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'FIPS186-3' == Outdated reference: draft-ietf-lamps-eai-addresses has been published as RFC 8398 == Outdated reference: draft-ietf-lamps-rfc5751-bis has been published as RFC 8551 ** Downref: Normative reference to an Informational RFC: RFC 2985 ** Obsolete normative reference: RFC 3447 (Obsoleted by RFC 8017) -- Duplicate reference: RFC5035, mentioned in 'RFC5652', was also mentioned in 'RFC5035'. ** Obsolete normative reference: RFC 5750 (Obsoleted by RFC 8550) ** Obsolete normative reference: RFC 5751 (Obsoleted by RFC 8551) ** Downref: Normative reference to an Informational RFC: RFC 6979 == Outdated reference: draft-ietf-curdle-pkix has been published as RFC 8410 -- Obsolete informational reference (is this intentional?): RFC 2313 (Obsoleted by RFC 2437) -- Duplicate reference: RFC2315, mentioned in 'RFC2315', was also mentioned in 'RFC2314'. -- Obsolete informational reference (is this intentional?): RFC 2630 (Obsoleted by RFC 3369, RFC 3370) -- Obsolete informational reference (is this intentional?): RFC 2632 (Obsoleted by RFC 3850) -- Duplicate reference: RFC5035, mentioned in 'RFC2633', was also mentioned in 'RFC5652'. -- Obsolete informational reference (is this intentional?): RFC 3850 (Obsoleted by RFC 5750) -- Obsolete informational reference (is this intentional?): RFC 3851 (Obsoleted by RFC 5751) -- Duplicate reference: RFC5035, mentioned in 'RFC3852', was also mentioned in 'RFC2633'. Summary: 6 errors (**), 0 flaws (~~), 9 warnings (==), 12 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 LAMPS J. Schaad 3 Internet-Draft August Cellars 4 Obsoletes: 5750 (if approved) B. Ramsdell 5 Intended status: Standards Track Brute Squad Labs, Inc. 6 Expires: December 23, 2018 S. Turner 7 sn3rd 8 June 21, 2018 10 Secure/Multipurpose Internet Mail Extensions (S/ MIME) Version 4.0 11 Certificate Handling 12 draft-ietf-lamps-rfc5750-bis-07 14 Abstract 16 This document specifies conventions for X.509 certificate usage by 17 Secure/Multipurpose Internet Mail Extensions (S/MIME) v4.0 agents. 18 S/MIME provides a method to send and receive secure MIME messages, 19 and certificates are an integral part of S/MIME agent processing. 20 S/MIME agents validate certificates as described in RFC 5280, the 21 Internet X.509 Public Key Infrastructure Certificate and CRL Profile. 22 S/MIME agents must meet the certificate processing requirements in 23 this document as well as those in RFC 5280. This document obsoletes 24 RFC 5750. 26 Contributing to this document 28 The source for this draft is being maintained in GitHub. Suggested 29 changes should be submitted as pull requests at . Instructions are on that page as well. Editorial 31 changes can be managed in GitHub, but any substantial issues need to 32 be discussed on the LAMPS mailing list. 34 Status of This Memo 36 This Internet-Draft is submitted in full conformance with the 37 provisions of BCP 78 and BCP 79. 39 Internet-Drafts are working documents of the Internet Engineering 40 Task Force (IETF). Note that other groups may also distribute 41 working documents as Internet-Drafts. The list of current Internet- 42 Drafts is at https://datatracker.ietf.org/drafts/current/. 44 Internet-Drafts are draft documents valid for a maximum of six months 45 and may be updated, replaced, or obsoleted by other documents at any 46 time. It is inappropriate to use Internet-Drafts as reference 47 material or to cite them other than as "work in progress." 48 This Internet-Draft will expire on December 23, 2018. 50 Copyright Notice 52 Copyright (c) 2018 IETF Trust and the persons identified as the 53 document authors. All rights reserved. 55 This document is subject to BCP 78 and the IETF Trust's Legal 56 Provisions Relating to IETF Documents 57 (https://trustee.ietf.org/license-info) in effect on the date of 58 publication of this document. Please review these documents 59 carefully, as they describe your rights and restrictions with respect 60 to this document. Code Components extracted from this document must 61 include Simplified BSD License text as described in Section 4.e of 62 the Trust Legal Provisions and are provided without warranty as 63 described in the Simplified BSD License. 65 This document may contain material from IETF Documents or IETF 66 Contributions published or made publicly available before November 67 10, 2008. The person(s) controlling the copyright in some of this 68 material may not have granted the IETF Trust the right to allow 69 modifications of such material outside the IETF Standards Process. 70 Without obtaining an adequate license from the person(s) controlling 71 the copyright in such materials, this document may not be modified 72 outside the IETF Standards Process, and derivative works of it may 73 not be created outside the IETF Standards Process, except to format 74 it for publication as an RFC or to translate it into languages other 75 than English. 77 Table of Contents 79 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 80 1.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 3 81 1.2. Conventions Used in This Document . . . . . . . . . . . . 4 82 1.3. Compatibility with Prior Practice S/MIME . . . . . . . . 5 83 1.4. Changes from S/MIME v3 to S/MIME v3.1 . . . . . . . . . . 5 84 1.5. Changes from S/MIME v3.1 to S/MIME v3.2 . . . . . . . . . 6 85 1.6. Changes since S/MIME 3.2 . . . . . . . . . . . . . . . . 7 86 2. CMS Options . . . . . . . . . . . . . . . . . . . . . . . . . 7 87 2.1. Certificate Revocation Lists . . . . . . . . . . . . . . 7 88 2.2. Certificate Choices . . . . . . . . . . . . . . . . . . . 8 89 2.2.1. Historical Note about CMS Certificates . . . . . . . 8 90 2.3. CertificateSet . . . . . . . . . . . . . . . . . . . . . 8 91 3. Using Distinguished Names for Internet Mail . . . . . . . . . 9 92 4. Certificate Processing . . . . . . . . . . . . . . . . . . . 11 93 4.1. Certificate Revocation Lists . . . . . . . . . . . . . . 11 94 4.2. Certificate Path Validation . . . . . . . . . . . . . . . 12 95 4.3. Certificate and CRL Signing Algorithms and Key Sizes . . 13 96 4.4. PKIX Certificate Extensions . . . . . . . . . . . . . . . 14 97 4.4.1. Basic Constraints . . . . . . . . . . . . . . . . . . 14 98 4.4.2. Key Usage Certificate Extension . . . . . . . . . . . 15 99 4.4.3. Subject Alternative Name . . . . . . . . . . . . . . 15 100 4.4.4. Extended Key Usage Extension . . . . . . . . . . . . 16 101 5. IANA Considertions . . . . . . . . . . . . . . . . . . . . . 16 102 6. Security Considerations . . . . . . . . . . . . . . . . . . . 16 103 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 18 104 7.1. Normative References . . . . . . . . . . . . . . . . . . 18 105 7.2. Informational References . . . . . . . . . . . . . . . . 21 106 Appendix A. Historic Considerations . . . . . . . . . . . . . . 24 107 A.1. Signature Algorithms and Key Sizes . . . . . . . . . . . 24 108 Appendix B. Moving S/MIME v2 Certificate Handling to Historic 109 Status . . . . . . . . . . . . . . . . . . . . . . . 25 110 Appendix C. Acknowledgments . . . . . . . . . . . . . . . . . . 