Internet DRAFT - draft-ietf-smime-3851bis

draft-ietf-smime-3851bis



S/MIME WG                                                   B. Ramsdell 
Internet Draft                                         Brute Squad Labs 
Intended Status: Standard Track                               S. Turner 
Obsoletes: 3851 (when approved)                                    IECA 
Expires: November 12, 2009                                 May 12, 2009 
 
 
                                      
    Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 
                          Message Specification 
                     draft-ietf-smime-3851bis-11.txt 


Status of this Memo 

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Copyright Notice 

   Copyright (c) 2009 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 in effect on the date of 
   publication of this document (http://trustee.ietf.org/license-info). 
   Please review these documents carefully, as they describe your rights 
   and restrictions with respect to this document. 

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. 

Discussion 

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

Table of Contents 

   1. Introduction................................................  3 
      1.1. Specification Overview.................................  4 
      1.2. Definitions............................................  5 
      1.3. Conventions used in this document......................  5 
      1.4. Compatibility with Prior Practice of S/MIME............  6 
      1.5. Changes From S/MIME v3 to S/MIME v3.1..................  6 
      1.6. Changes Since S/MIME v3.1..............................  7 
   2. CMS Options ................................................  8 
      2.1. DigestAlgorithmIdentifier..............................  8 
      2.2. SignatureAlgorithmIdentifier ..........................  9 
      2.3. KeyEncryptionAlgorithmIdentifier.......................  9 
      2.4. General Syntax......................................... 10 
      2.5. Attributes and the SignerInfo Type .................... 11 
      2.6. SignerIdentifier SignerInfo Type....................... 15 
      2.7. ContentEncryptionAlgorithmIdentifier................... 15 
   3. Creating S/MIME Messages.................................... 18 
      3.1. Preparing the MIME Entity for Signing, Enveloping or 
           Compressing............................................ 18 
 
 
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      3.2. The application/pkcs7-mime Media Type.................. 22 
      3.3. Creating an Enveloped-only Message .................... 25 
      3.4. Creating a Signed-only Message......................... 26 
      3.5. Creating a Compressed-only Message .................... 29 
      3.6. Multiple Operations.................................... 30 
      3.7. Creating a Certificate Management Message.............. 31 
      3.8. Registration Requests.................................. 31 
      3.9. Identifying an S/MIME Message.......................... 32 
   4. Certificate Processing ..................................... 32 
      4.1. Key Pair Generation.................................... 32 
      4.2. Signature Generation................................... 33 
      4.3. Signature Verification................................. 33 
      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. Normative References................................... 38 
      7.2. Informative References................................. 40 
   Appendix A. ASN.1 Module....................................... 42 
   Appendix B. Moving S/MIME v2 Message Specification to Historic 
               Status............................................. 46 
   Appendix C. Acknowledgments.................................... 46 
   Authors' Addresses............................................. 46 
    
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. 

 
 
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   Further, S/MIME can be used in automated message transfer agents that 
   use cryptographic security services that do not require any human 
   intervention, such as the signing of software-generated documents and 
   the encryption of FAX messages sent over the Internet. 

1.1. Specification Overview 

   This document describes a protocol for adding cryptographic signature 
   and encryption services to MIME data.  The MIME standard [MIME-SPEC] 
   provides a general structure for the content of Internet messages and 
   allows extensions for new content type based applications. 

   This specification defines how to create a MIME body part that has 
   been cryptographically enhanced according to the Cryptographic 
   Message Syntax (CMS) RFC 3852 and RFC 4853 [CMS], which is derived 
   from PKCS #7 [PKCS-7].  This specification also defines the 
   application/pkcs7-mime media type that can be used to transport those 
   body parts. 

   This document also discusses how to use the multipart/signed media 
   type defined in [MIME-SECURE] to transport S/MIME signed messages. 
   multipart/signed is used in conjunction with the application/pkcs7- 
   signature media type, which is used to transport a detached S/MIME 
   signature. 

   In order to create S/MIME messages, an S/MIME agent MUST follow the 
   specifications in this document, as well as the specifications listed 
   in the Cryptographic Message Syntax document [CMS], [CMSALG], 
   [RSAPSS], [RSAOAEP], and [CMS-SHA2]. 

