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'CHARSETS' -- Possible downref: Non-RFC (?) normative reference: ref. 'CMS' -- Possible downref: Non-RFC (?) normative reference: ref. 'ESS' -- Possible downref: Non-RFC (?) normative reference: ref. 'FIPS186-4' == Outdated reference: draft-ietf-curdle-cms-ecdh-new-curves has been published as RFC 8418 == Outdated reference: draft-ietf-lamps-rfc5750-bis has been published as RFC 8550 -- Possible downref: Non-RFC (?) normative reference: ref. 'MIME-SPEC' ** Obsolete normative reference: RFC 2138 (Obsoleted by RFC 2865) ** Obsolete normative reference: RFC 4288 (Obsoleted by RFC 6838) -- Duplicate reference: RFC5035, mentioned in 'RFC5652', was also mentioned in 'RFC5035'. ** Downref: Normative reference to an Informational RFC: RFC 5753 -- 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'. -- Obsolete informational reference (is this intentional?): RFC 5750 (Obsoleted by RFC 8550) -- Obsolete informational reference (is this intentional?): RFC 5751 (Obsoleted by RFC 8551) Summary: 3 errors (**), 0 flaws (~~), 14 warnings (==), 20 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: 5751 (if approved) B. Ramsdell 5 Intended status: Standards Track Brute Squad Labs, Inc. 6 Expires: October 9, 2017 S. Turner 7 sn3rd 8 April 7, 2017 10 Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 4.0 11 Message Specification 12 draft-ietf-lamps-rfc5751-bis-05 14 Abstract 16 This document defines Secure/Multipurpose Internet Mail Extensions 17 (S/MIME) version 4.0. S/MIME provides a consistent way to send and 18 receive secure MIME data. Digital signatures provide authentication, 19 message integrity, and non-repudiation with proof of origin. 20 Encryption provides data confidentiality. Compression can be used to 21 reduce data size. This document obsoletes RFC 5751. 23 Contributing to this document 25 The source for this draft is being maintained in GitHub. Suggested 26 changes should be submitted as pull requests at . Instructions are on that page as well. Editorial 28 changes can be managed in GitHub, but any substantial issues need to 29 be discussed on the LAMPS mailing list. 31 Status of This Memo 33 This Internet-Draft is submitted in full conformance with the 34 provisions of BCP 78 and BCP 79. 36 Internet-Drafts are working documents of the Internet Engineering 37 Task Force (IETF). Note that other groups may also distribute 38 working documents as Internet-Drafts. The list of current Internet- 39 Drafts is at http://datatracker.ietf.org/drafts/current/. 41 Internet-Drafts are draft documents valid for a maximum of six months 42 and may be updated, replaced, or obsoleted by other documents at any 43 time. It is inappropriate to use Internet-Drafts as reference 44 material or to cite them other than as "work in progress." 46 This Internet-Draft will expire on October 9, 2017. 48 Copyright Notice 50 Copyright (c) 2017 IETF Trust and the persons identified as the 51 document authors. All rights reserved. 53 This document is subject to BCP 78 and the IETF Trust's Legal 54 Provisions Relating to IETF Documents 55 (http://trustee.ietf.org/license-info) in effect on the date of 56 publication of this document. Please review these documents 57 carefully, as they describe your rights and restrictions with respect 58 to this document. Code Components extracted from this document must 59 include Simplified BSD License text as described in Section 4.e of 60 the Trust Legal Provisions and are provided without warranty as 61 described in the Simplified BSD License. 63 This document may contain material from IETF Documents or IETF 64 Contributions published or made publicly available before November 65 10, 2008. The person(s) controlling the copyright in some of this 66 material may not have granted the IETF Trust the right to allow 67 modifications of such material outside the IETF Standards Process. 68 Without obtaining an adequate license from the person(s) controlling 69 the copyright in such materials, this document may not be modified 70 outside the IETF Standards Process, and derivative works of it may 71 not be created outside the IETF Standards Process, except to format 72 it for publication as an RFC or to translate it into languages other 73 than English. 75 Table of Contents 77 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 78 1.1. Specification Overview . . . . . . . . . . . . . . . . . 4 79 1.2. Definitions . . . . . . . . . . . . . . . . . . . . . . . 5 80 1.3. Conventions Used in This Document . . . . . . . . . . . . 6 81 1.4. Compatibility with Prior Practice of S/MIME . . . . . . . 7 82 1.5. Changes from S/MIME v3 to S/MIME v3.1 . . . . . . . . . . 7 83 1.6. Changes from S/MIME v3.1 to S/MIME v3.2 . . . . . . . . . 8 84 1.7. Changes for S/MIME v4.0 . . . . . . . . . . . . . . . . . 9 85 2. CMS Options . . . . . . . . . . . . . . . . . . . . . . . . . 10 86 2.1. DigestAlgorithmIdentifier . . . . . . . . . . . . . . . . 10 87 2.2. SignatureAlgorithmIdentifier . . . . . . . . . . . . . . 10 88 2.3. KeyEncryptionAlgorithmIdentifier . . . . . . . . . . . . 11 89 2.4. General Syntax . . . . . . . . . . . . . . . . . . . . . 12 90 2.4.1. Data Content Type . . . . . . . . . . . . . . . . . . 12 91 2.4.2. SignedData Content Type . . . . . . . . . . . . . . . 12 92 2.4.3. EnvelopedData Content Type . . . . . . . . . . . . . 12 93 2.4.4. AuthEnvelopedData Content Type . . . . . . . . . . . 12 94 2.4.5. CompressedData Content Type . . . . . . . . . . . . . 12 95 2.5. Attributes and the SignerInfo Type . . . . . . . . . . . 13 96 2.5.1. Signing Time Attribute . . . . . . . . . . . . . . . 13 97 2.5.2. SMIME Capabilities Attribute . . . . . . . . . . . . 14 98 2.5.3. Encryption Key Preference Attribute . . . . . . . . . 15 99 2.6. SignerIdentifier SignerInfo Type . . . . . . . . . . . . 17 100 2.7. ContentEncryptionAlgorithmIdentifier . . . . . . . . . . 17 101 2.7.1. Deciding Which Encryption Method to Use . . . . . . . 17 102 2.7.2. Choosing Weak Encryption . . . . . . . . . . . . . . 19 103 2.7.3. Multiple Recipients . . . . . . . . . . . . . . . . . 19 104 3. Creating S/MIME Messages . . . . . . . . . . . . . . . . . . 19 105 3.1. Preparing the MIME Entity for Signing, Enveloping, or 106 Compressing . . . . . . . . . . . . . . . . . . . . . . . 20 107 3.1.1. Canonicalization . . . . . . . . . . . . . . . . . . 21 108 3.1.2. Transfer Encoding . . . . . . . . . . . . . . . . . . 22 109 3.1.3. Transfer Encoding for Signing Using multipart/signed 22 110 3.1.4. Sample Canonical MIME Entity . . . . . . . . . . . . 23 111 3.2. The application/pkcs7-mime Media Type . . . . . . . . . . 24 112 3.2.1. The name and filename Parameters . . . . . . . . . . 25 113 3.2.2. The smime-type Parameter . . . . . . . . . . . . . . 26 114 3.3. Creating an Enveloped-Only Message . . . . . . . . . . . 27 115 3.4. Creating an Authenticated Enveloped-Only Message . . . . 27 116 3.5. Creating a Signed-Only Message . . . . . . . . . . . . . 29 117 3.5.1. Choosing a Format for Signed-Only Messages . . . . . 29 118 3.5.2. Signing Using application/pkcs7-mime with SignedData 30 119 3.5.3. Signing Using the multipart/signed Format . . . . . . 31 120 3.6. Creating a Compressed-Only Message . . . . . . . . . . . 34 121 3.7. Multiple Operations . . . . . . . . . . . . . . . . . . . 34 122 3.8. Creating a Certificate Management Message . . . . . . . . 35 123 3.9. Registration Requests . . . . . . . . . . . . . . . . . . 36 124 3.10. Identifying an S/MIME Message . . . . . . . . . . . . . . 36 125 4. Certificate Processing . . . . . . . . . . . . . . . . . . . 36 126 4.1. Key Pair Generation . . . . . . . . . . . . . . . . . . . 37 127 4.2. Signature Generation . . . . . . . . . . . . . . . . . . 37 128 4.3. Signature Verification . . . . . . . . . . . . . . . . . 37 129 4.4. Encryption . . . . . . . . . . . . . . . . . . . . . . . 38 130 4.5. Decryption . . . . . . . . . . . . . . . . . . . . . . . 38 131 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 38 132 5.1. Media Type for application/pkcs7-mime . . . . . . . . . . 38 133 5.2. Media Type for application/pkcs7-signature . . . . . . . 39 134 5.3. Register authEnveloped-data smime-type . . . . . . . . . 40 135 6. IANA Considertions . . . . . . . . . . . . . . . . . . . . . 40 136 7. Security Considerations . . . . . . . . . . . . . . . . . . . 41 137 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 44 138 8.1. Normative References . . . . . . . . . . . . . . . . . . 44 139 8.2. Informative References . . . . . . . . . . . . . . . . . 48 140 Appendix A. ASN.1 Module . . . . . . . . . . . . . . . . . . . . 51 141 Appendix B. Historic Mail Considerations . . . . . . . . . . . . 53 142 B.1. DigestAlgorithmIdentifier . . . . . . . . . . . . . . . . 54 143 B.2. Signature Algorithms . . . . . . . . . . . . . . . . . . 54 144 B.3. ContentEncryptionAlgorithmIdentifier . . . . . . . . . . 56 145 B.4. KeyEncryptionAlgorithmIdentifier . . . . . . . . . . . . 56 146 Appendix C. Moving S/MIME v2 Message Specification to Historic 147 Status . . . . . . . . . . . . . . . . . . . . . . . 56 148 Appendix D. Acknowledgments . . . . . . . . . . . . . . . . . . 57 149 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 57 151 1. Introduction 153 S/MIME (Secure/Multipurpose Internet Mail Extensions) provides a 154 consistent way to send and receive secure MIME data. Based on the 155 popular Internet MIME standard, S/MIME provides the following 156 cryptographic security services for electronic messaging 157 applications: authentication, message integrity and non-repudiation 158 of origin (using digital signatures), and data confidentiality (using 159 encryption). As a supplementary service, S/MIME provides for message 160 compression. 162 S/MIME can be used by traditional mail user agents (MUAs) to add 163 cryptographic security services to mail that is sent, and to 164 interpret cryptographic security services in mail that is received. 165 However, S/MIME is not restricted to mail; it can be used with any 166 transport mechanism that transports MIME data, such as HTTP or SIP. 167 As such, S/MIME takes advantage of the object-based features of MIME 168 and allows secure messages to be exchanged in mixed-transport 169 systems. 171 Further, S/MIME can be used in automated message transfer agents that 172 use cryptographic security services that do not require any human 173 intervention, such as the signing of software-generated documents and 174 the encryption of FAX messages sent over the Internet. 176 1.1. Specification Overview 178 This document describes a protocol for adding cryptographic signature 179 and encryption services to MIME data. The MIME standard [MIME-SPEC] 180 provides a general structure for the content of Internet messages and 181 allows extensions for new content-type-based applications. 183 This specification defines how to create a MIME body part that has 184 been cryptographically enhanced according to the Cryptographic 185 Message Syntax (CMS) [CMS], which is derived from PKCS #7 [RFC2315]. 186 This specification also defines the application/pkcs7-mime media type 187 that can be used to transport those body parts. 189 This document also discusses how to use the multipart/signed media 190 type defined in [RFC1847] to transport S/MIME signed messages. 191 multipart/signed is used in conjunction with the 192 application/pkcs7-signature media type, which is used to transport a 193 detached S/MIME signature. 195 In order to create S/MIME messages, an S/MIME agent MUST follow the 196 specifications in this document, as well as the specifications listed 197 in the Cryptographic Message Syntax document [CMS], [RFC3370], 198 [RFC4056], [RFC3560], and [RFC5754]. 200 Throughout this specification, there are requirements and 201 recommendations made for how receiving agents handle incoming 202 messages. There are separate requirements and recommendations for 203 how sending agents create outgoing messages. In general, the best 204 strategy is to "be liberal in what you receive and conservative in 205 what you send". Most of the requirements are placed on the handling 206 of incoming messages, while the recommendations are mostly on the 207 creation of outgoing messages. 209 The separation for requirements on receiving agents and sending 210 agents also derives from the likelihood that there will be S/MIME 211 systems that involve software other than traditional Internet mail 212 clients. S/MIME can be used with any system that transports MIME 213 data. An automated process that sends an encrypted message might not 214 be able to receive an encrypted message at all, for example. Thus, 215 the requirements and recommendations for the two types of agents are 216 listed separately when appropriate. 218 1.2. Definitions 220 For the purposes of this specification, the following definitions 221 apply. 223 ASN.1: Abstract Syntax Notation One, as defined in ITU-T 224 Recommendations X.680, X.681, X.682 and X.683 225 [ASN.1]. 227 BER: Basic Encoding Rules for ASN.1, as defined in ITU- 228 T Recommendation X.690 [X.690]. 230 Certificate: A type that binds an entity's name to a public key 231 with a digital signature. 233 DER: Distinguished Encoding Rules for ASN.1, as defined 234 in ITU-T Recommendation X.690 [X.690]. 236 7-bit data: Text data with lines less than 998 characters 237 long, where none of the characters have the 8th 238 bit set, and there are no NULL characters. 239 and occur only as part of a end-of- 240 line delimiter. 242 8-bit data: Text data with lines less than 998 characters, and 243 where none of the characters are NULL characters. 244 and occur only as part of a 245 end-of-line delimiter. 247 Binary data: Arbitrary data. 249 Transfer encoding: A reversible transformation made on data so 8-bit 250 or binary data can be sent via a channel that only 251 transmits 7-bit data. 253 Receiving agent: Software that interprets and processes S/MIME CMS 254 objects, MIME body parts that contain CMS content 255 types, or both. 257 Sending agent: Software that creates S/MIME CMS content types, 258 MIME body parts that contain CMS content types, or 259 both. 261 S/MIME agent: User software that is a receiving agent, a sending 262 agent, or both. 264 Data Integrity Service: A security service that protects againist 265 unauthorized changes to data by ensuring that 266 changes to the data are detectable. [RFC4949] 268 Data Confidentiality: The property that data is not discolsed to 269 system entities unless they have been authorized 270 to know the data. [RFC4949] 272 Data Origination: The corroboration that the source of the data 273 received is as claimed. [RFC4949]. 