25 111 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26 113 1. Introduction 115 S/MIME (Secure/Multipurpose Internet Mail Extensions) v4.0, described 116 in [I-D.ietf-lamps-rfc5751-bis], provides a method to send and 117 receive secure MIME messages. Before using a public key to provide 118 security services, the S/MIME agent MUST verify that the public key 119 is valid. S/MIME agents MUST use PKIX certificates to validate 120 public keys as described in the Internet X.509 Public Key 121 Infrastructure (PKIX) Certificate and CRL Profile [RFC5280]. S/MIME 122 agents MUST meet the certificate processing requirements documented 123 in this document in addition to those stated in [RFC5280]. 125 This specification is compatible with the Cryptographic Message 126 Syntax (CMS) RFC 5652 [RFC5652] in that it uses the data types 127 defined by CMS. It also inherits all the varieties of architectures 128 for certificate-based key management supported by CMS. 130 This document obsoletes [RFC5750]. The most significant changes 131 revolve around changes in recommendations around the cryptographic 132 algorithms used by the specification. More details can be found in 133 Section 1.6. 135 1.1. Definitions 137 For the purposes of this document, the following definitions apply. 139 ASN.1: Abstract Syntax Notation One, as defined in ITU-T X.680 140 [X.680]. 142 Attribute certificate (AC): An X.509 AC is a separate structure from 143 a subject's public key X.509 certificate. A subject may have 144 multiple X.509 ACs associated with each of its public key X.509 145 certificates. Each X.509 AC binds one or more attributes with one of 146 the subject's public key X.509 certificates. The X.509 AC syntax is 147 defined in [RFC5755]. 149 Certificate: A type that binds an entity's name to a public key with 150 a digital signature. This type is defined in the Internet X.509 151 Public Key Infrastructure (PKIX) Certificate and CRL Profile 152 [RFC5280]. This type also contains the distinguished name of the 153 certificate issuer (the signer), an issuer-specific serial number, 154 the issuer's signature algorithm identifier, a validity period, and 155 extensions also defined in that document. 157 Certificate Revocation List (CRL): A type that contains information 158 about certificates whose validity an issuer has prematurely revoked. 159 The information consists of an issuer name, the time of issue, the 160 next scheduled time of issue, a list of certificate serial numbers 161 and their associated revocation times, and extensions as defined in 162 [RFC5280]. The CRL is signed by the issuer. The type intended by 163 this specification is the one defined in [RFC5280]. 165 Receiving agent: Software that interprets and processes S/MIME CMS 166 objects, MIME body parts that contain CMS objects, or both. 168 Sending agent: Software that creates S/MIME CMS objects, MIME body 169 parts that contain CMS objects, or both. 171 S/MIME agent: User software that is a receiving agent, a sending 172 agent, or both. 174 1.2. Conventions Used in This Document 176 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 177 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 178 "OPTIONAL" in this document are to be interpreted as described in BCP 179 14 [RFC2119] [RFC8174] when, and only when, they appear in all 180 capitals, as shown here. 182 We define the additional requirement levels: 184 SHOULD+ This term means the same as SHOULD. However, the authors 185 expect that a requirement marked as SHOULD+ will be promoted 186 at some future time to be a MUST. 188 SHOULD- This term means the same as SHOULD. However, the authors 189 expect that a requirement marked as SHOULD- will be demoted 190 to a MAY in a future version of this document. 192 MUST- This term means the same as MUST. However, the authors 193 expect that this requirement will no longer be a MUST in a 194 future document. Although its status will be determined at a 195 later time, it is reasonable to expect that if a future 196 revision of a document alters the status of a MUST- 197 requirement, it will remain at least a SHOULD or a SHOULD-. 199 The term RSA in this document almost always refers to the PKCS#1 v1.5 200 RSA signature algorithm even when not qualified as such. There are a 201 couple of places where it refers to the general RSA cryptographic 202 operation; these can be determined from the context where it is used. 204 1.3. Compatibility with Prior Practice S/MIME 206 S/MIME version 4.0 agents ought to attempt to have the greatest 207 interoperability possible with agents for prior versions of S/MIME. 209 S/MIME version 2 is described in RFC 2311 through RFC 2315 inclusive 210 [SMIMEv2], S/MIME version 3 is described in RFC 2630 through RFC 2634 211 inclusive and RFC 5035 [SMIMEv3], and S/MIME version 3.1 is described 212 in RFC 3850, RFC 3851, RFC 3852, RFC 2634, and RFC 5035 [SMIMEv3.1]. 213 RFC 2311 also has historical information about the development of 214 S/MIME. 216 Appendix A contains information about algorithms that were used for 217 prior versions of S/MIME but are no longer considered to meet modern 218 security standards. Support of these algorithms may be needed to 219 support historic S/MIME artifacts such as messages or files, but 220 SHOULD NOT be used for new artifacts. 222 1.4. Changes from S/MIME v3 to S/MIME v3.1 224 This section reflects the changes that were made when S/MIME v3.1 was 225 released. The RFC2119 langauage may have superceeded in later 226 versions. 228 Version 1 and version 2 CRLs MUST be supported. 230 Multiple certification authority (CA) certificates with the same 231 subject and public key, but with overlapping validity periods, MUST 232 be supported. 234 Version 2 attribute certificates SHOULD be supported, and version 1 235 attributes certificates MUST NOT be used. 237 The use of the MD2 digest algorithm for certificate signatures is 238 discouraged, and security language was added. 240 Clarified use of email address use in certificates. Certificates 241 that do not contain an email address have no requirements for 242 verifying the email address associated with the certificate. 244 Receiving agents SHOULD display certificate information when 245 displaying the results of signature verification. 247 Receiving agents MUST NOT accept a signature made with a certificate 248 that does not have at least one of the the digitalSignature or 249 nonRepudiation bits set. 251 Clarifications for the interpretation of the key usage and extended 252 key usage extensions. 254 1.5. Changes from S/MIME v3.1 to S/MIME v3.2 256 This section reflects the changes that were made when S/MIME v3.2 was 257 released. The RFC2119 langauage may have superceeded in later 258 versions. 260 Conventions Used in This Document: Moved to Section 1.2. Added 261 definitions for SHOULD+, SHOULD-, and MUST-. 263 Section 1.1: Updated ASN.1 definition and reference. 265 Section 1.3: Added text about v3.1 RFCs. 267 Section 3: Aligned email address text with RFC 5280. Updated note 268 to indicate emailAddress IA5String upper bound is 255 269 characters. Added text about matching email addresses. 