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Internet Engineering Task Force (IETF)                       B. RamsdellRequest for Comments: 5751                              Brute Squad LabsObsoletes:3851                                                S. TurnerCategory: Standards Track                                           IECAISSN: 2070-1721                                             January 2010Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2Message SpecificationAbstract   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 obsoletesRFC 3851.Status of This Memo   This is an Internet Standards Track document.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by   the Internet Engineering Steering Group (IESG).  Further   information on Internet Standards is available inSection 2 of   RFC 5741.   Information about the current status of this document, any   errata, and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc5751.Ramsdell & Turner            Standards Track                    [Page 1]

RFC 5751            S/MIME 3.2 Message Specification        January 2010Copyright Notice   Copyright (c) 2010 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.   This document may contain material from IETF Documents or IETF   Contributions published or made publicly available before November   10, 2008.  The person(s) controlling the copyright in some of this   material may not have granted the IETF Trust the right to allow   modifications of such material outside the IETF Standards Process.   Without obtaining an adequate license from the person(s) controlling   the copyright in such materials, this document may not be modified   outside the IETF Standards Process, and derivative works of it may   not be created outside the IETF Standards Process, except to format   it for publication as an RFC or to translate it into languages other   than English.Ramsdell & Turner            Standards Track                    [Page 2]

RFC 5751            S/MIME 3.2 Message Specification        January 2010Table of Contents1. Introduction ....................................................41.1. Specification Overview .....................................41.2. Definitions ................................................51.3. Conventions Used in This Document ..........................61.4. Compatibility with Prior Practice of S/MIME ................71.5. Changes from S/MIME v3 to S/MIME v3.1 ......................71.6. Changes since S/MIME v3.1 ..................................72. CMS Options .....................................................92.1. DigestAlgorithmIdentifier ..................................92.2. SignatureAlgorithmIdentifier ...............................92.3. KeyEncryptionAlgorithmIdentifier ..........................102.4. General Syntax ............................................112.5. Attributes and the SignerInfo Type ........................122.6. SignerIdentifier SignerInfo Type ..........................162.7. ContentEncryptionAlgorithmIdentifier ......................163. Creating S/MIME Messages .......................................18      3.1. Preparing the MIME Entity for Signing, Enveloping,           or Compressing ............................................193.2. The application/pkcs7-mime Media Type .....................233.3. Creating an Enveloped-Only Message ........................253.4. Creating a Signed-Only Message ............................263.5. Creating a Compressed-Only Message ........................303.6. Multiple Operations .......................................303.7. Creating a Certificate Management Message .................313.8. Registration Requests .....................................323.9. Identifying an S/MIME Message .............................324. Certificate Processing .........................................324.1. Key Pair Generation .......................................334.2. Signature Generation ......................................334.3. Signature Verification ....................................344.4. Encryption ................................................344.5. Decryption ................................................345. IANA Considerations ............................................345.1. Media Type for application/pkcs7-mime .....................345.2. Media Type for application/pkcs7-signature ................356. Security Considerations ........................................367. References .....................................................387.1. Reference Conventions .....................................387.2. Normative References ......................................397.3. Informative References ....................................41Appendix A. ASN.1 Module ..........................................43Appendix B. Moving S/MIME v2 Message Specification to Historic               Status ................................................45Appendix C. Acknowledgments .......................................45Ramsdell & Turner            Standards Track                    [Page 3]

RFC 5751            S/MIME 3.2 Message Specification        January 20101.  Introduction   S/MIME (Secure/Multipurpose Internet Mail Extensions) provides a   consistent way to send and receive secure MIME data.  Based on the   popular Internet MIME standard, S/MIME provides the following   cryptographic security services for electronic messaging   applications:  authentication, message integrity and non-repudiation   of origin (using digital signatures), and data confidentiality (using   encryption).  As a supplementary service, S/MIME provides for message   compression.   S/MIME can be used by traditional mail user agents (MUAs) to add   cryptographic security services to mail that is sent, and to   interpret cryptographic security services in mail that is received.   However, S/MIME is not restricted to mail; it can be used with any   transport mechanism that transports MIME data, such as HTTP or SIP.   As such, S/MIME takes advantage of the object-based features of MIME   and allows secure messages to be exchanged in mixed-transport   systems.   Further, S/MIME can be used in automated message transfer agents that   use cryptographic security services that do not require any human   intervention, such as the signing of software-generated documents and   the encryption of FAX messages sent over the Internet.1.1.  Specification Overview   This document describes a protocol for adding cryptographic signature   and encryption services to MIME data.  The MIME standard [MIME-SPEC]   provides a general structure for the content of Internet messages and   allows extensions for new content-type-based applications.   This specification defines how to create a MIME body part that has   been cryptographically enhanced according to the Cryptographic   Message Syntax (CMS)RFC 5652 [CMS], which is derived from PKCS #7   [PKCS-7].  This specification also defines the application/pkcs7-mime   media type that can be used to transport those body parts.   This document also discusses how to use the multipart/signed media   type defined in [MIME-SECURE] to transport S/MIME signed messages.   multipart/signed is used in conjunction with the application/pkcs7-   signature media type, which is used to transport a detached S/MIME   signature.Ramsdell & Turner            Standards Track                    [Page 4]

RFC 5751            S/MIME 3.2 Message Specification        January 2010   In order to create S/MIME messages, an S/MIME agent MUST follow the   specifications in this document, as well as the specifications listed   in the Cryptographic Message Syntax document [CMS], [CMSALG],   [RSAPSS], [RSAOAEP], and [CMS-SHA2].   Throughout this specification, there are requirements and   recommendations made for how receiving agents handle incoming   messages.  There are separate requirements and recommendations for   how sending agents create outgoing messages.  In general, the best   strategy is to "be liberal in what you receive and conservative in   what you send".  Most of the requirements are placed on the handling   of incoming messages, while the recommendations are mostly on the   creation of outgoing messages.   The separation for requirements on receiving agents and sending   agents also derives from the likelihood that there will be S/MIME   systems that involve software other than traditional Internet mail   clients.  S/MIME can be used with any system that transports MIME   data.  An automated process that sends an encrypted message might not   be able to receive an encrypted message at all, for example.  Thus,   the requirements and recommendations for the two types of agents are   listed separately when appropriate.1.2.  Definitions   For the purposes of this specification, the following definitions   apply.   ASN.1:             Abstract Syntax Notation One, as defined in ITU-T                      Recommendation X.680 [X.680].   BER:               Basic Encoding Rules for ASN.1, as defined in ITU-                      T Recommendation X.690 [X.690].   Certificate:       A type that binds an entity's name to a public key                      with a digital signature.   DER:               Distinguished Encoding Rules for ASN.1, as defined                      in ITU-T Recommendation X.690 [X.690].   7-bit data:        Text data with lines less than 998 characters                      long, where none of the characters have the 8th                      bit set, and there are no NULL characters.  <CR>                      and <LF> occur only as part of a <CR><LF> end-of-                      line delimiter.Ramsdell & Turner            Standards Track                    [Page 5]

RFC 5751            S/MIME 3.2 Message Specification        January 2010   8-bit data:        Text data with lines less than 998 characters, and                      where none of the characters are NULL characters.                      <CR> and <LF> occur only as part of a <CR><LF>                      end-of-line delimiter.   Binary data:       Arbitrary data.   Transfer encoding: A reversible transformation made on data so 8-bit                      or binary data can be sent via a channel that only                      transmits 7-bit data.   Receiving agent:   Software that interprets and processes S/MIME CMS                      objects, MIME body parts that contain CMS content                      types, or both.   Sending agent:     Software that creates S/MIME CMS content types,                      MIME body parts that contain CMS content types, or                      both.   S/MIME agent:      User software that is a receiving agent, a sending                      agent, or both.1.3.  Conventions Used in This Document   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described in [MUSTSHOULD].   We define some additional terms here:   SHOULD+   This term means the same as SHOULD.  However, the authors             expect that a requirement marked as SHOULD+ will be             promoted at some future time to be a MUST.   SHOULD-   This term means the same as SHOULD.  However, the authors             expect that a requirement marked as SHOULD- will be demoted             to a MAY in a future version of this document.   MUST-     This term means the same as MUST.  However, the authors             expect that this requirement will no longer be a MUST in a             future document.  Although its status will be determined at             a later time, it is reasonable to expect that if a future             revision of a document alters the status of a MUST-             requirement, it will remain at least a SHOULD or a SHOULD-.Ramsdell & Turner            Standards Track                    [Page 6]

RFC 5751            S/MIME 3.2 Message Specification        January 20101.4.  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 inRFC 2311 throughRFC 2315 inclusive   [SMIMEv2], S/MIME version 3 is described inRFC 2630 throughRFC 2634   inclusive andRFC 5035 [SMIMEv3], and S/MIME version 3.1 is described   inRFC 3850,RFC 3851,RFC 3852,RFC 2634, andRFC 5035 [SMIMEv3.1].RFC 2311 also has historical information about the development of   S/MIME.1.5.  Changes from S/MIME v3 to S/MIME v3.1   The RSA public key algorithm was changed to a MUST implement key   wrapping algorithm, and the Diffie-Hellman (DH) algorithm changed to   a SHOULD implement.   The AES symmetric encryption algorithm has been included as a SHOULD   implement.   The RSA public key algorithm was changed to a MUST implement   signature algorithm.   Ambiguous language about the use of "empty" SignedData messages to   transmit certificates was clarified to reflect that transmission of   Certificate Revocation Lists is also allowed.   The use of binary encoding for some MIME entities is now explicitly   discussed.   Header protection through the use of the message/rfc822 media type   has been added.   Use of the CompressedData CMS type is allowed, along with required   media type and file extension additions.1.6.  Changes since S/MIME v3.1   Editorial changes, e.g., replaced "MIME type" with "media type",   content-type with Content-Type.   Moved "Conventions Used in This Document" toSection 1.3.  Added   definitions for SHOULD+, SHOULD-, and MUST-.Section 1.1 andAppendix A: Added references to RFCs for RSASSA-PSS,   RSAES-OAEP, and SHA2 CMS algorithms.  Added CMS Multiple Signers   Clarification to CMS reference.Ramsdell & Turner            Standards Track                    [Page 7]

