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rfc:rfc3852

Network Working Group R. Housley Request for Comments: 3852 Vigil Security Obsoletes: 3369 July 2004 Category: Standards Track

                 Cryptographic Message Syntax (CMS)

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.

Copyright Notice

 Copyright (C) The Internet Society (2004).

Abstract

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

Table of Contents

 1.   Introduction. . . . . . . . . . . . . . . . . . . . . . . . .  3
      1.1.   Evolution of the CMS . . . . . . . . . . . . . . . . .  3
             1.1.1.  Changes Since PKCS #7 Version 1.5. . . . . . .  3
             1.1.2.  Changes Since RFC 2630 . . . . . . . . . . . .  4
             1.1.3.  Changes Since RFC 3369 . . . . . . . . . . . .  4
      1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . .  5
      1.3.  Version Numbers . . . . . . . . . . . . . . . . . . . .  5
 2.   General Overview. . . . . . . . . . . . . . . . . . . . . . .  5
 3.   General Syntax  . . . . . . . . . . . . . . . . . . . . . . .  6
 4.   Data Content Type . . . . . . . . . . . . . . . . . . . . . .  6
 5.   Signed-data Content Type. . . . . . . . . . . . . . . . . . .  7
      5.1.   SignedData Type. . . . . . . . . . . . . . . . . . . .  8
      5.2.   EncapsulatedContentInfo Type . . . . . . . . . . . . . 10
             5.2.1.   Compatibility with PKCS #7. . . . . . . . . . 11
      5.3.   SignerInfo Type. . . . . . . . . . . . . . . . . . . . 12
      5.4.   Message Digest Calculation Process . . . . . . . . . . 14
      5.5.   Signature Generation Process . . . . . . . . . . . . . 15
      5.6.   Signature Verification Process . . . . . . . . . . . . 15
 6.   Enveloped-data Content Type . . . . . . . . . . . . . . . . . 16
      6.1.   EnvelopedData Type . . . . . . . . . . . . . . . . . . 17

Housley Standards Track [Page 1] RFC 3852 Cryptographic Message Syntax July 2004

      6.2.   RecipientInfo Type . . . . . . . . . . . . . . . . . . 19
             6.2.1.   KeyTransRecipientInfo Type. . . . . . . . . . 20
             6.2.2.   KeyAgreeRecipientInfo Type. . . . . . . . . . 21
             6.2.3.   KEKRecipientInfo Type . . . . . . . . . . . . 24
             6.2.4.   PasswordRecipientInfo Type. . . . . . . . . . 25
             6.2.5.   OtherRecipientInfo Type . . . . . . . . . . . 26
      6.3.   Content-encryption Process . . . . . . . . . . . . . . 26
      6.4.   Key-encryption Process . . . . . . . . . . . . . . . . 27
 7.   Digested-data Content Type. . . . . . . . . . . . . . . . . . 27
 8.   Encrypted-data Content Type . . . . . . . . . . . . . . . . . 28
 9.   Authenticated-data Content Type . . . . . . . . . . . . . . . 29
      9.1.   AuthenticatedData Type . . . . . . . . . . . . . . . . 30
      9.2.   MAC Generation . . . . . . . . . . . . . . . . . . . . 32
      9.3.   MAC Verification . . . . . . . . . . . . . . . . . . . 33
 10.  Useful Types. . . . . . . . . . . . . . . . . . . . . . . . . 33
      10.1.  Algorithm Identifier Types . . . . . . . . . . . . . . 33
             10.1.1.  DigestAlgorithmIdentifier . . . . . . . . . . 34
             10.1.2.  SignatureAlgorithmIdentifier. . . . . . . . . 34
             10.1.3.  KeyEncryptionAlgorithmIdentifier. . . . . . . 34
             10.1.4.  ContentEncryptionAlgorithmIdentifier. . . . . 34
             10.1.5.  MessageAuthenticationCodeAlgorithm. . . . . . 35
             10.1.6.  KeyDerivationAlgorithmIdentifier. . . . . . . 35
      10.2.  Other Useful Types . . . . . . . . . . . . . . . . . . 35
             10.2.1.  RevocationInfoChoices . . . . . . . . . . . . 35
             10.2.2.  CertificateChoices. . . . . . . . . . . . . . 36
             10.2.3.  CertificateSet. . . . . . . . . . . . . . . . 37
             10.2.4.  IssuerAndSerialNumber . . . . . . . . . . . . 37
             10.2.5.  CMSVersion. . . . . . . . . . . . . . . . . . 38
             10.2.6.  UserKeyingMaterial. . . . . . . . . . . . . . 38
             10.2.7.  OtherKeyAttribute . . . . . . . . . . . . . . 38
 11.  Useful Attributes . . . . . . . . . . . . . . . . . . . . . . 38
      11.1.  Content Type . . . . . . . . . . . . . . . . . . . . . 39
      11.2.  Message Digest . . . . . . . . . . . . . . . . . . . . 39
      11.3.  Signing Time . . . . . . . . . . . . . . . . . . . . . 40
      11.4.  Countersignature . . . . . . . . . . . . . . . . . . . 41
 12.  ASN.1 Modules . . . . . . . . . . . . . . . . . . . . . . . . 42
      12.1.  CMS ASN.1 Module . . . . . . . . . . . . . . . . . . . 43
      12.2.  Version 1 Attribute Certificate ASN.1 Module . . . . . 50
 13.  References  . . . . . . . . . . . . . . . . . . . . . . . . . 51
      13.1.  Normative References . . . . . . . . . . . . . . . . . 51
      13.2.  Informative References . . . . . . . . . . . . . . . . 52
 14.  Security Considerations . . . . . . . . . . . . . . . . . . . 53
 15.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 55
 16.  Author's Address. . . . . . . . . . . . . . . . . . . . . . . 55
 17.  Full Copyright Statement. . . . . . . . . . . . . . . . . . . 56

Housley Standards Track [Page 2] RFC 3852 Cryptographic Message Syntax July 2004

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 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 working group
 [PROFILE].
 The CMS values are generated using ASN.1 [X.208-88], using BER-
 encoding [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 the 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

Housley Standards Track [Page 3] RFC 3852 Cryptographic Message Syntax July 2004

 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.
 S/MIME v2 signatures [OLDMSG], which are based on PKCS#7 version 1.5,
 are compatible with S/MIME v3 signatures [MSG], which are based on
 RFC 2630.  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

 This document 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.  This document 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 section 10.2.1 and section
 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.

Housley Standards Track [Page 4] RFC 3852 Cryptographic Message Syntax July 2004

 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.

1.2. 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-

Housley Standards Track [Page 5] RFC 3852 Cryptographic Message Syntax July 2004

 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 }

Housley Standards Track [Page 6] RFC 3852 Cryptographic Message Syntax July 2004

 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
 [OLDMSG] and RFC 3851 for S/MIME v3.1 [MSG].
 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.

Housley Standards Track [Page 7] RFC 3852 Cryptographic Message Syntax July 2004

    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 either by an issuer
 distinguished name along with an issuer-specific serial number or by
 a subject key identifier that uniquely identifies the certificate
 containing the public key.  The signer's certificate can be included
 in the SignedData certificates field.
 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:

Housley Standards Track [Page 8] RFC 3852 Cryptographic Message Syntax July 2004

       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

Housley Standards Track [Page 9] RFC 3852 Cryptographic Message Syntax July 2004

    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.  The
    details of the SignerInfo type are discussed in section 5.3.
    Since each signer can employ a 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.

Housley Standards Track [Page 10] RFC 3852 Cryptographic Message Syntax July 2004

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
 [OLDMSG] and RFC 3851 for S/MIME v3.1 [MSG].  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 both S/MIME v2 and S/MIME v3, 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 [ESS] signed receipt 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,

Housley Standards Track [Page 11] RFC 3852 Cryptographic Message Syntax July 2004

 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

Housley Standards Track [Page 12] RFC 3852 Cryptographic Message Syntax July 2004

 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
    reference, 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:

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       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

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 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

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 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.

6. Enveloped-data Content Type

 The enveloped-data content type consists of an encrypted content of
 any type and encrypted content-encryption keys for one or more
 recipients.  The combination of the encrypted content and one
 encrypted content-encryption key for a recipient is a "digital
 envelope" for that recipient.  Any type of content can be enveloped
 for an arbitrary number of recipients using any of the supported key
 management techniques for each recipient.
 The typical application of the enveloped-data content type will
 represent one or more recipients' digital envelopes on content of the
 data or signed-data content types.
 Enveloped-data is constructed by the following steps:
    1. A content-encryption key for a particular content-encryption
       algorithm is generated at random.
    2. The content-encryption key is encrypted for each recipient.
       The details of this encryption depend on the key management
       algorithm used, but four general techniques are supported:
       key transport:  the content-encryption key is encrypted in the
       recipient's public key;
       key agreement:  the recipient's public key and the sender's
       private key are used to generate a pairwise symmetric key, then
       the content-encryption key is encrypted in the pairwise
       symmetric key;

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       symmetric key-encryption keys:  the content-encryption key is
       encrypted in a previously distributed symmetric key-encryption
       key; and
       passwords: the content-encryption key is encrypted in a key-
       encryption key that is derived from a password or other shared
       secret value.
    3. For each recipient, the encrypted content-encryption key and
       other recipient-specific information are collected into a
       RecipientInfo value, defined in Section 6.2.
    4. The content is encrypted with the content-encryption key.
       Content encryption may require that the content be padded to a
       multiple of some block size; see Section 6.3.
    5. The RecipientInfo values for all the recipients are collected
       together with the encrypted content to form an EnvelopedData
       value as defined in Section 6.1.
    A recipient opens the digital envelope by decrypting one of the
    encrypted content-encryption keys and then decrypting the
    encrypted content with the recovered content-encryption key.
    This section is divided into four parts.  The first part describes
    the top-level type EnvelopedData, the second part describes the
    per-recipient information type RecipientInfo, and the third and
    fourth parts describe the content-encryption and key-encryption
    processes.