   Throughout this specification, there are requirements and 
   recommendations made for how receiving agents handle incoming 
   messages.  There are separate requirements and recommendations for 
   how sending agents create outgoing messages.  In general, the best 
   strategy is to "be liberal in what you receive and conservative in 
   what you send".  Most of the requirements are placed on the handling 
   of incoming messages while the recommendations are mostly on the 
   creation of outgoing messages. 

   The separation for requirements on receiving agents and sending 
   agents also derives from the likelihood that there will be S/MIME 
   systems that involve software other than traditional Internet mail 
   clients.  S/MIME can be used with any system that transports MIME 
   data.  An automated process that sends an encrypted message might not 
   be able to receive an encrypted message at all, for example.  Thus, 
   the requirements and recommendations for the two types of agents are 
   listed separately when appropriate. 
 
 
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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. 

   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]. 

 
 
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   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-. 

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, RFC 4853, and RFC 5035 
   [SMIMEv3.1].  RFC 2311 also has historical information about the 
   development of S/MIME. 

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

   The RSA public key algorithm was changed to a MUST implement key 
   wrapping algorithm, and the Diffie-Hellman algorithm changed to a 
   SHOULD implement. 

   The AES symmetric encryption algorithm has been included as a SHOULD 
   implement. 

   The RSA public key algorithm was changed to a MUST implement 
   signature algorithm. 

   Ambiguous language about the use of "empty" SignedData messages to 
   transmit certificates was clarified to reflect that transmission of 
   certificate revocation lists is also allowed. 

   The use of binary encoding for some MIME entities is now explicitly 
   discussed. 

 
 
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   Header protection through the use of the message/rfc822 media type 
   has been added. 

   Use of the CompressedData CMS type is allowed, along with required 
   media type and file extension additions. 

1.6. Changes Since S/MIME v3.1 

   Editorial changes, e.g., replaced "MIME type" with "media type", 
   content-type with Content-Type. 

   Moved "Conventions Used in This Document" to Section 1.3.  Added 
   definitions for SHOULD+, SHOULD-, and MUST-. 

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

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

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

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

   Sec 2.2 (signature algorithms): RSA with SHA-256 added as MUST, and 
   DSA with SHA-256 added as SHOULD+, RSA with SHA-1, DSA with SHA-1, 
   and RSA with MD5 changed to SHOULD-, and RSASSA-PSS with SHA-256 
   added as SHOULD+. Also added note about what S/MIME v3.1 clients 
   support. 

   Sec 2.3 (key encryption): DH changed to SHOULD- and RSAES-OAEP added 
   as SHOULD+.  Elaborated requirements for key wrap algorithm. 

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

   Sec 2.5.2: Replaced reference "sha1WithRSAEncryption" with 
   "sha256WithRSAEncryption", "DES-3EDE-CBC" with "AES-128 CBC", and 
   deleted the RC5 example. 

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

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


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

   Sec 2.7.1: Updated pointers from 2.7.2.1 through 2.7.2.4 to 2.7.1.1 
   to 2.7.1.2. 

   Sec 3.1.1: Removed text about MIME character sets. 

   Sec 3.2.2 and 3.6: Replaced "encrypted" with "enveloped". Update OID 
   example to use AES-128 CBC oid. 

   Sec 3.4.3.2: Replace micalg parameter for SHA-1 with sha-1. 

   Sec 4: Updated reference to CERT v3.2. 

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

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

   Sec 6: Updated Security Considerations. 

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

   App B: Added Appendix B to move S/MIME v2 to historic status. 

2. CMS Options 

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

2.1. DigestAlgorithmIdentifier 

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

 
 
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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. 

   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] 

 
 
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    - 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. 

   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 
 
 
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   is no signature information, to convey certificates.  Applying a 
   signature to a message provides authentication, message integrity, 
   and non-repudiation of origin. 

2.4.3. EnvelopedData Content Type 

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

2.4.4. CompressedData Content Type 

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

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

2.5. Attributes and the SignerInfo Type 

   The SignerInfo type allows the inclusion of unsigned and signed 
   attributes along with a signature. 

