Network Working Group                                         R. Housley
Request for Comments: 5652                                Vigil Security
Obsoletes: 3852                                           September 2009
Category: Standards Track


                   Cryptographic Message Syntax (CMS)

Abstract

   This document describes the Cryptographic Message Syntax (CMS).  This
   syntax is used to digitally sign, digest, authenticate, or encrypt
   arbitrary message content.

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

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Table of Contents

   1. Introduction ....................................................3
      1.1. Evolution of the CMS .......................................4
           1.1.1. Changes Since PKCS #7 Version 1.5 ...................4
           1.1.2. Changes Since RFC 2630 ..............................4
           1.1.3. Changes Since RFC 3369 ..............................5
           1.1.4. Changes Since RFC 3852 ..............................5
      1.2. Terminology ................................................5
      1.3. Version Numbers ............................................6
   2. General Overview ................................................6
   3. General Syntax ..................................................7
   4. Data Content Type ...............................................7
   5. Signed-data Content Type ........................................8
      5.1. SignedData Type ............................................9
      5.2. EncapsulatedContentInfo Type ..............................11
           5.2.1. Compatibility with PKCS #7 .........................12
      5.3. SignerInfo Type ...........................................13
      5.4. Message Digest Calculation Process ........................16
      5.5. Signature Generation Process ..............................16
      5.6. Signature Verification Process ............................17
   6. Enveloped-Data Content Type ....................................17
      6.1. EnvelopedData Type ........................................18
      6.2. RecipientInfo Type ........................................21
           6.2.1. KeyTransRecipientInfo Type .........................22
           6.2.2. KeyAgreeRecipientInfo Type .........................23
           6.2.3. KEKRecipientInfo Type ..............................25
           6.2.4. PasswordRecipientInfo Type .........................26
           6.2.5. OtherRecipientInfo Type ............................27
      6.3. Content-encryption Process ................................27
      6.4. Key-Encryption Process ....................................28
   7. Digested-Data Content Type .....................................28
   8. Encrypted-Data Content Type ....................................29
   9. Authenticated-Data Content Type ................................30
      9.1. AuthenticatedData Type ....................................31
      9.2. MAC Generation ............................................33
      9.3. MAC Verification ..........................................34
   10. Useful Types ..................................................34
      10.1. Algorithm Identifier Types ...............................35
           10.1.1. DigestAlgorithmIdentifier .........................35
           10.1.2. SignatureAlgorithmIdentifier ......................35
           10.1.3. KeyEncryptionAlgorithmIdentifier ..................35
           10.1.4. ContentEncryptionAlgorithmIdentifier ..............36
           10.1.5. MessageAuthenticationCodeAlgorithm ................36
           10.1.6. KeyDerivationAlgorithmIdentifier ..................36
      10.2. Other Useful Types .......................................36
           10.2.1. RevocationInfoChoices .............................36
           10.2.2. CertificateChoices ................................37

           10.2.3. CertificateSet ....................................38
           10.2.4. IssuerAndSerialNumber .............................38
           10.2.5. CMSVersion ........................................39
           10.2.6. UserKeyingMaterial ................................39
           10.2.7. OtherKeyAttribute .................................39
   11. Useful Attributes .............................................39
      11.1. Content Type .............................................40
      11.2. Message Digest ...........................................40
      11.3. Signing Time .............................................41
      11.4. Countersignature .........................................42
   12. ASN.1 Modules .................................................43
      12.1. CMS ASN.1 Module .........................................44
      12.2. Version 1 Attribute Certificate ASN.1 Module .............51
   13. References ....................................................52
      13.1. Normative References .....................................52
      13.2. Informative References ...................................53
   14. Security Considerations .......................................54
   15. Acknowledgments ...............................................56

1.  Introduction

   This document describes the Cryptographic Message Syntax (CMS).  This
   syntax is used to digitally sign, digest, authenticate, or encrypt
   arbitrary message content.