6.1. EnvelopedData Type

 The following object identifier identifies the enveloped-data content
 type:
    id-envelopedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
        us(840) rsadsi(113549) pkcs(1) pkcs7(7) 3 }
 The enveloped-data content type shall have ASN.1 type EnvelopedData:
  EnvelopedData ::= SEQUENCE {
   version CMSVersion,
   originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL,
   recipientInfos RecipientInfos,
   encryptedContentInfo EncryptedContentInfo,
   unprotectedAttrs [1] IMPLICIT UnprotectedAttributes OPTIONAL }

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  OriginatorInfo ::= SEQUENCE {
   certs [0] IMPLICIT CertificateSet OPTIONAL,
   crls [1] IMPLICIT RevocationInfoChoices OPTIONAL }
  RecipientInfos ::= SET SIZE (1..MAX) OF RecipientInfo
  EncryptedContentInfo ::= SEQUENCE {
   contentType ContentType,
   contentEncryptionAlgorithm ContentEncryptionAlgorithmIdentifier,
   encryptedContent [0] IMPLICIT EncryptedContent OPTIONAL }
  EncryptedContent ::= OCTET STRING
  UnprotectedAttributes ::= SET SIZE (1..MAX) OF Attribute
 The fields of type EnvelopedData have the following meanings:
    version is the syntax version number.  The appropriate value
    depends on originatorInfo, RecipientInfo, and unprotectedAttrs.
    The version MUST be assigned as follows:
       IF (originatorInfo is present) AND
          ((any certificates with a type of other are present) OR
          (any crls with a type of other are present))
       THEN version is 4
       ELSE
          IF ((originatorInfo is present) AND
             (any version 2 attribute certificates are present)) OR
             (any RecipientInfo structures include pwri) OR
             (any RecipientInfo structures include ori)
          THEN version is 3
          ELSE
             IF (originatorInfo is absent) OR
                (unprotectedAttrs is absent) OR
                (all RecipientInfo structures are version 0)
             THEN version is 0
             ELSE version is 2
    originatorInfo optionally provides information about the
    originator.  It is present only if required by the key management
    algorithm.  It may contain certificates and CRLs:
       certs is a collection of certificates.  certs may contain
       originator certificates associated with several different key
       management algorithms.  certs may also contain attribute
       certificates associated with the originator.  The certificates
       contained in certs are intended to be sufficient for all
       recipients to build certification paths from a recognized

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       "root" or "top-level certification authority."  However, certs
       may contain more certificates than necessary, and there may be
       certificates sufficient to make certification paths from two or
       more independent top-level certification authorities.
       Alternatively, certs may contain 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).
       crls is a collection of CRLs.  It is intended that the set
       contain information sufficient to determine whether or not the
       certificates in the certs field are valid, but such
       correspondence is not necessary.  There MAY be more CRLs than
       necessary, and there MAY also be fewer CRLs than necessary.
    recipientInfos is a collection of per-recipient information.
    There MUST be at least one element in the collection.
    encryptedContentInfo is the encrypted content information.
    unprotectedAttrs is a collection of attributes that are not
    encrypted.  The field is optional.  Useful attribute types are
    defined in Section 11.
 The fields of type EncryptedContentInfo have the following meanings:
    contentType indicates the type of content.
    contentEncryptionAlgorithm identifies the content-encryption
    algorithm, and any associated parameters, used to encrypt the
    content.  The content-encryption process is described in Section
    6.3.  The same content-encryption algorithm and content-encryption
    key are used for all recipients.
    encryptedContent is the result of encrypting the content.  The
    field is optional, and if the field is not present, its intended
    value must be supplied by other means.
 The recipientInfos field comes before the encryptedContentInfo field
 so that an EnvelopedData value may be processed in a single pass.

6.2. RecipientInfo Type

 Per-recipient information is represented in the type RecipientInfo.
 RecipientInfo has a different format for each of the supported key
 management techniques.  Any of the key management techniques can be

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 used for each recipient of the same encrypted content.  In all cases,
 the encrypted content-encryption key is transferred to one or more
 recipients.
 Since all implementations will not support every possible key
 management algorithm, all implementations MUST gracefully handle
 unimplemented algorithms when they are encountered.  For example, if
 a recipient receives a content-encryption key encrypted in their RSA
 public key using RSA-OAEP and the implementation only supports RSA
 PKCS #1 v1.5, then a graceful failure must be implemented.
 Implementations MUST support key transport, key agreement, and
 previously distributed symmetric key-encryption keys, as represented
 by ktri, kari, and kekri, respectively.  Implementations MAY support
 the password-based key management as represented by pwri.
 Implementations MAY support any other key management technique as
 represented by ori.  Since each recipient can employ a different key
 management technique and future specifications could define
 additional key management techniques, all implementations MUST
 gracefully handle unimplemented alternatives within the RecipientInfo
 CHOICE, all implementations MUST gracefully handle unimplemented
 versions of otherwise supported alternatives within the RecipientInfo
 CHOICE, and all implementations MUST gracefully handle unimplemented
 or unknown ori alternatives.
          RecipientInfo ::= CHOICE {
            ktri KeyTransRecipientInfo,
            kari [1] KeyAgreeRecipientInfo,
            kekri [2] KEKRecipientInfo,
            pwri [3] PasswordRecipientinfo,
            ori [4] OtherRecipientInfo }
          EncryptedKey ::= OCTET STRING

6.2.1. KeyTransRecipientInfo Type

 Per-recipient information using key transport is represented in the
 type KeyTransRecipientInfo.  Each instance of KeyTransRecipientInfo
 transfers the content-encryption key to one recipient.
    KeyTransRecipientInfo ::= SEQUENCE {
      version CMSVersion,  -- always set to 0 or 2
      rid RecipientIdentifier,
      keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
      encryptedKey EncryptedKey }

Housley Standards Track [Page 20] RFC 3852 Cryptographic Message Syntax July 2004

    RecipientIdentifier ::= CHOICE {
      issuerAndSerialNumber IssuerAndSerialNumber,
      subjectKeyIdentifier [0] SubjectKeyIdentifier }
 The fields of type KeyTransRecipientInfo have the following meanings:
    version is the syntax version number.  If the RecipientIdentifier
    is the CHOICE issuerAndSerialNumber, then the version MUST be 0.
    If the RecipientIdentifier is subjectKeyIdentifier, then the
    version MUST be 2.
    rid specifies the recipient's certificate or key that was used by
    the sender to protect the content-encryption key.  The content-
    encryption key is encrypted with the recipient's public key.  The
    RecipientIdentifier provides two alternatives for specifying the
    recipient's certificate, and thereby the recipient's public key.
    The recipient's certificate must contain a key transport public
    key.  Therefore, a recipient X.509 version 3 certificate that
    contains a key usage extension MUST assert the keyEncipherment
    bit.  The issuerAndSerialNumber alternative identifies the
    recipient's certificate by the issuer's distinguished name and the
    certificate serial number; the subjectKeyIdentifier identifies the
    recipient'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.  For
    recipient processing, implementations MUST support both of these
    alternatives for specifying the recipient's certificate.  For
    sender processing, implementations MUST support at least one of
    these alternatives.
    keyEncryptionAlgorithm identifies the key-encryption algorithm,
    and any associated parameters, used to encrypt the content-
    encryption key for the recipient.  The key-encryption process is
    described in Section 6.4.
    encryptedKey is the result of encrypting the content-encryption
    key for the recipient.

6.2.2. KeyAgreeRecipientInfo Type

 Recipient information using key agreement is represented in the type
 KeyAgreeRecipientInfo.  Each instance of KeyAgreeRecipientInfo will
 transfer the content-encryption key to one or more recipients that
 use the same key agreement algorithm and domain parameters for that
 algorithm.

Housley Standards Track [Page 21] RFC 3852 Cryptographic Message Syntax July 2004

    KeyAgreeRecipientInfo ::= SEQUENCE {
      version CMSVersion,  -- always set to 3
      originator [0] EXPLICIT OriginatorIdentifierOrKey,
      ukm [1] EXPLICIT UserKeyingMaterial OPTIONAL,
      keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
      recipientEncryptedKeys RecipientEncryptedKeys }
    OriginatorIdentifierOrKey ::= CHOICE {
      issuerAndSerialNumber IssuerAndSerialNumber,
      subjectKeyIdentifier [0] SubjectKeyIdentifier,
      originatorKey [1] OriginatorPublicKey }
    OriginatorPublicKey ::= SEQUENCE {
      algorithm AlgorithmIdentifier,
      publicKey BIT STRING }
    RecipientEncryptedKeys ::= SEQUENCE OF RecipientEncryptedKey
    RecipientEncryptedKey ::= SEQUENCE {
      rid KeyAgreeRecipientIdentifier,
      encryptedKey EncryptedKey }
    KeyAgreeRecipientIdentifier ::= CHOICE {
      issuerAndSerialNumber IssuerAndSerialNumber,
      rKeyId [0] IMPLICIT RecipientKeyIdentifier }
    RecipientKeyIdentifier ::= SEQUENCE {
      subjectKeyIdentifier SubjectKeyIdentifier,
      date GeneralizedTime OPTIONAL,
      other OtherKeyAttribute OPTIONAL }
    SubjectKeyIdentifier ::= OCTET STRING
 The fields of type KeyAgreeRecipientInfo have the following meanings:
    version is the syntax version number.  It MUST always be 3.
    originator is a CHOICE with three alternatives specifying the
    sender's key agreement public key.  The sender uses the
    corresponding private key and the recipient's public key to
    generate a pairwise key.  The content-encryption key is encrypted
    in the pairwise key.  The issuerAndSerialNumber alternative
    identifies the sender's certificate, and thereby the sender's
    public key, by the issuer's distinguished name and the certificate
    serial number.  The subjectKeyIdentifier alternative identifies
    the sender's certificate, and thereby the sender's public key, by
    a key identifier.  When an X.509 certificate is referenced, the
    key identifier matches the X.509 subjectKeyIdentifier extension

