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1 S/MIME WG Blake Ramsdell, SendMail
2 Internet Draft Sean Turner, IECA
3 Intended Status: Standard Track August 20, 2008
4 Obsoletes: 3851 (when approved)
5 Expires: February 20, 2009
7 Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2
8 Message Specification
9 draft-ietf-smime-3851bis-05.txt
11 Status of this Memo
13 By submitting this Internet-Draft, each author represents that any
14 applicable patent or other IPR claims of which he or she is aware
15 have been or will be disclosed, and any of which he or she becomes
16 aware will be disclosed, in accordance with Section 6 of BCP 79.
18 Internet-Drafts are working documents of the Internet Engineering
19 Task Force (IETF), its areas, and its working groups. Note that
20 other groups may also distribute working documents as Internet-
21 Drafts.
23 Internet-Drafts are draft documents valid for a maximum of six months
24 and may be updated, replaced, or obsoleted by other documents at any
25 time. It is inappropriate to use Internet-Drafts as reference
26 material or to cite them other than as "work in progress."
28 The list of current Internet-Drafts can be accessed at
29 http://www.ietf.org/ietf/1id-abstracts.txt
31 The list of Internet-Draft Shadow Directories can be accessed at
32 http://www.ietf.org/shadow.html
34 This Internet-Draft will expire on February 20, 2008.
36 Copyright Notice
38 Copyright (C) The IETF Trust (2008).
40 Abstract
42 This document defines Secure/Multipurpose Internet Mail Extensions
43 (S/MIME) version 3.2. S/MIME provides a consistent way to send and
44 receive secure MIME data. Digital signatures provide authentication,
45 message integrity, and non-repudiation with proof of origin.
47 Encryption provides data confidentiality. Compression can be used to
48 reduce data size. This document obsoletes RFC 3851.
50 Discussion
52 This draft is being discussed on the 'ietf-smime' mailing list. To
53 subscribe, send a message to ietf-smime-request@imc.org with the
54 single word subscribe in the body of the message. There is a Web site
55 for the mailing list at .
57 Table of Contents
59 1. Introduction...................................................3
60 1.1. Specification Overview....................................3
61 1.2. Definitions...............................................4
62 1.3. Conventions used in this document.........................5
63 1.4. Compatibility with Prior Practice of S/MIME...............6
64 1.5. Changes From S/MIME v3 to S/MIME v3.1.....................6
65 1.6. Changes Since S/MIME v3.1.................................6
66 2. CMS Options....................................................8
67 2.1. DigestAlgorithmIdentifier.................................8
68 2.2. SignatureAlgorithmIdentifier..............................8
69 2.3. KeyEncryptionAlgorithmIdentifier..........................9
70 2.4. General Syntax............................................9
71 2.5. Attributes and the SignerInfo Type.......................10
72 2.6. SignerIdentifier SignerInfo Type.........................14
73 2.7. ContentEncryptionAlgorithmIdentifier.....................15
74 3. Creating S/MIME Messages......................................17
75 3.1. Preparing the MIME Entity for Signing, Enveloping or
76 Compressing..............................................17
77 3.2. The application/pkcs7-mime Media Type....................22
78 3.3. Creating an Enveloped-only Message.......................24
79 3.4. Creating a Signed-only Message...........................25
80 3.5. Creating an Compressed-only Message......................29
81 3.6. Multiple Operations......................................30
82 3.7. Creating a Certificate Management Message................30
83 3.8. Registration Requests....................................31
84 3.9. Identifying an S/MIME Message............................31
85 4. Certificate Processing........................................32
86 4.1. Key Pair Generation......................................32
87 4.2. Signature Generation.....................................32
88 4.3. Signature Verification...................................32
89 4.4. Encryption...............................................33
90 4.5. Decryption...............................................33
91 5. IANA Considerations...........................................33
92 5.1. Media Type for application/pkcs7-mime....................34
93 5.2. Media Type for application/pkcs7-signature...............35
95 6. Security Considerations.......................................36
96 7. References....................................................38
97 7.1. Normative References.....................................38
98 7.2. Informative References...................................40
99 Appendix A. ASN.1 Module.........................................42
100 Appendix B. Moving S/MIME v2 Message Specification to
101 Historic Status......................................44
102 Appendix C. Acknowledgements.....................................44
104 1. Introduction
106 S/MIME (Secure/Multipurpose Internet Mail Extensions) provides a
107 consistent way to send and receive secure MIME data. Based on the
108 popular Internet MIME standard, S/MIME provides the following
109 cryptographic security services for electronic messaging
110 applications: authentication, message integrity and non-repudiation
111 of origin (using digital signatures), and data confidentiality (using
112 encryption). As a supplementary service, S/MIME provides for message
113 compression.
115 S/MIME can be used by traditional mail user agents (MUAs) to add
116 cryptographic security services to mail that is sent, and to
117 interpret cryptographic security services in mail that is received.
118 However, S/MIME is not restricted to mail; it can be used with any
119 transport mechanism that transports MIME data, such as HTTP or SIP.
120 As such, S/MIME takes advantage of the object-based features of MIME
121 and allows secure messages to be exchanged in mixed-transport
122 systems.
124 Further, S/MIME can be used in automated message transfer agents that
125 use cryptographic security services that do not require any human
126 intervention, such as the signing of software-generated documents and
127 the encryption of FAX messages sent over the Internet.
129 1.1. Specification Overview
131 This document describes a protocol for adding cryptographic signature
132 and encryption services to MIME data. The MIME standard [MIME-SPEC]
133 provides a general structure for the content of Internet messages and
134 allows extensions for new content type based applications.
136 This specification defines how to create a MIME body part that has
137 been cryptographically enhanced according to CMS [CMS], which is
138 derived from PKCS #7 [PKCS-7]. This specification also defines the
139 application/pkcs7-mime media type that can be used to transport those
140 body parts.
142 This document also discusses how to use the multipart/signed media
143 type defined in [MIME-SECURE] to transport S/MIME signed messages.
144 multipart/signed is used in conjunction with the application/pkcs7-
145 signature media type, which is used to transport a detached S/MIME
146 signature.
148 In order to create S/MIME messages, an S/MIME agent MUST follow the
149 specifications in this document, as well as the specifications listed
150 in the Cryptographic Message Syntax document [CMS], [CMSALG],
151 [RSAPSS], [RSAOAEP], and [CMS-SHA2].
153 Throughout this specification, there are requirements and
154 recommendations made for how receiving agents handle incoming
155 messages. There are separate requirements and recommendations for
156 how sending agents create outgoing messages. In general, the best
157 strategy is to "be liberal in what you receive and conservative in
158 what you send". Most of the requirements are placed on the handling
159 of incoming messages while the recommendations are mostly on the
160 creation of outgoing messages.
162 The separation for requirements on receiving agents and sending
163 agents also derives from the likelihood that there will be S/MIME
164 systems that involve software other than traditional Internet mail
165 clients. S/MIME can be used with any system that transports MIME
166 data. An automated process that sends an encrypted message might not
167 be able to receive an encrypted message at all, for example. Thus,
168 the requirements and recommendations for the two types of agents are
169 listed separately when appropriate.
171 1.2. Definitions
173 For the purposes of this specification, the following definitions
174 apply.
176 ASN.1: Abstract Syntax Notation One, as defined in ITU-T
177 Recommendation X.680 [X.680].
179 BER: Basic Encoding Rules for ASN.1, as defined in ITU-T
180 Recommendation X.690 [X.690].
182 Certificate: A type that binds an entity's name to a public key with
183 a digital signature.
185 DER: Distinguished Encoding Rules for ASN.1, as defined in ITU-T
186 Recommendation X.690 [X.690].
188 7-bit data: Text data with lines less than 998 characters long, where
189 none of the characters have the 8th bit set, and there are no NULL
190 characters. and occur only as part of a end of
191 line delimiter.
193 8-bit data: Text data with lines less than 998 characters, and where
194 none of the characters are NULL characters. and occur only
195 as part of a end of line delimiter.
197 Binary data: Arbitrary data.
199 Transfer Encoding: A reversible transformation made on data so 8-bit
200 or binary data can be sent via a channel that only transmits 7-bit
201 data.
203 Receiving agent: Software that interprets and processes S/MIME CMS
204 objects, MIME body parts that contain CMS content types, or both.
206 Sending agent: Software that creates S/MIME CMS content types, MIME
207 body parts that contain CMS content types, or both.
209 S/MIME agent: User software that is a receiving agent, a sending
210 agent, or both.
212 1.3. Conventions used in this document
214 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
215 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
216 document are to be interpreted as described in [MUSTSHOULD].
218 We define some additional terms here:
220 SHOULD+ This term means the same as SHOULD. However, the authors
221 expect that a requirement marked as SHOULD+ will be promoted at
222 some future time to be a MUST.
224 SHOULD- This term means the same as SHOULD. However, the authors
225 expect a requirement marked as SHOULD- will be demoted to a MAY
226 in a future version of this document.
228 MUST- This term means the same as MUST. However, the authors
229 expect that this requirement will no longer be a MUST in a future
230 document. Although its status will be determined at a later
231 time, it is reasonable to expect that if a future revision of a
232 document alters the status of a MUST- requirement, it will remain
233 at least a SHOULD or a SHOULD-.
235 1.4. Compatibility with Prior Practice of S/MIME
237 S/MIME version 3.2 agents SHOULD attempt to have the greatest
238 interoperability possible with agents for prior versions of S/MIME.
239 S/MIME version 2 is described in RFC 2311 through RFC 2315 inclusive
240 [SMIMEv2], S/MIME version 3 is described in RFC 2630 through RFC 2634
241 inclusive [SMIMEv3], and S/MIME version 3.1 is described in RFC 3850
242 through RFC 3852 and RFC 2634 [SMIMEv3.1]. RFC 2311 also has
243 historical information about the development of S/MIME.
