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'FIPS186-3' == Outdated reference: A later version (-18) exists of draft-ietf-lamps-eai-addresses-06 ** Downref: Normative reference to an Informational RFC: RFC 2985 ** Obsolete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reference: RFC 5751 (Obsoleted by RFC 8551) ** Downref: Normative reference to an Informational RFC: RFC 6979 == Outdated reference: A later version (-10) exists of draft-ietf-curdle-pkix-03 -- Obsolete informational reference (is this intentional?): RFC 2313 (Obsoleted by RFC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 (~~), 8 warnings (==), 13 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 Intended status: Standards Track B. Ramsdell 5 Expires: August 27, 2017 Brute Squad Labs, Inc. 6 S. Turner 7 sn3rd 8 February 23, 2017 10 Secure/Multipurpose Internet Mail Extensions (S/ MIME) Version 4.0 11 Certificate Handling 12 draft-ietf-lamps-rfc5750-bis-02 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 3850. 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 http://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 August 27, 2017. 50 Copyright Notice 52 Copyright (c) 2017 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 (http://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 . . . . . . . . . . . . . . . . 6 86 2. CMS Options . . . . . . . . . . . . . . . . . . . . . . . . . 7 87 2.1. Certificate Revocation Lists . . . . . . . . . . . . . . 7 88 2.2. Certificate Choices . . . . . . . . . . . . . . . . . . . 7 89 2.2.1. Historical Note about CMS Certificates . . . . . . . 7 90 2.3. CertificateSet . . . . . . . . . . . . . . . . . . . . . 8 91 3. Using Distinguished Names for Internet Mail . . . . . . . . . 9 92 4. Certificate Processing . . . . . . . . . . . . . . . . . . . 10 93 4.1. Certificate Revocation Lists . . . . . . . . . . . . . . 11 94 4.2. Certificate Path Validation . . . . . . . . . . . . . . . 11 95 4.3. Certificate and CRL Signing Algorithms and Key Sizes . . 12 96 4.4. PKIX Certificate Extensions . . . . . . . . . . . . . . . 13 97 4.4.1. Basic Constraints . . . . . . . . . . . . . . . . . . 14 98 4.4.2. Key Usage Certificate Extension . . . . . . . . . . . 14 99 4.4.3. Subject Alternative Name . . . . . . . . . . . . . . 15 100 4.4.4. Extended Key Usage Extension . . . . . . . . . . . . 15 101 5. Security Considerations . . . . . . . . . . . . . . . . . . . 15 102 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 103 6.1. Normative References . . . . . . . . . . . . . . . . . . 17 104 6.2. Informational References . . . . . . . . . . . . . . . . 20 105 Appendix A. Historic Considerations . . . . . . . . . . . . . . 23 106 A.1. Signature Algorithms and Key Sizes . . . . . . . . . . . 23 107 Appendix B. Moving S/MIME v2 Certificate Handling to Historic 108 Status . . . . . . . . . . . . . . . . . . . . . . . 24 109 Appendix C. Acknowledgments . . . . . . . . . . . . . . . . . . 24 110 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25 112 1. Introduction 114 S/MIME (Secure/Multipurpose Internet Mail Extensions) v4.0, described 115 in [I-D.ietf-lamps-rfc5751-bis], provides a method to send and 116 receive secure MIME messages. Before using a public key to provide 117 security services, the S/MIME agent MUST verify that the public key 118 is valid. S/MIME agents MUST use PKIX certificates to validate 119 public keys as described in the Internet X.509 Public Key 120 Infrastructure (PKIX) Certificate and CRL Profile [RFC5280]. S/MIME 121 agents MUST meet the certificate processing requirements documented 122 in this document in addition to those stated in [RFC5280]. 124 This specification is compatible with the Cryptographic Message 125 Syntax (CMS) RFC 5652 [RFC5652] in that it uses the data types 126 defined by CMS. It also inherits all the varieties of architectures 127 for certificate-based key management supported by CMS. 129 1.1. Definitions 131 For the purposes of this document, the following definitions apply. 133 ASN.1: Abstract Syntax Notation One, as defined in ITU-T X.680 134 [X.680]. 136 Attribute certificate (AC): An X.509 AC is a separate structure from 137 a subject's public key X.509 certificate. A subject may have 138 multiple X.509 ACs associated with each of its public key X.509 139 certificates. Each X.509 AC binds one or more attributes with one of 140 the subject's public key X.509 certificates. The X.509 AC syntax is 141 defined in [RFC5755]. 143 Certificate: A type that binds an entity's name to a public key with 144 a digital signature. This type is defined in the Internet X.509 145 Public Key Infrastructure (PKIX) Certificate and CRL Profile 146 [RFC5280]. This type also contains the distinguished name of the 147 certificate issuer (the signer), an issuer-specific serial number, 148 the issuer's signature algorithm identifier, a validity period, and 149 extensions also defined in that document. 151 Certificate Revocation List (CRL): A type that contains information 152 about certificates whose validity an issuer has prematurely revoked. 153 The information consists of an issuer name, the time of issue, the 154 next scheduled time of issue, a list of certificate serial numbers 155 and their associated revocation times, and extensions as defined in 156 [RFC5280]. The CRL is signed by the issuer. The type intended by 157 this specification is the one defined in [RFC5280]. 159 Receiving agent: Software that interprets and processes S/MIME CMS 160 objects, MIME body parts that contain CMS objects, or both. 162 Sending agent: Software that creates S/MIME CMS objects, MIME body 163 parts that contain CMS objects, or both. 165 S/MIME agent: User software that is a receiving agent, a sending 166 agent, or both. 168 1.2. Conventions Used in This Document 170 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 171 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 172 document are to be interpreted as described in [RFC2119]. 174 We define some additional terms here: 176 SHOULD+ This term means the same as SHOULD. However, the authors 177 expect that a requirement marked as SHOULD+ will be promoted 178 at some future time to be a MUST. 180 SHOULD- This term means the same as SHOULD. However, the authors 181 expect that a requirement marked as SHOULD- will be demoted 182 to a MAY in a future version of this document. 