   The CMS describes an encapsulation syntax for data protection.  It
   supports digital signatures and encryption.  The syntax allows
   multiple encapsulations; one encapsulation envelope can be nested
   inside another.  Likewise, one party can digitally sign some
   previously encapsulated data.  It also allows arbitrary attributes,
   such as signing time, to be signed along with the message content,
   and it provides for other attributes such as countersignatures to be
   associated with a signature.

   The CMS can support a variety of architectures for certificate-based
   key management, such as the one defined by the PKIX (Public Key
   Infrastructure using X.509) working group [PROFILE].

   The CMS values are generated using ASN.1 [X.208-88], using BER-
   encoding (Basic Encoding Rules) [X.209-88].  Values are typically
   represented as octet strings.  While many systems are capable of
   transmitting arbitrary octet strings reliably, it is well known that
   many electronic mail systems are not.  This document does not address
   mechanisms for encoding octet strings for reliable transmission in
   such environments.

1.1.  Evolution of the CMS

   The CMS is derived from PKCS #7 version 1.5, which is documented in
   RFC 2315 [PKCS#7].  PKCS #7 version 1.5 was developed outside of the
   IETF; it was originally published as an RSA Laboratories Technical
   Note in November 1993.  Since that time, the IETF has taken
   responsibility for the development and maintenance of the CMS.
   Today, several important IETF Standards-Track protocols make use of
   the CMS.

   This section describes that changes that the IETF has made to the CMS
   in each of the published versions.

1.1.1.  Changes Since PKCS #7 Version 1.5

   RFC 2630 [CMS1] was the first version of the CMS on the IETF
   Standards Track.  Wherever possible, backward compatibility with PKCS
   #7 version 1.5 is preserved; however, changes were made to
   accommodate version 1 attribute certificate transfer and to support
   algorithm-independent key management.  PKCS #7 version 1.5 included
   support only for key transport.  RFC 2630 adds support for key
   agreement and previously distributed symmetric key-encryption key
   techniques.

1.1.2.  Changes Since RFC 2630

   RFC 3369 [CMS2] obsoletes RFC 2630 [CMS1] and RFC 3211 [PWRI].
   Password-based key management is included in the CMS specification,
   and an extension mechanism to support new key management schemes
   without further changes to the CMS is specified.  Backward
   compatibility with RFC 2630 and RFC 3211 is preserved; however,
   version 2 attribute certificate transfer is added, and the use of
   version 1 attribute certificates is deprecated.

   Secure/Multipurpose Internet Mail Extensions (S/MIME) v2 signatures
   [MSG2], which are based on PKCS #7 version 1.5, are compatible with
   S/MIME v3 signatures [MSG3]and S/MIME v3.1 signatures [MSG3.1].
   However, there are some subtle compatibility issues with signatures
   based on PKCS #7 version 1.5.  These issues are discussed in Section
   5.2.1.  These issues remain with the current version of the CMS.

   Specific cryptographic algorithms are not discussed in this document,
   but they were discussed in RFC 2630.  The discussion of specific
   cryptographic algorithms has been moved to a separate document
   [CMSALG].  Separation of the protocol and algorithm specifications
   allows the IETF to update each document independently.  This
   specification does not require the implementation of any particular

   algorithms.  Rather, protocols that rely on the CMS are expected to
   choose appropriate algorithms for their environment.  The algorithms
   may be selected from [CMSALG] or elsewhere.

1.1.3.  Changes Since RFC 3369

   RFC 3852 [CMS3] obsoletes RFC 3369 [CMS2].  As discussed in the
   previous section, RFC 3369 introduced an extension mechanism to
   support new key management schemes without further changes to the
   CMS.  RFC 3852 introduces a similar extension mechanism to support
   additional certificate formats and revocation status information
   formats without further changes to the CMS.  These extensions are
   primarily documented in Sections 10.2.1 and 10.2.2.  Backward
   compatibility with earlier versions of the CMS is preserved.

   The use of version numbers is described in Section 1.3.