RFC 5751            S/MIME 3.2 Message Specification        January 2010Section 1.2: Updated references to ASN.1 to X.680 and BER and DER to   X.690.Section 1.4: Added references to S/MIME MSG 3.1 RFCs.Section 2.1 (digest algorithm): SHA-256 added as MUST, SHA-1 and MD5   made SHOULD-.Section 2.2 (signature algorithms): RSA with SHA-256 added as MUST,   and DSA with SHA-256 added as SHOULD+, RSA with SHA-1, DSA with   SHA-1, and RSA with MD5 changed to SHOULD-, and RSASSA-PSS with   SHA-256 added as SHOULD+.  Also added note about what S/MIME v3.1   clients support.Section 2.3 (key encryption): DH changed to SHOULD-, and RSAES-OAEP   added as SHOULD+.  Elaborated requirements for key wrap algorithm.Section 2.5.1: Added requirement that receiving agents MUST support   both GeneralizedTime and UTCTime.Section 2.5.2: Replaced reference "sha1WithRSAEncryption" with   "sha256WithRSAEncryption", "DES-3EDE-CBC" with "AES-128 CBC", and   deleted the RC5 example.Section 2.5.2.1: Deleted entire section (discussed deprecated RC2).Section 2.7, 2.7.1,Appendix A: references to RC2/40 removed.Section 2.7 (content encryption): AES-128 CBC added as MUST, AES-192   and AES-256 CBC SHOULD+, tripleDES now SHOULD-.Section 2.7.1: Updated pointers from 2.7.2.1 through 2.7.2.4 to   2.7.1.1 to 2.7.1.2.Section 3.1.1: Removed text about MIME character sets.Section 3.2.2 and 3.6: Replaced "encrypted" with "enveloped".  Update   OID example to use AES-128 CBC oid.Section 3.4.3.2: Replace micalg parameter for SHA-1 with sha-1.Section 4: Updated reference to CERT v3.2.Section 4.1: Updated RSA and DSA key size discussion.  Moved last   four sentences to security considerations.  Updated reference to   randomness requirements for security.Ramsdell & Turner            Standards Track                    [Page 8]

RFC 5751            S/MIME 3.2 Message Specification        January 2010Section 5: Added IANA registration templates to update media type   registry to point to this document as opposed toRFC 2311.Section 6: Updated security considerations.Section 7: Moved references fromAppendix B to this section.  Updated   references.  Added informational references to SMIMEv2, SMIMEv3, and   SMIMEv3.1.Appendix B: AddedAppendix B to move S/MIME v2 to Historic status.2.  CMS Options   CMS allows for a wide variety of options in content, attributes, and   algorithm support.  This section puts forth a number of support   requirements and recommendations in order to achieve a base level of   interoperability among all S/MIME implementations.  [CMSALG] and   [CMS-SHA2] provides additional details regarding the use of the   cryptographic algorithms.  [ESS] provides additional details   regarding the use of additional attributes.2.1.  DigestAlgorithmIdentifier   Sending and receiving agents MUST support SHA-256 [CMS-SHA2] and   SHOULD- support SHA-1 [CMSALG].  Receiving agents SHOULD- support MD5   [CMSALG] for the purpose of providing backward compatibility with   MD5-digested S/MIME v2 SignedData objects.2.2.  SignatureAlgorithmIdentifier   Receiving agents:      - MUST support RSA with SHA-256.      - SHOULD+ support DSA with SHA-256.      - SHOULD+ support RSASSA-PSS with SHA-256.      - SHOULD- support RSA with SHA-1.      - SHOULD- support DSA with SHA-1.      - SHOULD- support RSA with MD5.Ramsdell & Turner            Standards Track                    [Page 9]

RFC 5751            S/MIME 3.2 Message Specification        January 2010   Sending agents:      - MUST support RSA with SHA-256.      - SHOULD+ support DSA with SHA-256.      - SHOULD+ support RSASSA-PSS with SHA-256.      - SHOULD- support RSA with SHA-1 or DSA with SHA-1.      - SHOULD- support RSA with MD5.   SeeSection 4.1 for information on key size and algorithm references.   Note that S/MIME v3.1 clients support verifying id-dsa-with-sha1 and   rsaEncryption and might not implement sha256withRSAEncryption.  Note   that S/MIME v3 clients might only implement signing or signature   verification using id-dsa-with-sha1, and might also use id-dsa as an   AlgorithmIdentifier in this field.  Receiving clients SHOULD   recognize id-dsa as equivalent to id-dsa-with-sha1, and sending   clients MUST use id-dsa-with-sha1 if using that algorithm.  Also note   that S/MIME v2 clients are only required to verify digital signatures   using the rsaEncryption algorithm with SHA-1 or MD5, and might not   implement id-dsa-with-sha1 or id-dsa at all.2.3.  KeyEncryptionAlgorithmIdentifier   Receiving and sending agents:      - MUST support RSA Encryption, as specified in [CMSALG].      - SHOULD+ support RSAES-OAEP, as specified in [RSAOAEP].      - SHOULD- support DH ephemeral-static mode, as specified in        [CMSALG] and [SP800-57].   When DH ephemeral-static is used, a key wrap algorithm is also   specified in the KeyEncryptionAlgorithmIdentifier [CMS].  The   underlying encryption functions for the key wrap and content   encryption algorithm ([CMSALG] and [CMSAES]) and the key sizes for   the two algorithms MUST be the same (e.g., AES-128 key wrap algorithm   with AES-128 content encryption algorithm).  As AES-128 CBC is the   mandatory-to-implement content encryption algorithm, the AES-128 key   wrap algorithm MUST also be supported when DH ephemeral-static is   used.Ramsdell & Turner            Standards Track                   [Page 10]

RFC 5751            S/MIME 3.2 Message Specification        January 2010   Note that S/MIME v3.1 clients might only implement key encryption and   decryption using the rsaEncryption algorithm.  Note that S/MIME v3   clients might only implement key encryption and decryption using the   Diffie-Hellman algorithm.  Also note that S/MIME v2 clients are only   capable of decrypting content-encryption keys using the rsaEncryption   algorithm.2.4.  General Syntax   There are several CMS content types.  Of these, only the Data,   SignedData, EnvelopedData, and CompressedData content types are   currently used for S/MIME.2.4.1.  Data Content Type   Sending agents MUST use the id-data content type identifier to   identify the "inner" MIME message content.  For example, when   applying a digital signature to MIME data, the CMS SignedData   encapContentInfo eContentType MUST include the id-data object   identifier and the media type MUST be stored in the SignedData   encapContentInfo eContent OCTET STRING (unless the sending agent is   using multipart/signed, in which case the eContent is absent, perSection 3.4.3 of this document).  As another example, when applying   encryption to MIME data, the CMS EnvelopedData encryptedContentInfo   contentType MUST include the id-data object identifier and the   encrypted MIME content MUST be stored in the EnvelopedData   encryptedContentInfo encryptedContent OCTET STRING.2.4.2.  SignedData Content Type   Sending agents MUST use the SignedData content type to apply a   digital signature to a message or, in a degenerate case where there   is no signature information, to convey certificates.  Applying a   signature to a message provides authentication, message integrity,   and non-repudiation of origin.2.4.3.  EnvelopedData Content Type   This content type is used to apply data confidentiality to a message.   A sender needs to have access to a public key for each intended   message recipient to use this service.2.4.4.  CompressedData Content Type   This content type is used to apply data compression to a message.   This content type does not provide authentication, message integrity,   non-repudiation, or data confidentiality, and is only used to reduce   the message's size.Ramsdell & Turner            Standards Track                   [Page 11]

RFC 5751            S/MIME 3.2 Message Specification        January 2010   SeeSection 3.6 for further guidance on the use of this type in   conjunction with other CMS types.2.5.  Attributes and the SignerInfo Type   The SignerInfo type allows the inclusion of unsigned and signed   attributes along with a signature.   Receiving agents MUST be able to handle zero or one instance of each   of the signed attributes listed here.  Sending agents SHOULD generate   one instance of each of the following signed attributes in each   S/MIME message:      - Signing Time (section (Section 2.5.1 in this document)      - SMIME Capabilities (section (Section 2.5.2 in this document)      - Encryption Key Preference (section (Section 2.5.3 in this        document)      - Message Digest (section (Section 11.2 in [CMS])      - Content Type (section (Section 11.1 in [CMS])   Further, receiving agents SHOULD be able to handle zero or one   instance of the signingCertificate and signingCertificatev2 signed   attributes, as defined inSection 5 of RFC 2634 [ESS] andSection 3   of RFC 5035 [ESS].   Sending agents SHOULD generate one instance of the signingCertificate   or signingCertificatev2 signed attribute in each SignerInfo   structure.   Additional attributes and values for these attributes might be   defined in the future.  Receiving agents SHOULD handle attributes or   values that they do not recognize in a graceful manner.   Interactive sending agents that include signed attributes that are   not listed here SHOULD display those attributes to the user, so that   the user is aware of all of the data being signed.2.5.1.  Signing Time Attribute   The signing-time attribute is used to convey the time that a message   was signed.  The time of signing will most likely be created by a   message originator and therefore is only as trustworthy as the   originator.Ramsdell & Turner            Standards Track                   [Page 12]

