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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 (October 23, 1999) is 8951 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 472 looks like a reference -- Missing reference section? 'KEYM' on line 460 looks like a reference -- Missing reference section? 'CMS' on line 455 looks like a reference -- Missing reference section? 'MUSTSHOULD' on line 463 looks like a reference -- Missing reference section? 'RFC-822' on line 469 looks like a reference -- Missing reference section? 'DSS' on line 458 looks like a reference -- Missing reference section? 'CERTV2' on line 453 looks like a reference Summary: 7 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-08.txt Worldtalk 3 April 23, 1999 4 Expires October 23, 1999 6 S/MIME Version 3 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 20 material 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 [X.509] 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: 105 ietf-smime-request@imc.org 106 with the single word 107 subscribe 108 in the body of the message. There is a Web site for the mailing list 109 at . 111 2. CMS Options 113 The CMS message format allows for a wide variety of options in content 114 and algorithm support. This section puts forth a number of support 115 requirements and recommendations in order to achieve a base level of 116 interoperability among all S/MIME implementations. Most of the CMS 117 format for S/MIME messages is defined in [SMIME-MSG]. 119 2.1 CertificateRevocationLists 121 Receiving agents MUST support the Certificate Revocation List (CRL) 122 format defined in [KEYM]. If sending agents include CRLs in outgoing 123 messages, the CRL format defined in [KEYM] MUST be used. 125 All agents MUST be capable of performing revocation checks using CRLs 126 as specified in [KEYM]. All agents MUST perform revocation status 127 checking in accordance with [KEYM]. Receiving agents MUST recognize 128 CRLs in received S/MIME messages. 130 Agents SHOULD store CRLs received in messages for use in processing 131 later messages. 133 Agents MUST handle multiple valid Certificate Authority (CA) 134 certificates containing the same subject name and the same public keys 135 but with overlapping validity intervals. 137 2.2 CertificateChoices 139 Receiving agents MUST support PKIX v1 and PKIX v3 certificates. See 140 [KEYM] for details about the profile for certificate formats. End 141 entity certificates MAY include an Internet mail address, as described 142 in section 3.1. 144 Receiving agents SHOULD support X.509 attribute certificates. 146 2.2.1 Historical Note About CMS Certificates 148 The CMS message format supports a choice of certificate formats for 149 public key content types: PKIX, PKCS #6 Extended Certificates and 150 X.509 Attribute Certificates. The PKCS #6 format is not in widespread 151 use. In addition, PKIX certificate extensions address much of the same 152 functionality and flexibility as was intended in the PKCS #6. Thus, 153 sending and receiving agents MUST NOT use PKCS #6 extended 154 certificates. 156 2.3 CertificateSet 158 Receiving agents MUST be able to handle an arbitrary number of 159 certificates of arbitrary relationship to the message sender and to 160 each other in arbitrary order. In many cases, the certificates 161 included in a signed message may represent a chain of certification 162 from the sender to a particular root. There may be, however, 163 situations where the certificates in a signed message may be unrelated 164 and included for convenience. 166 Sending agents SHOULD include any certificates for the user's public 167 key(s) and associated issuer certificates. This increases the 168 likelihood that the intended recipient can establish trust in the 169 originator's public key(s). This is especially important when sending 170 a message to recipients that may not have access to the sender's 171 public key through any other means or when sending a signed message to 172 a new recipient. The inclusion of certificates in outgoing messages 173 can be omitted if S/MIME objects are sent within a group of 174 correspondents that has established access to each other's 175 certificates by some other means such as a shared directory or manual 176 certificate distribution. Receiving S/MIME agents SHOULD be able to 177 handle messages without certificates using a database or directory 178 lookup scheme. 180 A sending agent SHOULD include at least one chain of certificates up 181 to, but not including, a Certificate Authority (CA) that it believes 182 that the recipient may trust as authoritative. A receiving agent 183 SHOULD be able to handle an arbitrarily large number of certificates 184 and chains. 186 Agents MAY send CA certificates, that is, certificates that are self- 187 signed and can be considered the "root" of other chains. Note that 188 receiving agents SHOULD NOT simply trust any self-signed certificates 189 as valid CAs, but SHOULD use some other mechanism to determine if this 190 is a CA that should be trusted. Also note that in the case of DSA 191 certificates the parameters may be located in the root certificate. 192 This would require that the recipient possess the root certificate in 193 order to perform a signature verification, and is a valid example of a 194 case where transmitting the root certificate may be required. 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. Using Distinguished Names for Internet Mail 207 End-entity certificates MAY contain an Internet mail address as 208 described in [RFC-822]. The address must be an "addr-spec" as defined 209 in Section 6.1 of that specification. The email address SHOULD be in 210 the subjectAltName extension, and SHOULD NOT be in the subject 211 distinguished name. 213 Receiving agents MUST recognize email addresses in the subjectAltName 214 field. Receiving agents MUST recognize email addresses in the 215 Distinguished Name field in the PKCS #9 emailAddress attribute. 