184 MUST- This term means the same as MUST. However, the authors 185 expect that this requirement will no longer be a MUST in a 186 future document. Although its status will be determined at a 187 later time, it is reasonable to expect that if a future 188 revision of a document alters the status of a MUST- 189 requirement, it will remain at least a SHOULD or a SHOULD-. 191 1.3. Compatibility with Prior Practice S/MIME 193 S/MIME version 4.0 agents ought to attempt to have the greatest 194 interoperability possible with agents for prior versions of S/MIME. 196 S/MIME version 2 is described in RFC 2311 through RFC 2315 inclusive 197 [SMIMEv2], S/MIME version 3 is described in RFC 2630 through RFC 2634 198 inclusive and RFC 5035 [SMIMEv3], and S/MIME version 3.1 is described 199 in RFC 3850, RFC 3851, RFC 3852, RFC 2634, and RFC 5035 [SMIMEv3.1]. 200 RFC 2311 also has historical information about the development of 201 S/MIME. 203 Appendix A contains information about algorithms are were used for 204 prior versions of S/MIME but are no longer considered to meet modern 205 security standards. Support of these algorithms may be needed to 206 support historic S/MIME messages but SHOULD NOT be used for new mail. 208 1.4. Changes from S/MIME v3 to S/MIME v3.1 210 Version 1 and version 2 CRLs MUST be supported. 212 Multiple certification authority (CA) certificates with the same 213 subject and public key, but with overlapping validity periods, MUST 214 be supported. 216 Version 2 attribute certificates SHOULD be supported, and version 1 217 attributes certificates MUST NOT be used. 219 The use of the MD2 digest algorithm for certificate signatures is 220 discouraged, and security language was added. 222 Clarified use of email address use in certificates. Certificates 223 that do not contain an email address have no requirements for 224 verifying the email address associated with the certificate. 226 Receiving agents SHOULD display certificate information when 227 displaying the results of signature verification. 229 Receiving agents MUST NOT accept a signature made with a certificate 230 that does not have the digitalSignature or nonRepudiation bit set. 232 Clarifications for the interpretation of the key usage and extended 233 key usage extensions. 235 1.5. Changes from S/MIME v3.1 to S/MIME v3.2 237 Conventions Used in This Document: Moved to Section 1.2. Added 238 definitions for SHOULD+, SHOULD-, and MUST-. 240 Section 1.1: Updated ASN.1 definition and reference. 242 Section 1.3: Added text about v3.1 RFCs. 244 Section 3: Aligned email address text with RFC 5280. Updated note 245 to indicate emailAddress IA5String upper bound is 255 246 characters. Added text about matching email addresses. 248 Section 4.2: Added text to indicate how S/MIME agents locate the 249 correct user certificate. 251 Section 4.3: RSA with SHA-256 (PKCS #1 v1.5) added as MUST; DSA with 252 SHA-256 added as SHOULD+; RSA with SHA-1, DSA with SHA-1, 253 and RSA with MD5 changed to SHOULD-; and RSASSA-PSS with 254 SHA-256 added as SHOULD+. Updated key sizes and changed 255 pointer to PKIX RFCs. 257 Section 4.4.1: Aligned with PKIX on use of basic constraints 258 extension in CA certificates. Clarified which extension 259 is used to constrain end entities from using their keys 260 to perform issuing authority operations. 262 Section 5: Updated security considerations. 264 Section 7: Moved references from Appendix B to Section 6. Updated 265 the references. 267 Appendix A: Moved Appendix A to Appendix B. Added Appendix A to move 268 S/MIME v2 Certificate Handling to Historic Status. 270 1.6. Changes since S/MIME 3.2 272 Section 3: Require support for internationalized email addresses. 274 Section 4.3: Mandated support for ECDSA with P-256 and Ed25519. 275 Moved algorithms with SHA-1 and MD5 to historical status. 276 Moved DSA support to historical status. Increased lower 277 bounds on RSA key sizes. 279 Appendix A: Add a new appendix for algorithms that are now considered 280 to be historical. 282 2. CMS Options 284 The CMS message format allows for a wide variety of options in 285 content and algorithm support. This section puts forth a number of 286 support requirements and recommendations in order to achieve a base 287 level of interoperability among all S/MIME implementations. Most of 288 the CMS format for S/MIME messages is defined in [RFC5751]. 290 2.1. Certificate Revocation Lists 292 Receiving agents MUST support the Certificate Revocation List (CRL) 293 format defined in [RFC5280]. If sending agents include CRLs in 294 outgoing messages, the CRL format defined in [RFC5280] MUST be used. 295 In all cases, both v1 and v2 CRLs MUST be supported. 297 All agents MUST be capable of performing revocation checks using CRLs 298 as specified in [RFC5280]. All agents MUST perform revocation status 299 checking in accordance with [RFC5280]. Receiving agents MUST 300 recognize CRLs in received S/MIME messages. 302 Agents SHOULD store CRLs received in messages for use in processing 303 later messages. 305 2.2. Certificate Choices 307 Receiving agents MUST support v1 X.509 and v3 X.509 certificates as 308 profiled in [RFC5280]. End-entity certificates MAY include an 309 Internet mail address, as described in Section 3. 311 Receiving agents SHOULD support X.509 version 2 attribute 312 certificates. See [RFC5755] for details about the profile for 313 attribute certificates. 315 2.2.1. Historical Note about CMS Certificates 317 The CMS message format supports a choice of certificate formats for 318 public key content types: PKIX, PKCS #6 extended certificates 319 [PKCS6], and PKIX attribute certificates. 321 The PKCS #6 format is not in widespread use. In addition, PKIX 322 certificate extensions address much of the same functionality and 323 flexibility as was intended in the PKCS #6. Thus, sending and 324 receiving agents MUST NOT use PKCS #6 extended certificates. 326 X.509 version 1 attribute certificates are also not widely 327 implemented, and have been superseded with version 2 attribute 328 certificates. Sending agents MUST NOT send version 1 attribute 329 certificates. 