RFC 5751            S/MIME 3.2 Message Specification        January 2010   Sending agents MUST encode signing time through the year 2049 as   UTCTime; signing times in 2050 or later MUST be encoded as   GeneralizedTime.  When the UTCTime CHOICE is used, S/MIME agents MUST   interpret the year field (YY) as follows:      If YY is greater than or equal to 50, the year is interpreted as      19YY; if YY is less than 50, the year is interpreted as 20YY.   Receiving agents MUST be able to process signing-time attributes that   are encoded in either UTCTime or GeneralizedTime.2.5.2.  SMIME Capabilities Attribute   The SMIMECapabilities attribute includes signature algorithms (such   as "sha256WithRSAEncryption"), symmetric algorithms (such as "AES-128   CBC"), and key encipherment algorithms (such as "rsaEncryption").   There are also several identifiers that indicate support for other   optional features such as binary encoding and compression.  The   SMIMECapabilities were designed to be flexible and extensible so   that, in the future, a means of identifying other capabilities and   preferences such as certificates can be added in a way that will not   cause current clients to break.   If present, the SMIMECapabilities attribute MUST be a   SignedAttribute; it MUST NOT be an UnsignedAttribute.  CMS defines   SignedAttributes as a SET OF Attribute.  The SignedAttributes in a   signerInfo MUST NOT include multiple instances of the   SMIMECapabilities attribute.  CMS defines the ASN.1 syntax for   Attribute to include attrValues SET OF AttributeValue.  A   SMIMECapabilities attribute MUST only include a single instance of   AttributeValue.  There MUST NOT be zero or multiple instances of   AttributeValue present in the attrValues SET OF AttributeValue.   The semantics of the SMIMECapabilities attribute specify a partial   list as to what the client announcing the SMIMECapabilities can   support.  A client does not have to list every capability it   supports, and need not list all its capabilities so that the   capabilities list doesn't get too long.  In an SMIMECapabilities   attribute, the object identifiers (OIDs) are listed in order of their   preference, but SHOULD be separated logically along the lines of   their categories (signature algorithms, symmetric algorithms, key   encipherment algorithms, etc.).   The structure of the SMIMECapabilities attribute is to facilitate   simple table lookups and binary comparisons in order to determine   matches.  For instance, the DER-encoding for the SMIMECapability for   AES-128 CBC MUST be identically encoded regardless of the   implementation.  Because of the requirement for identical encoding,Ramsdell & Turner            Standards Track                   [Page 13]

RFC 5751            S/MIME 3.2 Message Specification        January 2010   individuals documenting algorithms to be used in the   SMIMECapabilities attribute SHOULD explicitly document the correct   byte sequence for the common cases.   For any capability, the associated parameters for the OID MUST   specify all of the parameters necessary to differentiate between two   instances of the same algorithm.   The OIDs that correspond to algorithms SHOULD use the same OID as the   actual algorithm, except in the case where the algorithm usage is   ambiguous from the OID.  For instance, in an earlier specification,   rsaEncryption was ambiguous because it could refer to either a   signature algorithm or a key encipherment algorithm.  In the event   that an OID is ambiguous, it needs to be arbitrated by the maintainer   of the registered SMIMECapabilities list as to which type of   algorithm will use the OID, and a new OID MUST be allocated under the   smimeCapabilities OID to satisfy the other use of the OID.   The registered SMIMECapabilities list specifies the parameters for   OIDs that need them, most notably key lengths in the case of   variable-length symmetric ciphers.  In the event that there are no   differentiating parameters for a particular OID, the parameters MUST   be omitted, and MUST NOT be encoded as NULL.  Additional values for   the SMIMECapabilities attribute might be defined in the future.   Receiving agents MUST handle a SMIMECapabilities object that has   values that it does not recognize in a graceful manner.Section 2.7.1 explains a strategy for caching capabilities.2.5.3.  Encryption Key Preference Attribute   The encryption key preference attribute allows the signer to   unambiguously describe which of the signer's certificates has the   signer's preferred encryption key.  This attribute is designed to   enhance behavior for interoperating with those clients that use   separate keys for encryption and signing.  This attribute is used to   convey to anyone viewing the attribute which of the listed   certificates is appropriate for encrypting a session key for future   encrypted messages.   If present, the SMIMEEncryptionKeyPreference attribute MUST be a   SignedAttribute; it MUST NOT be an UnsignedAttribute.  CMS defines   SignedAttributes as a SET OF Attribute.  The SignedAttributes in a   signerInfo MUST NOT include multiple instances of the   SMIMEEncryptionKeyPreference attribute.  CMS defines the ASN.1 syntax   for Attribute to include attrValues SET OF AttributeValue.  A   SMIMEEncryptionKeyPreference attribute MUST only include a singleRamsdell & Turner            Standards Track                   [Page 14]

RFC 5751            S/MIME 3.2 Message Specification        January 2010   instance of AttributeValue.  There MUST NOT be zero or multiple   instances of AttributeValue present in the attrValues SET OF   AttributeValue.   The sending agent SHOULD include the referenced certificate in the   set of certificates included in the signed message if this attribute   is used.  The certificate MAY be omitted if it has been previously   made available to the receiving agent.  Sending agents SHOULD use   this attribute if the commonly used or preferred encryption   certificate is not the same as the certificate used to sign the   message.   Receiving agents SHOULD store the preference data if the signature on   the message is valid and the signing time is greater than the   currently stored value.  (As with the SMIMECapabilities, the clock   skew SHOULD be checked and the data not used if the skew is too   great.)  Receiving agents SHOULD respect the sender's encryption key   preference attribute if possible.  This, however, represents only a   preference and the receiving agent can use any certificate in   replying to the sender that is valid.Section 2.7.1 explains a strategy for caching preference data.2.5.3.1.  Selection of Recipient Key Management Certificate   In order to determine the key management certificate to be used when   sending a future CMS EnvelopedData message for a particular   recipient, the following steps SHOULD be followed:   - If an SMIMEEncryptionKeyPreference attribute is found in a     SignedData object received from the desired recipient, this     identifies the X.509 certificate that SHOULD be used as the X.509     key management certificate for the recipient.   - If an SMIMEEncryptionKeyPreference attribute is not found in a     SignedData object received from the desired recipient, the set of     X.509 certificates SHOULD be searched for a X.509 certificate with     the same subject name as the signer of a X.509 certificate that can     be used for key management.   - Or use some other method of determining the user's key management     key.  If a X.509 key management certificate is not found, then     encryption cannot be done with the signer of the message.  If     multiple X.509 key management certificates are found, the S/MIME     agent can make an arbitrary choice between them.Ramsdell & Turner            Standards Track                   [Page 15]

RFC 5751            S/MIME 3.2 Message Specification        January 20102.6.  SignerIdentifier SignerInfo Type   S/MIME v3.2 implementations MUST support both issuerAndSerialNumber   and subjectKeyIdentifier.  Messages that use the subjectKeyIdentifier   choice cannot be read by S/MIME v2 clients.   It is important to understand that some certificates use a value for   subjectKeyIdentifier that is not suitable for uniquely identifying a   certificate.  Implementations MUST be prepared for multiple   certificates for potentially different entities to have the same   value for subjectKeyIdentifier, and MUST be prepared to try each   matching certificate during signature verification before indicating   an error condition.2.7.  ContentEncryptionAlgorithmIdentifier   Sending and receiving agents:      - MUST support encryption and decryption with AES-128 CBC        [CMSAES].      - SHOULD+ support encryption and decryption with AES-192 CBC and        AES-256 CBC [CMSAES].      - SHOULD- support encryption and decryption with DES EDE3 CBC,        hereinafter called "tripleDES" [CMSALG].2.7.1.  Deciding Which Encryption Method to Use   When a sending agent creates an encrypted message, it has to decide   which type of encryption to use.  The decision process involves using   information garnered from the capabilities lists included in messages   received from the recipient, as well as out-of-band information such   as private agreements, user preferences, legal restrictions, and so   on.Section 2.5.2 defines a method by which a sending agent can   optionally announce, among other things, its decrypting capabilities   in its order of preference.  The following method for processing and   remembering the encryption capabilities attribute in incoming signed   messages SHOULD be used.      - If the receiving agent has not yet created a list of        capabilities for the sender's public key, then, after verifying        the signature on the incoming message and checking the        timestamp, the receiving agent SHOULD create a new list        containing at least the signing time and the symmetric        capabilities.Ramsdell & Turner            Standards Track                   [Page 16]