   Since the publication of RFC 3369, a few errata have been noted.
   These errata are posted on the RFC Editor web site.  These errors
   have been corrected in this document.

   The text in Section 11.4 that describes the counter signature
   unsigned attribute is clarified.  Hopefully, the revised text is
   clearer about the portion of the SignerInfo signature that is covered
   by a countersignature.

1.1.4.  Changes Since RFC 3852

   This document obsoletes RFC 3852 [CMS3].  The primary reason for the
   publication of this document is to advance the CMS along the
   standards maturity ladder.

   This document includes the clarifications that were originally
   published in RFC 4853 [CMSMSIG] regarding the proper handling of the
   SignedData protected content type when more than one digital
   signature is present.

   Since the publication of RFC 3852, a few errata have been noted.
   These errata are posted on the RFC Editor web site.  These errors
   have been corrected in this document.

1.2.  Terminology

   In this document, the key words MUST, MUST NOT, REQUIRED, SHOULD,
   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL are to be interpreted as
   described in [STDWORDS].

1.3.  Version Numbers

   Each of the major data structures includes a version number as the
   first item in the data structure.  The version numbers are intended
   to avoid ASN.1 decode errors.  Some implementations do not check the
   version number prior to attempting a decode, and if a decode error
   occurs, then the version number is checked as part of the error
   handling routine.  This is a reasonable approach; it places error
   processing outside of the fast path.  This approach is also forgiving
   when an incorrect version number is used by the sender.

   Most of the initial version numbers were assigned in PKCS #7 version
   1.5.  Others were assigned when the structure was initially created.
   Whenever a structure is updated, a higher version number is assigned.
   However, to ensure maximum interoperability, the higher version
   number is only used when the new syntax feature is employed.  That
   is, the lowest version number that supports the generated syntax is
   used.

2.  General Overview

   The CMS is general enough to support many different content types.
   This document defines one protection content, ContentInfo.
   ContentInfo encapsulates a single identified content type, and the
   identified type may provide further encapsulation.  This document
   defines six content types: data, signed-data, enveloped-data,
   digested-data, encrypted-data, and authenticated-data.  Additional
   content types can be defined outside this document.

   An implementation that conforms to this specification MUST implement
   the protection content, ContentInfo, and MUST implement the data,
   signed-data, and enveloped-data content types.  The other content
   types MAY be implemented.

   As a general design philosophy, each content type permits single pass
   processing using indefinite-length Basic Encoding Rules (BER)
   encoding.  Single-pass operation is especially helpful if content is
   large, stored on tapes, or is "piped" from another process.  Single-
   pass operation has one significant drawback: it is difficult to
   perform encode operations using the Distinguished Encoding Rules
   (DER) [X.509-88] encoding in a single pass since the lengths of the
   various components may not be known in advance.  However, signed
   attributes within the signed-data content type and authenticated
   attributes within the authenticated-data content type need to be
   transmitted in DER form to ensure that recipients can verify a
   content that contains one or more unrecognized attributes.  Signed
   attributes and authenticated attributes are the only data types used
   in the CMS that require DER encoding.

3.  General Syntax

   The following object identifier identifies the content information
   type:

      id-ct-contentInfo OBJECT IDENTIFIER ::= { iso(1) member-body(2)
         us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) ct(1) 6 }

   The CMS associates a content type identifier with a content.  The
   syntax MUST have ASN.1 type ContentInfo:

      ContentInfo ::= SEQUENCE {
        contentType ContentType,
        content [0] EXPLICIT ANY DEFINED BY contentType }

      ContentType ::= OBJECT IDENTIFIER

   The fields of ContentInfo have the following meanings:

      contentType indicates the type of the associated content.  It is
      an object identifier; it is a unique string of integers assigned
      by an authority that defines the content type.

      content is the associated content.  The type of content can be
      determined uniquely by contentType.  Content types for data,
      signed-data, enveloped-data, digested-data, encrypted-data, and
      authenticated-data are defined in this document.  If additional
      content types are defined in other documents, the ASN.1 type
      defined SHOULD NOT be a CHOICE type.

