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(See Section 2.2 of https://www.ietf.org/id-info/checklist for how to handle the case when there are no actions for IANA.) ** The document seems to lack an Authors' Addresses Section. ** The document seems to lack a both a reference to RFC 2119 and the recommended RFC 2119 boilerplate, even if it appears to use RFC 2119 keywords. RFC 2119 keyword, line 31: '...S/MIME agent MUST certify that the pub...' RFC 2119 keyword, line 34: '...Profile [KEYM]. S/MIME agents MUST meet the S/MIME-specific...' RFC 2119 keyword, line 96: '...draft, the terms MUST, MUST NOT, SHOUL...' RFC 2119 keyword, line 123: '...Receiving agents MUST support for the ...' RFC 2119 keyword, line 125: '...outgoing messages, the CRL format defined in [KEYM] MUST be used....' (51 more instances...) Miscellaneous warnings: ---------------------------------------------------------------------------- -- 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 (December 14, 1998) is 9258 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? 'SMIME-MSG' on line 470 looks like a reference -- Missing reference section? 'KEYM' on line 461 looks like a reference -- Missing reference section? 'CMS' on line 455 looks like a reference -- Missing reference section? 'MUSTSHOULD' on line 464 looks like a reference -- Missing reference section? 'RFC-822' on line 467 looks like a reference -- Missing reference section? 'CERTV2' on line 491 looks like a reference -- Missing reference section? 'CRMF' on line 458 looks like a reference Summary: 9 errors (**), 0 flaws (~~), 3 warnings (==), 9 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Internet Draft Editor: Blake Ramsdell, 2 draft-ietf-smime-cert-06.txt Worldtalk 3 December 14, 1998 4 Expires in six months 6 S/MIME Version 3 Certificate Handling 8 Status of this memo 10 This document is an Internet-Draft. Internet-Drafts are working 11 documents of the Internet Engineering Task Force (IETF), its areas, 12 and its working groups. Note that other groups may also distribute 13 working documents as Internet-Drafts. 15 Internet-Drafts are draft documents valid for a maximum of six months 16 and may be updated, replaced, or obsoleted by other documents at any 17 time. It is inappropriate to use Internet-Drafts as reference material 18 or to cite them other than as "work in progress." 20 To view the entire list of current Internet-Drafts, please check the 21 "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow 22 Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern Europe), 23 ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific Rim), 24 ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast). 26 1. Overview 28 S/MIME (Secure/Multipurpose Internet Mail Extensions), described in 29 [SMIME-MSG], provides a method to send and receive secure MIME 30 messages. Before using a public key to provide security services, the 31 S/MIME agent MUST certify that the public key is valid. S/MIME agents 32 MUST use PKIX certificates to validate public keys as described in the 33 Internet X.509 Public Key Infrastructure (PKIX) Certificate and CRL 34 Profile [KEYM]. S/MIME agents MUST meet the S/MIME-specific 35 requirements documented in this I-D in addition to those stated in 36 [KEYM]. 38 This specification is compatible with the Cryptographic Message Syntax 39 [CMS] in that it uses the data types defined by CMS. It also inherits 40 all the varieties of architectures for certificate-based key 41 management supported by CMS. Note that the method S/MIME messages make 42 certificate requests is defined in [SMIME-MSG]. 44 1.1 Definitions 46 For the purposes of this draft, the following definitions apply. 48 ASN.1: Abstract Syntax Notation One, as defined in ITU-T X.680-689. 50 Attribute Certificate (AC): An X.509 AC is a separate structure from a 51 subject's public key X.509 Certificate. A subject may have multiple 52 X.509 ACs associated with each of its public key X.509 Certificates. 53 Each X.509 AC binds one or more Attributes with one of the subject's 54 public key X.509 Certificates. The X.509 AC syntax is defined in 55 [X.509] 57 BER: Basic Encoding Rules for ASN.1, as defined in ITU-T X.690. 