271 Section 4.2: Added text to indicate how S/MIME agents locate the 272 correct user certificate. 274 Section 4.3: RSA with SHA-256 (PKCS #1 v1.5) added as MUST; DSA with 275 SHA-256 added as SHOULD+; RSA with SHA-1, DSA with SHA-1, 276 and RSA with MD5 changed to SHOULD-; and RSASSA-PSS with 277 SHA-256 added as SHOULD+. Updated key sizes and changed 278 pointer to PKIX RFCs. 280 Section 4.4.1: Aligned with PKIX on use of basic constraints 281 extension in CA certificates. Clarified which extension 282 is used to constrain end entities from using their keys 283 to perform issuing authority operations. 285 Section 5: Updated security considerations. 287 Section 7: Moved references from Appendix B to Section 6. Updated 288 the references. 290 Appendix A: Moved Appendix A to Appendix B. Added Appendix A to move 291 S/MIME v2 Certificate Handling to Historic Status. 293 1.6. Changes since S/MIME 3.2 295 This section reflects the changes that were made when S/MIME v4.0 was 296 released. The RFC2119 langauage may have superceeded in later 297 versions. 299 Section 3: Require support for internationalized email addresses. 301 Section 4.3: Mandated support for ECDSA with P-256 and Ed25519. 302 Moved algorithms with SHA-1 and MD5 to historical status. 303 Moved DSA support to historical status. Increased lower 304 bounds on RSA key sizes. 306 Appendix A: Add a new appendix for algorithms that are now considered 307 to be historical. 309 2. CMS Options 311 The CMS message format allows for a wide variety of options in 312 content and algorithm support. This section puts forth a number of 313 support requirements and recommendations in order to achieve a base 314 level of interoperability among all S/MIME implementations. Most of 315 the CMS format for S/MIME messages is defined in 316 [I-D.ietf-lamps-rfc5751-bis]. 318 2.1. Certificate Revocation Lists 320 Receiving agents MUST support the Certificate Revocation List (CRL) 321 format defined in [RFC5280]. If sending agents include CRLs in 322 outgoing messages, the CRL format defined in [RFC5280] MUST be used. 323 Receiving agents MUST support both v1 and v2 CRLs. 325 All agents MUST be capable of performing revocation checks using CRLs 326 as specified in [RFC5280]. All agents MUST perform revocation status 327 checking in accordance with [RFC5280]. Receiving agents MUST 328 recognize CRLs in received S/MIME messages. 330 Agents SHOULD store CRLs received in messages for use in processing 331 later messages. 333 2.2. Certificate Choices 335 Receiving agents MUST support v1 X.509 and v3 X.509 certificates as 336 profiled in [RFC5280]. End-entity certificates MAY include an 337 Internet mail address, as described in Section 3. 339 Receiving agents SHOULD support X.509 version 2 attribute 340 certificates. See [RFC5755] for details about the profile for 341 attribute certificates. 343 2.2.1. Historical Note about CMS Certificates 345 The CMS message format supports a choice of certificate formats for 346 public key content types: PKIX, PKCS #6 extended certificates 347 [PKCS6], and PKIX attribute certificates. 349 The PKCS #6 format is not in widespread use. In addition, PKIX 350 certificate extensions address much of the same functionality and 351 flexibility as was intended in the PKCS #6. Thus, sending and 352 receiving agents MUST NOT use PKCS #6 extended certificates. 353 Receiving agents MUST be able to parse and process a message 354 containing PKCS #6 extended certificates although ignoring those 355 certificates is expected behavior. 357 X.509 version 1 attribute certificates are also not widely 358 implemented, and have been superseded with version 2 attribute 359 certificates. Sending agents MUST NOT send version 1 attribute 360 certificates. 362 2.3. CertificateSet 364 Receiving agents MUST be able to handle an arbitrary number of 365 certificates of arbitrary relationship to the message sender and to 366 each other in arbitrary order. In many cases, the certificates 367 included in a signed message may represent a chain of certification 368 from the sender to a particular root. There may be, however, 369 situations where the certificates in a signed message may be 370 unrelated and included for convenience. 372 Sending agents SHOULD include any certificates for the user's public 373 key(s) and associated issuer certificates. This increases the 374 likelihood that the intended recipient can establish trust in the 375 originator's public key(s). This is especially important when 376 sending a message to recipients that may not have access to the 377 sender's public key through any other means or when sending a signed 378 message to a new recipient. The inclusion of certificates in 379 outgoing messages can be omitted if S/MIME objects are sent within a 380 group of correspondents that has established access to each other's 381 certificates by some other means such as a shared directory or manual 382 certificate distribution. Receiving S/MIME agents SHOULD be able to 383 handle messages without certificates by using a database or directory 384 lookup scheme to find them. 386 A sending agent SHOULD include at least one chain of certificates up 387 to, but not including, a certification authority (CA) that it 388 believes that the recipient may trust as authoritative. A receiving 389 agent MUST be able to handle an arbitrarily large number of 390 certificates and chains. 392 Agents MAY send CA certificates, that is, cross-certificates, self- 393 issued certificates, and self-signed certificates. Note that 394 receiving agents SHOULD NOT simply trust any self-signed certificates 395 as valid CAs, but SHOULD use some other mechanism to determine if 396 this is a CA that should be trusted. Also note that when 397 certificates contain Digital Signature Algorithm (DSA) public keys 398 the parameters may be located in the root certificate. This would 399 require that the recipient possess both the end-entity certificate 400 and the root certificate to perform a signature verification, and is 401 a valid example of a case where transmitting the root certificate may 402 be required. 404 Receiving agents MUST support chaining based on the distinguished 405 name fields. Other methods of building certificate chains MAY be 406 supported. 408 Receiving agents SHOULD support the decoding of X.509 attribute 409 certificates included in CMS objects. All other issues regarding the 410 generation and use of X.509 attribute certificates are outside of the 411 scope of this specification. One specification that addresses 412 attribute certificate use is defined in [RFC3114]. 