RFC 5751            S/MIME 3.2 Message Specification        January 2010      - If such a list already exists, the receiving agent SHOULD verify        that the signing time in the incoming message is greater than        the signing time stored in the list and that the signature is        valid.  If so, the receiving agent SHOULD update both the        signing time and capabilities in the list.  Values of the        signing time that lie far in the future (that is, a greater        discrepancy than any reasonable clock skew), or a capabilities        list in messages whose signature could not be verified, MUST NOT        be accepted.   The list of capabilities SHOULD be stored for future use in creating   messages.   Before sending a message, the sending agent MUST decide whether it is   willing to use weak encryption for the particular data in the   message.  If the sending agent decides that weak encryption is   unacceptable for this data, then the sending agent MUST NOT use a   weak algorithm.  The decision to use or not use weak encryption   overrides any other decision in this section about which encryption   algorithm to use.   Sections2.7.1.1 through2.7.1.2 describe the decisions a sending   agent SHOULD use in deciding which type of encryption will be applied   to a message.  These rules are ordered, so the sending agent SHOULD   make its decision in the order given.2.7.1.1.  Rule 1: Known Capabilities   If the sending agent has received a set of capabilities from the   recipient for the message the agent is about to encrypt, then the   sending agent SHOULD use that information by selecting the first   capability in the list (that is, the capability most preferred by the   intended recipient) that the sending agent knows how to encrypt.  The   sending agent SHOULD use one of the capabilities in the list if the   agent reasonably expects the recipient to be able to decrypt the   message.2.7.1.2.  Rule 2: Unknown Capabilities, Unknown Version of S/MIME   If the following two conditions are met:      - the sending agent has no knowledge of the encryption        capabilities of the recipient, and      - the sending agent has no knowledge of the version of S/MIME of        the recipient,Ramsdell & Turner            Standards Track                   [Page 17]

RFC 5751            S/MIME 3.2 Message Specification        January 2010   then the sending agent SHOULD use AES-128 because it is a stronger   algorithm and is required by S/MIME v3.2.  If the sending agent   chooses not to use AES-128 in this step, it SHOULD use tripleDES.2.7.2.  Choosing Weak Encryption   All algorithms that use 40-bit keys are considered by many to be weak   encryption.  A sending agent that is controlled by a human SHOULD   allow a human sender to determine the risks of sending data using a   weak encryption algorithm before sending the data, and possibly allow   the human to use a stronger encryption method such as tripleDES or   AES.2.7.3.  Multiple Recipients   If a sending agent is composing an encrypted message to a group of   recipients where the encryption capabilities of some of the   recipients do not overlap, the sending agent is forced to send more   than one message.  Please note that if the sending agent chooses to   send a message encrypted with a strong algorithm, and then send the   same message encrypted with a weak algorithm, someone watching the   communications channel could learn the contents of the strongly   encrypted message simply by decrypting the weakly encrypted message.3.  Creating S/MIME Messages   This section describes the S/MIME message formats and how they are   created.  S/MIME messages are a combination of MIME bodies and CMS   content types.  Several media types as well as several CMS content   types are used.  The data to be secured is always a canonical MIME   entity.  The MIME entity and other data, such as certificates and   algorithm identifiers, are given to CMS processing facilities that   produce a CMS object.  Finally, the CMS object is wrapped in MIME.   The Enhanced Security Services for S/MIME [ESS] document provides   descriptions of how nested, secured S/MIME messages are formatted.   ESS provides a description of how a triple-wrapped S/MIME message is   formatted using multipart/signed and application/pkcs7-mime for the   signatures.   S/MIME provides one format for enveloped-only data, several formats   for signed-only data, and several formats for signed and enveloped   data.  Several formats are required to accommodate several   environments, in particular for signed messages.  The criteria for   choosing among these formats are also described.   The reader of this section is expected to understand MIME as   described in [MIME-SPEC] and [MIME-SECURE].Ramsdell & Turner            Standards Track                   [Page 18]

RFC 5751            S/MIME 3.2 Message Specification        January 20103.1.  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 theRFC-822   header.  Note that S/MIME can also be used to secure MIME entities   used in applications other than Internet mail.  If protection of theRFC-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 Sections3.2,3.4, and elsewhere.   The procedure for preparing a MIME entity is given in [MIME-SPEC].   The same procedure is used here with some additional restrictions   when signing.  The description of the procedures from [MIME-SPEC] is   repeated here, but it is suggested that the reader refer to that   document for the exact procedure.  This section also describes   additional requirements.   A single procedure is used for creating MIME entities that are to   have any combination of signing, enveloping, and compressing applied.   Some additional steps are recommended to defend against known   corruptions that can occur during mail transport that are of   particular importance for clear-signing using the multipart/signed   format.  It is recommended that these additional steps be performed   on enveloped messages, or signed and enveloped messages, so that the   message can be forwarded to any environment without modification.   These steps are descriptive rather than prescriptive.  The   implementer is free to use any procedure as long as the result is the   same.   Step 1.  The MIME entity is prepared according to the local            conventions.   Step 2.  The leaf parts of the MIME entity are converted to canonical            form.   Step 3.  Appropriate transfer encoding is applied to the leaves of            the MIME entity.Ramsdell & Turner            Standards Track                   [Page 19]

RFC 5751            S/MIME 3.2 Message Specification        January 2010   When an S/MIME message is received, the security services on the   message are processed, and the result is the MIME entity.  That MIME   entity is typically passed to a MIME-capable user agent where it is   further decoded and presented to the user or receiving application.   In order to protect outer, non-content-related message header fields   (for instance, the "Subject", "To", "From", and "Cc" fields), the   sending client MAY wrap a full MIME message in a message/rfc822   wrapper in order to apply S/MIME security services to these header   fields.  It is up to the receiving client to decide how to present   this "inner" header along with the unprotected "outer" header.   When an S/MIME message is received, if the top-level protected MIME   entity has a Content-Type of message/rfc822, it can be assumed that   the intent was to provide header protection.  This entity SHOULD be   presented as the top-level message, taking into account header   merging issues as previously discussed.3.1.1.  Canonicalization   Each MIME entity MUST be converted to a canonical form that is   uniquely and unambiguously representable in the environment where the   signature is created and the environment where the signature will be   verified.  MIME entities MUST be canonicalized for enveloping and   compressing as well as signing.   The exact details of canonicalization depend on the actual media type   and subtype of an entity, and are not described here.  Instead, the   standard for the particular media type SHOULD be consulted.  For   example, canonicalization of type text/plain is different from   canonicalization of audio/basic.  Other than text types, most types   have only one representation regardless of computing platform or   environment that can be considered their canonical representation.   In general, canonicalization will be performed by the non-security   part of the sending agent rather than the S/MIME implementation.   The most common and important canonicalization is for text, which is   often represented differently in different environments.  MIME   entities of major type "text" MUST have both their line endings and   character set canonicalized.  The line ending MUST be the pair of   characters <CR><LF>, and the charset SHOULD be a registered charset   [CHARSETS].  The details of the canonicalization are specified in   [MIME-SPEC].   Note that some charsets such as ISO-2022 have multiple   representations for the same characters.  When preparing such text   for signing, the canonical representation specified for the charset   MUST be used.Ramsdell & Turner            Standards Track                   [Page 20]

RFC 5751            S/MIME 3.2 Message Specification        January 20103.1.2.  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   inSection 3.1.3 for all MIME entities they secure.  The reason for   securing only 7-bit MIME entities, even for enveloped data that are   not exposed to the transport, is that it allows the MIME entity to be   handled in any environment without changing it.  For example, a   trusted gateway might remove the envelope, but not the signature, of   a message, and then forward the signed message on to the end   recipient so that they can verify the signatures directly.  If the   transport internal to the site is not 8-bit clean, such as on a wide-   area network with a single mail gateway, verifying the signature will   not be possible unless the original MIME entity was only 7-bit data.   S/MIME implementations that "know" that all intended recipients are   capable of handling inner (all but the outermost) binary MIME objects   SHOULD use binary encoding as opposed to a 7-bit-safe transfer   encoding for the inner entities.  The use of a 7-bit-safe encoding   (such as base64) would unnecessarily expand the message size.   Implementations MAY "know" that recipient implementations are capable   of handling inner binary MIME entities either by interpreting the id-   cap-preferBinaryInside SMIMECapabilities attribute, by prior   agreement, or by other means.   If one or more intended recipients are unable to handle inner binary   MIME objects, or if this capability is unknown for any of the   intended recipients, S/MIME implementations SHOULD use transfer   encoding described inSection 3.1.3 for all MIME entities they   secure.3.1.3.  Transfer Encoding for Signing Using multipart/signed   If a multipart/signed entity is ever to be transmitted over the   standard Internet SMTP infrastructure or other transport that is   constrained to 7-bit text, it MUST have transfer encoding applied so   that it is represented as 7-bit text.  MIME entities that are 7-bit   data already need no transfer encoding.  Entities such as 8-bit text   and binary data can be encoded with quoted-printable or base-64   transfer encoding.Ramsdell & Turner            Standards Track                   [Page 21]

RFC 5751            S/MIME 3.2 Message Specification        January 2010   The primary reason for the 7-bit requirement is that the Internet   mail transport infrastructure cannot guarantee transport of 8-bit or   binary data.  Even though many segments of the transport   infrastructure now handle 8-bit and even binary data, it is sometimes   not possible to know whether the transport path is 8-bit clean.  If a   mail message with 8-bit data were to encounter a message transfer   agent that cannot transmit 8-bit or binary data, the agent has three   options, none of which are acceptable for a clear-signed message:    - The agent could change the transfer encoding; this would      invalidate the signature.    - The agent could transmit the data anyway, which would most likely      result in the 8th bit being corrupted; this too would invalidate      the signature.    - The agent could return the message to the sender.   [MIME-SECURE] prohibits an agent from changing the transfer encoding   of the first part of a multipart/signed message.  If a compliant   agent that cannot transmit 8-bit or binary data encounters a   multipart/signed message with 8-bit or binary data in the first part,   it would have to return the message to the sender as undeliverable.3.1.4.  Sample Canonical MIME Entity   This example shows a multipart/mixed message with full transfer   encoding.  This message contains a text part and an attachment.  The   sample message text includes characters that are not US-ASCII and   thus need to be transfer encoded.  Though not shown here, the end of   each line is <CR><LF>.  The line ending of the MIME headers, the   text, and the transfer encoded parts, all MUST be <CR><LF>.   Note that this example is not of an S/MIME message.      Content-Type: multipart/mixed; boundary=bar      --bar      Content-Type: text/plain; charset=iso-8859-1      Content-Transfer-Encoding: quoted-printable      =A1Hola Michael!      How do you like the new S/MIME specification?      It's generally a good idea to encode lines that begin with      From=20because some mail transport agents will insert a greater-      than (>) sign, thus invalidating the signature.Ramsdell & Turner            Standards Track                   [Page 22]