245 1.5. Changes From S/MIME v3 to S/MIME v3.1
247 The RSA public key algorithm was changed to a MUST implement key
248 wrapping algorithm, and the Diffie-Hellman algorithm changed to a
249 SHOULD implement.
251 The AES symmetric encryption algorithm has been included as a SHOULD
252 implement.
254 The RSA public key algorithm was changed to a MUST implement
255 signature algorithm.
257 Ambiguous language about the use of "empty" SignedData messages to
258 transmit certificates was clarified to reflect that transmission of
259 certificate revocation lists is also allowed.
261 The use of binary encoding for some MIME entities is now explicitly
262 discussed.
264 Header protection through the use of the message/rfc822 media type
265 has been added.
267 Use of the CompressedData CMS type is allowed, along with required
268 media type and file extension additions.
270 1.6. Changes Since S/MIME v3.1
272 Editorial changes, e.g., replaced "MIME type" with "media type",
273 content-type with Content-Type.
275 Moved "Conventions Used in This Document" to Section 1.2. Added
276 definitions for SHOULD+, SHOULD-, and MUST-.
278 Sec 1.1 and Appendix A: Added references to RFCs for RSA-PSS, RSA-
279 OAEP, and SHA2 CMS Algorithms. Added CMS Multiple Signers
280 Clarification to CMS reference.
282 Sec 1.2: Updated references to ASN.1 to X.680 and BER and DER to
283 X.690.
285 Sec 1.3: Added references to S/MIME MSG 3.1 RFCs.
287 Sec 2.1 (digest algorithm): SHA-256 added as MUST, SHA-1 and MD5 made
288 SHOULD-.
290 Sec 2.2 (signature algorithms): RSA with SHA-256 and DSA with SHA-256
291 added as MUSTs, RSA with SHA-1, DSA with SHA-1, and RSA with MD5
292 changed to SHOULD-, and RSA-PSS with SHA-256 added as SHOULD+. Also
293 added note about what S/MIME v3.1 clients support.
295 Sec 2.3 (key encryption): DH changed to SHOULD- and RSA-OAEP added as
296 SHOULD+.
298 Sec 2.5.1: Added requirement that receiving agents MUST support both
299 GeneralizedTime and UTCTime.
301 Sec 2.5.2: Replaced reference "sha1WithRSAEncrption" with
302 "sha256WithRSAEncryption", "DES-3EDE-CBC" and "AES-128 CBC", and
303 deleted the RC5 example.
305 Sec 2.5.2.1, 2.7, 2.7.1, Appendix A: references to RC2/40 removed.
307 Sec 2.7 (content encryption): AES-128 CBC added as MUST, AES-192 and
308 AES-256 CBC SHOULD+, tripleDES now SHOULD-.
310 Sec 2.7.1: Updated pointers from 2.7.2.1 through 2.7.2.4 to 2.7.1.1
311 to 2.7.1.2.
313 Sec 3.2.2: Replaced "encrypted" with "enveloped." Update OID example
314 to use AES-128 CBC oid.
316 Sec 4: Updated reference to CERT v3.2.
318 Sec 4.1: Updated RSA and DSA key size discussion. Moved last four
319 sentences to security considerations. Updated reference to randomness
320 requirements for security.
322 Sec 5: Added IANA registration templates to update media type
323 registry to point to this document as opposed to RFC 2311.
325 Sec 6: Updated Security Considerations.
327 Sec 7: Moved references from Appendix B to this section. Update
328 references. Added informational references to SMIMEv2, SMIMEv3, and
329 SMIMEv3.1.
331 App B: Added Appendix B to move S/MIME v2 to historic status.
333 2. CMS Options
335 CMS allows for a wide variety of options in content, attributes, and
336 algorithm support. This section puts forth a number of support
337 requirements and recommendations in order to achieve a base level of
338 interoperability among all S/MIME implementations. [CMSALG] and [CMS-
339 SHA2] provides additional details regarding the use of the
340 cryptographic algorithms. [ESS] provides additional details
341 regarding the use of additional attributes.
343 2.1. DigestAlgorithmIdentifier
345 Sending and receiving agents MUST support SHA-256 [CMS-SHA2] and
346 SHOULD- support SHA-1 [CMSALG]. Receiving agents SHOULD- support MD5
347 [CMSALG] for the purpose of providing backward compatibility with
348 MD5-digested S/MIME v2 SignedData objects.
350 2.2. SignatureAlgorithmIdentifier
352 Receiving agents:
354 - MUST support RSA with SHA-256, as specified in [CMS-SHA2]
356 - SHOULD+ support DSA with SHA-256, as specified in [CMS-SHA2]
358 - SHOULD+ support RSA-PSS with SHA-256, as specified in [RSAPSS]
360 - SHOULD- support RSA with SHA-1, as specified in [CMSALG]
362 - SHOULD- support DSA with SHA-1, as specified in [CMSALG]
364 - SHOULD- support RSA with MD5, as specified in [CMSALG].
366 Sending agents:
368 - MUST support RSA with SHA-256, as specified in [CMS-SHA2]
370 - SHOULD+ support DSA with SHA-256, as specified in [CMS-SHA2]
372 - SHOULD+ support RSA-PSS with SHA-256, as specified in [RSAPSS]
374 - SHOULD- support RSA with SHA-1 or DSA with SHA-1, as specified in
375 [CMSALG]
377 - SHOULD- support RSA with MD5, as specified in [CMSALG].
379 Note that S/MIME v3.1 clients support verifying id-dsa-with-sha1 and
380 rsaEncryption and might not implement sha256withRSAEncryption. Note
381 that S/MIME v3 clients might only implement signing or signature
382 verification using id-dsa-with-sha1, and might also use id-dsa as an
383 AlgorithmIdentifier in this field. Receiving clients SHOULD
384 recognize id-dsa as equivalent to id-dsa-with-sha1, and sending
385 clients MUST use id-dsa-with-sha1 if using that algorithm. Also note
386 that S/MIME v2 clients are only required to verify digital signatures
387 using the rsaEncryption algorithm with SHA-1 or MD5, and might not
388 implement id-dsa-with-sha1 or id-dsa at all.
390 2.3. KeyEncryptionAlgorithmIdentifier
392 Receiving and sending agents:
394 - MUST support RSA Encryption, as specified in [CMSALG]
396 - SHOULD+ support RSA-OAEP, as specified in [RSAOAEP]
398 - SHOULD- support DH ephemeral-static mode, as specified
399 in [CMSALG].
401 Note that S/MIME v3.1 clients might only implement key encryption and
402 decryption using the rsaEncryption algorithm. Note that S/MIME v3
403 clients might only implement key encryption and decryption using the
404 Diffie-Hellman algorithm. Also note that S/MIME v2 clients are only
405 capable of decrypting content-encryption keys using the rsaEncryption
406 algorithm.
408 2.4. General Syntax
410 There are several CMS content types. Of these, only the Data,
411 SignedData, EnvelopedData, and CompressedData content types are
412 currently used for S/MIME.
414 2.4.1. Data Content Type
416 Sending agents MUST use the id-data content type identifier to
417 identify the "inner" MIME message content. For example, when
418 applying a digital signature to MIME data, the CMS SignedData
419 encapContentInfo eContentType MUST include the id-data object
420 identifier and the media type MUST be stored in the SignedData
421 encapContentInfo eContent OCTET STRING (unless the sending agent is
422 using multipart/signed, in which case the eContent is absent, per
423 section 3.4.3 of this document). As another example, when applying
424 encryption to MIME data, the CMS EnvelopedData encryptedContentInfo
425 contentType MUST include the id-data object identifier and the
426 encrypted MIME content MUST be stored in the EnvelopedData
427 encryptedContentInfo encryptedContent OCTET STRING.
429 2.4.2. SignedData Content Type
431 Sending agents MUST use the SignedData content type to apply a
432 digital signature to a message or, in a degenerate case where there
433 is no signature information, to convey certificates. Applying a
434 signature to a message provides authentication, message integrity,
435 and non-repudiation of origin.
437 2.4.3. EnvelopedData Content Type
439 This content type is used to apply data confidentiality to a message.
440 A sender needs to have access to a public key for each intended
441 message recipient to use this service.
443 2.4.4. CompressedData Content Type
445 This content type is used to apply data compression to a message.
446 This content type does not provide authentication, message integrity,
447 non-repudiation, or data confidentiality, and is only used to reduce
448 the message's size.
450 See section 3.6 for further guidance on the use of this type in
451 conjunction with other CMS types.
453 2.5. Attributes and the SignerInfo Type
455 The SignerInfo type allows the inclusion of unsigned and signed
456 attributes along with a signature.
458 Receiving agents MUST be able to handle zero or one instance of each
459 of the signed attributes listed here. Sending agents SHOULD generate
460 one instance of each of the following signed attributes in each
461 S/MIME message:
463 - signingTime (section 2.5.1 in this document)
465 - sMIMECapabilities (section 2.5.2 in this document)
467 - sMIMEEncryptionKeyPreference (section 2.5.3 in this document)
469 - id-messageDigest (section 11.2 in [CMS])
471 - id-contentType (section 11.1 in [CMS])
473 Further, receiving agents SHOULD be able to handle zero or one
474 instance in the signingCertificate signed attribute, as defined in
475 section 5 of [ESS].
477 Sending agents SHOULD generate one instance of the signingCertificate
478 signed attribute in each SignerInfo structure.
480 Additional attributes and values for these attributes might be
481 defined in the future. Receiving agents SHOULD handle attributes or
482 values that they do not recognize in a graceful manner.
484 Interactive sending agents that include signed attributes that are
485 not listed here SHOULD display those attributes to the user, so that
486 the user is aware of all of the data being signed.
488 2.5.1. Signing-Time Attribute
490 The signing-time attribute is used to convey the time that a message
491 was signed. The time of signing will most likely be created by a
492 message originator and therefore is only as trustworthy as the
493 originator.
495 Sending agents MUST encode signing time through the year 2049 as
496 UTCTime; signing times in 2050 or later MUST be encoded as
497 GeneralizedTime. When the UTCTime CHOICE is used, S/MIME agents MUST
498 interpret the year field (YY) as follows:
500 If YY is greater than or equal to 50, the year is interpreted as
501 19YY; if YY is less than 50, the year is interpreted as 20YY.