414 3. Using Distinguished Names for Internet Mail 416 End-entity certificates MAY contain an Internet mail address. Email 417 addresses restricted to 7-bit ASCII characters use the pkcs-9-at- 418 emailAddress OID (see below) and are encoded as described in 419 Section 4.2.1.6 of [RFC5280]. Internationalized Email address names 420 use the OID defined in [I-D.ietf-lamps-eai-addresses] and are encoded 421 as described there. The email address SHOULD be in the 422 subjectAltName extension, and SHOULD NOT be in the subject 423 distinguished name. 425 Receiving agents MUST recognize and accept certificates that contain 426 no email address. Agents are allowed to provide an alternative 427 mechanism for associating an email address with a certificate that 428 does not contain an email address, such as through the use of the 429 agent's address book, if available. Receiving agents MUST recognize 430 both ASCII and internationalized email addresses in the 431 subjectAltName field. Receiving agents MUST recognize email 432 addresses in the Distinguished Name field in the PKCS #9 [RFC2985] 433 emailAddress attribute: 435 pkcs-9-at-emailAddress OBJECT IDENTIFIER ::= 436 { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 1 } 438 Note that this attribute MUST be encoded as IA5String and has an 439 upper bound of 255 characters. The right side of the email address 440 SHOULD be treated as ASCII-case-insensitive. 442 Comparing of email addresses is fraught with peril. 443 [I-D.ietf-lamps-eai-addresses] defines the procedure for doing 444 comparison of Internationalized email addresses. For ASCII email 445 addresses the domain component (right-hand side of the '@') MUST be 446 compared using a case-insensitive function. The local name component 447 (left-hand side of the '@') SHOULD be compared using a case- 448 insensitive function. Some localities may perform other 449 transformations on the local name component before doing the 450 comparison, however an S/MIME client cannot know what specific 451 localities do. 453 Sending agents SHOULD make the address in the From or Sender header 454 in a mail message match an Internet mail address in the signer's 455 certificate. Receiving agents MUST check that the address in the 456 From or Sender header of a mail message matches an Internet mail 457 address in the signer's certificate, if mail addresses are present in 458 the certificate. A receiving agent SHOULD provide some explicit 459 alternate processing of the message if this comparison fails; this 460 might be done by displaying or logging a message that shows the 461 recipient the mail addresses in the certificate or other certificate 462 details. 464 A receiving agent SHOULD display a subject name or other certificate 465 details when displaying an indication of successful or unsuccessful 466 signature verification. 468 All subject and issuer names MUST be populated (i.e., not an empty 469 SEQUENCE) in S/MIME-compliant X.509 certificates, except that the 470 subject distinguished name (DN) in a user's (i.e., end-entity) 471 certificate MAY be an empty SEQUENCE in which case the subjectAltName 472 extension will include the subject's identifier and MUST be marked as 473 critical. 475 4. Certificate Processing 477 S/MIME agents need to provide some certificate retrieval mechanism in 478 order to gain access to certificates for recipients of digital 479 envelopes. There are many ways to implement certificate retrieval 480 mechanisms. [X.500] directory service is an excellent example of a 481 certificate retrieval-only mechanism that is compatible with classic 482 X.500 Distinguished Names. The IETF has published [RFC8162] which 483 describes an experimental protocol to retrieve certificates from the 484 Domain Name System (DNS). Until such mechanisms are widely used, 485 their utility may be limited by the small number of the 486 correspondent's certificates that can be retrieved. At a minimum, 487 for initial S/MIME deployment, a user agent could automatically 488 generate a message to an intended recipient requesting the 489 recipient's certificate in a signed return message. 491 Receiving and sending agents SHOULD also provide a mechanism to allow 492 a user to "store and protect" certificates for correspondents in such 493 a way so as to guarantee their later retrieval. In many 494 environments, it may be desirable to link the certificate retrieval/ 495 storage mechanisms together in some sort of certificate database. In 496 its simplest form, a certificate database would be local to a 497 particular user and would function in a similar way as an "address 498 book" that stores a user's frequent correspondents. In this way, the 499 certificate retrieval mechanism would be limited to the certificates 500 that a user has stored (presumably from incoming messages). A 501 comprehensive certificate retrieval/storage solution might combine 502 two or more mechanisms to allow the greatest flexibility and utility 503 to the user. For instance, a secure Internet mail agent might resort 504 to checking a centralized certificate retrieval mechanism for a 505 certificate if it cannot be found in a user's local certificate 506 storage/retrieval database. 508 Receiving and sending agents SHOULD provide a mechanism for the 509 import and export of certificates, using a CMS certs-only message. 510 This allows for import and export of full certificate chains as 511 opposed to just a single certificate. This is described in 512 [RFC5751]. 514 Agents MUST handle multiple valid certification authority (CA) 515 certificates containing the same subject name and the same public 516 keys but with overlapping validity intervals. 518 4.1. Certificate Revocation Lists 520 In general, it is always better to get the latest CRL information 521 from a CA than to get information stored in an incoming messages. A 522 receiving agent SHOULD have access to some CRL retrieval mechanism in 523 order to gain access to certificate revocation information when 524 validating certification paths. A receiving or sending agent SHOULD 525 also provide a mechanism to allow a user to store incoming 526 certificate revocation information for correspondents in such a way 527 so as to guarantee its later retrieval. 529 Receiving and sending agents SHOULD retrieve and utilize CRL 530 information every time a certificate is verified as part of a 531 certification path validation even if the certificate was already 532 verified in the past. However, in many instances (such as off-line 533 verification) access to the latest CRL information may be difficult 534 or impossible. The use of CRL information, therefore, may be 535 dictated by the value of the information that is protected. The 536 value of the CRL information in a particular context is beyond the 537 scope of this specification but may be governed by the policies 538 associated with particular certification paths. 540 All agents MUST be capable of performing revocation checks using CRLs 541 as specified in [RFC5280]. All agents MUST perform revocation status 542 checking in accordance with [RFC5280]. Receiving agents MUST 543 recognize CRLs in received S/MIME messages. 