Housley Standards Track [Page 22] RFC 3852 Cryptographic Message Syntax July 2004

    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.  The originatorKey alternative
    includes the algorithm identifier and sender's key agreement
    public key.  This alternative permits originator anonymity since
    the public key is not certified.  Implementations MUST support all
    three alternatives for specifying the sender's public key.
    ukm is optional.  With some key agreement algorithms, the sender
    provides a User Keying Material (UKM) to ensure that a different
    key is generated each time the same two parties generate a
    pairwise key.  Implementations MUST accept a KeyAgreeRecipientInfo
    SEQUENCE that includes a ukm field.  Implementations that do not
    support key agreement algorithms that make use of UKMs MUST
    gracefully handle the presence of UKMs.
    keyEncryptionAlgorithm identifies the key-encryption algorithm,
    and any associated parameters, used to encrypt the content-
    encryption key with the key-encryption key.  The key-encryption
    process is described in Section 6.4.
    recipientEncryptedKeys includes a recipient identifier and
    encrypted key for one or more recipients.  The
    KeyAgreeRecipientIdentifier is a CHOICE with two alternatives
    specifying the recipient's certificate, and thereby the
    recipient's public key, that was used by the sender to generate a
    pairwise key-encryption key.  The recipient's certificate must
    contain a key agreement public key.  Therefore, a recipient X.509
    version 3 certificate that contains a key usage extension MUST
    assert the keyAgreement bit.  The content-encryption key is
    encrypted in the pairwise key-encryption key.  The
    issuerAndSerialNumber alternative identifies the recipient's
    certificate by the issuer's distinguished name and the certificate
    serial number; the RecipientKeyIdentifier is described below.  The
    encryptedKey is the result of encrypting the content-encryption
    key in the pairwise key-encryption key generated using the key
    agreement algorithm.  Implementations MUST support both
    alternatives for specifying the recipient's certificate.
 The fields of type RecipientKeyIdentifier have the following
 meanings:
    subjectKeyIdentifier identifies the recipient'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

Housley Standards Track [Page 23] RFC 3852 Cryptographic Message Syntax July 2004

    specify the certificate format and their use with the CMS must
    include details on matching the key identifier to the appropriate
    certificate field.
    date is optional.  When present, the date specifies which of the
    recipient's previously distributed UKMs was used by the sender.
    other is optional.  When present, this field contains additional
    information used by the recipient to locate the public keying
    material used by the sender.

6.2.3. KEKRecipientInfo Type

 Recipient information using previously distributed symmetric keys is
 represented in the type KEKRecipientInfo.  Each instance of
 KEKRecipientInfo will transfer the content-encryption key to one or
 more recipients who have the previously distributed key-encryption
 key.
    KEKRecipientInfo ::= SEQUENCE {
      version CMSVersion,  -- always set to 4
      kekid KEKIdentifier,
      keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
      encryptedKey EncryptedKey }
    KEKIdentifier ::= SEQUENCE {
      keyIdentifier OCTET STRING,
      date GeneralizedTime OPTIONAL,
      other OtherKeyAttribute OPTIONAL }
 The fields of type KEKRecipientInfo have the following meanings:
    version is the syntax version number.  It MUST always be 4.
    kekid specifies a symmetric key-encryption key that was previously
    distributed to the sender and one or more recipients.
    keyEncryptionAlgorithm identifies the key-encryption algorithm,
    and any associated parameters, used to encrypt the content-
    encryption key with the key-encryption key.  The key-encryption
    process is described in Section 6.4.
    encryptedKey is the result of encrypting the content-encryption
    key in the key-encryption key.

Housley Standards Track [Page 24] RFC 3852 Cryptographic Message Syntax July 2004

 The fields of type KEKIdentifier have the following meanings:
    keyIdentifier identifies the key-encryption key that was
    previously distributed to the sender and one or more recipients.
    date is optional.  When present, the date specifies a single key-
    encryption key from a set that was previously distributed.
    other is optional.  When present, this field contains additional
    information used by the recipient to determine the key-encryption
    key used by the sender.

6.2.4. PasswordRecipientInfo Type

 Recipient information using a password or shared secret value is
 represented in the type PasswordRecipientInfo.  Each instance of
 PasswordRecipientInfo will transfer the content-encryption key to one
 or more recipients who possess the password or shared secret value.
 The PasswordRecipientInfo Type is specified in RFC 3211 [PWRI].  The
 PasswordRecipientInfo structure is repeated here for completeness.
    PasswordRecipientInfo ::= SEQUENCE {
      version CMSVersion,   -- Always set to 0
      keyDerivationAlgorithm [0] KeyDerivationAlgorithmIdentifier
                                   OPTIONAL,
      keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
      encryptedKey EncryptedKey }
 The fields of type PasswordRecipientInfo have the following meanings:
    version is the syntax version number.  It MUST always be 0.
    keyDerivationAlgorithm identifies the key-derivation algorithm,
    and any associated parameters, used to derive the key-encryption
    key from the password or shared secret value.  If this field is
    absent, the key-encryption key is supplied from an external
    source, for example a hardware crypto token such as a smart card.
    keyEncryptionAlgorithm identifies the encryption algorithm, and
    any associated parameters, used to encrypt the content-encryption
    key with the key-encryption key.
    encryptedKey is the result of encrypting the content-encryption
    key with the key-encryption key.

Housley Standards Track [Page 25] RFC 3852 Cryptographic Message Syntax July 2004

6.2.5. OtherRecipientInfo Type

 Recipient information for additional key management techniques are
 represented in the type OtherRecipientInfo.  The OtherRecipientInfo
 type allows key management techniques beyond key transport, key
 agreement, previously distributed symmetric key-encryption keys, and
 password-based key management to be specified in future documents.
 An object identifier uniquely identifies such key management
 techniques.
    OtherRecipientInfo ::= SEQUENCE {
      oriType OBJECT IDENTIFIER,
      oriValue ANY DEFINED BY oriType }
 The fields of type OtherRecipientInfo have the following meanings:
    oriType identifies the key management technique.
    oriValue contains the protocol data elements needed by a recipient
    using the identified key management technique.

6.3. Content-encryption Process

 The content-encryption key for the desired content-encryption
 algorithm is randomly generated.  The data to be protected is padded
 as described below, then the padded data is encrypted using the
 content-encryption key.  The encryption operation maps an arbitrary
 string of octets (the data) to another string of octets (the
 ciphertext) under control of a content-encryption key.  The encrypted
 data is included in the EnvelopedData encryptedContentInfo
 encryptedContent OCTET STRING.
 Some content-encryption algorithms assume the input length is a
 multiple of k octets, where k is greater than one.  For such
 algorithms, the input shall be padded at the trailing end with k-(lth
 mod k) octets all having value k-(lth mod k), where lth is the length
 of the input.  In other words, the input is padded at the trailing
 end with one of the following strings:
                   01 -- if lth mod k = k-1
                02 02 -- if lth mod k = k-2
                    .
                    .
                    .
          k k ... k k -- if lth mod k = 0

Housley Standards Track [Page 26] RFC 3852 Cryptographic Message Syntax July 2004

 The padding can be removed unambiguously since all input is padded,
 including input values that are already a multiple of the block size,
 and no padding string is a suffix of another.  This padding method is
 well defined if and only if k is less than 256.

6.4. Key-encryption Process

 The input to the key-encryption process -- the value supplied to the
 recipient's key-encryption algorithm -- is just the "value" of the
 content-encryption key.
 Any of the aforementioned key management techniques can be used for
 each recipient of the same encrypted content.

7. Digested-data Content Type

 The digested-data content type consists of content of any type and a
 message digest of the content.
 Typically, the digested-data content type is used to provide content
 integrity, and the result generally becomes an input to the
 enveloped-data content type.
 The following steps construct digested-data:
    1. A message digest is computed on the content with a message-
       digest algorithm.
    2. The message-digest algorithm and the message digest are
       collected together with the content into a DigestedData value.
 A recipient verifies the message digest by comparing the message
 digest to an independently computed message digest.
 The following object identifier identifies the digested-data content
 type:
    id-digestedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
        us(840) rsadsi(113549) pkcs(1) pkcs7(7) 5 }

Housley Standards Track [Page 27] RFC 3852 Cryptographic Message Syntax July 2004

 The digested-data content type shall have ASN.1 type DigestedData:
    DigestedData ::= SEQUENCE {
      version CMSVersion,
      digestAlgorithm DigestAlgorithmIdentifier,
      encapContentInfo EncapsulatedContentInfo,
      digest Digest }
    Digest ::= OCTET STRING
 The fields of type DigestedData have the following meanings:
    version is the syntax version number.  If the encapsulated content
    type is id-data, then the value of version MUST be 0; however, if
    the encapsulated content type is other than id-data, then the
    value of version MUST be 2.
    digestAlgorithm identifies the message digest algorithm, and any
    associated parameters, under which the content is digested.  The
    message-digesting process is the same as in Section 5.4 in the
    case when there are no signed attributes.
    encapContentInfo is the content that is digested, as defined in
    section 5.2.
    digest is the result of the message-digesting process.
 The ordering of the digestAlgorithm field, the encapContentInfo
 field, and the digest field makes it possible to process a
 DigestedData value in a single pass.