331 2.3. CertificateSet 333 Receiving agents MUST be able to handle an arbitrary number of 334 certificates of arbitrary relationship to the message sender and to 335 each other in arbitrary order. In many cases, the certificates 336 included in a signed message may represent a chain of certification 337 from the sender to a particular root. There may be, however, 338 situations where the certificates in a signed message may be 339 unrelated and included for convenience. 341 Sending agents SHOULD include any certificates for the user's public 342 key(s) and associated issuer certificates. This increases the 343 likelihood that the intended recipient can establish trust in the 344 originator's public key(s). This is especially important when 345 sending a message to recipients that may not have access to the 346 sender's public key through any other means or when sending a signed 347 message to a new recipient. The inclusion of certificates in 348 outgoing messages can be omitted if S/MIME objects are sent within a 349 group of correspondents that has established access to each other's 350 certificates by some other means such as a shared directory or manual 351 certificate distribution. Receiving S/MIME agents SHOULD be able to 352 handle messages without certificates using a database or directory 353 lookup scheme. 355 A sending agent SHOULD include at least one chain of certificates up 356 to, but not including, a certification authority (CA) that it 357 believes that the recipient may trust as authoritative. A receiving 358 agent MUST be able to handle an arbitrarily large number of 359 certificates and chains. 361 Agents MAY send CA certificates, that is, cross-certificates, self- 362 issued certificates, and self-signed certificates. Note that 363 receiving agents SHOULD NOT simply trust any self-signed certificates 364 as valid CAs, but SHOULD use some other mechanism to determine if 365 this is a CA that should be trusted. Also note that when 366 certificates contain Digital Signature Algorithm (DSA) public keys 367 the parameters may be located in the root certificate. This would 368 require that the recipient possess both the end-entity certificate 369 and the root certificate to perform a signature verification, and is 370 a valid example of a case where transmitting the root certificate may 371 be required. 373 Receiving agents MUST support chaining based on the distinguished 374 name fields. Other methods of building certificate chains MAY be 375 supported. 377 Receiving agents SHOULD support the decoding of X.509 attribute 378 certificates included in CMS objects. All other issues regarding the 379 generation and use of X.509 attribute certificates are outside of the 380 scope of this specification. One specification that addresses 381 attribute certificate use is defined in [RFC3114]. 383 3. Using Distinguished Names for Internet Mail 385 End-entity certificates MAY contain an Internet mail address. Email 386 addresses retricted to 7-bit ASCII characters are encoded as 387 described in Section 4.2.1.6 of [RFC5280]. Internationalized Email 388 address names are encoded as described in 389 [I-D.ietf-lamps-eai-addresses]. The email address SHOULD be in the 390 subjectAltName extension, and SHOULD NOT be in the subject 391 distinguished name. 393 Receiving agents MUST recognize and accept certificates that contain 394 no email address. Agents are allowed to provide an alternative 395 mechanism for associating an email address with a certificate that 396 does not contain an email address, such as through the use of the 397 agent's address book, if available. Receiving agents MUST recognize 398 both ASCII and internationalized email addresses in the 399 subjectAltName field. Receiving agents MUST recognize email 400 addresses in the Distinguished Name field in the PKCS #9 [RFC2985] 401 emailAddress attribute: 403 pkcs-9-at-emailAddress OBJECT IDENTIFIER ::= 404 { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 1 } 406 Note that this attribute MUST be encoded as IA5String and has an 407 upper bound of 255 characters. The right side of the email address 408 SHOULD be treated as ASCII-case-insensitive. 410 Comparing of email addresses is fraught with peril. 411 [I-D.ietf-lamps-eai-addresses] defines the procedure for doing 412 comparison of Internationalized email addresses. For ASCII email 413 addresses the domain component (right-hand side of the '@') MUST be 414 compared using a case-insensitive function. The local name component 415 (left-hand side of the '@') SHOULD be compared using a case- 416 insensitive function. Some localities may perform other 417 transformations on the local name component before doing the 418 comparison, however an S/MIME client cannot know what specific 419 localities do. 421 Sending agents SHOULD make the address in the From or Sender header 422 in a mail message match an Internet mail address in the signer's 423 certificate. Receiving agents MUST check that the address in the 424 From or Sender header of a mail message matches an Internet mail 425 address in the signer's certificate, if mail addresses are present in 426 the certificate. A receiving agent SHOULD provide some explicit 427 alternate processing of the message if this comparison fails, which 428 may be to display a message that shows the recipient the addresses in 429 the certificate or other certificate details. 431 A receiving agent SHOULD display a subject name or other certificate 432 details when displaying an indication of successful or unsuccessful 433 signature verification. 435 All subject and issuer names MUST be populated (i.e., not an empty 436 SEQUENCE) in S/MIME-compliant X.509 certificates, except that the 437 subject distinguished name (DN) in a user's (i.e., end-entity) 438 certificate MAY be an empty SEQUENCE in which case the subjectAltName 439 extension will include the subject's identifier and MUST be marked as 440 critical. 