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rfc:rfc5751

Internet Engineering Task Force (IETF) B. Ramsdell Request for Comments: 5751 Brute Squad Labs Obsoletes: 3851 S. Turner Category: Standards Track IECA ISSN: 2070-1721 January 2010

 Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2
                       Message Specification

Abstract

 This document defines Secure/Multipurpose Internet Mail Extensions
 (S/MIME) version 3.2.  S/MIME provides a consistent way to send and
 receive secure MIME data.  Digital signatures provide authentication,
 message integrity, and non-repudiation with proof of origin.
 Encryption provides data confidentiality.  Compression can be used to
 reduce data size.  This document obsoletes RFC 3851.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by
 the Internet Engineering Steering Group (IESG).  Further
 information on Internet Standards is available in Section 2 of
 RFC 5741.
 Information about the current status of this document, any
 errata, and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc5751.

Ramsdell & Turner Standards Track [Page 1] RFC 5751 S/MIME 3.2 Message Specification January 2010

Copyright Notice

 Copyright (c) 2010 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Ramsdell & Turner Standards Track [Page 2] RFC 5751 S/MIME 3.2 Message Specification January 2010

Table of Contents

 1. Introduction ....................................................4
    1.1. Specification Overview .....................................4
    1.2. Definitions ................................................5
    1.3. Conventions Used in This Document ..........................6
    1.4. Compatibility with Prior Practice of S/MIME ................7
    1.5. Changes from S/MIME v3 to S/MIME v3.1 ......................7
    1.6. Changes since S/MIME v3.1 ..................................7
 2. CMS Options .....................................................9
    2.1. DigestAlgorithmIdentifier ..................................9
    2.2. SignatureAlgorithmIdentifier ...............................9
    2.3. KeyEncryptionAlgorithmIdentifier ..........................10
    2.4. General Syntax ............................................11
    2.5. Attributes and the SignerInfo Type ........................12
    2.6. SignerIdentifier SignerInfo Type ..........................16
    2.7. ContentEncryptionAlgorithmIdentifier ......................16
 3. Creating S/MIME Messages .......................................18
    3.1. Preparing the MIME Entity for Signing, Enveloping,
         or Compressing ............................................19
    3.2. The application/pkcs7-mime Media Type .....................23
    3.3. Creating an Enveloped-Only Message ........................25
    3.4. Creating a Signed-Only Message ............................26
    3.5. Creating a Compressed-Only Message ........................30
    3.6. Multiple Operations .......................................30
    3.7. Creating a Certificate Management Message .................31
    3.8. Registration Requests .....................................32
    3.9. Identifying an S/MIME Message .............................32
 4. Certificate Processing .........................................32
    4.1. Key Pair Generation .......................................33
    4.2. Signature Generation ......................................33
    4.3. Signature Verification ....................................34
    4.4. Encryption ................................................34
    4.5. Decryption ................................................34
 5. IANA Considerations ............................................34
    5.1. Media Type for application/pkcs7-mime .....................34
    5.2. Media Type for application/pkcs7-signature ................35
 6. Security Considerations ........................................36
 7. References .....................................................38
    7.1. Reference Conventions .....................................38
    7.2. Normative References ......................................39
    7.3. Informative References ....................................41
 Appendix A. ASN.1 Module ..........................................43
 Appendix B. Moving S/MIME v2 Message Specification to Historic
             Status ................................................45
 Appendix C. Acknowledgments .......................................45

Ramsdell & Turner Standards Track [Page 3] RFC 5751 S/MIME 3.2 Message Specification January 2010

1. Introduction

 S/MIME (Secure/Multipurpose Internet Mail Extensions) provides a
 consistent way to send and receive secure MIME data.  Based on the
 popular Internet MIME standard, S/MIME provides the following
 cryptographic security services for electronic messaging
 applications:  authentication, message integrity and non-repudiation
 of origin (using digital signatures), and data confidentiality (using
 encryption).  As a supplementary service, S/MIME provides for message
 compression.
 S/MIME can be used by traditional mail user agents (MUAs) to add
 cryptographic security services to mail that is sent, and to
 interpret cryptographic security services in mail that is received.
 However, S/MIME is not restricted to mail; it can be used with any
 transport mechanism that transports MIME data, such as HTTP or SIP.
 As such, S/MIME takes advantage of the object-based features of MIME
 and allows secure messages to be exchanged in mixed-transport
 systems.
 Further, S/MIME can be used in automated message transfer agents that
 use cryptographic security services that do not require any human
 intervention, such as the signing of software-generated documents and
 the encryption of FAX messages sent over the Internet.

1.1. Specification Overview

 This document describes a protocol for adding cryptographic signature
 and encryption services to MIME data.  The MIME standard [MIME-SPEC]
 provides a general structure for the content of Internet messages and
 allows extensions for new content-type-based applications.
 This specification defines how to create a MIME body part that has
 been cryptographically enhanced according to the Cryptographic
 Message Syntax (CMS) RFC 5652 [CMS], which is derived from PKCS #7
 [PKCS-7].  This specification also defines the application/pkcs7-mime
 media type that can be used to transport those body parts.
 This document also discusses how to use the multipart/signed media
 type defined in [MIME-SECURE] to transport S/MIME signed messages.
 multipart/signed is used in conjunction with the application/pkcs7-
 signature media type, which is used to transport a detached S/MIME
 signature.

Ramsdell & Turner Standards Track [Page 4] RFC 5751 S/MIME 3.2 Message Specification January 2010

 In order to create S/MIME messages, an S/MIME agent MUST follow the
 specifications in this document, as well as the specifications listed
 in the Cryptographic Message Syntax document [CMS], [CMSALG],
 [RSAPSS], [RSAOAEP], and [CMS-SHA2].
 Throughout this specification, there are requirements and
 recommendations made for how receiving agents handle incoming
 messages.  There are separate requirements and recommendations for
 how sending agents create outgoing messages.  In general, the best
 strategy is to "be liberal in what you receive and conservative in
 what you send".  Most of the requirements are placed on the handling
 of incoming messages, while the recommendations are mostly on the
 creation of outgoing messages.
 The separation for requirements on receiving agents and sending
 agents also derives from the likelihood that there will be S/MIME
 systems that involve software other than traditional Internet mail
 clients.  S/MIME can be used with any system that transports MIME
 data.  An automated process that sends an encrypted message might not
 be able to receive an encrypted message at all, for example.  Thus,
 the requirements and recommendations for the two types of agents are
 listed separately when appropriate.

1.2. Definitions

 For the purposes of this specification, the following definitions
 apply.
 ASN.1:             Abstract Syntax Notation One, as defined in ITU-T
                    Recommendation X.680 [X.680].
 BER:               Basic Encoding Rules for ASN.1, as defined in ITU-
                    T Recommendation X.690 [X.690].
 Certificate:       A type that binds an entity's name to a public key
                    with a digital signature.
 DER:               Distinguished Encoding Rules for ASN.1, as defined
                    in ITU-T Recommendation X.690 [X.690].
 7-bit data:        Text data with lines less than 998 characters
                    long, where none of the characters have the 8th
                    bit set, and there are no NULL characters.  <CR>
                    and <LF> occur only as part of a <CR><LF> end-of-
                    line delimiter.

Ramsdell & Turner Standards Track [Page 5] RFC 5751 S/MIME 3.2 Message Specification January 2010

 8-bit data:        Text data with lines less than 998 characters, and
                    where none of the characters are NULL characters.
                    <CR> and <LF> occur only as part of a <CR><LF>
                    end-of-line delimiter.
 Binary data:       Arbitrary data.
 Transfer encoding: A reversible transformation made on data so 8-bit
                    or binary data can be sent via a channel that only
                    transmits 7-bit data.
 Receiving agent:   Software that interprets and processes S/MIME CMS
                    objects, MIME body parts that contain CMS content
                    types, or both.
 Sending agent:     Software that creates S/MIME CMS content types,
                    MIME body parts that contain CMS content types, or
                    both.
 S/MIME agent:      User software that is a receiving agent, a sending
                    agent, or both.

1.3. Conventions Used in This Document

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [MUSTSHOULD].
 We define some additional terms here:
 SHOULD+   This term means the same as SHOULD.  However, the authors
           expect that a requirement marked as SHOULD+ will be
           promoted at some future time to be a MUST.
 SHOULD-   This term means the same as SHOULD.  However, the authors
           expect that a requirement marked as SHOULD- will be demoted
           to a MAY in a future version of this document.
 MUST-     This term means the same as MUST.  However, the authors
           expect that this requirement will no longer be a MUST in a
           future document.  Although its status will be determined at
           a later time, it is reasonable to expect that if a future
           revision of a document alters the status of a MUST-
           requirement, it will remain at least a SHOULD or a SHOULD-.

Ramsdell & Turner Standards Track [Page 6] RFC 5751 S/MIME 3.2 Message Specification January 2010

1.4. Compatibility with Prior Practice of S/MIME

 S/MIME version 3.2 agents ought to attempt to have the greatest
 interoperability possible with agents for prior versions of S/MIME.
 S/MIME version 2 is described in RFC 2311 through RFC 2315 inclusive
 [SMIMEv2], S/MIME version 3 is described in RFC 2630 through RFC 2634
 inclusive and RFC 5035 [SMIMEv3], and S/MIME version 3.1 is described
 in RFC 3850, RFC 3851, RFC 3852, RFC 2634, and RFC 5035 [SMIMEv3.1].
 RFC 2311 also has historical information about the development of
 S/MIME.

1.5. Changes from S/MIME v3 to S/MIME v3.1

 The RSA public key algorithm was changed to a MUST implement key
 wrapping algorithm, and the Diffie-Hellman (DH) algorithm changed to
 a SHOULD implement.
 The AES symmetric encryption algorithm has been included as a SHOULD
 implement.
 The RSA public key algorithm was changed to a MUST implement
 signature algorithm.
 Ambiguous language about the use of "empty" SignedData messages to
 transmit certificates was clarified to reflect that transmission of
 Certificate Revocation Lists is also allowed.
 The use of binary encoding for some MIME entities is now explicitly
 discussed.
 Header protection through the use of the message/rfc822 media type
 has been added.
 Use of the CompressedData CMS type is allowed, along with required
 media type and file extension additions.

1.6. Changes since S/MIME v3.1

 Editorial changes, e.g., replaced "MIME type" with "media type",
 content-type with Content-Type.
 Moved "Conventions Used in This Document" to Section 1.3.  Added
 definitions for SHOULD+, SHOULD-, and MUST-.
 Section 1.1 and Appendix A: Added references to RFCs for RSASSA-PSS,
 RSAES-OAEP, and SHA2 CMS algorithms.  Added CMS Multiple Signers
 Clarification to CMS reference.

