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Versions: (RFC 3851) 00 01 02 03 04 05 06 07 08 09 10 11 RFC 5751

S/MIME WG                              Blake Ramsdell, Brute Squad Labs
Internet Draft                                        Sean Turner, IECA
Intended Status: Standard Track                         October 2, 2008
Obsoletes: 3851 (when approved)
Expires: April 2, 2009



     Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2
                           Message Specification
                      draft-ietf-smime-3851bis-08.txt


Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
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   This Internet-Draft will expire on April 2, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2008).

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.



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   Encryption provides data confidentiality.  Compression can be used to
   reduce data size.  This document obsoletes RFC 3851.

Discussion

   This draft is being discussed on the 'ietf-smime' mailing list. To
   subscribe, send a message to ietf-smime-request@imc.org with the
   single word subscribe in the body of the message. There is a Web site
   for the mailing list at <http://www.imc.org/ietf-smime/>.

Table of Contents

   1. Introduction...................................................3
      1.1. Specification Overview....................................3
      1.2. Definitions...............................................4
      1.3. Conventions used in this document.........................5
      1.4. Compatibility with Prior Practice of S/MIME...............6
      1.5. Changes From S/MIME v3 to S/MIME v3.1.....................6
      1.6. Changes Since S/MIME v3.1.................................6
   2. CMS Options....................................................8
      2.1. DigestAlgorithmIdentifier.................................8
      2.2. SignatureAlgorithmIdentifier..............................8
      2.3. KeyEncryptionAlgorithmIdentifier..........................9
      2.4. General Syntax...........................................10
      2.5. Attributes and the SignerInfo Type.......................11
      2.6. SignerIdentifier SignerInfo Type.........................15
      2.7. ContentEncryptionAlgorithmIdentifier.....................15
   3. Creating S/MIME Messages......................................17
      3.1. Preparing the MIME Entity for Signing, Enveloping or
           Compressing..............................................18
      3.2. The application/pkcs7-mime Media Type....................22
      3.3. Creating an Enveloped-only Message.......................24
      3.4. Creating a Signed-only Message...........................25
      3.5. Creating an Compressed-only Message......................29
      3.6. Multiple Operations......................................30
      3.7. Creating a Certificate Management Message................30
      3.8. Registration Requests....................................31
      3.9. Identifying an S/MIME Message............................31
   4. Certificate Processing........................................32
      4.1. Key Pair Generation......................................32
      4.2. Signature Generation.....................................33
      4.3. Signature Verification...................................33
      4.4. Encryption...............................................33
      4.5. Decryption...............................................33
   5. IANA Considerations...........................................34
      5.1. Media Type for application/pkcs7-mime....................34
      5.2. Media Type for application/pkcs7-signature...............35


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   6. Security Considerations.......................................36
   7. References....................................................38
      7.1. Normative References.....................................38
      7.2. Informative References...................................40
   Appendix A. ASN.1 Module.........................................42
   Appendix B. Moving S/MIME v2 Message Specification to
               Historic Status......................................44

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 3852 and RFC 4853 [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.



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   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.

   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].




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   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.

   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-.



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1.4. Compatibility with Prior Practice of S/MIME

   S/MIME version 3.2 agents SHOULD 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, RFC 4853, 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 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-.

   Sec 1.1 and Appendix A: Added references to RFCs for RSA-PSS, RSA-
   OAEP, and SHA2 CMS Algorithms.  Added CMS Multiple Signers
   Clarification to CMS reference.


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   Sec 1.2: Updated references to ASN.1 to X.680 and BER and DER to
   X.690.

   Sec 1.4: Added references to S/MIME MSG 3.1 RFCs.

   Sec 2.1 (digest algorithm): SHA-256 added as MUST, SHA-1 and MD5 made
   SHOULD-.

   Sec 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 RSA-PSS with SHA-256 added
   as SHOULD+. Also added note about what S/MIME v3.1 clients support.

   Sec 2.3 (key encryption): DH changed to SHOULD- and RSA-OAEP added as
   SHOULD+.

   Sec 2.5.1: Added requirement that receiving agents MUST support both
   GeneralizedTime and UTCTime.

   Sec 2.5.2: Replaced reference "sha1WithRSAEncrption" with
   "sha256WithRSAEncryption", "DES-3EDE-CBC" with "AES-128 CBC", and
   deleted the RC5 example.