442 4. Certificate Processing 444 S/MIME agents need to provide some certificate retrieval mechanism in 445 order to gain access to certificates for recipients of digital 446 envelopes. There are many ways to implement certificate retrieval 447 mechanisms. [X.500] directory service is an excellent example of a 448 certificate retrieval-only mechanism that is compatible with classic 449 X.500 Distinguished Names. Another method under consideration by the 450 IETF is to provide certificate retrieval services as part of the 451 existing Domain Name System (DNS). Until such mechanisms are widely 452 used, their utility may be limited by the small number of the 453 correspondent's certificates that can be retrieved. At a minimum, 454 for initial S/MIME deployment, a user agent could automatically 455 generate a message to an intended recipient requesting the 456 recipient's certificate in a signed return message. 458 Receiving and sending agents SHOULD also provide a mechanism to allow 459 a user to "store and protect" certificates for correspondents in such 460 a way so as to guarantee their later retrieval. In many 461 environments, it may be desirable to link the certificate retrieval/ 462 storage mechanisms together in some sort of certificate database. In 463 its simplest form, a certificate database would be local to a 464 particular user and would function in a similar way as an "address 465 book" that stores a user's frequent correspondents. In this way, the 466 certificate retrieval mechanism would be limited to the certificates 467 that a user has stored (presumably from incoming messages). A 468 comprehensive certificate retrieval/storage solution may combine two 469 or more mechanisms to allow the greatest flexibility and utility to 470 the user. For instance, a secure Internet mail agent may resort to 471 checking a centralized certificate retrieval mechanism for a 472 certificate if it cannot be found in a user's local certificate 473 storage/retrieval database. 475 Receiving and sending agents SHOULD provide a mechanism for the 476 import and export of certificates, using a CMS certs-only message. 477 This allows for import and export of full certificate chains as 478 opposed to just a single certificate. This is described in 479 [RFC5751]. 481 Agents MUST handle multiple valid certification authority (CA) 482 certificates containing the same subject name and the same public 483 keys but with overlapping validity intervals. 485 4.1. Certificate Revocation Lists 487 In general, it is always better to get the latest CRL information 488 from a CA than to get information stored away from incoming messages. 489 A receiving agent SHOULD have access to some CRL retrieval mechanism 490 in order to gain access to certificate revocation information when 491 validating certification paths. A receiving or sending agent SHOULD 492 also provide a mechanism to allow a user to store incoming 493 certificate revocation information for correspondents in such a way 494 so as to guarantee its later retrieval. 496 Receiving and sending agents SHOULD retrieve and utilize CRL 497 information every time a certificate is verified as part of a 498 certification path validation even if the certificate was already 499 verified in the past. However, in many instances (such as off-line 500 verification) access to the latest CRL information may be difficult 501 or impossible. The use of CRL information, therefore, may be 502 dictated by the value of the information that is protected. The 503 value of the CRL information in a particular context is beyond the 504 scope of this specification but may be governed by the policies 505 associated with particular certification paths. 507 All agents MUST be capable of performing revocation checks using CRLs 508 as specified in [RFC5280]. All agents MUST perform revocation status 509 checking in accordance with [RFC5280]. Receiving agents MUST 510 recognize CRLs in received S/MIME messages. 512 4.2. Certificate Path Validation 514 In creating a user agent for secure messaging, certificate, CRL, and 515 certification path validation SHOULD be highly automated while still 516 acting in the best interests of the user. Certificate, CRL, and path 517 validation MUST be performed as per [RFC5280] when validating a 518 correspondent's public key. This is necessary before using a public 519 key to provide security services such as verifying a signature, 520 encrypting a content-encryption key (e.g., RSA), or forming a 521 pairwise symmetric key (e.g., Diffie-Hellman) to be used to encrypt 522 or decrypt a content-encryption key. 524 Certificates and CRLs are made available to the path validation 525 procedure in two ways: a) incoming messages, and b) certificate and 526 CRL retrieval mechanisms. Certificates and CRLs in incoming messages 527 are not required to be in any particular order nor are they required 528 to be in any way related to the sender or recipient of the message 529 (although in most cases they will be related to the sender). 530 Incoming certificates and CRLs SHOULD be cached for use in path 531 validation and optionally stored for later use. This temporary 532 certificate and CRL cache SHOULD be used to augment any other 533 certificate and CRL retrieval mechanisms for path validation on 534 incoming signed messages. 536 When verifying a signature and the certificates that are included in 537 the message, if a signingCertificate attribute from RFC 2634 [ESS] or 538 a signingCertificateV2 attribute from RFC 5035 [ESS] is found in an 539 S/MIME message, it SHALL be used to identify the signer's 540 certificate. Otherwise, the certificate is identified in an S/MIME 541 message, either using the issuerAndSerialNumber, which identifies the 542 signer's certificate by the issuer's distinguished name and the 543 certificate serial number, or the subjectKeyIdentifier, which 544 identifies the signer's certificate by a key identifier. 546 When decrypting an encrypted message, if a 547 SMIMEEncryptionKeyPreference attribute is found in an encapsulating 548 SignedData, it SHALL be used to identify the originator's certificate 549 found in OriginatorInfo. See [RFC5652] for the CMS fields that 550 reference the originator's and recipient's certificates. 552 4.3. Certificate and CRL Signing Algorithms and Key Sizes 554 Certificates and Certificate Revocation Lists (CRLs) are signed by 555 the certificate issuer. Receiving agents: 557 - MUST support ECDSA with curve P-256 with SHA-256. 559 - MUST support EdDSA with curve 25519 using PureEdDSA mode. 561 - MUST- support RSA with SHA-256. 563 - SHOULD support RSASSA-PSS with SHA-256. 