Ramsdell & Turner Standards Track [Page 7] RFC 5751 S/MIME 3.2 Message Specification January 2010

 Section 1.2: Updated references to ASN.1 to X.680 and BER and DER to
 X.690.
 Section 1.4: Added references to S/MIME MSG 3.1 RFCs.
 Section 2.1 (digest algorithm): SHA-256 added as MUST, SHA-1 and MD5
 made SHOULD-.
 Section 2.2 (signature algorithms): RSA with SHA-256 added as MUST,
 and DSA with SHA-256 added as SHOULD+, RSA with SHA-1, DSA with
 SHA-1, and RSA with MD5 changed to SHOULD-, and RSASSA-PSS with
 SHA-256 added as SHOULD+.  Also added note about what S/MIME v3.1
 clients support.
 Section 2.3 (key encryption): DH changed to SHOULD-, and RSAES-OAEP
 added as SHOULD+.  Elaborated requirements for key wrap algorithm.
 Section 2.5.1: Added requirement that receiving agents MUST support
 both GeneralizedTime and UTCTime.
 Section 2.5.2: Replaced reference "sha1WithRSAEncryption" with
 "sha256WithRSAEncryption", "DES-3EDE-CBC" with "AES-128 CBC", and
 deleted the RC5 example.
 Section 2.5.2.1: Deleted entire section (discussed deprecated RC2).
 Section 2.7, 2.7.1, Appendix A: references to RC2/40 removed.
 Section 2.7 (content encryption): AES-128 CBC added as MUST, AES-192
 and AES-256 CBC SHOULD+, tripleDES now SHOULD-.
 Section 2.7.1: Updated pointers from 2.7.2.1 through 2.7.2.4 to
 2.7.1.1 to 2.7.1.2.
 Section 3.1.1: Removed text about MIME character sets.
 Section 3.2.2 and 3.6: Replaced "encrypted" with "enveloped".  Update
 OID example to use AES-128 CBC oid.
 Section 3.4.3.2: Replace micalg parameter for SHA-1 with sha-1.
 Section 4: Updated reference to CERT v3.2.
 Section 4.1: Updated RSA and DSA key size discussion.  Moved last
 four sentences to security considerations.  Updated reference to
 randomness requirements for security.

Ramsdell & Turner Standards Track [Page 8] RFC 5751 S/MIME 3.2 Message Specification January 2010

 Section 5: Added IANA registration templates to update media type
 registry to point to this document as opposed to RFC 2311.
 Section 6: Updated security considerations.
 Section 7: Moved references from Appendix B to this section.  Updated
 references.  Added informational references to SMIMEv2, SMIMEv3, and
 SMIMEv3.1.
 Appendix B: Added Appendix B to move S/MIME v2 to Historic status.

2. CMS Options

 CMS allows for a wide variety of options in content, attributes, and
 algorithm support.  This section puts forth a number of support
 requirements and recommendations in order to achieve a base level of
 interoperability among all S/MIME implementations.  [CMSALG] and
 [CMS-SHA2] provides additional details regarding the use of the
 cryptographic algorithms.  [ESS] provides additional details
 regarding the use of additional attributes.

2.1. DigestAlgorithmIdentifier

 Sending and receiving agents MUST support SHA-256 [CMS-SHA2] and
 SHOULD- support SHA-1 [CMSALG].  Receiving agents SHOULD- support MD5
 [CMSALG] for the purpose of providing backward compatibility with
 MD5-digested S/MIME v2 SignedData objects.

2.2. SignatureAlgorithmIdentifier

 Receiving agents:
  1. MUST support RSA with SHA-256.
  1. SHOULD+ support DSA with SHA-256.
  1. SHOULD+ support RSASSA-PSS with SHA-256.
  1. SHOULD- support RSA with SHA-1.
  1. SHOULD- support DSA with SHA-1.
  1. SHOULD- support RSA with MD5.

Ramsdell & Turner Standards Track [Page 9] RFC 5751 S/MIME 3.2 Message Specification January 2010

 Sending agents:
  1. MUST support RSA with SHA-256.
  1. SHOULD+ support DSA with SHA-256.
  1. SHOULD+ support RSASSA-PSS with SHA-256.
  1. SHOULD- support RSA with SHA-1 or DSA with SHA-1.
  1. SHOULD- support RSA with MD5.
 See Section 4.1 for information on key size and algorithm references.
 Note that S/MIME v3.1 clients support verifying id-dsa-with-sha1 and
 rsaEncryption and might not implement sha256withRSAEncryption.  Note
 that S/MIME v3 clients might only implement signing or signature
 verification using id-dsa-with-sha1, and might also use id-dsa as an
 AlgorithmIdentifier in this field.  Receiving clients SHOULD
 recognize id-dsa as equivalent to id-dsa-with-sha1, and sending
 clients MUST use id-dsa-with-sha1 if using that algorithm.  Also note
 that S/MIME v2 clients are only required to verify digital signatures
 using the rsaEncryption algorithm with SHA-1 or MD5, and might not
 implement id-dsa-with-sha1 or id-dsa at all.

2.3. KeyEncryptionAlgorithmIdentifier

 Receiving and sending agents:
  1. MUST support RSA Encryption, as specified in [CMSALG].
  1. SHOULD+ support RSAES-OAEP, as specified in [RSAOAEP].
  1. SHOULD- support DH ephemeral-static mode, as specified in

[CMSALG] and [SP800-57].

 When DH ephemeral-static is used, a key wrap algorithm is also
 specified in the KeyEncryptionAlgorithmIdentifier [CMS].  The
 underlying encryption functions for the key wrap and content
 encryption algorithm ([CMSALG] and [CMSAES]) and the key sizes for
 the two algorithms MUST be the same (e.g., AES-128 key wrap algorithm
 with AES-128 content encryption algorithm).  As AES-128 CBC is the
 mandatory-to-implement content encryption algorithm, the AES-128 key
 wrap algorithm MUST also be supported when DH ephemeral-static is
 used.

Ramsdell & Turner Standards Track [Page 10] RFC 5751 S/MIME 3.2 Message Specification January 2010

 Note that S/MIME v3.1 clients might only implement key encryption and
 decryption using the rsaEncryption algorithm.  Note that S/MIME v3
 clients might only implement key encryption and decryption using the
 Diffie-Hellman algorithm.  Also note that S/MIME v2 clients are only
 capable of decrypting content-encryption keys using the rsaEncryption
 algorithm.

2.4. General Syntax

 There are several CMS content types.  Of these, only the Data,
 SignedData, EnvelopedData, and CompressedData content types are
 currently used for S/MIME.

2.4.1. Data Content Type

 Sending agents MUST use the id-data content type identifier to
 identify the "inner" MIME message content.  For example, when
 applying a digital signature to MIME data, the CMS SignedData
 encapContentInfo eContentType MUST include the id-data object
 identifier and the media type MUST be stored in the SignedData
 encapContentInfo eContent OCTET STRING (unless the sending agent is
 using multipart/signed, in which case the eContent is absent, per
 Section 3.4.3 of this document).  As another example, when applying
 encryption to MIME data, the CMS EnvelopedData encryptedContentInfo
 contentType MUST include the id-data object identifier and the
 encrypted MIME content MUST be stored in the EnvelopedData
 encryptedContentInfo encryptedContent OCTET STRING.

2.4.2. SignedData Content Type

 Sending agents MUST use the SignedData content type to apply a
 digital signature to a message or, in a degenerate case where there
 is no signature information, to convey certificates.  Applying a
 signature to a message provides authentication, message integrity,
 and non-repudiation of origin.

2.4.3. EnvelopedData Content Type

 This content type is used to apply data confidentiality to a message.
 A sender needs to have access to a public key for each intended
 message recipient to use this service.

2.4.4. CompressedData Content Type

 This content type is used to apply data compression to a message.
 This content type does not provide authentication, message integrity,
 non-repudiation, or data confidentiality, and is only used to reduce
 the message's size.

Ramsdell & Turner Standards Track [Page 11] RFC 5751 S/MIME 3.2 Message Specification January 2010

 See Section 3.6 for further guidance on the use of this type in
 conjunction with other CMS types.

2.5. Attributes and the SignerInfo Type

 The SignerInfo type allows the inclusion of unsigned and signed
 attributes along with a signature.
 Receiving agents MUST be able to handle zero or one instance of each
 of the signed attributes listed here.  Sending agents SHOULD generate
 one instance of each of the following signed attributes in each
 S/MIME message:
  1. Signing Time (section (Section 2.5.1 in this document)
  1. SMIME Capabilities (section (Section 2.5.2 in this document)
  1. Encryption Key Preference (section (Section 2.5.3 in this

document)

  1. Message Digest (section (Section 11.2 in [CMS])
  1. Content Type (section (Section 11.1 in [CMS])
 Further, receiving agents SHOULD be able to handle zero or one
 instance of the signingCertificate and signingCertificatev2 signed
 attributes, as defined in Section 5 of RFC 2634 [ESS] and Section 3
 of RFC 5035 [ESS].
 Sending agents SHOULD generate one instance of the signingCertificate
 or signingCertificatev2 signed attribute in each SignerInfo
 structure.
 Additional attributes and values for these attributes might be
 defined in the future.  Receiving agents SHOULD handle attributes or
 values that they do not recognize in a graceful manner.
 Interactive sending agents that include signed attributes that are
 not listed here SHOULD display those attributes to the user, so that
 the user is aware of all of the data being signed.

2.5.1. Signing Time Attribute

 The signing-time attribute is used to convey the time that a message
 was signed.  The time of signing will most likely be created by a
 message originator and therefore is only as trustworthy as the
 originator.

Ramsdell & Turner Standards Track [Page 12] RFC 5751 S/MIME 3.2 Message Specification January 2010

 Sending agents MUST encode signing time through the year 2049 as
 UTCTime; signing times in 2050 or later MUST be encoded as
 GeneralizedTime.  When the UTCTime CHOICE is used, S/MIME agents MUST
 interpret the year field (YY) as follows:
    If YY is greater than or equal to 50, the year is interpreted as
    19YY; if YY is less than 50, the year is interpreted as 20YY.
 Receiving agents MUST be able to process signing-time attributes that
 are encoded in either UTCTime or GeneralizedTime.