503 Receiving agents MUST be able to process signing-time attributes that
504 are encoded in either UTCTime or GeneralizedTime.
506 2.5.2. SMIMECapabilities Attribute
508 The SMIMECapabilities attribute includes signature algorithms (such
509 as "sha256WithRSAEncryption"), symmetric algorithms (such as "AES-128
510 CBC"), and key encipherment algorithms (such as "rsaEncryption").
511 There are also several identifiers which indicate support for other
512 optional features such as binary encoding and compression. The
513 SMIMECapabilities were designed to be flexible and extensible so
514 that, in the future, a means of identifying other capabilities and
515 preferences such as certificates can be added in a way that will not
516 cause current clients to break.
518 If present, the SMIMECapabilities attribute MUST be a
519 SignedAttribute; it MUST NOT be an UnsignedAttribute. CMS defines
520 SignedAttributes as a SET OF Attribute. The SignedAttributes in a
521 signerInfo MUST NOT include multiple instances of the
522 SMIMECapabilities attribute. CMS defines the ASN.1 syntax for
523 Attribute to include attrValues SET OF AttributeValue. A
524 SMIMECapabilities attribute MUST only include a single instance of
525 AttributeValue. There MUST NOT be zero or multiple instances of
526 AttributeValue present in the attrValues SET OF AttributeValue.
528 The semantics of the SMIMECapabilities attribute specify a partial
529 list as to what the client announcing the SMIMECapabilities can
530 support. A client does not have to list every capability it
531 supports, and need not list all its capabilities so that the
532 capabilities list doesn't get too long. In an SMIMECapabilities
533 attribute, the object identifiers (OIDs) are listed in order of their
534 preference, but SHOULD be separated logically along the lines of
535 their categories (signature algorithms, symmetric algorithms, key
536 encipherment algorithms, etc.)
538 The structure of the SMIMECapabilities attribute is to facilitate
539 simple table lookups and binary comparisons in order to determine
540 matches. For instance, the DER-encoding for the SMIMECapability for
541 AES-128 CBC MUST be identically encoded regardless of the
542 implementation. Because of the requirement for identical encoding,
543 individuals documenting algorithms to be used in the
544 SMIMECapabilities attribute SHOULD explicitly document the correct
545 byte sequence for the common cases.
547 For any capability, the associated parameters for the OID MUST
548 specify all of the parameters necessary to differentiate between two
549 instances of the same algorithm.
551 The OIDs that correspond to algorithms SHOULD use the same OID as the
552 actual algorithm, except in the case where the algorithm usage is
553 ambiguous from the OID. For instance, in an earlier specification,
554 rsaEncryption was ambiguous because it could refer to either a
555 signature algorithm or a key encipherment algorithm. In the event
556 that an OID is ambiguous, it needs to be arbitrated by the maintainer
557 of the registered SMIMECapabilities list as to which type of
558 algorithm will use the OID, and a new OID MUST be allocated under the
559 smimeCapabilities OID to satisfy the other use of the OID.
561 The registered SMIMECapabilities list specifies the parameters for
562 OIDs that need them, most notably key lengths in the case of
563 variable-length symmetric ciphers. In the event that there are no
564 differentiating parameters for a particular OID, the parameters MUST
565 be omitted, and MUST NOT be encoded as NULL. Additional values for
566 the SMIMECapabilities attribute might be defined in the future.
567 Receiving agents MUST handle a SMIMECapabilities object that has
568 values that it does not recognize in a graceful manner.
570 Section 2.7.1 explains a strategy for caching capabilities.
572 2.5.3. Encryption Key Preference Attribute
574 The encryption key preference attribute allows the signer to
575 unambiguously describe which of the signer's certificates has the
576 signer's preferred encryption key. This attribute is designed to
577 enhance behavior for interoperating with those clients that use
578 separate keys for encryption and signing. This attribute is used to
579 convey to anyone viewing the attribute which of the listed
580 certificates is appropriate for encrypting a session key for future
581 encrypted messages.
583 If present, the SMIMEEncryptionKeyPreference attribute MUST be a
584 SignedAttribute; it MUST NOT be an UnsignedAttribute. CMS defines
585 SignedAttributes as a SET OF Attribute. The SignedAttributes in a
586 signerInfo MUST NOT include multiple instances of the
587 SMIMEEncryptionKeyPreference attribute. CMS defines the ASN.1 syntax
588 for Attribute to include attrValues SET OF AttributeValue. A
589 SMIMEEncryptionKeyPreference attribute MUST only include a single
590 instance of AttributeValue. There MUST NOT be zero or multiple
591 instances of AttributeValue present in the attrValues SET OF
592 AttributeValue.
594 The sending agent SHOULD include the referenced certificate in the
595 set of certificates included in the signed message if this attribute
596 is used. The certificate MAY be omitted if it has been previously
597 made available to the receiving agent. Sending agents SHOULD use
598 this attribute if the commonly used or preferred encryption
599 certificate is not the same as the certificate used to sign the
600 message.
602 Receiving agents SHOULD store the preference data if the signature on
603 the message is valid and the signing time is greater than the
604 currently stored value. (As with the SMIMECapabilities, the clock
605 skew SHOULD be checked and the data not used if the skew is too
606 great.) Receiving agents SHOULD respect the sender's encryption key
607 preference attribute if possible. This, however, represents only a
608 preference and the receiving agent can use any certificate in
609 replying to the sender that is valid.
611 Section 2.7.1 explains a strategy for caching preference data.
613 2.5.3.1. Selection of Recipient Key Management Certificate
615 In order to determine the key management certificate to be used when
616 sending a future CMS EnvelopedData message for a particular
617 recipient, the following steps SHOULD be followed:
619 - If an SMIMEEncryptionKeyPreference attribute is found in a
620 SignedData object received from the desired recipient, this
621 identifies the X.509 certificate that SHOULD be used as the X.509
622 key management certificate for the recipient.
624 - If an SMIMEEncryptionKeyPreference attribute is not found in a
625 SignedData object received from the desired recipient, the set of
626 X.509 certificates SHOULD be searched for a X.509 certificate
627 with the same subject name as the signer of a X.509 certificate
628 which can be used for key management.
630 - Or use some other method of determining the user's key management
631 key. If a X.509 key management certificate is not found, then
632 encryption cannot be done with the signer of the message. If
633 multiple X.509 key management certificates are found, the S/MIME
634 agent can make an arbitrary choice between them.
636 2.6. SignerIdentifier SignerInfo Type
638 S/MIME v3.2 implementations MUST support both issuerAndSerialNumber
639 as well as subjectKeyIdentifier. Messages that use the
640 subjectKeyIdentifier choice cannot be read by S/MIME v2 clients.
642 It is important to understand that some certificates use a value for
643 subjectKeyIdentifier that is not suitable for uniquely identifying a
644 certificate. Implementations MUST be prepared for multiple
645 certificates for potentially different entities to have the same
646 value for subjectKeyIdentifier, and MUST be prepared to try each
647 matching certificate during signature verification before indicating
648 an error condition.
650 2.7. ContentEncryptionAlgorithmIdentifier
652 Sending and receiving agents:
654 - MUST support encryption and decryption with AES-128 CBC [CMSAES]
656 - SHOULD+ support encryption and decryption with AES-192 CBC and
657 AES-256 CBC [CMSAES]
659 - SHOULD- support encryption and decryption with DES EDE3 CBC,
660 hereinafter called "tripleDES" [CMSALG].
662 2.7.1. Deciding Which Encryption Method To Use
664 When a sending agent creates an encrypted message, it has to decide
665 which type of encryption to use. The decision process involves using
666 information garnered from the capabilities lists included in messages
667 received from the recipient, as well as out-of-band information such
668 as private agreements, user preferences, legal restrictions, and so
669 on.
671 Section 2.5.2 defines a method by which a sending agent can
672 optionally announce, among other things, its decrypting capabilities
673 in its order of preference. The following method for processing and
674 remembering the encryption capabilities attribute in incoming signed
675 messages SHOULD be used.
677 - If the receiving agent has not yet created a list of capabilities
678 for the sender's public key, then, after verifying the signature
679 on the incoming message and checking the timestamp, the receiving
680 agent SHOULD create a new list containing at least the signing
681 time and the symmetric capabilities.
683 - If such a list already exists, the receiving agent SHOULD verify
684 that the signing time in the incoming message is greater than the
685 signing time stored in the list and that the signature is valid.
686 If so, the receiving agent SHOULD update both the signing time
687 and capabilities in the list. Values of the signing time that
688 lie far in the future (that is, a greater discrepancy than any
689 reasonable clock skew), or a capabilities list in messages whose
690 signature could not be verified, MUST NOT be accepted.
692 The list of capabilities SHOULD be stored for future use in creating
693 messages.
695 Before sending a message, the sending agent MUST decide whether it is
696 willing to use weak encryption for the particular data in the
697 message. If the sending agent decides that weak encryption is
698 unacceptable for this data, then the sending agent MUST NOT use a
699 weak algorithm. The decision to use or not use weak encryption
700 overrides any other decision in this section about which encryption
701 algorithm to use.
703 Sections 2.7.1.1 through 2.7.1.2 describe the decisions a sending
704 agent SHOULD use in deciding which type of encryption will be applied
705 to a message. These rules are ordered, so the sending agent SHOULD
706 make its decision in the order given.
708 2.7.1.1. Rule 1: Known Capabilities
710 If the sending agent has received a set of capabilities from the
711 recipient for the message the agent is about to encrypt, then the
712 sending agent SHOULD use that information by selecting the first
713 capability in the list (that is, the capability most preferred by the
714 intended recipient) that the sending agent knows how to encrypt. The
715 sending agent SHOULD use one of the capabilities in the list if the
716 agent reasonably expects the recipient to be able to decrypt the
717 message.