275 1.3. Conventions Used in This Document 277 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 278 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 279 document are to be interpreted as described in [RFC2119]. 281 We define the additional requirement levels: 283 SHOULD+ This term means the same as SHOULD. However, the authors 284 expect that a requirement marked as SHOULD+ will be 285 promoted at some future time to be a MUST. 287 SHOULD- This term means the same as SHOULD. However, the authors 288 expect that a requirement marked as SHOULD- will be demoted 289 to a MAY in a future version of this document. 291 MUST- This term means the same as MUST. However, the authors 292 expect that this requirement will no longer be a MUST in a 293 future document. Although its status will be determined at 294 a later time, it is reasonable to expect that if a future 295 revision of a document alters the status of a MUST- 296 requirement, it will remain at least a SHOULD or a SHOULD-. 298 The term RSA in this document almost always refers to the PKCS#1 v1.5 299 RSA signature or encryption algorithms even when not qualified as 300 such. There are a couple of places where it refers to the general 301 RSA cryptographic operation, these can be determined from the context 302 where it is used. 304 1.4. Compatibility with Prior Practice of S/MIME 306 S/MIME version 4.0 agents ought to attempt to have the greatest 307 interoperability possible with agents for prior versions of S/MIME. 308 S/MIME version 2 is described in RFC 2311 through RFC 2315 inclusive 309 [SMIMEv2], S/MIME version 3 is described in RFC 2630 through RFC 2634 310 inclusive and RFC 5035 [SMIMEv3], S/MIME version 3.1 is described in 311 RFC 3850, RFC 3851, RFC 3852, RFC 2634, and RFC 5035 [SMIMEv3.1], and 312 S/MIME version 3.2 is described in [SMIMEv3.2]. [RFC2311] also has 313 historical information about the development of S/MIME. 315 1.5. Changes from S/MIME v3 to S/MIME v3.1 317 The RSA public key algorithm was changed to a MUST implement key 318 wrapping algorithm, and the Diffie-Hellman (DH) algorithm changed to 319 a SHOULD implement. 321 The AES symmetric encryption algorithm has been included as a SHOULD 322 implement. 324 The RSA public key algorithm was changed to a MUST implement 325 signature algorithm. 327 Ambiguous language about the use of "empty" SignedData messages to 328 transmit certificates was clarified to reflect that transmission of 329 Certificate Revocation Lists is also allowed. 331 The use of binary encoding for some MIME entities is now explicitly 332 discussed. 334 Header protection through the use of the message/rfc822 media type 335 has been added. 337 Use of the CompressedData CMS type is allowed, along with required 338 media type and file extension additions. 340 1.6. Changes from S/MIME v3.1 to S/MIME v3.2 342 Editorial changes, e.g., replaced "MIME type" with "media type", 343 content-type with Content-Type. 345 Moved "Conventions Used in This Document" to Section 1.3. Added 346 definitions for SHOULD+, SHOULD-, and MUST-. 348 Section 1.1 and Appendix A: Added references to RFCs for RSASSA-PSS, 349 RSAES-OAEP, and SHA2 CMS algorithms. Added CMS Multiple Signers 350 Clarification to CMS reference. 352 Section 1.2: Updated references to ASN.1 to X.680 and BER and DER to 353 X.690. 355 Section 1.4: Added references to S/MIME MSG 3.1 RFCs. 357 Section 2.1 (digest algorithm): SHA-256 added as MUST, SHA-1 and MD5 358 made SHOULD-. 360 Section 2.2 (signature algorithms): RSA with SHA-256 added as MUST, 361 and DSA with SHA-256 added as SHOULD+, RSA with SHA-1, DSA with 362 SHA-1, and RSA with MD5 changed to SHOULD-, and RSASSA-PSS with 363 SHA-256 added as SHOULD+. Also added note about what S/MIME v3.1 364 clients support. 366 Section 2.3 (key encryption): DH changed to SHOULD-, and RSAES-OAEP 367 added as SHOULD+. Elaborated requirements for key wrap algorithm. 369 Section 2.5.1: Added requirement that receiving agents MUST support 370 both GeneralizedTime and UTCTime. 372 Section 2.5.2: Replaced reference "sha1WithRSAEncryption" with 373 "sha256WithRSAEncryption", "DES-3EDE-CBC" with "AES-128 CBC", and 374 deleted the RC5 example. 376 Section 2.5.2.1: Deleted entire section (discussed deprecated RC2). 378 Section 2.7, 2.7.1, Appendix A: references to RC2/40 removed. 380 Section 2.7 (content encryption): AES-128 CBC added as MUST, AES-192 381 and AES-256 CBC SHOULD+, tripleDES now SHOULD-. 383 Section 2.7.1: Updated pointers from 2.7.2.1 through 2.7.2.4 to 384 2.7.1.1 to 2.7.1.2. 386 Section 3.1.1: Removed text about MIME character sets. 388 Section 3.2.2 and 3.6: Replaced "encrypted" with "enveloped". Update 389 OID example to use AES-128 CBC oid. 391 Section 3.4.3.2: Replace micalg parameter for SHA-1 with sha-1. 393 Section 4: Updated reference to CERT v3.2. 395 Section 4.1: Updated RSA and DSA key size discussion. Moved last 396 four sentences to security considerations. Updated reference to 397 randomness requirements for security. 399 Section 5: Added IANA registration templates to update media type 400 registry to point to this document as opposed to RFC 2311. 402 Section 7: Updated security considerations. 404 Section 7: Moved references from Appendix B to this section. Updated 405 references. Added informational references to SMIMEv2, SMIMEv3, and 406 SMIMEv3.1. 408 Appendix C: Added Appendix C to move S/MIME v2 to Historic status. 410 1.7. Changes for S/MIME v4.0 412 - Add the use of AuthEnvelopedData, including defining and 413 registering an smime-type value (Section 2.4.4 and Section 3.4). 415 - Update the content encryption algorithms (Section 2.7 and 416 Section 2.7.1.2): Add AES-256 GCM, add ChaCha200-Poly1305, remove 417 AES-192 CBC, mark tripleDES as historic. 419 - Update the set of signature algorithms (Section 2.2): Add EdDSA 420 and ECDSA, mark DSA as historic 422 - Update the set of digest algorithms (Section 2.1): Add SHA-512, 423 mark SHA-1 as historic. 425 - Update the size of keys to be used for RSA encryption and RSA 426 signing (Section 4). 428 - Create Appendix B which deals with considerations for dealing with 429 historic email messages. 431 2. CMS Options 433 CMS allows for a wide variety of options in content, attributes, and 434 algorithm support. This section puts forth a number of support 435 requirements and recommendations in order to achieve a base level of 436 interoperability among all S/MIME implementations. [RFC3370] and 437 [RFC5754] provides additional details regarding the use of the 438 cryptographic algorithms. [ESS] provides additional details 439 regarding the use of additional attributes. 441 2.1. DigestAlgorithmIdentifier 443 The algorithms here are used for digesting the body of the message 444 and are not the same as the digest algorithms used as part the 445 signature algorithms. The result of this is placed in the message- 446 digest attribute of the signed attributes. It is RECOMMENDED that 447 the algorithm used for digesting the body of the message be of 448 similar or greater strength than the signature algorithm. 450 Sending and Receiving agents: 452 - MUST support SHA-256. 454 - MUST support SHA-512. 456 [RFC5754] provides the details for using these algorithms with 457 S/MIME. 459 2.2. SignatureAlgorithmIdentifier 461 Receiving agents: 463 - MUST support ECDSA with curve P-256 and SHA-256. 465 - MUST support EdDSA with curve 25519 using PureEdDSA mode. 467 - MUST- support RSA with SHA-256. 469 - SHOULD support RSASSA-PSS with SHA-256. 471 - MUST NOT support EdDSA using the pre-hash mode. 473 Sending agents: 475 - MUST support at least one of the following algorithms: ECDSA with 476 curve P-256 and SHA-256, or EdDSA with curve 25519 using PureEdDSA 477 mode. 479 - MUST- support RSA with SHA-256. 481 - SHOULD support RSASSA-PSS with SHA-256. 483 - MUST NOT support EdDSA using the pre-hash mode. 485 Both ECDSA and EdDSA are included in the list of required algorithms 486 for political reasons. NIST is unable to provide the seeds that were 487 used to create their standardized curves, this means that there is a 488 section of the community which believes that there might be a 489 backdoor to these curves. The EdDSA curves were, in part, created in 490 response to this feeling. However, there are still significant 491 sections of the industry which need to have NIST approved algorithms. 492 For this reason, both sets of curves are represented in the recieving 493 agent list, but there is only a requirement for curve in the sending 494 agent list. 496 See Section 4.1 for information on key size and algorithm references. 498 2.3. KeyEncryptionAlgorithmIdentifier 500 Receiving and sending agents: 502 - MUST support ECDH ephemeral-static mode for P-256, as specified in 503 [RFC5753]. 505 - MUST support ECDH ephemeral-static mode for X25519 using HKDF-256 506 for the KDF, as specified in 507 [I-D.ietf-curdle-cms-ecdh-new-curves]. 509 - MUST- support RSA Encryption, as specified in [RFC3370]. 511 - SHOULD+ support RSAES-OAEP, as specified in [RFC3560]. 513 When ECDH ephemeral-static is used, a key wrap algorithm is also 514 specified in the KeyEncryptionAlgorithmIdentifier [RFC5652]. The 515 underlying encryption functions for the key wrap and content 516 encryption algorithm ([RFC3370] and [RFC3565]) and the key sizes for 517 the two algorithms MUST be the same (e.g., AES-128 key wrap algorithm 518 with AES-128 content encryption algorithm). As both 128 and 256 bit 519 AES modes are mandatory-to-implment as content encryption algorithms 520 (Section 2.7), both the AES-128 and AES-256 key wrap algorithms MUST 521 be supported when ECDH ephemeral-static is used. 523 Appendix B provides information on algorithms support in older 524 versions of S/MIME. 526 2.4. General Syntax 528 There are several CMS content types. Of these, only the Data, 529 SignedData, EnvelopedData, AuthEnvelopedData, and CompressedData 530 content types are currently used for S/MIME. 532 2.4.1. Data Content Type 534 Sending agents MUST use the id-data content type identifier to 535 identify the "inner" MIME message content. For example, when 536 applying a digital signature to MIME data, the CMS SignedData 537 encapContentInfo eContentType MUST include the id-data object 538 identifier and the media type MUST be stored in the SignedData 539 encapContentInfo eContent OCTET STRING (unless the sending agent is 540 using multipart/signed, in which case the eContent is absent, per 541 Section 3.5.3 of this document). As another example, when applying 542 encryption to MIME data, the CMS EnvelopedData encryptedContentInfo 543 contentType MUST include the id-data object identifier and the 544 encrypted MIME content MUST be stored in the EnvelopedData 545 encryptedContentInfo encryptedContent OCTET STRING. 547 2.4.2. SignedData Content Type 549 Sending agents MUST use the SignedData content type to apply a 550 digital signature to a message or, in a degenerate case where there 551 is no signature information, to convey certificates. Applying a 552 signature to a message provides authentication, message integrity, 553 and non-repudiation of origin. 555 2.4.3. EnvelopedData Content Type 557 This content type is used to apply data confidentiality to a message. 558 A sender needs to have access to a public key for each intended 559 message recipient to use this service. 561 2.4.4. AuthEnvelopedData Content Type 563 This content type is used to apply data confidentiality and message 564 integrity to a message. This content type does not provide 565 authentication or non-repudiation. A sender needs to have access to 566 a public key for each intended message recipient to use this service. 568 2.4.5. CompressedData Content Type 570 This content type is used to apply data compression to a message. 571 This content type does not provide authentication, message integrity, 572 non-repudiation, or data confidentiality, and is only used to reduce 573 the message's size. 575 See Section 3.7 for further guidance on the use of this type in 576 conjunction with other CMS types. 578 2.5. Attributes and the SignerInfo Type 580 The SignerInfo type allows the inclusion of unsigned and signed 581 attributes along with a signature. 583 Receiving agents MUST be able to handle zero or one instance of each 584 of the signed attributes listed here. Sending agents SHOULD generate 585 one instance of each of the following signed attributes in each 586 S/MIME message: 588 - Signing Time (Section 2.5.1 in this document) 590 - SMIME Capabilities (Section 2.5.2 in this document) 592 - Encryption Key Preference (Section 2.5.3 in this document) 594 - Message Digest (Section 11.2 in [RFC5652]) 596 - Content Type (Section 11.1 in [RFC5652]) 598 Further, receiving agents SHOULD be able to handle zero or one 599 instance of the signingCertificate and signingCertificatev2 signed 600 attributes, as defined in Section 5 of RFC 2634 [ESS] and Section 3 601 of RFC 5035 [ESS]. 603 Sending agents SHOULD generate one instance of the signingCertificate 604 or signingCertificatev2 signed attribute in each SignerInfo 605 structure. 607 Additional attributes and values for these attributes might be 608 defined in the future. Receiving agents SHOULD handle attributes or 609 values that they do not recognize in a graceful manner. 611 Interactive sending agents that include signed attributes that are 612 not listed here SHOULD display those attributes to the user, so that 613 the user is aware of all of the data being signed. 615 2.5.1. Signing Time Attribute 617 The signing-time attribute is used to convey the time that a message 618 was signed. The time of signing will most likely be created by a 619 message originator and therefore is only as trustworthy as the 620 originator. 622 Sending agents MUST encode signing time through the year 2049 as 623 UTCTime; signing times in 2050 or later MUST be encoded as 624 GeneralizedTime. When the UTCTime CHOICE is used, S/MIME agents MUST 625 interpret the year field (YY) as follows: 627 If YY is greater than or equal to 50, the year is interpreted as 628 19YY; if YY is less than 50, the year is interpreted as 20YY. 630 Receiving agents MUST be able to process signing-time attributes that 631 are encoded in either UTCTime or GeneralizedTime. 