545 4.2. Certificate Path Validation 547 In creating a user agent for secure messaging, certificate, CRL, and 548 certification path validation should be highly automated while still 549 acting in the best interests of the user. Certificate, CRL, and path 550 validation MUST be performed as per [RFC5280] when validating a 551 correspondent's public key. This is necessary before using a public 552 key to provide security services such as verifying a signature, 553 encrypting a content-encryption key (e.g., RSA), or forming a 554 pairwise symmetric key (e.g., Diffie-Hellman) to be used to encrypt 555 or decrypt a content-encryption key. 557 Certificates and CRLs are made available to the path validation 558 procedure in two ways: a) incoming messages, and b) certificate and 559 CRL retrieval mechanisms. Certificates and CRLs in incoming messages 560 are not required to be in any particular order nor are they required 561 to be in any way related to the sender or recipient of the message 562 (although in most cases they will be related to the sender). 563 Incoming certificates and CRLs SHOULD be cached for use in path 564 validation and optionally stored for later use. This temporary 565 certificate and CRL cache SHOULD be used to augment any other 566 certificate and CRL retrieval mechanisms for path validation on 567 incoming signed messages. 569 When verifying a signature and the certificates that are included in 570 the message, if a signingCertificate attribute from RFC 2634 [ESS] or 571 a signingCertificateV2 attribute from RFC 5035 [ESS] is found in an 572 S/MIME message, it SHALL be used to identify the signer's 573 certificate. Otherwise, the certificate is identified in an S/MIME 574 message, either using the issuerAndSerialNumber, which identifies the 575 signer's certificate by the issuer's distinguished name and the 576 certificate serial number, or the subjectKeyIdentifier, which 577 identifies the signer's certificate by a key identifier. 579 When decrypting an encrypted message, if a 580 SMIMEEncryptionKeyPreference attribute is found in an encapsulating 581 SignedData, it SHALL be used to identify the originator's certificate 582 found in OriginatorInfo. See [RFC5652] for the CMS fields that 583 reference the originator's and recipient's certificates. 585 4.3. Certificate and CRL Signing Algorithms and Key Sizes 587 Certificates and Certificate Revocation Lists (CRLs) are signed by 588 the certificate issuer. Receiving agents: 590 - MUST support ECDSA with curve P-256 with SHA-256. 592 - MUST support EdDSA with curve 25519 using PureEdDSA mode. 594 - MUST- support RSA PKCS#1 v1.5 with SHA-256. 596 - SHOULD support RSASSA-PSS with SHA-256. 598 Implementations SHOULD use deterministic generation for the parameter 599 'k' for ECDSA as outlined in [RFC6979]. EdDSA is defined to generate 600 this parameter deterministically. 602 The following are the RSA and RSASSA-PSS key size requirements for 603 S/MIME receiving agents during certificate and CRL signature 604 verification: 606 key size <= 2047 : SHOULD NOT (see Historic Considerations) 607 2048 <= key size <= 4096 : MUST (see Security Considerations) 608 4096 < key size : MAY (see Security Considerations) 610 The signature algorithm object identifiers for RSA PKCS#1 v1.5 and 611 RSASSA-PSS with SHA-256 using 1024-bit through 3072-bit public keys 612 are specified in [RFC4055] and the signature algorithm definition is 613 found in [FIPS186-2] with Change Notice 1. 615 The signature algorithm object identifiers for RSA PKCS#1 v1.5 and 616 RSASSA-PSS with SHA-256 using 4096-bit public keys are specified in 617 [RFC4055] and the signature algorithm definition is found in 618 [RFC3447]. 620 For RSASSA-PSS with SHA-256 see [RFC4056]. 622 For ECDSA see [RFC5758] and [RFC6090]. The first reference provides 623 the signature algorithm's object identifier and the second provides 624 the signature algorithm's definition. Curves other than curve P-256 625 MAY be used as well. 627 For EdDSA see [I-D.ietf-curdle-pkix] and [RFC8032]. The first 628 reference provides the signature algorithm's object identifier and 629 the second provides the signature algorithm's definition. Other 630 curves than curve 25519 MAY be used as well. 632 4.4. PKIX Certificate Extensions 634 PKIX describes an extensible framework in which the basic certificate 635 information can be extended and describes how such extensions can be 636 used to control the process of issuing and validating certificates. 637 The LAMPS Working Group has ongoing efforts to identify and create 638 extensions that have value in particular certification environments. 639 Further, there are active efforts underway to issue PKIX certificates 640 for business purposes. This document identifies the minimum required 641 set of certificate extensions that have the greatest value in the 642 S/MIME environment. The syntax and semantics of all the identified 643 extensions are defined in [RFC5280]. 645 Sending and receiving agents MUST correctly handle the basic 646 constraints, key usage, authority key identifier, subject key 647 identifier, and subject alternative names certificate extensions when 648 they appear in end-entity and CA certificates. Some mechanism SHOULD 649 exist to gracefully handle other certificate extensions when they 650 appear in end-entity or CA certificates. 652 Certificates issued for the S/MIME environment SHOULD NOT contain any 653 critical extensions (extensions that have the critical field set to 654 TRUE) other than those listed here. These extensions SHOULD be 655 marked as non-critical unless the proper handling of the extension is 656 deemed critical to the correct interpretation of the associated 657 certificate. Other extensions may be included, but those extensions 658 SHOULD NOT be marked as critical. 660 Interpretation and syntax for all extensions MUST follow [RFC5280], 661 unless otherwise specified here. 663 4.4.1. Basic Constraints 665 The basic constraints extension serves to delimit the role and 666 position that an issuing authority or end-entity certificate plays in 667 a certification path. 669 For example, certificates issued to CAs and subordinate CAs contain a 670 basic constraints extension that identifies them as issuing authority 671 certificates. End-entity certificates contain the key usage 672 extension that restrains end-entities from using the key when 673 performing issuing authority operations (see Section 4.4.2). 675 As per [RFC5280], certificates MUST contain a basicConstraints 676 extension in CA certificates, and SHOULD NOT contain that extension 677 in end-entity certificates. 679 4.4.2. Key Usage Certificate Extension 681 The key usage extension serves to limit the technical purposes for 682 which a public key listed in a valid certificate may be used. 683 Issuing authority certificates may contain a key usage extension that 684 restricts the key to signing certificates, certificate revocation 685 lists, and other data. 687 For example, a certification authority may create subordinate issuer 688 certificates that contain a key usage extension that specifies that 689 the corresponding public key can be used to sign end user 690 certificates and sign CRLs. 692 If a key usage extension is included in a PKIX certificate, then it 693 MUST be marked as critical. 695 S/MIME receiving agents MUST NOT accept the signature of a message if 696 it was verified using a certificate that contains the key usage 697 extension without at least one of the digitalSignature or 698 nonRepudiation bits set. Sometimes S/MIME is used as a secure 699 message transport for applications beyond interpersonal messaging; in 700 such cases, the S/MIME-enabled application can specify additional 701 requirements concerning the digitalSignature or nonRepudiation bits 702 within this extension. 