[Docs] [txt|pdf] [draft-ietf-smime-...] [Diff1] [Diff2] [IPR] [Errata]

PROPOSED STANDARD
Errata Exist
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:

      - MUST support RSA with SHA-256.

      - SHOULD+ support DSA with SHA-256.

      - SHOULD+ support RSASSA-PSS with SHA-256.

      - SHOULD- support RSA with SHA-1.

      - SHOULD- support DSA with SHA-1.

      - SHOULD- support RSA with MD5.








Ramsdell & Turner            Standards Track                    [Page 9]

RFC 5751            S/MIME 3.2 Message Specification        January 2010


   Sending agents:

      - MUST support RSA with SHA-256.

      - SHOULD+ support DSA with SHA-256.

      - SHOULD+ support RSASSA-PSS with SHA-256.

      - SHOULD- support RSA with SHA-1 or DSA with SHA-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:

      - MUST support RSA Encryption, as specified in [CMSALG].

      - SHOULD+ support RSAES-OAEP, as specified in [RSAOAEP].

      - 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:

      - Signing Time (section (Section 2.5.1 in this document)

      - SMIME Capabilities (section (Section 2.5.2 in this document)

      - Encryption Key Preference (section (Section 2.5.3 in this
        document)

      - Message Digest (section (Section 11.2 in [CMS])

      - 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:

   - 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.

   - 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.

   - 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:

      - MUST support encryption and decryption with AES-128 CBC
        [CMSAES].

      - SHOULD+ support encryption and decryption with AES-192 CBC and
        AES-256 CBC [CMSAES].

      - 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.

      - 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


      - 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:

      - the sending agent has no knowledge of the encryption
        capabilities of the recipient, and

      - 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:

    - The agent could change the transfer encoding; this would
      invalidate the signature.

    - The agent could transmit the data anyway, which would most likely
      result in the 8th bit being corrupted; this too would invalidate
      the signature.

    - 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

      --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

      --bar
      Content-Type: image/jpeg
      Content-Transfer-Encoding: base64

      iQCVAwUBMJrRF2N9oWBghPDJAQE9UQQAtl7LuRVndBjrk4EqYBIb3h5QXIX/LC//
      jJV5bNvkZIGPIcEmI5iFd9boEgvpirHtIREEqLQRkYNoBActFBZmh9GC3C041WGq
      uMbrbxc+nIs1TIKlA08rVi9ig/2Yh7LFrK5Ein57U/W72vgSxLhe/zhdfolT9Brn
      HOxEa44b+EI=

      --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:

      - application/pkcs7-mime with SignedData.

      - 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

       --boundary42
       Content-Type: text/plain

       This is a clear-signed message.

       --boundary42
       Content-Type: application/pkcs7-signature; name=smime.p7s
       Content-Transfer-Encoding: base64
       Content-Disposition: attachment; filename=smime.p7s

       ghyHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6
       4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj
       n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
       7GhIGfHfYT64VQbnj756

      --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:

      - Compression of binary encoded encrypted data is discouraged,
        since it will not yield significant compression.  Base64
        encrypted data could very well benefit, however.

      - 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:

      - The Million Message Attack described in RFC 3218 [MMA].

      - The Diffie-Hellman "small-subgroup" attacks described in RFC
        2785 [DHSUB].

      - 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

   -- 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) };

   --  id-aa is the arc with all new authenticated and unauthenticated
   --  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)}

   -- S/MIME Capabilities provides a method of broadcasting the
   -- symmetric capabilities understood.  Algorithms SHOULD be ordered
   -- 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

   -- Encryption Key Preference provides a method of broadcasting the
   -- 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
   }

   -- receipentKeyId is spelt incorrectly, but kept for historical
   -- 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 }

   -- The preferBinaryInside OID indicates an ability to receive
   -- messages with binary encoding inside the CMS wrapper.
   -- The preferBinaryInside attribute's value field is ABSENT.

   id-cap-preferBinaryInside  OBJECT IDENTIFIER ::= { id-cap 1 }

   --  The following list OIDs to be used with S/MIME V3

   -- Signature Algorithms Not Found in [CMSALG], [CMS-SHA2], [RSAPSS],
   -- and [RSAOAEP]

   --
   -- md2WithRSAEncryption OBJECT IDENTIFIER ::=
   --    {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1)
   --     2}

   --
   -- Other Signed Attributes
   --
   -- signingTime OBJECT IDENTIFIER ::=
   --    {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
   --     5}
   --    See [CMS] for a description of how to encode the attribute
   --    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]


Html markup produced by rfcmarkup 1.109, available from https://tools.ietf.org/tools/rfcmarkup/