633 2.5.2. SMIME Capabilities Attribute 635 The SMIMECapabilities attribute includes signature algorithms (such 636 as "sha256WithRSAEncryption"), symmetric algorithms (such as "AES-128 637 CBC"), authenticated symmetric algorithms (such as "AES-128 GCM") and 638 key encipherment algorithms (such as "rsaEncryption"). The presence 639 of an algorthm based SMIME Capability attribute in this sequence 640 implies that the sender can deal with the algorithm as well as 641 undertanding the ASN.1 structures associated with that algorithm. 642 There are also several identifiers that indicate support for other 643 optional features such as binary encoding and compression. The 644 SMIMECapabilities were designed to be flexible and extensible so 645 that, in the future, a means of identifying other capabilities and 646 preferences such as certificates can be added in a way that will not 647 cause current clients to break. 649 If present, the SMIMECapabilities attribute MUST be a 650 SignedAttribute; it MUST NOT be an UnsignedAttribute. CMS defines 651 SignedAttributes as a SET OF Attribute. The SignedAttributes in a 652 signerInfo MUST NOT include multiple instances of the 653 SMIMECapabilities attribute. CMS defines the ASN.1 syntax for 654 Attribute to include attrValues SET OF AttributeValue. A 655 SMIMECapabilities attribute MUST only include a single instance of 656 AttributeValue. There MUST NOT be zero or multiple instances of 657 AttributeValue present in the attrValues SET OF AttributeValue. 659 The semantics of the SMIMECapabilities attribute specify a partial 660 list as to what the client announcing the SMIMECapabilities can 661 support. A client does not have to list every capability it 662 supports, and need not list all its capabilities so that the 663 capabilities list doesn't get too long. In an SMIMECapabilities 664 attribute, the object identifiers (OIDs) are listed in order of their 665 preference, but SHOULD be separated logically along the lines of 666 their categories (signature algorithms, symmetric algorithms, key 667 encipherment algorithms, etc.). 669 The structure of the SMIMECapabilities attribute is to facilitate 670 simple table lookups and binary comparisons in order to determine 671 matches. For instance, the DER-encoding for the SMIMECapability for 672 AES-128 CBC MUST be identically encoded regardless of the 673 implementation. Because of the requirement for identical encoding, 674 individuals documenting algorithms to be used in the 675 SMIMECapabilities attribute SHOULD explicitly document the correct 676 byte sequence for the common cases. 678 For any capability, the associated parameters for the OID MUST 679 specify all of the parameters necessary to differentiate between two 680 instances of the same algorithm. 682 The OIDs that correspond to algorithms SHOULD use the same OID as the 683 actual algorithm, except in the case where the algorithm usage is 684 ambiguous from the OID. For instance, in an earlier specification, 685 rsaEncryption was ambiguous because it could refer to either a 686 signature algorithm or a key encipherment algorithm. In the event 687 that an OID is ambiguous, it needs to be arbitrated by the maintainer 688 of the registered SMIMECapabilities list as to which type of 689 algorithm will use the OID, and a new OID MUST be allocated under the 690 smimeCapabilities OID to satisfy the other use of the OID. 692 The registered SMIMECapabilities list specifies the parameters for 693 OIDs that need them, most notably key lengths in the case of 694 variable-length symmetric ciphers. In the event that there are no 695 differentiating parameters for a particular OID, the parameters MUST 696 be omitted, and MUST NOT be encoded as NULL. Additional values for 697 the SMIMECapabilities attribute might be defined in the future. 698 Receiving agents MUST handle a SMIMECapabilities object that has 699 values that it does not recognize in a graceful manner. 701 Section 2.7.1 explains a strategy for caching capabilities. 703 2.5.3. Encryption Key Preference Attribute 705 The encryption key preference attribute allows the signer to 706 unambiguously describe which of the signer's certificates has the 707 signer's preferred encryption key. This attribute is designed to 708 enhance behavior for interoperating with those clients that use 709 separate keys for encryption and signing. This attribute is used to 710 convey to anyone viewing the attribute which of the listed 711 certificates is appropriate for encrypting a session key for future 712 encrypted messages. 714 If present, the SMIMEEncryptionKeyPreference attribute MUST be a 715 SignedAttribute; it MUST NOT be an UnsignedAttribute. CMS defines 716 SignedAttributes as a SET OF Attribute. The SignedAttributes in a 717 signerInfo MUST NOT include multiple instances of the 718 SMIMEEncryptionKeyPreference attribute. CMS defines the ASN.1 syntax 719 for Attribute to include attrValues SET OF AttributeValue. A 720 SMIMEEncryptionKeyPreference attribute MUST only include a single 721 instance of AttributeValue. There MUST NOT be zero or multiple 722 instances of AttributeValue present in the attrValues SET OF 723 AttributeValue. 725 The sending agent SHOULD include the referenced certificate in the 726 set of certificates included in the signed message if this attribute 727 is used. The certificate MAY be omitted if it has been previously 728 made available to the receiving agent. Sending agents SHOULD use 729 this attribute if the commonly used or preferred encryption 730 certificate is not the same as the certificate used to sign the 731 message. 733 Receiving agents SHOULD store the preference data if the signature on 734 the message is valid and the signing time is greater than the 735 currently stored value. (As with the SMIMECapabilities, the clock 736 skew SHOULD be checked and the data not used if the skew is too 737 great.) Receiving agents SHOULD respect the sender's encryption key 738 preference attribute if possible. This, however, represents only a 739 preference and the receiving agent can use any certificate in 740 replying to the sender that is valid. 742 Section 2.7.1 explains a strategy for caching preference data. 744 2.5.3.1. Selection of Recipient Key Management Certificate 746 In order to determine the key management certificate to be used when 747 sending a future CMS EnvelopedData message for a particular 748 recipient, the following steps SHOULD be followed: 750 - If an SMIMEEncryptionKeyPreference attribute is found in a 751 SignedData object received from the desired recipient, this 752 identifies the X.509 certificate that SHOULD be used as the X.509 753 key management certificate for the recipient. 755 - If an SMIMEEncryptionKeyPreference attribute is not found in a 756 SignedData object received from the desired recipient, the set of 757 X.509 certificates SHOULD be searched for a X.509 certificate with 758 the same subject name as the signer of a X.509 certificate that 759 can be used for key management. 761 - Or use some other method of determining the user's key management 762 key. If a X.509 key management certificate is not found, then 763 encryption cannot be done with the signer of the message. If 764 multiple X.509 key management certificates are found, the S/MIME 765 agent can make an arbitrary choice between them. 767 2.6. SignerIdentifier SignerInfo Type 769 S/MIME v4.0 implementations MUST support both issuerAndSerialNumber 770 and subjectKeyIdentifier. Messages that use the subjectKeyIdentifier 771 choice cannot be read by S/MIME v2 clients. 773 It is important to understand that some certificates use a value for 774 subjectKeyIdentifier that is not suitable for uniquely identifying a 775 certificate. Implementations MUST be prepared for multiple 776 certificates for potentially different entities to have the same 777 value for subjectKeyIdentifier, and MUST be prepared to try each 778 matching certificate during signature verification before indicating 779 an error condition. 781 2.7. ContentEncryptionAlgorithmIdentifier 783 Sending and receiving agents: 785 - MUST support encryption and decryption with AES-128 GCM and 786 AES-256 GCM [RFC5084]. 788 - MUST- support encryption and decryption with AES-128 CBC 789 [RFC3565]. 791 - SHOULD+ support encryption and decryption with ChaCha20-Poly1305 792 [RFC7905]. 794 2.7.1. Deciding Which Encryption Method to Use 796 When a sending agent creates an encrypted message, it has to decide 797 which type of encryption to use. The decision process involves using 798 information garnered from the capabilities lists included in messages 799 received from the recipient, as well as out-of-band information such 800 as private agreements, user preferences, legal restrictions, and so 801 on. 803 Section 2.5.2 defines a method by which a sending agent can 804 optionally announce, among other things, its decrypting capabilities 805 in its order of preference. The following method for processing and 806 remembering the encryption capabilities attribute in incoming signed 807 messages SHOULD be used. 809 - If the receiving agent has not yet created a list of capabilities 810 for the sender's public key, then, after verifying the signature 811 on the incoming message and checking the timestamp, the receiving 812 agent SHOULD create a new list containing at least the signing 813 time and the symmetric capabilities. 815 - If such a list already exists, the receiving agent SHOULD verify 816 that the signing time in the incoming message is greater than the 817 signing time stored in the list and that the signature is valid. 818 If so, the receiving agent SHOULD update both the signing time and 819 capabilities in the list. Values of the signing time that lie far 820 in the future (that is, a greater discrepancy than any reasonable 821 clock skew), or a capabilities list in messages whose signature 822 could not be verified, MUST NOT be accepted. 824 The list of capabilities SHOULD be stored for future use in creating 825 messages. 827 Before sending a message, the sending agent MUST decide whether it is 828 willing to use weak encryption for the particular data in the 829 message. If the sending agent decides that weak encryption is 830 unacceptable for this data, then the sending agent MUST NOT use a 831 weak algorithm. The decision to use or not use weak encryption 832 overrides any other decision in this section about which encryption 833 algorithm to use. 835 Section 2.7.1.1 and Section 2.7.1.2 describe the decisions a sending 836 agent SHOULD use in deciding which type of encryption will be applied 837 to a message. These rules are ordered, so the sending agent SHOULD 838 make its decision in the order given. 840 2.7.1.1. Rule 1: Known Capabilities 842 If the sending agent has received a set of capabilities from the 843 recipient for the message the agent is about to encrypt, then the 844 sending agent SHOULD use that information by selecting the first 845 capability in the list (that is, the capability most preferred by the 846 intended recipient) that the sending agent knows how to encrypt. The 847 sending agent SHOULD use one of the capabilities in the list if the 848 agent reasonably expects the recipient to be able to decrypt the 849 message. 851 2.7.1.2. Rule 2: Unknown Capabilities, Unknown Version of S/MIME 853 If the following two conditions are met: 855 - the sending agent has no knowledge of the encryption capabilities 856 of the recipient, and 858 - the sending agent has no knowledge of the version of S/MIME of the 859 recipient, 861 then the sending agent SHOULD use AES-256 GCM because it is a 862 stronger algorithm and is required by S/MIME v4.0. If the sending 863 agent chooses not to use AES-256 GCM in this step, it SHOULD use 864 AES-128 CBC. 866 2.7.2. Choosing Weak Encryption 868 All algorithms that use 112-bit keys are considered by many to be 869 weak encryption. A sending agent that is controlled by a human 870 SHOULD allow a human sender to determine the risks of sending data 871 using a weak encryption algorithm before sending the data, and 872 possibly allow the human to use a stronger encryption method such as 873 AES GCM or AES CBC. 875 2.7.3. Multiple Recipients 877 If a sending agent is composing an encrypted message to a group of 878 recipients where the encryption capabilities of some of the 879 recipients do not overlap, the sending agent is forced to send more 880 than one message. Please note that if the sending agent chooses to 881 send a message encrypted with a strong algorithm, and then send the 882 same message encrypted with a weak algorithm, someone watching the 883 communications channel could learn the contents of the strongly 884 encrypted message simply by decrypting the weakly encrypted message. 886 3. Creating S/MIME Messages 888 This section describes the S/MIME message formats and how they are 889 created. S/MIME messages are a combination of MIME bodies and CMS 890 content types. Several media types as well as several CMS content 891 types are used. The data to be secured is always a canonical MIME 892 entity. The MIME entity and other data, such as certificates and 893 algorithm identifiers, are given to CMS processing facilities that 894 produce a CMS object. Finally, the CMS object is wrapped in MIME. 895 The Enhanced Security Services for S/MIME [ESS] document provides 896 descriptions of how nested, secured S/MIME messages are formatted. 897 ESS provides a description of how a triple-wrapped S/MIME message is 898 formatted using multipart/signed and application/pkcs7-mime for the 899 signatures. 901 S/MIME provides one format for enveloped-only data, several formats 902 for signed-only data, and several formats for signed and enveloped 903 data. Several formats are required to accommodate several 904 environments, in particular for signed messages. The criteria for 905 choosing among these formats are also described. 907 The reader of this section is expected to understand MIME as 908 described in [MIME-SPEC] and [RFC1847]. 910 3.1. Preparing the MIME Entity for Signing, Enveloping, or Compressing 912 S/MIME is used to secure MIME entities. A MIME entity can be a sub- 913 part, sub-parts of a message, or the whole message with all its sub- 914 parts. A MIME entity that is the whole message includes only the 915 MIME message headers and MIME body, and does not include the RFC-822 916 header. Note that S/MIME can also be used to secure MIME entities 917 used in applications other than Internet mail. If protection of the 918 RFC-822 header is required, the use of the message/rfc822 media type 919 is explained later in this section. 921 The MIME entity that is secured and described in this section can be 922 thought of as the "inside" MIME entity. That is, it is the 923 "innermost" object in what is possibly a larger MIME message. 