719 2.7.1.2. Rule 2: Unknown Capabilities, Unknown Version of S/MIME
721 If the following two conditions are met:
723 - The sending agent has no knowledge of the encryption capabilities
724 of the recipient, and
726 - The sending agent has no knowledge of the version of S/MIME of the
727 recipient,
729 then the sending agent SHOULD use AES-128 because it is a stronger
730 algorithm and is required by S/MIME v3.2. If the sending agent
731 chooses not to use AES-128 in this step, it SHOULD use tripleDES.
733 2.7.2. Choosing Weak Encryption
735 All algorithms that use 40 bit keys are considered by many to be weak
736 encryption. A sending agent that is controlled by a human SHOULD
737 allow a human sender to determine the risks of sending data using a
738 weak encryption algorithm before sending the data, and possibly allow
739 the human to use a stronger encryption method such as tripleDES or
740 AES.
742 2.7.3. Multiple Recipients
744 If a sending agent is composing an encrypted message to a group of
745 recipients where the encryption capabilities of some of the
746 recipients do not overlap, the sending agent is forced to send more
747 than one message. Please note that if the sending agent chooses to
748 send a message encrypted with a strong algorithm, and then send the
749 same message encrypted with a weak algorithm, someone watching the
750 communications channel could learn the contents of the strongly-
751 encrypted message simply by decrypting the weakly-encrypted message.
753 3. Creating S/MIME Messages
755 This section describes the S/MIME message formats and how they are
756 created. S/MIME messages are a combination of MIME bodies and CMS
757 content types. Several media types as well as several CMS content
758 types are used. The data to be secured is always a canonical MIME
759 entity. The MIME entity and other data, such as certificates and
760 algorithm identifiers, are given to CMS processing facilities which
761 produce a CMS object. Finally, the CMS object is wrapped in MIME.
762 The Enhanced Security Services for S/MIME [ESS] document provides
763 descriptions of how nested, secured S/MIME messages are formatted.
764 ESS provides a description of how a triple-wrapped S/MIME message is
765 formatted using multipart/signed and application/pkcs7-mime for the
766 signatures.
768 S/MIME provides one format for enveloped-only data, several formats
769 for signed-only data, and several formats for signed and enveloped
770 data. Several formats are required to accommodate several
771 environments, in particular for signed messages. The criteria for
772 choosing among these formats are also described.
774 The reader of this section is expected to understand MIME as
775 described in [MIME-SPEC] and [MIME-SECURE].
777 3.1. Preparing the MIME Entity for Signing, Enveloping or Compressing
779 S/MIME is used to secure MIME entities. A MIME entity can be a sub-
780 part, sub-parts of a message, or the whole message with all its sub-
781 parts. A MIME entity that is the whole message includes only the
782 MIME message headers and MIME body, and does not include the RFC-822
783 header. Note that S/MIME can also be used to secure MIME entities
784 used in applications other than Internet mail. If protection of the
785 RFC-822 header is required, the use of the message/rfc822 media type
786 is explained later in this section.
788 The MIME entity that is secured and described in this section can be
789 thought of as the "inside" MIME entity. That is, it is the
790 "innermost" object in what is possibly a larger MIME message.
791 Processing "outside" MIME entities into CMS content types is
792 described in Section 3.2, 3.4, and elsewhere.
794 The procedure for preparing a MIME entity is given in [MIME-SPEC].
795 The same procedure is used here with some additional restrictions
796 when signing. Description of the procedures from [MIME-SPEC] are
797 repeated here, but it is suggested that the reader refer to that
798 document for the exact procedure. This section also describes
799 additional requirements.
801 A single procedure is used for creating MIME entities that are to
802 have any combination of signing, enveloping, and compressing applied.
803 Some additional steps are recommended to defend against known
804 corruptions that can occur during mail transport that are of
805 particular importance for clear-signing using the multipart/signed
806 format. It is recommended that these additional steps be performed
807 on enveloped messages, or signed and enveloped messages, so that the
808 message can be forwarded to any environment without modification.
810 These steps are descriptive rather than prescriptive. The
811 implementer is free to use any procedure as long as the result is the
812 same.
814 Step 1. The MIME entity is prepared according to the local
815 conventions.
817 Step 2. The leaf parts of the MIME entity are converted to
818 canonical form.
820 Step 3. Appropriate transfer encoding is applied to the leaves
821 of the MIME entity.
823 When an S/MIME message is received, the security services on the
824 message are processed, and the result is the MIME entity. That MIME
825 entity is typically passed to a MIME-capable user agent where it is
826 further decoded and presented to the user or receiving application.
828 In order to protect outer, non-content related message header fields
829 (for instance, the "Subject", "To", "From" and "Cc" fields), the
830 sending client MAY wrap a full MIME message in a message/rfc822
831 wrapper in order to apply S/MIME security services to these header
832 fields. It is up to the receiving client to decide how to present
833 this "inner" header along with the unprotected "outer" header.
835 When an S/MIME message is received, if the top-level protected MIME
836 entity has a Content-Type of message/rfc822, it can be assumed that
837 the intent was to provide header protection. This entity SHOULD be
838 presented as the top-level message, taking into account header
839 merging issues as previously discussed.
841 3.1.1. Canonicalization
843 Each MIME entity MUST be converted to a canonical form that is
844 uniquely and unambiguously representable in the environment where the
845 signature is created and the environment where the signature will be
846 verified. MIME entities MUST be canonicalized for enveloping and
847 compressing as well as signing.
849 The exact details of canonicalization depend on the actual media type
850 and subtype of an entity, and are not described here. Instead, the
851 standard for the particular media type SHOULD be consulted. For
852 example, canonicalization of type text/plain is different from
853 canonicalization of audio/basic. Other than text types, most types
854 have only one representation regardless of computing platform or
855 environment which can be considered their canonical representation.
856 In general, canonicalization will be performed by the non-security
857 part of the sending agent rather than the S/MIME implementation.
859 The most common and important canonicalization is for text, which is
860 often represented differently in different environments. MIME
861 entities of major type "text" MUST have both their line endings and
862 character set canonicalized. The line ending MUST be the pair of
863 characters , and the charset SHOULD be a registered charset
864 [CHARSETS]. The details of the canonicalization are specified in
865 [MIME-SPEC]. The chosen charset SHOULD be named in the charset
866 parameter so that the receiving agent can unambiguously determine the
867 charset used.
869 Note that some charsets such as ISO-2022 have multiple
870 representations for the same characters. When preparing such text
871 for signing, the canonical representation specified for the charset
872 MUST be used.
874 3.1.2. Transfer Encoding
876 When generating any of the secured MIME entities below, except the
877 signing using the multipart/signed format, no transfer encoding is
878 required at all. S/MIME implementations MUST be able to deal with
879 binary MIME objects. If no Content-Transfer-Encoding header field is
880 present, the transfer encoding is presumed to be 7BIT.
882 S/MIME implementations SHOULD however use transfer encoding described
883 in section 3.1.3 for all MIME entities they secure. The reason for
884 securing only 7-bit MIME entities, even for enveloped data that are
885 not exposed to the transport, is that it allows the MIME entity to be
886 handled in any environment without changing it. For example, a
887 trusted gateway might remove the envelope, but not the signature, of
888 a message, and then forward the signed message on to the end
889 recipient so that they can verify the signatures directly. If the
890 transport internal to the site is not 8-bit clean, such as on a wide-
891 area network with a single mail gateway, verifying the signature will
892 not be possible unless the original MIME entity was only 7-bit data.
894 S/MIME implementations which "know" that all intended recipient(s)
895 are capable of handling inner (all but the outermost) binary MIME
896 objects SHOULD use binary encoding as opposed to a 7-bit-safe
897 transfer encoding for the inner entities. The use of a 7-bit-safe
898 encoding (such as base64) would unnecessarily expand the message
899 size. Implementations MAY "know" that recipient implementations are
900 capable of handling inner binary MIME entities either by interpreting
901 the id-cap-preferBinaryInside sMIMECapabilities attribute, by prior
902 agreement, or by other means.
904 If one or more intended recipients are unable to handle inner binary
905 MIME objects, or if this capability is unknown for any of the
906 intended recipients, S/MIME implementations SHOULD use transfer
907 encoding described in section 3.1.3 for all MIME entities they
908 secure.
910 3.1.3. Transfer Encoding for Signing Using multipart/signed
912 If a multipart/signed entity is ever to be transmitted over the
913 standard Internet SMTP infrastructure or other transport that is
914 constrained to 7-bit text, it MUST have transfer encoding applied so
915 that it is represented as 7-bit text. MIME entities that are 7-bit
916 data already need no transfer encoding. Entities such as 8-bit text
917 and binary data can be encoded with quoted-printable or base-64
918 transfer encoding.
920 The primary reason for the 7-bit requirement is that the Internet
921 mail transport infrastructure cannot guarantee transport of 8-bit or
922 binary data. Even though many segments of the transport
923 infrastructure now handle 8-bit and even binary data, it is sometimes
924 not possible to know whether the transport path is 8-bit clean. If a
925 mail message with 8-bit data were to encounter a message transfer
926 agent that can not transmit 8-bit or binary data, the agent has three
927 options, none of which are acceptable for a clear-signed message:
929 - The agent could change the transfer encoding; this would
930 invalidate the signature.
932 - The agent could transmit the data anyway, which would most likely
933 result in the 8th bit being corrupted; this too would invalidate
934 the signature.
936 - The agent could return the message to the sender.
938 [MIME-SECURE] prohibits an agent from changing the transfer encoding
939 of the first part of a multipart/signed message. If a compliant
940 agent that can not transmit 8-bit or binary data encounters a
941 multipart/signed message with 8-bit or binary data in the first part,
942 it would have to return the message to the sender as undeliverable.
944 3.1.4. Sample Canonical MIME Entity
946 This example shows a multipart/mixed message with full transfer
947 encoding. This message contains a text part and an attachment. The
948 sample message text includes characters that are not US-ASCII and
949 thus need to be transfer encoded. Though not shown here, the end of
950 each line is . The line ending of the MIME headers, the
951 text, and transfer encoded parts, all MUST be .
