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rfc:rfc2630

Network Working Group R. Housley Request for Comments: 2630 SPYRUS Category: Standards Track June 1999

                    Cryptographic Message Syntax

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 (1999).  All Rights Reserved.

Abstract

 This document describes the Cryptographic Message Syntax.  This
 syntax is used to digitally sign, digest, authenticate, or encrypt
 arbitrary messages.
 The Cryptographic Message Syntax is derived from PKCS #7 version 1.5
 as specified in RFC 2315 [PKCS#7].  Wherever possible, backward
 compatibility is preserved; however, changes were necessary to
 accommodate attribute certificate transfer and key agreement
 techniques for key management.

Housley Standards Track [Page 1] RFC 2630 Cryptographic Message Syntax June 1999

Table of Contents

 1   Introduction .................................................  4
 2   General Overview .............................................  4
 3   General Syntax ...............................................  5
 4   Data Content Type ............................................  5
 5   Signed-data Content Type .....................................  6
     5.1  SignedData Type .........................................  7
     5.2  EncapsulatedContentInfo Type ............................  8
     5.3  SignerInfo Type .........................................  9
     5.4  Message Digest Calculation Process ...................... 11
     5.5  Message Signature Generation Process .................... 12
     5.6  Message Signature Verification Process .................. 12
 6   Enveloped-data Content Type .................................. 12
     6.1  EnvelopedData Type ...................................... 14
     6.2  RecipientInfo Type ...................................... 15
          6.2.1  KeyTransRecipientInfo Type ....................... 16
          6.2.2  KeyAgreeRecipientInfo Type ....................... 17
          6.2.3  KEKRecipientInfo Type ............................ 19
     6.3  Content-encryption Process .............................. 20
     6.4  Key-encryption Process .................................. 20
 7   Digested-data Content Type ................................... 21
 8   Encrypted-data Content Type .................................. 22
 9   Authenticated-data Content Type .............................. 23
     9.1  AuthenticatedData Type .................................. 23
     9.2  MAC Generation .......................................... 25
     9.3  MAC Verification ........................................ 26
 10  Useful Types ................................................. 27
     10.1  Algorithm Identifier Types ............................. 27
           10.1.1  DigestAlgorithmIdentifier ...................... 27
           10.1.2  SignatureAlgorithmIdentifier ................... 27
           10.1.3  KeyEncryptionAlgorithmIdentifier ............... 28
           10.1.4  ContentEncryptionAlgorithmIdentifier ........... 28
           10.1.5  MessageAuthenticationCodeAlgorithm ............. 28
     10.2  Other Useful Types ..................................... 28
           10.2.1  CertificateRevocationLists ..................... 28
           10.2.2  CertificateChoices ............................. 29
           10.2.3  CertificateSet ................................. 29
           10.2.4  IssuerAndSerialNumber .......................... 30
           10.2.5  CMSVersion ..................................... 30
           10.2.6  UserKeyingMaterial ............................. 30
           10.2.7  OtherKeyAttribute .............................. 30

Housley Standards Track [Page 2] RFC 2630 Cryptographic Message Syntax June 1999

 11  Useful Attributes ............................................ 31
     11.1  Content Type ........................................... 31
     11.2  Message Digest ......................................... 32
     11.3  Signing Time ........................................... 32
     11.4  Countersignature ....................................... 34
 12  Supported Algorithms ......................................... 35
     12.1  Digest Algorithms ...................................... 35
           12.1.1  SHA-1 .......................................... 35
           12.1.2  MD5 ............................................ 35
     12.2  Signature Algorithms ................................... 36
           12.2.1  DSA ............................................ 36
           12.2.2  RSA ............................................ 36
     12.3  Key Management Algorithms .............................. 36
           12.3.1  Key Agreement Algorithms ....................... 36
                   12.3.1.1  X9.42 Ephemeral-Static Diffie-Hellman. 37
           12.3.2  Key Transport Algorithms ....................... 38
                   12.3.2.1  RSA .................................. 39
           12.3.3  Symmetric Key-Encryption Key Algorithms ........ 39
                   12.3.3.1  Triple-DES Key Wrap .................. 40
                   12.3.3.2  RC2 Key Wrap ......................... 41
    12.4  Content Encryption Algorithms ........................... 41
          12.4.1  Triple-DES CBC .................................. 42
          12.4.2  RC2 CBC ......................................... 42
    12.5  Message Authentication Code Algorithms .................. 42
          12.5.1  HMAC with SHA-1 ................................. 43
    12.6  Triple-DES and RC2 Key Wrap Algorithms .................. 43
          12.6.1  Key Checksum .................................... 44
          12.6.2  Triple-DES Key Wrap ............................. 44
          12.6.3  Triple-DES Key Unwrap ........................... 44
          12.6.4  RC2 Key Wrap .................................... 45
          12.6.5  RC2 Key Unwrap .................................. 46
 Appendix A:  ASN.1 Module ........................................ 47
 References ....................................................... 55
 Security Considerations .......................................... 56
 Acknowledgments .................................................. 58
 Author's Address ................................................. 59
 Full Copyright Statement ......................................... 60

