 4.4.2 Key Usage Certificate Extension 409 The key usage extension serves to limit the technical purposes for 410 which a public key listed in a valid certificate may be used. Issuing 411 authority certificates may contain a key usage extension that 412 restricts the key to signing certificates, certificate revocation 413 lists and other data. 415 For example, a certification authority may create subordinate issuer 416 certificates which contain a key usage extension which specifies that 417 the corresponding public key can be used to sign end user certificates 418 and sign CRLs. 420 If a key usage extension is included in a PKIX certificate, then it 421 MUST be marked as critical. 423 S/MIME receiving agents MUST NOT accept the signature of a message if 424 it was verified using a certificate which contains the key usage 425 extension without either the digitalSignature or nonRepudiation bit 426 set. Sometimes S/MIME is used as a secure message transport for 427 applications beyond interpersonal messaging. In such cases, the 428 S/MIME-enabled application can specify additional requirements 429 concerning the digitalSignature or nonRepudiation bits within this 430 extension. 432 4.4.2.1 Key Usage in Diffie-Hellman Key Exchange Certificates 434 For Diffie-Hellman key exchange certificates (certificates in which 435 the subject public key algorithm is dhpublicnumber), if the keyUsage 436 keyAgreement bit is set to 1 AND if the public key is to be used to 437 form a pairwise key to decrypt data, then the S/MIME agent MUST only 438 use the public key if the keyUsage encipherOnly bit is set to 0. If 439 the keyUsage keyAgreement bit is set to 1 AND if the key is to be used 440 to form a pairwise key to encrypt data, then the S/MIME agent MUST 441 only use the public key if the keyUsage decipherOnly bit is set to 0. 443 4.4.3 Subject Alternative Name Extension 445 The subject alternative name extension is used in S/MIME as the 446 preferred means to convey the RFC-2822 email address(es) that 447 correspond to the entity for this certificate. Any RFC-2822 email 448 addresses present MUST be encoded using the rfc822Name CHOICE of the 449 GeneralName type. Since the SubjectAltName type is a SEQUENCE OF 450 GeneralName, multiple RFC-2822 email addresses MAY be present. 452 5. Security Considerations 454 All of the security issues faced by any cryptographic application must 455 be faced by a S/MIME agent. Among these issues are protecting the 456 user's private key, preventing various attacks, and helping the user 457 avoid mistakes such as inadvertently encrypting a message for the 458 wrong recipient. The entire list of security considerations is beyond 459 the scope of this document, but some significant concerns are listed 460 here. 462 When processing certificates, there are many situations where the 463 processing might fail. Because the processing may be done by a user 464 agent, a security gateway, or other program, there is no single way to 465 handle such failures. Just because the methods to handle the failures 466 has not been listed, however, the reader should not assume that they 467 are not important. The opposite is true: if a certificate is not 468 provably valid and associated with the message, the processing 469 software should take immediate and noticable steps to inform the end 470 user about it. 472 Some of the many places where signature and certificate checking might 473 fail include: 475 - no Internet mail addresses in a certificate matches the sender of a 476 message,if the certificate contains at least one mail address 477 - no certificate chain leads to a trusted CA 478 - no ability to check the CRL for a certificate 479 - an invalid CRL was received 480 - the CRL being checked is expired 481 - the certificate is expired 482 - the certificate has been revoked 484 There are certainly other instances where a certificate may be 485 invalid, and it is the responsibility of the processing software to 486 check them all thoroughly, and to decide what to do if the check 487 fails. 489 At the Selected Areas in Cryptography '95 conference in May 1995, 490 Rogier and Chauvaud presented an attack on MD2 that can nearly find 491 collisions [RC95]. Collisions occur when one can find two different 492 messages that generate the same message digest. A checksum operation 493 in MD2 is the only remaining obstacle to the success of the attack. 494 For this reason, the use of MD2 for new applications is discouraged. 495 It is still reasonable to use MD2 to verify existing signatures, as 496 the ability to find collisions in MD2 does not enable an attacker to 497 find new messages having a previously computed hash value. 499 A. References 501 [CERTV2] "S/MIME Version 2 Certificate Handling", RFC 2312 503 [CMS] "Cryptographic Message Syntax", RFC 3369 505 [CMSALG] "Cryptographic Message Syntax (CMS) Algorithms", RFC 3370 507 [KEYM] "Internet X.509 Public Key Infrastructure Certificate and CRL 508 Profile", RFC 3280 510 [KEYMAC] "An Internet Attribute Certificate Profile for 511 Authorization", RFC 3281 513 [KEYMALG] "Algorithms and Identifiers for the Internet X.509 Public 514 Key Infrastructure Certificate and CRL Profile ", RFC 3279 516 [MUSTSHOULD] "Key words for use in RFCs to Indicate Requirement 517 Levels", RFC 2119 519 [PKCS9] "PKCS #9: Selected Object Classes and Attribute Types Version 520 2.0", RFC 2985 522 [RC95] Rogier, N. and Chauvaud, P., "The compression function of MD2 523 is not collision free," Presented at Selected Areas in Cryptography 524 '95, May 1995 526 [RFC-2822], "Internet Message Format", RFC 2822 528 [SECLABEL] "Implementing Company Classification Policy with the S/MIME 529 Security Label", RFC 3114 531 [SMIME-MSG] "S/MIME Version 3 Message Specification ", Internet Draft 532 draft-ietf-smime-msg 534 [X.500] ITU-T Recommendation X.500 (1997) | ISO/IEC 9594-1:1997, 535 Information technology - Open Systems Interconnection - The Directory: 536 Overview of concepts, models and services 538 [X.501] ITU-T Recommendation X.501 (1997) | ISO/IEC 9594-2:1997, 539 Information technology - Open Systems Interconnection - The Directory: 540 Models 542 [X.509] ITU-T Recommendation X.509 (1997) | ISO/IEC 9594-8:1997, 543 Information technology - Open Systems Interconnection - The Directory: 544 Authentication framework 546 [X.520] ITU-T Recommendation X.520 (1997) | ISO/IEC 9594-6:1997, 547 Information technology - Open Systems Interconnection - The Directory: 548 Selected attribute types. 550 B. Acknowledgements 552 [tbd] 554 C. Editor's address 556 Blake Ramsdell 557 Brute Squad Labs 558 Suite 217-C 559 16451 Redmond Way 560 Redmond, WA 98052-4482 562 blake@brutesquadlabs.com 564 D. Changes from last draft 566 Clarifications for the use of email addresses in certificates (David 567 P. Kemp) 569 nonRepudiation and digitalSignature key usage language clarification 570 (Russ Housley) 572 Updated references to CMS and CMSALG to point to RFCs (Blake Ramsdell)