953 Note that this example is not of an S/MIME message.
955 Content-Type: multipart/mixed; boundary=bar
957 --bar
958 Content-Type: text/plain; charset=iso-8859-1
959 Content-Transfer-Encoding: quoted-printable
961 =A1Hola Michael!
963 How do you like the new S/MIME specification?
965 It's generally a good idea to encode lines that begin with
966 From=20because some mail transport agents will insert a greater-
967 than (>) sign, thus invalidating the signature.
969 Also, in some cases it might be desirable to encode any =20
970 trailing whitespace that occurs on lines in order to ensure =20
971 that the message signature is not invalidated when passing =20
972 a gateway that modifies such whitespace (like BITNET). =20
973 --bar
974 Content-Type: image/jpeg
975 Content-Transfer-Encoding: base64
977 iQCVAwUBMJrRF2N9oWBghPDJAQE9UQQAtl7LuRVndBjrk4EqYBIb3h5QXIX/LC//
978 jJV5bNvkZIGPIcEmI5iFd9boEgvpirHtIREEqLQRkYNoBActFBZmh9GC3C041WGq
979 uMbrbxc+nIs1TIKlA08rVi9ig/2Yh7LFrK5Ein57U/W72vgSxLhe/zhdfolT9Brn
980 HOxEa44b+EI=
982 --bar--
984 3.2. The application/pkcs7-mime Media Type
986 The application/pkcs7-mime media type is used to carry CMS content
987 types including EnvelopedData, SignedData, and CompressedData. The
988 details of constructing these entities are described in subsequent
989 sections. This section describes the general characteristics of the
990 application/pkcs7-mime media type.
992 The carried CMS object always contains a MIME entity that is prepared
993 as described in section 3.1 if the eContentType is id-data. Other
994 contents MAY be carried when the eContentType contains different
995 values. See [ESS] for an example of this with signed receipts.
997 Since CMS content types are binary data, in most cases base-64
998 transfer encoding is appropriate, in particular, when used with SMTP
999 transport. The transfer encoding used depends on the transport
1000 through which the object is to be sent, and is not a characteristic
1001 of the media type.
1003 Note that this discussion refers to the transfer encoding of the CMS
1004 object or "outside" MIME entity. It is completely distinct from, and
1005 unrelated to, the transfer encoding of the MIME entity secured by the
1006 CMS object, the "inside" object, which is described in section 3.1.
1008 Because there are several types of application/pkcs7-mime objects, a
1009 sending agent SHOULD do as much as possible to help a receiving agent
1010 know about the contents of the object without forcing the receiving
1011 agent to decode the ASN.1 for the object. The Content-Type header
1012 field of all application/pkcs7-mime objects SHOULD include the
1013 optional "smime-type" parameter, as described in the following
1014 sections.
1016 3.2.1. The name and filename Parameters
1018 For the application/pkcs7-mime, sending agents SHOULD emit the
1019 optional "name" parameter to the Content-Type field for compatibility
1020 with older systems. Sending agents SHOULD also emit the optional
1021 Content-Disposition field [CONTDISP] with the "filename" parameter.
1022 If a sending agent emits the above parameters, the value of the
1023 parameters SHOULD be a file name with the appropriate extension:
1025 Media Type File Extension
1026 application/pkcs7-mime (SignedData, EnvelopedData) .p7m
1027 application/pkcs7-mime (degenerate SignedData .p7c
1028 certificate management message)
1029 application/pkcs7-mime (CompressedData) .p7z
1030 application/pkcs7-signature (SignedData) .p7s
1032 In addition, the file name SHOULD be limited to eight characters
1033 followed by a three letter extension. The eight character filename
1034 base can be any distinct name; the use of the filename base "smime"
1035 SHOULD be used to indicate that the MIME entity is associated with
1036 S/MIME.
1038 Including a file name serves two purposes. It facilitates easier use
1039 of S/MIME objects as files on disk. It also can convey type
1040 information across gateways. When a MIME entity of type
1041 application/pkcs7-mime (for example) arrives at a gateway that has no
1042 special knowledge of S/MIME, it will default the entity's media type
1043 to application/octet-stream and treat it as a generic attachment,
1044 thus losing the type information. However, the suggested filename
1045 for an attachment is often carried across a gateway. This often
1046 allows the receiving systems to determine the appropriate application
1047 to hand the attachment off to, in this case, a stand-alone S/MIME
1048 processing application. Note that this mechanism is provided as a
1049 convenience for implementations in certain environments. A proper
1050 S/MIME implementation MUST use the media types and MUST NOT rely on
1051 the file extensions.
1053 3.2.2. The smime-type parameter
1055 The application/pkcs7-mime content type defines the optional "smime-
1056 type" parameter. The intent of this parameter is to convey details
1057 about the security applied (signed or enveloped) along with
1058 information about the contained content. This specification defines
1059 the following smime-types.
1061 Name CMS type Inner Content
1062 enveloped-data EnvelopedData id-data
1063 signed-data SignedData id-data
1064 certs-only SignedData none
1065 compressed-data CompressedData id-data
1067 In order for consistency to be obtained with future specifications,
1068 the following guidelines SHOULD be followed when assigning a new
1069 smime-type parameter.
1071 1. If both signing and encryption can be applied to the content,
1072 then two values for smime-type SHOULD be assigned "signed-*" and
1073 "encrypted-*". If one operation can be assigned then this can be
1074 omitted. Thus since "certs-only" can only be signed, "signed-"
1075 is omitted.
1077 2. A common string for a content OID SHOULD be assigned. We use
1078 "data" for the id-data content OID when MIME is the inner
1079 content.
1081 3. If no common string is assigned, then the common string of
1082 "OID." is recommended (for example,
1083 "OID.2.16.840.1.101.3.4.1.2" would be AES-128 CBC).
1085 It is explicitly intended that this field be a suitable hint for mail
1086 client applications to indicate whether a message is "signed" or
1087 "encrypted" without having to tunnel into the CMS payload.
1089 3.3. Creating an Enveloped-only Message
1091 This section describes the format for enveloping a MIME entity
1092 without signing it. It is important to note that sending enveloped
1093 but not signed messages does not provide for data integrity. It is
1094 possible to replace ciphertext in such a way that the processed
1095 message will still be valid, but the meaning can be altered.
1097 Step 1. The MIME entity to be enveloped is prepared according to
1098 section 3.1.
1100 Step 2. The MIME entity and other required data is processed
1101 into a CMS object of type EnvelopedData. In addition to
1102 encrypting a copy of the content-encryption key for each
1103 recipient, a copy of the content-encryption key SHOULD be
1104 encrypted for the originator and included in the EnvelopedData
1105 (see [CMS] Section 6).
1107 Step 3. The EnvelopedData object is wrapped in a CMS ContentInfo
1108 object.
1110 Step 4. The ContentInfo object is inserted into an
1111 application/pkcs7-mime MIME entity.
1113 The smime-type parameter for enveloped-only messages is "enveloped-
1114 data". The file extension for this type of message is ".p7m".
1116 A sample message would be:
1118 Content-Type: application/pkcs7-mime; smime-type=enveloped-data;
1119 name=smime.p7m
1120 Content-Transfer-Encoding: base64
1121 Content-Disposition: attachment; filename=smime.p7m
1123 rfvbnj756tbBghyHhHUujhJhjH77n8HHGT9HG4VQpfyF467GhIGfHfYT6
1124 7n8HHGghyHhHUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H
1125 f8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
1126 0GhIGfHfQbnj756YT64V
1128 3.4. Creating a Signed-only Message
1130 There are two formats for signed messages defined for S/MIME:
1132 - application/pkcs7-mime with SignedData; and,
1134 - multipart/signed.
1136 In general, the multipart/signed form is preferred for sending, and
1137 receiving agents MUST be able to handle both.
1139 3.4.1. Choosing a Format for Signed-only Messages
1141 There are no hard-and-fast rules when a particular signed-only format
1142 is chosen because it depends on the capabilities of all the receivers
1143 and the relative importance of receivers with S/MIME facilities being
1144 able to verify the signature versus the importance of receivers
1145 without S/MIME software being able to view the message.
1147 Messages signed using the multipart/signed format can always be
1148 viewed by the receiver whether they have S/MIME software or not. They
1149 can also be viewed whether they are using a MIME-native user agent or
1150 they have messages translated by a gateway. In this context, "be
1151 viewed" means the ability to process the message essentially as if it
1152 were not a signed message, including any other MIME structure the
1153 message might have.
1155 Messages signed using the SignedData format cannot be viewed by a
1156 recipient unless they have S/MIME facilities. However, the
1157 SignedData format protects the message content from being changed by
1158 benign intermediate agents. Such agents might do line wrapping or
1159 content-transfer encoding changes which would break the signature.
1161 3.4.2. Signing Using application/pkcs7-mime with SignedData
1163 This signing format uses the application/pkcs7-mime media type. The
1164 steps to create this format are:
1166 Step 1. The MIME entity is prepared according to section 3.1.
1168 Step 2. The MIME entity and other required data is processed
1169 into a CMS object of type SignedData.
1171 Step 3. The SignedData object is wrapped in a CMS ContentInfo
1172 object.
1174 Step 4. The ContentInfo object is inserted into an
1175 application/pkcs7-mime MIME entity.
1177 The smime-type parameter for messages using application/pkcs7-mime
1178 with SignedData is "signed-data". The file extension for this type
1179 of message is ".p7m".
1181 A sample message would be:
1183 Content-Type: application/pkcs7-mime; smime-type=signed-data;
1184 name=smime.p7m
1185 Content-Transfer-Encoding: base64
1186 Content-Disposition: attachment; filename=smime.p7m
1188 567GhIGfHfYT6ghyHhHUujpfyF4f8HHGTrfvhJhjH776tbB9HG4VQbnj7
1189 77n8HHGT9HG4VQpfyF467GhIGfHfYT6rfvbnj756tbBghyHhHUujhJhjH
1190 HUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H7n8HHGghyHh
1191 6YT64V0GhIGfHfQbnj75
1193 3.4.3. Signing Using the multipart/signed Format
1195 This format is a clear-signing format. Recipients without any S/MIME
1196 or CMS processing facilities are able to view the message. It makes
1197 use of the multipart/signed media type described in [MIME-SECURE].