4.  Data Content Type

   The following object identifier identifies the data content type:

      id-data OBJECT IDENTIFIER ::= { iso(1) member-body(2)
         us(840) rsadsi(113549) pkcs(1) pkcs7(7) 1 }

   The data content type is intended to refer to arbitrary octet
   strings, such as ASCII text files; the interpretation is left to the
   application.  Such strings need not have any internal structure
   (although they could have their own ASN.1 definition or other
   structure).

   S/MIME uses id-data to identify MIME-encoded content.  The use of
   this content identifier is specified in RFC 2311 for S/MIME v2
   [MSG2], RFC 2633 for S/MIME v3 [MSG3], and RFC 3851 for S/MIME v3.1
   [MSG3.1].

   The data content type is generally encapsulated in the signed-data,
   enveloped-data, digested-data, encrypted-data, or authenticated-data
   content type.

5.  Signed-data Content Type

   The signed-data content type consists of a content of any type and
   zero or more signature values.  Any number of signers in parallel can
   sign any type of content.

   The typical application of the signed-data content type represents
   one signer's digital signature on content of the data content type.
   Another typical application disseminates certificates and certificate
   revocation lists (CRLs).

   The process by which signed-data is constructed involves the
   following steps:

   1.  For each signer, a message digest, or hash value, is computed on
       the content with a signer-specific message-digest algorithm.  If
       the signer is signing any information other than the content, the
       message digest of the content and the other information are
       digested with the signer's message digest algorithm (see Section
       5.4), and the result becomes the "message digest."

   2.  For each signer, the message digest is digitally signed using the
       signer's private key.

   3.  For each signer, the signature value and other signer-specific
       information are collected into a SignerInfo value, as defined in
       Section 5.3.  Certificates and CRLs for each signer, and those
       not corresponding to any signer, are collected in this step.

   4.  The message digest algorithms for all the signers and the
       SignerInfo values for all the signers are collected together with
       the content into a SignedData value, as defined in Section 5.1.

   A recipient independently computes the message digest.  This message
   digest and the signer's public key are used to verify the signature
   value.  The signer's public key is referenced in one of two ways.  It
   can be referenced by an issuer distinguished name along with an
   issuer-specific serial number to uniquely identify the certificate
   that contains the public key.  Alternatively, it can be referenced by
   a subject key identifier, which accommodates both certified and
   uncertified public keys.  While not required, the signer's
   certificate can be included in the SignedData certificates field.

   When more than one signature is present, the successful validation of
   one signature associated with a given signer is usually treated as a
   successful signature by that signer.  However, there are some
   application environments where other rules are needed.  An
   application that employs a rule other than one valid signature for
   each signer must specify those rules.  Also, where simple matching of
   the signer identifier is not sufficient to determine whether the
   signatures were generated by the same signer, the application
   specification must describe how to determine which signatures were
   generated by the same signer.  Support of different communities of
   recipients is the primary reason that signers choose to include more
   than one signature.  For example, the signed-data content type might
   include signatures generated with the RSA signature algorithm and
   with the Elliptic Curve Digital Signature Algorithm (ECDSA) signature
   algorithm.  This allows recipients to verify the signature associated
   with one algorithm or the other.

   This section is divided into six parts.  The first part describes the
   top-level type SignedData, the second part describes
   EncapsulatedContentInfo, the third part describes the per-signer
   information type SignerInfo, and the fourth, fifth, and sixth parts
   describe the message digest calculation, signature generation, and
   signature verification processes, respectively.