RFC 5751            S/MIME 3.2 Message Specification        January 2010      Also, in some cases it might be desirable to encode any =20      trailing whitespace that occurs on lines in order to ensure =20      that the message signature is not invalidated when passing =20      a gateway that modifies such whitespace (like BITNET). =20      --bar      Content-Type: image/jpeg      Content-Transfer-Encoding: base64      iQCVAwUBMJrRF2N9oWBghPDJAQE9UQQAtl7LuRVndBjrk4EqYBIb3h5QXIX/LC//      jJV5bNvkZIGPIcEmI5iFd9boEgvpirHtIREEqLQRkYNoBActFBZmh9GC3C041WGq      uMbrbxc+nIs1TIKlA08rVi9ig/2Yh7LFrK5Ein57U/W72vgSxLhe/zhdfolT9Brn      HOxEa44b+EI=      --bar--3.2.  The application/pkcs7-mime Media Type   The application/pkcs7-mime media type is used to carry CMS content   types including EnvelopedData, SignedData, and CompressedData.  The   details of constructing these entities are described in subsequent   sections.  This section describes the general characteristics of the   application/pkcs7-mime media type.   The carried CMS object always contains a MIME entity that is prepared   as described inSection 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 inSection 3.1.   Because there are several types of application/pkcs7-mime objects, a   sending agent SHOULD do as much as possible to help a receiving agent   know about the contents of the object without forcing the receiving   agent to decode the ASN.1 for the object.  The Content-Type header   field of all application/pkcs7-mime objects SHOULD include the   optional "smime-type" parameter, as described in the following   sections.Ramsdell & Turner            Standards Track                   [Page 23]

RFC 5751            S/MIME 3.2 Message Specification        January 20103.2.1.  The name and filename Parameters   For the application/pkcs7-mime, sending agents SHOULD emit the   optional "name" parameter to the Content-Type field for compatibility   with older systems.  Sending agents SHOULD also emit the optional   Content-Disposition field [CONTDISP] with the "filename" parameter.   If a sending agent emits the above parameters, the value of the   parameters SHOULD be a file name with the appropriate extension:   Media Type                                            File Extension     application/pkcs7-mime (SignedData, EnvelopedData)      .p7m     application/pkcs7-mime (degenerate SignedData           .p7c        certificate management message)     application/pkcs7-mime (CompressedData)                 .p7z     application/pkcs7-signature (SignedData)                .p7s   In addition, the file name SHOULD be limited to eight characters   followed by a three-letter extension.  The eight-character filename   base can be any distinct name; the use of the filename base "smime"   SHOULD be used to indicate that the MIME entity is associated with   S/MIME.   Including a file name serves two purposes.  It facilitates easier use   of S/MIME objects as files on disk.  It also can convey type   information across gateways.  When a MIME entity of type   application/pkcs7-mime (for example) arrives at a gateway that has no   special knowledge of S/MIME, it will default the entity's media type   to application/octet-stream and treat it as a generic attachment,   thus losing the type information.  However, the suggested filename   for an attachment is often carried across a gateway.  This often   allows the receiving systems to determine the appropriate application   to hand the attachment off to, in this case, a stand-alone S/MIME   processing application.  Note that this mechanism is provided as a   convenience for implementations in certain environments.  A proper   S/MIME implementation MUST use the media types and MUST NOT rely on   the file extensions.3.2.2.  The smime-type Parameter   The application/pkcs7-mime content type defines the optional "smime-   type" parameter.  The intent of this parameter is to convey details   about the security applied (signed or enveloped) along with   information about the contained content.  This specification defines   the following smime-types.Ramsdell & Turner            Standards Track                   [Page 24]

RFC 5751            S/MIME 3.2 Message Specification        January 2010      Name                   CMS Type                Inner Content      enveloped-data         EnvelopedData           id-data      signed-data            SignedData              id-data      certs-only             SignedData              none      compressed-data        CompressedData          id-data   In order for consistency to be obtained with future specifications,   the following guidelines SHOULD be followed when assigning a new   smime-type parameter.      1. If both signing and encryption can be applied to the content,         then two values for smime-type SHOULD be assigned "signed-*"         and "enveloped-*".  If one operation can be assigned, then this         can be omitted.  Thus, since "certs-only" can only be signed,         "signed-" is omitted.      2. A common string for a content OID SHOULD be assigned.  We use         "data" for the id-data content OID when MIME is the inner         content.      3. If no common string is assigned, then the common string of         "OID.<oid>" is recommended (for example,         "OID.2.16.840.1.101.3.4.1.2" would be AES-128 CBC).   It is explicitly intended that this field be a suitable hint for mail   client applications to indicate whether a message is "signed" or   "enveloped" without having to tunnel into the CMS payload.3.3.  Creating an Enveloped-Only Message   This section describes the format for enveloping a MIME entity   without signing it.  It is important to note that sending enveloped   but not signed messages does not provide for data integrity.  It is   possible to replace ciphertext in such a way that the processed   message will still be valid, but the meaning can be altered.   Step 1.  The MIME entity to be enveloped is prepared according toSection 3.1.   Step 2.  The MIME entity and other required data is processed into a            CMS object of type EnvelopedData.  In addition to encrypting            a copy of the content-encryption key for each recipient, a            copy of the content-encryption key SHOULD be encrypted for            the originator and included in the EnvelopedData (see [CMS],            Section 6).   Step 3.  The EnvelopedData object is wrapped in a CMS ContentInfo            object.Ramsdell & Turner            Standards Track                   [Page 25]

RFC 5751            S/MIME 3.2 Message Specification        January 2010   Step 4.  The ContentInfo object is inserted into an            application/pkcs7-mime MIME entity.   The smime-type parameter for enveloped-only messages is "enveloped-   data".  The file extension for this type of message is ".p7m".   A sample message would be:      Content-Type: application/pkcs7-mime; smime-type=enveloped-data;           name=smime.p7m      Content-Transfer-Encoding: base64      Content-Disposition: attachment; filename=smime.p7m      rfvbnj756tbBghyHhHUujhJhjH77n8HHGT9HG4VQpfyF467GhIGfHfYT6      7n8HHGghyHhHUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H      f8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4      0GhIGfHfQbnj756YT64V3.4.  Creating a Signed-Only Message   There are two formats for signed messages defined for S/MIME:      - application/pkcs7-mime with SignedData.      - multipart/signed.   In general, the multipart/signed form is preferred for sending, and   receiving agents MUST be able to handle both.3.4.1.  Choosing a Format for Signed-Only Messages   There are no hard-and-fast rules as to when a particular signed-only   format is chosen.  It depends on the capabilities of all the   receivers and the relative importance of receivers with S/MIME   facilities being able to verify the signature versus the importance   of receivers without S/MIME software being able to view the message.   Messages signed using the multipart/signed format can always be   viewed by the receiver whether or not they have S/MIME software.   They can also be viewed whether they are using a MIME-native user   agent or they have messages translated by a gateway.  In this   context, "be viewed" means the ability to process the message   essentially as if it were not a signed message, including any other   MIME structure the message might have.Ramsdell & Turner            Standards Track                   [Page 26]

RFC 5751            S/MIME 3.2 Message Specification        January 2010   Messages signed using the SignedData format cannot be viewed by a   recipient unless they have S/MIME facilities.  However, the   SignedData format protects the message content from being changed by   benign intermediate agents.  Such agents might do line wrapping or   content-transfer encoding changes that would break the signature.3.4.2.  Signing Using application/pkcs7-mime with SignedData   This signing format uses the application/pkcs7-mime media type.  The   steps to create this format are:   Step 1.  The MIME entity is prepared according toSection 3.1.   Step 2.  The MIME entity and other required data are processed into a            CMS object of type SignedData.   Step 3.  The SignedData object is wrapped in a CMS ContentInfo            object.   Step 4.  The ContentInfo object is inserted into an            application/pkcs7-mime MIME entity.   The smime-type parameter for messages using application/pkcs7-mime   with SignedData is "signed-data".  The file extension for this type   of message is ".p7m".   A sample message would be:      Content-Type: application/pkcs7-mime; smime-type=signed-data;           name=smime.p7m      Content-Transfer-Encoding: base64      Content-Disposition: attachment; filename=smime.p7m      567GhIGfHfYT6ghyHhHUujpfyF4f8HHGTrfvhJhjH776tbB9HG4VQbnj7      77n8HHGT9HG4VQpfyF467GhIGfHfYT6rfvbnj756tbBghyHhHUujhJhjH      HUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H7n8HHGghyHh      6YT64V0GhIGfHfQbnj753.4.3.  Signing Using the multipart/signed Format   This format is a clear-signing format.  Recipients without any S/MIME   or CMS processing facilities are able to view the message.  It makes   use of the multipart/signed media type described in [MIME-SECURE].   The multipart/signed media type has two parts.  The first part   contains the MIME entity that is signed; the second part contains the   "detached signature" CMS SignedData object in which the   encapContentInfo eContent field is absent.Ramsdell & Turner            Standards Track                   [Page 27]