704 If the key usage extension is not specified, receiving clients MUST 705 presume that both the digitalSignature and nonRepudiation bits are 706 set. 708 4.4.3. Subject Alternative Name 710 The subject alternative name extension is used in S/MIME as the 711 preferred means to convey the email address(es) that correspond(s) to 712 the entity for this certificate. If the local portion of the email 713 address is ASCII, it MUST be encoded using the rfc822Name CHOICE of 714 the GeneralName type as described in [RFC5280], Section 4.2.1.6. If 715 the local portion of the email address is not ASCII, it MUST be 716 encoded using the otherName CHOICE of the GeneralName type as 717 described in [I-D.ietf-lamps-eai-addresses], Section 3. Since the 718 SubjectAltName type is a SEQUENCE OF GeneralName, multiple email 719 addresses MAY be present. 721 4.4.4. Extended Key Usage Extension 723 The extended key usage extension also serves to limit the technical 724 purposes for which a public key listed in a valid certificate may be 725 used. The set of technical purposes for the certificate therefore 726 are the intersection of the uses indicated in the key usage and 727 extended key usage extensions. 729 For example, if the certificate contains a key usage extension 730 indicating digital signature and an extended key usage extension that 731 includes the email protection OID, then the certificate may be used 732 for signing but not encrypting S/MIME messages. If the certificate 733 contains a key usage extension indicating digital signature but no 734 extended key usage extension, then the certificate may also be used 735 to sign but not encrypt S/MIME messages. 737 If the extended key usage extension is present in the certificate, 738 then interpersonal message S/MIME receiving agents MUST check that it 739 contains either the emailProtection or the anyExtendedKeyUsage OID as 740 defined in [RFC5280]. S/MIME uses other than interpersonal messaging 741 MAY require the explicit presence of the extended key usage extension 742 or other OIDs to be present in the extension or both. 744 5. IANA Considertions 746 This document has no new IANA considerations. 748 6. Security Considerations 750 All of the security issues faced by any cryptographic application 751 must be faced by a S/MIME agent. Among these issues are protecting 752 the user's private key, preventing various attacks, and helping the 753 user avoid mistakes such as inadvertently encrypting a message for 754 the wrong recipient. The entire list of security considerations is 755 beyond the scope of this document, but some significant concerns are 756 listed here. 758 When processing certificates, there are many situations where the 759 processing might fail. Because the processing may be done by a user 760 agent, a security gateway, or other program, there is no single way 761 to handle such failures. Just because the methods to handle the 762 failures have not been listed, however, the reader should not assume 763 that they are not important. The opposite is true: if a certificate 764 is not provably valid and associated with the message, the processing 765 software should take immediate and noticeable steps to inform the end 766 user about it. 768 Some of the many places where signature and certificate checking 769 might fail include: 771 - no Internet mail addresses in a certificate match the sender of a 772 message, if the certificate contains at least one mail address 774 - no certificate chain leads to a trusted CA 776 - no ability to check the CRL for a certificate 778 - an invalid CRL was received 780 - the CRL being checked is expired 782 - the certificate is expired 784 - the certificate has been revoked 786 There are certainly other instances where a certificate may be 787 invalid, and it is the responsibility of the processing software to 788 check them all thoroughly, and to decide what to do if the check 789 fails. 791 It is possible for there to be multiple unexpired CRLs for a CA. If 792 an agent is consulting CRLs for certificate validation, it SHOULD 793 make sure that the most recently issued CRL for that CA is consulted, 794 since an S/MIME message sender could deliberately include an older 795 unexpired CRL in an S/MIME message. This older CRL might not include 796 recently revoked certificates, which might lead an agent to accept a 797 certificate that has been revoked in a subsequent CRL. 799 When determining the time for a certificate validity check, agents 800 have to be careful to use a reliable time. In most cases the time 801 used SHOULD be the current time, some exceptions to this would be: 803 - The time the message was received is stored in a secure manner and 804 is used at a later time to validate the message. 806 - The time in a SigningTime attribute found in a counter signature 807 attribute which has been successfully validated. 809 The SigningTime attribute could be deliberately set to direct the 810 receiving agent to check a CRL that could have out-of-date revocation 811 status for a certificate, or cause an improper result when checking 812 the Validity field of a certificate. This could be done either by 813 the sender of the message, or an attacker which has compromised the 814 key of the sender. 816 In addition to the Security Considerations identified in [RFC5280], 817 caution should be taken when processing certificates that have not 818 first been validated to a trust anchor. Certificates could be 819 manufactured by untrusted sources for the purpose of mounting denial 820 of service or other attacks. For example, keys selected to require 821 excessive cryptographic processing, or extensive lists of CRL 822 Distribution Point (CDP) and/or Authority Information Access (AIA) 823 addresses in the certificate, could be used to mount denial-of- 824 service attacks. Similarly, attacker-specified CDP and/or AIA 825 addresses could be included in fake certificates to allow the 826 originator to detect receipt of the message even if signature 827 verification fails. 829 RSA keys of less than 2048 bits are now considered by many experts to 830 be cryptographically insecure (due to advances in computing power), 831 and SHOULD no longer be used to sign certificates or CRLs. Such keys 832 were previously considered secure, so processing previously received 833 signed and encrypted mail may require processing certificates or CRLs 834 signed with weak keys. Implementations that wish to support previous 835 versions of S/MIME or process old messages need to consider the 836 security risks that result from accepting certificates and CRLs with 837 smaller key sizes (e.g., spoofed certificates) versus the costs of 838 denial of service. If an implementation supports verification of 839 certificates or CRLs generated with RSA and DSA keys of less than 840 2048 bits, it MUST warn the user. Implementers should consider 841 providing a stronger warning for weak signatures on certificates and 842 CRLs associated with newly received messages than the one provided 843 for certificates and CRLs associated with previously stored messages. 