5.1.  SignedData Type

   The following object identifier identifies the signed-data content
   type:

      id-signedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
         us(840) rsadsi(113549) pkcs(1) pkcs7(7) 2 }

   The signed-data content type shall have ASN.1 type SignedData:

      SignedData ::= SEQUENCE {
        version CMSVersion,
        digestAlgorithms DigestAlgorithmIdentifiers,
        encapContentInfo EncapsulatedContentInfo,
        certificates [0] IMPLICIT CertificateSet OPTIONAL,
        crls [1] IMPLICIT RevocationInfoChoices OPTIONAL,
        signerInfos SignerInfos }

      DigestAlgorithmIdentifiers ::= SET OF DigestAlgorithmIdentifier

      SignerInfos ::= SET OF SignerInfo

   The fields of type SignedData have the following meanings:

      version is the syntax version number.  The appropriate value
      depends on certificates, eContentType, and SignerInfo.  The
      version MUST be assigned as follows:

         IF ((certificates is present) AND
            (any certificates with a type of other are present)) OR
            ((crls is present) AND
            (any crls with a type of other are present))
         THEN version MUST be 5
         ELSE
            IF (certificates is present) AND
               (any version 2 attribute certificates are present)
            THEN version MUST be 4
            ELSE
               IF ((certificates is present) AND
                  (any version 1 attribute certificates are present)) OR
                  (any SignerInfo structures are version 3) OR
                  (encapContentInfo eContentType is other than id-data)
               THEN version MUST be 3
               ELSE version MUST be 1

      digestAlgorithms is a collection of message digest algorithm
      identifiers.  There MAY be any number of elements in the
      collection, including zero.  Each element identifies the message
      digest algorithm, along with any associated parameters, used by
      one or more signer.  The collection is intended to list the
      message digest algorithms employed by all of the signers, in any
      order, to facilitate one-pass signature verification.
      Implementations MAY fail to validate signatures that use a digest
      algorithm that is not included in this set.  The message digesting
      process is described in Section 5.4.

      encapContentInfo is the signed content, consisting of a content
      type identifier and the content itself.  Details of the
      EncapsulatedContentInfo type are discussed in Section 5.2.

      certificates is a collection of certificates.  It is intended that
      the set of certificates be sufficient to contain certification
      paths from a recognized "root" or "top-level certification
      authority" to all of the signers in the signerInfos field.  There
      may be more certificates than necessary, and there may be
      certificates sufficient to contain certification paths from two or
      more independent top-level certification authorities.  There may
      also be fewer certificates than necessary, if it is expected that
      recipients have an alternate means of obtaining necessary

      certificates (e.g., from a previous set of certificates).  The
      signer's certificate MAY be included.  The use of version 1
      attribute certificates is strongly discouraged.

      crls is a collection of revocation status information.  It is
      intended that the collection contain information sufficient to
      determine whether the certificates in the certificates field are
      valid, but such correspondence is not necessary.  Certificate
      revocation lists (CRLs) are the primary source of revocation
      status information.  There MAY be more CRLs than necessary, and
      there MAY also be fewer CRLs than necessary.

      signerInfos is a collection of per-signer information.  There MAY
      be any number of elements in the collection, including zero.  When
      the collection represents more than one signature, the successful
      validation of one of signature from a given signer ought to be
      treated as a successful signature by that signer.  However, there
      are some application environments where other rules are needed.
      The details of the SignerInfo type are discussed in Section 5.3.
      Since each signer can employ a different digital signature
      technique, and future specifications could update the syntax, all
      implementations MUST gracefully handle unimplemented versions of
      SignerInfo.  Further, since all implementations will not support
      every possible signature algorithm, all implementations MUST
      gracefully handle unimplemented signature algorithms when they are
      encountered.

5.2.  EncapsulatedContentInfo Type

   The content is represented in the type EncapsulatedContentInfo:

      EncapsulatedContentInfo ::= SEQUENCE {
        eContentType ContentType,
        eContent [0] EXPLICIT OCTET STRING OPTIONAL }

      ContentType ::= OBJECT IDENTIFIER

   The fields of type EncapsulatedContentInfo have the following
   meanings:

      eContentType is an object identifier.  The object identifier
      uniquely specifies the content type.

      eContent is the content itself, carried as an octet string.  The
      eContent need not be DER encoded.