8. Encrypted-data Content Type

 The encrypted-data content type consists of encrypted content of any
 type.  Unlike the enveloped-data content type, the encrypted-data
 content type has neither recipients nor encrypted content-encryption
 keys.  Keys MUST be managed by other means.
 The typical application of the encrypted-data content type will be to
 encrypt the content of the data content type for local storage,
 perhaps where the encryption key is derived from a password.
 The following object identifier identifies the encrypted-data content
 type:
    id-encryptedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
        us(840) rsadsi(113549) pkcs(1) pkcs7(7) 6 }

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 The encrypted-data content type shall have ASN.1 type EncryptedData:
    EncryptedData ::= SEQUENCE {
      version CMSVersion,
      encryptedContentInfo EncryptedContentInfo,
      unprotectedAttrs [1] IMPLICIT UnprotectedAttributes OPTIONAL }
 The fields of type EncryptedData have the following meanings:
    version is the syntax version number.  If unprotectedAttrs is
    present, then version MUST be 2.  If unprotectedAttrs is absent,
    then version MUST be 0.
    encryptedContentInfo is the encrypted content information, as
    defined in Section 6.1.
    unprotectedAttrs is a collection of attributes that are not
    encrypted.  The field is optional.  Useful attribute types are
    defined in Section 11.

9. Authenticated-data Content Type

 The authenticated-data content type consists of content of any type,
 a message authentication code (MAC), and encrypted authentication
 keys for one or more recipients.  The combination of the MAC and one
 encrypted authentication key for a recipient is necessary for that
 recipient to verify the integrity of the content.  Any type of
 content can be integrity protected for an arbitrary number of
 recipients.
 The process by which authenticated-data is constructed involves the
 following steps:
    1. A message-authentication key for a particular message-
       authentication algorithm is generated at random.
    2. The message-authentication key is encrypted for each recipient.
       The details of this encryption depend on the key management
       algorithm used.
    3. For each recipient, the encrypted message-authentication key
       and other recipient-specific information are collected into a
       RecipientInfo value, defined in Section 6.2.
    4. Using the message-authentication key, the originator computes a
       MAC value on the content.  If the originator is authenticating
       any information in addition to the content (see Section 9.2), a
       message digest is calculated on the content, the message digest

Housley Standards Track [Page 29] RFC 3852 Cryptographic Message Syntax July 2004

       of the content and the other information are authenticated
       using the message-authentication key, and the result becomes
       the "MAC value."

9.1. AuthenticatedData Type

 The following object identifier identifies the authenticated-data
 content type:
    id-ct-authData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
       us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16)
       ct(1) 2 }
 The authenticated-data content type shall have ASN.1 type
 AuthenticatedData:
    AuthenticatedData ::= SEQUENCE {
      version CMSVersion,
      originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL,
      recipientInfos RecipientInfos,
      macAlgorithm MessageAuthenticationCodeAlgorithm,
      digestAlgorithm [1] DigestAlgorithmIdentifier OPTIONAL,
      encapContentInfo EncapsulatedContentInfo,
      authAttrs [2] IMPLICIT AuthAttributes OPTIONAL,
      mac MessageAuthenticationCode,
      unauthAttrs [3] IMPLICIT UnauthAttributes OPTIONAL }
    AuthAttributes ::= SET SIZE (1..MAX) OF Attribute
    UnauthAttributes ::= SET SIZE (1..MAX) OF Attribute
    MessageAuthenticationCode ::= OCTET STRING
 The fields of type AuthenticatedData have the following meanings:
    version is the syntax version number.  The version MUST be
    assigned as follows:
       IF (originatorInfo is present) AND
          ((any certificates with a type of other are present) OR
          (any crls with a type of other are present))
       THEN version is 3
       ELSE
          IF ((originatorInfo is present) AND
             (any version 2 attribute certificates are present))
          THEN version is 1
          ELSE version is 0

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    originatorInfo optionally provides information about the
    originator.  It is present only if required by the key management
    algorithm.  It MAY contain certificates, attribute certificates,
    and CRLs, as defined in Section 6.1.
    recipientInfos is a collection of per-recipient information, as
    defined in Section 6.1.  There MUST be at least one element in the
    collection.
    macAlgorithm is a message authentication code (MAC) algorithm
    identifier.  It identifies the MAC algorithm, along with any
    associated parameters, used by the originator.  Placement of the
    macAlgorithm field facilitates one-pass processing by the
    recipient.
    digestAlgorithm identifies the message digest algorithm, and any
    associated parameters, used to compute a message digest on the
    encapsulated content if authenticated attributes are present.  The
    message digesting process is described in Section 9.2.  Placement
    of the digestAlgorithm field facilitates one-pass processing by
    the recipient.  If the digestAlgorithm field is present, then the
    authAttrs field MUST also be present.
    encapContentInfo is the content that is authenticated, as defined
    in section 5.2.
    authAttrs is a collection of authenticated attributes.  The
    authAttrs structure is optional, but it MUST be present if the
    content type of the EncapsulatedContentInfo value being
    authenticated is not id-data.  If the authAttrs field is present,
    then the digestAlgorithm field MUST also be present.  The
    AuthAttributes structure MUST be DER encoded, even if the rest of
    the structure is BER encoded.  Useful attribute types are defined
    in Section 11.  If the authAttrs 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 authenticated.
       Section 11.1 defines the content-type attribute.
       A message-digest attribute, having as its value the message
       digest of the content.  Section 11.2 defines the message-digest
       attribute.
    mac is the message authentication code.

Housley Standards Track [Page 31] RFC 3852 Cryptographic Message Syntax July 2004

    unauthAttrs is a collection of attributes that are not
    authenticated.  The field is optional.  To date, no attributes
    have been defined for use as unauthenticated attributes, but other
    useful attribute types are defined in Section 11.

9.2. MAC Generation

 The MAC calculation process computes a message authentication code
 (MAC) on either the content being authenticated or a message digest
 of content being authenticated together with the originator's
 authenticated attributes.
 If authAttrs field is absent, the input to the MAC calculation
 process is the value of the encapContentInfo eContent OCTET STRING.
 Only the octets comprising the value of the eContent OCTET STRING are
 input to the MAC algorithm; the tag and the length octets are
 omitted.  This has the advantage that the length of the content being
 authenticated need not be known in advance of the MAC generation
 process.
 If authAttrs field is present, the content-type attribute (as
 described in Section 11.1) and the message-digest attribute (as
 described in section 11.2) MUST be included, and the input to the MAC
 calculation process is the DER encoding of authAttrs.  A separate
 encoding of the authAttrs field is performed for message digest
 calculation.  The IMPLICIT [2] tag in the authAttrs field is not used
 for the DER encoding, rather an EXPLICIT SET OF tag is used.  That
 is, the DER encoding of the SET OF tag, rather than of the IMPLICIT
 [2] tag, is to be included in the message digest calculation along
 with the length and content octets of the authAttrs value.
 The message digest calculation process computes a message digest on
 the content being authenticated.  The initial input to the message
 digest calculation process is the "value" of the encapsulated content
 being authenticated.  Specifically, the input is the encapContentInfo
 eContent OCTET STRING to which the authentication process is applied.
 Only the octets comprising the value of the encapContentInfo eContent
 OCTET STRING are input to the message digest algorithm, not the tag
 or the length octets.  This has the advantage that the length of the
 content being authenticated need not be known in advance.  Although
 the encapContentInfo eContent OCTET STRING tag and length octets are
 not included in the message digest calculation, they are still
 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 contents of any length that have
 the same message digest.

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 The input to the MAC calculation process includes the MAC input data,
 defined above, and an authentication key conveyed in a recipientInfo
 structure.  The details of MAC calculation depend on the MAC
 algorithm employed (e.g., HMAC).  The object identifier, along with
 any parameters, that specifies the MAC algorithm employed by the
 originator is carried in the macAlgorithm field.  The MAC value
 generated by the originator is encoded as an OCTET STRING and carried
 in the mac field.

9.3. MAC Verification

 The input to the MAC verification process includes the input data
 (determined based on the presence or absence of the authAttrs field,
 as defined in 9.2), and the authentication key conveyed in
 recipientInfo.  The details of the MAC verification process depend on
 the MAC algorithm employed.
 The recipient MUST NOT rely on any MAC values or message digest
 values computed by the originator.  The content is authenticated as
 described in section 9.2.  If the originator includes authenticated
 attributes, then the content of the authAttrs is authenticated as
 described in section 9.2.  For authentication to succeed, the MAC
 value calculated by the recipient MUST be the same as the value of
 the mac field.  Similarly, for authentication to succeed when the
 authAttrs field is present, the content message digest value
 calculated by the recipient MUST be the same as the message digest
 value included in the authAttrs message-digest attribute.
 If the AuthenticatedData includes authAttrs, then the content-type
 attribute value MUST match the AuthenticatedData encapContentInfo
 eContentType value.

10. Useful Types

 This section is divided into two parts.  The first part defines
 algorithm identifiers, and the second part defines other useful
 types.

10.1. Algorithm Identifier Types

 All of the algorithm identifiers have the same type:
 AlgorithmIdentifier.  The definition of AlgorithmIdentifier is taken
 from X.509 [X.509-88].
 There are many alternatives for each algorithm type.