565 - MUST NOT support EdDSA using Pre-hash mode. 567 Implementations SHOULD use deterministic generation for the parameter 568 'k' for ECDSA as outlined in [RFC6979]. EdDSA is defined to generate 569 this parameter deterministically. 571 The following are the RSA and RSASSA-PSS key size requirements for 572 S/MIME receiving agents during certificate and CRL signature 573 verification: 575 key size <= 2047 : SHOULD NOT (see Historic Considerations) 576 2048 <= key size <= 4096 : MUST (see Security Considerations) 577 4096 < key size : MAY (see Security Considerations) 579 For 1024-bit through 3072-bit RSA with SHA-256 see [RFC4055] and 580 [FIPS186-2] with Change Notice 1, and for 4096-bit RSA with SHA-256 581 see [RFC4055] and [RFC3447]. In either case, the first reference 582 provides the signature algorithm's object identifier and the second 583 provides the signature algorithm's definition. 585 For RSASSA-PSS with SHA-256 see [RFC4056]. 587 For ECDSA see [RFC5758] and [RFC6090]. The first reference provides 588 the signature algorithm's object identifier and the second provides 589 the signature algorithm's definition. Curves other than curve P-256 590 MAY be used as well. 592 For EdDSA see [I-D.ietf-curdle-pkix] and [I-D.irtf-cfrg-eddsa]. The 593 first reference provides the signature algorithm's object identifier 594 and the second provides the signature algorithm's definition. Other 595 curves than curve 25519 MAY be used as well. 597 4.4. PKIX Certificate Extensions 599 PKIX describes an extensible framework in which the basic certificate 600 information can be extended and describes how such extensions can be 601 used to control the process of issuing and validating certificates. 602 The PKIX Working Group has ongoing efforts to identify and create 603 extensions that have value in particular certification environments. 604 Further, there are active efforts underway to issue PKIX certificates 605 for business purposes. This document identifies the minimum required 606 set of certificate extensions that have the greatest value in the 607 S/MIME environment. The syntax and semantics of all the identified 608 extensions are defined in [RFC5280]. 610 Sending and receiving agents MUST correctly handle the basic 611 constraints, key usage, authority key identifier, subject key 612 identifier, and subject alternative names certificate extensions when 613 they appear in end-entity and CA certificates. Some mechanism SHOULD 614 exist to gracefully handle other certificate extensions when they 615 appear in end-entity or CA certificates. 617 Certificates issued for the S/MIME environment SHOULD NOT contain any 618 critical extensions (extensions that have the critical field set to 619 TRUE) other than those listed here. These extensions SHOULD be 620 marked as non-critical unless the proper handling of the extension is 621 deemed critical to the correct interpretation of the associated 622 certificate. Other extensions may be included, but those extensions 623 SHOULD NOT be marked as critical. 625 Interpretation and syntax for all extensions MUST follow [RFC5280], 626 unless otherwise specified here. 628 4.4.1. Basic Constraints 630 The basic constraints extension serves to delimit the role and 631 position that an issuing authority or end-entity certificate plays in 632 a certification path. 634 For example, certificates issued to CAs and subordinate CAs contain a 635 basic constraint extension that identifies them as issuing authority 636 certificates. End-entity certificates contain the key usage 637 extension that restrains end entities from using the key when 638 performing issuing authority operations (see Section 4.4.2). 640 As per [RFC5280], certificates MUST contain a basicConstraints 641 extension in CA certificates, and SHOULD NOT contain that extension 642 in end- entity certificates. 644 4.4.2. Key Usage Certificate Extension 646 The key usage extension serves to limit the technical purposes for 647 which a public key listed in a valid certificate may be used. 648 Issuing authority certificates may contain a key usage extension that 649 restricts the key to signing certificates, certificate revocation 650 lists, and other data. 652 For example, a certification authority may create subordinate issuer 653 certificates that contain a key usage extension that specifies that 654 the corresponding public key can be used to sign end user 655 certificates and sign CRLs. 657 If a key usage extension is included in a PKIX certificate, then it 658 MUST be marked as critical. 660 S/MIME receiving agents MUST NOT accept the signature of a message if 661 it was verified using a certificate that contains the key usage 662 extension without either the digitalSignature or nonRepudiation bit 663 set. Sometimes S/MIME is used as a secure message transport for 664 applications beyond interpersonal messaging. In such cases, the 665 S/MIME-enabled application can specify additional requirements 666 concerning the digitalSignature or nonRepudiation bits within this 667 extension. 669 If the key usage extension is not specified, receiving clients MUST 670 presume that the digitalSignature and nonRepudiation bits are set. 672 4.4.3. Subject Alternative Name 674 The subject alternative name extension is used in S/MIME as the 675 preferred means to convey the email address(es) that correspond(s) to 676 the entity for this certificate. Any ASCII email addresses present 677 MUST be encoded using the rfc822Name CHOICE of the GeneralName type 678 as described in [RFC5280], Section 4.2.1.6. Any internationalized 679 email addresses present MUST be encoded using the otherName CHOICE of 680 the GeneralName type as described in [I-D.ietf-lamps-eai-addresses], 681 Section 3. Since the SubjectAltName type is a SEQUENCE OF 682 GeneralName, multiple email addresses MAY be present. 684 4.4.4. Extended Key Usage Extension 686 The extended key usage extension also serves to limit the technical 687 purposes for which a public key listed in a valid certificate may be 688 used. The set of technical purposes for the certificate therefore 689 are the intersection of the uses indicated in the key usage and 690 extended key usage extensions. 