2.5.2. SMIME Capabilities Attribute

 The SMIMECapabilities attribute includes signature algorithms (such
 as "sha256WithRSAEncryption"), symmetric algorithms (such as "AES-128
 CBC"), and key encipherment algorithms (such as "rsaEncryption").
 There are also several identifiers that indicate support for other
 optional features such as binary encoding and compression.  The
 SMIMECapabilities were designed to be flexible and extensible so
 that, in the future, a means of identifying other capabilities and
 preferences such as certificates can be added in a way that will not
 cause current clients to break.
 If present, the SMIMECapabilities attribute MUST be a
 SignedAttribute; it MUST NOT be an UnsignedAttribute.  CMS defines
 SignedAttributes as a SET OF Attribute.  The SignedAttributes in a
 signerInfo MUST NOT include multiple instances of the
 SMIMECapabilities attribute.  CMS defines the ASN.1 syntax for
 Attribute to include attrValues SET OF AttributeValue.  A
 SMIMECapabilities attribute MUST only include a single instance of
 AttributeValue.  There MUST NOT be zero or multiple instances of
 AttributeValue present in the attrValues SET OF AttributeValue.
 The semantics of the SMIMECapabilities attribute specify a partial
 list as to what the client announcing the SMIMECapabilities can
 support.  A client does not have to list every capability it
 supports, and need not list all its capabilities so that the
 capabilities list doesn't get too long.  In an SMIMECapabilities
 attribute, the object identifiers (OIDs) are listed in order of their
 preference, but SHOULD be separated logically along the lines of
 their categories (signature algorithms, symmetric algorithms, key
 encipherment algorithms, etc.).
 The structure of the SMIMECapabilities attribute is to facilitate
 simple table lookups and binary comparisons in order to determine
 matches.  For instance, the DER-encoding for the SMIMECapability for
 AES-128 CBC MUST be identically encoded regardless of the
 implementation.  Because of the requirement for identical encoding,

Ramsdell & Turner Standards Track [Page 13] RFC 5751 S/MIME 3.2 Message Specification January 2010

 individuals documenting algorithms to be used in the
 SMIMECapabilities attribute SHOULD explicitly document the correct
 byte sequence for the common cases.
 For any capability, the associated parameters for the OID MUST
 specify all of the parameters necessary to differentiate between two
 instances of the same algorithm.
 The OIDs that correspond to algorithms SHOULD use the same OID as the
 actual algorithm, except in the case where the algorithm usage is
 ambiguous from the OID.  For instance, in an earlier specification,
 rsaEncryption was ambiguous because it could refer to either a
 signature algorithm or a key encipherment algorithm.  In the event
 that an OID is ambiguous, it needs to be arbitrated by the maintainer
 of the registered SMIMECapabilities list as to which type of
 algorithm will use the OID, and a new OID MUST be allocated under the
 smimeCapabilities OID to satisfy the other use of the OID.
 The registered SMIMECapabilities list specifies the parameters for
 OIDs that need them, most notably key lengths in the case of
 variable-length symmetric ciphers.  In the event that there are no
 differentiating parameters for a particular OID, the parameters MUST
 be omitted, and MUST NOT be encoded as NULL.  Additional values for
 the SMIMECapabilities attribute might be defined in the future.
 Receiving agents MUST handle a SMIMECapabilities object that has
 values that it does not recognize in a graceful manner.
 Section 2.7.1 explains a strategy for caching capabilities.

2.5.3. Encryption Key Preference Attribute

 The encryption key preference attribute allows the signer to
 unambiguously describe which of the signer's certificates has the
 signer's preferred encryption key.  This attribute is designed to
 enhance behavior for interoperating with those clients that use
 separate keys for encryption and signing.  This attribute is used to
 convey to anyone viewing the attribute which of the listed
 certificates is appropriate for encrypting a session key for future
 encrypted messages.
 If present, the SMIMEEncryptionKeyPreference attribute MUST be a
 SignedAttribute; it MUST NOT be an UnsignedAttribute.  CMS defines
 SignedAttributes as a SET OF Attribute.  The SignedAttributes in a
 signerInfo MUST NOT include multiple instances of the
 SMIMEEncryptionKeyPreference attribute.  CMS defines the ASN.1 syntax
 for Attribute to include attrValues SET OF AttributeValue.  A
 SMIMEEncryptionKeyPreference attribute MUST only include a single

Ramsdell & Turner Standards Track [Page 14] RFC 5751 S/MIME 3.2 Message Specification January 2010

 instance of AttributeValue.  There MUST NOT be zero or multiple
 instances of AttributeValue present in the attrValues SET OF
 AttributeValue.
 The sending agent SHOULD include the referenced certificate in the
 set of certificates included in the signed message if this attribute
 is used.  The certificate MAY be omitted if it has been previously
 made available to the receiving agent.  Sending agents SHOULD use
 this attribute if the commonly used or preferred encryption
 certificate is not the same as the certificate used to sign the
 message.
 Receiving agents SHOULD store the preference data if the signature on
 the message is valid and the signing time is greater than the
 currently stored value.  (As with the SMIMECapabilities, the clock
 skew SHOULD be checked and the data not used if the skew is too
 great.)  Receiving agents SHOULD respect the sender's encryption key
 preference attribute if possible.  This, however, represents only a
 preference and the receiving agent can use any certificate in
 replying to the sender that is valid.
 Section 2.7.1 explains a strategy for caching preference data.

2.5.3.1. Selection of Recipient Key Management Certificate

 In order to determine the key management certificate to be used when
 sending a future CMS EnvelopedData message for a particular
 recipient, the following steps SHOULD be followed:
  1. If an SMIMEEncryptionKeyPreference attribute is found in a

SignedData object received from the desired recipient, this

   identifies the X.509 certificate that SHOULD be used as the X.509
   key management certificate for the recipient.
  1. If an SMIMEEncryptionKeyPreference attribute is not found in a

SignedData object received from the desired recipient, the set of

   X.509 certificates SHOULD be searched for a X.509 certificate with
   the same subject name as the signer of a X.509 certificate that can
   be used for key management.
  1. Or use some other method of determining the user's key management

key. If a X.509 key management certificate is not found, then

   encryption cannot be done with the signer of the message.  If
   multiple X.509 key management certificates are found, the S/MIME
   agent can make an arbitrary choice between them.

Ramsdell & Turner Standards Track [Page 15] RFC 5751 S/MIME 3.2 Message Specification January 2010

2.6. SignerIdentifier SignerInfo Type

 S/MIME v3.2 implementations MUST support both issuerAndSerialNumber
 and subjectKeyIdentifier.  Messages that use the subjectKeyIdentifier
 choice cannot be read by S/MIME v2 clients.
 It is important to understand that some certificates use a value for
 subjectKeyIdentifier that is not suitable for uniquely identifying a
 certificate.  Implementations MUST be prepared for multiple
 certificates for potentially different entities to have the same
 value for subjectKeyIdentifier, and MUST be prepared to try each
 matching certificate during signature verification before indicating
 an error condition.

2.7. ContentEncryptionAlgorithmIdentifier

 Sending and receiving agents:
  1. MUST support encryption and decryption with AES-128 CBC

[CMSAES].

  1. SHOULD+ support encryption and decryption with AES-192 CBC and

AES-256 CBC [CMSAES].

  1. SHOULD- support encryption and decryption with DES EDE3 CBC,

hereinafter called "tripleDES" [CMSALG].

2.7.1. Deciding Which Encryption Method to Use

 When a sending agent creates an encrypted message, it has to decide
 which type of encryption to use.  The decision process involves using
 information garnered from the capabilities lists included in messages
 received from the recipient, as well as out-of-band information such
 as private agreements, user preferences, legal restrictions, and so
 on.
 Section 2.5.2 defines a method by which a sending agent can
 optionally announce, among other things, its decrypting capabilities
 in its order of preference.  The following method for processing and
 remembering the encryption capabilities attribute in incoming signed
 messages SHOULD be used.
  1. If the receiving agent has not yet created a list of

capabilities for the sender's public key, then, after verifying

      the signature on the incoming message and checking the
      timestamp, the receiving agent SHOULD create a new list
      containing at least the signing time and the symmetric
      capabilities.

Ramsdell & Turner Standards Track [Page 16] RFC 5751 S/MIME 3.2 Message Specification January 2010

  1. If such a list already exists, the receiving agent SHOULD verify

that the signing time in the incoming message is greater than

      the signing time stored in the list and that the signature is
      valid.  If so, the receiving agent SHOULD update both the
      signing time and capabilities in the list.  Values of the
      signing time that lie far in the future (that is, a greater
      discrepancy than any reasonable clock skew), or a capabilities
      list in messages whose signature could not be verified, MUST NOT
      be accepted.
 The list of capabilities SHOULD be stored for future use in creating
 messages.
 Before sending a message, the sending agent MUST decide whether it is
 willing to use weak encryption for the particular data in the
 message.  If the sending agent decides that weak encryption is
 unacceptable for this data, then the sending agent MUST NOT use a
 weak algorithm.  The decision to use or not use weak encryption
 overrides any other decision in this section about which encryption
 algorithm to use.
 Sections 2.7.1.1 through 2.7.1.2 describe the decisions a sending
 agent SHOULD use in deciding which type of encryption will be applied
 to a message.  These rules are ordered, so the sending agent SHOULD
 make its decision in the order given.

2.7.1.1. Rule 1: Known Capabilities

 If the sending agent has received a set of capabilities from the
 recipient for the message the agent is about to encrypt, then the
 sending agent SHOULD use that information by selecting the first
 capability in the list (that is, the capability most preferred by the
 intended recipient) that the sending agent knows how to encrypt.  The
 sending agent SHOULD use one of the capabilities in the list if the
 agent reasonably expects the recipient to be able to decrypt the
 message.

2.7.1.2. Rule 2: Unknown Capabilities, Unknown Version of S/MIME

 If the following two conditions are met:
  1. the sending agent has no knowledge of the encryption

capabilities of the recipient, and

  1. the sending agent has no knowledge of the version of S/MIME of

the recipient,

Ramsdell & Turner Standards Track [Page 17] RFC 5751 S/MIME 3.2 Message Specification January 2010

 then the sending agent SHOULD use AES-128 because it is a stronger
 algorithm and is required by S/MIME v3.2.  If the sending agent
 chooses not to use AES-128 in this step, it SHOULD use tripleDES.

2.7.2. Choosing Weak Encryption

 All algorithms that use 40-bit keys are considered by many to be weak
 encryption.  A sending agent that is controlled by a human SHOULD
 allow a human sender to determine the risks of sending data using a
 weak encryption algorithm before sending the data, and possibly allow
 the human to use a stronger encryption method such as tripleDES or
 AES.