Housley Standards Track [Page 3] RFC 2630 Cryptographic Message Syntax June 1999

1 Introduction

 This document describes the Cryptographic Message Syntax.  This
 syntax is used to digitally sign, digest, authenticate, or encrypt
 arbitrary messages.
 The Cryptographic Message Syntax describes an encapsulation syntax
 for data protection.  It supports digital signatures, message
 authentication codes, and encryption.  The syntax allows multiple
 encapsulation, so 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 Cryptographic Message Syntax can support a variety of
 architectures for certificate-based key management, such as the one
 defined by the PKIX working group.
 The Cryptographic Message Syntax 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.

2 General Overview

 The Cryptographic Message Syntax (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 if desired.
 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 4] RFC 2630 Cryptographic Message Syntax June 1999

 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 require DER
 encoding.  Signed attributes and authenticated attributes must 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 CMS data types
 that require DER encoding.

3 General Syntax

 The Cryptographic Message Syntax (CMS) associates a content type
 identifier with a content.  The syntax shall have ASN.1 type
 ContentInfo:
    ContentInfo ::= SEQUENCE {
      contentType ContentType,
      content [0] EXPLICIT ANY DEFINED BY contentType }
    ContentType ::= OBJECT IDENTIFIER
 The fields of ContentInfo have the following meanings:
    contentType indicates the type of the associated content.  It is
    an object identifier; it is a unique string of integers assigned
    by an authority that defines the content type.
    content is the associated content.  The type of content can be
    determined uniquely by contentType.  Content types for data,
    signed-data, enveloped-data, digested-data, encrypted-data, and
    authenticated-data are defined in this document.  If additional
    content types are defined in other documents, the ASN.1 type
    defined should not be a CHOICE type.

4 Data Content Type

 The following object identifier identifies the data content type:
    id-data OBJECT IDENTIFIER ::= { iso(1) member-body(2)
        us(840) rsadsi(113549) pkcs(1) pkcs7(7) 1 }
 The data content type is intended to refer to arbitrary octet
 strings, such as ASCII text files; the interpretation is left to the
 application.  Such strings need not have any internal structure

Housley Standards Track [Page 5] RFC 2630 Cryptographic Message Syntax June 1999

 (although they could have their own ASN.1 definition or other
 structure).
 The data content type is generally encapsulated in the signed-data,
 enveloped-data, digested-data, encrypted-data, or authenticated-data
 content type.

5 Signed-data Content Type

 The signed-data content type consists of a content of any type and
 zero or more signature values.  Any number of signers in parallel can
 sign any type of content.
 The typical application of the signed-data content type represents
 one signer's digital signature on content of the data content type.
 Another typical application disseminates certificates and certificate
 revocation lists (CRLs).
 The process by which signed-data is constructed involves the
 following steps:
    1.  For each signer, a message digest, or hash value, is computed
    on the content with a signer-specific message-digest algorithm.
    If the signer is signing any information other than the content,
    the message digest of the content and the other information are
    digested with the signer's message digest algorithm (see Section
    5.4), and the result becomes the "message digest."
    2.  For each signer, the message digest is digitally signed using
    the signer's private key.
    3.  For each signer, the signature value and other signer-specific
    information are collected into a SignerInfo value, as defined in
    Section 5.3.  Certificates and CRLs for each signer, and those not
    corresponding to any signer, are collected in this step.
    4.  The message digest algorithms for all the signers and the
    SignerInfo values for all the signers are collected together with
    the content into a SignedData value, as defined in Section 5.1.
 A recipient independently computes the message digest.  This message
 digest and the signer's public key are used to verify the signature
 value.  The signer's public key is referenced 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 may be included
 in the SignedData certificates field.