924 Processing "outside" MIME entities into CMS content types is 925 described in Section 3.2, Section 3.5, and elsewhere. 927 The procedure for preparing a MIME entity is given in [MIME-SPEC]. 928 The same procedure is used here with some additional restrictions 929 when signing. The description of the procedures from [MIME-SPEC] is 930 repeated here, but it is suggested that the reader refer to that 931 document for the exact procedure. This section also describes 932 additional requirements. 934 A single procedure is used for creating MIME entities that are to 935 have any combination of signing, enveloping, and compressing applied. 936 Some additional steps are recommended to defend against known 937 corruptions that can occur during mail transport that are of 938 particular importance for clear-signing using the multipart/signed 939 format. It is recommended that these additional steps be performed 940 on enveloped messages, or signed and enveloped messages, so that the 941 message can be forwarded to any environment without modification. 943 These steps are descriptive rather than prescriptive. The 944 implementer is free to use any procedure as long as the result is the 945 same. 947 Step 1. The MIME entity is prepared according to the local 948 conventions. 950 Step 2. The leaf parts of the MIME entity are converted to canonical 951 form. 953 Step 3. Appropriate transfer encoding is applied to the leaves of 954 the MIME entity. 956 When an S/MIME message is received, the security services on the 957 message are processed, and the result is the MIME entity. That MIME 958 entity is typically passed to a MIME-capable user agent where it is 959 further decoded and presented to the user or receiving application. 961 In order to protect outer, non-content-related message header fields 962 (for instance, the "Subject", "To", "From", and "Cc" fields), the 963 sending client MAY wrap a full MIME message in a message/rfc822 964 wrapper in order to apply S/MIME security services to these header 965 fields. It is up to the receiving client to decide how to present 966 this "inner" header along with the unprotected "outer" header. 968 When an S/MIME message is received, if the top-level protected MIME 969 entity has a Content-Type of message/rfc822, it can be assumed that 970 the intent was to provide header protection. This entity SHOULD be 971 presented as the top-level message, taking into account header 972 merging issues as previously discussed. 974 3.1.1. Canonicalization 976 Each MIME entity MUST be converted to a canonical form that is 977 uniquely and unambiguously representable in the environment where the 978 signature is created and the environment where the signature will be 979 verified. MIME entities MUST be canonicalized for enveloping and 980 compressing as well as signing. 982 The exact details of canonicalization depend on the actual media type 983 and subtype of an entity, and are not described here. Instead, the 984 standard for the particular media type SHOULD be consulted. For 985 example, canonicalization of type text/plain is different from 986 canonicalization of audio/basic. Other than text types, most types 987 have only one representation regardless of computing platform or 988 environment that can be considered their canonical representation. 989 In general, canonicalization will be performed by the non-security 990 part of the sending agent rather than the S/MIME implementation. 992 The most common and important canonicalization is for text, which is 993 often represented differently in different environments. MIME 994 entities of major type "text" MUST have both their line endings and 995 character set canonicalized. The line ending MUST be the pair of 996 characters , and the charset SHOULD be a registered charset 997 [CHARSETS]. The details of the canonicalization are specified in 998 [MIME-SPEC]. 1000 Note that some charsets such as ISO-2022 have multiple 1001 representations for the same characters. When preparing such text 1002 for signing, the canonical representation specified for the charset 1003 MUST be used. 1005 3.1.2. Transfer Encoding 1007 When generating any of the secured MIME entities below, except the 1008 signing using the multipart/signed format, no transfer encoding is 1009 required at all. S/MIME implementations MUST be able to deal with 1010 binary MIME objects. If no Content-Transfer-Encoding header field is 1011 present, the transfer encoding is presumed to be 7BIT. 1013 S/MIME implementations SHOULD however use transfer encoding described 1014 in Section 3.1.3 for all MIME entities they secure. The reason for 1015 securing only 7-bit MIME entities, even for enveloped data that are 1016 not exposed to the transport, is that it allows the MIME entity to be 1017 handled in any environment without changing it. For example, a 1018 trusted gateway might remove the envelope, but not the signature, of 1019 a message, and then forward the signed message on to the end 1020 recipient so that they can verify the signatures directly. If the 1021 transport internal to the site is not 8-bit clean, such as on a wide- 1022 area network with a single mail gateway, verifying the signature will 1023 not be possible unless the original MIME entity was only 7-bit data. 1025 S/MIME implementations that "know" that all intended recipients are 1026 capable of handling inner (all but the outermost) binary MIME objects 1027 SHOULD use binary encoding as opposed to a 7-bit-safe transfer 1028 encoding for the inner entities. The use of a 7-bit-safe encoding 1029 (such as base64) would unnecessarily expand the message size. 1030 Implementations MAY "know" that recipient implementations are capable 1031 of handling inner binary MIME entities either by interpreting the id- 1032 cap-preferBinaryInside SMIMECapabilities attribute, by prior 1033 agreement, or by other means. 1035 If one or more intended recipients are unable to handle inner binary 1036 MIME objects, or if this capability is unknown for any of the 1037 intended recipients, S/MIME implementations SHOULD use transfer 1038 encoding described in Section 3.1.3 for all MIME entities they 1039 secure. 1041 3.1.3. Transfer Encoding for Signing Using multipart/signed 1043 If a multipart/signed entity is ever to be transmitted over the 1044 standard Internet SMTP infrastructure or other transport that is 1045 constrained to 7-bit text, it MUST have transfer encoding applied so 1046 that it is represented as 7-bit text. MIME entities that are 7-bit 1047 data already need no transfer encoding. Entities such as 8-bit text 1048 and binary data can be encoded with quoted-printable or base-64 1049 transfer encoding. 1051 The primary reason for the 7-bit requirement is that the Internet 1052 mail transport infrastructure cannot guarantee transport of 8-bit or 1053 binary data. Even though many segments of the transport 1054 infrastructure now handle 8-bit and even binary data, it is sometimes 1055 not possible to know whether the transport path is 8-bit clean. If a 1056 mail message with 8-bit data were to encounter a message transfer 1057 agent that cannot transmit 8-bit or binary data, the agent has three 1058 options, none of which are acceptable for a clear-signed message: 1060 - The agent could change the transfer encoding; this would 1061 invalidate the signature. 1063 - The agent could transmit the data anyway, which would most likely 1064 result in the 8th bit being corrupted; this too would invalidate 1065 the signature. 1067 - The agent could return the message to the sender. 1069 [RFC1847] prohibits an agent from changing the transfer encoding of 1070 the first part of a multipart/signed message. If a compliant agent 1071 that cannot transmit 8-bit or binary data encounters a 1072 multipart/signed message with 8-bit or binary data in the first part, 1073 it would have to return the message to the sender as undeliverable. 1075 3.1.4. Sample Canonical MIME Entity 1077 This example shows a multipart/mixed message with full transfer 1078 encoding. This message contains a text part and an attachment. The 1079 sample message text includes characters that are not ASCII and thus 1080 need to be transfer encoded. Though not shown here, the end of each 1081 line is . The line ending of the MIME headers, the text, and 1082 the transfer encoded parts, all MUST be . 1084 Note that this example is not of an S/MIME message. 1086 Content-Type: multipart/mixed; boundary=bar 1088 --bar 1089 Content-Type: text/plain; charset=iso-8859-1 1090 Content-Transfer-Encoding: quoted-printable 1092 =A1Hola Michael! 1094 How do you like the new S/MIME specification? 1096 It's generally a good idea to encode lines that begin with 1097 From=20because some mail transport agents will insert a greater- 1098 than (>) sign, thus invalidating the signature. 1100 Also, in some cases it might be desirable to encode any =20 1101 trailing whitespace that occurs on lines in order to ensure =20 1102 that the message signature is not invalidated when passing =20 1103 a gateway that modifies such whitespace (like BITNET). =20 1105 --bar 1106 Content-Type: image/jpeg 1107 Content-Transfer-Encoding: base64 1109 iQCVAwUBMJrRF2N9oWBghPDJAQE9UQQAtl7LuRVndBjrk4EqYBIb3h5QXIX/LC// 1110 jJV5bNvkZIGPIcEmI5iFd9boEgvpirHtIREEqLQRkYNoBActFBZmh9GC3C041WGq 1111 uMbrbxc+nIs1TIKlA08rVi9ig/2Yh7LFrK5Ein57U/W72vgSxLhe/zhdfolT9Brn 1112 HOxEa44b+EI= 1114 --bar-- 1116 3.2. The application/pkcs7-mime Media Type 1118 The application/pkcs7-mime media type is used to carry CMS content 1119 types including EnvelopedData, SignedData, and CompressedData. The 1120 details of constructing these entities are described in subsequent 1121 sections. This section describes the general characteristics of the 1122 application/pkcs7-mime media type. 1124 The carried CMS object always contains a MIME entity that is prepared 1125 as described in Section 3.1 if the eContentType is id-data. Other 1126 contents MAY be carried when the eContentType contains different 1127 values. See [ESS] for an example of this with signed receipts. 1129 Since CMS content types are binary data, in most cases base-64 1130 transfer encoding is appropriate, in particular, when used with SMTP 1131 transport. The transfer encoding used depends on the transport 1132 through which the object is to be sent, and is not a characteristic 1133 of the media type. 1135 Note that this discussion refers to the transfer encoding of the CMS 1136 object or "outside" MIME entity. It is completely distinct from, and 1137 unrelated to, the transfer encoding of the MIME entity secured by the 1138 CMS object, the "inside" object, which is described in Section 3.1. 1140 Because there are several types of application/pkcs7-mime objects, a 1141 sending agent SHOULD do as much as possible to help a receiving agent 1142 know about the contents of the object without forcing the receiving 1143 agent to decode the ASN.1 for the object. The Content-Type header 1144 field of all application/pkcs7-mime objects SHOULD include the 1145 optional "smime-type" parameter, as described in the following 1146 sections. 1148 3.2.1. The name and filename Parameters 1150 For the application/pkcs7-mime, sending agents SHOULD emit the 1151 optional "name" parameter to the Content-Type field for compatibility 1152 with older systems. Sending agents SHOULD also emit the optional 1153 Content-Disposition field [RFC2138] with the "filename" parameter. 1154 If a sending agent emits the above parameters, the value of the 1155 parameters SHOULD be a file name with the appropriate extension: 1157 Media Type File 1158 Extension 1159 application/pkcs7-mime (SignedData, EnvelopedData) .p7m 1160 application/pkcs7-mime (degenerate SignedData certificate .p7c 1161 management message) 1162 application/pkcs7-mime (CompressedData) .p7z 1163 application/pkcs7-signature (SignedData) .p7s 1165 In addition, the file name SHOULD be limited to eight characters 1166 followed by a three-letter extension. The eight-character filename 1167 base can be any distinct name; the use of the filename base "smime" 1168 SHOULD be used to indicate that the MIME entity is associated with 1169 S/MIME. 1171 Including a file name serves two purposes. It facilitates easier use 1172 of S/MIME objects as files on disk. It also can convey type 1173 information across gateways. When a MIME entity of type 1174 application/pkcs7-mime (for example) arrives at a gateway that has no 1175 special knowledge of S/MIME, it will default the entity's media type 1176 to application/octet-stream and treat it as a generic attachment, 1177 thus losing the type information. However, the suggested filename 1178 for an attachment is often carried across a gateway. This often 1179 allows the receiving systems to determine the appropriate application 1180 to hand the attachment off to, in this case, a stand-alone S/MIME 1181 processing application. Note that this mechanism is provided as a 1182 convenience for implementations in certain environments. A proper 1183 S/MIME implementation MUST use the media types and MUST NOT rely on 1184 the file extensions. 1186 3.2.2. The smime-type Parameter 1188 The application/pkcs7-mime content type defines the optional "smime- 1189 type" parameter. The intent of this parameter is to convey details 1190 about the security applied (signed or enveloped) along with 1191 information about the contained content. This specification defines 1192 the following smime-types. 1194 Name CMS Type Inner Content 1195 enveloped-data EnvelopedData id-data 1196 signed-data SignedData id-data 1197 certs-only SignedData id-data 1198 compressed-data CompressedData id-data 1199 authEnveloped-data AuthEnvelopedData id-data 1201 In order for consistency to be obtained with future specifications, 1202 the following guidelines SHOULD be followed when assigning a new 1203 smime-type parameter. 1205 1. If both signing and encryption can be applied to the content, 1206 then three values for smime-type SHOULD be assigned "signed-*", 1207 "authEnv-*", and "enveloped-*". If one operation can be 1208 assigned, then this can be omitted. Thus, since "certs-only" can 1209 only be signed, "signed-" is omitted. 1211 2. A common string for a content OID SHOULD be assigned. We use 1212 "data" for the id-data content OID when MIME is the inner 1213 content. 1215 3. If no common string is assigned, then the common string of 1216 "OID." is recommended (for example, 1217 "OID.2.16.840.1.101.3.4.1.2" would be AES-128 CBC). 