2.7.3. Multiple Recipients

 If a sending agent is composing an encrypted message to a group of
 recipients where the encryption capabilities of some of the
 recipients do not overlap, the sending agent is forced to send more
 than one message.  Please note that if the sending agent chooses to
 send a message encrypted with a strong algorithm, and then send the
 same message encrypted with a weak algorithm, someone watching the
 communications channel could learn the contents of the strongly
 encrypted message simply by decrypting the weakly encrypted message.

3. Creating S/MIME Messages

 This section describes the S/MIME message formats and how they are
 created.  S/MIME messages are a combination of MIME bodies and CMS
 content types.  Several media types as well as several CMS content
 types are used.  The data to be secured is always a canonical MIME
 entity.  The MIME entity and other data, such as certificates and
 algorithm identifiers, are given to CMS processing facilities that
 produce a CMS object.  Finally, the CMS object is wrapped in MIME.
 The Enhanced Security Services for S/MIME [ESS] document provides
 descriptions of how nested, secured S/MIME messages are formatted.
 ESS provides a description of how a triple-wrapped S/MIME message is
 formatted using multipart/signed and application/pkcs7-mime for the
 signatures.
 S/MIME provides one format for enveloped-only data, several formats
 for signed-only data, and several formats for signed and enveloped
 data.  Several formats are required to accommodate several
 environments, in particular for signed messages.  The criteria for
 choosing among these formats are also described.
 The reader of this section is expected to understand MIME as
 described in [MIME-SPEC] and [MIME-SECURE].

Ramsdell & Turner Standards Track [Page 18] RFC 5751 S/MIME 3.2 Message Specification January 2010

3.1. Preparing the MIME Entity for Signing, Enveloping, or Compressing

 S/MIME is used to secure MIME entities.  A MIME entity can be a sub-
 part, sub-parts of a message, or the whole message with all its sub-
 parts.  A MIME entity that is the whole message includes only the
 MIME message headers and MIME body, and does not include the RFC-822
 header.  Note that S/MIME can also be used to secure MIME entities
 used in applications other than Internet mail.  If protection of the
 RFC-822 header is required, the use of the message/rfc822 media type
 is explained later in this section.
 The MIME entity that is secured and described in this section can be
 thought of as the "inside" MIME entity.  That is, it is the
 "innermost" object in what is possibly a larger MIME message.
 Processing "outside" MIME entities into CMS content types is
 described in Sections 3.2, 3.4, and elsewhere.
 The procedure for preparing a MIME entity is given in [MIME-SPEC].
 The same procedure is used here with some additional restrictions
 when signing.  The description of the procedures from [MIME-SPEC] is
 repeated here, but it is suggested that the reader refer to that
 document for the exact procedure.  This section also describes
 additional requirements.
 A single procedure is used for creating MIME entities that are to
 have any combination of signing, enveloping, and compressing applied.
 Some additional steps are recommended to defend against known
 corruptions that can occur during mail transport that are of
 particular importance for clear-signing using the multipart/signed
 format.  It is recommended that these additional steps be performed
 on enveloped messages, or signed and enveloped messages, so that the
 message can be forwarded to any environment without modification.
 These steps are descriptive rather than prescriptive.  The
 implementer is free to use any procedure as long as the result is the
 same.
 Step 1.  The MIME entity is prepared according to the local
          conventions.
 Step 2.  The leaf parts of the MIME entity are converted to canonical
          form.
 Step 3.  Appropriate transfer encoding is applied to the leaves of
          the MIME entity.

Ramsdell & Turner Standards Track [Page 19] RFC 5751 S/MIME 3.2 Message Specification January 2010

 When an S/MIME message is received, the security services on the
 message are processed, and the result is the MIME entity.  That MIME
 entity is typically passed to a MIME-capable user agent where it is
 further decoded and presented to the user or receiving application.
 In order to protect outer, non-content-related message header fields
 (for instance, the "Subject", "To", "From", and "Cc" fields), the
 sending client MAY wrap a full MIME message in a message/rfc822
 wrapper in order to apply S/MIME security services to these header
 fields.  It is up to the receiving client to decide how to present
 this "inner" header along with the unprotected "outer" header.
 When an S/MIME message is received, if the top-level protected MIME
 entity has a Content-Type of message/rfc822, it can be assumed that
 the intent was to provide header protection.  This entity SHOULD be
 presented as the top-level message, taking into account header
 merging issues as previously discussed.

3.1.1. Canonicalization

 Each MIME entity MUST be converted to a canonical form that is
 uniquely and unambiguously representable in the environment where the
 signature is created and the environment where the signature will be
 verified.  MIME entities MUST be canonicalized for enveloping and
 compressing as well as signing.
 The exact details of canonicalization depend on the actual media type
 and subtype of an entity, and are not described here.  Instead, the
 standard for the particular media type SHOULD be consulted.  For
 example, canonicalization of type text/plain is different from
 canonicalization of audio/basic.  Other than text types, most types
 have only one representation regardless of computing platform or
 environment that can be considered their canonical representation.
 In general, canonicalization will be performed by the non-security
 part of the sending agent rather than the S/MIME implementation.
 The most common and important canonicalization is for text, which is
 often represented differently in different environments.  MIME
 entities of major type "text" MUST have both their line endings and
 character set canonicalized.  The line ending MUST be the pair of
 characters <CR><LF>, and the charset SHOULD be a registered charset
 [CHARSETS].  The details of the canonicalization are specified in
 [MIME-SPEC].
 Note that some charsets such as ISO-2022 have multiple
 representations for the same characters.  When preparing such text
 for signing, the canonical representation specified for the charset
 MUST be used.

Ramsdell & Turner Standards Track [Page 20] RFC 5751 S/MIME 3.2 Message Specification January 2010

3.1.2. Transfer Encoding

 When generating any of the secured MIME entities below, except the
 signing using the multipart/signed format, no transfer encoding is
 required at all.  S/MIME implementations MUST be able to deal with
 binary MIME objects.  If no Content-Transfer-Encoding header field is
 present, the transfer encoding is presumed to be 7BIT.
 S/MIME implementations SHOULD however use transfer encoding described
 in Section 3.1.3 for all MIME entities they secure.  The reason for
 securing only 7-bit MIME entities, even for enveloped data that are
 not exposed to the transport, is that it allows the MIME entity to be
 handled in any environment without changing it.  For example, a
 trusted gateway might remove the envelope, but not the signature, of
 a message, and then forward the signed message on to the end
 recipient so that they can verify the signatures directly.  If the
 transport internal to the site is not 8-bit clean, such as on a wide-
 area network with a single mail gateway, verifying the signature will
 not be possible unless the original MIME entity was only 7-bit data.
 S/MIME implementations that "know" that all intended recipients are
 capable of handling inner (all but the outermost) binary MIME objects
 SHOULD use binary encoding as opposed to a 7-bit-safe transfer
 encoding for the inner entities.  The use of a 7-bit-safe encoding
 (such as base64) would unnecessarily expand the message size.
 Implementations MAY "know" that recipient implementations are capable
 of handling inner binary MIME entities either by interpreting the id-
 cap-preferBinaryInside SMIMECapabilities attribute, by prior
 agreement, or by other means.
 If one or more intended recipients are unable to handle inner binary
 MIME objects, or if this capability is unknown for any of the
 intended recipients, S/MIME implementations SHOULD use transfer
 encoding described in Section 3.1.3 for all MIME entities they
 secure.

3.1.3. Transfer Encoding for Signing Using multipart/signed

 If a multipart/signed entity is ever to be transmitted over the
 standard Internet SMTP infrastructure or other transport that is
 constrained to 7-bit text, it MUST have transfer encoding applied so
 that it is represented as 7-bit text.  MIME entities that are 7-bit
 data already need no transfer encoding.  Entities such as 8-bit text
 and binary data can be encoded with quoted-printable or base-64
 transfer encoding.

Ramsdell & Turner Standards Track [Page 21] RFC 5751 S/MIME 3.2 Message Specification January 2010

 The primary reason for the 7-bit requirement is that the Internet
 mail transport infrastructure cannot guarantee transport of 8-bit or
 binary data.  Even though many segments of the transport
 infrastructure now handle 8-bit and even binary data, it is sometimes
 not possible to know whether the transport path is 8-bit clean.  If a
 mail message with 8-bit data were to encounter a message transfer
 agent that cannot transmit 8-bit or binary data, the agent has three
 options, none of which are acceptable for a clear-signed message:
  1. The agent could change the transfer encoding; this would

invalidate the signature.

  1. The agent could transmit the data anyway, which would most likely

result in the 8th bit being corrupted; this too would invalidate

    the signature.
  1. The agent could return the message to the sender.
 [MIME-SECURE] prohibits an agent from changing the transfer encoding
 of the first part of a multipart/signed message.  If a compliant
 agent that cannot transmit 8-bit or binary data encounters a
 multipart/signed message with 8-bit or binary data in the first part,
 it would have to return the message to the sender as undeliverable.

3.1.4. Sample Canonical MIME Entity

 This example shows a multipart/mixed message with full transfer
 encoding.  This message contains a text part and an attachment.  The
 sample message text includes characters that are not US-ASCII and
 thus need to be transfer encoded.  Though not shown here, the end of
 each line is <CR><LF>.  The line ending of the MIME headers, the
 text, and the transfer encoded parts, all MUST be <CR><LF>.
 Note that this example is not of an S/MIME message.
    Content-Type: multipart/mixed; boundary=bar
  1. -bar

Content-Type: text/plain; charset=iso-8859-1

    Content-Transfer-Encoding: quoted-printable
    =A1Hola Michael!
    How do you like the new S/MIME specification?
    It's generally a good idea to encode lines that begin with
    From=20because some mail transport agents will insert a greater-
    than (>) sign, thus invalidating the signature.