59 Certificate: A type that binds an entity's distinguished name to a 60 public key with a digital signature. This type is defined in the 61 Internet X.509 Public Key Infrastructure (PKIX) Certificate and CRL 62 Profile [KEYM]. This type also contains the distinguished name of the 63 certificate issuer (the signer), an issuer-specific serial number, the 64 issuer's signature algorithm identifier, a validity period, and 65 extensions also defined in that document. 67 Certificate Revocation List (CRL): A type that contains information 68 about certificates whose validity an issuer has prematurely revoked. 69 The information consists of an issuer name, the time of issue, the 70 next scheduled time of issue, a list of certificate serial numbers and 71 their associated revocation times, and extensions as defined in 72 [KEYM]. The CRL is signed by the issuer. The type intended by this 73 specification is the one defined in [KEYM]. 75 DER: Distinguished Encoding Rules for ASN.1, as defined in ITU-T 76 X.690. 78 Receiving agent: software that interprets and processes S/MIME CMS 79 objects, MIME body parts that contain CMS objects, or both. 81 Sending agent: software that creates S/MIME CMS objects, MIME body 82 parts that contain CMS objects, or both. 84 S/MIME agent: user software that is a receiving agent, a sending 85 agent, or both. 87 1.2 Compatibility with Prior Practice of S/MIME 89 S/MIME version 3 agents should attempt to have the greatest 90 interoperability possible with S/MIME version 2 agents. S/MIME version 91 2 is described in RFC 2311 through RFC 2315, inclusive. RFC 2311 also 92 has historical information about the development of S/MIME. 94 1.3 Terminology 96 Throughout this draft, the terms MUST, MUST NOT, SHOULD, and SHOULD 97 NOT are used in capital letters. This conforms to the definitions in 98 [MUSTSHOULD]. [MUSTSHOULD] defines the use of these key words to help 99 make the intent of standards track documents as clear as possible. The 100 same key words are used in this document to help implementors achieve 101 interoperability. 103 1.4 Discussion of This Draft 105 This draft is being discussed on the "ietf-smime" mailing list. 106 To subscribe, send a message to: 107 ietf-smime-request@imc.org 108 with the single word 109 subscribe 110 in the body of the message. There is a Web site for the mailing list 111 at . 113 2. CMS Options 115 The CMS message format allows for a wide variety of options in content 116 and algorithm support. This section puts forth a number of support 117 requirements and recommendations in order to achieve a base level of 118 interoperability among all S/MIME implementations. Most of the CMS 119 format for S/MIME messages is defined in [SMIME-MSG]. 121 2.1 CertificateRevocationLists 123 Receiving agents MUST support for the Certificate Revocation List 124 (CRL) format defined in [KEYM]. If sending agents include CRLs in 125 outgoing messages, the CRL format defined in [KEYM] MUST be used. 127 All agents MUST validate CRLs and check certificates against CRLs, if 128 available, in accordance with [KEYM]. 130 Receiving agents MUST recognize CRLs in received S/MIME messages. 132 Agents MUST use revocation information included as a CRL in an S/MIME 133 message when verifying the signature and certificate path validity in 134 that message. Agents SHOULD store CRLs received in messages for use in 135 processing later messages. 137 Agents MUST handle multiple valid Certificate Authority (CA) 138 certificates containing the same subject name and the same public keys 139 but with overlapping validity intervals. 141 2.2 CertificateChoices 143 Receiving agents MUST support PKIX v1 and PKIX v3 certificates. See 144 [KEYM] for details about the profile for certificate formats. End 145 entity certificates MAY include an Internet mail address, as described 146 in section 3.1. 148 Receiving agents SHOULD support X.509 attribute certificates. 150 2.2.1 Historical Note About CMS Certificates 152 The CMS message format supports a choice of certificate formats for 153 public key content types: PKIX, PKCS #6 Extended Certificates and 154 X.509 Attribute Certificates. The PKCS #6 format is not in widespread 155 use. In addition, PKIX certificate extensions address much of the same 156 functionality and flexibility as was intended in the PKCS #6. Thus, 157 sending and receiving agents MUST NOT use PKCS #6 extended 158 certificates. 