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10.1.1. DigestAlgorithmIdentifier

 The DigestAlgorithmIdentifier type identifies a message-digest
 algorithm.  Examples include SHA-1, MD2, and MD5.  A message-digest
 algorithm maps an octet string (the content) to another octet string
 (the message digest).
    DigestAlgorithmIdentifier ::= AlgorithmIdentifier

10.1.2. SignatureAlgorithmIdentifier

 The SignatureAlgorithmIdentifier type identifies a signature
 algorithm.  Examples include RSA, DSA, and ECDSA.  A signature
 algorithm supports signature generation and verification operations.
 The signature generation operation uses the message digest and the
 signer's private key to generate a signature value.  The signature
 verification operation uses the message digest and the signer's
 public key to determine whether or not a signature value is valid.
 Context determines which operation is intended.
    SignatureAlgorithmIdentifier ::= AlgorithmIdentifier

10.1.3. KeyEncryptionAlgorithmIdentifier

 The KeyEncryptionAlgorithmIdentifier type identifies a key-encryption
 algorithm used to encrypt a content-encryption key.  The encryption
 operation maps an octet string (the key) to another octet string (the
 encrypted key) under control of a key-encryption key.  The decryption
 operation is the inverse of the encryption operation.  Context
 determines which operation is intended.
 The details of encryption and decryption depend on the key management
 algorithm used.  Key transport, key agreement, previously distributed
 symmetric key-encrypting keys, and symmetric key-encrypting keys
 derived from passwords are supported.
    KeyEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier

10.1.4. ContentEncryptionAlgorithmIdentifier

 The ContentEncryptionAlgorithmIdentifier type identifies a content-
 encryption algorithm.  Examples include Triple-DES and RC2.  A
 content-encryption algorithm supports encryption and decryption
 operations.  The encryption operation maps an octet string (the
 plaintext) to another octet string (the ciphertext) under control of

Housley Standards Track [Page 34] RFC 3852 Cryptographic Message Syntax July 2004

 a content-encryption key.  The decryption operation is the inverse of
 the encryption operation.  Context determines which operation is
 intended.
    ContentEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier

10.1.5. MessageAuthenticationCodeAlgorithm

 The MessageAuthenticationCodeAlgorithm type identifies a message
 authentication code (MAC) algorithm.  Examples include DES-MAC and
 HMAC-SHA-1.  A MAC algorithm supports generation and verification
 operations.  The MAC generation and verification operations use the
 same symmetric key.  Context determines which operation is intended.
    MessageAuthenticationCodeAlgorithm ::= AlgorithmIdentifier

10.1.6. KeyDerivationAlgorithmIdentifier

 The KeyDerivationAlgorithmIdentifier type is specified in RFC 3211
 [PWRI].  The KeyDerivationAlgorithmIdentifier definition is repeated
 here for completeness.
 Key derivation algorithms convert a password or shared secret value
 into a key-encryption key.
    KeyDerivationAlgorithmIdentifier ::= AlgorithmIdentifier

10.2. Other Useful Types

 This section defines types that are used other places in the
 document.  The types are not listed in any particular order.

10.2.1. RevocationInfoChoices

 The RevocationInfoChoices type gives a set of revocation status
 information alternatives.  It is intended that the set contain
 information sufficient to determine whether the certificates and
 attribute certificates with which the set is associated are revoked.
 However, there MAY be more revocation status information than
 necessary or there MAY be less revocation status information than
 necessary.  X.509 Certificate revocation lists (CRLs) [X.509-97] are
 the primary source of revocation status information, but any other
 revocation information format can be supported.  The
 OtherRevocationInfoFormat alternative is provided to support any
 other revocation information format without further modifications to
 the CMS.  For example, Online Certificate Status Protocol (OCSP)
 Responses [OCSP] can be supported using the
 OtherRevocationInfoFormat.

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 The CertificateList may contain a CRL, an Authority Revocation List
 (ARL), a Delta CRL, or an Attribute Certificate Revocation List.  All
 of these lists share a common syntax.
 The CertificateList type gives a certificate revocation list (CRL).
 CRLs are specified in X.509 [X.509-97], and they are profiled for use
 in the Internet in RFC 3280 [PROFILE].
 The definition of CertificateList is taken from X.509.
    RevocationInfoChoices ::= SET OF RevocationInfoChoice
    RevocationInfoChoice ::= CHOICE {
      crl CertificateList,
      other [1] IMPLICIT OtherRevocationInfoFormat }
    OtherRevocationInfoFormat ::= SEQUENCE {
      otherRevInfoFormat OBJECT IDENTIFIER,
      otherRevInfo ANY DEFINED BY otherRevInfoFormat }

10.2.2. CertificateChoices

 The CertificateChoices type gives either a PKCS #6 extended
 certificate [PKCS#6], an X.509 certificate, a version 1 X.509
 attribute certificate (ACv1) [X.509-97], a version 2 X.509 attribute
 certificate (ACv2) [X.509-00], or any other certificate format.  The
 PKCS #6 extended certificate is obsolete.  The PKCS #6 certificate is
 included for backward compatibility, and PKCS #6 certificates SHOULD
 NOT be used.  The ACv1 is also obsolete.  ACv1 is included for
 backward compatibility, and ACv1 SHOULD NOT be used.  The Internet
 profile of X.509 certificates is specified in the "Internet X.509
 Public Key Infrastructure: Certificate and CRL Profile" [PROFILE].
 The Internet profile of ACv2 is specified in the "An Internet
 Attribute Certificate Profile for Authorization" [ACPROFILE].  The
 OtherCertificateFormat alternative is provided to support any other
 certificate format without further modifications to the CMS.
 The definition of Certificate is taken from X.509.
 The definitions of AttributeCertificate are taken from X.509-1997 and
 X.509-2000.  The definition from X.509-1997 is assigned to
 AttributeCertificateV1 (see section 12.2), and the definition from
 X.509-2000 is assigned to AttributeCertificateV2.

Housley Standards Track [Page 36] RFC 3852 Cryptographic Message Syntax July 2004

    CertificateChoices ::= CHOICE {
     certificate Certificate,
     extendedCertificate [0] IMPLICIT ExtendedCertificate, -- Obsolete
     v1AttrCert [1] IMPLICIT AttributeCertificateV1,       -- Obsolete
     v2AttrCert [2] IMPLICIT AttributeCertificateV2,
     other [3] IMPLICIT OtherCertificateFormat }
    OtherCertificateFormat ::= SEQUENCE {
     otherCertFormat OBJECT IDENTIFIER,
     otherCert ANY DEFINED BY otherCertFormat }

10.2.3. CertificateSet

 The CertificateSet type provides a set of certificates.  It is
 intended that the set be sufficient to contain certification paths
 from a recognized "root" or "top-level certification authority" to
 all of the sender certificates with which the set is associated.
 However, there may be more certificates than necessary, or there MAY
 be fewer than necessary.
 The precise meaning of a "certification path" is outside the scope of
 this document.  However, [PROFILE] provides a definition for X.509
 certificates.  Some applications may impose upper limits on the
 length of a certification path; others may enforce certain
 relationships between the subjects and issuers of certificates within
 a certification path.
    CertificateSet ::= SET OF CertificateChoices

10.2.4. IssuerAndSerialNumber

 The IssuerAndSerialNumber type identifies a certificate, and thereby
 an entity and a public key, by the distinguished name of the
 certificate issuer and an issuer-specific certificate serial number.
 The definition of Name is taken from X.501 [X.501-88], and the
 definition of CertificateSerialNumber is taken from X.509 [X.509-97].
    IssuerAndSerialNumber ::= SEQUENCE {
      issuer Name,
      serialNumber CertificateSerialNumber }
    CertificateSerialNumber ::= INTEGER

Housley Standards Track [Page 37] RFC 3852 Cryptographic Message Syntax July 2004

10.2.5. CMSVersion

 The CMSVersion type gives a syntax version number, for compatibility
 with future revisions of this specification.
    CMSVersion ::= INTEGER
                   { v0(0), v1(1), v2(2), v3(3), v4(4), v5(5) }

10.2.6. UserKeyingMaterial

 The UserKeyingMaterial type gives a syntax for user keying material
 (UKM).  Some key agreement algorithms require UKMs to ensure that a
 different key is generated each time the same two parties generate a
 pairwise key.  The sender provides a UKM for use with a specific key
 agreement algorithm.
    UserKeyingMaterial ::= OCTET STRING

10.2.7. OtherKeyAttribute

 The OtherKeyAttribute type gives a syntax for the inclusion of other
 key attributes that permit the recipient to select the key used by
 the sender.  The attribute object identifier must be registered along
 with the syntax of the attribute itself.  Use of this structure
 should be avoided since it might impede interoperability.
    OtherKeyAttribute ::= SEQUENCE {
      keyAttrId OBJECT IDENTIFIER,
      keyAttr ANY DEFINED BY keyAttrId OPTIONAL }

11. Useful Attributes

 This section defines attributes that may be used with signed-data,
 enveloped-data, encrypted-data, or authenticated-data.  The syntax of
 Attribute is compatible with X.501 [X.501-88] and RFC 3280 [PROFILE].
 Some of the attributes defined in this section were originally
 defined in PKCS #9 [PKCS#9]; others were originally defined in a
 previous version of this specification [CMS1].  The attributes are
 not listed in any particular order.
 Additional attributes are defined in many places, notably the S/MIME
 Version 3 Message Specification [MSG] and the Enhanced Security
 Services for S/MIME [ESS], which also include recommendations on the
 placement of these attributes.

Housley Standards Track [Page 38] RFC 3852 Cryptographic Message Syntax July 2004

11.1. Content Type

 The content-type attribute type specifies the content type of the
 ContentInfo within signed-data or authenticated-data.  The content-
 type attribute type MUST be present whenever signed attributes are
 present in signed-data or authenticated attributes present in
 authenticated-data.  The content-type attribute value MUST match the
 encapContentInfo eContentType value in the signed-data or
 authenticated-data.
 The content-type attribute MUST be a signed attribute or an
 authenticated attribute; it MUST NOT be an unsigned attribute,
 unauthenticated attribute, or unprotected attribute.
 The following object identifier identifies the content-type
 attribute:
    id-contentType OBJECT IDENTIFIER ::= { iso(1) member-body(2)
        us(840) rsadsi(113549) pkcs(1) pkcs9(9) 3 }
 Content-type attribute values have ASN.1 type ContentType:
    ContentType ::= OBJECT IDENTIFIER
 Even though the syntax is defined as a SET OF AttributeValue, a
 content-type attribute MUST have a single attribute value; zero or
 multiple instances of AttributeValue are not permitted.
 The SignedAttributes and AuthAttributes syntaxes are each defined as
 a SET OF Attributes.  The SignedAttributes in a signerInfo MUST NOT
 include multiple instances of the content-type attribute.  Similarly,
 the AuthAttributes in an AuthenticatedData MUST NOT include multiple
 instances of the content-type attribute.