   Receiving agents MUST be able to handle zero or one instance of each 
   of the signed attributes listed here.  Sending agents SHOULD generate 
   one instance of each of the following signed attributes in each 
   S/MIME message: 

    - signingTime (section 2.5.1 in this document) 

    - sMIMECapabilities (section 2.5.2 in this document) 

    - sMIMEEncryptionKeyPreference (section 2.5.3 in this document) 

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

    - id-contentType (section 11.1 in [CMS]) 

   Further, receiving agents SHOULD be able to handle zero or one 
   instance of the signingCertificate and signingCertificatev2 signed 
   attributes, as defined in section 5 of RFC 2634 [ESS] and section 3 
   of RFC 5035 [ESS]. 



 
 
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   Sending agents SHOULD generate one instance of the signingCertificate 
   or signingCertificatev2 signed attribute in each SignerInfo 
   structure. 

   Additional attributes and values for these attributes might be 
   defined in the future.  Receiving agents SHOULD handle attributes or 
   values that they do not recognize in a graceful manner. 

   Interactive sending agents that include signed attributes that are 
   not listed here SHOULD display those attributes to the user, so that 
   the user is aware of all of the data being signed. 

2.5.1. Signing-Time Attribute 

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

   Sending agents MUST encode signing time through the year 2049 as 
   UTCTime; signing times in 2050 or later MUST be encoded as 
   GeneralizedTime.  When the UTCTime CHOICE is used, S/MIME agents MUST 
   interpret the year field (YY) as follows: 

      If YY is greater than or equal to 50, the year is interpreted as 
      19YY; if YY is less than 50, the year is interpreted as 20YY. 

   Receiving agents MUST be able to process signing-time attributes that 
   are encoded in either UTCTime or GeneralizedTime. 

2.5.2. SMIMECapabilities Attribute 

   The SMIMECapabilities attribute includes signature algorithms (such 
   as "sha256WithRSAEncryption"), symmetric algorithms (such as "AES-128 
   CBC"), and key encipherment algorithms (such as "rsaEncryption").  
   There are also several identifiers which indicate support for other 
   optional features such as binary encoding and compression.  The 
   SMIMECapabilities were designed to be flexible and extensible so 
   that, in the future, a means of identifying other capabilities and 
   preferences such as certificates can be added in a way that will not 
   cause current clients to break. 

   If present, the SMIMECapabilities attribute MUST be a 
   SignedAttribute; it MUST NOT be an UnsignedAttribute.  CMS defines 
   SignedAttributes as a SET OF Attribute.  The SignedAttributes in a 
   signerInfo MUST NOT include multiple instances of the 
   SMIMECapabilities attribute.  CMS defines the ASN.1 syntax for 
 
 
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   Attribute to include attrValues SET OF AttributeValue.  A 
   SMIMECapabilities attribute MUST only include a single instance of 
   AttributeValue.  There MUST NOT be zero or multiple instances of 
   AttributeValue present in the attrValues SET OF AttributeValue. 

   The semantics of the SMIMECapabilities attribute specify a partial 
   list as to what the client announcing the SMIMECapabilities can 
   support.  A client does not have to list every capability it 
   supports, and need not list all its capabilities so that the 
   capabilities list doesn't get too long.  In an SMIMECapabilities 
   attribute, the object identifiers (OIDs) are listed in order of their 
   preference, but SHOULD be separated logically along the lines of 
   their categories (signature algorithms, symmetric algorithms, key 
   encipherment algorithms, etc.) 

   The structure of the SMIMECapabilities attribute is to facilitate 
   simple table lookups and binary comparisons in order to determine 
   matches.  For instance, the DER-encoding for the SMIMECapability for 
   AES-128 CBC MUST be identically encoded regardless of the 
   implementation.  Because of the requirement for identical encoding, 
   individuals documenting algorithms to be used in the 
   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. 

 
 
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   Section 2.7.1 explains a strategy for caching capabilities. 

2.5.3. Encryption Key Preference Attribute 

   The encryption key preference attribute allows the signer to 
   unambiguously describe which of the signer's certificates has the 
   signer's preferred encryption key.  This attribute is designed to 
   enhance behavior for interoperating with those clients that use 
   separate keys for encryption and signing.  This attribute is used to 
   convey to anyone viewing the attribute which of the listed 
   certificates is appropriate for encrypting a session key for future 
   encrypted messages. 