160 2.3 CertificateSet 162 Receiving agents MUST be able to handle an arbitrary number of 163 certificates of arbitrary relationship to the message sender and to 164 each other in arbitrary order. In many cases, the certificates 165 included in a signed message may represent a chain of certification 166 from the sender to a particular root. There may be, however, 167 situations where the certificates in a signed message may be unrelated 168 and included for convenience. 170 Sending agents SHOULD include any certificates for the user's public 171 key(s) and associated issuer certificates. This increases the 172 likelihood that the intended recipient can establish trust in the 173 originator's public key(s). This is especially important when sending 174 a message to recipients that may not have access to the sender's 175 public key through any other means or when sending a signed message to 176 a new recipient. The inclusion of certificates in outgoing messages 177 can be omitted if S/MIME objects are sent within a group of 178 correspondents that has established access to each other's 179 certificates by some other means such as a shared directory or manual 180 certificate distribution. Receiving S/MIME agents SHOULD be able to 181 handle messages without certificates using a database or directory 182 lookup scheme. 184 A sending agent SHOULD include at least one chain of certificates up 185 to, but not including, a Certificate Authority (CA) that it believes 186 that the recipient may trust as authoritative. A receiving agent 187 SHOULD be able to handle an arbitrarily large number of certificates 188 and chains. 190 Agents MAY send CA certificates, that is, certificates that are self- 191 signed and can be considered the "root" of other chains. Note that 192 receiving agents SHOULD NOT simply trust any self-signed certificates 193 as valid CAs, but SHOULD use some other mechanism to determine if this 194 is a CA that should be trusted. 196 Receiving agents MUST support chaining based on the distinguished name 197 fields. Other methods of building certificate chains may be supported 198 but are not currently recommended. 200 Receiving agents SHOULD support the decoding of X.509 attribute 201 certificates included in CMS objects. All other issues regarding the 202 generation and use of X.509 attribute certificates are outside of the 203 scope of this specification. 205 3. Distinguished Names in Certificates 207 3.1 Using Distinguished Names for Internet Mail 209 End-entity certificates MAY contain an Internet mail address as 210 described in [RFC-822]. The address must be an "addr-spec" as defined 211 in Section 6.1 of that specification. The email address SHOULD be in 212 the subjectAltName extension, and SHOULD NOT be in the subject 213 distinguished name. 215 Receiving agents MUST recognize email addresses in the subjectAltName 216 field. Receiving agents MUST recognize email addresses in the 217 Distinguished Name field. 219 Sending agents SHOULD make the address in the From or Sender header in 220 a mail message match an Internet mail address in the signer's 221 certificate. Receiving agents MUST check that the address in the From 222 or Sender header of a mail message matches an Internet mail address in 223 the signer's certificate, if mail addresses are present in the 224 certificate. A receiving agent SHOULD provide some explicit alternate 225 processing of the message if this comparison fails, which may be to 226 display a message that shows the recipient the addresses in the 227 certificate or other certificate details. 229 All subject and issuer names MUST be populated (i.e. not an empty 230 SEQUENCE) in S/MIME-compliant PKIX certificates, except that the 231 subject DN in a user's (i.e. end-entity) certificate MAY be an empty 232 SEQUENCE in which case the subjectAltName extension will include the 233 subject's identifier and MUST be marked as critical. 235 4. Certificate Processing 237 A receiving agent needs to provide some certificate retrieval 238 mechanism in order to gain access to certificates for recipients of 239 digital envelopes. There are many ways to implement certificate 240 retrieval mechanisms. X.500 directory service is an excellent example 241 of a certificate retrieval-only mechanism that is compatible with 242 classic X.500 Distinguished Names. The PKIX Working Group is 243 investigating other mechanisms such as directory servers. Another 244 method under consideration by the IETF is to provide certificate 245 retrieval services as part of the existing Domain Name System (DNS). 