692 For example, if the certificate contains a key usage extension 693 indicating digital signature and an extended key usage extension that 694 includes the email protection OID, then the certificate may be used 695 for signing but not encrypting S/MIME messages. If the certificate 696 contains a key usage extension indicating digital signature but no 697 extended key usage extension, then the certificate may also be used 698 to sign but not encrypt S/MIME messages. 700 If the extended key usage extension is present in the certificate, 701 then interpersonal message S/MIME receiving agents MUST check that it 702 contains either the emailProtection or the anyExtendedKeyUsage OID as 703 defined in [RFC5280]. S/MIME uses other than interpersonal messaging 704 MAY require the explicit presence of the extended key usage extension 705 or other OIDs to be present in the extension or both. 707 5. Security Considerations 709 All of the security issues faced by any cryptographic application 710 must be faced by a S/MIME agent. Among these issues are protecting 711 the user's private key, preventing various attacks, and helping the 712 user avoid mistakes such as inadvertently encrypting a message for 713 the wrong recipient. The entire list of security considerations is 714 beyond the scope of this document, but some significant concerns are 715 listed here. 717 When processing certificates, there are many situations where the 718 processing might fail. Because the processing may be done by a user 719 agent, a security gateway, or other program, there is no single way 720 to handle such failures. Just because the methods to handle the 721 failures have not been listed, however, the reader should not assume 722 that they are not important. The opposite is true: if a certificate 723 is not provably valid and associated with the message, the processing 724 software should take immediate and noticeable steps to inform the end 725 user about it. 727 Some of the many places where signature and certificate checking 728 might fail include: 730 - no Internet mail addresses in a certificate match the sender of a 731 message, if the certificate contains at least one mail address 733 - no certificate chain leads to a trusted CA 735 - no ability to check the CRL for a certificate 737 - an invalid CRL was received 739 - the CRL being checked is expired 741 - the certificate is expired 743 - the certificate has been revoked 745 There are certainly other instances where a certificate may be 746 invalid, and it is the responsibility of the processing software to 747 check them all thoroughly, and to decide what to do if the check 748 fails. 750 It is possible for there to be multiple unexpired CRLs for a CA. If 751 an agent is consulting CRLs for certificate validation, it SHOULD 752 make sure that the most recently issued CRL for that CA is consulted, 753 since an S/MIME message sender could deliberately include an older 754 unexpired CRL in an S/MIME message. This older CRL might not include 755 recently revoked certificates, which might lead an agent to accept a 756 certificate that has been revoked in a subsequent CRL. 758 When determining the time for a certificate validity check, agents 759 have to be careful to use a reliable time. Unless it is from a 760 trusted agent, this time MUST NOT be the SigningTime attribute found 761 in an S/MIME message. For most sending agents, the SigningTime 762 attribute could be deliberately set to direct the receiving agent to 763 check a CRL that could have out-of-date revocation status for a 764 certificate, or cause an improper result when checking the Validity 765 field of a certificate. 767 In addition to the Security Considerations identified in [RFC5280], 768 caution should be taken when processing certificates that have not 769 first been validated to a trust anchor. Certificates could be 770 manufactured by untrusted sources for the purpose of mounting denial 771 of service or other attacks. For example, keys selected to require 772 excessive cryptographic processing, or extensive lists of CRL 773 Distribution Point (CDP) and/or Authority Information Access (AIA) 774 addresses in the certificate, could be used to mount denial-of- 775 service attacks. Similarly, attacker-specified CDP and/or AIA 776 addresses could be included in fake certificates to allow the 777 originator to detect receipt of the message even if signature 778 verification fails. 780 RSA keys of less than 2048 bits are now considered by many experts to 781 be cryptographically insecure (due to advances in computing power), 782 and should no longer be used to sign certificates or CRLs. Such keys 783 were previously considered secure, so processing previously received 784 signed and encrypted mail may require processing certificates or CRLs 785 signed with weak keys. Implementations that wish to support previous 786 versions of S/MIME or process old messages need to consider the 787 security risks that result from accepting certificates and CRLs with 788 smaller key sizes (e.g., spoofed certificates) versus the costs of 789 denial of service. If an implementation supports verification of 790 certificates or CRLs generated with RSA and DSA keys of less than 791 2048 bits, it MUST warn the user. Implementers should consider 792 providing a stronger warning for weak signatures on certificates and 793 CRLs associated with newly received messages than the one provided 794 for certificates and CRLs associated with previously stored messages. 795 Server implementations (e.g., secure mail list servers) where user 796 warnings are not appropriate SHOULD reject messages with weak 797 cryptography. 799 If an implementation is concerned about compliance with National 800 Institute of Standards and Technology (NIST) key size 801 recommendations, then see [SP800-57]. 803 6. References 805 6.1. Normative References 807 [FIPS186-2] 808 National Institute of Standards and Technology (NIST), 809 "Digital Signature Standard (DSS) [With Change Notice 1]", 810 Federal Information Processing Standards 811 Publication 186-2, January 2000. 813 [FIPS186-3] 814 National Institute of Standards and Technology (NIST), 815 "Digital Signature Standard (DSS)", Federal Information 816 Processing Standards Publication 186-3, June 2009. 818 [I-D.ietf-lamps-eai-addresses] 819 Melnikov, A. and W. Chuang, "Internationalized Email 820 Addresses in X.509 certificates", draft-ietf-lamps-eai- 821 addresses-06 (work in progress), February 2017. 