   The optional omission of the eContent within the
   EncapsulatedContentInfo field makes it possible to construct
   "external signatures".  In the case of external signatures, the
   content being signed is absent from the EncapsulatedContentInfo value
   included in the signed-data content type.  If the eContent value
   within EncapsulatedContentInfo is absent, then the signatureValue is
   calculated and the eContentType is assigned as though the eContent
   value was present.

   In the degenerate case where there are no signers, the
   EncapsulatedContentInfo value being "signed" is irrelevant.  In this
   case, the content type within the EncapsulatedContentInfo value being
   "signed" MUST be id-data (as defined in Section 4), and the content
   field of the EncapsulatedContentInfo value MUST be omitted.

5.2.1.  Compatibility with PKCS #7

   This section contains a word of warning to implementers that wish to
   support both the CMS and PKCS #7 [PKCS#7] SignedData content types.
   Both the CMS and PKCS #7 identify the type of the encapsulated
   content with an object identifier, but the ASN.1 type of the content
   itself is variable in PKCS #7 SignedData content type.

   PKCS #7 defines content as:

      content [0] EXPLICIT ANY DEFINED BY contentType OPTIONAL

   The CMS defines eContent as:

      eContent [0] EXPLICIT OCTET STRING OPTIONAL

   The CMS definition is much easier to use in most applications, and it
   is compatible with both S/MIME v2 and S/MIME v3.  S/MIME signed
   messages using the CMS and PKCS #7 are compatible because identical
   signed message formats are specified in RFC 2311 for S/MIME v2
   [MSG2], RFC 2633 for S/MIME v3 [MSG3], and RFC 3851 for S/MIME v3.1
   [MSG3.1].  S/MIME v2 encapsulates the MIME content in a Data type
   (that is, an OCTET STRING) carried in the SignedData contentInfo
   content ANY field, and S/MIME v3 carries the MIME content in the
   SignedData encapContentInfo eContent OCTET STRING.  Therefore, in
   S/MIME v2, S/MIME v3, and S/MIME v3.1, the MIME content is placed in
   an OCTET STRING and the message digest is computed over the identical
   portions of the content.  That is, the message digest is computed
   over the octets comprising the value of the OCTET STRING, neither the
   tag nor length octets are included.

   There are incompatibilities between the CMS and PKCS #7 SignedData
   types when the encapsulated content is not formatted using the Data
   type.  For example, when an RFC 2634 signed receipt [ESS] is
   encapsulated in the CMS SignedData type, then the Receipt SEQUENCE is
   encoded in the SignedData encapContentInfo eContent OCTET STRING and
   the message digest is computed using the entire Receipt SEQUENCE
   encoding (including tag, length and value octets).  However, if an
   RFC 2634 signed receipt is encapsulated in the PKCS #7 SignedData
   type, then the Receipt SEQUENCE is DER encoded [X.509-88] in the
   SignedData contentInfo content ANY field (a SEQUENCE, not an OCTET
   STRING).  Therefore, the message digest is computed using only the
   value octets of the Receipt SEQUENCE encoding.

   The following strategy can be used to achieve backward compatibility
   with PKCS #7 when processing SignedData content types.  If the
   implementation is unable to ASN.1 decode the SignedData type using
   the CMS SignedData encapContentInfo eContent OCTET STRING syntax,
   then the implementation MAY attempt to decode the SignedData type
   using the PKCS #7 SignedData contentInfo content ANY syntax and
   compute the message digest accordingly.