1219 It is explicitly intended that this field be a suitable hint for mail 1220 client applications to indicate whether a message is "signed", 1221 "authEnveloped" or "enveloped" without having to tunnel into the CMS 1222 payload. 1224 A registry for additional smime-type parameter values has been 1225 defined in [RFC7114]. 1227 3.3. Creating an Enveloped-Only Message 1229 This section describes the format for enveloping a MIME entity 1230 without signing it. It is important to note that sending enveloped 1231 but not signed messages does not provide for data integrity. It is 1232 possible to replace ciphertext in such a way that the processed 1233 message will still be valid, but the meaning can be altered. 1235 Step 1. The MIME entity to be enveloped is prepared according to 1236 Section 3.1. 1238 Step 2. The MIME entity and other required data is processed into a 1239 CMS object of type EnvelopedData. In addition to encrypting 1240 a copy of the content-encryption key for each recipient, a 1241 copy of the content-encryption key SHOULD be encrypted for 1242 the originator and included in the EnvelopedData (see 1243 [RFC5652], Section 6). 1245 Step 3. The EnvelopedData object is wrapped in a CMS ContentInfo 1246 object. 1248 Step 4. The ContentInfo object is inserted into an 1249 application/pkcs7-mime MIME entity. 1251 The smime-type parameter for enveloped-only messages is "enveloped- 1252 data". The file extension for this type of message is ".p7m". 1254 A sample message would be: 1256 Content-Type: application/pkcs7-mime; name=smime.p7m; 1257 smime-type=enveloped-data 1258 Content-Transfer-Encoding: base64 1259 Content-Disposition: attachment; filename=smime.p7m 1261 MIIBHgYJKoZIhvcNAQcDoIIBDzCCAQsCAQAxgcAwgb0CAQAwJjASMRAwDgYDVQQDEw 1262 dDYXJsUlNBAhBGNGvHgABWvBHTbi7NXXHQMA0GCSqGSIb3DQEBAQUABIGAC3EN5nGI 1263 iJi2lsGPcP2iJ97a4e8kbKQz36zg6Z2i0yx6zYC4mZ7mX7FBs3IWg+f6KgCLx3M1eC 1264 bWx8+MDFbbpXadCDgO8/nUkUNYeNxJtuzubGgzoyEd8Ch4H/dd9gdzTd+taTEgS0ip 1265 dSJuNnkVY4/M652jKKHRLFf02hosdR8wQwYJKoZIhvcNAQcBMBQGCCqGSIb3DQMHBA 1266 gtaMXpRwZRNYAgDsiSf8Z9P43LrY4OxUk660cu1lXeCSFOSOpOJ7FuVyU= 1268 3.4. Creating an Authenticated Enveloped-Only Message 1270 This section describes the format for enveloping a MIME entity 1271 without signing it. Authenticated enveloped messages provide 1272 confidentiality and data integrity. It is important to note that 1273 sending authenticated enveloped messages does not provide for 1274 authentication when using S/MIME. It is possible to replace 1275 ciphertext in such a way that the processed message will still be 1276 valid, but the meaning can be altered. However this is substantially 1277 more difficult than it is for an enveloped-only message as the 1279 Step 1. The MIME entity to be enveloped is prepared according to 1280 Section 3.1. 1282 Step 2. The MIME entity and other required data is processed into a 1283 CMS object of type AuthEnvelopedData. In addition to 1284 encrypting a copy of the content-encryption key for each 1285 recipient, a copy of the content-encryption key SHOULD be 1286 encrypted for the originator and included in the 1287 AuthEnvelopedData (see [RFC5083]). 1289 Step 3. The AuthEnvelopedData object is wrapped in a CMS ContentInfo 1290 object. 1292 Step 4. The ContentInfo object is inserted into an 1293 application/pkcs7-mime MIME entity. 1295 The smime-type parameter for authenticated enveloped-only messages is 1296 "authEnveloped-data". The file extension for this type of message is 1297 ".p7m". 1299 A sample message would be: 1301 Content-Type: application/pkcs7-mime; smime-type=authEnveloped-data; 1302 name=smime.p7m 1303 Content-Transfer-Encoding: base64 1304 Content-Disposition: attachment; filename=smime.p7m 1306 MIIDWQYLKoZIhvcNAQkQARegggNIMIIDRAIBADGBvjCBuwIBADAmMBIxEDAO 1307 BgNVBAMTB0NhcmxSU0ECEEY0a8eAAFa8EdNuLs1dcdAwCwYJKoZIhvcNAQEB 1308 BIGAgyZJo0ERTxA4xdTri5P5tVMyh0RARepTUCORZvlUbcUlaI8IpJZH3/J1 1309 Fv6MxTRS4O/K+ZcTlQmYeWLQvwdltQdOIP3mhpqXzTnOYhTK1IDtF2zx75Lg 1310 vE+ilpcLIzXfJB4RCBPtBWaHAof4Wb+VMQvLkk9OolX4mRSH1LPktgAwggJq 1311 BgkqhkiG9w0BBwEwGwYJYIZIAWUDBAEGMA4EDGPizioC9OHSsnNx4oCCAj7Y 1312 Cb8rOy8+55106newEJohC/aDgWbJhrMKzSOwa7JraXOV3HXD3NvKbl665dRx 1313 vmDwSCNaLCRU5q8/AxQx2SvnAbM+JKcEfC/VFdd4SiHNiUECAApLku2rMi5B 1314 WrhW/FXmx9d+cjum2BRwB3wj0q1wajdB0/kVRbQwg697dnlYyUog4vpJERjr 1315 7KAkawZx1RMHaM18wgZjUNpCBXFS3chQi9mTBp2i2Hf5iZ8OOtTx+rCQUmI6 1316 Jhy03vdcPCCARBjn3v0d3upZYDZddMA41CB9fKnnWFjadV1KpYwv80tqsEfx 1317 Vo0lJQ5VtJ8MHJiBpLVKadRIZ4iH2ULC0JtN5mXE1SrFKh7cqbJ4+7nqSRL3 1318 oBTud3rX41DGshOjpqcYHT4sqYlgZkc6dp0g1+hF1p3cGmjHdpysV2NVSUev 1319 ghHbvSqhIsXFzRSWKiZOigmlkv3R5LnjpYyP4brM62Jl7y0qborvV4dNMz7m 1320 D+5YxSlH0KAe8z6TT3LHuQdN7QCkFoiUSCaNhpAFaakkGIpqcqLhpOK4lXxt 1321 kptCG93eUwNCcTxtx6bXufPR5TUHohvZvfeqMp42kL37FJC/A8ZHoOxXy8+X 1322 X5QYxCQNuofWlvnIWv0Nr8w65x6lgVjPYmd/cHwzQKBTBMXN6pBud/PZL5zF 1323 tw3QHlQkBR+UflMWZKeN9L0KdQ27mQlCo5gQS85aifxoiiA2v9+0hxZw91rP 1324 IW4D+GS7oMMoKj8ZNyCJJsyf5smRZ+WxeBoolb3+TiGcBBCsRnfe6noLZiFO 1325 6Zeu2ZwE 1327 3.5. Creating a Signed-Only Message 1329 There are two formats for signed messages defined for S/MIME: 1331 - application/pkcs7-mime with SignedData. 1333 - multipart/signed. 1335 In general, the multipart/signed form is preferred for sending, and 1336 receiving agents MUST be able to handle both. 1338 3.5.1. Choosing a Format for Signed-Only Messages 1340 There are no hard-and-fast rules as to when a particular signed-only 1341 format is chosen. It depends on the capabilities of all the 1342 receivers and the relative importance of receivers with S/MIME 1343 facilities being able to verify the signature versus the importance 1344 of receivers without S/MIME software being able to view the message. 1346 Messages signed using the multipart/signed format can always be 1347 viewed by the receiver whether or not they have S/MIME software. 1348 They can also be viewed whether they are using a MIME-native user 1349 agent or they have messages translated by a gateway. In this 1350 context, "be viewed" means the ability to process the message 1351 essentially as if it were not a signed message, including any other 1352 MIME structure the message might have. 1354 Messages signed using the SignedData format cannot be viewed by a 1355 recipient unless they have S/MIME facilities. However, the 1356 SignedData format protects the message content from being changed by 1357 benign intermediate agents. Such agents might do line wrapping or 1358 content-transfer encoding changes that would break the signature. 1360 3.5.2. Signing Using application/pkcs7-mime with SignedData 1362 This signing format uses the application/pkcs7-mime media type. The 1363 steps to create this format are: 1365 Step 1. The MIME entity is prepared according to Section 3.1. 1367 Step 2. The MIME entity and other required data are processed into a 1368 CMS object of type SignedData. 1370 Step 3. The SignedData object is wrapped in a CMS ContentInfo 1371 object. 1373 Step 4. The ContentInfo object is inserted into an 1374 application/pkcs7-mime MIME entity. 1376 The smime-type parameter for messages using application/pkcs7-mime 1377 with SignedData is "signed-data". The file extension for this type 1378 of message is ".p7m". 1380 A sample message would be: 1382 Content-Type: application/pkcs7-mime; smime-type=signed-data; 1383 name=smime.p7m 1384 Content-Transfer-Encoding: base64 1385 Content-Disposition: attachment; filename=smime.p7m 1387 MIIDmQYJKoZIhvcNAQcCoIIDijCCA4YCAQExCTAHBgUrDgMCGjAtBgkqhkiG9w0BBw 1388 GgIAQeDQpUaGlzIGlzIHNvbWUgc2FtcGxlIGNvbnRlbnQuoIIC4DCCAtwwggKboAMC 1389 AQICAgDIMAkGByqGSM44BAMwEjEQMA4GA1UEAxMHQ2FybERTUzAeFw05OTA4MTcwMT 1390 EwNDlaFw0zOTEyMzEyMzU5NTlaMBMxETAPBgNVBAMTCEFsaWNlRFNTMIIBtjCCASsG 1391 ByqGSM44BAEwggEeAoGBAIGNze2D6gqeOT7CSCij5EeT3Q7XqA7sU8WrhAhP/5Thc0 1392 h+DNbzREjR/p+vpKGJL+HZMMg23j+bv7dM3F9piuR10DcMkQiVm96nXvn89J8v3UOo 1393 i1TxP7AHCEdNXYjDw7Wz41UIddU5dhDEeL3/nbCElzfy5FEbteQJllzzflvbAhUA4k 1394 emGkVmuBPG2o+4NyErYov3k80CgYAmONAUiTKqOfs+bdlLWWpMdiM5BAI1XPLLGjDD 1395 HlBd3ZtZ4s2qBT1YwHuiNrhuB699ikIlp/R1z0oIXks+kPht6pzJIYo7dhTpzi5dow 1396 fNI4W4LzABfG1JiRGJNkS9+MiVSlNWteL5c+waYTYfEX/Cve3RUP+YdMLRgUpgObo2 1397 OQOBhAACgYBc47ladRSWC6l63eM/qeysXty9txMRNKYWiSgRI9k0hmd1dRMSPUNbb+ 1398 VRv/qJ8qIbPiR9PQeNW2PIu0WloErjhdbOBoA/6CN+GvIkq1MauCcNHu8Iv2YUgFxi 1399 rGX6FYvxuzTU0pY39mFHssQyhPB+QUD9RqdjTjPypeL08oPluKOBgTB/MAwGA1UdEw 1400 EB/wQCMAAwDgYDVR0PAQH/BAQDAgbAMB8GA1UdIwQYMBaAFHBEPoIub4feStN14z0g 1401 vEMrk/EfMB0GA1UdDgQWBBS+bKGz48H37UNwpM4TAeL945f+zTAfBgNVHREEGDAWgR 1402 RBbGljZURTU0BleGFtcGxlLmNvbTAJBgcqhkjOOAQDAzAAMC0CFFUMpBkfQiuJcSIz 1403 jYNqtT1na79FAhUAn2FTUlQLXLLd2ud2HeIQUltDXr0xYzBhAgEBMBgwEjEQMA4GA1 1404 UEAxMHQ2FybERTUwICAMgwBwYFKw4DAhowCQYHKoZIzjgEAwQuMCwCFD1cSW6LIUFz 1405 eXle3YI5SKSBer/sAhQmCq7s/CTFHOEjgASeUjbMpx5g6A== 1407 3.5.3. Signing Using the multipart/signed Format 1409 This format is a clear-signing format. Recipients without any S/MIME 1410 or CMS processing facilities are able to view the message. It makes 1411 use of the multipart/signed media type described in [RFC1847]. The 1412 multipart/signed media type has two parts. The first part contains 1413 the MIME entity that is signed; the second part contains the 1414 "detached signature" CMS SignedData object in which the 1415 encapContentInfo eContent field is absent. 1417 3.5.3.1. The application/pkcs7-signature Media Type 1419 This media type always contains a CMS ContentInfo containing a single 1420 CMS object of type SignedData. The SignedData encapContentInfo 1421 eContent field MUST be absent. The signerInfos field contains the 1422 signatures for the MIME entity. 1424 The file extension for signed-only messages using application/pkcs7- 1425 signature is ".p7s". 1427 3.5.3.2. Creating a multipart/signed Message 1429 Step 1. The MIME entity to be signed is prepared according to 1430 Section 3.1, taking special care for clear-signing. 1432 Step 2. The MIME entity is presented to CMS processing in order to 1433 obtain an object of type SignedData in which the 1434 encapContentInfo eContent field is absent. 1436 Step 3. The MIME entity is inserted into the first part of a 1437 multipart/signed message with no processing other than that 1438 described in Section 3.1. 1440 Step 4. Transfer encoding is applied to the "detached signature" CMS 1441 SignedData object, and it is inserted into a MIME entity of 1442 type application/pkcs7-signature. 1444 Step 5. The MIME entity of the application/pkcs7-signature is 1445 inserted into the second part of the multipart/signed 1446 entity. 1448 The multipart/signed Content-Type has two required parameters: the 1449 protocol parameter and the micalg parameter. 1451 The protocol parameter MUST be "application/pkcs7-signature". Note 1452 that quotation marks are required around the protocol parameter 1453 because MIME requires that the "/" character in the parameter value 1454 MUST be quoted. 1456 The micalg parameter allows for one-pass processing when the 1457 signature is being verified. The value of the micalg parameter is 1458 dependent on the message digest algorithm(s) used in the calculation 1459 of the Message Integrity Check. If multiple message digest 1460 algorithms are used, they MUST be separated by commas per [RFC1847]. 1461 The values to be placed in the micalg parameter SHOULD be from the 1462 following: 1464 Algorithm Value Used 1465 MD5 md5 1466 SHA-1 sha-1 1467 SHA-224 sha-224 1468 SHA-256 sha-256 1469 SHA-384 sha-384 1470 SHA-512 sha-512 1471 Any other (defined separately in algorithm profile or "unknown" if 1472 not defined) 1474 (Historical note: some early implementations of S/MIME emitted and 1475 expected "rsa-md5", "rsa-sha1", and "sha1" for the micalg parameter.) 1476 Receiving agents SHOULD be able to recover gracefully from a micalg 1477 parameter value that they do not recognize. Future names for this 1478 parameter will be consistent with the IANA "Hash Function Textual 1479 Names" registry. 1481 3.5.3.3. Sample multipart/signed Message 1483 Content-Type: multipart/signed; 1484 micalg=SHA1; 1485 boundary="----=_NextBoundry____Fri,_06_Sep_2002_00:25:21"; 1486 protocol="application/pkcs7-signature" 1488 This is a multi-part message in MIME format. 1490 ------=_NextBoundry____Fri,_06_Sep_2002_00:25:21 1492 This is some sample content. 1493 ------=_NextBoundry____Fri,_06_Sep_2002_00:25:21 1494 Content-Type: application/pkcs7-signature; name=smime.p7s 1495 Content-Transfer-Encoding: base64 1496 Content-Disposition: attachment; filename=smime.p7s 1498 MIIDdwYJKoZIhvcNAQcCoIIDaDCCA2QCAQExCTAHBgUrDgMCGjALBgkqhkiG9w0BBw 1499 GgggLgMIIC3DCCApugAwIBAgICAMgwCQYHKoZIzjgEAzASMRAwDgYDVQQDEwdDYXJs 1500 RFNTMB4XDTk5MDgxNzAxMTA0OVoXDTM5MTIzMTIzNTk1OVowEzERMA8GA1UEAxMIQW 1501 xpY2VEU1MwggG2MIIBKwYHKoZIzjgEATCCAR4CgYEAgY3N7YPqCp45PsJIKKPkR5Pd 1502 DteoDuxTxauECE//lOFzSH4M1vNESNH+n6+koYkv4dkwyDbeP5u/t0zcX2mK5HXQNw 1503 yRCJWb3qde+fz0ny/dQ6iLVPE/sAcIR01diMPDtbPjVQh11Tl2EMR4vf+dsISXN/Lk 1504 URu15AmWXPN+W9sCFQDiR6YaRWa4E8baj7g3IStii/eTzQKBgCY40BSJMqo5+z5t2U 1505 tZakx2IzkEAjVc8ssaMMMeUF3dm1nizaoFPVjAe6I2uG4Hr32KQiWn9HXPSgheSz6Q 1506 +G3qnMkhijt2FOnOLl2jB80jhbgvMAF8bUmJEYk2RL34yJVKU1a14vlz7BphNh8Rf8 1507 K97dFQ/5h0wtGBSmA5ujY5A4GEAAKBgFzjuVp1FJYLqXrd4z+p7Kxe3L23ExE0phaJ 1508 KBEj2TSGZ3V1ExI9Q1tv5VG/+onyohs+JH09B41bY8i7RaWgSuOF1s4GgD/oI34a8i 1509 SrUxq4Jw0e7wi/ZhSAXGKsZfoVi/G7NNTSljf2YUeyxDKE8H5BQP1Gp2NOM/Kl4vTy 1510 g+W4o4GBMH8wDAYDVR0TAQH/BAIwADAOBgNVHQ8BAf8EBAMCBsAwHwYDVR0jBBgwFo 1511 AUcEQ+gi5vh95K03XjPSC8QyuT8R8wHQYDVR0OBBYEFL5sobPjwfftQ3CkzhMB4v3j 1512 l/7NMB8GA1UdEQQYMBaBFEFsaWNlRFNTQGV4YW1wbGUuY29tMAkGByqGSM44BAMDMA 1513 AwLQIUVQykGR9CK4lxIjONg2q1PWdrv0UCFQCfYVNSVAtcst3a53Yd4hBSW0NevTFj 1514 MGECAQEwGDASMRAwDgYDVQQDEwdDYXJsRFNTAgIAyDAHBgUrDgMCGjAJBgcqhkjOOA 1515 QDBC4wLAIUM/mGf6gkgp9Z0XtRdGimJeB/BxUCFGFFJqwYRt1WYcIOQoGiaowqGzVI 1517 ------=_NextBoundry____Fri,_06_Sep_2002_00:25:21-- 1519 The content that is digested (the first part of the multipart/signed) 1520 consists of the bytes: 1522 54 68 69 73 20 69 73 20 73 6f 6d 65 20 73 61 6d 70 6c 65 20 63 6f 6e 1523 74 65 6e 74 2e 0d 0a 1525 3.6. Creating a Compressed-Only Message 1527 This section describes the format for compressing a MIME entity. 1528 Please note that versions of S/MIME prior to version 3.1 did not 1529 specify any use of CompressedData, and will not recognize it. The 1530 use of a capability to indicate the ability to receive CompressedData 1531 is described in [RFC3274] and is the preferred method for 1532 compatibility. 1534 Step 1. The MIME entity to be compressed is prepared according to 1535 Section 3.1. 1537 Step 2. The MIME entity and other required data are processed into a 1538 CMS object of type CompressedData. 