   Sec 2.5.2.1: Deleted entire section (discussed deprecated RC2).

   Sec 2.7, 2.7.1, Appendix A: references to RC2/40 removed.

   Sec 2.7 (content encryption): AES-128 CBC added as MUST, AES-192 and
   AES-256 CBC SHOULD+, tripleDES now SHOULD-.

   Sec 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.

   Sec 3.2.2: Replaced "encrypted" with "enveloped". Update OID example
   to use AES-128 CBC oid.

   Sec 4: Updated reference to CERT v3.2.

   Sec 4.1: Updated RSA and DSA key size discussion. Moved last four
   sentences to security considerations. Updated reference to randomness
   requirements for security.

   Sec 5: Added IANA registration templates to update media type
   registry to point to this document as opposed to RFC 2311.

   Sec 6: Updated Security Considerations.



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   Sec 7: Moved references from Appendix B to this section. Updated
   references. Added informational references to SMIMEv2, SMIMEv3, and
   SMIMEv3.1.

   App 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 RSA-PSS with SHA-256

    - SHOULD- support RSA with SHA-1

    - SHOULD- support DSA with SHA-1

    - SHOULD- support RSA with MD5.











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   Sending agents:

    - MUST support RSA with SHA-256

    - SHOULD+ support DSA with SHA-256

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

    - SHOULD- support DH ephemeral-static mode, as specified
       in [CMSALG].

   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.







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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.

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





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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:

    - signingTime (section 2.5.1 in this document)

    - sMIMECapabilities (section 2.5.2 in this document)

    - sMIMEEncryptionKeyPreference (section 2.5.3 in this document)

    - id-messageDigest (section 11.2 in [CMS])

    - id-contentType (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 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.

   Sending agents MUST encode signing time through the year 2049 as
   UTCTime; signing times in 2050 or later MUST be encoded as



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   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. SMIMECapabilities 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 which 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,
   individuals documenting algorithms to be used in the


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   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
   instance of AttributeValue.  There MUST NOT be zero or multiple


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   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
      which 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.





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2.6. SignerIdentifier SignerInfo Type

   S/MIME v3.2 implementations MUST support both issuerAndSerialNumber
   as well as 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.



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    - 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,




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   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 which
   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].


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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 Section 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.  Description of the procedures from [MIME-SPEC] are
   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.




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   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 which 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].  The chosen charset SHOULD be named in the charset
   parameter so that the receiving agent can unambiguously determine the
   charset used.




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   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.

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 which "know" that all intended recipient(s)
   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


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   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.

   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 can not 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 can not 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 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!



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      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.

      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.




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   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.

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.





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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.

     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
      "encrypted-*".  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
   "encrypted" 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.



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      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.

      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; and,

    - 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 when a particular signed-only format
   is chosen because 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.



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   Messages signed using the multipart/signed format can always be
   viewed by the receiver whether they have S/MIME software or not. 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.

   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 which 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 is 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




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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.

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.


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   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:

     Algorithm   Value used

     MD5         md5
     SHA-1       sha1
     SHA-224     sha224
     SHA-256     sha256
     SHA-384     sha384
     SHA-512     sha512
     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" and "rsa-sha1" for the micalg parameter.)
   Receiving agents SHOULD be able to recover gracefully from a micalg
   parameter value that they do not recognize.

   The SHA-224, SHA-384, and SHA-512 algorithms [FIPS180-3] are not
   currently recommended in S/MIME, and are included here for
   completeness.

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



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      n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
      7GhIGfHfYT64VQbnj756

     --boundary42--

   The content that is digested (the first part of the multipart/signed)
   are 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

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 is 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


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3.6. Multiple Operations

   The signed-only, encrypted-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.

   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 were 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 which creates a CMS
      object of type SignedData.  The SignedData encapContentInfo



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      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".

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 certificate 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


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   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].

   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, 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 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 512-2048-bit RSA-PSS with SHA-256 see [RSAPSS].