844 Server implementations (e.g., secure mail list servers) where user 845 warnings are not appropriate SHOULD reject messages with weak 846 cryptography. 848 If an implementation is concerned about compliance with National 849 Institute of Standards and Technology (NIST) key size 850 recommendations, then see [SP800-57]. 852 7. References 854 7.1. Normative References 856 [FIPS186-2] 857 National Institute of Standards and Technology (NIST), 858 "Digital Signature Standard (DSS) [With Change Notice 1]", 859 Federal Information Processing Standards 860 Publication 186-2, January 2000. 862 [FIPS186-3] 863 National Institute of Standards and Technology (NIST), 864 "Digital Signature Standard (DSS)", Federal Information 865 Processing Standards Publication 186-3, June 2009. 867 [I-D.ietf-lamps-eai-addresses] 868 Melnikov, A. and W. Chuang, "Internationalized Email 869 Addresses in X.509 certificates", draft-ietf-lamps-eai- 870 addresses-18 (work in progress), March 2018. 872 [I-D.ietf-lamps-rfc5751-bis] 873 Schaad, J., Ramsdell, B., and S. Turner, "Secure/ 874 Multipurpose Internet Mail Extensions (S/MIME) Version 4.0 875 Message Specification", draft-ietf-lamps-rfc5751-bis-10 876 (work in progress), June 2018. 878 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 879 Requirement Levels", BCP 14, RFC 2119, 880 DOI 10.17487/RFC2119, March 1997, 881 . 883 [RFC2634] Hoffman, P., Ed., "Enhanced Security Services for S/MIME", 884 RFC 2634, DOI 10.17487/RFC2634, June 1999, 885 . 887 [RFC2985] Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object 888 Classes and Attribute Types Version 2.0", RFC 2985, 889 DOI 10.17487/RFC2985, November 2000, 890 . 892 [RFC3279] Bassham, L., Polk, W., and R. Housley, "Algorithms and 893 Identifiers for the Internet X.509 Public Key 894 Infrastructure Certificate and Certificate Revocation List 895 (CRL) Profile", RFC 3279, DOI 10.17487/RFC3279, April 896 2002, . 898 [RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography 899 Standards (PKCS) #1: RSA Cryptography Specifications 900 Version 2.1", RFC 3447, DOI 10.17487/RFC3447, February 901 2003, . 903 [RFC4055] Schaad, J., Kaliski, B., and R. Housley, "Additional 904 Algorithms and Identifiers for RSA Cryptography for use in 905 the Internet X.509 Public Key Infrastructure Certificate 906 and Certificate Revocation List (CRL) Profile", RFC 4055, 907 DOI 10.17487/RFC4055, June 2005, 908 . 910 [RFC4056] Schaad, J., "Use of the RSASSA-PSS Signature Algorithm in 911 Cryptographic Message Syntax (CMS)", RFC 4056, 912 DOI 10.17487/RFC4056, June 2005, 913 . 915 [RFC5035] Schaad, J., "Enhanced Security Services (ESS) Update: 916 Adding CertID Algorithm Agility", RFC 5035, 917 DOI 10.17487/RFC5035, August 2007, 918 . 920 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 921 Housley, R., and W. Polk, "Internet X.509 Public Key 922 Infrastructure Certificate and Certificate Revocation List 923 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, 924 . 926 [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, 927 RFC 5652, DOI 10.17487/RFC5652, September 2009, 928 . 930 [RFC5750] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 931 Mail Extensions (S/MIME) Version 3.2 Certificate 932 Handling", RFC 5750, DOI 10.17487/RFC5750, January 2010, 933 . 935 [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 936 Mail Extensions (S/MIME) Version 3.2 Message 937 Specification", RFC 5751, DOI 10.17487/RFC5751, January 938 2010, . 940 [RFC5755] Farrell, S., Housley, R., and S. Turner, "An Internet 941 Attribute Certificate Profile for Authorization", 942 RFC 5755, DOI 10.17487/RFC5755, January 2010, 943 . 945 [RFC5758] Dang, Q., Santesson, S., Moriarty, K., Brown, D., and T. 946 Polk, "Internet X.509 Public Key Infrastructure: 947 Additional Algorithms and Identifiers for DSA and ECDSA", 948 RFC 5758, DOI 10.17487/RFC5758, January 2010, 949 . 951 [RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature 952 Algorithm (DSA) and Elliptic Curve Digital Signature 953 Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August 954 2013, . 956 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 957 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 958 May 2017, . 960 [SMIMEv3.2] 961 "S/MIME version 3.2". 963 This group of documents represents S/MIME version 3.2. 964 This set of documents are [RFC2634], [RFC5750], [[This 965 Document]], [RFC5652], and [RFC5035]. 967 [SMIMEv4.0] 968 "S/MIME version 4.0". 970 This group of documents represents S/MIME version 4.0. 971 This set of documents are [RFC2634], 972 [I-D.ietf-lamps-rfc5751-bis], [[This Document]], 973 [RFC5652], and [RFC5035]. 975 [X.680] "Information Technology - Abstract Syntax Notation One 976 (ASN.1): Specification of basic notation. ITU-T 977 Recommendation X.680 (2002) | ISO/IEC 8824-1:2002.". 979 7.2. Informational References 981 [ESS] "Enhanced Security Services for S/ MIME". 983 This is the set of documents dealing with enhanced 984 security services and refers to [RFC2634] and [RFC5035]. 986 [I-D.ietf-curdle-pkix] 987 Josefsson, S. and J. Schaad, "Algorithm Identifiers for 988 Ed25519, Ed448, X25519 and X448 for use in the Internet 989 X.509 Public Key Infrastructure", draft-ietf-curdle- 990 pkix-10 (work in progress), May 2018. 992 [PKCS6] RSA Laboratories, "PKCS #6: Extended-Certificate Syntax 993 Standard", November 1993. 995 [RFC2311] Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L., and 996 L. Repka, "S/MIME Version 2 Message Specification", 997 RFC 2311, DOI 10.17487/RFC2311, March 1998, 998 . 1000 [RFC2312] Dusse, S., Hoffman, P., Ramsdell, B., and J. Weinstein, 1001 "S/MIME Version 2 Certificate Handling", RFC 2312, 1002 DOI 10.17487/RFC2312, March 1998, 1003 . 1005 [RFC2313] Kaliski, B., "PKCS #1: RSA Encryption Version 1.5", 1006 RFC 2313, DOI 10.17487/RFC2313, March 1998, 1007 . 1009 [RFC2314] Kaliski, B., "PKCS #10: Certification Request Syntax 1010 Version 1.5", RFC 2314, DOI 10.17487/RFC2314, March 1998, 1011 . 1013 [RFC2315] Kaliski, B., "PKCS #7: Cryptographic Message Syntax 1014 Version 1.5", RFC 2315, DOI 10.17487/RFC2315, March 1998, 1015 . 1017 [RFC2630] Housley, R., "Cryptographic Message Syntax", RFC 2630, 1018 DOI 10.17487/RFC2630, June 1999, 1019 . 1021 [RFC2631] Rescorla, E., "Diffie-Hellman Key Agreement Method", 1022 RFC 2631, DOI 10.17487/RFC2631, June 1999, 1023 . 1025 [RFC2632] Ramsdell, B., Ed., "S/MIME Version 3 Certificate 1026 Handling", RFC 2632, DOI 10.17487/RFC2632, June 1999, 1027 . 1029 [RFC2633] Ramsdell, B., Ed., "S/MIME Version 3 Message 1030 Specification", RFC 2633, DOI 10.17487/RFC2633, June 1999, 1031 . 1033 [RFC3114] Nicolls, W., "Implementing Company Classification Policy 1034 with the S/MIME Security Label", RFC 3114, 1035 DOI 10.17487/RFC3114, May 2002, 1036 . 1038 [RFC3850] Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail 1039 Extensions (S/MIME) Version 3.1 Certificate Handling", 1040 RFC 3850, DOI 10.17487/RFC3850, July 2004, 1041 . 