Ramsdell & Turner Standards Track [Page 22] RFC 5751 S/MIME 3.2 Message Specification January 2010

    Also, in some cases it might be desirable to encode any =20
    trailing whitespace that occurs on lines in order to ensure =20
    that the message signature is not invalidated when passing =20
    a gateway that modifies such whitespace (like BITNET). =20
  1. -bar

Content-Type: image/jpeg

    Content-Transfer-Encoding: base64
    iQCVAwUBMJrRF2N9oWBghPDJAQE9UQQAtl7LuRVndBjrk4EqYBIb3h5QXIX/LC//
    jJV5bNvkZIGPIcEmI5iFd9boEgvpirHtIREEqLQRkYNoBActFBZmh9GC3C041WGq
    uMbrbxc+nIs1TIKlA08rVi9ig/2Yh7LFrK5Ein57U/W72vgSxLhe/zhdfolT9Brn
    HOxEa44b+EI=
  1. -bar–

3.2. The application/pkcs7-mime Media Type

 The application/pkcs7-mime media type is used to carry CMS content
 types including EnvelopedData, SignedData, and CompressedData.  The
 details of constructing these entities are described in subsequent
 sections.  This section describes the general characteristics of the
 application/pkcs7-mime media type.
 The carried CMS object always contains a MIME entity that is prepared
 as described in Section 3.1 if the eContentType is id-data.  Other
 contents MAY be carried when the eContentType contains different
 values.  See [ESS] for an example of this with signed receipts.
 Since CMS content types are binary data, in most cases base-64
 transfer encoding is appropriate, in particular, when used with SMTP
 transport.  The transfer encoding used depends on the transport
 through which the object is to be sent, and is not a characteristic
 of the media type.
 Note that this discussion refers to the transfer encoding of the CMS
 object or "outside" MIME entity.  It is completely distinct from, and
 unrelated to, the transfer encoding of the MIME entity secured by the
 CMS object, the "inside" object, which is described in Section 3.1.
 Because there are several types of application/pkcs7-mime objects, a
 sending agent SHOULD do as much as possible to help a receiving agent
 know about the contents of the object without forcing the receiving
 agent to decode the ASN.1 for the object.  The Content-Type header
 field of all application/pkcs7-mime objects SHOULD include the
 optional "smime-type" parameter, as described in the following
 sections.

Ramsdell & Turner Standards Track [Page 23] RFC 5751 S/MIME 3.2 Message Specification January 2010

3.2.1. The name and filename Parameters

 For the application/pkcs7-mime, sending agents SHOULD emit the
 optional "name" parameter to the Content-Type field for compatibility
 with older systems.  Sending agents SHOULD also emit the optional
 Content-Disposition field [CONTDISP] with the "filename" parameter.
 If a sending agent emits the above parameters, the value of the
 parameters SHOULD be a file name with the appropriate extension:
 Media Type                                            File Extension
   application/pkcs7-mime (SignedData, EnvelopedData)      .p7m
   application/pkcs7-mime (degenerate SignedData           .p7c
      certificate management message)
   application/pkcs7-mime (CompressedData)                 .p7z
   application/pkcs7-signature (SignedData)                .p7s
 In addition, the file name SHOULD be limited to eight characters
 followed by a three-letter extension.  The eight-character filename
 base can be any distinct name; the use of the filename base "smime"
 SHOULD be used to indicate that the MIME entity is associated with
 S/MIME.
 Including a file name serves two purposes.  It facilitates easier use
 of S/MIME objects as files on disk.  It also can convey type
 information across gateways.  When a MIME entity of type
 application/pkcs7-mime (for example) arrives at a gateway that has no
 special knowledge of S/MIME, it will default the entity's media type
 to application/octet-stream and treat it as a generic attachment,
 thus losing the type information.  However, the suggested filename
 for an attachment is often carried across a gateway.  This often
 allows the receiving systems to determine the appropriate application
 to hand the attachment off to, in this case, a stand-alone S/MIME
 processing application.  Note that this mechanism is provided as a
 convenience for implementations in certain environments.  A proper
 S/MIME implementation MUST use the media types and MUST NOT rely on
 the file extensions.

3.2.2. The smime-type Parameter

 The application/pkcs7-mime content type defines the optional "smime-
 type" parameter.  The intent of this parameter is to convey details
 about the security applied (signed or enveloped) along with
 information about the contained content.  This specification defines
 the following smime-types.

Ramsdell & Turner Standards Track [Page 24] RFC 5751 S/MIME 3.2 Message Specification January 2010

    Name                   CMS Type                Inner Content
    enveloped-data         EnvelopedData           id-data
    signed-data            SignedData              id-data
    certs-only             SignedData              none
    compressed-data        CompressedData          id-data
 In order for consistency to be obtained with future specifications,
 the following guidelines SHOULD be followed when assigning a new
 smime-type parameter.
    1. If both signing and encryption can be applied to the content,
       then two values for smime-type SHOULD be assigned "signed-*"
       and "enveloped-*".  If one operation can be assigned, then this
       can be omitted.  Thus, since "certs-only" can only be signed,
       "signed-" is omitted.
    2. A common string for a content OID SHOULD be assigned.  We use
       "data" for the id-data content OID when MIME is the inner
       content.
    3. If no common string is assigned, then the common string of
       "OID.<oid>" is recommended (for example,
       "OID.2.16.840.1.101.3.4.1.2" would be AES-128 CBC).
 It is explicitly intended that this field be a suitable hint for mail
 client applications to indicate whether a message is "signed" or
 "enveloped" without having to tunnel into the CMS payload.

3.3. Creating an Enveloped-Only Message

 This section describes the format for enveloping a MIME entity
 without signing it.  It is important to note that sending enveloped
 but not signed messages does not provide for data integrity.  It is
 possible to replace ciphertext in such a way that the processed
 message will still be valid, but the meaning can be altered.
 Step 1.  The MIME entity to be enveloped is prepared according to
          Section 3.1.
 Step 2.  The MIME entity and other required data is processed into a
          CMS object of type EnvelopedData.  In addition to encrypting
          a copy of the content-encryption key for each recipient, a
          copy of the content-encryption key SHOULD be encrypted for
          the originator and included in the EnvelopedData (see [CMS],
          Section 6).
 Step 3.  The EnvelopedData object is wrapped in a CMS ContentInfo
          object.

Ramsdell & Turner Standards Track [Page 25] RFC 5751 S/MIME 3.2 Message Specification January 2010

 Step 4.  The ContentInfo object is inserted into an
          application/pkcs7-mime MIME entity.
 The smime-type parameter for enveloped-only messages is "enveloped-
 data".  The file extension for this type of message is ".p7m".
 A sample message would be:
    Content-Type: application/pkcs7-mime; smime-type=enveloped-data;
         name=smime.p7m
    Content-Transfer-Encoding: base64
    Content-Disposition: attachment; filename=smime.p7m
    rfvbnj756tbBghyHhHUujhJhjH77n8HHGT9HG4VQpfyF467GhIGfHfYT6
    7n8HHGghyHhHUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H
    f8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
    0GhIGfHfQbnj756YT64V

3.4. Creating a Signed-Only Message

 There are two formats for signed messages defined for S/MIME:
  1. application/pkcs7-mime with SignedData.
  1. multipart/signed.
 In general, the multipart/signed form is preferred for sending, and
 receiving agents MUST be able to handle both.

3.4.1. Choosing a Format for Signed-Only Messages

 There are no hard-and-fast rules as to when a particular signed-only
 format is chosen.  It depends on the capabilities of all the
 receivers and the relative importance of receivers with S/MIME
 facilities being able to verify the signature versus the importance
 of receivers without S/MIME software being able to view the message.
 Messages signed using the multipart/signed format can always be
 viewed by the receiver whether or not they have S/MIME software.
 They can also be viewed whether they are using a MIME-native user
 agent or they have messages translated by a gateway.  In this
 context, "be viewed" means the ability to process the message
 essentially as if it were not a signed message, including any other
 MIME structure the message might have.

Ramsdell & Turner Standards Track [Page 26] RFC 5751 S/MIME 3.2 Message Specification January 2010

 Messages signed using the SignedData format cannot be viewed by a
 recipient unless they have S/MIME facilities.  However, the
 SignedData format protects the message content from being changed by
 benign intermediate agents.  Such agents might do line wrapping or
 content-transfer encoding changes that would break the signature.

3.4.2. Signing Using application/pkcs7-mime with SignedData

 This signing format uses the application/pkcs7-mime media type.  The
 steps to create this format are:
 Step 1.  The MIME entity is prepared according to Section 3.1.
 Step 2.  The MIME entity and other required data are processed into a
          CMS object of type SignedData.
 Step 3.  The SignedData object is wrapped in a CMS ContentInfo
          object.
 Step 4.  The ContentInfo object is inserted into an
          application/pkcs7-mime MIME entity.
 The smime-type parameter for messages using application/pkcs7-mime
 with SignedData is "signed-data".  The file extension for this type
 of message is ".p7m".
 A sample message would be:
    Content-Type: application/pkcs7-mime; smime-type=signed-data;
         name=smime.p7m
    Content-Transfer-Encoding: base64
    Content-Disposition: attachment; filename=smime.p7m
    567GhIGfHfYT6ghyHhHUujpfyF4f8HHGTrfvhJhjH776tbB9HG4VQbnj7
    77n8HHGT9HG4VQpfyF467GhIGfHfYT6rfvbnj756tbBghyHhHUujhJhjH
    HUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H7n8HHGghyHh
    6YT64V0GhIGfHfQbnj75

3.4.3. Signing Using the multipart/signed Format

 This format is a clear-signing format.  Recipients without any S/MIME
 or CMS processing facilities are able to view the message.  It makes
 use of the multipart/signed media type described in [MIME-SECURE].
 The multipart/signed media type has two parts.  The first part
 contains the MIME entity that is signed; the second part contains the
 "detached signature" CMS SignedData object in which the
 encapContentInfo eContent field is absent.

Ramsdell & Turner Standards Track [Page 27] RFC 5751 S/MIME 3.2 Message Specification January 2010

3.4.3.1. The application/pkcs7-signature Media Type

 This media type always contains a CMS ContentInfo containing a single
 CMS object of type SignedData.  The SignedData encapContentInfo
 eContent field MUST be absent.  The signerInfos field contains the
 signatures for the MIME entity.
 The file extension for signed-only messages using application/pkcs7-
 signature is ".p7s".