11.2. Message Digest

 The message-digest attribute type specifies the message digest of the
 encapContentInfo eContent OCTET STRING being signed in signed-data
 (see section 5.4) or authenticated in authenticated-data (see section
 9.2).  For signed-data, the message digest is computed using the
 signer's message digest algorithm.  For authenticated-data, the
 message digest is computed using the originator's message digest
 algorithm.
 Within signed-data, the message-digest signed attribute type MUST be
 present when there are any signed attributes present.  Within
 authenticated-data, the message-digest authenticated attribute type
 MUST be present when there are any authenticated attributes present.

Housley Standards Track [Page 39] RFC 3852 Cryptographic Message Syntax July 2004

 The message-digest attribute MUST be a signed attribute or an
 authenticated attribute; it MUST NOT be an unsigned attribute,
 unauthenticated attribute, or unprotected attribute.
 The following object identifier identifies the message-digest
 attribute:
    id-messageDigest OBJECT IDENTIFIER ::= { iso(1) member-body(2)
        us(840) rsadsi(113549) pkcs(1) pkcs9(9) 4 }
 Message-digest attribute values have ASN.1 type MessageDigest:
    MessageDigest ::= OCTET STRING
 A message-digest attribute MUST have a single attribute value, even
 though the syntax is defined as a SET OF AttributeValue.  There MUST
 NOT be zero or multiple instances of AttributeValue present.
 The SignedAttributes syntax and AuthAttributes syntax are each
 defined as a SET OF Attributes.  The SignedAttributes in a signerInfo
 MUST include only one instance of the message-digest attribute.
 Similarly, the AuthAttributes in an AuthenticatedData MUST include
 only one instance of the message-digest attribute.

11.3. Signing Time

 The signing-time attribute type specifies the time at which the
 signer (purportedly) performed the signing process.  The signing-time
 attribute type is intended for use in signed-data.
 The signing-time attribute MUST be a signed attribute or an
 authenticated attribute; it MUST NOT be an unsigned attribute,
 unauthenticated attribute, or unprotected attribute.
 The following object identifier identifies the signing-time
 attribute:
    id-signingTime OBJECT IDENTIFIER ::= { iso(1) member-body(2)
        us(840) rsadsi(113549) pkcs(1) pkcs9(9) 5 }
 Signing-time attribute values have ASN.1 type SigningTime:
    SigningTime ::= Time
    Time ::= CHOICE {
      utcTime UTCTime,
      generalizedTime GeneralizedTime }

Housley Standards Track [Page 40] RFC 3852 Cryptographic Message Syntax July 2004

 Note: The definition of Time matches the one specified in the 1997
 version of X.509 [X.509-97].
 Dates between 1 January 1950 and 31 December 2049 (inclusive) MUST be
 encoded as UTCTime.  Any dates with year values before 1950 or after
 2049 MUST be encoded as GeneralizedTime.
 UTCTime values MUST be expressed in Coordinated Universal Time
 (formerly known as Greenwich Mean Time (GMT) and Zulu clock time) and
 MUST include seconds (i.e., times are YYMMDDHHMMSSZ), even where the
 number of seconds is zero.  Midnight MUST be represented as
 "YYMMDD000000Z".  Century information is implicit, and the century
 MUST be determined as follows:
    Where YY is greater than or equal to 50, the year MUST be
    interpreted as 19YY; and
    Where YY is less than 50, the year MUST be interpreted as 20YY.
 GeneralizedTime values MUST be expressed in Coordinated Universal
 Time and MUST include seconds (i.e., times are YYYYMMDDHHMMSSZ), even
 where the number of seconds is zero.  GeneralizedTime values MUST NOT
 include fractional seconds.
 A signing-time attribute MUST have a single attribute value, even
 though the syntax is defined as a SET OF AttributeValue.  There MUST
 NOT be zero or multiple instances of AttributeValue present.
 The SignedAttributes syntax and the AuthAttributes syntax are each
 defined as a SET OF Attributes.  The SignedAttributes in a signerInfo
 MUST NOT include multiple instances of the signing-time attribute.
 Similarly, the AuthAttributes in an AuthenticatedData MUST NOT
 include multiple instances of the signing-time attribute.
 No requirement is imposed concerning the correctness of the signing
 time, and acceptance of a purported signing time is a matter of a
 recipient's discretion.  It is expected, however, that some signers,
 such as time-stamp servers, will be trusted implicitly.

11.4. Countersignature

 The countersignature attribute type specifies one or more signatures
 on the contents octets of the signature OCTET STRING in a SignerInfo
 value of the signed-data.  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.  Thus, the countersignature
 attribute type countersigns (signs in serial) another signature.

Housley Standards Track [Page 41] RFC 3852 Cryptographic Message Syntax July 2004

 The countersignature attribute MUST be an unsigned attribute; it MUST
 NOT be a signed attribute, an authenticated attribute, an
 unauthenticated attribute, or an unprotected attribute.
 The following object identifier identifies the countersignature
 attribute:
    id-countersignature OBJECT IDENTIFIER ::= { iso(1) member-body(2)
        us(840) rsadsi(113549) pkcs(1) pkcs9(9) 6 }
 Countersignature attribute values have ASN.1 type Countersignature:
    Countersignature ::= SignerInfo
 Countersignature values have the same meaning as SignerInfo values
 for ordinary signatures, except that:
    1. The signedAttributes field MUST NOT contain a content-type
       attribute; there is no content type for countersignatures.
    2. The signedAttributes field MUST contain a message-digest
       attribute if it contains any other attributes.
    3. The input to the message-digesting process is the contents
       octets of the DER encoding of the signatureValue field of the
       SignerInfo value with which the attribute is associated.
 A countersignature attribute can have multiple attribute values.  The
 syntax is defined as a SET OF AttributeValue, and there MUST be one
 or more instances of AttributeValue present.
 The UnsignedAttributes syntax is defined as a SET OF Attributes.  The
 UnsignedAttributes in a signerInfo may include multiple instances of
 the countersignature attribute.
 A countersignature, since it has type SignerInfo, can itself contain
 a countersignature attribute.  Thus, it is possible to construct an
 arbitrarily long series of countersignatures.

12. ASN.1 Modules

 Section 12.1 contains the ASN.1 module for the CMS, and section 12.2
 contains the ASN.1 module for the Version 1 Attribute Certificate.

Housley Standards Track [Page 42] RFC 3852 Cryptographic Message Syntax July 2004

12.1. CMS ASN.1 Module

 CryptographicMessageSyntax2004
   { iso(1) member-body(2) us(840) rsadsi(113549)
     pkcs(1) pkcs-9(9) smime(16) modules(0) cms-2004(24) }
 DEFINITIONS IMPLICIT TAGS ::=
 BEGIN
  1. - EXPORTS All
  2. - The types and values defined in this module are exported for use
  3. - in the other ASN.1 modules. Other applications may use them for
  4. - their own purposes.
 IMPORTS
  1. - Imports from RFC 3280 [PROFILE], Appendix A.1

AlgorithmIdentifier, Certificate, CertificateList,

         CertificateSerialNumber, Name
            FROM PKIX1Explicit88
                 { iso(1) identified-organization(3) dod(6)
                   internet(1) security(5) mechanisms(5) pkix(7)
                   mod(0) pkix1-explicit(18) }
  1. - Imports from RFC 3281 [ACPROFILE], Appendix B

AttributeCertificate

            FROM PKIXAttributeCertificate
                 { iso(1) identified-organization(3) dod(6)
                   internet(1) security(5) mechanisms(5) pkix(7)
                   mod(0) attribute-cert(12) }
  1. - Imports from Appendix B of this document

AttributeCertificateV1

            FROM AttributeCertificateVersion1
                 { iso(1) member-body(2) us(840) rsadsi(113549)
                   pkcs(1) pkcs-9(9) smime(16) modules(0)
                   v1AttrCert(15) } ;
  1. - Cryptographic Message Syntax
 ContentInfo ::= SEQUENCE {
   contentType ContentType,
   content [0] EXPLICIT ANY DEFINED BY contentType }
 ContentType ::= OBJECT IDENTIFIER

Housley Standards Track [Page 43] RFC 3852 Cryptographic Message Syntax July 2004

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

Housley Standards Track [Page 44] RFC 3852 Cryptographic Message Syntax July 2004

 EnvelopedData ::= SEQUENCE {
   version CMSVersion,
   originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL,
   recipientInfos RecipientInfos,
   encryptedContentInfo EncryptedContentInfo,
   unprotectedAttrs [1] IMPLICIT UnprotectedAttributes OPTIONAL }
 OriginatorInfo ::= SEQUENCE {
   certs [0] IMPLICIT CertificateSet OPTIONAL,
   crls [1] IMPLICIT RevocationInfoChoices OPTIONAL }
 RecipientInfos ::= SET SIZE (1..MAX) OF RecipientInfo
 EncryptedContentInfo ::= SEQUENCE {
   contentType ContentType,
   contentEncryptionAlgorithm ContentEncryptionAlgorithmIdentifier,
   encryptedContent [0] IMPLICIT EncryptedContent OPTIONAL }
 EncryptedContent ::= OCTET STRING
 UnprotectedAttributes ::= SET SIZE (1..MAX) OF Attribute
 RecipientInfo ::= CHOICE {
   ktri KeyTransRecipientInfo,
   kari [1] KeyAgreeRecipientInfo,
   kekri [2] KEKRecipientInfo,
   pwri [3] PasswordRecipientInfo,
   ori [4] OtherRecipientInfo }
 EncryptedKey ::= OCTET STRING
 KeyTransRecipientInfo ::= SEQUENCE {
   version CMSVersion,  -- always set to 0 or 2
   rid RecipientIdentifier,
   keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
   encryptedKey EncryptedKey }
 RecipientIdentifier ::= CHOICE {
   issuerAndSerialNumber IssuerAndSerialNumber,
   subjectKeyIdentifier [0] SubjectKeyIdentifier }
 KeyAgreeRecipientInfo ::= SEQUENCE {
   version CMSVersion,  -- always set to 3
   originator [0] EXPLICIT OriginatorIdentifierOrKey,
   ukm [1] EXPLICIT UserKeyingMaterial OPTIONAL,
   keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
   recipientEncryptedKeys RecipientEncryptedKeys }

