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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-00.txt Brute Squad Labs 3 February 8, 2002 4 Expires August 8, 2002 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 [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: 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. 129 All agents MUST be capable of performing revocation checks using CRLs 130 as specified in [KEYM]. All agents MUST perform revocation status 131 checking in accordance with [KEYM]. Receiving agents MUST recognize 132 CRLs in received S/MIME messages. 134 Agents SHOULD store CRLs received in messages for use in processing 135 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. Also note that in the case of DSA 195 certificates the parameters may be located in the root certificate. 196 This would require that the recipient possess the root certificate in 197 order to perform a signature verification, and is a valid example of a 198 case where transmitting the root certificate may be required. 200 Receiving agents MUST support chaining based on the distinguished name 201 fields. Other methods of building certificate chains may be supported 202 but are not currently recommended. 204 Receiving agents SHOULD support the decoding of X.509 attribute 205 certificates included in CMS objects. All other issues regarding the 206 generation and use of X.509 attribute certificates are outside of the 207 scope of this specification. 209 3. Using Distinguished Names for Internet Mail 211 End-entity certificates MAY contain an Internet mail address as 212 described in [RFC-2822]. The address must be an "addr-spec" as defined 213 in Section 3.4.1 of that specification. The email address SHOULD be in 214 the subjectAltName extension, and SHOULD NOT be in the subject 215 distinguished name. 217 Receiving agents MUST recognize email addresses in the subjectAltName 218 field. Receiving agents MUST recognize email addresses in the 219 Distinguished Name field in the PKCS #9 emailAddress attribute. 221 Sending agents SHOULD make the address in the From or Sender header in 222 a mail message match an Internet mail address in the signer's 223 certificate. Receiving agents MUST check that the address in the From 224 or Sender header of a mail message matches an Internet mail address in 225 the signer's certificate, if mail addresses are present in the 226 certificate. A receiving agent SHOULD provide some explicit alternate 227 processing of the message if this comparison fails, which may be to 228 display a message that shows the recipient the addresses in the 229 certificate or other certificate details. 231 All subject and issuer names MUST be populated (i.e. not an empty 232 SEQUENCE) in S/MIME-compliant PKIX certificates, except that the 233 subject DN in a user's (i.e. end-entity) certificate MAY be an empty 234 SEQUENCE in which case the subjectAltName extension will include the 235 subject's identifier and MUST be marked as critical. 237 4. Certificate Processing 239 A receiving agent needs to provide some certificate retrieval 240 mechanism in order to gain access to certificates for recipients of 241 digital envelopes. There are many ways to implement certificate 242 retrieval mechanisms. X.500 directory service is an excellent example 243 of a certificate retrieval-only mechanism that is compatible with 244 classic X.500 Distinguished Names. The PKIX Working Group is 245 investigating other mechanisms such as directory servers. Another 246 method under consideration by the IETF is to provide certificate 247 retrieval services as part of the existing Domain Name System (DNS). 248 Until such mechanisms are widely used, their utility may be limited by 249 the small number of correspondent's certificates that can be 250 retrieved. At a minimum, for initial S/MIME deployment, a user agent 251 could automatically generate a message to an intended recipient 252 requesting that recipient's certificate in a signed return message. 254 Receiving and sending agents SHOULD also provide a mechanism to allow 255 a user to "store and protect" certificates for correspondents in such 256 a way so as to guarantee their later retrieval. In many environments, 257 it may be desirable to link the certificate retrieval/storage 258 mechanisms together in some sort of certificate database. In its 259 simplest form, a certificate database would be local to a particular 260 user and would function in a similar way as a "address book" that 261 stores a user's frequent correspondents. In this way, the certificate 262 retrieval mechanism would be limited to the certificates that a user 263 has stored (presumably from incoming messages). A comprehensive 264 certificate retrieval/storage solution may combine two or more 265 mechanisms to allow the greatest flexibility and utility to the user. 266 For instance, a secure Internet mail agent may resort to checking a 267 centralized certificate retrieval mechanism for a certificate if it 268 can not be found in a user's local certificate storage/retrieval 269 database. 271 Receiving and sending agents SHOULD provide a mechanism for the import 272 and export of certificates, using a CMS certs-only message. This 273 allows for import and export of full certificate chains as opposed to 274 just a single certificate. This is described in [SMIME-MSG]. 