1043 [RFC3851] Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail 1044 Extensions (S/MIME) Version 3.1 Message Specification", 1045 RFC 3851, DOI 10.17487/RFC3851, July 2004, 1046 . 1048 [RFC3852] Housley, R., "Cryptographic Message Syntax (CMS)", 1049 RFC 3852, DOI 10.17487/RFC3852, July 2004, 1050 . 1052 [RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic 1053 Curve Cryptography Algorithms", RFC 6090, 1054 DOI 10.17487/RFC6090, February 2011, 1055 . 1057 [RFC6151] Turner, S. and L. Chen, "Updated Security Considerations 1058 for the MD5 Message-Digest and the HMAC-MD5 Algorithms", 1059 RFC 6151, DOI 10.17487/RFC6151, March 2011, 1060 . 1062 [RFC6194] Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security 1063 Considerations for the SHA-0 and SHA-1 Message-Digest 1064 Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011, 1065 . 1067 [RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital 1068 Signature Algorithm (EdDSA)", RFC 8032, 1069 DOI 10.17487/RFC8032, January 2017, 1070 . 1072 [RFC8162] Hoffman, P. and J. Schlyter, "Using Secure DNS to 1073 Associate Certificates with Domain Names for S/MIME", 1074 RFC 8162, DOI 10.17487/RFC8162, May 2017, 1075 . 1077 [SMIMEv2] "S/MIME version v2". 1079 This group of documents represents S/MIME version 2. This 1080 set of documents are [RFC2311], [RFC2312], [RFC2313], 1081 [RFC2314], and [RFC2315]. 1083 [SMIMEv3] "S/MIME version 3". 1085 This group of documents represents S/MIME version 3. This 1086 set of documents are [RFC2630], [RFC2631], [RFC2632], 1087 [RFC2633], [RFC2634], and [RFC5035]. 1089 [SMIMEv3.1] 1090 "S/MIME version 3.1". 1092 This group of documents represents S/MIME version 3.1. 1093 This set of documents are [RFC2634], [RFC3850], [RFC3851], 1094 [RFC3852], and [RFC5035]. 1096 [SP800-57] 1097 National Institute of Standards and Technology (NIST), 1098 "Special Publication 800-57: Recommendation for Key 1099 Management", August 2005. 1101 [X.500] "ITU-T Recommendation X.500 (1997) | ISO/IEC 9594- 1:1997, 1102 Information technology - Open Systems Interconnection - 1103 The Directory: Overview of concepts, models and 1104 services.". 1106 Appendix A. Historic Considerations 1108 A.1. Signature Algorithms and Key Sizes 1110 There are a number of problems with validating certificates on 1111 sufficiently historic messages. For this reason it is strongly 1112 suggested that UAs treat these certificates differently from those on 1113 current messages. These problems include: 1115 - CAs are not required to keep certificates on a CRL beyond one 1116 update after a certificate has expired. This means that unless 1117 CRLs are cached as part of the message it is not always possible 1118 to check if a certificate has been revoked. The same problems 1119 exist with OCSP responses as they may be based on a CRL rather 1120 than on the certificate database. 1122 - RSA and DSA keys of less than 2048 bits are now considered by many 1123 experts to be cryptographically insecure (due to advances in 1124 computing power). Such keys were previously considered secure, so 1125 processing of historic certificates will often result in the use 1126 of weak keys. Implementations that wish to support previous 1127 versions of S/MIME or process old messages need to consider the 1128 security risks that result from smaller key sizes (e.g., spoofed 1129 messages) versus the costs of denial of service. 1131 [SMIMEv3.1] set the lower limit on suggested key sizes for 1132 creating and validation at 1024 bits. Prior to that the lower 1133 bound on key sizes was 512 bits. 1135 - Hash functions used to validate signatures on historic messages 1136 may longer be considered to be secure (see below). While there 1137 are not currently any known practical pre-image or second pre- 1138 image attacks against MD5 or SHA-1, the fact they are no longer 1139 considered to be collision resistant implies that the security 1140 level of any signature that is created with that these hash 1141 algorithms should also be considered as suspect. 1143 The following algorithms have been called out for some level of 1144 support by previous S/MIME specifications: 1146 - RSA with MD5 was dropped in [SMIMEv4.0]. MD5 is no longer 1147 considered to be secure as it is no longer collision-resistant. 1148 Details can be found in [RFC6151]. 1150 - RSA and DSA with SHA-1 were dropped in [SMIMEv4.0]. SHA-1 is no 1151 longer considered to be secure as it is no longer collision- 1152 resistant. The IETF statement on SHA-1 can be found in [RFC6194] 1153 but it is out-of-date relative to the most recent advances. 1155 - DSA with SHA-256 support was dropped in [SMIMEv4.0]. DSA was 1156 dropped as part of a general movement from finite fields to 1157 elliptic curves. Issues have come up dealing with non- 1158 deterministic generation of the parameter 'k' (see [RFC6979]). 1160 For 512-bit RSA with SHA-1 see [RFC3279] and [FIPS186-2] without 1161 Change Notice 1, for 512-bit RSA with SHA-256 see [RFC4055] and 1162 [FIPS186-2] without Change Notice 1. 1164 For 512-bit DSA with SHA-1 see [RFC3279] and [FIPS186-2] without 1165 Change Notice 1, for 512-bit DSA with SHA-256 see [RFC5758] and 1166 [FIPS186-2] without Change Notice 1, for 1024-bit DSA with SHA-1 see 1167 [RFC3279] and [FIPS186-2] with Change Notice 1, for 1024-bit through 1168 3072 DSA with SHA-256 see [RFC5758] and [FIPS186-3]. In either case, 1169 the first reference provides the signature algorithm's object 1170 identifier and the second provides the signature algorithm's 1171 definition. 1173 Appendix B. Moving S/MIME v2 Certificate Handling to Historic Status 1175 The S/MIME v3 [SMIMEv3], v3.1 [SMIMEv3.1], v3.2 [SMIMEv3.2], and v4.0 1176 (this document) are backward compatible with the S/MIME v2 1177 Certificate Handling Specification [SMIMEv2], with the exception of 1178 the algorithms (dropped RC2/40 requirement and added DSA and RSASSA- 1179 PSS requirements). Therefore, it is recommended that RFC 2312 1180 [SMIMEv2] be moved to Historic status. 1182 Appendix C. Acknowledgments 1184 Many thanks go out to the other authors of the S/MIME v2 RFC: Steve 1185 Dusse, Paul Hoffman, and Jeff Weinstein. Without v2, there wouldn't 1186 be a v3, v3.1, v3.2 or v4.0. 1188 A number of the members of the S/MIME Working Group have also worked 1189 very hard and contributed to this document. Any list of people is 1190 doomed to omission, and for that I apologize. In alphabetical order, 1191 the following people stand out in my mind because they made direct 1192 contributions to this document. 1194 Bill Flanigan, Trevor Freeman, Elliott Ginsburg, Alfred Hoenes, Paul 1195 Hoffman, Russ Housley, David P. Kemp, Michael Myers, John Pawling, 1196 and Denis Pinkas. 1198 The version 4 update to the S/MIME documents was done under the 1199 auspices of the LAMPS Working Group. 1201 Authors' Addresses 1203 Jim Schaad 1204 August Cellars 1206 Email: ietf@augustcellars.com 1208 Blake Ramsdell 1209 Brute Squad Labs, Inc. 1211 Email: blaker@gmail.com 1213 Sean Turner 1214 sn3rd 1216 Email: sean@sn3rd.com