   The following strategy can be used to achieve backward compatibility
   with PKCS #7 when creating a SignedData content type in which the
   encapsulated content is not formatted using the Data type.
   Implementations MAY examine the value of the eContentType, and then
   adjust the expected DER encoding of eContent based on the object
   identifier value.  For example, to support Microsoft Authenticode
   [MSAC], the following information MAY be included:

      eContentType Object Identifier is set to { 1 3 6 1 4 1 311 2 1 4 }

      eContent contains DER-encoded Authenticode signing information

5.3.  SignerInfo Type

   Per-signer information is represented in the type SignerInfo:

      SignerInfo ::= SEQUENCE {
        version CMSVersion,
        sid SignerIdentifier,
        digestAlgorithm DigestAlgorithmIdentifier,
        signedAttrs [0] IMPLICIT SignedAttributes OPTIONAL,
        signatureAlgorithm SignatureAlgorithmIdentifier,
        signature SignatureValue,
        unsignedAttrs [1] IMPLICIT UnsignedAttributes OPTIONAL }

      SignerIdentifier ::= CHOICE {
        issuerAndSerialNumber IssuerAndSerialNumber,
        subjectKeyIdentifier [0] SubjectKeyIdentifier }

      SignedAttributes ::= SET SIZE (1..MAX) OF Attribute

      UnsignedAttributes ::= SET SIZE (1..MAX) OF Attribute

      Attribute ::= SEQUENCE {
        attrType OBJECT IDENTIFIER,
        attrValues SET OF AttributeValue }

      AttributeValue ::= ANY

      SignatureValue ::= OCTET STRING

   The fields of type SignerInfo have the following meanings:

      version is the syntax version number.  If the SignerIdentifier is
      the CHOICE issuerAndSerialNumber, then the version MUST be 1.  If
      the SignerIdentifier is subjectKeyIdentifier, then the version
      MUST be 3.

      sid specifies the signer's certificate (and thereby the signer's
      public key).  The signer's public key is needed by the recipient
      to verify the signature.  SignerIdentifier provides two
      alternatives for specifying the signer's public key.  The
      issuerAndSerialNumber alternative identifies the signer's
      certificate by the issuer's distinguished name and the certificate
      serial number; the subjectKeyIdentifier identifies the signer's
      certificate by a key identifier.  When an X.509 certificate is
      referenced, the key identifier matches the X.509
      subjectKeyIdentifier extension value.  When other certificate
      formats are referenced, the documents that specify the certificate
      format and their use with the CMS must include details on matching
      the key identifier to the appropriate certificate field.
      Implementations MUST support the reception of the
      issuerAndSerialNumber and subjectKeyIdentifier forms of
      SignerIdentifier.  When generating a SignerIdentifier,
      implementations MAY support one of the forms (either
      issuerAndSerialNumber or subjectKeyIdentifier) and always use it,
      or implementations MAY arbitrarily mix the two forms.  However,
      subjectKeyIdentifier MUST be used to refer to a public key
      contained in a non-X.509 certificate.

      digestAlgorithm identifies the message digest algorithm, and any
      associated parameters, used by the signer.  The message digest is
      computed on either the content being signed or the content

      together with the signed attributes using the process described in
      Section 5.4.  The message digest algorithm SHOULD be among those
      listed in the digestAlgorithms field of the associated SignerData.
      Implementations MAY fail to validate signatures that use a digest
      algorithm that is not included in the SignedData digestAlgorithms
      set.

      signedAttrs is a collection of attributes that are signed.  The
      field is optional, but it MUST be present if the content type of
      the EncapsulatedContentInfo value being signed is not id-data.
      SignedAttributes MUST be DER encoded, even if the rest of the
      structure is BER encoded.  Useful attribute types, such as signing
      time, are defined in Section 11.  If the field is present, it MUST
      contain, at a minimum, the following two attributes:

         A content-type attribute having as its value the content type
         of the EncapsulatedContentInfo value being signed.  Section
         11.1 defines the content-type attribute.  However, the
         content-type attribute MUST NOT be used as part of a
         countersignature unsigned attribute as defined in Section 11.4.