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4.2. Signature Generation

   The following are the requirements for an S/MIME agent generated RSA
   signatures:

    512 <= key size <  1024 : MAY     (see Security Considerations)
   1024 <= key size <= 2048 : SHOULD  (see Security Considerations)
   2048 <  key size         : MAY     (see Security Considerations)

   The following are the requirements for an S/MIME agent generated DSA
   signatures:

    512 <= key size <= 1023 : MAY     (see Security Considerations)
   1024  = key size         : SHOULD- (see Security Considerations)

4.3. Signature Verification

   The following are the requirements for S/MIME receiving agents during
   signature verification of RSA signatures:

    512 <= key size <= 2048 : MUST    (see Security Considerations)
   2048 <  key size         : MAY     (see Security Considerations)

   The following are the requirements for S/MIME receiving agents during
   signature verification of DSA signatures:

    512 <= key size <= 1023 : MAY     (see Security Considerations)
   1024  = key size         : SHOULD- (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 algorithms:

    512 <= key size <  1024 : MAY    (see Security Considerations)
   1024 <= key size <= 2048 : SHOULD (see Security Considerations)
   2048 <  key size         : MAY    (see Security Considerations)

   The following are the requirements for an S/MIME agent when
   establishing keys for content encryption using the DH algorithms:

    512 <= key size <= 1023 : MAY     (see Security Considerations)
   1024  = key size         : SHOULD- (see Security Considerations)






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4.5. Decryption

   The following are the requirements for an S/MIME agent when
   establishing keys for content decryption using the RSA algorithms:

    512 <= key size <= 2048 : MUST   (see Security Considerations)
   2048 <  key size         : MAY    (see Security Considerations)

   The following are the requirements for an S/MIME agent when
   establishing keys for content decryption using the DH algorithms:

    512 <= key size <= 1023 : MAY     (see Security Considerations)
   1024  = key size         : SHOULD- (see Security Considerations)

5. IANA Considerations

   The following is intended to provide sufficient information to update
   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

   Optional Parameters: smime-type/signed-data
                        smime-type/enveloped-data
                        smime-type/compressed-data
                        smime-type/certs-only


   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


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   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

   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




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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 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 100 bits of security.
   The standards to offer the same level of security for DSA and DH are
   not yet available.  In particular, [FIPS186-2] without Change Notice
   allowed DSA key sizes between 512 and 1024 bits and [FIPS186-2] with
   Change Notice 1 only allowed DSA key sizes of 1024 bits.  A revision
   to support larger key sizes is being developed, and once it is
   available, implementors ought to support DSA key sizes comparable to
   the RSA key sizes recommended in this specification.  The key sizes
   that must be supported to conform to this specification seem
   appropriate for the Internet based on [STRENGTH].  Of course, there
   are environments, such as financial and medical system, that may
   select different key sizes.  For this reason, an implementation MAY
   support key sizes beyond those recommended in this specification.



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   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 which would result in excessive
   cryptographic processing, for example keys larger than those mandated
   in this specification, which could swamp the processing element.
   Agents which 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.

   Today, 512-bit RSA, DSA, and DH keys are considered by many experts
   to be cryptographically insecure.

   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.  When feasible, sending and receiving agents SHOULD
   inform senders and recipients of the relative cryptographic strength
   of messages.

   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.

   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 NIST key size
   recommendations, then see [SP800-57].






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7. References

7.1. Normative References

   [CERT32]      Ramsdell, B., and S. Turner, "S/MIME Version 3.2
                 Certificate Handling",
                 draft-ietf-smime-3850-bis-08.txt, work-in-progress.

   [CHARSETS]    Character sets assigned by IANA.  See
                 http://www.iana.org/assignments/character-sets

   [CMS]         Housley, R., "Cryptographic Message Syntax (CMS)", RFC
                 3852, July 2004.

                 Housley, R., "Cryptographic Message Syntax (CMS)
                 Multiple Signer Clarification", RFC 4853, April 2007.

   [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", draft-ietf-smime-sha2-08.txt, work in
                 progress.

   [CONTDISP]    Troost, R., Dorner, S., and K. Moore, "Communicating
                 Presentation Information in Internet Messages: The
                 Content-Disposition Header Field", RFC 2183, August
                 1997.

   [ESS]         Hoffman, P., "Enhanced Security Services for S/MIME",
                 RFC 2634, June 1999.

                 Schaad, J., "ESS Update: Adding CertID Algorithm
                 Agility", RFC 5035, August 2007.

   [FIPS180-3]   National Institute of Standards and Technology (NIST),
                 "Secure Hash Standard (SHS)", (draft) FIPS Publication
                 180-3, June 2007.



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   [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,
                 (draft) March 2006.

   [MIME-SPEC]   Freed, N. and N. Borenstein, "Multipurpose Internet
                 Mail Extensions (MIME) Part One: Format of Internet
                 Message Bodies", RFC 2045, November 1996.