1540 Step 3. The CompressedData object is wrapped in a CMS ContentInfo 1541 object. 1543 Step 4. The ContentInfo object is inserted into an 1544 application/pkcs7-mime MIME entity. 1546 The smime-type parameter for compressed-only messages is "compressed- 1547 data". The file extension for this type of message is ".p7z". 1549 A sample message would be: 1551 Content-Type: application/pkcs7-mime; smime-type=compressed-data; 1552 name=smime.p7z 1553 Content-Transfer-Encoding: base64 1554 Content-Disposition: attachment; filename=smime.p7z 1556 eNoLycgsVgCi4vzcVIXixNyCnFSF5Py8ktS8Ej0AlCkKVA== 1558 3.7. Multiple Operations 1560 The signed-only, enveloped-only, and compressed-only MIME formats can 1561 be nested. This works because these formats are all MIME entities 1562 that encapsulate other MIME entities. 1564 An S/MIME implementation MUST be able to receive and process 1565 arbitrarily nested S/MIME within reasonable resource limits of the 1566 recipient computer. 1568 It is possible to apply any of the signing, encrypting, and 1569 compressing operations in any order. It is up to the implementer and 1570 the user to choose. When signing first, the signatories are then 1571 securely obscured by the enveloping. When enveloping first the 1572 signatories are exposed, but it is possible to verify signatures 1573 without removing the enveloping. This can be useful in an 1574 environment where automatic signature verification is desired, as no 1575 private key material is required to verify a signature. 1577 There are security ramifications to choosing whether to sign first or 1578 encrypt first. A recipient of a message that is encrypted and then 1579 signed can validate that the encrypted block was unaltered, but 1580 cannot determine any relationship between the signer and the 1581 unencrypted contents of the message. A recipient of a message that 1582 is signed then encrypted can assume that the signed message itself 1583 has not been altered, but that a careful attacker could have changed 1584 the unauthenticated portions of the encrypted message. 1586 When using compression, keep the following guidelines in mind: 1588 - Compression of binary encoded encrypted data is discouraged, since 1589 it will not yield significant compression. Base64 encrypted data 1590 could very well benefit, however. 1592 - If a lossy compression algorithm is used with signing, you will 1593 need to compress first, then sign. 1595 3.8. Creating a Certificate Management Message 1597 The certificate management message or MIME entity is used to 1598 transport certificates and/or Certificate Revocation Lists, such as 1599 in response to a registration request. 1601 Step 1. The certificates and/or Certificate Revocation Lists are 1602 made available to the CMS generating process that creates a 1603 CMS object of type SignedData. The SignedData 1604 encapContentInfo eContent field MUST be absent and 1605 signerInfos field MUST be empty. 1607 Step 2. The SignedData object is wrapped in a CMS ContentInfo 1608 object. 1610 Step 3. The ContentInfo object is enclosed in an 1611 application/pkcs7-mime MIME entity. 1613 The smime-type parameter for a certificate management message is 1614 "certs-only". The file extension for this type of message is ".p7c". 1616 3.9. Registration Requests 1618 A sending agent that signs messages MUST have a certificate for the 1619 signature so that a receiving agent can verify the signature. There 1620 are many ways of getting certificates, such as through an exchange 1621 with a certification authority, through a hardware token or diskette, 1622 and so on. 1624 S/MIME v2 [SMIMEv2] specified a method for "registering" public keys 1625 with certificate authorities using an application/pkcs10 body part. 1626 Since that time, the IETF PKIX Working Group has developed other 1627 methods for requesting certificates. However, S/MIME v4.0 does not 1628 require a particular certificate request mechanism. 1630 3.10. Identifying an S/MIME Message 1632 Because S/MIME takes into account interoperation in non-MIME 1633 environments, several different mechanisms are employed to carry the 1634 type information, and it becomes a bit difficult to identify S/MIME 1635 messages. The following table lists criteria for determining whether 1636 or not a message is an S/MIME message. A message is considered an 1637 S/MIME message if it matches any of the criteria listed below. 1639 The file suffix in the table below comes from the "name" parameter in 1640 the Content-Type header field, or the "filename" parameter on the 1641 Content-Disposition header field. These parameters that give the 1642 file suffix are not listed below as part of the parameter section. 1644 Media type parameters file suffix 1645 application/pkcs7-mime n/a n/a 1646 multipart/signed protocol= n/a 1647 "application/pkcs7-signature" 1648 application/octet-stream n/a p7m, p7s, 1649 p7c, p7z 1651 4. Certificate Processing 1653 A receiving agent MUST provide some certificate retrieval mechanism 1654 in order to gain access to certificates for recipients of digital 1655 envelopes. This specification does not cover how S/MIME agents 1656 handle certificates, only what they do after a certificate has been 1657 validated or rejected. S/MIME certificate issues are covered in 1658 [RFC5750]. 1660 At a minimum, for initial S/MIME deployment, a user agent could 1661 automatically generate a message to an intended recipient requesting 1662 that recipient's certificate in a signed return message. Receiving 1663 and sending agents SHOULD also provide a mechanism to allow a user to 1664 "store and protect" certificates for correspondents in such a way so 1665 as to guarantee their later retrieval. 1667 4.1. Key Pair Generation 1669 All generated key pairs MUST be generated from a good source of non- 1670 deterministic random input [RFC4086] and the private key MUST be 1671 protected in a secure fashion. 1673 An S/MIME user agent MUST NOT generate asymmetric keys less than 2048 1674 bits for use with an RSA signature algorithm. 1676 For 2048-bit through 4096-bit RSA with SHA-256 see [RFC5754] and 1677 [FIPS186-4]. The first reference provides the signature algorithm's 1678 object identifier, and the second provides the signature algorithm's 1679 definition. 1681 For RSASSA-PSS with SHA-256, see [RFC4056]. For RSAES-OAEP, see 1682 [RFC3560]. 1684 4.2. Signature Generation 1686 The following are the requirements for an S/MIME agent generated RSA 1687 and RSASSA-PSS signatures: 1689 key size <= 2047 : SHOULD NOT (Note 1) 1690 2048 <= key size <= 4096 : SHOULD (see Security Considerations) 1691 4096 < key size : MAY (see Security Considerations) 1693 Note 1: see Historical Mail Considerations in Section 6. 1694 Note 2: see Security Considerations in Appendix B. 1696 Key sizes for ECDSA and EdDSA are fixed by the curve. 1698 4.3. Signature Verification 1700 The following are the requirements for S/MIME receiving agents during 1701 signature verification of RSA and RSASSA-PSS signatures: 1703 key size <= 2047 : SHOULD NOT (Note 1) 1704 2048 <= key size <= 4096 : MUST (Note 2) 1705 4096 < key size : MAY (Note 2) 1707 Note 1: see Historical Mail Considerations in Section 6. 1708 Note 2: see Security Considerations in Appendix B. 1710 Key sizes for ECDSA and EdDSA are fixed by the curve. 1712 4.4. Encryption 1714 The following are the requirements for an S/MIME agent when 1715 establishing keys for content encryption using the RSA, and RSA-OAEP 1716 algorithms: 1718 key size <= 2047 : SHOULD NOT (Note 1) 1719 2048 <= key size <= 4096 : SHOULD (Note 2) 1720 4096 < key size : MAY (Note 2) 1722 Note 1: see Historical Mail Considerations in Section 6. 1723 Note 2: see Security Considerations in Appendix B. 1725 Key sizes for ECDH are fixed by the curve. 1727 4.5. Decryption 1729 The following are the requirements for an S/MIME agent when 1730 establishing keys for content decryption using the RSA and RSAES-OAEP 1731 algorithms: 1733 key size <= 2047 : MAY (Note 1) 1734 2048 <= key size <= 4096 : MUST (Note 2) 1735 4096 < key size : MAY (Note 2) 1737 Note 1: see Historical Mail Considerations in Section 6. 1738 Note 2: see Security Considerations in Appendix B. 1740 Key sizes for ECDH are fixed by the curve. 1742 5. IANA Considerations 1744 The following information updates the media type registration for 1745 application/pkcs7-mime and application/pkcs7-signature to refer to 1746 this document as opposed to RFC 2311. 1748 Note that other documents can define additional MIME media types for 1749 S/MIME. 1751 5.1. Media Type for application/pkcs7-mime 1752 Type name: application 1754 Subtype Name: pkcs7-mime 1756 Required Parameters: NONE 1758 Optional Parameters: smime-type/signed-data 1759 smime-type/enveloped-data 1760 smime-type/compressed-data 1761 smime-type/certs-only 1762 name 1764 Encoding Considerations: See Section 3 of this document 1766 Security Considerations: See Section 6 of this document 1768 Interoperability Considerations: See Sections 1-6 of this document 1770 Published Specification: RFC 2311, RFC 2633, and this document 1772 Applications that use this media type: Security applications 1774 Additional information: NONE 1776 Person & email to contact for further information: iesg@ietf.org 1778 Intended usage: COMMON 1780 Restrictions on usage: NONE 1782 Author: Sean Turner 1784 Change Controller: S/MIME working group delegated from the IESG 1786 5.2. Media Type for application/pkcs7-signature 1787 Type name: application 1789 Subtype Name: pkcs7-signature 1791 Required Parameters: NONE 1793 Optional Parameters: NONE 1795 Encoding Considerations: See Section 3 of this document 1797 Security Considerations: See Section 6 of this document 1799 Interoperability Considerations: See Sections 1-6 of this document 1801 Published Specification: RFC 2311, RFC 2633, and this document 1803 Applications that use this media type: Security applications 1805 Additional information: NONE 1807 Person & email to contact for further information: iesg@ietf.org 1809 Intended usage: COMMON 1811 Restrictions on usage: NONE 1813 Author: Sean Turner 1815 Change Controller: S/MIME working group delegated from the IESG 1817 5.3. Register authEnveloped-data smime-type 1819 IANA is required to register the following value in the "Parameter 1820 Values for the smime-type Parameter" registry. The values to be 1821 registered are: 1823 smime-type value: authEnveloped-data 1825 Reference: [[This Document, Section 3.2.2]] 1827 6. IANA Considertions 1829 This document has no new IANA considerations. 1831 7. Security Considerations 1833 Cryptographic algorithms will be broken or weakened over time. 1834 Implementers and users need to check that the cryptographic 1835 algorithms listed in this document continue to provide the expected 1836 level of security. The IETF from time to time may issue documents 1837 dealing with the current state of the art. For example: 1839 - The Million Message Attack described in RFC 3218 [RFC3218]. 1841 - The Diffie-Hellman "small-subgroup" attacks described in RFC 2785 1842 [RFC2785]. 1844 - The attacks against hash algorithms described in RFC 4270 1845 [RFC4270]. 1847 This specification uses Public-Key Cryptography technologies. It is 1848 assumed that the private key is protected to ensure that it is not 1849 accessed or altered by unauthorized parties. 1851 It is impossible for most people or software to estimate the value of 1852 a message's content. Further, it is impossible for most people or 1853 software to estimate the actual cost of recovering an encrypted 1854 message content that is encrypted with a key of a particular size. 1855 Further, it is quite difficult to determine the cost of a failed 1856 decryption if a recipient cannot process a message's content. Thus, 1857 choosing between different key sizes (or choosing whether to just use 1858 plaintext) is also impossible for most people or software. However, 1859 decisions based on these criteria are made all the time, and 1860 therefore this specification gives a framework for using those 1861 estimates in choosing algorithms. 1863 The choice of 2048 bits as an RSA asymmetric key size in this 1864 specification is based on the desire to provide at least 100 bits of 1865 security. The key sizes that must be supported to conform to this 1866 specification seem appropriate for the Internet based on [RFC3766]. 1867 Of course, there are environments, such as financial and medical 1868 systems, that may select different key sizes. For this reason, an 1869 implementation MAY support key sizes beyond those recommended in this 1870 specification. 1872 Receiving agents that validate signatures and sending agents that 1873 encrypt messages need to be cautious of cryptographic processing 1874 usage when validating signatures and encrypting messages using keys 1875 larger than those mandated in this specification. An attacker could 1876 send certificates with keys that would result in excessive 1877 cryptographic processing, for example, keys larger than those 1878 mandated in this specification, which could swamp the processing 1879 element. Agents that use such keys without first validating the 1880 certificate to a trust anchor are advised to have some sort of 1881 cryptographic resource management system to prevent such attacks. 1883 Using weak cryptography in S/MIME offers little actual security over 1884 sending plaintext. However, other features of S/MIME, such as the 1885 specification of AES and the ability to announce stronger 1886 cryptographic capabilities to parties with whom you communicate, 1887 allow senders to create messages that use strong encryption. Using 1888 weak cryptography is never recommended unless the only alternative is 1889 no cryptography. 1891 RSA and DSA keys of less than 2048 bits are now considered by many 1892 experts to be cryptographically insecure (due to advances in 1893 computing power), and should no longer be used to protect messages. 1894 Such keys were previously considered secure, so processing previously 1895 received signed and encrypted mail will often result in the use of 1896 weak keys. Implementations that wish to support previous versions of 1897 S/MIME or process old messages need to consider the security risks 1898 that result from smaller key sizes (e.g., spoofed messages) versus 1899 the costs of denial of service. If an implementation supports 1900 verification of digital signatures generated with RSA and DSA keys of 1901 less than 1024 bits, it MUST warn the user. Implementers should 1902 consider providing different warnings for newly received messages and 1903 previously stored messages. Server implementations (e.g., secure 1904 mail list servers) where user warnings are not appropriate SHOULD 1905 reject messages with weak signatures. 