1198 The multipart/signed media type has two parts. The first part
1199 contains the MIME entity that is signed; the second part contains the
1200 "detached signature" CMS SignedData object in which the
1201 encapContentInfo eContent field is absent.
1203 3.4.3.1. The application/pkcs7-signature Media Type
1205 This media type always contains a CMS ContentInfo containing a single
1206 CMS object of type SignedData. The SignedData encapContentInfo
1207 eContent field MUST be absent. The signerInfos field contains the
1208 signatures for the MIME entity.
1210 The file extension for signed-only messages using application/pkcs7-
1211 signature is ".p7s".
1213 3.4.3.2. Creating a multipart/signed Message
1215 Step 1. The MIME entity to be signed is prepared according to
1216 section 3.1, taking special care for clear-signing.
1218 Step 2. The MIME entity is presented to CMS processing in order
1219 to obtain an object of type SignedData in which the
1220 encapContentInfo eContent field is absent.
1222 Step 3. The MIME entity is inserted into the first part of a
1223 multipart/signed message with no processing other than that
1224 described in section 3.1.
1226 Step 4. Transfer encoding is applied to the "detached signature"
1227 CMS SignedData object and it is inserted into a MIME entity of
1228 type application/pkcs7-signature.
1230 Step 5. The MIME entity of the application/pkcs7-signature is
1231 inserted into the second part of the multipart/signed entity.
1233 The multipart/signed Content-Type has two required parameters: the
1234 protocol parameter and the micalg parameter.
1236 The protocol parameter MUST be "application/pkcs7-signature". Note
1237 that quotation marks are required around the protocol parameter
1238 because MIME requires that the "/" character in the parameter value
1239 MUST be quoted.
1241 The micalg parameter allows for one-pass processing when the
1242 signature is being verified. The value of the micalg parameter is
1243 dependent on the message digest algorithm(s) used in the calculation
1244 of the Message Integrity Check. If multiple message digest
1245 algorithms are used they MUST be separated by commas per [MIME-
1246 SECURE]. The values to be placed in the micalg parameter SHOULD be
1247 from the following:
1249 Algorithm Value used
1251 MD5 md5
1252 SHA-1 sha1
1253 SHA-224 sha224
1254 SHA-256 sha256
1255 SHA-384 sha384
1256 SHA-512 sha512
1257 Any other (defined separately in algorithm profile or "unknown"
1258 if not defined)
1260 (Historical note: some early implementations of S/MIME emitted and
1261 expected "rsa-md5" and "rsa-sha1" for the micalg parameter.)
1262 Receiving agents SHOULD be able to recover gracefully from a micalg
1263 parameter value that they do not recognize.
1265 The SHA-224, SHA-384, and SHA-512 algorithms [FIPS180-3] are not
1266 currently recommended in S/MIME, and are included here for
1267 completeness.
1269 3.4.3.3. Sample multipart/signed Message
1271 Content-Type: multipart/signed;
1272 protocol="application/pkcs7-signature";
1273 micalg=sha1; boundary=boundary42
1275 --boundary42
1276 Content-Type: text/plain
1278 This is a clear-signed message.
1280 --boundary42
1281 Content-Type: application/pkcs7-signature; name=smime.p7s
1282 Content-Transfer-Encoding: base64
1283 Content-Disposition: attachment; filename=smime.p7s
1285 ghyHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6
1286 4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj
1287 n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
1288 7GhIGfHfYT64VQbnj756
1290 --boundary42--
1292 The content that is digested (the first part of the multipart/signed)
1293 are the bytes:
1295 43 6f 6e 74 65 6e 74 2d 54 79 70 65 3a 20 74 65 78 74 2f 70 6c 61 69
1296 6e 0d 0a 0d 0a 54 68 69 73 20 69 73 20 61 20 63 6c 65 61 72 2d 73 69
1297 67 6e 65 64 20 6d 65 73 73 61 67 65 2e 0d 0a
1299 3.5. Creating an Compressed-only Message
1301 This section describes the format for compressing a MIME entity.
1302 Please note that versions of S/MIME prior to version 3.1 did not
1303 specify any use of CompressedData, and will not recognize it. The
1304 use of a capability to indicate the ability to receive CompressedData
1305 is described in [CMSCOMPR] and is the preferred method for
1306 compatibility.
1308 Step 1. The MIME entity to be compressed is prepared according
1309 to section 3.1.
1311 Step 2. The MIME entity and other required data is processed
1312 into a CMS object of type CompressedData.
1314 Step 3. The CompressedData object is wrapped in a CMS
1315 ContentInfo object.
1317 Step 4. The ContentInfo object is inserted into an
1318 application/pkcs7-mime MIME entity.
1320 The smime-type parameter for compressed-only messages is "compressed-
1321 data". The file extension for this type of message is ".p7z".
1323 A sample message would be:
1325 Content-Type: application/pkcs7-mime; smime-type=compressed-data;
1326 name=smime.p7z
1327 Content-Transfer-Encoding: base64
1328 Content-Disposition: attachment; filename=smime.p7z
1330 rfvbnj756tbBghyHhHUujhJhjH77n8HHGT9HG4VQpfyF467GhIGfHfYT6
1331 7n8HHGghyHhHUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H
1332 f8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
1333 0GhIGfHfQbnj756YT64V
1335 3.6. Multiple Operations
1337 The signed-only, encrypted-only, and compressed-only MIME formats can
1338 be nested. This works because these formats are all MIME entities
1339 that encapsulate other MIME entities.
1341 An S/MIME implementation MUST be able to receive and process
1342 arbitrarily nested S/MIME within reasonable resource limits of the
1343 recipient computer.
1345 It is possible to apply any of the signing, encrypting, and
1346 compressing operations in any order. It is up to the implementer and
1347 the user to choose. When signing first, the signatories are then
1348 securely obscured by the enveloping. When enveloping first the
1349 signatories are exposed, but it is possible to verify signatures
1350 without removing the enveloping. This can be useful in an
1351 environment were automatic signature verification is desired, as no
1352 private key material is required to verify a signature.
1354 There are security ramifications to choosing whether to sign first or
1355 encrypt first. A recipient of a message that is encrypted and then
1356 signed can validate that the encrypted block was unaltered, but
1357 cannot determine any relationship between the signer and the
1358 unencrypted contents of the message. A recipient of a message that
1359 is signed-then-encrypted can assume that the signed message itself
1360 has not been altered, but that a careful attacker could have changed
1361 the unauthenticated portions of the encrypted message.
1363 When using compression, keep the following guidelines in mind:
1365 - Compression of binary encoded encrypted data is discouraged, since
1366 it will not yield significant compression. Base64 encrypted data
1367 could very well benefit, however.
1369 - If a lossy compression algorithm is used with signing, you will
1370 need to compress first, then sign.
1372 3.7. Creating a Certificate Management Message
1374 The certificate management message or MIME entity is used to
1375 transport certificates and/or certificate revocation lists, such as
1376 in response to a registration request.
1378 Step 1. The certificates and/or certificate revocation lists are
1379 made available to the CMS generating process which creates a CMS
1380 object of type SignedData. The SignedData encapContentInfo
1381 eContent field MUST be absent and signerInfos field MUST be
1382 empty.
1384 Step 2. The SignedData object is wrapped in a CMS ContentInfo
1385 object.
1387 Step 3. The ContentInfo object is enclosed in an
1388 application/pkcs7-mime MIME entity.
1390 The smime-type parameter for a certificate management message is
1391 "certs-only". The file extension for this type of message is ".p7c".
1393 3.8. Registration Requests
1395 A sending agent that signs messages MUST have a certificate for the
1396 signature so that a receiving agent can verify the signature. There
1397 are many ways of getting certificates, such as through an exchange
1398 with a certificate authority, through a hardware token or diskette,
1399 and so on.
1401 S/MIME v2 [SMIMEv2] specified a method for "registering" public keys
1402 with certificate authorities using an application/pkcs10 body part.
1403 Since that time, the IETF PKIX Working Group has developed other
1404 methods for requesting certificates. However, S/MIME v3.2 does not
1405 require a particular certificate request mechanism.
1407 3.9. Identifying an S/MIME Message
1409 Because S/MIME takes into account interoperation in non-MIME
1410 environments, several different mechanisms are employed to carry the
1411 type information, and it becomes a bit difficult to identify S/MIME
1412 messages. The following table lists criteria for determining whether
1413 or not a message is an S/MIME message. A message is considered an
1414 S/MIME message if it matches any of the criteria listed below.
1416 The file suffix in the table below comes from the "name" parameter in
1417 the Content-Type header field, or the "filename" parameter on the
1418 Content-Disposition header field. These parameters that give the
1419 file suffix are not listed below as part of the parameter section.
1421 Media type: application/pkcs7-mime
1422 parameters: any
1423 file suffix: any
1425 Media type: multipart/signed
1426 parameters: protocol="application/pkcs7-signature"
1427 file suffix: any
1428 Media type: application/octet-stream
1429 parameters: any
1430 file suffix: p7m, p7s, p7c, p7z
1432 4. Certificate Processing
1434 A receiving agent MUST provide some certificate retrieval mechanism
1435 in order to gain access to certificates for recipients of digital
1436 envelopes. This specification does not cover how S/MIME agents
1437 handle certificates, only what they do after a certificate has been
1438 validated or rejected. S/MIME certificate issues are covered in
1439 [CERT32].
1441 At a minimum, for initial S/MIME deployment, a user agent could
1442 automatically generate a message to an intended recipient requesting
1443 that recipient's certificate in a signed return message. Receiving
1444 and sending agents SHOULD also provide a mechanism to allow a user to
1445 "store and protect" certificates for correspondents in such a way so
1446 as to guarantee their later retrieval.