823 [I-D.ietf-lamps-rfc5751-bis] 824 Schaad, J., Ramsdell, B., and S. Turner, "Secure/ 825 Multipurpose Internet Mail Extensions (S/MIME) Version 4.0 826 Message Specification", draft-ietf-lamps-rfc5751-bis-02 827 (work in progress), October 2016. 829 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 830 Requirement Levels", BCP 14, RFC 2119, 831 DOI 10.17487/RFC2119, March 1997, 832 . 834 [RFC2634] Hoffman, P., Ed., "Enhanced Security Services for S/MIME", 835 RFC 2634, DOI 10.17487/RFC2634, June 1999, 836 . 838 [RFC2985] Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object 839 Classes and Attribute Types Version 2.0", RFC 2985, 840 DOI 10.17487/RFC2985, November 2000, 841 . 843 [RFC3279] Bassham, L., Polk, W., and R. Housley, "Algorithms and 844 Identifiers for the Internet X.509 Public Key 845 Infrastructure Certificate and Certificate Revocation List 846 (CRL) Profile", RFC 3279, DOI 10.17487/RFC3279, April 847 2002, . 849 [RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography 850 Standards (PKCS) #1: RSA Cryptography Specifications 851 Version 2.1", RFC 3447, DOI 10.17487/RFC3447, February 852 2003, . 854 [RFC4055] Schaad, J., Kaliski, B., and R. Housley, "Additional 855 Algorithms and Identifiers for RSA Cryptography for use in 856 the Internet X.509 Public Key Infrastructure Certificate 857 and Certificate Revocation List (CRL) Profile", RFC 4055, 858 DOI 10.17487/RFC4055, June 2005, 859 . 861 [RFC4056] Schaad, J., "Use of the RSASSA-PSS Signature Algorithm in 862 Cryptographic Message Syntax (CMS)", RFC 4056, 863 DOI 10.17487/RFC4056, June 2005, 864 . 866 [RFC5035] Schaad, J., "Enhanced Security Services (ESS) Update: 867 Adding CertID Algorithm Agility", RFC 5035, 868 DOI 10.17487/RFC5035, August 2007, 869 . 871 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 872 Housley, R., and W. Polk, "Internet X.509 Public Key 873 Infrastructure Certificate and Certificate Revocation List 874 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, 875 . 877 [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, 878 RFC 5652, DOI 10.17487/RFC5652, September 2009, 879 . 881 [RFC5750] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 882 Mail Extensions (S/MIME) Version 3.2 Certificate 883 Handling", RFC 5750, DOI 10.17487/RFC5750, January 2010, 884 . 886 [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 887 Mail Extensions (S/MIME) Version 3.2 Message 888 Specification", RFC 5751, DOI 10.17487/RFC5751, January 889 2010, . 891 [RFC5755] Farrell, S., Housley, R., and S. Turner, "An Internet 892 Attribute Certificate Profile for Authorization", 893 RFC 5755, DOI 10.17487/RFC5755, January 2010, 894 . 896 [RFC5758] Dang, Q., Santesson, S., Moriarty, K., Brown, D., and T. 897 Polk, "Internet X.509 Public Key Infrastructure: 898 Additional Algorithms and Identifiers for DSA and ECDSA", 899 RFC 5758, DOI 10.17487/RFC5758, January 2010, 900 . 902 [RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature 903 Algorithm (DSA) and Elliptic Curve Digital Signature 904 Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August 905 2013, . 907 [SMIMEv3.2] 908 "S/MIME version 3.2". 910 This group of documents represents S/MIME version 3.2. 911 This set of documents are [RFC2634], [RFC5750], [[This 912 Document]], [RFC5652], and [RFC5035]. 914 [SMIMEv4.0] 915 "S/MIME version 4.0". 917 This group of documents represents S/MIME version 4.0. 918 This set of documents are [RFC2634], 919 [I-D.ietf-lamps-rfc5751-bis], [[This Document]], 920 [RFC5652], and [RFC5035]. 922 [X.680] "Information Technology - Abstract Syntax Notation One 923 (ASN.1): Specification of basic notation. ITU-T 924 Recommendation X.680 (2002) | ISO/IEC 8824-1:2002.". 926 6.2. Informational References 928 [ESS] "Enhanced Security Services for S/ MIME". 930 This is the set of documents dealing with enhanged 931 security services and refers to [RFC2634] and [RFC5035]. 933 [I-D.ietf-curdle-pkix] 934 Josefsson, S. and J. Schaad, "Algorithm Identifiers for 935 Ed25519, Ed25519ph, Ed448, Ed448ph, X25519 and X448 for 936 use in the Internet X.509 Public Key Infrastructure", 937 draft-ietf-curdle-pkix-03 (work in progress), November 938 2016. 940 [I-D.irtf-cfrg-eddsa] 941 Josefsson, S. and I. Liusvaara, "Edwards-curve Digital 942 Signature Algorithm (EdDSA)", draft-irtf-cfrg-eddsa-08 943 (work in progress), August 2016. 945 [PKCS6] RSA Laboratories, "PKCS #6: Extended-Certificate Syntax 946 Standard", November 1993. 948 [RFC2311] Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L., and 949 L. Repka, "S/MIME Version 2 Message Specification", 950 RFC 2311, DOI 10.17487/RFC2311, March 1998, 951 . 953 [RFC2312] Dusse, S., Hoffman, P., Ramsdell, B., and J. Weinstein, 954 "S/MIME Version 2 Certificate Handling", RFC 2312, 955 DOI 10.17487/RFC2312, March 1998, 956 . 958 [RFC2313] Kaliski, B., "PKCS #1: RSA Encryption Version 1.5", 959 RFC 2313, DOI 10.17487/RFC2313, March 1998, 960 . 962 [RFC2314] Kaliski, B., "PKCS #10: Certification Request Syntax 963 Version 1.5", RFC 2314, DOI 10.17487/RFC2314, March 1998, 964 . 966 [RFC2315] Kaliski, B., "PKCS #7: Cryptographic Message Syntax 967 Version 1.5", RFC 2315, DOI 10.17487/RFC2315, March 1998, 968 . 970 [RFC2630] Housley, R., "Cryptographic Message Syntax", RFC 2630, 971 DOI 10.17487/RFC2630, June 1999, 972 . 974 [RFC2631] Rescorla, E., "Diffie-Hellman Key Agreement Method", 975 RFC 2631, DOI 10.17487/RFC2631, June 1999, 976 . 978 [RFC2632] Ramsdell, B., Ed., "S/MIME Version 3 Certificate 979 Handling", RFC 2632, DOI 10.17487/RFC2632, June 1999, 980 . 982 [RFC2633] Ramsdell, B., Ed., "S/MIME Version 3 Message 983 Specification", RFC 2633, DOI 10.17487/RFC2633, June 1999, 984 . 986 [RFC3114] Nicolls, W., "Implementing Company Classification Policy 987 with the S/MIME Security Label", RFC 3114, 988 DOI 10.17487/RFC3114, May 2002, 989 . 991 [RFC3850] Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail 992 Extensions (S/MIME) Version 3.1 Certificate Handling", 993 RFC 3850, DOI 10.17487/RFC3850, July 2004, 994 . 996 [RFC3851] Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail 997 Extensions (S/MIME) Version 3.1 Message Specification", 998 RFC 3851, DOI 10.17487/RFC3851, July 2004, 999 . 1001 [RFC3852] Housley, R., "Cryptographic Message Syntax (CMS)", 1002 RFC 3852, DOI 10.17487/RFC3852, July 2004, 1003 . 