Housley Standards Track [Page 6] RFC 2630 Cryptographic Message Syntax June 1999

 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 CertificateRevocationLists 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.  If no attribute
    certificates are present in the certificates field, the
    encapsulated content type is id-data, and all of the elements of
    SignerInfos are version 1, then the value of version shall be 1.
    Alternatively, if attribute certificates are present, the
    encapsulated content type is other than id-data, or any of the
    elements of SignerInfos are version 3, then the value of version
    shall be 3.
    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.  The message
    digesting process is described in Section 5.4.

Housley Standards Track [Page 7] RFC 2630 Cryptographic Message Syntax June 1999

    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 chains 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 chains 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).  As discussed above, if
    attribute certificates are present, then the value of version
    shall be 3.
    crls is a collection of certificate revocation lists (CRLs).  It
    is intended that the set contain information sufficient to
    determine whether or not the certificates in the certificates
    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.
    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.

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

Housley Standards Track [Page 8] RFC 2630 Cryptographic Message Syntax June 1999

 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" should be id-data (as defined in section 4), and the content
 field of the EncapsulatedContentInfo value should be omitted.

5.3 SignerInfo Type

 Per-signer information is represented in the type SignerInfo:
    SignerInfo ::= SEQUENCE {
      version CMSVersion,
      sid SignerIdentifier,
      digestAlgorithm DigestAlgorithmIdentifier,
      signedAttrs [0] IMPLICIT SignedAttributes OPTIONAL,
      signatureAlgorithm SignatureAlgorithmIdentifier,
      signature SignatureValue,
      unsignedAttrs [1] IMPLICIT UnsignedAttributes OPTIONAL }
    SignerIdentifier ::= CHOICE {
      issuerAndSerialNumber IssuerAndSerialNumber,
      subjectKeyIdentifier [0] SubjectKeyIdentifier }
    SignedAttributes ::= SET SIZE (1..MAX) OF Attribute
    UnsignedAttributes ::= SET SIZE (1..MAX) OF Attribute
    Attribute ::= SEQUENCE {
      attrType OBJECT IDENTIFIER,
      attrValues SET OF AttributeValue }
    AttributeValue ::= ANY
    SignatureValue ::= OCTET STRING
 The fields of type SignerInfo have the following meanings:
    version is the syntax version number.  If the SignerIdentifier is
    the CHOICE issuerAndSerialNumber, then the version shall be 1.  If

Housley Standards Track [Page 9] RFC 2630 Cryptographic Message Syntax June 1999

    the SignerIdentifier is subjectKeyIdentifier, then the version
    shall 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 the X.509 subjectKeyIdentifier extension value.
    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.
    signedAttributes 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.
    Each SignedAttribute in the SET must be DER encoded.  Useful
    attribute types, such as signing time, are defined in Section 11.
    If the field is present, it must contain, at a minimum, the
    following two attributes:
       A content-type attribute having as its value the content type
       of the EncapsulatedContentInfo value being signed.  Section
       11.1 defines the content-type attribute.  The content-type
       attribute is not required when 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.
    unsignedAttributes is a collection of attributes that are not
    signed.  The field is optional.  Useful attribute types, such as
    countersignatures, are defined in Section 11.

Housley Standards Track [Page 10] RFC 2630 Cryptographic Message Syntax June 1999

 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.

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 signedAttributes field is present.  When the field is
 absent, the result is just the message digest of the content as
 described above.  When the field is present, however, the result is
 the message digest of the complete DER encoding of the
 SignedAttributes value contained in the signedAttributes field.
 Since the SignedAttributes value, when present, must contain the
 content type and the content message digest attributes, those values
 are indirectly included in the result.  The content type attribute is
 not required when used as part of a countersignature unsigned
 attribute as defined in section 11.4.  A separate encoding of the
 signedAttributes field is performed for message digest calculation.
 The IMPLICIT [0] tag in the signedAttributes 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 [0]
 tag, is to be included in the message digest calculation along with
 the length and content octets of the SignedAttributes value.
 When the signedAttributes field is absent, then 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.

Housley Standards Track [Page 11] RFC 2630 Cryptographic Message Syntax June 1999

 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 messages of any
 length that have the same message digest.

5.5 Message 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 is encoded as an OCTET STRING and
 carried in the signature field.

5.6 Message Signature Verification Process

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

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 three key
 management techniques for each recipient.