1448 4.1. Key Pair Generation
1450 All generated key pairs MUST be generated from a good source of non-
1451 deterministic random input [RANDOM] and the private key MUST be
1452 protected in a secure fashion.
1454 An S/MIME user agent MUST NOT generate asymmetric keys less than 512
1455 bits for use with the RSA or DSA signature algorithms.
1457 4.2. Signature Generation
1459 The following are the requirements for an S/MIME agent generated RSA
1460 signatures:
1462 512 <= key size < 1024 : MAY (see Security Considerations)
1463 1024 <= key size <= 2048 : SHOULD (see Security Considerations)
1464 2048 < key size : MAY (see Security Considerations)
1466 The following are the requirements for an S/MIME agent generated DSA
1467 signatures:
1469 512 <= key size <= 1024 : SHOULD (see Security Considerations)
1471 4.3. Signature Verification
1473 The following are the requirements for S/MIME receiving agents during
1474 signature verification of RSA signatures:
1476 512 <= key size <= 2048 : MUST (see Security Considerations)
1477 2048 < key size : MAY (see Security Considerations)
1479 The following are the requirements for S/MIME receiving agents during
1480 signature verification of DSA signatures:
1482 512 <= key size <= 1024 : SHOULD (see Security Considerations)
1484 4.4. Encryption
1486 The following are the requirements for an S/MIME agent when
1487 establishing keys for content encryption using the RSA algorithms:
1489 512 <= key size < 1024 : MAY (see Security Considerations)
1490 1024 <= key size <= 2048 : SHOULD (see Security Considerations)
1491 2048 < key size : MAY (see Security Considerations)
1493 The following are the requirements for an S/MIME agent when
1494 establishing keys for content encryption using the DH algorithms:
1496 512 <= key size <= 1024 : SHOULD (see Security Considerations)
1498 4.5. Decryption
1500 The following are the requirements for an S/MIME agent when
1501 establishing keys for content decryption using the RSA algorithms:
1503 512 <= key size <= 2048 : MUST (see Security Considerations)
1504 2048 < key size : MAY (see Security Considerations)
1506 The following are the requirements for an S/MIME agent when
1507 establishing keys for content decryption using the DH algorithms:
1509 512 <= key size <= 1024 : SHOULD (see Security Considerations)
1511 5. IANA Considerations
1513 The following is intended to provide sufficient information to update
1514 the media type registration for application/pkcs7-mime and
1515 application/pkcs7-signature to refer to this document as opposed to
1516 RFC 2311.
1518 Note that other documents can define additional MIME media types for
1519 S/MIME.
1521 5.1. Media Type for application/pkcs7-mime
1523 Type name: application
1525 Subtype Name: pkcs7-mime
1527 Required Parameters: NONE
1529 Optional Parameters: smime-type/signed-data
1530 smime-type/enveloped-data
1531 smime-type/compressed-data
1532 smime-type/certs-only
1534 Encoding Considerations: See Section 3 of this document
1536 Security Considerations: See Section 6 of this document
1538 Interoperability Considerations: See Sections 1-6 of this document
1540 Published Specification: RFC 2311, RFC 2633, and this document
1542 Applications that use this media type: Security applications
1544 Additional information: NONE
1546 Person & email to contact for further information: S/MIME working
1547 group chairs smime-chairs@tools.ietf.org
1549 Intended usage: COMMON
1551 Restrictions on usage: NONE
1553 Author: Sean Turner
1555 Change Controller: S/MIME working group delegated from the IESG
1557 5.2. Media Type for application/pkcs7-signature
1559 Type name: application
1561 Subtype Name: pkcs7-signature
1563 Required Parameters: NONE
1565 Optional Parameters: NONE
1567 Encoding Considerations: See Section 3 of this document
1569 Security Considerations: See Section 6 of this document
1571 Interoperability Considerations: See Sections 1-6 of this document
1573 Published Specification: RFC 2311, RFC 2633, and this document
1575 Applications that use this media type: Security applications
1577 Additional information: NONE
1579 Person & email to contact for further information: S/MIME working
1580 group chairs smime-chairs@tools.ietf.org
1582 Intended usage: COMMON
1584 Restrictions on usage: NONE
1586 Author: Sean Turner
1588 Change Controller: S/MIME working group delegated from the IESG
1590 6. Security Considerations
1592 Cryptographic algorithms will be broken or weakened over time.
1593 Implementers and users need to check that the cryptographic
1594 algorithms listed in this document continue to provide the expected
1595 level of security. The IETF from time to time may issue documents
1596 dealing with the current state of the art. For example:
1598 - The Million Message Attack described in RFC 3218 [MMA].
1600 - The Diffie-Hellman "small-subgroup" attacks described in
1601 RFC 2785 [DHSUB].
1603 - The attacks against hash algorithms described in
1604 RFC 4270 [HASH-ATTACK]
1606 This specification uses Public-Key Cryptography technologies. It is
1607 assumed that the private is protected to ensure that it is not
1608 accessed or altered by unauthorized parties.
1610 It is impossible for most people or software to estimate the value of
1611 a message's content. Further, it is impossible for most people or
1612 software to estimate the actual cost of recovering an encrypted
1613 message content that is encrypted with a key of a particular size.
1614 Further, it is quite difficult to determine the cost of a failed
1615 decryption if a recipient cannot process a message's content. Thus,
1616 choosing between different key sizes (or choosing whether to just use
1617 plaintext) is also impossible for most people or software. However,
1618 decisions based on these criteria are made all the time, and
1619 therefore this specification gives a framework for using those
1620 estimates in choosing algorithms.
1622 The choice of 1024 bits as the RSA, DSA, and DH asymmetric key size
1623 in this specification is based on the desire to provide 80 bits of
1624 security. This key size seems prudent for the Internet based on
1625 Section 4.3 of [STRENGTH]. There are other environments (e.g.,
1626 government, financial, and medical) that may consider this key size
1627 to be inadequate. Likewise, there are other environments that may
1628 consider this key size to be excessive.
1630 Receiving agents that validate signatures and sending agents that
1631 encrypt messages, need to be cautious of cryptographic processing
1632 usage when validating signatures and encrypting messages using keys
1633 larger than those mandated in this specification. An attacker could
1634 send certificates with keys which would result in excessive
1635 cryptographic processing, for example keys larger than those mandated
1636 in this specification, which could swamp the processing element.
1638 Agents which use such keys without first validating the certificate
1639 to a trust anchor are advised to have some sort of cryptographic
1640 resource management system to prevent such attacks.
1642 Today, 512-bit RSA, DSA, and DH keys are considered by many experts
1643 to be cryptographically insecure.
1645 Using weak cryptography in S/MIME offers little actual security over
1646 sending plaintext. However, other features of S/MIME, such as the
1647 specification of AES and the ability to announce stronger
1648 cryptographic capabilities to parties with whom you communicate,
1649 allow senders to create messages that use strong encryption. Using
1650 weak cryptography is never recommended unless the only alternative is
1651 no cryptography. When feasible, sending and receiving agents SHOULD
1652 inform senders and recipients of the relative cryptographic strength
1653 of messages.
1655 Implementers SHOULD be aware that multiple active key pairs can be
1656 associated with a single individual. For example, one key pair can
1657 be used to support confidentiality, while a different key pair can be
1658 used for digital signatures.
1660 If a sending agent is sending the same message using different
1661 strengths of cryptography, an attacker watching the communications
1662 channel might be able to determine the contents of the strongly-
1663 encrypted message by decrypting the weakly-encrypted version. In
1664 other words, a sender SHOULD NOT send a copy of a message using
1665 weaker cryptography than they would use for the original of the
1666 message.
1668 Modification of the ciphertext can go undetected if authentication is
1669 not also used, which is the case when sending EnvelopedData without
1670 wrapping it in SignedData or enclosing SignedData within it.
1672 7. References
1674 7.1. Normative References
1676 [CERT32] Ramsdell, B., and S. Turner, "S/MIME Version 3.2
1677 Certificate Handling", work in progress.
1679 [CHARSETS] Character sets assigned by IANA. See
1680 http://www.iana.org/assignments/character-sets
1682 [CMS] Housley, R., "Cryptographic Message Syntax (CMS)", RFC
1683 3852, July 2004.
1685 Housley, R., "Cryptographic Message Syntax (CMS)
1686 Multiple Signer Clarification", RFC 4852, April 2007.
1688 [CMSAES] Schaad, J., "Use of the Advanced Encryption Standard
1689 (AES) Encryption Algorithm in Cryptographic Message
1690 Syntax (CMS)", RFC 3565, July 2003.
1692 [CMSALG] Housley, R., "Cryptographic Message Syntax (CMS)
1693 Algorithms", RFC 3370, August 2002.
1695 [CMSCOMPR] Gutmann, P., "Compressed Data Content Type for
1696 Cryptographic Message Syntax (CMS)", RFC 3274, June
1697 2002.
1699 [CMS-SHA2] Turner. S., "Using SHA2 Algorithms with Cryptographic
1700 Message Syntax", work in progress.
1702 [CONTDISP] Troost, R., Dorner, S., and K. Moore, "Communicating
1703 Presentation Information in Internet Messages: The
1704 Content-Disposition Header Field", RFC 2183, August
1705 1997.
1707 [ESS] Hoffman, P., "Enhanced Security Services for S/MIME",
1708 RFC 2634, June 1999.
1710 [FIPS180-3] National Institute of Standards and Technology (NIST),
1711 "Secure Hash Standard (SHS)", FIPS Publication 180-3,
1712 June 2007.
1714 [MIME-SPEC] Freed, N. and N. Borenstein, "Multipurpose Internet
1715 Mail Extensions (MIME) Part One: Format of Internet
1716 Message Bodies", RFC 2045, November 1996.
1718 Freed, N. and N. Borenstein, "Multipurpose Internet
1719 Mail Extensions (MIME) Part Two: Media Types", RFC
1720 2046, November 1996.