3.4.3.2. Creating a multipart/signed Message

 Step 1.  The MIME entity to be signed is prepared according to
          Section 3.1, taking special care for clear-signing.
 Step 2.  The MIME entity is presented to CMS processing in order to
          obtain an object of type SignedData in which the
          encapContentInfo eContent field is absent.
 Step 3.  The MIME entity is inserted into the first part of a
          multipart/signed message with no processing other than that
          described in Section 3.1.
 Step 4.  Transfer encoding is applied to the "detached signature" CMS
          SignedData object, and it is inserted into a MIME entity of
          type application/pkcs7-signature.
 Step 5.  The MIME entity of the application/pkcs7-signature is
          inserted into the second part of the multipart/signed
          entity.
 The multipart/signed Content-Type has two required parameters: the
 protocol parameter and the micalg parameter.
 The protocol parameter MUST be "application/pkcs7-signature".  Note
 that quotation marks are required around the protocol parameter
 because MIME requires that the "/" character in the parameter value
 MUST be quoted.
 The micalg parameter allows for one-pass processing when the
 signature is being verified.  The value of the micalg parameter is
 dependent on the message digest algorithm(s) used in the calculation
 of the Message Integrity Check.  If multiple message digest
 algorithms are used, they MUST be separated by commas per [MIME-
 SECURE].  The values to be placed in the micalg parameter SHOULD be
 from the following:

Ramsdell & Turner Standards Track [Page 28] RFC 5751 S/MIME 3.2 Message Specification January 2010

    Algorithm   Value Used
    MD5         md5
    SHA-1       sha-1
    SHA-224     sha-224
    SHA-256     sha-256
    SHA-384     sha-384
    SHA-512     sha-512
    Any other   (defined separately in algorithm profile or "unknown"
                 if not defined)
 (Historical note: some early implementations of S/MIME emitted and
 expected "rsa-md5", "rsa-sha1", and "sha1" for the micalg parameter.)
 Receiving agents SHOULD be able to recover gracefully from a micalg
 parameter value that they do not recognize.  Future names for this
 parameter will be consistent with the IANA "Hash Function Textual
 Names" registry.

3.4.3.3. Sample multipart/signed Message

     Content-Type: multipart/signed;
        protocol="application/pkcs7-signature";
        micalg=sha1; boundary=boundary42
  1. -boundary42

Content-Type: text/plain

     This is a clear-signed message.
  1. -boundary42

Content-Type: application/pkcs7-signature; name=smime.p7s

     Content-Transfer-Encoding: base64
     Content-Disposition: attachment; filename=smime.p7s
     ghyHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6
     4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj
     n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
     7GhIGfHfYT64VQbnj756
  1. -boundary42–
 The content that is digested (the first part of the multipart/signed)
 consists of the bytes:
 43 6f 6e 74 65 6e 74 2d 54 79 70 65 3a 20 74 65 78 74 2f 70 6c 61 69
 6e 0d 0a 0d 0a 54 68 69 73 20 69 73 20 61 20 63 6c 65 61 72 2d 73 69
 67 6e 65 64 20 6d 65 73 73 61 67 65 2e 0d 0a

Ramsdell & Turner Standards Track [Page 29] RFC 5751 S/MIME 3.2 Message Specification January 2010

3.5. Creating a Compressed-Only Message

 This section describes the format for compressing a MIME entity.
 Please note that versions of S/MIME prior to version 3.1 did not
 specify any use of CompressedData, and will not recognize it.  The
 use of a capability to indicate the ability to receive CompressedData
 is described in [CMSCOMPR] and is the preferred method for
 compatibility.
 Step 1.  The MIME entity to be compressed is prepared according to
          Section 3.1.
 Step 2.  The MIME entity and other required data are processed into a
          CMS object of type CompressedData.
 Step 3.  The CompressedData object is wrapped in a CMS ContentInfo
          object.
 Step 4.  The ContentInfo object is inserted into an
          application/pkcs7-mime MIME entity.
 The smime-type parameter for compressed-only messages is "compressed-
 data".  The file extension for this type of message is ".p7z".
 A sample message would be:
 Content-Type: application/pkcs7-mime; smime-type=compressed-data;
    name=smime.p7z
 Content-Transfer-Encoding: base64
 Content-Disposition: attachment; filename=smime.p7z
 rfvbnj756tbBghyHhHUujhJhjH77n8HHGT9HG4VQpfyF467GhIGfHfYT6
 7n8HHGghyHhHUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H
 f8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
 0GhIGfHfQbnj756YT64V

3.6. Multiple Operations

 The signed-only, enveloped-only, and compressed-only MIME formats can
 be nested.  This works because these formats are all MIME entities
 that encapsulate other MIME entities.
 An S/MIME implementation MUST be able to receive and process
 arbitrarily nested S/MIME within reasonable resource limits of the
 recipient computer.

Ramsdell & Turner Standards Track [Page 30] RFC 5751 S/MIME 3.2 Message Specification January 2010

 It is possible to apply any of the signing, encrypting, and
 compressing operations in any order.  It is up to the implementer and
 the user to choose.  When signing first, the signatories are then
 securely obscured by the enveloping.  When enveloping first the
 signatories are exposed, but it is possible to verify signatures
 without removing the enveloping.  This can be useful in an
 environment where automatic signature verification is desired, as no
 private key material is required to verify a signature.
 There are security ramifications to choosing whether to sign first or
 encrypt first.  A recipient of a message that is encrypted and then
 signed can validate that the encrypted block was unaltered, but
 cannot determine any relationship between the signer and the
 unencrypted contents of the message.  A recipient of a message that
 is signed then encrypted can assume that the signed message itself
 has not been altered, but that a careful attacker could have changed
 the unauthenticated portions of the encrypted message.
 When using compression, keep the following guidelines in mind:
  1. Compression of binary encoded encrypted data is discouraged,

since it will not yield significant compression. Base64

      encrypted data could very well benefit, however.
  1. If a lossy compression algorithm is used with signing, you will

need to compress first, then sign.

3.7. Creating a Certificate Management Message

 The certificate management message or MIME entity is used to
 transport certificates and/or Certificate Revocation Lists, such as
 in response to a registration request.
 Step 1.  The certificates and/or Certificate Revocation Lists are
          made available to the CMS generating process that creates a
          CMS object of type SignedData.  The SignedData
          encapContentInfo eContent field MUST be absent and
          signerInfos field MUST be empty.
 Step 2.  The SignedData object is wrapped in a CMS ContentInfo
          object.
 Step 3.  The ContentInfo object is enclosed in an
          application/pkcs7-mime MIME entity.
 The smime-type parameter for a certificate management message is
 "certs-only".  The file extension for this type of message is ".p7c".

Ramsdell & Turner Standards Track [Page 31] RFC 5751 S/MIME 3.2 Message Specification January 2010

3.8. Registration Requests

 A sending agent that signs messages MUST have a certificate for the
 signature so that a receiving agent can verify the signature.  There
 are many ways of getting certificates, such as through an exchange
 with a certification authority, through a hardware token or diskette,
 and so on.
 S/MIME v2 [SMIMEv2] specified a method for "registering" public keys
 with certificate authorities using an application/pkcs10 body part.
 Since that time, the IETF PKIX Working Group has developed other
 methods for requesting certificates.  However, S/MIME v3.2 does not
 require a particular certificate request mechanism.

3.9. Identifying an S/MIME Message

 Because S/MIME takes into account interoperation in non-MIME
 environments, several different mechanisms are employed to carry the
 type information, and it becomes a bit difficult to identify S/MIME
 messages.  The following table lists criteria for determining whether
 or not a message is an S/MIME message.  A message is considered an
 S/MIME message if it matches any of the criteria listed below.
 The file suffix in the table below comes from the "name" parameter in
 the Content-Type header field, or the "filename" parameter on the
 Content-Disposition header field.  These parameters that give the
 file suffix are not listed below as part of the parameter section.
 Media type:  application/pkcs7-mime
 parameters:  any
 file suffix: any
 Media type:  multipart/signed
 parameters:  protocol="application/pkcs7-signature"
 file suffix: any
 Media type:  application/octet-stream
 parameters:  any
 file suffix: p7m, p7s, p7c, p7z

4. Certificate Processing

 A receiving agent MUST provide some certificate retrieval mechanism
 in order to gain access to certificates for recipients of digital
 envelopes.  This specification does not cover how S/MIME agents
 handle certificates, only what they do after a certificate has been
 validated or rejected.  S/MIME certificate issues are covered in
 [CERT32].

Ramsdell & Turner Standards Track [Page 32] RFC 5751 S/MIME 3.2 Message Specification January 2010

 At a minimum, for initial S/MIME deployment, a user agent could
 automatically generate a message to an intended recipient requesting
 that recipient's certificate in a signed return message.  Receiving
 and sending agents SHOULD also provide a mechanism to allow a user to
 "store and protect" certificates for correspondents in such a way so
 as to guarantee their later retrieval.

4.1. Key Pair Generation

 All generated key pairs MUST be generated from a good source of non-
 deterministic random input [RANDOM] and the private key MUST be
 protected in a secure fashion.
 An S/MIME user agent MUST NOT generate asymmetric keys less than 512
 bits for use with the RSA or DSA signature algorithms.
 For 512-bit RSA with SHA-1 see [CMSALG] and [FIPS186-2] without
 Change Notice 1, for 512-bit RSA with SHA-256 see [CMS-SHA2] and
 [FIPS186-2] without Change Notice 1, and for 1024-bit through
 2048-bit RSA with SHA-256 see [CMS-SHA2] and [FIPS186-2] with Change
 Notice 1.  The first reference provides the signature algorithm's
 object identifier, and the second provides the signature algorithm's
 definition.
 For 512-bit DSA with SHA-1 see [CMSALG] and [FIPS186-2] without
 Change Notice 1, for 512-bit DSA with SHA-256 see [CMS-SHA2] and
 [FIPS186-2] without Change Notice 1, for 1024-bit DSA with SHA-1 see
 [CMSALG] and [FIPS186-2] with Change Notice 1, for 1024-bit and above
 DSA with SHA-256 see [CMS-SHA2] and [FIPS186-3].  The first reference
 provides the signature algorithm's object identifier and the second
 provides the signature algorithm's definition.
 For RSASSA-PSS with SHA-256, see [RSAPSS].  For 1024-bit DH, see
 [CMSALG].  For 1024-bit and larger DH, see [SP800-56A]; regardless,
 use the KDF, which is from X9.42, specified in [CMSALG].  For RSAES-
 OAEP, see [RSAOAEP].