1907 Implementers SHOULD be aware that multiple active key pairs can be 1908 associated with a single individual. For example, one key pair can 1909 be used to support confidentiality, while a different key pair can be 1910 used for digital signatures. 1912 If a sending agent is sending the same message using different 1913 strengths of cryptography, an attacker watching the communications 1914 channel might be able to determine the contents of the strongly 1915 encrypted message by decrypting the weakly encrypted version. In 1916 other words, a sender SHOULD NOT send a copy of a message using 1917 weaker cryptography than they would use for the original of the 1918 message. 1920 Modification of the ciphertext in EnvelopedData can go undetected if 1921 authentication is not also used, which is the case when sending 1922 EnvelopedData without wrapping it in SignedData or enclosing 1923 SignedData within it. This is one of the reasons for moving from 1924 EnvelopedData to AuthEnvelopedData as the authenticated encryption 1925 algorithms provide the authentication without needing the SignedData 1926 layer. 1928 If an implementation is concerned about compliance with National 1929 Institute of Standards and Technology (NIST) key size 1930 recommendations, then see [SP800-57]. 1932 If messaging environments make use of the fact that a message is 1933 signed to change the behavior of message processing (examples would 1934 be running rules or UI display hints), without first verifying that 1935 the message is actually signed and knowing the state of the 1936 signature, this can lead to incorrect handling of the message. 1937 Visual indicators on messages may need to have the signature 1938 validation code checked periodically if the indicator is supposed to 1939 give information on the current status of a message. 1941 Many people assume that the use of an authenticated encryption 1942 algorithm is all that is needed to be in a situtation where the 1943 sender of the message will be authenticated. In almost all cases 1944 this is not a correct statement. There are a number of preconditions 1945 that need to hold for an authenticated encryption algorithm to 1946 provide this service: 1948 - The starting key must be bound to a single entity. The use of a 1949 group key only would allow for the statement that a message was 1950 sent by one of the entities that held the key but will not 1951 identify a specific entity. 1953 - The message must have exactly one sender and one recipient. 1954 Having more than one recipient would allow for the second 1955 recipient to create a message that the first recipient would 1956 believe is from the sender by stripping them as a recipient from 1957 the message. 1959 - A direct path needs to exist from the starting key to the key used 1960 as the content encryption key (CEK) which guarantees that no third 1961 party could have seen the resulting CEK. This means that one 1962 needs to be using an algorithm that is called a "Direct 1963 Encryption" or a "Direct Key Agreement" algorithm in other 1964 contexts. This means that the starting key is used directly as 1965 the CEK key, or that the starting key is used to create a secret 1966 which then is transformed into the CEK via a KDF step. 1968 S/MIME implementations almost universally use ephemeral-static rather 1969 than static-static key agreement and do not use a shared secret for 1970 encryption, this means that the first precondition is not met. There 1971 is a document [RFC6278] which defined how to use static-static key 1972 agreement with CMS so that is readably doable. Currently, all S/MIME 1973 key agreement methods derive a KEK and wrap a CEK. This violates the 1974 third precondition above. New key agreement algorithms that directly 1975 created the CEK without creating an intervening KEK would need to be 1976 defined. 1978 Even when all of the preconditions are met and origination of a 1979 message is established by the use of an authenticated encryption 1980 algorithm, users need to be aware that there is no way to prove this 1981 to a third party. This is because either of the parties can 1982 successfully create the message (or just alter the content) based on 1983 the fact that the CEK is going to be known to both parties. Thus the 1984 origination is always built on a presumption that "I did not send 1985 this message to myself." 1987 All of the authenticated encryption algorithms in this document use 1988 counter mode for the encryption portion of the algorithm. This means 1989 that the length of the plain text will always be known as the cipher 1990 text length and the plain text length are always the same. This 1991 information can enable passive observers to infer information based 1992 solely on the length of the message. Applications for which this is 1993 a concern need to provide some type of padding so that the length of 1994 the message does not provide this information. 1996 8. References 1998 8.1. Normative References 2000 [ASN.1] "Information Technology - Abstract Syntax Notation 2001 (ASN.1)". 2003 ASN.1 syntax consists of the following references [X.680], 2004 [X.681], [X.682], and [X.683]. 2006 [CHARSETS] 2007 "Character sets assigned by IANA.", 2008 . 2010 [CMS] "Cryptograhic Message Syntax". 2012 This is the set of documents dealing with the 2013 cryptographic message syntax and refers to [RFC5652] and 2014 [RFC5083]. 2016 [ESS] "Enhanced Security Services for S/MIME". 2018 This is the set of documents dealing with enhanged 2019 security services and refers to [RFC2634] and [RFC5035]. 2021 [FIPS186-4] 2022 National Institute of Standards and Technology (NIST), 2023 "Digital Signature Standard (DSS)", Federal Information 2024 Processing Standards Publication 186-4, July 2013. 2026 [I-D.ietf-curdle-cms-ecdh-new-curves] 2027 Housley, R., "Use of the Elliptic Curve Diffie-Hellamn Key 2028 Agreement Algorithm with X25519 and X448 in the 2029 Cryptographic Message Syntax (CMS)", draft-ietf-curdle- 2030 cms-ecdh-new-curves-02 (work in progress), March 2017. 2032 [I-D.ietf-lamps-rfc5750-bis] 2033 Schaad, J., Ramsdell, B., and S. Turner, "Secure/ 2034 Multipurpose Internet Mail Extensions (S/ MIME) Version 2035 4.0 Certificate Handling", draft-ietf-lamps-rfc5750-bis-03 2036 (work in progress), March 2017. 2038 [MIME-SPEC] 2039 "MIME Message Specifications". 2041 This is the set of documents that define how to use MIME. 2042 This set of documents is [RFC2045], [RFC2046], [RFC2047], 2043 [RFC2049], [RFC4288], and [RFC4289]. 2045 [RFC1847] Galvin, J., Murphy, S., Crocker, S., and N. Freed, 2046 "Security Multiparts for MIME: Multipart/Signed and 2047 Multipart/Encrypted", RFC 1847, DOI 10.17487/RFC1847, 2048 October 1995, . 2050 [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 2051 Extensions (MIME) Part One: Format of Internet Message 2052 Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996, 2053 . 2055 [RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 2056 Extensions (MIME) Part Two: Media Types", RFC 2046, 2057 DOI 10.17487/RFC2046, November 1996, 2058 . 2060 [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions) 2061 Part Three: Message Header Extensions for Non-ASCII Text", 2062 RFC 2047, DOI 10.17487/RFC2047, November 1996, 2063 . 2065 [RFC2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 2066 Extensions (MIME) Part Five: Conformance Criteria and 2067 Examples", RFC 2049, DOI 10.17487/RFC2049, November 1996, 2068 . 2070 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 2071 Requirement Levels", BCP 14, RFC 2119, 2072 DOI 10.17487/RFC2119, March 1997, 2073 . 2075 [RFC2138] Rigney, C., Rubens, A., Simpson, W., and S. Willens, 2076 "Remote Authentication Dial In User Service (RADIUS)", 2077 RFC 2138, DOI 10.17487/RFC2138, April 1997, 2078 . 2080 [RFC2634] Hoffman, P., Ed., "Enhanced Security Services for S/MIME", 2081 RFC 2634, DOI 10.17487/RFC2634, June 1999, 2082 . 2084 [RFC3274] Gutmann, P., "Compressed Data Content Type for 2085 Cryptographic Message Syntax (CMS)", RFC 3274, 2086 DOI 10.17487/RFC3274, June 2002, 2087 . 2089 [RFC3370] Housley, R., "Cryptographic Message Syntax (CMS) 2090 Algorithms", RFC 3370, DOI 10.17487/RFC3370, August 2002, 2091 . 2093 [RFC3560] Housley, R., "Use of the RSAES-OAEP Key Transport 2094 Algorithm in Cryptographic Message Syntax (CMS)", 2095 RFC 3560, DOI 10.17487/RFC3560, July 2003, 2096 . 2098 [RFC3565] Schaad, J., "Use of the Advanced Encryption Standard (AES) 2099 Encryption Algorithm in Cryptographic Message Syntax 2100 (CMS)", RFC 3565, DOI 10.17487/RFC3565, July 2003, 2101 . 2103 [RFC4056] Schaad, J., "Use of the RSASSA-PSS Signature Algorithm in 2104 Cryptographic Message Syntax (CMS)", RFC 4056, 2105 DOI 10.17487/RFC4056, June 2005, 2106 . 2108 [RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker, 2109 "Randomness Requirements for Security", BCP 106, RFC 4086, 2110 DOI 10.17487/RFC4086, June 2005, 2111 . 2113 [RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and 2114 Registration Procedures", RFC 4288, DOI 10.17487/RFC4288, 2115 December 2005, . 2117 [RFC4289] Freed, N. and J. Klensin, "Multipurpose Internet Mail 2118 Extensions (MIME) Part Four: Registration Procedures", 2119 BCP 13, RFC 4289, DOI 10.17487/RFC4289, December 2005, 2120 . 2122 [RFC5035] Schaad, J., "Enhanced Security Services (ESS) Update: 2123 Adding CertID Algorithm Agility", RFC 5035, 2124 DOI 10.17487/RFC5035, August 2007, 2125 . 2127 [RFC5083] Housley, R., "Cryptographic Message Syntax (CMS) 2128 Authenticated-Enveloped-Data Content Type", RFC 5083, 2129 DOI 10.17487/RFC5083, November 2007, 2130 . 2132 [RFC5084] Housley, R., "Using AES-CCM and AES-GCM Authenticated 2133 Encryption in the Cryptographic Message Syntax (CMS)", 2134 RFC 5084, DOI 10.17487/RFC5084, November 2007, 2135 . 2137 [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, 2138 RFC 5652, DOI 10.17487/RFC5652, September 2009, 2139 . 2141 [RFC5753] Turner, S. and D. Brown, "Use of Elliptic Curve 2142 Cryptography (ECC) Algorithms in Cryptographic Message 2143 Syntax (CMS)", RFC 5753, DOI 10.17487/RFC5753, January 2144 2010, . 2146 [RFC5754] Turner, S., "Using SHA2 Algorithms with Cryptographic 2147 Message Syntax", RFC 5754, DOI 10.17487/RFC5754, January 2148 2010, . 2150 [SMIMEv4.0] 2151 "S/MIME version 4.0". 2153 This group of documents represents S/MIME version 4.0. 2154 This set of documents are [RFC2634], 2155 [I-D.ietf-lamps-rfc5750-bis], [[This Document]], 2156 [RFC5652], and [RFC5035]. 2158 [X.680] "Information Technology - Abstract Syntax Notation One 2159 (ASN.1): Specification of basic notation. ITU-T 2160 Recommendation X.680 (2002)", ITU-T X.680, ISO/ 2161 IEC 8824-1:2008, November 2008. 2163 [X.681] "Information Technology - Abstract Syntax Notation One 2164 (ASN.1): Information object specification", ITU-T X.681, 2165 ISO/IEC 8824-2:2008, November 2008. 2167 [X.682] "Information Technology - Abstract Syntax Notation One 2168 (ASN.1): Constraint specification", ITU-T X.682, ISO/ 2169 IEC 8824-3:2008, November 2008. 2171 [X.683] "Information Technology - Abstract Syntax Notation One 2172 (ASN.1): Parameteriztion of ASN.1 specifications", 2173 ITU-T X.683, ISO/IEC 8824-4:2008, November 2008. 2175 [X.690] "Information Technology - ASN.1 encoding rules: 2176 Specification of Basic Encoding Rules (BER), Canonical 2177 Encoding Rules (CER) and Distinguished Encoding Rules 2178 (DER).", ITU-T X.690, ISO/IEC 8825-1:2002, July 2002. 2180 8.2. Informative References 2182 [FIPS186-2] 2183 National Institute of Standards and Technology (NIST), 2184 "Digital Signature Standard (DSS) [With Change Notice 1]", 2185 Federal Information Processing Standards 2186 Publication 186-2, January 2000. 2188 [RFC2268] Rivest, R., "A Description of the RC2(r) Encryption 2189 Algorithm", RFC 2268, DOI 10.17487/RFC2268, March 1998, 2190 . 2192 [RFC2311] Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L., and 2193 L. Repka, "S/MIME Version 2 Message Specification", 2194 RFC 2311, DOI 10.17487/RFC2311, March 1998, 2195 . 2197 [RFC2312] Dusse, S., Hoffman, P., Ramsdell, B., and J. Weinstein, 2198 "S/MIME Version 2 Certificate Handling", RFC 2312, 2199 DOI 10.17487/RFC2312, March 1998, 2200 . 2202 [RFC2313] Kaliski, B., "PKCS #1: RSA Encryption Version 1.5", 2203 RFC 2313, DOI 10.17487/RFC2313, March 1998, 2204 . 2206 [RFC2314] Kaliski, B., "PKCS #10: Certification Request Syntax 2207 Version 1.5", RFC 2314, DOI 10.17487/RFC2314, March 1998, 2208 . 2210 [RFC2315] Kaliski, B., "PKCS #7: Cryptographic Message Syntax 2211 Version 1.5", RFC 2315, DOI 10.17487/RFC2315, March 1998, 2212 . 2214 [RFC2630] Housley, R., "Cryptographic Message Syntax", RFC 2630, 2215 DOI 10.17487/RFC2630, June 1999, 2216 . 2218 [RFC2631] Rescorla, E., "Diffie-Hellman Key Agreement Method", 2219 RFC 2631, DOI 10.17487/RFC2631, June 1999, 2220 . 2222 [RFC2632] Ramsdell, B., Ed., "S/MIME Version 3 Certificate 2223 Handling", RFC 2632, DOI 10.17487/RFC2632, June 1999, 2224 . 2226 [RFC2633] Ramsdell, B., Ed., "S/MIME Version 3 Message 2227 Specification", RFC 2633, DOI 10.17487/RFC2633, June 1999, 2228 . 2230 [RFC2785] Zuccherato, R., "Methods for Avoiding the "Small-Subgroup" 2231 Attacks on the Diffie-Hellman Key Agreement Method for S/ 2232 MIME", RFC 2785, DOI 10.17487/RFC2785, March 2000, 2233 . 2235 [RFC3218] Rescorla, E., "Preventing the Million Message Attack on 2236 Cryptographic Message Syntax", RFC 3218, 2237 DOI 10.17487/RFC3218, January 2002, 2238 . 2240 [RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For 2241 Public Keys Used For Exchanging Symmetric Keys", BCP 86, 2242 RFC 3766, DOI 10.17487/RFC3766, April 2004, 2243 . 2245 [RFC3850] Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail 2246 Extensions (S/MIME) Version 3.1 Certificate Handling", 2247 RFC 3850, DOI 10.17487/RFC3850, July 2004, 2248 . 2250 [RFC3851] Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail 2251 Extensions (S/MIME) Version 3.1 Message Specification", 2252 RFC 3851, DOI 10.17487/RFC3851, July 2004, 2253 . 2255 [RFC3852] Housley, R., "Cryptographic Message Syntax (CMS)", 2256 RFC 3852, DOI 10.17487/RFC3852, July 2004, 2257 . 2259 [RFC4134] Hoffman, P., Ed., "Examples of S/MIME Messages", RFC 4134, 2260 DOI 10.17487/RFC4134, July 2005, 2261 . 