   If present, the SMIMEEncryptionKeyPreference attribute MUST be a 
   SignedAttribute; it MUST NOT be an UnsignedAttribute.  CMS defines 
   SignedAttributes as a SET OF Attribute.  The SignedAttributes in a 
   signerInfo MUST NOT include multiple instances of the 
   SMIMEEncryptionKeyPreference attribute.  CMS defines the ASN.1 syntax 
   for Attribute to include attrValues SET OF AttributeValue.  A 
   SMIMEEncryptionKeyPreference attribute MUST only include a single 
   instance of AttributeValue.  There MUST NOT be zero or multiple 
   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. 





 
 
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2.5.3.1. Selection of Recipient Key Management Certificate 

   In order to determine the key management certificate to be used when 
   sending a future CMS EnvelopedData message for a particular 
   recipient, the following steps SHOULD be followed: 

    - If an SMIMEEncryptionKeyPreference attribute is found in a 
      SignedData object received from the desired recipient, this 
      identifies the X.509 certificate that SHOULD be used as the X.509 
      key management certificate for the recipient. 

    - If an SMIMEEncryptionKeyPreference attribute is not found in a 
      SignedData object received from the desired recipient, the set of 
      X.509 certificates SHOULD be searched for a X.509 certificate 
      with the same subject name as the signer of a X.509 certificate 
      which can be used for key management. 

    - Or use some other method of determining the user's key management 
      key.  If a X.509 key management certificate is not found, then 
      encryption cannot be done with the signer of the message.  If 
      multiple X.509 key management certificates are found, the S/MIME 
      agent can make an arbitrary choice between them. 

2.6. SignerIdentifier SignerInfo Type 

   S/MIME v3.2 implementations MUST support both issuerAndSerialNumber 
   as well as subjectKeyIdentifier.  Messages that use the 
   subjectKeyIdentifier choice cannot be read by S/MIME v2 clients. 

   It is important to understand that some certificates use a value for 
   subjectKeyIdentifier that is not suitable for uniquely identifying a 
   certificate.  Implementations MUST be prepared for multiple 
   certificates for potentially different entities to have the same 
   value for subjectKeyIdentifier, and MUST be prepared to try each 
   matching certificate during signature verification before indicating 
   an error condition. 

2.7. ContentEncryptionAlgorithmIdentifier 

   Sending and receiving agents: 

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

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


 
 
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    - 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. 

    - 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 

 
 
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   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,  

   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. 
 
 
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3. Creating S/MIME Messages 

   This section describes the S/MIME message formats and how they are 
   created.  S/MIME messages are a combination of MIME bodies and CMS 
   content types.  Several media types as well as several CMS content 
   types are used.  The data to be secured is always a canonical MIME 
   entity.  The MIME entity and other data, such as certificates and 
   algorithm identifiers, are given to CMS processing facilities which 
   produce a CMS object.  Finally, the CMS object is wrapped in MIME. 
   The Enhanced Security Services for S/MIME [ESS] document provides 
   descriptions of how nested, secured S/MIME messages are formatted. 
   ESS provides a description of how a triple-wrapped S/MIME message is 
   formatted using multipart/signed and application/pkcs7-mime for the 
   signatures. 

   S/MIME provides one format for enveloped-only data, several formats 
   for signed-only data, and several formats for signed and enveloped 
   data.  Several formats are required to accommodate several 
   environments, in particular for signed messages.  The criteria for 
   choosing among these formats are also described. 

   The reader of this section is expected to understand MIME as 
   described in [MIME-SPEC] and [MIME-SECURE]. 

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

   S/MIME is used to secure MIME entities.  A MIME entity can be a sub-
   part, sub-parts of a message, or the whole message with all its sub-
   parts.  A MIME entity that is the whole message includes only the 
   MIME message headers and MIME body, and does not include the RFC-822 
   header. Note that S/MIME can also be used to secure MIME entities 
   used in applications other than Internet mail.  If protection of the 
   RFC-822 header is required, the use of the message/rfc822 media type 
   is explained later in this section. 

   The MIME entity that is secured and described in this section can be 
   thought of as the "inside" MIME entity.  That is, it is the 
   "innermost" object in what is possibly a larger MIME message. 
   Processing "outside" MIME entities into CMS content types is 
   described in Section 3.2, 3.4, and elsewhere. 