246 Until such mechanisms are widely used, their utility may be limited by 247 the small number of correspondent's certificates that can be 248 retrieved. At a minimum, for initial S/MIME deployment, a user agent 249 could automatically generate a message to an intended recipient 250 requesting that recipient's certificate in a signed return message. 252 Receiving and sending agents SHOULD also provide a mechanism to allow 253 a user to "store and protect" certificates for correspondents in such 254 a way so as to guarantee their later retrieval. In many environments, 255 it may be desirable to link the certificate retrieval/storage 256 mechanisms together in some sort of certificate database. In its 257 simplest form, a certificate database would be local to a particular 258 user and would function in a similar way as a "address book" that 259 stores a user's frequent correspondents. In this way, the certificate 260 retrieval mechanism would be limited to the certificates that a user 261 has stored (presumably from incoming messages). A comprehensive 262 certificate retrieval/storage solution may combine two or more 263 mechanisms to allow the greatest flexibility and utility to the user. 264 For instance, a secure Internet mail agent may resort 265 to checking a centralized certificate retrieval mechanism for a 266 certificate if it can not be found in a user's local certificate 267 storage/retrieval database. 269 Receiving and sending agents SHOULD provide a mechanism for the import 270 and export of certificates, using a CMS certs-only message. This 271 allows for import and export of full certificate chains as opposed to 272 just a single certificate. This is described in [SMIME-MSG]. 274 4.1 Certificate Revocation Lists 276 In general, it is always better to get the latest CRL information from 277 a CA than to get information stored away from incoming messages. A 278 receiving agent SHOULD have access to some certificate-revocation list 279 (CRL) retrieval mechanism in order to gain access to certificate- 280 revocation information when validating certificate chains. A receiving 281 or sending agent SHOULD also provide a mechanism to allow a user to 282 store incoming certificate-revocation information for correspondents 283 in such a way so as to guarantee its later retrieval. 285 Receiving and sending agents SHOULD retrieve and utilize CRL 286 information every time a certificate is verified as part of a 287 certificate chain validation even if the certificate was already 288 verified in the past. However, in many instances (such as off-line 289 verification) access to the latest CRL information may be difficult or 290 impossible. The use of CRL information, therefore, may be dictated by 291 the value of the information that is protected. The value of the CRL 292 information in a particular context is beyond the scope of this draft 293 but may be governed by the policies associated with particular 294 certificate hierarchies. 296 4.2 Certificate Chain Validation 298 In creating a user agent for secure messaging, certificate, CRL, and 299 certificate chain validation SHOULD be highly automated while still 300 acting in the best interests of the user. Certificate, CRL, and chain 301 validation MUST be performed as per [KEYM] when validating a 302 correspondent's public key. This is necessary before using a public 303 key to provide security services such as: verifying a signature; 304 encrypting a content-encryption key (ex: RSA); or forming a pairwise 305 symmetric key (ex: Diffie-Hellman) to be used to encrypt or decrypt a 306 content-encryption key. 308 Certificates and CRLs are made available to the chain validation 309 procedure in two ways: a) incoming messages, and b) certificate and 310 CRL retrieval mechanisms. Certificates and CRLs in incoming messages 311 are not required to be in any particular order nor are they required 312 to be in any way related to the sender or recipient of the message 313 (although in most cases they will be related to the sender). Incoming 314 certificates and CRLs SHOULD be cached for use in chain validation and 315 optionally stored for later use. This temporary certificate and CRL 316 cache SHOULD be used to augment any other certificate and CRL 317 retrieval mechanisms for chain validation on incoming signed messages. 