         A message-digest attribute, having as its value the message
         digest of the content.  Section 11.2 defines the message-digest
         attribute.

      signatureAlgorithm identifies the signature algorithm, and any
      associated parameters, used by the signer to generate the digital
      signature.

      signature is the result of digital signature generation, using the
      message digest and the signer's private key.  The details of the
      signature depend on the signature algorithm employed.

      unsignedAttrs is a collection of attributes that are not signed.
      The field is optional.  Useful attribute types, such as
      countersignatures, are defined in Section 11.

   The fields of type SignedAttribute and UnsignedAttribute have the
   following meanings:

      attrType indicates the type of attribute.  It is an object
      identifier.

      attrValues is a set of values that comprise the attribute.  The
      type of each value in the set can be determined uniquely by
      attrType.  The attrType can impose restrictions on the number of
      items in the set.

5.4.  Message Digest Calculation Process

   The message digest calculation process computes a message digest on
   either the content being signed or the content together with the
   signed attributes.  In either case, the initial input to the message
   digest calculation process is the "value" of the encapsulated content
   being signed.  Specifically, the initial input is the
   encapContentInfo eContent OCTET STRING to which the signing process
   is applied.  Only the octets comprising the value of the eContent
   OCTET STRING are input to the message digest algorithm, not the tag
   or the length octets.

   The result of the message digest calculation process depends on
   whether the signedAttrs field is present.  When the field is absent,
   the result is just the message digest of the content as described
   above.  When the field is present, however, the result is the message
   digest of the complete DER encoding of the SignedAttrs value
   contained in the signedAttrs field.  Since the SignedAttrs value,
   when present, must contain the content-type and the message-digest
   attributes, those values are indirectly included in the result.  The
   content-type attribute MUST NOT be included in a countersignature
   unsigned attribute as defined in Section 11.4.  A separate encoding
   of the signedAttrs field is performed for message digest calculation.
   The IMPLICIT [0] tag in the signedAttrs is not used for the DER
   encoding, rather an EXPLICIT SET OF tag is used.  That is, the DER
   encoding of the EXPLICIT SET OF tag, rather than of the IMPLICIT [0]
   tag, MUST be included in the message digest calculation along with
   the length and content octets of the SignedAttributes value.

   When the signedAttrs field is absent, only the octets comprising the
   value of the SignedData encapContentInfo eContent OCTET STRING (e.g.,
   the contents of a file) are input to the message digest calculation.
   This has the advantage that the length of the content being signed
   need not be known in advance of the signature generation process.

   Although the encapContentInfo eContent OCTET STRING tag and length
   octets are not included in the message digest calculation, they are
   protected by other means.  The length octets are protected by the
   nature of the message digest algorithm since it is computationally
   infeasible to find any two distinct message contents of any length
   that have the same message digest.

5.5.  Signature Generation Process

   The input to the signature generation process includes the result of
   the message digest calculation process and the signer's private key.
   The details of the signature generation depend on the signature
   algorithm employed.  The object identifier, along with any

   parameters, that specifies the signature algorithm employed by the
   signer is carried in the signatureAlgorithm field.  The signature
   value generated by the signer MUST be encoded as an OCTET STRING and
   carried in the signature field.

5.6.  Signature Verification Process

   The input to the signature verification process includes the result
   of the message digest calculation process and the signer's public
   key.  The recipient MAY obtain the correct public key for the signer
   by any means, but the preferred method is from a certificate obtained
   from the SignedData certificates field.  The selection and validation
   of the signer's public key MAY be based on certification path
   validation (see [PROFILE]) as well as other external context, but is
   beyond the scope of this document.  The details of the signature
   verification depend on the signature algorithm employed.

   The recipient MUST NOT rely on any message digest values computed by
   the originator.  If the SignedData signerInfo includes
   signedAttributes, then the content message digest MUST be calculated
   as described in Section 5.4.  For the signature to be valid, the
   message digest value calculated by the recipient MUST be the same as
   the value of the messageDigest attribute included in the
   signedAttributes of the SignedData signerInfo.

   If the SignedData signerInfo includes signedAttributes, then the
   content-type attribute value MUST match the SignedData
   encapContentInfo eContentType value.