4.2. Signature Generation

 The following are the requirements for an S/MIME agent generated RSA,
 RSASSA-PSS, and DSA signatures:
         key size <= 1023 : SHOULD NOT (see Security Considerations)
 1024 <= key size <= 2048 : SHOULD     (see Security Considerations)
 2048 <  key size         : MAY        (see Security Considerations)

Ramsdell & Turner Standards Track [Page 33] RFC 5751 S/MIME 3.2 Message Specification January 2010

4.3. Signature Verification

 The following are the requirements for S/MIME receiving agents during
 signature verification of RSA, RSASSA-PSS, and DSA signatures:
         key size <= 1023 : MAY        (see Security Considerations)
 1024 <= key size <= 2048 : MUST       (see Security Considerations)
 2048 <  key size         : MAY        (see Security Considerations)

4.4. Encryption

 The following are the requirements for an S/MIME agent when
 establishing keys for content encryption using the RSA, RSA-OAEP, and
 DH algorithms:
         key size <= 1023 : SHOULD NOT (see Security Considerations)
 1024 <= key size <= 2048 : SHOULD     (see Security Considerations)
 2048 <  key size         : MAY        (see Security Considerations)

4.5. Decryption

 The following are the requirements for an S/MIME agent when
 establishing keys for content decryption using the RSA, RSAES-OAEP,
 and DH algorithms:
         key size <= 1023 : MAY        (see Security Considerations)
 1024 <= key size <= 2048 : MUST       (see Security Considerations)
 2048 <  key size         : MAY        (see Security Considerations)

5. IANA Considerations

 The following information updates the media type registration for
 application/pkcs7-mime and application/pkcs7-signature to refer to
 this document as opposed to RFC 2311.
 Note that other documents can define additional MIME media types for
 S/MIME.

5.1. Media Type for application/pkcs7-mime

 Type name: application
 Subtype Name: pkcs7-mime
 Required Parameters: NONE

Ramsdell & Turner Standards Track [Page 34] RFC 5751 S/MIME 3.2 Message Specification January 2010

 Optional Parameters: smime-type/signed-data
                      smime-type/enveloped-data
                      smime-type/compressed-data
                      smime-type/certs-only
                      name
 Encoding Considerations: See Section 3 of this document
 Security Considerations: See Section 6 of this document
 Interoperability Considerations: See Sections 1-6 of this document
 Published Specification: RFC 2311, RFC 2633, and this document
 Applications that use this media type: Security applications
 Additional information: NONE
 Person & email to contact for further information:
    S/MIME working group chairs smime-chairs@tools.ietf.org
 Intended usage: COMMON
 Restrictions on usage: NONE
 Author: Sean Turner
 Change Controller: S/MIME working group delegated from the IESG

5.2. Media Type for application/pkcs7-signature

 Type name: application
 Subtype Name: pkcs7-signature
 Required Parameters: NONE
 Optional Parameters: NONE
 Encoding Considerations: See Section 3 of this document
 Security Considerations: See Section 6 of this document
 Interoperability Considerations: See Sections 1-6 of this document
 Published Specification: RFC 2311, RFC 2633, and this document
 Applications that use this media type: Security applications

Ramsdell & Turner Standards Track [Page 35] RFC 5751 S/MIME 3.2 Message Specification January 2010

 Additional information: NONE
 Person & email to contact for further information:
    S/MIME working group chairs smime-chairs@tools.ietf.org
 Intended usage: COMMON
 Restrictions on usage: NONE
 Author: Sean Turner
 Change Controller: S/MIME working group delegated from the IESG

6. Security Considerations

 Cryptographic algorithms will be broken or weakened over time.
 Implementers and users need to check that the cryptographic
 algorithms listed in this document continue to provide the expected
 level of security.  The IETF from time to time may issue documents
 dealing with the current state of the art.  For example:
  1. The Million Message Attack described in RFC 3218 [MMA].
  1. The Diffie-Hellman "small-subgroup" attacks described in RFC

2785 [DHSUB].

  1. The attacks against hash algorithms described in RFC 4270 [HASH-

ATTACK].

 This specification uses Public-Key Cryptography technologies.  It is
 assumed that the private key is protected to ensure that it is not
 accessed or altered by unauthorized parties.
 It is impossible for most people or software to estimate the value of
 a message's content.  Further, it is impossible for most people or
 software to estimate the actual cost of recovering an encrypted
 message content that is encrypted with a key of a particular size.
 Further, it is quite difficult to determine the cost of a failed
 decryption if a recipient cannot process a message's content.  Thus,
 choosing between different key sizes (or choosing whether to just use
 plaintext) is also impossible for most people or software.  However,
 decisions based on these criteria are made all the time, and
 therefore this specification gives a framework for using those
 estimates in choosing algorithms.
 The choice of 2048 bits as the RSA asymmetric key size in this
 specification is based on the desire to provide at least 100 bits of
 security.  The key sizes that must be supported to conform to this

Ramsdell & Turner Standards Track [Page 36] RFC 5751 S/MIME 3.2 Message Specification January 2010

 specification seem appropriate for the Internet based on [STRENGTH].
 Of course, there are environments, such as financial and medical
 systems, that may select different key sizes.  For this reason, an
 implementation MAY support key sizes beyond those recommended in this
 specification.
 Receiving agents that validate signatures and sending agents that
 encrypt messages need to be cautious of cryptographic processing
 usage when validating signatures and encrypting messages using keys
 larger than those mandated in this specification.  An attacker could
 send certificates with keys that would result in excessive
 cryptographic processing, for example, keys larger than those
 mandated in this specification, which could swamp the processing
 element.  Agents that use such keys without first validating the
 certificate to a trust anchor are advised to have some sort of
 cryptographic resource management system to prevent such attacks.
 Using weak cryptography in S/MIME offers little actual security over
 sending plaintext.  However, other features of S/MIME, such as the
 specification of AES and the ability to announce stronger
 cryptographic capabilities to parties with whom you communicate,
 allow senders to create messages that use strong encryption.  Using
 weak cryptography is never recommended unless the only alternative is
 no cryptography.
 RSA and DSA keys of less than 1024 bits are now considered by many
 experts to be cryptographically insecure (due to advances in
 computing power), and should no longer be used to protect messages.
 Such keys were previously considered secure, so processing previously
 received signed and encrypted mail will often result in the use of
 weak keys.  Implementations that wish to support previous versions of
 S/MIME or process old messages need to consider the security risks
 that result from smaller key sizes (e.g., spoofed messages) versus
 the costs of denial of service.  If an implementation supports
 verification of digital signatures generated with RSA and DSA keys of
 less than 1024 bits, it MUST warn the user.  Implementers should
 consider providing different warnings for newly received messages and
 previously stored messages.  Server implementations (e.g., secure
 mail list servers) where user warnings are not appropriate SHOULD
 reject messages with weak signatures.
 Implementers SHOULD be aware that multiple active key pairs can be
 associated with a single individual.  For example, one key pair can
 be used to support confidentiality, while a different key pair can be
 used for digital signatures.

Ramsdell & Turner Standards Track [Page 37] RFC 5751 S/MIME 3.2 Message Specification January 2010

 If a sending agent is sending the same message using different
 strengths of cryptography, an attacker watching the communications
 channel might be able to determine the contents of the strongly
 encrypted message by decrypting the weakly encrypted version.  In
 other words, a sender SHOULD NOT send a copy of a message using
 weaker cryptography than they would use for the original of the
 message.
 Modification of the ciphertext can go undetected if authentication is
 not also used, which is the case when sending EnvelopedData without
 wrapping it in SignedData or enclosing SignedData within it.
 If an implementation is concerned about compliance with National
 Institute of Standards and Technology (NIST) key size
 recommendations, then see [SP800-57].
 If messaging environments make use of the fact that a message is
 signed to change the behavior of message processing (examples would
 be running rules or UI display hints), without first verifying that
 the message is actually signed and knowing the state of the
 signature, this can lead to incorrect handling of the message.
 Visual indicators on messages may need to have the signature
 validation code checked periodically if the indicator is supposed to
 give information on the current status of a message.

7. References

7.1. Reference Conventions

 [CMS] refers to [RFC5652].
 [ESS] refers to [RFC2634] and [RFC5035].
 [MIME] refers to [RFC2045], [RFC2046],  [RFC2047], [RFC2049],
 [RFC4288], and [RFC4289].
 [SMIMEv2] refers to [RFC2311], [RFC2312], [RFC2313], [RFC2314], and
 [RFC2315].
 [SMIMEv3] refers to [RFC2630], [RFC2631], [RFC2632], [RFC2633],
 [RFC2634], and [RFC5035].
 [SMIMv3.1] refers to [RFC2634], [RFC3850], [RFC3851], [RFC3852], and
 [RFC5035].

Ramsdell & Turner Standards Track [Page 38] RFC 5751 S/MIME 3.2 Message Specification January 2010

7.2. Normative References

 [CERT32]      Ramsdell, B. and S. Turner, "Secure/Multipurpose
               Internet Mail Extensions (S/MIME) Version 3.2
               Certificate Handling", RFC 5750, January 2010.
 [CHARSETS]    Character sets assigned by IANA.  See
               http://www.iana.org/assignments/character-sets.
 [CMSAES]      Schaad, J., "Use of the Advanced Encryption Standard
               (AES) Encryption Algorithm in Cryptographic Message
               Syntax (CMS)", RFC 3565, July 2003.
 [CMSALG]      Housley, R., "Cryptographic Message Syntax (CMS)
               Algorithms", RFC 3370, August 2002.
 [CMSCOMPR]    Gutmann, P., "Compressed Data Content Type for
               Cryptographic Message Syntax (CMS)", RFC 3274, June
               2002.
 [CMS-SHA2]    Turner, S., "Using SHA2 Algorithms with Cryptographic
               Message Syntax", RFC 5754, January 2010.
 [CONTDISP]    Troost, R., Dorner, S., and K. Moore, Ed.,
               "Communicating Presentation Information in Internet
               Messages: The Content-Disposition Header Field", RFC
               2183, August 1997.
 [FIPS186-2]   National Institute of Standards and Technology (NIST),
               "Digital Signature Standard (DSS)", FIPS Publication
               186-2, January 2000. [With Change Notice 1].
 [FIPS186-3]   National Institute of Standards and Technology (NIST),
               FIPS Publication 186-3: Digital Signature Standard,
               June 2009.
 [MIME-SECURE] Galvin, J., Murphy, S., Crocker, S., and N. Freed,
               "Security Multiparts for MIME: Multipart/Signed and
               Multipart/Encrypted", RFC 1847, October 1995.
 [MUSTSHOULD]  Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RANDOM]      Eastlake, D., 3rd, Schiller, J., and S. Crocker,
               "Randomness Requirements for Security", BCP 106, RFC
               4086, June 2005.