   The procedure for preparing a MIME entity is given in [MIME-SPEC]. 
   The same procedure is used here with some additional restrictions 
   when signing.  Description of the procedures from [MIME-SPEC] are 
   repeated here, but it is suggested that the reader refer to that 
   document for the exact procedure.  This section also describes 
   additional requirements. 
 
 
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   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. 

   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. 
 
 
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   The exact details of canonicalization depend on the actual media type 
   and subtype of an entity, and are not described here.  Instead, the 
   standard for the particular media type SHOULD be consulted.  For 
   example, canonicalization of type text/plain is different from 
   canonicalization of audio/basic.  Other than text types, most types 
   have only one representation regardless of computing platform or 
   environment which can be considered their canonical representation. 
   In general, canonicalization will be performed by the non-security 
   part of the sending agent rather than the S/MIME implementation. 

   The most common and important canonicalization is for text, which is 
   often represented differently in different environments.  MIME 
   entities of major type "text" MUST have both their line endings and 
   character set canonicalized.  The line ending MUST be the pair of 
   characters <CR><LF>, and the charset SHOULD be a registered charset 
   [CHARSETS].  The details of the canonicalization are specified in 
   [MIME-SPEC]. 

   Note that some charsets such as ISO-2022 have multiple 
   representations for the same characters.  When preparing such text 
   for signing, the canonical representation specified for the charset 
   MUST be used. 

3.1.2. Transfer Encoding 

   When generating any of the secured MIME entities below, except the 
   signing using the multipart/signed format, no transfer encoding is 
   required at all.  S/MIME implementations MUST be able to deal with 
   binary MIME objects.  If no Content-Transfer-Encoding header field is 
   present, the transfer encoding is presumed to be 7BIT. 

   S/MIME implementations SHOULD however use transfer encoding described 
   in section 3.1.3 for all MIME entities they secure.  The reason for 
   securing only 7-bit MIME entities, even for enveloped data that are 
   not exposed to the transport, is that it allows the MIME entity to be 
   handled in any environment without changing it.  For example, a 
   trusted gateway might remove the envelope, but not the signature, of 
   a message, and then forward the signed message on to the end 
   recipient so that they can verify the signatures directly.  If the 
   transport internal to the site is not 8-bit clean, such as on a wide-
   area network with a single mail gateway, verifying the signature will 
   not be possible unless the original MIME entity was only 7-bit data. 

   S/MIME implementations which "know" that all intended recipient(s) 
   are capable of handling inner (all but the outermost) binary MIME 
   objects SHOULD use binary encoding as opposed to a 7-bit-safe 
   transfer encoding for the inner entities.  The use of a 7-bit-safe 
 
 
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   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. 

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

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

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

    - The agent could return the message to the sender. 

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


 
 
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3.1.4. Sample Canonical MIME Entity 

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

   Note that this example is not of an S/MIME message. 

      Content-Type: multipart/mixed; boundary=bar 

      --bar 
      Content-Type: text/plain; charset=iso-8859-1 
      Content-Transfer-Encoding: quoted-printable 

      =A1Hola Michael! 

      How do you like the new S/MIME specification? 

      It's generally a good idea to encode lines that begin with 
      From=20because some mail transport agents will insert a greater-
      than (>) sign, thus invalidating the signature. 

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

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

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

      --bar-- 

3.2. The application/pkcs7-mime Media Type 

   The application/pkcs7-mime media type is used to carry CMS content 
   types including EnvelopedData, SignedData, and CompressedData.  The 
   details of constructing these entities are described in subsequent 

 
 
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   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. 

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" 

 
 
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   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. 

     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. 


 
 
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      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. 

      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 

 
 
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3.4. Creating a Signed-only Message 

   There are two formats for signed messages defined for S/MIME: 

    - application/pkcs7-mime with SignedData; and, 

    - multipart/signed. 

   In general, the multipart/signed form is preferred for sending, and 
   receiving agents MUST be able to handle both. 

3.4.1. Choosing a Format for Signed-only Messages 

   There are no hard-and-fast rules when a particular signed-only format 
   is chosen because it depends on the capabilities of all the receivers 
   and the relative importance of receivers with S/MIME facilities being 
   able to verify the signature versus the importance of receivers 
   without S/MIME software being able to view the message. 