Housley Standards Track [Page 45] RFC 3852 Cryptographic Message Syntax July 2004

 OriginatorIdentifierOrKey ::= CHOICE {
   issuerAndSerialNumber IssuerAndSerialNumber,
   subjectKeyIdentifier [0] SubjectKeyIdentifier,
   originatorKey [1] OriginatorPublicKey }
 OriginatorPublicKey ::= SEQUENCE {
   algorithm AlgorithmIdentifier,
   publicKey BIT STRING }
 RecipientEncryptedKeys ::= SEQUENCE OF RecipientEncryptedKey
 RecipientEncryptedKey ::= SEQUENCE {
   rid KeyAgreeRecipientIdentifier,
   encryptedKey EncryptedKey }
 KeyAgreeRecipientIdentifier ::= CHOICE {
   issuerAndSerialNumber IssuerAndSerialNumber,
   rKeyId [0] IMPLICIT RecipientKeyIdentifier }
 RecipientKeyIdentifier ::= SEQUENCE {
   subjectKeyIdentifier SubjectKeyIdentifier,
   date GeneralizedTime OPTIONAL,
   other OtherKeyAttribute OPTIONAL }
 SubjectKeyIdentifier ::= OCTET STRING
 KEKRecipientInfo ::= SEQUENCE {
   version CMSVersion,  -- always set to 4
   kekid KEKIdentifier,
   keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
   encryptedKey EncryptedKey }
 KEKIdentifier ::= SEQUENCE {
   keyIdentifier OCTET STRING,
   date GeneralizedTime OPTIONAL,
   other OtherKeyAttribute OPTIONAL }
 PasswordRecipientInfo ::= SEQUENCE {
   version CMSVersion,   -- always set to 0
   keyDerivationAlgorithm [0] KeyDerivationAlgorithmIdentifier
                              OPTIONAL,
   keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
   encryptedKey EncryptedKey }
 OtherRecipientInfo ::= SEQUENCE {
   oriType OBJECT IDENTIFIER,
   oriValue ANY DEFINED BY oriType }

Housley Standards Track [Page 46] RFC 3852 Cryptographic Message Syntax July 2004

 DigestedData ::= SEQUENCE {
   version CMSVersion,
   digestAlgorithm DigestAlgorithmIdentifier,
   encapContentInfo EncapsulatedContentInfo,
   digest Digest }
 Digest ::= OCTET STRING
 EncryptedData ::= SEQUENCE {
   version CMSVersion,
   encryptedContentInfo EncryptedContentInfo,
   unprotectedAttrs [1] IMPLICIT UnprotectedAttributes OPTIONAL }
 AuthenticatedData ::= SEQUENCE {
   version CMSVersion,
   originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL,
   recipientInfos RecipientInfos,
   macAlgorithm MessageAuthenticationCodeAlgorithm,
   digestAlgorithm [1] DigestAlgorithmIdentifier OPTIONAL,
   encapContentInfo EncapsulatedContentInfo,
   authAttrs [2] IMPLICIT AuthAttributes OPTIONAL,
   mac MessageAuthenticationCode,
   unauthAttrs [3] IMPLICIT UnauthAttributes OPTIONAL }
 AuthAttributes ::= SET SIZE (1..MAX) OF Attribute
 UnauthAttributes ::= SET SIZE (1..MAX) OF Attribute
 MessageAuthenticationCode ::= OCTET STRING
 DigestAlgorithmIdentifier ::= AlgorithmIdentifier
 SignatureAlgorithmIdentifier ::= AlgorithmIdentifier
 KeyEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier
 ContentEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier
 MessageAuthenticationCodeAlgorithm ::= AlgorithmIdentifier
 KeyDerivationAlgorithmIdentifier ::= AlgorithmIdentifier
 RevocationInfoChoices ::= SET OF RevocationInfoChoice
 RevocationInfoChoice ::= CHOICE {
   crl CertificateList,
   other [1] IMPLICIT OtherRevocationInfoFormat }

Housley Standards Track [Page 47] RFC 3852 Cryptographic Message Syntax July 2004

 OtherRevocationInfoFormat ::= SEQUENCE {
   otherRevInfoFormat OBJECT IDENTIFIER,
   otherRevInfo ANY DEFINED BY otherRevInfoFormat }
 CertificateChoices ::= CHOICE {
   certificate Certificate,
   extendedCertificate [0] IMPLICIT ExtendedCertificate,  -- Obsolete
   v1AttrCert [1] IMPLICIT AttributeCertificateV1,        -- Obsolete
   v2AttrCert [2] IMPLICIT AttributeCertificateV2,
   other [3] IMPLICIT OtherCertificateFormat }
 AttributeCertificateV2 ::= AttributeCertificate
 OtherCertificateFormat ::= SEQUENCE {
   otherCertFormat OBJECT IDENTIFIER,
   otherCert ANY DEFINED BY otherCertFormat }
 CertificateSet ::= SET OF CertificateChoices
 IssuerAndSerialNumber ::= SEQUENCE {
   issuer Name,
   serialNumber CertificateSerialNumber }
 CMSVersion ::= INTEGER  { v0(0), v1(1), v2(2), v3(3), v4(4), v5(5) }
 UserKeyingMaterial ::= OCTET STRING
 OtherKeyAttribute ::= SEQUENCE {
   keyAttrId OBJECT IDENTIFIER,
   keyAttr ANY DEFINED BY keyAttrId OPTIONAL }
  1. - Content Type Object Identifiers
 id-ct-contentInfo OBJECT IDENTIFIER ::= { iso(1) member-body(2)
     us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) ct(1) 6 }
 id-data OBJECT IDENTIFIER ::= { iso(1) member-body(2)
     us(840) rsadsi(113549) pkcs(1) pkcs7(7) 1 }
 id-signedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
     us(840) rsadsi(113549) pkcs(1) pkcs7(7) 2 }
 id-envelopedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
     us(840) rsadsi(113549) pkcs(1) pkcs7(7) 3 }
 id-digestedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
     us(840) rsadsi(113549) pkcs(1) pkcs7(7) 5 }

Housley Standards Track [Page 48] RFC 3852 Cryptographic Message Syntax July 2004

 id-encryptedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
     us(840) rsadsi(113549) pkcs(1) pkcs7(7) 6 }
 id-ct-authData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
     us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) ct(1) 2 }
  1. - The CMS Attributes
 MessageDigest ::= OCTET STRING
 SigningTime  ::= Time
 Time ::= CHOICE {
   utcTime UTCTime,
   generalTime GeneralizedTime }
 Countersignature ::= SignerInfo
  1. - Attribute Object Identifiers
 id-contentType OBJECT IDENTIFIER ::= { iso(1) member-body(2)
     us(840) rsadsi(113549) pkcs(1) pkcs9(9) 3 }
 id-messageDigest OBJECT IDENTIFIER ::= { iso(1) member-body(2)
     us(840) rsadsi(113549) pkcs(1) pkcs9(9) 4 }
 id-signingTime OBJECT IDENTIFIER ::= { iso(1) member-body(2)
     us(840) rsadsi(113549) pkcs(1) pkcs9(9) 5 }
 id-countersignature OBJECT IDENTIFIER ::= { iso(1) member-body(2)
     us(840) rsadsi(113549) pkcs(1) pkcs9(9) 6 }
  1. - Obsolete Extended Certificate syntax from PKCS#6
 ExtendedCertificateOrCertificate ::= CHOICE {
   certificate Certificate,
   extendedCertificate [0] IMPLICIT ExtendedCertificate }
 ExtendedCertificate ::= SEQUENCE {
   extendedCertificateInfo ExtendedCertificateInfo,
   signatureAlgorithm SignatureAlgorithmIdentifier,
   signature Signature }

Housley Standards Track [Page 49] RFC 3852 Cryptographic Message Syntax July 2004

 ExtendedCertificateInfo ::= SEQUENCE {
   version CMSVersion,
   certificate Certificate,
   attributes UnauthAttributes }
 Signature ::= BIT STRING
 END -- of CryptographicMessageSyntax2004

12.2. Version 1 Attribute Certificate ASN.1 Module

 AttributeCertificateVersion1
     { iso(1) member-body(2) us(840) rsadsi(113549)
       pkcs(1) pkcs-9(9) smime(16) modules(0) v1AttrCert(15) }
 DEFINITIONS EXPLICIT TAGS ::=
 BEGIN
  1. - EXPORTS All
 IMPORTS
  1. - Imports from RFC 3280 [PROFILE], Appendix A.1

AlgorithmIdentifier, Attribute, CertificateSerialNumber,

         Extensions, UniqueIdentifier
            FROM PKIX1Explicit88
                 { iso(1) identified-organization(3) dod(6)
                   internet(1) security(5) mechanisms(5) pkix(7)
                   mod(0) pkix1-explicit(18) }
  1. - Imports from RFC 3280 [PROFILE], Appendix A.2

GeneralNames

            FROM PKIX1Implicit88
                 { iso(1) identified-organization(3) dod(6)
                   internet(1) security(5) mechanisms(5) pkix(7)
                   mod(0) pkix1-implicit(19) }
  1. - Imports from RFC 3281 [ACPROFILE], Appendix B

AttCertValidityPeriod, IssuerSerial

            FROM PKIXAttributeCertificate
                 { iso(1) identified-organization(3) dod(6)
                   internet(1) security(5) mechanisms(5) pkix(7)
                   mod(0) attribute-cert(12) } ;
  1. - Definition extracted from X.509-1997 [X.509-97], but
  2. - different type names are used to avoid collisions.