276 4.1 Certificate Revocation Lists 278 In general, it is always better to get the latest CRL information from 279 a CA than to get information stored away from incoming messages. A 280 receiving agent SHOULD have access to some certificate-revocation list 281 (CRL) retrieval mechanism in order to gain access to certificate- 282 revocation information when validating certificate chains. A receiving 283 or sending agent SHOULD also provide a mechanism to allow a user to 284 store incoming certificate-revocation information for correspondents 285 in such a way so as to guarantee its later retrieval. 287 Receiving and sending agents SHOULD retrieve and utilize CRL 288 information every time a certificate is verified as part of a 289 certificate chain validation even if the certificate was already 290 verified in the past. However, in many instances (such as off-line 291 verification) access to the latest CRL information may be difficult or 292 impossible. The use of CRL information, therefore, may be dictated by 293 the value of the information that is protected. The value of the CRL 294 information in a particular context is beyond the scope of this draft 295 but may be governed by the policies associated with particular 296 certificate hierarchies. 298 All agents MUST be capable of performing revocation checks using CRLs 299 as specified in [KEYM]. All agents MUST perform revocation status 300 checking in accordance with [KEYM]. Receiving agents MUST recognize 301 CRLs in received S/MIME messages. 303 4.2 Certificate Chain Validation 305 In creating a user agent for secure messaging, certificate, CRL, and 306 certificate chain validation SHOULD be highly automated while still 307 acting in the best interests of the user. Certificate, CRL, and chain 308 validation MUST be performed as per [KEYM] when validating a 309 correspondent's public key. This is necessary before using a public 310 key to provide security services such as: verifying a signature; 311 encrypting a content-encryption key (ex: RSA); or forming a pairwise 312 symmetric key (ex: Diffie-Hellman) to be used to encrypt or decrypt a 313 content-encryption key. 315 Certificates and CRLs are made available to the chain validation 316 procedure in two ways: a) incoming messages, and b) certificate and 317 CRL retrieval mechanisms. Certificates and CRLs in incoming messages 318 are not required to be in any particular order nor are they required 319 to be in any way related to the sender or recipient of the message 320 (although in most cases they will be related to the sender). Incoming 321 certificates and CRLs SHOULD be cached for use in chain validation and 322 optionally stored for later use. This temporary certificate and CRL 323 cache SHOULD be used to augment any other certificate and CRL 324 retrieval mechanisms for chain validation on incoming signed messages. 326 4.3 Certificate and CRL Signing Algorithms 328 Certificates and Certificate-Revocation Lists (CRLs) are signed by the 329 certificate issuer. A receiving agent MUST be capable of verifying the 330 signatures on certificates and CRLs made with id-dsa-with-sha1 331 [CMSALG]. 333 A receiving agent MUST be capable of verifying the signatures on 334 certificates and CRLs made with md2WithRSAEncryption, 335 md5WithRSAEncryption and sha-1WithRSAEncryption signature algorithms 336 with key sizes from 512 bits to 2048 bits described in [CMSALG]. 338 4.4 PKIX Certificate Extensions 340 PKIX describes an extensible framework in which the basic certificate 341 information can be extended and how such extensions can be used to 342 control the process of issuing and validating certificates. The PKIX 343 Working Group has ongoing efforts to identify and create extensions 344 which have value in particular certification environments. Further, 345 there are active efforts underway to issue PKIX certificates for 346 business purposes. This document identifies the minimum required set 347 of certificate extensions which have the greatest value in the S/MIME 348 environment. The syntax and semantics of all the identified extensions 349 are defined in [KEYM]. 351 Sending and receiving agents MUST correctly handle the Basic 352 Constraints Certificate Extension, the Key Usage Certificate 353 Extension, authorityKeyID, subjectKeyID, and the subjectAltNames when 354 they appear in end-user certificates. Some mechanism SHOULD exist to 355 handle the defined certificate extensions when they appear in 356 intermediate or CA certificates. 358 Certificates issued for the S/MIME environment SHOULD NOT contain any 359 critical extensions (extensions that have the critical field set to 360 TRUE) other than those listed here. These extensions SHOULD be marked 361 as non-critical unless the proper handling of the extension is deemed 362 critical to the correct interpretation of the associated certificate. 363 Other extensions may be included, but those extensions SHOULD NOT be 364 marked as critical. 366 Interpretation and syntax for all extensions MUST follow [KEYM], 367 unless otherwise specified here. 369 4.4.1 Basic Constraints Certificate Extension 371 The basic constraints extension serves to delimit the role and 372 position of an issuing authority or end-entity certificate plays in a 373 chain of certificates. 375 For example, certificates issued to CAs and subordinate CAs contain a 376 basic constraint extension that identifies them as issuing authority 377 certificates. End-entity certificates contain an extension that 378 constrains the certificate from being an issuing authority 379 certificate. 