   Messages signed using the multipart/signed format can always be 
   viewed by the receiver whether they have S/MIME software or not. They 
   can also be viewed whether they are using a MIME-native user agent or 
   they have messages translated by a gateway.  In this context, "be 
   viewed" means the ability to process the message essentially as if it 
   were not a signed message, including any other MIME structure the 
   message might have. 

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

3.4.2. Signing Using application/pkcs7-mime with SignedData 

   This signing format uses the application/pkcs7-mime media type.  The 
   steps to create this format are: 

      Step 1.  The MIME entity is prepared according to section 3.1. 

      Step 2.  The MIME entity and other required data is processed 
      into a CMS object of type SignedData. 

      Step 3.  The SignedData object is wrapped in a CMS ContentInfo 
      object. 


 
 
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      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. 

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. 
 
 
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      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: 

     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. 



 
 
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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) 
   are the bytes: 

   43 6f 6e 74 65 6e 74 2d 54 79 70 65 3a 20 74 65 78 74 2f 70 6c 61 69 
   6e 0d 0a 0d 0a 54 68 69 73 20 69 73 20 61 20 63 6c 65 61 72 2d 73 69 
   67 6e 65 64 20 6d 65 73 73 61 67 65 2e 0d 0a 

3.5. Creating a Compressed-only Message 

   This section describes the format for compressing a MIME entity. 
   Please note that versions of S/MIME prior to version 3.1 did not 
   specify any use of CompressedData, and will not recognize it.  The 
   use of a capability to indicate the ability to receive CompressedData 
   is described in [CMSCOMPR] and is the preferred method for 
   compatibility. 

      Step 1.  The MIME entity to be compressed is prepared according 
      to section 3.1. 

      Step 2.  The MIME entity and other required data is processed 
      into a CMS object of type CompressedData. 

      Step 3.  The CompressedData object is wrapped in a CMS 
      ContentInfo object. 
 
 
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      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. 

   It is possible to apply any of the signing, encrypting, and 
   compressing operations in any order.  It is up to the implementer and 
   the user to choose.  When signing first, the signatories are then 
   securely obscured by the enveloping.  When enveloping first the 
   signatories are exposed, but it is possible to verify signatures 
   without removing the enveloping.  This can be useful in an 
   environment were automatic signature verification is desired, as no 
   private key material is required to verify a signature. 

   There are security ramifications to choosing whether to sign first or 
   encrypt first.  A recipient of a message that is encrypted and then 
   signed can validate that the encrypted block was unaltered, but 
   cannot determine any relationship between the signer and the 
   unencrypted contents of the message.  A recipient of a message that 
   is signed-then-encrypted can assume that the signed message itself 
   has not been altered, but that a careful attacker could have changed 
   the unauthenticated portions of the encrypted message. 


 
 
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   When using compression, keep the following guidelines in mind: 

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

    - If a lossy compression algorithm is used with signing, you will 
      need to compress first, then sign. 

3.7. Creating a Certificate Management Message 

   The certificate management message or MIME entity is used to 
   transport certificates and/or certificate revocation lists, such as 
   in response to a registration request. 

      Step 1.  The certificates and/or certificate revocation lists are 
      made available to the CMS generating process which creates a CMS 
      object of type SignedData.  The SignedData encapContentInfo 
      eContent field MUST be absent and signerInfos field MUST be 
      empty. 

      Step 2.  The SignedData object is wrapped in a CMS ContentInfo 
      object. 

      Step 3.  The ContentInfo object is enclosed in an 
      application/pkcs7-mime MIME entity. 

   The smime-type parameter for a certificate management message is 
   "certs-only".  The file extension for this type of message is ".p7c". 

3.8. Registration Requests 

   A sending agent that signs messages MUST have a certificate for the 
   signature so that a receiving agent can verify the signature.  There 
   are many ways of getting certificates, such as through an exchange 
   with a certificate authority, through a hardware token or diskette, 
   and so on. 

   S/MIME v2 [SMIMEv2] specified a method for "registering" public keys 
   with certificate authorities using an application/pkcs10 body part. 
   Since that time, the IETF PKIX Working Group has developed other 
   methods for requesting certificates.  However, S/MIME v3.2 does not 
   require a particular certificate request mechanism. 




 
 
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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]. 

   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. 
 
 
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   An S/MIME user agent MUST NOT generate asymmetric keys less than 512 
   bits for use with the RSA or DSA signature algorithms. 