319 4.3 Certificate and CRL Signing Algorithms 321 Certificates and Certificate-Revocation Lists (CRLs) are signed by the 322 certificate issuer. A receiving agent MUST be capable of verifying the 323 signatures on certificates and CRLs made with id-dsa-with-sha1. 325 A receiving agent SHOULD be capable of verifying the signatures on 326 certificates and CRLs made with md2WithRSAEncryption, 327 md5WithRSAEncryption and sha-1WithRSAEncryption signature algorithms 328 with key sizes from 512 bits to 2048 bits described in [SMIME-MSG]. 330 4.4 PKIX Certificate Extensions 332 PKIX describes an extensible framework in which the basic certificate 333 information can be extended and how such extensions can be used to 334 control the process of issuing and validating certificates. The PKIX 335 Working Group has ongoing efforts to identify and create extensions 336 which have value in particular certification environments. As such, 337 there is 338 still a fair amount of profiling work to be done before there is 339 widespread agreement on which extensions will be used. Further, there 340 are active efforts underway to issue PKIX certificates for business 341 purposes. This draft identifies the minumum required set of 342 certificate extensions which have the greatest value in the S/MIME 343 environment. The basicConstraints, and keyUsage extensions are defined 344 in [KEYM]. 346 Sending and receiving agents MUST correctly handle the Basic 347 Constraints Certificate Extension, the Key Usage Certificate 348 Extension, authorityKeyID, subjectKeyID, and the subjectAltNames when 349 they appear in end-user certificates. Some mechanism SHOULD exist to 350 handle the defined certificate extensions when they appear in 351 intermediate or CA certificates. 353 Certificates issued for the S/MIME environment SHOULD NOT contain any 354 critical extensions (extensions that have the critical field set to 355 TRUE) other than those listed here. These extensions SHOULD be marked 356 as non-critical unless the proper handling of the extension is deemed 357 critical to the correct interpretation of the associated certificate. 358 Other extensions may be included, but those extensions SHOULD NOT be 359 marked as critical. 361 Interpretation and syntax for all extensions MUST follow [KEYM], 362 unless otherwise specified here. 364 4.4.1 Basic Constraints Certificate Extension 366 The basic constraints extension serves to delimit the role and 367 position of an issuing authority or end-entity certificate plays in a 368 chain of certificates. 370 For example, certificates issued to CAs and subordinate CAs contain a 371 basic constraint extension that identifies them as issuing authority 372 certificates. End-entity certificates contain an extension that 373 constrains the certificate from being an issuing authority 374 certificate. 376 Certificates SHOULD contain a basicConstraints extension. 378 4.4.2 Key Usage Certificate Extension 380 The key usage extension serves to limit the technical purposes for 381 which a public key listed in a valid certificate may be used. Issuing 382 authority certificates may contain a key usage extension that 383 restricts the key to signing certificates, certificate revocation 384 lists and other data. 386 For example, a certification authority may create subordinate issuer 387 certificates which contain a keyUsage extension which specifies that 388 the corresponding public key can be used to sign end user certs and 389 sign CRLs. 391 If a key usage extension is included in a PKIX certificate, then it 392 MUST be marked as critical. 394 4.4.2.1 Key Usage in Diffie-Hellman Key Exchange Certificates 396 For Diffie-Hellman key exchange certificates (certificates in which 397 the subject public key algorithm is dhpublicnumber), more 398 clarification is needed as to how to interpret the key usage 399 extension. If the keyUsage keyAgreement bit is set to 1 AND if the 400 public key is to be used to form a pairwise key to decrypt data, then 401 the S/MIME agent MUST only use the public key if the keyUsage 402 encipherOnly bit is set to 0. If the keyUsage keyAgreement bit is set 403 to 1 AND if the key is to be used to form a pairwise key to encrypt 404 data, then the S/MIME agent MUST only use the public key if the 405 keyUsage decipherOnly bit is set to 0. 