                 Freed, N. and N. Borenstein, "Multipurpose Internet
                 Mail Extensions (MIME) Part Two: Media Types", RFC
                 2046, November 1996.

                 Moore, K., "MIME (Multipurpose Internet Mail
                 Extensions) Part Three: Message Header Extensions for
                 Non-ASCII Text", RFC 2047, November 1996.

                 Freed, N., and J. Klensin, "Multipurpose Internet Mail
                 Extensions (MIME) Part Four: Registration Procedures",
                 BCP 13, RFC 4289, December 2005.

                 Freed, N., and J. Klensin, "Media Type Specifications
                 and Registration Procedures", BCP 13, RFC 4288,
                 December 2005.

                 Freed, N. and N. Borenstein, "Multipurpose Internet
                 Mail Extensions (MIME) Part Five: Conformance Criteria
                 and Examples", RFC 2049, November 1996.

   [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 3rd, D., Crocker, S., and J. Schiller,
                 "Randomness Requirements for Security", BCP 106, RFC
                 4086, June 2005.

   [RSAPSS]      Schaad, J., "Use of RSASA-PSS Signature Algorithm in
                 Cryptographic Message Syntax (CMS)", RFC 4056, June
                 2005.



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   [RSAOAEP]     Housley, R. "Use of the RSAES-OAEP Key Transport
                 Algorithm in the Cryptographic Message Syntax (CMS)",
                 RFC 3560, July 2003

   [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).

7.2. 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., Schneier, B., "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.

   [SMIMEv2]     Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L.,
                 and L. Repka, "S/MIME Version 2 Message
                 Specification", RFC 2311, March 1998.

                 Dusse, S., Hoffman, P., Ramsdell, B., and J.
                 Weinstein, "S/MIME Version 2 Certificate Handling",
                 RFC 2312, March 1998.

                 Kaliski, B., "PKCS #1: RSA Encryption Version 1.5",
                 RFC 2313, March 1998.

                 Kaliski, B., "PKCS #10: Certificate Request Syntax
                 Version 1.5", RFC 2314, March 1998.




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                 Kaliski, B., "PKCS #7: Certificate Message Syntax
                 Version 1.5", RFC 2315, March 1998.

   [SMIMEv3]     Housley, R., "Cryptographic Message Syntax", RFC 2630,
                 June 1999.

                 Rescorla, E., "Diffie-Hellman Key Agreement Method",
                 RFC 2631, June 1999.

                 Ramsdell, B., "S/MIME Version 3 Certificate Handling",
                 RFC 2632, June 1999.

                 Ramsdell, B., "S/MIME Version 3 Message
                 Specification", RFC 2633, June 1999.

                 Hoffman, P., "Enhanced Security Services for S/MIME",
                 RFC 2634, June 1999.

                 Schaad, J., "ESS Update: Adding CertID Algorithm
                 Agility", RFC 5035, August 2007.

   [SMIMEv3.1]   Housley, R., "Cryptographic Message Syntax", RFC 3852,
                 July 2004.

                 Housley, R., "Cryptographic Message Syntax (CMS)
                 Multiple Signer Clarification", RFC 4853, April 2007.

                 Ramsdell, B., "S/MIME Version 3.1 Certificate
                 Handling", RFC 3850, July 2004.

                 Ramsdell, B., "S/MIME Version 3.1 Message
                 Specification", RFC 3851, July 2004.

                 Hoffman, P., "Enhanced Security Services for S/MIME",
                 RFC 2634, June 1999.

                 Schaad, J., "ESS Update: Adding CertID Algorithm
                 Agility", RFC 5035, August 2007.

   [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.



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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 the by 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.




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   id-aa-encrypKeyPref OBJECT IDENTIFIER ::= {id-aa 11}

   SMIMEEncryptionKeyPreference ::= CHOICE {
      issuerAndSerialNumber   [0] IssuerAndSerialNumber,
      receipentKeyId          [1] RecipientKeyIdentifier,
      subjectAltKeyIdentifier [2] SubjectKeyIdentifier
   }

   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 the 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





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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 RSA-PSS requirements). Therefore, it is
   recommended that RFC 2311 [SMIMEv2] be moved to Historic status.

Appendix C. Acknowledgements

   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 due to the fact that 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.

Author's 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









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