RFC 5751            S/MIME 3.2 Message Specification        January 20103.4.3.1.  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 toSection 3.1, taking special care for clear-signing.   Step 2.  The MIME entity is presented to CMS processing in order to            obtain an object of type SignedData in which the            encapContentInfo eContent field is absent.   Step 3.  The MIME entity is inserted into the first part of a            multipart/signed message with no processing other than that            described inSection 3.1.   Step 4.  Transfer encoding is applied to the "detached signature" CMS            SignedData object, and it is inserted into a MIME entity of            type application/pkcs7-signature.   Step 5.  The MIME entity of the application/pkcs7-signature is            inserted into the second part of the multipart/signed            entity.   The multipart/signed Content-Type has two required parameters: the   protocol parameter and the micalg parameter.   The protocol parameter MUST be "application/pkcs7-signature".  Note   that quotation marks are required around the protocol parameter   because MIME requires that the "/" character in the parameter value   MUST be quoted.   The micalg parameter allows for one-pass processing when the   signature is being verified.  The value of the micalg parameter is   dependent on the message digest algorithm(s) used in the calculation   of the Message Integrity Check.  If multiple message digest   algorithms are used, they MUST be separated by commas per [MIME-   SECURE].  The values to be placed in the micalg parameter SHOULD be   from the following:Ramsdell & Turner            Standards Track                   [Page 28]

RFC 5751            S/MIME 3.2 Message Specification        January 2010      Algorithm   Value Used      MD5         md5      SHA-1       sha-1      SHA-224     sha-224      SHA-256     sha-256      SHA-384     sha-384      SHA-512     sha-512      Any other   (defined separately in algorithm profile or "unknown"                   if not defined)   (Historical note: some early implementations of S/MIME emitted and   expected "rsa-md5", "rsa-sha1", and "sha1" for the micalg parameter.)   Receiving agents SHOULD be able to recover gracefully from a micalg   parameter value that they do not recognize.  Future names for this   parameter will be consistent with the IANA "Hash Function Textual   Names" registry.3.4.3.3.  Sample multipart/signed Message       Content-Type: multipart/signed;          protocol="application/pkcs7-signature";          micalg=sha1; boundary=boundary42       --boundary42       Content-Type: text/plain       This is a clear-signed message.       --boundary42       Content-Type: application/pkcs7-signature; name=smime.p7s       Content-Transfer-Encoding: base64       Content-Disposition: attachment; filename=smime.p7s       ghyHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6       4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj       n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4       7GhIGfHfYT64VQbnj756      --boundary42--   The content that is digested (the first part of the multipart/signed)   consists of the bytes:   43 6f 6e 74 65 6e 74 2d 54 79 70 65 3a 20 74 65 78 74 2f 70 6c 61 69   6e 0d 0a 0d 0a 54 68 69 73 20 69 73 20 61 20 63 6c 65 61 72 2d 73 69   67 6e 65 64 20 6d 65 73 73 61 67 65 2e 0d 0aRamsdell & Turner            Standards Track                   [Page 29]

RFC 5751            S/MIME 3.2 Message Specification        January 20103.5.  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 toSection 3.1.   Step 2.  The MIME entity and other required data are processed into a            CMS object of type CompressedData.   Step 3.  The CompressedData object is wrapped in a CMS ContentInfo            object.   Step 4.  The ContentInfo object is inserted into an            application/pkcs7-mime MIME entity.   The smime-type parameter for compressed-only messages is "compressed-   data".  The file extension for this type of message is ".p7z".   A sample message would be:   Content-Type: application/pkcs7-mime; smime-type=compressed-data;      name=smime.p7z   Content-Transfer-Encoding: base64   Content-Disposition: attachment; filename=smime.p7z   rfvbnj756tbBghyHhHUujhJhjH77n8HHGT9HG4VQpfyF467GhIGfHfYT6   7n8HHGghyHhHUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H   f8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4   0GhIGfHfQbnj756YT64V3.6.  Multiple Operations   The signed-only, enveloped-only, and compressed-only MIME formats can   be nested.  This works because these formats are all MIME entities   that encapsulate other MIME entities.   An S/MIME implementation MUST be able to receive and process   arbitrarily nested S/MIME within reasonable resource limits of the   recipient computer.Ramsdell & Turner            Standards Track                   [Page 30]

RFC 5751            S/MIME 3.2 Message Specification        January 2010   It is possible to apply any of the signing, encrypting, and   compressing operations in any order.  It is up to the implementer and   the user to choose.  When signing first, the signatories are then   securely obscured by the enveloping.  When enveloping first the   signatories are exposed, but it is possible to verify signatures   without removing the enveloping.  This can be useful in an   environment where automatic signature verification is desired, as no   private key material is required to verify a signature.   There are security ramifications to choosing whether to sign first or   encrypt first.  A recipient of a message that is encrypted and then   signed can validate that the encrypted block was unaltered, but   cannot determine any relationship between the signer and the   unencrypted contents of the message.  A recipient of a message that   is signed then encrypted can assume that the signed message itself   has not been altered, but that a careful attacker could have changed   the unauthenticated portions of the encrypted message.   When using compression, keep the following guidelines in mind:      - Compression of binary encoded encrypted data is discouraged,        since it will not yield significant compression.  Base64        encrypted data could very well benefit, however.      - If a lossy compression algorithm is used with signing, you will        need to compress first, then sign.3.7.  Creating a Certificate Management Message   The certificate management message or MIME entity is used to   transport certificates and/or Certificate Revocation Lists, such as   in response to a registration request.   Step 1.  The certificates and/or Certificate Revocation Lists are            made available to the CMS generating process that creates a            CMS object of type SignedData.  The SignedData            encapContentInfo eContent field MUST be absent and            signerInfos field MUST be empty.   Step 2.  The SignedData object is wrapped in a CMS ContentInfo            object.   Step 3.  The ContentInfo object is enclosed in an            application/pkcs7-mime MIME entity.   The smime-type parameter for a certificate management message is   "certs-only".  The file extension for this type of message is ".p7c".Ramsdell & Turner            Standards Track                   [Page 31]

RFC 5751            S/MIME 3.2 Message Specification        January 20103.8.  Registration Requests   A sending agent that signs messages MUST have a certificate for the   signature so that a receiving agent can verify the signature.  There   are many ways of getting certificates, such as through an exchange   with a certification authority, through a hardware token or diskette,   and so on.   S/MIME v2 [SMIMEv2] specified a method for "registering" public keys   with certificate authorities using an application/pkcs10 body part.   Since that time, the IETF PKIX Working Group has developed other   methods for requesting certificates.  However, S/MIME v3.2 does not   require a particular certificate request mechanism.3.9.  Identifying an S/MIME Message   Because S/MIME takes into account interoperation in non-MIME   environments, several different mechanisms are employed to carry the   type information, and it becomes a bit difficult to identify S/MIME   messages.  The following table lists criteria for determining whether   or not a message is an S/MIME message.  A message is considered an   S/MIME message if it matches any of the criteria listed below.   The file suffix in the table below comes from the "name" parameter in   the Content-Type header field, or the "filename" parameter on the   Content-Disposition header field.  These parameters that give the   file suffix are not listed below as part of the parameter section.   Media type:  application/pkcs7-mime   parameters:  any   file suffix: any   Media type:  multipart/signed   parameters:  protocol="application/pkcs7-signature"   file suffix: any   Media type:  application/octet-stream   parameters:  any   file suffix: p7m, p7s, p7c, p7z4.  Certificate Processing   A receiving agent MUST provide some certificate retrieval mechanism   in order to gain access to certificates for recipients of digital   envelopes.  This specification does not cover how S/MIME agents   handle certificates, only what they do after a certificate has been   validated or rejected.  S/MIME certificate issues are covered in   [CERT32].Ramsdell & Turner            Standards Track                   [Page 32]

RFC 5751            S/MIME 3.2 Message Specification        January 2010   At a minimum, for initial S/MIME deployment, a user agent could   automatically generate a message to an intended recipient requesting   that recipient's certificate in a signed return message.  Receiving   and sending agents SHOULD also provide a mechanism to allow a user to   "store and protect" certificates for correspondents in such a way so   as to guarantee their later retrieval.4.1.  Key Pair Generation   All generated key pairs MUST be generated from a good source of non-   deterministic random input [RANDOM] and the private key MUST be   protected in a secure fashion.   An S/MIME user agent MUST NOT generate asymmetric keys less than 512   bits for use with the RSA or DSA signature algorithms.   For 512-bit RSA with SHA-1 see [CMSALG] and [FIPS186-2] without   Change Notice 1, for 512-bit RSA with SHA-256 see [CMS-SHA2] and   [FIPS186-2] without Change Notice 1, and for 1024-bit through   2048-bit RSA with SHA-256 see [CMS-SHA2] and [FIPS186-2] with Change   Notice 1.  The first reference provides the signature algorithm's   object identifier, and the second provides the signature algorithm's   definition.   For 512-bit DSA with SHA-1 see [CMSALG] and [FIPS186-2] without   Change Notice 1, for 512-bit DSA with SHA-256 see [CMS-SHA2] and   [FIPS186-2] without Change Notice 1, for 1024-bit DSA with SHA-1 see   [CMSALG] and [FIPS186-2] with Change Notice 1, for 1024-bit and above   DSA with SHA-256 see [CMS-SHA2] and [FIPS186-3].  The first reference   provides the signature algorithm's object identifier and the second   provides the signature algorithm's definition.   For RSASSA-PSS with SHA-256, see [RSAPSS].  For 1024-bit DH, see   [CMSALG].  For 1024-bit and larger DH, see [SP800-56A]; regardless,   use the KDF, which is from X9.42, specified in [CMSALG].  For RSAES-   OAEP, see [RSAOAEP].4.2.  Signature Generation   The following are the requirements for an S/MIME agent generated RSA,   RSASSA-PSS, and DSA signatures:           key size <= 1023 : SHOULD NOT (see Security Considerations)   1024 <= key size <= 2048 : SHOULD     (see Security Considerations)   2048 <  key size         : MAY        (see Security Considerations)Ramsdell & Turner            Standards Track                   [Page 33]