407 4.4.3 Subject Alternative Name Extension 409 The subject alternative name extension is used in S/MIME as the 410 preferred means to convey the RFC-822 email address(es) that 411 correspond to the entity for this certificate. Any RFC-822 email 412 addresses present MUST be encoded using the rfc822Name CHOICE of the 413 GeneralName type. Since the SubjectAltName type is a SEQUENCE OF 414 GeneralName, multiple RFC-822 email addresses MAY be present. 416 5. Security Considerations 418 All of the security issues faced by any cryptographic application must 419 be faced by a S/MIME agent. Among these issues are protecting the 420 user's private key, preventing various attacks, and helping the user 421 avoid mistakes such as inadvertently encrypting a message for the 422 wrong recipient. The entire list of security considerations is beyond 423 the scope of this document, but some significant concerns are listed 424 here. 426 When processing certificates, there are many situations where the 427 processing might fail. Because the processing may be done by a user 428 agent, a security gateway, or other program, there is no single way to 429 handle such failures. Just because the methods to handle the failures 430 has not been listed, however, the reader should not assume that they 431 are not important. The opposite is true: if a certificate is not 432 provably valid and associated with the message, the processing 433 software should take immediate and noticable steps to inform the end 434 user about it. 436 Some of the many places where signature and certificate checking might 437 fail include: 438 - no Internet mail addresses in a certificate match the sender of a 439 message 440 - no certificate chain leads to a trusted CA 441 - no ability to check the CRL for a certificate 442 - an invalid CRL was received 443 - the CRL being checked is expired 444 - the certificate is expired 445 - the certificate has been revoked 446 There are certainly other instances where a certificate may be 447 invalid, and it is the responsibility of the processing software to 448 check them all thoroughly, and to decide what to do if the check 449 fails. 451 A. References 453 [CERTV2] "S/MIME Version 2 Certificate Handling", RFC 2312 455 [CMS] "Cryptographic Message Syntax", Internet Draft draft-housley- 456 smime-cms 458 [CRMF] "Certificate Request Message Format", Internet Draft draft-ietf- 459 pkix-crmf 461 [KEYM] "Internet X.509 Public Key Infrastructure Certificate and CRL 462 Profile", Internet-Draft draft-ietf-pkix-ipki-part1 464 [MUSTSHOULD] "Key words for use in RFCs to Indicate Requirement 465 Levels", RFC 2119 467 [RFC-822], "Standard For The Format Of ARPA Internet Text Messages", 468 RFC 822. 470 [SMIME-MSG] "S/MIME Version 3 Message Specification ", Internet Draft 471 draft-ietf-smime-msg 473 [X.500] ITU-T Recommendation X.500 (1997) | ISO/IEC 9594-1:1997, 474 Information technology - Open Systems Interconnection - The Directory: 475 Overview of concepts, models and services 477 [X.501] ITU-T Recommendation X.501 (1997) | ISO/IEC 9594-2:1997, 478 Information technology - Open Systems Interconnection - The Directory: 479 Models 481 [X.509] ITU-T Recommendation X.509 (1997) | ISO/IEC 9594-8:1997, 482 Information technology - Open Systems Interconnection - The Directory: 483 Authentication framework 485 [X.520] ITU-T Recommendation X.520 (1997) | ISO/IEC 9594-6:1997, 486 Information technology - Open Systems Interconnection - The Directory: 487 Selected attribute types. 489 B. Acknowledgements 491 This document is largely based on [CERTV2] written by Steve Dusse, 492 Paul Hoffman, Blake Ramsdell, and Jeff Weinstein. 494 Significant comments and additions were made by John Pawling and Jim 495 Schaad. 497 C. Changes from last draft 499 Removed section 3.2 (required name attributes) (WG Consensus) 500 Added definitions for "agents" (Bill Flanigan, Paul Hoffman) 502 D. Editor�s address 504 Blake Ramsdell 505 Worldtalk 506 13122 NE 20th St., Suite C 507 Bellevue, WA 98005 508 (425) 882-8861 509 blaker@deming.com