











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.





































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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.



















































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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











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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.

















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   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.













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   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 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.









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   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.











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   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.

















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   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.













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   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.







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   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.









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   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,







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   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









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   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.













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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.







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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 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].









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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 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.













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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 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.







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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.













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   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.







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      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.









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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.















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      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.







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   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.















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   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.









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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:











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      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









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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.













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   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".









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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].







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   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)













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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













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   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







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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



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







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   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.













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   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].















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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.











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   [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.











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   [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.























































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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}









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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

















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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





















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