RFC 5751            S/MIME 3.2 Message Specification        January 20104.3.  Signature Verification   The following are the requirements for S/MIME receiving agents during   signature verification of RSA, RSASSA-PSS, and DSA signatures:           key size <= 1023 : MAY        (see Security Considerations)   1024 <= key size <= 2048 : MUST       (see Security Considerations)   2048 <  key size         : MAY        (see Security Considerations)4.4.  Encryption   The following are the requirements for an S/MIME agent when   establishing keys for content encryption using the RSA, RSA-OAEP, and   DH algorithms:           key size <= 1023 : SHOULD NOT (see Security Considerations)   1024 <= key size <= 2048 : SHOULD     (see Security Considerations)   2048 <  key size         : MAY        (see Security Considerations)4.5.  Decryption   The following are the requirements for an S/MIME agent when   establishing keys for content decryption using the RSA, RSAES-OAEP,   and DH algorithms:           key size <= 1023 : MAY        (see Security Considerations)   1024 <= key size <= 2048 : MUST       (see Security Considerations)   2048 <  key size         : MAY        (see Security Considerations)5.  IANA Considerations   The following information updates the media type registration for   application/pkcs7-mime and application/pkcs7-signature to refer to   this document as opposed toRFC 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: NONERamsdell & Turner            Standards Track                   [Page 34]

RFC 5751            S/MIME 3.2 Message Specification        January 2010   Optional Parameters: smime-type/signed-data                        smime-type/enveloped-data                        smime-type/compressed-data                        smime-type/certs-only                        name   Encoding Considerations: SeeSection 3 of this document   Security Considerations: SeeSection 6 of this document   Interoperability Considerations: See Sections1-6 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 IESG5.2.  Media Type for application/pkcs7-signature   Type name: application   Subtype Name: pkcs7-signature   Required Parameters: NONE   Optional Parameters: NONE   Encoding Considerations: SeeSection 3 of this document   Security Considerations: SeeSection 6 of this document   Interoperability Considerations: See Sections1-6 of this document   Published Specification:RFC 2311,RFC 2633, and this document   Applications that use this media type: Security applicationsRamsdell & Turner            Standards Track                   [Page 35]

RFC 5751            S/MIME 3.2 Message Specification        January 2010   Additional information: NONE   Person & email to contact for further information:      S/MIME working group chairs smime-chairs@tools.ietf.org   Intended usage: COMMON   Restrictions on usage: NONE   Author: Sean Turner   Change Controller: S/MIME working group delegated from the IESG6.  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 inRFC 3218 [MMA].      - The Diffie-Hellman "small-subgroup" attacks described inRFC2785 [DHSUB].      - The attacks against hash algorithms described inRFC 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 thisRamsdell & Turner            Standards Track                   [Page 36]

RFC 5751            S/MIME 3.2 Message Specification        January 2010   specification seem appropriate for the Internet based on [STRENGTH].   Of course, there are environments, such as financial and medical   systems, that may select different key sizes.  For this reason, an   implementation MAY support key sizes beyond those recommended in this   specification.   Receiving agents that validate signatures and sending agents that   encrypt messages need to be cautious of cryptographic processing   usage when validating signatures and encrypting messages using keys   larger than those mandated in this specification.  An attacker could   send certificates with keys that would result in excessive   cryptographic processing, for example, keys larger than those   mandated in this specification, which could swamp the processing   element.  Agents that use such keys without first validating the   certificate to a trust anchor are advised to have some sort of   cryptographic resource management system to prevent such attacks.   Using weak cryptography in S/MIME offers little actual security over   sending plaintext.  However, other features of S/MIME, such as the   specification of AES and the ability to announce stronger   cryptographic capabilities to parties with whom you communicate,   allow senders to create messages that use strong encryption.  Using   weak cryptography is never recommended unless the only alternative is   no cryptography.   RSA and DSA keys of less than 1024 bits are now considered by many   experts to be cryptographically insecure (due to advances in   computing power), and should no longer be used to protect messages.   Such keys were previously considered secure, so processing previously   received signed and encrypted mail will often result in the use of   weak keys.  Implementations that wish to support previous versions of   S/MIME or process old messages need to consider the security risks   that result from smaller key sizes (e.g., spoofed messages) versus   the costs of denial of service.  If an implementation supports   verification of digital signatures generated with RSA and DSA keys of   less than 1024 bits, it MUST warn the user.  Implementers should   consider providing different warnings for newly received messages and   previously stored messages.  Server implementations (e.g., secure   mail list servers) where user warnings are not appropriate SHOULD   reject messages with weak signatures.   Implementers SHOULD be aware that multiple active key pairs can be   associated with a single individual.  For example, one key pair can   be used to support confidentiality, while a different key pair can be   used for digital signatures.Ramsdell & Turner            Standards Track                   [Page 37]

RFC 5751            S/MIME 3.2 Message Specification        January 2010   If a sending agent is sending the same message using different   strengths of cryptography, an attacker watching the communications   channel might be able to determine the contents of the strongly   encrypted message by decrypting the weakly encrypted version.  In   other words, a sender SHOULD NOT send a copy of a message using   weaker cryptography than they would use for the original of the   message.   Modification of the ciphertext can go undetected if authentication is   not also used, which is the case when sending EnvelopedData without   wrapping it in SignedData or enclosing SignedData within it.   If an implementation is concerned about compliance with National   Institute of Standards and Technology (NIST) key size   recommendations, then see [SP800-57].   If messaging environments make use of the fact that a message is   signed to change the behavior of message processing (examples would   be running rules or UI display hints), without first verifying that   the message is actually signed and knowing the state of the   signature, this can lead to incorrect handling of the message.   Visual indicators on messages may need to have the signature   validation code checked periodically if the indicator is supposed to   give information on the current status of a message.7.  References7.1.  Reference Conventions   [CMS] refers to [RFC5652].   [ESS] refers to [RFC2634] and [RFC5035].   [MIME] refers to [RFC2045], [RFC2046],  [RFC2047], [RFC2049],   [RFC4288], and [RFC4289].   [SMIMEv2] refers to [RFC2311], [RFC2312], [RFC2313], [RFC2314], and   [RFC2315].   [SMIMEv3] refers to [RFC2630], [RFC2631], [RFC2632], [RFC2633],   [RFC2634], and [RFC5035].   [SMIMv3.1] refers to [RFC2634], [RFC3850], [RFC3851], [RFC3852], and   [RFC5035].Ramsdell & Turner            Standards Track                   [Page 38]

RFC 5751            S/MIME 3.2 Message Specification        January 20107.2.  Normative References   [CERT32]      Ramsdell, B. and S. Turner, "Secure/Multipurpose                 Internet Mail Extensions (S/MIME) Version 3.2                 Certificate Handling",RFC 5750, January 2010.   [CHARSETS]    Character sets assigned by IANA.  Seehttp://www.iana.org/assignments/character-sets.   [CMSAES]      Schaad, J., "Use of the Advanced Encryption Standard                 (AES) Encryption Algorithm in Cryptographic Message                 Syntax (CMS)",RFC 3565, July 2003.   [CMSALG]      Housley, R., "Cryptographic Message Syntax (CMS)                 Algorithms",RFC 3370, August 2002.   [CMSCOMPR]    Gutmann, P., "Compressed Data Content Type for                 Cryptographic Message Syntax (CMS)",RFC 3274, June                 2002.   [CMS-SHA2]    Turner, S., "Using SHA2 Algorithms with Cryptographic                 Message Syntax",RFC 5754, January 2010.   [CONTDISP]    Troost, R., Dorner, S., and K. Moore, Ed.,                 "Communicating Presentation Information in Internet                 Messages: The Content-Disposition Header Field",RFC2183, August 1997.   [FIPS186-2]   National Institute of Standards and Technology (NIST),                 "Digital Signature Standard (DSS)", FIPS Publication                 186-2, January 2000. [With Change Notice 1].   [FIPS186-3]   National Institute of Standards and Technology (NIST),                 FIPS Publication 186-3: Digital Signature Standard,                 June 2009.   [MIME-SECURE] Galvin, J., Murphy, S., Crocker, S., and N. Freed,                 "Security Multiparts for MIME: Multipart/Signed and                 Multipart/Encrypted",RFC 1847, October 1995.   [MUSTSHOULD]  Bradner, S., "Key words for use in RFCs to Indicate                 Requirement Levels",BCP 14,RFC 2119, March 1997.   [RANDOM]      Eastlake, D., 3rd, Schiller, J., and S. Crocker,                 "Randomness Requirements for Security",BCP 106,RFC4086, June 2005.Ramsdell & Turner            Standards Track                   [Page 39]