Ramsdell & Turner Standards Track [Page 39] RFC 5751 S/MIME 3.2 Message Specification January 2010

 [RFC2045]     Freed, N. and N. Borenstein, "Multipurpose Internet
               Mail Extensions (MIME) Part One: Format of Internet
               Message Bodies", RFC 2045, November 1996.
 [RFC2046]     Freed, N. and N. Borenstein, "Multipurpose Internet
               Mail Extensions (MIME) Part Two: Media Types", RFC
               2046, November 1996.
 [RFC2047]     Moore, K., "MIME (Multipurpose Internet Mail
               Extensions) Part Three: Message Header Extensions for
               Non-ASCII Text", RFC 2047, November 1996.
 [RFC2049]     Freed, N. and N. Borenstein, "Multipurpose Internet
               Mail Extensions (MIME) Part Five: Conformance Criteria
               and Examples", RFC 2049, November 1996.
 [RFC2634]     Hoffman, P. Ed., "Enhanced Security Services for
               S/MIME", RFC 2634, June 1999.
 [RFC4288]     Freed, N. and J. Klensin, "Media Type Specifications
               and Registration Procedures", BCP 13, RFC 4288,
               December 2005.
 [RFC4289]     Freed, N. and J. Klensin, "Multipurpose Internet Mail
               Extensions (MIME) Part Four: Registration Procedures",
               BCP 13, RFC 4289, December 2005.
 [RFC5035]     Schaad, J., "Enhanced Security Services (ESS) Update:
               Adding CertID Algorithm Agility", RFC 5035, August
               2007.
 [RFC5652]     Housley, R., "Cryptographic Message Syntax (CMS)", RFC
               5652, September 2009.
 [RSAOAEP]     Housley, R. "Use of the RSAES-OAEP Key Transport
               Algorithm in the Cryptographic Message Syntax (CMS)",
               RFC 3560, July 2003.
 [RSAPSS]      Schaad, J., "Use of the RSASSA-PSS Signature Algorithm
               in Cryptographic Message Syntax (CMS)", RFC 4056, June
               2005.
 [SP800-56A]   National Institute of Standards and Technology (NIST),
               Special Publication 800-56A: Recommendation Pair-Wise
               Key Establishment Schemes Using Discrete Logarithm
               Cryptography (Revised), March 2007.

Ramsdell & Turner Standards Track [Page 40] RFC 5751 S/MIME 3.2 Message Specification January 2010

 [X.680]       ITU-T Recommendation X.680 (2002) | ISO/IEC
               8824-1:2002. Information Technology - Abstract Syntax
               Notation One (ASN.1):  Specification of basic notation.
 [X.690]       ITU-T Recommendation X.690 (2002) | ISO/IEC
               8825-1:2002.  Information Technology - ASN.1 encoding
               rules: Specification of Basic Encoding Rules (BER),
               Canonical Encoding Rules (CER) and Distinguished
               Encoding Rules (DER).

7.3. Informative References

 [DHSUB]       Zuccherato, R., "Methods for Avoiding the "Small-
               Subgroup" Attacks on the Diffie-Hellman Key Agreement
               Method for S/MIME", RFC 2785, March 2000.
 [HASH-ATTACK] Hoffman, P. and B. Schneier, "Attacks on Cryptographic
               Hashes in Internet Protocols", RFC 4270, November 2005.
 [MMA]         Rescorla, E., "Preventing the Million Message Attack on
               Cryptographic Message Syntax", RFC 3218, January 2002.
 [PKCS-7]      Kaliski, B., "PKCS #7: Cryptographic Message Syntax
               Version 1.5", RFC 2315, March 1998.
 [RFC2311]     Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L.,
               and L. Repka, "S/MIME Version 2 Message Specification",
               RFC 2311, March 1998.
 [RFC2312]     Dusse, S., Hoffman, P., Ramsdell, B., and J.
               Weinstein, "S/MIME Version 2 Certificate Handling", RFC
               2312, March 1998.
 [RFC2313]     Kaliski, B., "PKCS #1: RSA Encryption Version 1.5", RFC
               2313, March 1998.
 [RFC2314]     Kaliski, B., "PKCS #10: Certification Request Syntax
               Version 1.5", RFC 2314, March 1998.
 [RFC2315]     Kaliski, B., "PKCS #7: Certification Message Syntax
               Version 1.5", RFC 2315, March 1998.
 [RFC2630]     Housley, R., "Cryptographic Message Syntax", RFC 2630,
               June 1999.
 [RFC2631]     Rescorla, E., "Diffie-Hellman Key Agreement Method",
               RFC 2631, June 1999.

Ramsdell & Turner Standards Track [Page 41] RFC 5751 S/MIME 3.2 Message Specification January 2010

 [RFC2632]     Ramsdell, B., Ed., "S/MIME Version 3 Certificate
               Handling", RFC 2632, June 1999.
 [RFC2633]     Ramsdell, B., Ed., "S/MIME Version 3 Message
               Specification", RFC 2633, June 1999.
 [RFC3850]     Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail
               Extensions (S/MIME) Version 3.1 Certificate Handling",
               RFC 3850, July 2004.
 [RFC3851]     Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail
               Extensions (S/MIME) Version 3.1 Message Specification",
               RFC 3851, July 2004.
 [RFC3852]     Housley, R., "Cryptographic Message Syntax (CMS)", RFC
               3852, July 2004.
 [SP800-57]    National Institute of Standards and Technology (NIST),
               Special Publication 800-57: Recommendation for Key
               Management, August 2005.
 [STRENGTH]    Orman, H., and P. Hoffman, "Determining Strengths For
               Public Keys Used For Exchanging Symmetric Keys", BCP
               86, RFC 3766, April 2004.

Ramsdell & Turner Standards Track [Page 42] RFC 5751 S/MIME 3.2 Message Specification January 2010

Appendix A. ASN.1 Module

 Note: The ASN.1 module contained herein is unchanged from RFC 3851
 [SMIMEv3.1] with the exception of a change to the prefersBinaryInside
 ASN.1 comment.  This module uses the 1988 version of ASN.1.
 SecureMimeMessageV3dot1
   { iso(1) member-body(2) us(840) rsadsi(113549)
          pkcs(1) pkcs-9(9) smime(16) modules(0) msg-v3dot1(21) }
 DEFINITIONS IMPLICIT TAGS ::=
 BEGIN
 IMPORTS
  1. - Cryptographic Message Syntax [CMS]

SubjectKeyIdentifier, IssuerAndSerialNumber,

    RecipientKeyIdentifier
        FROM  CryptographicMessageSyntax
              { iso(1) member-body(2) us(840) rsadsi(113549)
                pkcs(1) pkcs-9(9) smime(16) modules(0) cms-2001(14) };
  1. - id-aa is the arc with all new authenticated and unauthenticated
  2. - attributes produced by the S/MIME Working Group
 id-aa OBJECT IDENTIFIER ::= {iso(1) member-body(2) usa(840)
         rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) attributes(2)}
  1. - S/MIME Capabilities provides a method of broadcasting the
  2. - symmetric capabilities understood. Algorithms SHOULD be ordered
  3. - by preference and grouped by type
 smimeCapabilities OBJECT IDENTIFIER ::= {iso(1) member-body(2)
         us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 15}
 SMIMECapability ::= SEQUENCE {
    capabilityID OBJECT IDENTIFIER,
    parameters ANY DEFINED BY capabilityID OPTIONAL }
 SMIMECapabilities ::= SEQUENCE OF SMIMECapability
  1. - Encryption Key Preference provides a method of broadcasting the
  2. - preferred encryption certificate.
 id-aa-encrypKeyPref OBJECT IDENTIFIER ::= {id-aa 11}

Ramsdell & Turner Standards Track [Page 43] RFC 5751 S/MIME 3.2 Message Specification January 2010

 SMIMEEncryptionKeyPreference ::= CHOICE {
    issuerAndSerialNumber   [0] IssuerAndSerialNumber,
    receipentKeyId          [1] RecipientKeyIdentifier,
    subjectAltKeyIdentifier [2] SubjectKeyIdentifier
 }
  1. - receipentKeyId is spelt incorrectly, but kept for historical
  2. - reasons.
 id-smime OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
         rsadsi(113549) pkcs(1) pkcs9(9) 16 }
 id-cap  OBJECT IDENTIFIER ::= { id-smime 11 }
  1. - The preferBinaryInside OID indicates an ability to receive
  2. - messages with binary encoding inside the CMS wrapper.
  3. - The preferBinaryInside attribute's value field is ABSENT.
 id-cap-preferBinaryInside  OBJECT IDENTIFIER ::= { id-cap 1 }
  1. - The following list OIDs to be used with S/MIME V3
  1. - Signature Algorithms Not Found in [CMSALG], [CMS-SHA2], [RSAPSS],
  2. - and [RSAOAEP]
  1. -
  2. - md2WithRSAEncryption OBJECT IDENTIFIER ::=
  3. - {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1)
  4. - 2}
  1. -
  2. - Other Signed Attributes
  3. -
  4. - signingTime OBJECT IDENTIFIER ::=
  5. - {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
  6. - 5}
  7. - See [CMS] for a description of how to encode the attribute
  8. - value.
 SMIMECapabilitiesParametersForRC2CBC ::= INTEGER
 --        (RC2 Key Length (number of bits))
 END

Ramsdell & Turner Standards Track [Page 44] RFC 5751 S/MIME 3.2 Message Specification January 2010

Appendix B. Moving S/MIME v2 Message Specification to Historic Status

 The S/MIME v3 [SMIMEv3], v3.1 [SMIMEv3.1], and v3.2 (this document)
 are backwards compatible with the S/MIME v2 Message Specification
 [SMIMEv2], with the exception of the algorithms (dropped RC2/40
 requirement and added DSA and RSASSA-PSS requirements).  Therefore,
 it is recommended that RFC 2311 [SMIMEv2] be moved to Historic
 status.

Appendix C. Acknowledgments

 Many thanks go out to the other authors of the S/MIME version 2
 Message Specification RFC: Steve Dusse, Paul Hoffman, Laurence
 Lundblade, and Lisa Repka.  Without v2, there wouldn't be a v3, v3.1,
 or v3.2.
 A number of the members of the S/MIME Working Group have also worked
 very hard and contributed to this document.  Any list of people is
 doomed to omission, and for that I apologize.  In alphabetical order,
 the following people stand out in my mind because they made direct
 contributions to this document:
 Tony Capel, Piers Chivers, Dave Crocker, Bill Flanigan, Peter
 Gutmann, Alfred Hoenes, Paul Hoffman, Russ Housley, William Ottaway,
 John Pawling, and Jim Schaad.

Authors' Addresses

 Blake Ramsdell
 Brute Squad Labs, Inc.
 EMail: blaker@gmail.com
 Sean Turner
 IECA, Inc.
 3057 Nutley Street, Suite 106
 Fairfax, VA 22031
 USA
 EMail: turners@ieca.com

Ramsdell & Turner Standards Track [Page 45]

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