1722 Moore, K., "MIME (Multipurpose Internet Mail
1723 Extensions) Part Three: Message Header Extensions for
1724 Non-ASCII Text", RFC 2047, November 1996.
1726 Freed, N., and J. Klensin, , "Multipurpose Internet
1727 Mail Extensions (MIME) Part Four: Registration
1728 Procedures", BCP 13, RFC 4289, December 2005.
1730 Freed, N., and J. Klensin, "Media Type Specifications
1731 and Registration Procedures ", BCP 13, RFC 4288,
1732 December 2005.
1734 Freed, N. and N. Borenstein, "Multipurpose Internet
1735 Mail Extensions (MIME) Part Five: Conformance Criteria
1736 and Examples", RFC 2049, November 1996.
1738 [MIME-SECURE] Galvin, J., Murphy, S., Crocker, S., and N. Freed,
1739 "Security Multiparts for MIME: Multipart/Signed and
1740 Multipart/Encrypted", RFC 1847, October 1995.
1742 [MUSTSHOULD] Bradner, S., "Key words for use in RFCs to Indicate
1743 Requirement Levels", BCP 14, RFC 2119, March 1997.
1745 [RANDOM] Eastlake 3rd, D., Crocker, S., and J. Schiller,
1746 "Randomness Requirements for Security", BCP 106, RFC
1747 4086, June 2005.
1749 [RSAPSS] Schaad, J., "Use of RSASA-PSS Signature Algorithm in
1750 Cryptographic Message Syntax (CMS)", RFC 4056, June
1751 2005.
1753 [RSAOAEP] Housley, R. "Use of the RSAES-OAEP Key Transport
1754 Algorithm in the Cryptographic Message Syntax (CMS)",
1755 RFC 3560, July 2003
1757 [X.680] ITU-T Recommendation X.680 (2002) | ISO/IEC 8824-
1758 1:2002. Information Technology - Abstract Syntax
1759 Notation One (ASN.1): Specification of basic
1760 notation.
1762 [X.690] ITU-T Recommendation X.690 (2002) | ISO/IEC 8825-
1763 1:2002. Information Technology - ASN.1 encoding
1764 rules: Specification of Basic Encoding Rules (BER),
1765 Canonical Encoding Rules (CER) and Distinguished
1766 Encoding Rules (DER).
1768 7.2. Informative References
1770 [DHSUB] Zuccherato, R., "Methods for Avoiding the "Small-
1771 Subgroup" Attacks on the Diffie-Hellman Key Agreement
1772 Method for S/MIME", RFC 2785, March 2000.
1774 [HASH-ATTACK] Hoffman, P., Schneier, B., "Attacks on Cryptographic
1775 Hashes in Internet Protocols", RFC 4270, November
1776 2005.
1778 [MMA] Rescorla, E., "Preventing the Million Message Attack
1779 on Cryptographic Message Syntax", RFC 3218, January
1780 2002.
1782 [PKCS-7] Kaliski, B., "PKCS #7: Cryptographic Message Syntax
1783 Version 1.5", RFC 2315, March 1998.
1785 [SMIMEv2] Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L.,
1786 and L. Repka, "S/MIME Version 2 Message
1787 Specification", RFC 2311, March 1998.
1789 Dusse, S., Hoffman, P., Ramsdell, B., and J.
1790 Weinstein, "S/MIME Version 2 Certificate Handling",
1791 RFC 2312, March 1998.
1793 Kaliski, B., "PKCS #1: RSA Encryption Version 1.5",
1794 RFC 2313, March 1998.
1796 Kaliski, B., "PKCS #10: Certificate Request Syntax
1797 Version 1.5", RFC 2314, March 1998.
1799 Kaliski, B., "PKCS #7: Certificate Message Syntax
1800 Version 1.5", RFC 2314, March 1998.
1802 [SMIMEv3] Housley, R., "Cryptographic Message Syntax", RFC 2630,
1803 June 1999.
1805 Rescorla, E., "Diffie-Hellman Key Agreement Method",
1806 RFC 2631, June 1999.
1808 Ramsdell, B., "S/MIME Version 3 Certificate Handling",
1809 RFC 2632, June 1999.
1811 Ramsdell, B., "S/MIME Version 3 Message
1812 Specification", RFC 2633, June 1999.
1814 Hoffman, P., "Enhanced Security Services for S/MIME",
1815 RFC 2634, June 1999.
1817 [SMIMEv3.1] Housley, R., "Cryptographic Message Syntax", RFC 3852,
1818 July 2004.
1820 Ramsdell, B., "S/MIME Version 3.1 Certificate
1821 Handling", RFC 3850, July 2004.
1823 Ramsdell, B., "S/MIME Version 3.1 Message
1824 Specification", RFC 3851, July 2004.
1826 Hoffman, P., "Enhanced Security Services for S/MIME",
1827 RFC 2634, June 1999.
1829 [STRENGTH] Orman, H., and P. Hoffman, "Determining Strengths For
1830 Public Keys Used For Exchanging Symmetric Keys", BCP
1831 86, RFC 3766, April 2004.
1833 Appendix A. ASN.1 Module
1835 NOTE: The ASN.1 module contained herein is unchanged from RFC 3851
1836 [SMIMEv3.1] with the exception of a minor change to the
1837 prefersBinaryInside ASN.1 comment. This modules use the 1988 version
1838 of ASN.1.
1840 SecureMimeMessageV3dot1
1842 { iso(1) member-body(2) us(840) rsadsi(113549)
1843 pkcs(1) pkcs-9(9) smime(16) modules(0) msg-v3dot1(21) }
1845 DEFINITIONS IMPLICIT TAGS ::=
1847 BEGIN
1849 IMPORTS
1851 -- Cryptographic Message Syntax [CMS]
1852 SubjectKeyIdentifier, IssuerAndSerialNumber,
1853 RecipientKeyIdentifier
1854 FROM CryptographicMessageSyntax
1855 { iso(1) member-body(2) us(840) rsadsi(113549)
1856 pkcs(1) pkcs-9(9) smime(16) modules(0) cms-2001(14) };
1858 -- id-aa is the arc with all new authenticated and unauthenticated
1859 -- attributes produced the by S/MIME Working Group
1861 id-aa OBJECT IDENTIFIER ::= {iso(1) member-body(2) usa(840)
1862 rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) attributes(2)}
1864 -- S/MIME Capabilities provides a method of broadcasting the
1865 -- symmetric capabilities understood. Algorithms SHOULD be ordered
1866 -- by preference and grouped by type
1868 smimeCapabilities OBJECT IDENTIFIER ::= {iso(1) member-body(2)
1869 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 15}
1871 SMIMECapability ::= SEQUENCE {
1872 capabilityID OBJECT IDENTIFIER,
1873 parameters ANY DEFINED BY capabilityID OPTIONAL }
1875 SMIMECapabilities ::= SEQUENCE OF SMIMECapability
1877 -- Encryption Key Preference provides a method of broadcasting the
1878 -- preferred encryption certificate.
1880 id-aa-encrypKeyPref OBJECT IDENTIFIER ::= {id-aa 11}
1882 SMIMEEncryptionKeyPreference ::= CHOICE {
1883 issuerAndSerialNumber [0] IssuerAndSerialNumber,
1884 receipentKeyId [1] RecipientKeyIdentifier,
1885 subjectAltKeyIdentifier [2] SubjectKeyIdentifier
1886 }
1888 id-smime OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
1889 rsadsi(113549) pkcs(1) pkcs9(9) 16 }
1891 id-cap OBJECT IDENTIFIER ::= { id-smime 11 }
1893 -- The preferBinaryInside OID indicates an ability to receive
1894 -- messages with binary encoding inside the CMS wrapper
1896 id-cap-preferBinaryInside OBJECT IDENTIFIER ::= { id-cap 1 }
1898 -- The following list the OIDs to be used with S/MIME V3
1900 -- Signature Algorithms Not Found in [CMSALG]
1902 --
1904 -- md2WithRSAEncryption OBJECT IDENTIFIER ::=
1905 -- {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1)
1906 -- 2}
1908 --
1910 -- Other Signed Attributes
1911 --
1912 -- signingTime OBJECT IDENTIFIER ::=
1913 -- {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
1914 -- 5}
1915 -- See [CMS] for a description of how to encode the attribute
1916 -- value.
1918 SMIMECapabilitiesParametersForRC2CBC ::= INTEGER
1919 -- (RC2 Key Length (number of bits))
1921 END
1923 Appendix B. Moving S/MIME v2 Message Specification to Historic Status
1925 The S/MIME v3 [SMIMEv3], v3.1 [SMIMEv3.1], and v3.2 (this document)
1926 Message Specifications are backwards S/MIME v2 Message Specification
1927 [SMIMEv2], with the exception of the algorithms (dropped RC2/40
1928 requirement and added DSA and RSA-PSS requirements). Therefore, it is
1929 recommended that RFC 2311 [SMIMEv2] be moved to Historic status.
1931 Appendix C. Acknowledgements
1933 Many thanks go out to the other authors of the S/MIME Version 2
1934 Message Specification RFC: Steve Dusse, Paul Hoffman, Laurence
1935 Lundblade and Lisa Repka. Without v2, there wouldn't be a v3, v3.1 or
1936 v3.2.
1938 A number of the members of the S/MIME Working Group have also worked
1939 very hard and contributed to this document. Any list of people is
1940 doomed to omission, and for that I apologize. In alphabetical order,
1941 the following people stand out in my mind due to the fact that they
1942 made direct contributions to this document.
1944 Tony Capel, Piers Chivers, Dave Crocker, Bill Flanigan, Peter
1945 Gutmann, Alfred Hoenes, Paul Hoffman, Russ Housley, William Ottaway,
1946 John Pawling, and Jim Schaad.
1948 Author's Addresses
1950 Blake Ramsdell
1951 SendMail
1953 Email: blake@sendmail.com
1955 Sean Turner
1957 IECA, Inc.
1958 3057 Nutley Street, Suite 106
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