2263 [RFC4270] Hoffman, P. and B. Schneier, "Attacks on Cryptographic 2264 Hashes in Internet Protocols", RFC 4270, 2265 DOI 10.17487/RFC4270, November 2005, 2266 . 2268 [RFC4949] Shirey, R., "Internet Security Glossary, Version 2", 2269 FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007, 2270 . 2272 [RFC5750] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 2273 Mail Extensions (S/MIME) Version 3.2 Certificate 2274 Handling", RFC 5750, DOI 10.17487/RFC5750, January 2010, 2275 . 2277 [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 2278 Mail Extensions (S/MIME) Version 3.2 Message 2279 Specification", RFC 5751, DOI 10.17487/RFC5751, January 2280 2010, . 2282 [RFC6151] Turner, S. and L. Chen, "Updated Security Considerations 2283 for the MD5 Message-Digest and the HMAC-MD5 Algorithms", 2284 RFC 6151, DOI 10.17487/RFC6151, March 2011, 2285 . 2287 [RFC6194] Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security 2288 Considerations for the SHA-0 and SHA-1 Message-Digest 2289 Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011, 2290 . 2292 [RFC6278] Herzog, J. and R. Khazan, "Use of Static-Static Elliptic 2293 Curve Diffie-Hellman Key Agreement in Cryptographic 2294 Message Syntax", RFC 6278, DOI 10.17487/RFC6278, June 2295 2011, . 2297 [RFC7114] Leiba, B., "Creation of a Registry for smime-type 2298 Parameter Values", RFC 7114, DOI 10.17487/RFC7114, January 2299 2014, . 2301 [RFC7905] Langley, A., Chang, W., Mavrogiannopoulos, N., 2302 Strombergson, J., and S. Josefsson, "ChaCha20-Poly1305 2303 Cipher Suites for Transport Layer Security (TLS)", 2304 RFC 7905, DOI 10.17487/RFC7905, June 2016, 2305 . 2307 [SMIMEv2] "S/MIME version v2". 2309 This group of documents represents S/MIME version 2. This 2310 set of documents are [RFC2311], [RFC2312], [RFC2313], 2311 [RFC2314], and [RFC2315]. 2313 [SMIMEv3] "S/MIME version 3". 2315 This group of documents represents S/MIME version 3. This 2316 set of documents are [RFC2630], [RFC2631], [RFC2632], 2317 [RFC2633], [RFC2634], and [RFC5035]. 2319 [SMIMEv3.1] 2320 "S/MIME version 3.1". 2322 This group of documents represents S/MIME version 3.1. 2323 This set of documents are [RFC2634], [RFC3850], [RFC3851], 2324 [RFC3852], and [RFC5035]. 2326 [SMIMEv3.2] 2327 "S/MIME version 3.2". 2329 This group of documents represents S/MIME version 3.2. 2330 This set of documents are [RFC2634], [RFC5750], [RFC5751], 2331 [RFC5652], and [RFC5035]. 2333 [SP800-56A] 2334 National Institute of Standards and Technology (NIST), 2335 "Special Publication 800-56A Revision 2: Recommendation 2336 Pair-Wise Key Establishment Schemes Using Discrete 2337 Logarithm Cryptography", May 2013. 2339 [SP800-57] 2340 National Institute of Standards and Technology (NIST), 2341 "Special Publication 800-57: Recommendation for Key 2342 Management", August 2005. 2344 [TripleDES] 2345 Tuchman, W., "Hellman Presents No Shortcut Solutions to 2346 DES"", IEEE Spectrum v. 16, n. 7, pp 40-41, July 1979. 2348 Appendix A. ASN.1 Module 2350 Note: The ASN.1 module contained herein is unchanged from RFC 3851 2351 [SMIMEv3.1] with the exception of a change to the prefersBinaryInside 2352 ASN.1 comment. This module uses the 1988 version of ASN.1. 2354 SecureMimeMessageV3dot1 2355 { iso(1) member-body(2) us(840) rsadsi(113549) 2356 pkcs(1) pkcs-9(9) smime(16) modules(0) msg-v3dot1(21) } 2358 DEFINITIONS IMPLICIT TAGS ::= 2360 BEGIN 2362 IMPORTS 2364 -- Cryptographic Message Syntax [CMS] 2365 SubjectKeyIdentifier, IssuerAndSerialNumber, 2366 RecipientKeyIdentifier 2367 FROM CryptographicMessageSyntax 2368 { iso(1) member-body(2) us(840) rsadsi(113549) 2369 pkcs(1) pkcs-9(9) smime(16) modules(0) cms-2001(14) }; 2371 -- id-aa is the arc with all new authenticated and unauthenticated 2372 -- attributes produced by the S/MIME Working Group 2374 id-aa OBJECT IDENTIFIER ::= {iso(1) member-body(2) usa(840) 2375 rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) attributes(2)} 2377 -- S/MIME Capabilities provides a method of broadcasting the 2378 -- symmetric capabilities understood. Algorithms SHOULD be ordered 2379 -- by preference and grouped by type 2381 smimeCapabilities OBJECT IDENTIFIER ::= {iso(1) member-body(2) 2382 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 15} 2384 SMIMECapability ::= SEQUENCE { 2385 capabilityID OBJECT IDENTIFIER, 2386 parameters ANY DEFINED BY capabilityID OPTIONAL } 2388 SMIMECapabilities ::= SEQUENCE OF SMIMECapability 2390 -- Encryption Key Preference provides a method of broadcasting the 2391 -- preferred encryption certificate. 2393 id-aa-encrypKeyPref OBJECT IDENTIFIER ::= {id-aa 11} 2395 SMIMEEncryptionKeyPreference ::= CHOICE { 2396 issuerAndSerialNumber [0] IssuerAndSerialNumber, 2397 receipentKeyId [1] RecipientKeyIdentifier, 2398 subjectAltKeyIdentifier [2] SubjectKeyIdentifier 2399 } 2401 -- receipentKeyId is spelt incorrectly, but kept for historical 2402 -- reasons. 2404 id-smime OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) 2405 rsadsi(113549) pkcs(1) pkcs9(9) 16 } 2407 id-cap OBJECT IDENTIFIER ::= { id-smime 11 } 2409 -- The preferBinaryInside OID indicates an ability to receive 2410 -- messages with binary encoding inside the CMS wrapper. 2411 -- The preferBinaryInside attribute's value field is ABSENT. 2413 id-cap-preferBinaryInside OBJECT IDENTIFIER ::= { id-cap 1 } 2415 -- The following list OIDs to be used with S/MIME V3 2417 -- Signature Algorithms Not Found in [RFC3370], [RFC5754], [RFC4056], 2418 -- and [RFC3560] 2420 -- 2421 -- md2WithRSAEncryption OBJECT IDENTIFIER ::= 2422 -- {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 2423 -- 2} 2425 -- 2426 -- Other Signed Attributes 2427 -- 2428 -- signingTime OBJECT IDENTIFIER ::= 2429 -- {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 2430 -- 5} 2431 -- See [CMS] for a description of how to encode the attribute 2432 -- value. 2434 SMIMECapabilitiesParametersForRC2CBC ::= INTEGER 2435 -- (RC2 Key Length (number of bits)) 2437 END 2439 Appendix B. Historic Mail Considerations 2441 Over the course of updating the S/MIME specifications, the set of 2442 recommended algorithms has been modified each time the document has 2443 been updated. This means that if a user has historic emails and 2444 their user agent has been updated to only support the current set of 2445 recommended algorithms some of those old emails will no longer be 2446 accessible. It is strongly suggested that user agents implement some 2447 of the following algorithms for dealing with historic emails. 2449 This appendix contains a number of references to documents that have 2450 been obsoleted or replaced, this is intentional as frequently the 2451 updated documents do not have the same information in them. 2453 B.1. DigestAlgorithmIdentifier 2455 The following algorithms have been called our for some level of 2456 support by previous S/MIME specifications: 2458 - SHA-1 was dropped in [SMIMEv4.0]. SHA-1 is no longer considerd to 2459 be secure as it is no longer collision-resistant. The IETF 2460 statement on SHA-1 can be found in [RFC6194] but it is out-of-date 2461 relative to the most recient advances. 2463 - MD5 was dropped in [SMIMEv4.0]. MD5 is no longer considered to be 2464 secure as it is no longer collision-resistant. Details can be 2465 found in [RFC6151]. 2467 B.2. Signature Algorithms 2469 There are a number of problems with validating signatures on 2470 sufficently historic messages. For this reason it is strongly 2471 suggested that UAs treat these signatures differently from those on 2472 current messages. These problems include: 2474 - CAs are not required to keep certificates on a CRL beyond one 2475 update after a certificate has expired. This means that unless 2476 CRLs are cached as part of the message it is not always possible 2477 to check if a certificate has been revoked. The same problems 2478 exist with OCSP responses as they may be based on a CRL rather 2479 than on the certificate database. 2481 - RSA and DSA keys of less than 2048 bits are now considered by many 2482 experts to be cryptographically insecure (due to advances in 2483 computing power). Such keys were previously considered secure, so 2484 processing of historic signed messages will often result in the 2485 use of weak keys. Implementations that wish to support previous 2486 versions of S/MIME or process old messages need to consider the 2487 security risks that result from smaller key sizes (e.g., spoofed 2488 messages) versus the costs of denial of service. 2490 [SMIMEv3.1] set the lower limit on suggested key sizes for 2491 creating and validation at 1024 bits. Prior to that the lower 2492 bound on key sizes was 512 bits. 2494 - Hash functions used to validate signatures on historic messages 2495 may longer be considered to be secure. (See below.) While there 2496 are not currently any known practical pre-image or second pre- 2497 image attacks against MD5 or SHA-1, the fact they are no longer 2498 considered to be collision resistent the security levels of the 2499 signatures are generally considered suspect. 2501 - The previous two issues apply to the certificates used to validate 2502 the binding of the public key to the identity that signed the 2503 message as well. 2505 The following algorithms have been called out for some level of 2506 support by previous S/MIME specifications: 2508 - RSA with MD5 was dropped in [SMIMEv4.0]. MD5 is no longer 2509 considered to be secure as it is no longer collision-resistant. 2510 Details can be found in [RFC6151]. 2512 - RSA and DSA with SHA-1 were dropped in [SMIMEv4.0]. SHA-1 is no 2513 longer considered to be secure as it is no longer collision- 2514 resistant. The IETF statment on SHA-1 can be found in [RFC6194] 2515 but it is out-of-date relative to the most recent advances. 2517 - DSA with SHA-256 was dropped in [SMIMEv4.0]. DSA has been 2518 replaced by elliptic curve versions. 2520 As requirements for manditory to implement has changed over time, 2521 some issues have been created that can cause interopatability 2522 problems: 2524 - S/MIME v2 clients are only required to verify digital signatures 2525 using the rsaEncryption algorithm with SHA-1 or MD5, and might not 2526 implement id-dsa-with-sha1 or id-dsa at all. 2528 - S/MIME v3 clients might only implement signing or signature 2529 verification using id-dsa-with-sha1, and might also use id-dsa as 2530 an AlgorithmIdentifier in this field. 2532 - Note that S/MIME v3.1 clients support verifying id-dsa-with-sha1 2533 and rsaEncryption and might not implement sha256withRSAEncryption. 2535 NOTE: Receiving clients SHOULD recognize id-dsa as equivalent to id- 2536 dsa-with-sha1, and sending clients MUST use id-dsa-with-sha1 if using 2537 that algorithm. 2539 For 512-bit RSA with SHA-1 see [RFC3370] and [FIPS186-2] without 2540 Change Notice 1, for 512-bit RSA with SHA-256 see [RFC5754] and 2541 [FIPS186-2] without Change Notice 1, and for 1024-bit through 2542 2048-bit RSA with SHA-256 see [RFC5754] and [FIPS186-2] with Change 2543 Notice 1. The first reference provides the signature algorithm's 2544 object identifier, and the second provides the signature algorithm's 2545 definition. 2547 For 512-bit DSA with SHA-1 see [RFC3370] and [FIPS186-2] without 2548 Change Notice 1, for 512-bit DSA with SHA-256 see [RFC5754] and 2550 [FIPS186-2] without Change Notice 1, for 1024-bit DSA with SHA-1 see 2551 [RFC3370] and [FIPS186-2] with Change Notice 1, for 1024-bit and 2552 above DSA with SHA-256 see [RFC5754] and [FIPS186-4]. The first 2553 reference provides the signature algorithm's object identifier and 2554 the second provides the signature algorithm's definition. 2556 B.3. ContentEncryptionAlgorithmIdentifier 2558 The following algorithms have been called out for some level of 2559 support by previous S/MIME specifications: 2561 - RC2/40 [RFC2268] was dropped in [SMIMEv3.2]. The algorithm is 2562 known to be insecure and, if supported, should only be used to 2563 decrypt existing email. 2565 - DES EDE3 CBC [TripleDES], also known as "tripleDES" is dropped in 2566 [SMIMEv4.0]. This algorithms is removed from the supported list 2567 due to the fact that it has a 64-bit block size and the fact that 2568 it offers less that 128-bits of security. This algorithm should 2569 be supported only to decrypt existing email, it should not be used 2570 to encrypt new emails. 2572 B.4. KeyEncryptionAlgorithmIdentifier 2574 The following algorithms have been called out for some level of 2575 support by previous S/MIME specifications: 2577 - DH ephemeral-static mode, as specified in [RFC3370] and 2578 [SP800-57], was dropped in [SMIMEv4.0]. 2580 - RSA key sizes have been increased over time. Decrypting old mail 2581 with smaller key sizes is reasonable, however new mail should use 2582 the updated key sizes. 2584 For 1024-bit DH, see [RFC3370]. For 1024-bit and larger DH, see 2585 [SP800-56A]; regardless, use the KDF, which is from X9.42, specified 2586 in [RFC3370]. 2588 Appendix C. Moving S/MIME v2 Message Specification to Historic Status 2590 The S/MIME v3 [SMIMEv3], v3.1 [SMIMEv3.1], and v3.2 [SMIMEv3.2] are 2591 backwards compatible with the S/MIME v2 Message Specification 2592 [SMIMEv2], with the exception of the algorithms (dropped RC2/40 2593 requirement and added DSA and RSASSA-PSS requirements). Therefore, 2594 it is recommended that RFC 2311 [SMIMEv2] be moved to Historic 2595 status. 2597 Appendix D. Acknowledgments 2599 Many thanks go out to the other authors of the S/MIME version 2 2600 Message Specification RFC: Steve Dusse, Paul Hoffman, Laurence 2601 Lundblade, and Lisa Repka. Without v2, there wouldn't be a v3, v3.1, 2602 v3.2 or v4.0. 2604 Some of the examples in this document were stolen from [RFC4134]. 2605 Thanks go the the people who wrote and verified the examples in that 2606 document. 2608 A number of the members of the S/MIME Working Group have also worked 2609 very hard and contributed to this document. Any list of people is 2610 doomed to omission, and for that I apologize. In alphabetical order, 2611 the following people stand out in my mind because they made direct 2612 contributions to various versions of this document: 2614 Tony Capel, Piers Chivers, Dave Crocker, Bill Flanigan, Peter 2615 Gutmann, Alfred Hoenes, Paul Hoffman, Russ Housley, William Ottaway, 2616 and John Pawling. 2618 The version 4 update to the S/MIME documents was done under the 2619 auspices of the LAMPS Working Group. 2621 Authors' Addresses 2623 Jim Schaad 2624 August Cellars 2626 Email: ietf@augustcellars.com 2628 Blake Ramsdell 2629 Brute Squad Labs, Inc. 2631 Email: blaker@gmail.com 2633 Sean Turner 2634 sn3rd 2636 Email: sean@sn3rd.com