   For 512-bit RSA with SHA-1 see [CMSALG] and [FIPS186-2] without 
   Change Notice 1, for 512-bit RSA with SHA-256 see [CMS-SHA2] and 
   [FIPS186-2] without Change Notice 1, for 1024-bit through 2048-bit 
   RSA with SHA-256 see [CMS-SHA2] and [FIPS186-2] with Change Notice 1. 
   The first reference provides the signature algorithm's object 
   identifier and the second provides the signature algorithm's 
   definition. 

   For 512-bit DSA with SHA-1 see [CMSALG] and [FIPS186-2] without 
   Change Notice 1, for 512-bit DSA with SHA-256 see [CMS-SHA2] and 
   [FIPS186-2] without Change Notice 1, for 1024-bit DSA with SHA-1 see 
   [CMSALG] and [FIPS186-2] with Change Notice 1, for 1024-bit 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 DH see [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) 

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) 








 
 
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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 is intended to provide sufficient information to update 
   the media type registration for application/pkcs7-mime and 
   application/pkcs7-signature to refer to this document as opposed to 
   RFC 2311. 

   Note that other documents can define additional MIME media types for 
   S/MIME. 

5.1. Media Type for application/pkcs7-mime 

   Type name: application 

   Subtype Name: pkcs7-mime 

   Required Parameters: NONE 

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

    
   Encoding Considerations: See Section 3 of this document 

 
 
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   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 

   Additional information: NONE 

   Person & email to contact for further information: S/MIME working 
   group chairs smime-chairs@tools.ietf.org 

   Intended usage: COMMON 
 
 
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   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 
   specification seem appropriate for the Internet based on [STRENGTH].  
   Of course, there are environments, such as financial and medical 
   system, that may select different key sizes.  For this reason, an 
   implementation MAY support key sizes beyond those recommended in this 
   specification. 

 
 
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   Receiving agents that validate signatures and sending agents that 
   encrypt messages, need to be cautious of cryptographic processing 
   usage when validating signatures and encrypting messages using keys 
   larger than those mandated in this specification.  An attacker could 
   send certificates with keys which would result in excessive 
   cryptographic processing, for example keys larger than those mandated 
   in this specification, which could swamp the processing element.  
   Agents which use such keys without first validating the certificate 
   to a trust anchor are advised to have some sort of cryptographic 
   resource management system to prevent such attacks. 

   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. 

   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. 
 
 
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   Modification of the ciphertext can go undetected if authentication is 
   not also used, which is the case when sending EnvelopedData without 
   wrapping it in SignedData or enclosing SignedData within it. 

   If an implementation is concerned about compliance with NIST key size 
   recommendations, then see [SP800-57]. 

   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, can lead to incorrect handling of the message.  Visual 
   indicators on messages may need to have the signature validation code 
   check periodically if the indicator is supposed to give information 
   on the current status of a message. 

7. References 

7.1. Normative References 

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

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

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

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

   [CMSAES]      Schaad, J., "Use of the Advanced Encryption Standard 
                 (AES) Encryption Algorithm in Cryptographic Message 
                 Syntax (CMS)", RFC 3565, July 2003. 

   [CMSALG]      Housley, R., "Cryptographic Message Syntax (CMS) 
                 Algorithms", RFC 3370, August 2002. 

   [CMSCOMPR]    Gutmann, P., "Compressed Data Content Type for 
                 Cryptographic Message Syntax (CMS)", RFC 3274, June 
                 2002. 

   [CMS-SHA2]    Turner. S., "Using SHA2 Algorithms with Cryptographic 
                 Message Syntax", draft-ietf-smime-sha2-11.txt, work in 
                 progress. 
 
 
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   [CONTDISP]    Troost, R., Dorner, S., and K. Moore, "Communicating 
                 Presentation Information in Internet Messages: The 
                 Content-Disposition Header Field", RFC 2183, August 
                 1997. 

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

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

   [FIPS186-2]   National Institute of Standards and Technology (NIST), 
                 "Digital Signature Standard (DSS)", FIPS Publication 
                 186-2, January 2000. [With Change Notice 1]. 

   [FIPS186-3]   National Institute of Standards and Technology (NIST), 
                 FIPS Publication 186-3: Digital Signature Standard, 
                 (draft) March 2006. 