217 Sending agents SHOULD make the address in the From or Sender header in 218 a mail message match an Internet mail address in the signer's 219 certificate. Receiving agents MUST check that the address in the From 220 or Sender header of a mail message matches an Internet mail address in 221 the signer's certificate, if mail addresses are present in the 222 certificate. A receiving agent SHOULD provide some explicit alternate 223 processing of the message if this comparison fails, which may be to 224 display a message that shows the recipient the addresses in the 225 certificate or other certificate details. 227 All subject and issuer names MUST be populated (i.e. not an empty 228 SEQUENCE) in S/MIME-compliant PKIX certificates, except that the 229 subject DN in a user's (i.e. end-entity) certificate MAY be an empty 230 SEQUENCE in which case the subjectAltName extension will include the 231 subject's identifier and MUST be marked as critical. 233 4. Certificate Processing 235 A receiving agent needs to provide some certificate retrieval 236 mechanism in order to gain access to certificates for recipients of 237 digital envelopes. There are many ways to implement certificate 238 retrieval mechanisms. X.500 directory service is an excellent example 239 of a certificate retrieval-only mechanism that is compatible with 240 classic X.500 Distinguished Names. The PKIX Working Group is 241 investigating other mechanisms such as directory servers. Another 242 method under consideration by the IETF is to provide certificate 243 retrieval services as part of the existing Domain Name System (DNS). 244 Until such mechanisms are widely used, their utility may be limited by 245 the small number of correspondent's certificates that can be 246 retrieved. At a minimum, for initial S/MIME deployment, a user agent 247 could automatically generate a message to an intended recipient 248 requesting that recipient's certificate in a signed return message. 250 Receiving and sending agents SHOULD also provide a mechanism to allow 251 a user to "store and protect" certificates for correspondents in such 252 a way so as to guarantee their later retrieval. In many environments, 253 it may be desirable to link the certificate retrieval/storage 254 mechanisms together in some sort of certificate database. In its 255 simplest form, a certificate database would be local to a particular 256 user and would function in a similar way as a "address book" that 257 stores a user's frequent correspondents. In this way, the certificate 258 retrieval mechanism would be limited to the certificates that a user 259 has stored (presumably from incoming messages). A comprehensive 260 certificate retrieval/storage solution may combine two or more 261 mechanisms to allow the greatest flexibility and utility to the user. 262 For instance, a secure Internet mail agent may resort 263 to checking a centralized certificate retrieval mechanism for a 264 certificate if it can not be found in a user's local certificate 265 storage/retrieval database. 267 Receiving and sending agents SHOULD provide a mechanism for the import 268 and export of certificates, using a CMS certs-only message. This 269 allows for import and export of full certificate chains as opposed to 270 just a single certificate. This is described in [SMIME-MSG]. 272 4.1 Certificate Revocation Lists 274 In general, it is always better to get the latest CRL information from 275 a CA than to get information stored away from incoming messages. A 276 receiving agent SHOULD have access to some certificate-revocation list 277 (CRL) retrieval mechanism in order to gain access to certificate- 278 revocation information when validating certificate chains. A receiving 279 or sending agent SHOULD also provide a mechanism to allow a user to 280 store incoming certificate-revocation information for correspondents 281 in such a way so as to guarantee its later retrieval. 283 Receiving and sending agents SHOULD retrieve and utilize CRL 284 information every time a certificate is verified as part of a 285 certificate chain validation even if the certificate was already 286 verified in the past. However, in many instances (such as off-line 287 verification) access to the latest CRL information may be difficult or 288 impossible. The use of CRL information, therefore, may be dictated by 289 the value of the information that is protected. The value of the CRL 290 information in a particular context is beyond the scope of this draft 291 but may be governed by the policies associated with particular 292 certificate hierarchies. 294 All agents MUST be capable of performing revocation checks using CRLs 295 as specified in [KEYM]. All agents MUST perform revocation status 296 checking in accordance with [KEYM]. Receiving agents MUST recognize 297 CRLs in received S/MIME messages. 