Housley Standards Track [Page 50] RFC 3852 Cryptographic Message Syntax July 2004

 AttributeCertificateV1 ::= SEQUENCE {
   acInfo AttributeCertificateInfoV1,
   signatureAlgorithm AlgorithmIdentifier,
   signature BIT STRING }
 AttributeCertificateInfoV1 ::= SEQUENCE {
   version AttCertVersionV1 DEFAULT v1,
   subject CHOICE {
     baseCertificateID [0] IssuerSerial,
       -- associated with a Public Key Certificate
     subjectName [1] GeneralNames },
       -- associated with a name
   issuer GeneralNames,
   signature AlgorithmIdentifier,
   serialNumber CertificateSerialNumber,
   attCertValidityPeriod AttCertValidityPeriod,
   attributes SEQUENCE OF Attribute,
   issuerUniqueID UniqueIdentifier OPTIONAL,
   extensions Extensions OPTIONAL }
 AttCertVersionV1 ::= INTEGER { v1(0) }
 END -- of AttributeCertificateVersion1

13. References

13.1. Normative References

 [ACPROFILE]  Farrell, S. and R. Housley, "An Internet Attribute
              Certificate Profile for Authorization", RFC 3281, April
              2002.
 [PROFILE]    Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
              X.509 Public Key Infrastructure Certificate and
              Certificate Revocation List (CRL) Profile", RFC 3280,
              April 2002.
 [STDWORDS]   Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
 [X.208-88]   CCITT.  Recommendation X.208: Specification of Abstract
              Syntax Notation One (ASN.1).  1988.
 [X.209-88]   CCITT.  Recommendation X.209: Specification of Basic
              Encoding Rules for Abstract Syntax Notation One (ASN.1).
              1988.

Housley Standards Track [Page 51] RFC 3852 Cryptographic Message Syntax July 2004

 [X.501-88]   CCITT.  Recommendation X.501: The Directory - Models.
              1988.
 [X.509-88]   CCITT.  Recommendation X.509: The Directory -
              Authentication Framework.  1988.
 [X.509-97]   ITU-T.  Recommendation X.509: The Directory -
              Authentication Framework.  1997.
 [X.509-00]   ITU-T.  Recommendation X.509: The Directory -
              Authentication Framework.  2000.

13.2. Informative References

 [CMS1]       Housley, R., "Cryptographic Message Syntax", RFC 2630,
              June 1999.
 [CMS2]       Housley, R., "Cryptographic Message Syntax (CMS)", RFC
              3369, August 2002.
 [CMSALG]     Housley, R., "Cryptographic Message Syntax (CMS)
              Algorithms", RFC 3370, August 2002.
 [ESS]        Hoffman, P., "Enhanced Security Services for S/MIME",
              RFC 2634, June 1999.
 [MSAC]       Microsoft Development Network (MSDN) Library,
              "Authenticode", April 2004 Release.
 [MSG]        Ramsdell, B., "S/MIME Version 3.1 Message
              Specification", RFC 3851, July 2004.
 [OCSP]       Myers, M., Ankney, R., Malpani, A., Galperin, S. and C.
              Adams, "X.509 Internet Public Key Infrastructure Online
              Certificate Status Protocol - OCSP", RFC 2560, June
              1999.
 [OLDMSG]     Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L., and
              L. Repka, "S/MIME Version 2 Message Specification", RFC
              2311, March 1998.
 [PKCS#6]     RSA Laboratories.  PKCS #6: Extended-Certificate Syntax
              Standard, Version 1.5.  November 1993.
 [PKCS#7]     Kaliski, B., "PKCS #7: Cryptographic Message Syntax
              Version 1.5", RFC 2315, March 1998.

Housley Standards Track [Page 52] RFC 3852 Cryptographic Message Syntax July 2004

 [PKCS#9]     RSA Laboratories.  PKCS #9: Selected Attribute Types,
              Version 1.1.  November 1993.
 [PWRI]       Gutmann, P., "Password-based Encryption for CMS", RFC
              3211, December 2001.
 [RANDOM]     Eastlake 3rd, D., Crocker, S., and J. Schiller,
              "Randomness Recommendations for Security", RFC 1750,
              December 1994.

14. Security Considerations

 The Cryptographic Message Syntax provides a method for digitally
 signing data, digesting data, encrypting data, and authenticating
 data.
 Implementations must protect the signer's private key.  Compromise of
 the signer's private key permits masquerade.
 Implementations must protect the key management private key, the
 key-encryption key, and the content-encryption key.  Compromise of
 the key management private key or the key-encryption key may result
 in the disclosure of all contents protected with that key.
 Similarly, compromise of the content-encryption key may result in
 disclosure of the associated encrypted content.
 Implementations must protect the key management private key and the
 message-authentication key.  Compromise of the key management private
 key permits masquerade of authenticated data.  Similarly, compromise
 of the message-authentication key may result in undetectable
 modification of the authenticated content.
 The key management technique employed to distribute message-
 authentication keys must itself provide data origin authentication,
 otherwise the contents are delivered with integrity from an unknown
 source.  Neither RSA [PKCS#1, NEWPKCS#1] nor Ephemeral-Static
 Diffie-Hellman [DH-X9.42] provide the necessary data origin
 authentication.  Static-Static Diffie-Hellman [DH-X9.42] does provide
 the necessary data origin authentication when both the originator and
 recipient public keys are bound to appropriate identities in X.509
 certificates.
 When more than two parties share the same message-authentication key,
 data origin authentication is not provided.  Any party that knows the
 message-authentication key can compute a valid MAC, therefore the
 contents could originate from any one of the parties.

Housley Standards Track [Page 53] RFC 3852 Cryptographic Message Syntax July 2004

 Implementations must randomly generate content-encryption keys,
 message-authentication keys, initialization vectors (IVs), and
 padding.  Also, the generation of public/private key pairs relies on
 a random numbers.  The use of inadequate pseudo-random number
 generators (PRNGs) to generate cryptographic keys can result in
 little or no security.  An attacker may find it much easier to
 reproduce the PRNG environment that produced the keys, searching the
 resulting small set of possibilities, rather than brute force
 searching the whole key space.  The generation of quality random
 numbers is difficult.  RFC 1750 [RANDOM] offers important guidance in
 this area.
 When using key agreement algorithms or previously distributed
 symmetric key-encryption keys, a key-encryption key is used to
 encrypt the content-encryption key.  If the key-encryption and
 content-encryption algorithms are different, the effective security
 is determined by the weaker of the two algorithms.  If, for example,
 content is encrypted with Triple-DES using a 168-bit Triple-DES
 content-encryption key, and the content-encryption key is wrapped
 with RC2 using a 40-bit RC2 key-encryption key, then at most 40 bits
 of protection is provided.  A trivial search to determine the value
 of the 40-bit RC2 key can recover the Triple-DES key, and then the
 Triple-DES key can be used to decrypt the content.  Therefore,
 implementers must ensure that key-encryption algorithms are as strong
 or stronger than content-encryption algorithms.
 Implementers should be aware that cryptographic algorithms become
 weaker with time.  As new cryptoanalysis techniques are developed and
 computing performance improves, the work factor to break a particular
 cryptographic algorithm will be reduced.  Therefore, cryptographic
 algorithm implementations should be modular, allowing new algorithms
 to be readily inserted.  That is, implementors should be prepared for
 the set of algorithms that must be supported to change over time.
 The countersignature unsigned attribute includes a digital signature
 that is computed on the content signature value, thus the
 countersigning process need not know the original signed content.
 This structure permits implementation efficiency advantages; however,
 this structure may also permit the countersigning of an inappropriate
 signature value.  Therefore, implementations that perform
 countersignatures should either verify the original signature value
 prior to countersigning it (this verification requires processing of
 the original content), or implementations should perform
 countersigning in a context that ensures that only appropriate
 signature values are countersigned.

Housley Standards Track [Page 54] RFC 3852 Cryptographic Message Syntax July 2004

15. Acknowledgments

 This document is the result of contributions from many professionals.
 I appreciate the hard work of all members of the IETF S/MIME Working
 Group.  I extend a special thanks to Rich Ankney, Simon Blake-Wilson,
 Tim Dean, Steve Dusse, Carl Ellison, Peter Gutmann, Bob Jueneman,
 Stephen Henson, Paul Hoffman, Scott Hollenbeck, Don Johnson, Burt
 Kaliski, John Linn, John Pawling, Blake Ramsdell, Francois Rousseau,
 Jim Schaad, Dave Solo, Paul Timmel, and Sean Turner for their efforts
 and support.

16. Author's Address

 Russell Housley
 Vigil Security, LLC
 918 Spring Knoll Drive
 Herndon, VA 20170
 USA
 EMail: housley@vigilsec.com

Housley Standards Track [Page 55] RFC 3852 Cryptographic Message Syntax July 2004

17. Full Copyright Statement

 Copyright (C) The Internet Society (2004).  This document is subject
 to the rights, licenses and restrictions contained in BCP 78, and
 except as set forth therein, the authors retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

 The IETF takes no position regarding the validity or scope of any
 Intellectual Property Rights or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
 this document or the extent to which any license under such rights
 might or might not be available; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
 on the procedures with respect to rights in RFC documents can be
 found in BCP 78 and BCP 79.
 Copies of IPR disclosures made to the IETF Secretariat and any
 assurances of licenses to be made available, or the result of an
 attempt made to obtain a general license or permission for the use of
 such proprietary rights by implementers or users of this
 specification can be obtained from the IETF on-line IPR repository at
 http://www.ietf.org/ipr.
 The IETF invites any interested party to bring to its attention any
 copyrights, patents or patent applications, or other proprietary
 rights that may cover technology that may be required to implement
 this standard.  Please address the information to the IETF at ietf-
 ipr@ietf.org.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Housley Standards Track [Page 56]

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