381 Certificates SHOULD contain a basicConstraints extension in CA 382 certificates, and SHOULD NOT contain that extension in end entity 383 certificates. 385 4.4.2 Key Usage Certificate Extension 387 The key usage extension serves to limit the technical purposes for 388 which a public key listed in a valid certificate may be used. Issuing 389 authority certificates may contain a key usage extension that 390 restricts the key to signing certificates, certificate revocation 391 lists and other data. 393 For example, a certification authority may create subordinate issuer 394 certificates which contain a keyUsage extension which specifies that 395 the corresponding public key can be used to sign end user certs and 396 sign CRLs. 398 If a key usage extension is included in a PKIX certificate, then it 399 MUST be marked as critical. 401 4.4.2.1 Key Usage in Diffie-Hellman Key Exchange Certificates 403 For Diffie-Hellman key exchange certificates (certificates in which 404 the subject public key algorithm is dhpublicnumber), if the keyUsage 405 keyAgreement bit is set to 1 AND if the public key is to be used to 406 form a pairwise key to decrypt data, then the S/MIME agent MUST only 407 use the public key if the keyUsage encipherOnly bit is set to 0. If 408 the keyUsage keyAgreement bit is set to 1 AND if the key is to be used 409 to form a pairwise key to encrypt data, then the S/MIME agent MUST 410 only use the public key if the keyUsage decipherOnly bit is set to 0. 412 4.4.3 Subject Alternative Name Extension 414 The subject alternative name extension is used in S/MIME as the 415 preferred means to convey the RFC-2822 email address(es) that 416 correspond to the entity for this certificate. Any RFC-2822 email 417 addresses present MUST be encoded using the rfc822Name CHOICE of the 418 GeneralName type. Since the SubjectAltName type is a SEQUENCE OF 419 GeneralName, multiple RFC-2822 email addresses MAY be present. 421 5. Security Considerations 423 All of the security issues faced by any cryptographic application must 424 be faced by a S/MIME agent. Among these issues are protecting the 425 user's private key, preventing various attacks, and helping the user 426 avoid mistakes such as inadvertently encrypting a message for the 427 wrong recipient. The entire list of security considerations is beyond 428 the scope of this document, but some significant concerns are listed 429 here. 431 When processing certificates, there are many situations where the 432 processing might fail. Because the processing may be done by a user 433 agent, a security gateway, or other program, there is no single way to 434 handle such failures. Just because the methods to handle the failures 435 has not been listed, however, the reader should not assume that they 436 are not important. The opposite is true: if a certificate is not 437 provably valid and associated with the message, the processing 438 software should take immediate and noticable steps to inform the end 439 user about it. 441 Some of the many places where signature and certificate checking might 442 fail include: 444 - no Internet mail addresses in a certificate match the sender of a 445 message 446 - no certificate chain leads to a trusted CA 447 - no ability to check the CRL for a certificate 448 - an invalid CRL was received 449 - the CRL being checked is expired 450 - the certificate is expired 451 - the certificate has been revoked 453 There are certainly other instances where a certificate may be 454 invalid, and it is the responsibility of the processing software to 455 check them all thoroughly, and to decide what to do if the check 456 fails. 458 [TBD] -- PKCS #1 v1.5 warnings? 460 A. References 462 [CERTV2] "S/MIME Version 2 Certificate Handling", RFC 2312 464 [CMS] "Cryptographic Message Syntax", Internet Draft draft-housley- 465 smime-cms 467 [CMSALG] "Cryptographic Message Syntax (CMS) Algorithms", Internet- 468 Draft draft-ietf-smime-cmsalg 470 [KEYM] "Internet X.509 Public Key Infrastructure Certificate and CRL 471 Profile", RFC 2459 473 [MUSTSHOULD] "Key words for use in RFCs to Indicate Requirement 474 Levels", RFC 2119 476 [RFC-2822], "Internet Message Format", RFC 2822 478 [SMIME-MSG] "S/MIME Version 3 Message Specification ", Internet Draft 479 draft-ietf-smime-msg 481 [X.500] ITU-T Recommendation X.500 (1997) | ISO/IEC 9594-1:1997, 482 Information technology - Open Systems Interconnection - The Directory: 483 Overview of concepts, models and services 485 [X.501] ITU-T Recommendation X.501 (1997) | ISO/IEC 9594-2:1997, 486 Information technology - Open Systems Interconnection - The Directory: 487 Models 489 [X.509] ITU-T Recommendation X.509 (1997) | ISO/IEC 9594-8:1997, 490 Information technology - Open Systems Interconnection - The Directory: 491 Authentication framework 493 [X.520] ITU-T Recommendation X.520 (1997) | ISO/IEC 9594-6:1997, 494 Information technology - Open Systems Interconnection - The Directory: 495 Selected attribute types. 497 B. Acknowledgements 499 [tbd] 501 C. Changes from last draft 503 Edited as plain text due to unfortunate loss of original Word 504 document, so wrapping may change randomly, but strange 8bit characters 505 should no longer appear mysteriously (Blake Ramsdell) 507 D. Editor's address 509 Blake Ramsdell 510 Brute Squad Labs 511 Suite 217-C 512 16451 Redmond Way 513 Redmond, WA 98052-4482 515 blake@brutesquadlabs.com