299 4.2 Certificate Chain Validation 301 In creating a user agent for secure messaging, certificate, CRL, and 302 certificate chain validation SHOULD be highly automated while still 303 acting in the best interests of the user. Certificate, CRL, and chain 304 validation MUST be performed as per [KEYM] when validating a 305 correspondent's public key. This is necessary before using a public 306 key to provide security services such as: verifying a signature; 307 encrypting a content-encryption key (ex: RSA); or forming a pairwise 308 symmetric key (ex: Diffie-Hellman) to be used to encrypt or decrypt a 309 content-encryption key. 311 Certificates and CRLs are made available to the chain validation 312 procedure in two ways: a) incoming messages, and b) certificate and 313 CRL retrieval mechanisms. Certificates and CRLs in incoming messages 314 are not required to be in any particular order nor are they required 315 to be in any way related to the sender or recipient of the message 316 (although in most cases they will be related to the sender). Incoming 317 certificates and CRLs SHOULD be cached for use in chain validation and 318 optionally stored for later use. This temporary certificate and CRL 319 cache SHOULD be used to augment any other certificate and CRL 320 retrieval mechanisms for chain validation on incoming signed messages. 322 4.3 Certificate and CRL Signing Algorithms 324 Certificates and Certificate-Revocation Lists (CRLs) are signed by the 325 certificate issuer. A receiving agent MUST be capable of verifying the 326 signatures on certificates and CRLs made with id-dsa-with-sha1 [DSS]. 328 A receiving agent SHOULD be capable of verifying the signatures on 329 certificates and CRLs made with md2WithRSAEncryption, 330 md5WithRSAEncryption and sha-1WithRSAEncryption signature algorithms 331 with key sizes from 512 bits to 2048 bits described in [PKCS#1V2]. 333 4.4 PKIX Certificate Extensions 335 PKIX describes an extensible framework in which the basic certificate 336 information can be extended and how such extensions can be used to 337 control the process of issuing and validating certificates. The PKIX 338 Working Group has ongoing efforts to identify and create extensions 339 which have value in particular certification environments. Further, 340 there are active efforts underway to issue PKIX certificates for 341 business purposes. This document identifies the minumum required set 342 of certificate extensions which have the greatest value in the S/MIME 343 environment. The syntax and semantics of all the identified extensions 344 are defined 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 in CA 377 certificates, and SHOULD NOT contain that extension in end entity 378 certificates. 380 4.4.2 Key Usage Certificate Extension 382 The key usage extension serves to limit the technical purposes for 383 which a public key listed in a valid certificate may be used. Issuing 384 authority certificates may contain a key usage extension that 385 restricts the key to signing certificates, certificate revocation 386 lists and other data. 388 For example, a certification authority may create subordinate issuer 389 certificates which contain a keyUsage extension which specifies that 390 the corresponding public key can be used to sign end user certs and 391 sign CRLs. 393 If a key usage extension is included in a PKIX certificate, then it 394 MUST be marked as critical. 396 4.4.2.1 Key Usage in Diffie-Hellman Key Exchange Certificates 398 For Diffie-Hellman key exchange certificates (certificates in which 399 the subject public key algorithm is dhpublicnumber), if the keyUsage 400 keyAgreement bit is set to 1 AND if the public key is to be used to 401 form a pairwise key to decrypt data, then the S/MIME agent MUST only 402 use the public key if the keyUsage encipherOnly bit is set to 0. If 403 the keyUsage keyAgreement bit is set to 1 AND if the key is to be used 404 to form a pairwise key to encrypt data, then the S/MIME agent MUST 405 only use the public key if the 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 [DSS] NIST FIPS PUB 186, "Digital Signature Standard", 18 May 1994. 460 [KEYM] "Internet X.509 Public Key Infrastructure Certificate and CRL 461 Profile", Internet-Draft draft-ietf-pkix-ipki-part1 463 [MUSTSHOULD] "Key words for use in RFCs to Indicate Requirement 464 Levels", RFC 2119 466 [PKCS#1V2], "PKCS #1: RSA Cryptography Specifications Version 2.0", 467 RFC 2437 469 [RFC-822], "Standard For The Format Of ARPA Internet Text Messages", 470 RFC 822. 472 [SMIME-MSG] "S/MIME Version 3 Message Specification ", Internet Draft 473 draft-ietf-smime-msg 475 [X.500] ITU-T Recommendation X.500 (1997) | ISO/IEC 9594-1:1997, 476 Information technology - Open Systems Interconnection - The Directory: 477 Overview of concepts, models and services 479 [X.501] ITU-T Recommendation X.501 (1997) | ISO/IEC 9594-2:1997, 480 Information technology - Open Systems Interconnection - The Directory: 481 Models 483 [X.509] ITU-T Recommendation X.509 (1997) | ISO/IEC 9594-8:1997, 484 Information technology - Open Systems Interconnection - The Directory: 485 Authentication framework 487 [X.520] ITU-T Recommendation X.520 (1997) | ISO/IEC 9594-6:1997, 488 Information technology - Open Systems Interconnection - The Directory: 489 Selected attribute types. 491 B. Acknowledgements 493 Many thanks go out to the other authors of the S/MIME v2 RFC: Steve 494 Dusse, Paul Hoffman and Jeff Weinstein. Without v2, there wouldn't be 495 a v3. 497 A number of the members of the S/MIME Working Group have also worked 498 very hard and contributed to this document. Any list of people is 499 doomed to omission and for that I apologize. In alphabetical order, 500 the following people stand out in my mind due to the fact that they 501 made direct contributions to this document. 503 Bill Flanigan 504 Elliott Ginsburg 505 Paul Hoffman 506 Russ Housley 507 Michael Myers 508 John Pawling 509 Denis Pinkas 510 Jim Schaad 512 C. Changes from last draft 514 Changed section 1.3 to contain correct reference language for 515 MUSTSHOULD (Thomas Narten) 516 Changed section D to update author's address (Blake Ramsdell) 517 Changed in section B to actual list (Blake Ramsdell) 519 D. Editor�s address 521 Blake Ramsdell 522 Worldtalk 523 17720 NE 65th St Ste 201 524 Redmond, WA 98052 525 +1 425 376 0225 526 blaker@deming.com