1005 [RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic 1006 Curve Cryptography Algorithms", RFC 6090, 1007 DOI 10.17487/RFC6090, February 2011, 1008 . 1010 [RFC6151] Turner, S. and L. Chen, "Updated Security Considerations 1011 for the MD5 Message-Digest and the HMAC-MD5 Algorithms", 1012 RFC 6151, DOI 10.17487/RFC6151, March 2011, 1013 . 1015 [RFC6194] Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security 1016 Considerations for the SHA-0 and SHA-1 Message-Digest 1017 Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011, 1018 . 1020 [SMIMEv2] "S/MIME version v2". 1022 This group of documents represents S/MIME version 2. This 1023 set of documents are [RFC2311], [RFC2312], [RFC2313], 1024 [RFC2314], and [RFC2315]. 1026 [SMIMEv3] "S/MIME version 3". 1028 This group of documents represents S/MIME version 3. This 1029 set of documents are [RFC2630], [RFC2631], [RFC2632], 1030 [RFC2633], [RFC2634], and [RFC5035]. 1032 [SMIMEv3.1] 1033 "S/MIME version 3.1". 1035 This group of documents represents S/MIME version 3.1. 1036 This set of documents are [RFC2634], [RFC3850], [RFC3851], 1037 [RFC3852], and [RFC5035]. 1039 [SP800-57] 1040 National Institute of Standards and Technology (NIST), 1041 "Special Publication 800-57: Recommendation for Key 1042 Management", August 2005. 1044 [X.500] "ITU-T Recommendation X.500 (1997) | ISO/IEC 9594- 1:1997, 1045 Information technology - Open Systems Interconnection - 1046 The Directory: Overview of concepts, models and 1047 services.". 1049 Appendix A. Historic Considerations 1051 A.1. Signature Algorithms and Key Sizes 1053 There are a number of problems with validating certificates on 1054 sufficiently historic messages. For this reason it is strongly 1055 suggested that UAs treat these certificates differently from those on 1056 current messages. These problems include: 1058 - CAs are not required to keep certificates on a CRL beyond one 1059 update after a certificate has expired. This means that unless 1060 CRLs are cached as part of the message it is not always possible 1061 to check if a certificate has been revoked. The same problems 1062 exist with OCSP responses as they may be based on a CRL rather 1063 than on the certificate database. 1065 - RSA and DSA keys of less than 2048 bits are now considered by many 1066 experts to be cryptographically insecure (due to advances in 1067 computing power). Such keys were previously considered secure, so 1068 processing of historic certificates will often result in the use 1069 of weak keys. Implementations that wish to support previous 1070 versions of S/MIME or process old messages need to consider the 1071 security risks that result from smaller key sizes (e.g., spoofed 1072 messages) versus the costs of denial of service. 1074 [SMIMEv3.1] set the lower limit on suggested key sizes for 1075 creating and validation at 1024 bits. Prior to that the lower 1076 bound on key sizes was 512 bits. 1078 - Hash functions used to validate signatures on historic messages 1079 may longer be considered to be secure (see below). While there 1080 are not currently any known practical pre-image or second pre- 1081 image attacks against MD5 or SHA-1, the fact they are no longer 1082 considered to be collision resistent the security levels of the 1083 signatures are generally considered suspect. 1085 The following algorithms have been called out for some level of 1086 support by previous S/MIME specifications: 1088 - RSA with MD5 was dropped in [SMIMEv4.0]. MD5 is no longer 1089 considered to be secure as it is no longer collision-resistant. 1090 Details can be found in [RFC6151]. 1092 - RSA and DSA with SHA-1 were dropped in [SMIMEv4.0]. SHA-1 is 1093 nolonger considered to be secure as it is no longer collision- 1094 resistant. The IETF statement on SHA-1 can be found in [RFC6194] 1095 but it is out-of-date relative to the most recent advances. 1097 - DSA with SHA-256 support was dropped in [SMIMEv4.0]. DSA was 1098 dropped as part of a general movement from discrete logarithms to 1099 elliptic curves. Issues have come up dealing with small group 1100 attacks and with non-deterministic generation of the parameter 'k' 1101 (see [RFC6979]). 1103 For 512-bit RSA with SHA-1 see [RFC3279] and [FIPS186-2] without 1104 Change Notice 1, for 512-bit RSA with SHA-256 see [RFC4055] and 1105 [FIPS186-2] without Change Notice 1. 1107 For 512-bit DSA with SHA-1 see [RFC3279] and [FIPS186-2] without 1108 Change Notice 1, for 512-bit DSA with SHA-256 see [RFC5758] and 1109 [FIPS186-2] without Change Notice 1, for 1024-bit DSA with SHA-1 see 1110 [RFC3279] and [FIPS186-2] with Change Notice 1, for 1024-bit through 1111 3072 DSA with SHA-256 see [RFC5758] and [FIPS186-3]. In either case, 1112 the first reference provides the signature algorithm's object 1113 identifier and the second provides the signature algorithm's 1114 definition. 1116 Appendix B. Moving S/MIME v2 Certificate Handling to Historic Status 1118 The S/MIME v3 [SMIMEv3], v3.1 [SMIMEv3.1], v3.2 [SMIMEv3.2], and v4.0 1119 (this document) are backwards compatible with the S/MIME v2 1120 Certificate Handling Specification [SMIMEv2], with the exception of 1121 the algorithms (dropped RC2/40 requirement and added DSA and RSASSA- 1122 PSS requirements). Therefore, it is recommended that RFC 2312 1123 [SMIMEv2] be moved to Historic status. 1125 Appendix C. Acknowledgments 1127 Many thanks go out to the other authors of the S/MIME v2 RFC: Steve 1128 Dusse, Paul Hoffman, and Jeff Weinstein. Without v2, there wouldn't 1129 be a v3, v3.1, or v3.2. 1131 A number of the members of the S/MIME Working Group have also worked 1132 very hard and contributed to this document. Any list of people is 1133 doomed to omission, and for that I apologize. In alphabetical order, 1134 the following people stand out in my mind because they made direct 1135 contributions to this document. 1137 Bill Flanigan, Trevor Freeman, Elliott Ginsburg, Alfred Hoenes, Paul 1138 Hoffman, Russ Housley, David P. Kemp, Michael Myers, John Pawling, 1139 and Denis Pinkas. 1141 Authors' Addresses 1143 Jim Schaad 1144 August Cellars 1146 Email: ietf@augustcellars.com 1148 Blake Ramsdell 1149 Brute Squad Labs, Inc. 1151 Email: blaker@gmail.com 1153 Sean Turner 1154 sn3rd 1156 Email: sean@sn3rd.com