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

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

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

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

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

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

   [MIME-SECURE] Galvin, J., Murphy, S., Crocker, S., and N. Freed, 
                 "Security Multiparts for MIME: Multipart/Signed and 
                 Multipart/Encrypted", RFC 1847, October 1995. 

 
 
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   [MUSTSHOULD]  Bradner, S., "Key words for use in RFCs to Indicate 
                 Requirement Levels", BCP 14, RFC 2119, March 1997. 

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

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

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

   [X.680]       ITU-T Recommendation X.680 (2002) | ISO/IEC 8824-
                 1:2002. Information Technology - Abstract Syntax 
                 Notation One (ASN.1):  Specification of basic 
                 notation. 

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

7.2. Informative References 

   [DHSUB]       Zuccherato, R., "Methods for Avoiding the "Small-
                 Subgroup" Attacks on the Diffie-Hellman Key Agreement 
                 Method for S/MIME", RFC 2785, March 2000. 

   [HASH-ATTACK] Hoffman, P., Schneier, B., "Attacks on Cryptographic 
                 Hashes in Internet Protocols", RFC 4270, November 
                 2005. 

   [MMA]         Rescorla, E., "Preventing the Million Message Attack 
                 on Cryptographic Message Syntax", RFC 3218, January 
                 2002. 

   [PKCS-7]      Kaliski, B., "PKCS #7: Cryptographic Message Syntax 
                 Version 1.5", RFC 2315, March 1998. 





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

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

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

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

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

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

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

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

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

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

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













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

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

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

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

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

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

   [SP800-57]    National Institute of Standards and Technology (NIST), 
                 Special Publication 800-57: Recommendation for Key 
                 Management, August 2005. 

   [STRENGTH]    Orman, H., and P. Hoffman, "Determining Strengths For 
                 Public Keys Used For Exchanging Symmetric Keys", BCP 
                 86, RFC 3766, April 2004. 

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 

   -- 
   --  Copyright (c) 2009 IETF Trust and the persons identified as 
   --  authors of the code.  All rights reserved. 
   -- 
 
 
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   --  Redistribution and use in source and binary forms, with or  
   --  without modification, are permitted provided that the following 
   --  conditions are met: 
   -- 
   -- - Redistributions of source code must retain the above copyright 
   --   notice, this list of conditions and the following disclaimer. 
   -- 
   -- - Redistributions in binary form must reproduce the above 
   --   copyright notice, this list of conditions and the following 
   --   disclaimer in the documentation and/or other materials provided 
   --   with the distribution. 
   -- 
   -- - Neither the name of Internet Society, IETF or IETF Trust, nor 
   --   the names of specific contributors, may be used to endorse or 
   --   promote products derived from this software without specific 
   --   prior written permission. 
   -- 
   -- 
   -- 
   --  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND 
   --  CONTRIBUTORS 'AS IS' AND ANY EXPRESS OR IMPLIED WARRANTIES, 
   --  INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF 
   --  MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE 
   --  DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR 
   --  CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, 
   --  SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT 
   --  LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS 
   --  OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER 
   --  CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, 
   --  STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 
   --  ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF 
   --  ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 
   -- 
   --  This version of the ASN.1 module is part of RFC XXXX; 
   --  see the RFC itself for full legal notices. 
   -- 

   //RFC EDITOR NOTE: Replace XXXX with this RFC's #. 

   -- 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) }; 


 
 
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   --  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} 

   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 


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

   -- 

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

   -- 

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

   SMIMECapabilitiesParametersForRC2CBC ::= INTEGER 
   --        (RC2 Key Length (number of bits)) 

   END 
























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

   The S/MIME v3 [SMIMEv3], v3.1 [SMIMEv3.1], and v3.2 (this document) 
   are backwards compatible with the S/MIME v2 Message Specification 
   [SMIMEv2], with the exception of the algorithms (dropped RC2/40 
   requirement and added DSA and 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 due to the fact that they 
   made direct contributions to this document. 

   Tony Capel, Piers Chivers, Dave Crocker, Bill Flanigan, Peter 
   Gutmann, Alfred Hoenes, Paul Hoffman, Russ Housley, William Ottaway, 
   John Pawling, and Jim Schaad. 

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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