RFC 5751            S/MIME 3.2 Message Specification        January 2010   [RFC2045]     Freed, N. and N. Borenstein, "Multipurpose Internet                 Mail Extensions (MIME) Part One: Format of Internet                 Message Bodies",RFC 2045, November 1996.   [RFC2046]     Freed, N. and N. Borenstein, "Multipurpose Internet                 Mail Extensions (MIME) Part Two: Media Types",RFC2046, November 1996.   [RFC2047]     Moore, K., "MIME (Multipurpose Internet Mail                 Extensions) Part Three: Message Header Extensions for                 Non-ASCII Text",RFC 2047, November 1996.   [RFC2049]     Freed, N. and N. Borenstein, "Multipurpose Internet                 Mail Extensions (MIME) Part Five: Conformance Criteria                 and Examples",RFC 2049, November 1996.   [RFC2634]     Hoffman, P. Ed., "Enhanced Security Services for                 S/MIME",RFC 2634, June 1999.   [RFC4288]     Freed, N. and J. Klensin, "Media Type Specifications                 and Registration Procedures",BCP 13,RFC 4288,                 December 2005.   [RFC4289]     Freed, N. and J. Klensin, "Multipurpose Internet Mail                 Extensions (MIME) Part Four: Registration Procedures",BCP 13,RFC 4289, December 2005.   [RFC5035]     Schaad, J., "Enhanced Security Services (ESS) Update:                 Adding CertID Algorithm Agility",RFC 5035, August                 2007.   [RFC5652]     Housley, R., "Cryptographic Message Syntax (CMS)",RFC5652, September 2009.   [RSAOAEP]     Housley, R. "Use of the RSAES-OAEP Key Transport                 Algorithm in the Cryptographic Message Syntax (CMS)",RFC 3560, July 2003.   [RSAPSS]      Schaad, J., "Use of the RSASSA-PSS Signature Algorithm                 in Cryptographic Message Syntax (CMS)",RFC 4056, June                 2005.   [SP800-56A]   National Institute of Standards and Technology (NIST),                 Special Publication 800-56A: Recommendation Pair-Wise                 Key Establishment Schemes Using Discrete Logarithm                 Cryptography (Revised), March 2007.Ramsdell & Turner            Standards Track                   [Page 40]

RFC 5751            S/MIME 3.2 Message Specification        January 2010   [X.680]       ITU-T Recommendation X.680 (2002) | ISO/IEC                 8824-1:2002. Information Technology - Abstract Syntax                 Notation One (ASN.1):  Specification of basic notation.   [X.690]       ITU-T Recommendation X.690 (2002) | ISO/IEC                 8825-1:2002.  Information Technology - ASN.1 encoding                 rules: Specification of Basic Encoding Rules (BER),                 Canonical Encoding Rules (CER) and Distinguished                 Encoding Rules (DER).7.3.  Informative References   [DHSUB]       Zuccherato, R., "Methods for Avoiding the "Small-                 Subgroup" Attacks on the Diffie-Hellman Key Agreement                 Method for S/MIME",RFC 2785, March 2000.   [HASH-ATTACK] Hoffman, P. and B. Schneier, "Attacks on Cryptographic                 Hashes in Internet Protocols",RFC 4270, November 2005.   [MMA]         Rescorla, E., "Preventing the Million Message Attack on                 Cryptographic Message Syntax",RFC 3218, January 2002.   [PKCS-7]      Kaliski, B., "PKCS #7: Cryptographic Message Syntax                 Version 1.5",RFC 2315, March 1998.   [RFC2311]     Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L.,                 and L. Repka, "S/MIME Version 2 Message Specification",RFC 2311, March 1998.   [RFC2312]     Dusse, S., Hoffman, P., Ramsdell, B., and J.                 Weinstein, "S/MIME Version 2 Certificate Handling",RFC2312, March 1998.   [RFC2313]     Kaliski, B., "PKCS #1: RSA Encryption Version 1.5",RFC2313, March 1998.   [RFC2314]     Kaliski, B., "PKCS #10: Certification Request Syntax                 Version 1.5",RFC 2314, March 1998.   [RFC2315]     Kaliski, B., "PKCS #7: Certification Message Syntax                 Version 1.5",RFC 2315, March 1998.   [RFC2630]     Housley, R., "Cryptographic Message Syntax",RFC 2630,                 June 1999.   [RFC2631]     Rescorla, E., "Diffie-Hellman Key Agreement Method",RFC 2631, June 1999.Ramsdell & Turner            Standards Track                   [Page 41]

RFC 5751            S/MIME 3.2 Message Specification        January 2010   [RFC2632]     Ramsdell, B., Ed., "S/MIME Version 3 Certificate                 Handling",RFC 2632, June 1999.   [RFC2633]     Ramsdell, B., Ed., "S/MIME Version 3 Message                 Specification",RFC 2633, June 1999.   [RFC3850]     Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail                 Extensions (S/MIME) Version 3.1 Certificate Handling",RFC 3850, July 2004.   [RFC3851]     Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail                 Extensions (S/MIME) Version 3.1 Message Specification",RFC 3851, July 2004.   [RFC3852]     Housley, R., "Cryptographic Message Syntax (CMS)",RFC3852, July 2004.   [SP800-57]    National Institute of Standards and Technology (NIST),                 Special Publication 800-57: Recommendation for Key                 Management, August 2005.   [STRENGTH]    Orman, H., and P. Hoffman, "Determining Strengths For                 Public Keys Used For Exchanging Symmetric Keys",BCP86,RFC 3766, April 2004.Ramsdell & Turner            Standards Track                   [Page 42]

RFC 5751            S/MIME 3.2 Message Specification        January 2010Appendix A.  ASN.1 Module   Note: The ASN.1 module contained herein is unchanged fromRFC 3851   [SMIMEv3.1] with the exception of a change to the prefersBinaryInside   ASN.1 comment.  This module uses the 1988 version of ASN.1.   SecureMimeMessageV3dot1     { iso(1) member-body(2) us(840) rsadsi(113549)            pkcs(1) pkcs-9(9) smime(16) modules(0) msg-v3dot1(21) }   DEFINITIONS IMPLICIT TAGS ::=   BEGIN   IMPORTS   -- Cryptographic Message Syntax [CMS]      SubjectKeyIdentifier, IssuerAndSerialNumber,      RecipientKeyIdentifier          FROM  CryptographicMessageSyntax                { iso(1) member-body(2) us(840) rsadsi(113549)                  pkcs(1) pkcs-9(9) smime(16) modules(0) cms-2001(14) };   --  id-aa is the arc with all new authenticated and unauthenticated   --  attributes produced by the S/MIME Working Group   id-aa OBJECT IDENTIFIER ::= {iso(1) member-body(2) usa(840)           rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) attributes(2)}   -- S/MIME Capabilities provides a method of broadcasting the   -- symmetric capabilities understood.  Algorithms SHOULD be ordered   -- by preference and grouped by type   smimeCapabilities OBJECT IDENTIFIER ::= {iso(1) member-body(2)           us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 15}   SMIMECapability ::= SEQUENCE {      capabilityID OBJECT IDENTIFIER,      parameters ANY DEFINED BY capabilityID OPTIONAL }   SMIMECapabilities ::= SEQUENCE OF SMIMECapability   -- Encryption Key Preference provides a method of broadcasting the   -- preferred encryption certificate.   id-aa-encrypKeyPref OBJECT IDENTIFIER ::= {id-aa 11}Ramsdell & Turner            Standards Track                   [Page 43]

RFC 5751            S/MIME 3.2 Message Specification        January 2010   SMIMEEncryptionKeyPreference ::= CHOICE {      issuerAndSerialNumber   [0] IssuerAndSerialNumber,      receipentKeyId          [1] RecipientKeyIdentifier,      subjectAltKeyIdentifier [2] SubjectKeyIdentifier   }   -- receipentKeyId is spelt incorrectly, but kept for historical   -- reasons.   id-smime OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)           rsadsi(113549) pkcs(1) pkcs9(9) 16 }   id-cap  OBJECT IDENTIFIER ::= { id-smime 11 }   -- The preferBinaryInside OID indicates an ability to receive   -- messages with binary encoding inside the CMS wrapper.   -- The preferBinaryInside attribute's value field is ABSENT.   id-cap-preferBinaryInside  OBJECT IDENTIFIER ::= { id-cap 1 }   --  The following list OIDs to be used with S/MIME V3   -- Signature Algorithms Not Found in [CMSALG], [CMS-SHA2], [RSAPSS],   -- and [RSAOAEP]   --   -- md2WithRSAEncryption OBJECT IDENTIFIER ::=   --    {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1)   --     2}   --   -- Other Signed Attributes   --   -- signingTime OBJECT IDENTIFIER ::=   --    {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)   --     5}   --    See [CMS] for a description of how to encode the attribute   --    value.   SMIMECapabilitiesParametersForRC2CBC ::= INTEGER   --        (RC2 Key Length (number of bits))   ENDRamsdell & Turner            Standards Track                   [Page 44]

RFC 5751            S/MIME 3.2 Message Specification        January 2010Appendix 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 thatRFC 2311 [SMIMEv2] be moved to Historic   status.Appendix C.  Acknowledgments   Many thanks go out to the other authors of the S/MIME version 2   Message Specification RFC: Steve Dusse, Paul Hoffman, Laurence   Lundblade, and Lisa Repka.  Without v2, there wouldn't be a v3, v3.1,   or v3.2.   A number of the members of the S/MIME Working Group have also worked   very hard and contributed to this document.  Any list of people is   doomed to omission, and for that I apologize.  In alphabetical order,   the following people stand out in my mind because they made direct   contributions to this document:   Tony Capel, Piers Chivers, Dave Crocker, Bill Flanigan, Peter   Gutmann, Alfred Hoenes, Paul Hoffman, Russ Housley, William Ottaway,   John Pawling, and Jim Schaad.Authors' Addresses   Blake Ramsdell   Brute Squad Labs, Inc.   EMail: blaker@gmail.com   Sean Turner   IECA, Inc.   3057 Nutley Street, Suite 106   Fairfax, VA 22031   USA   EMail: turners@ieca.comRamsdell & Turner            Standards Track                   [Page 45]

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