Housley Standards Track [Page 12] RFC 2630 Cryptographic Message Syntax June 1999

 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 three 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; and
       symmetric key-encryption keys:  the content-encryption key is
       encrypted in a previously distributed symmetric key-encryption
       key.
    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.

Housley Standards Track [Page 13] RFC 2630 Cryptographic Message Syntax June 1999

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 }
    OriginatorInfo ::= SEQUENCE {
      certs [0] IMPLICIT CertificateSet OPTIONAL,
      crls [1] IMPLICIT CertificateRevocationLists OPTIONAL }
    RecipientInfos ::= SET 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.  If originatorInfo is
    present, then version shall be 2.  If any of the RecipientInfo
    structures included have a version other than 0, then the version
    shall be 2.  If unprotectedAttrs is present, then version shall be
    2.  If originatorInfo is absent, all of the RecipientInfo
    structures are version 0, and unprotectedAttrs is absent, then
    version shall be 0.
    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

Housley Standards Track [Page 14] RFC 2630 Cryptographic Message Syntax June 1999

       management algorithms.  certs may also contain attribute
       certificates associated with the originator.  The certificates
       contained in certs are intended to be sufficient to make chains
       from a recognized "root" or "top-level certification authority"
       to all recipients.  However, certs may contain more
       certificates than necessary, and there may be certificates
       sufficient to make chains 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 is 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 the three key management

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 techniques that are supported: key transport, key agreement, and
 previously distributed symmetric key-encryption keys.  Any of the
 three key management techniques can be used for each recipient of the
 same encrypted content.  In all cases, the content-encryption key is
 transferred to one or more recipient in encrypted form.
    RecipientInfo ::= CHOICE {
      ktri KeyTransRecipientInfo,
      kari [1] KeyAgreeRecipientInfo,
      kekri [2] KEKRecipientInfo }
    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 }
    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 shall be 0.
    If the RecipientIdentifier is subjectKeyIdentifier, then the
    version shall be 2.
    rid specifies the recipient's certificate or key that was used by
    the sender to protect the content-encryption 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.  The content-encryption key is encrypted with the recipient's
    public key.  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 the X.509 subjectKeyIdentifier
    extension value.

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    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 recipient that
 uses the same key agreement algorithm and domain parameters for that
 algorithm.
    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

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 The fields of type KeyAgreeRecipientInfo have the following meanings:
    version is the syntax version number.  It shall 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
    the X.509 subjectKeyIdentifier extension value.  The originatorKey
    alternative includes the algorithm identifier and sender's key
    agreement public key. Permitting originator anonymity since the
    public key is not certified.
    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.
    keyEncryptionAlgorithm identifies the key-encryption algorithm,
    and any associated parameters, used to encrypt the content-
    encryption key in 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.  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.
 The fields of type RecipientKeyIdentifier have the following
 meanings:
    subjectKeyIdentifier identifies the recipient's certificate by the
    X.509 subjectKeyIdentifier extension value.

Housley Standards Track [Page 18] RFC 2630 Cryptographic Message Syntax June 1999

    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 shall 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.
 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.

Housley Standards Track [Page 19] RFC 2630 Cryptographic Message Syntax June 1999

    other is optional.  When present, this field contains additional
    information used by the recipient to determine the key-encryption
    key used by the sender.

6.3 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.
 The input to the content-encryption process is the "value" of the
 content being enveloped.  Only the value octets of the envelopedData
 encryptedContentInfo encryptedContent OCTET STRING are encrypted; the
 OCTET STRING tag and length octets are not encrypted.
 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
 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 three key management techniques can be used for each
 recipient of the same encrypted content.

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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 }
 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 shall be 0; however, if
    the encapsulated content type is other than id-data, then the
    value of version shall 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.

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    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 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 }
 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 shall be 2.  If unprotectedAttrs is absent,
    then version shall 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.

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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 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 }

Housley Standards Track [Page 23] RFC 2630 Cryptographic Message Syntax June 1999

 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,
      authenticatedAttributes [2] IMPLICIT AuthAttributes OPTIONAL,
      mac MessageAuthenticationCode,
      unauthenticatedAttributes [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.  It shall be 0.
    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
    authenticatedAttributes field must also be present.

Housley Standards Track [Page 24] RFC 2630 Cryptographic Message Syntax June 1999

    encapContentInfo is the content that is authenticated, as defined
    in section 5.2.
    authenticatedAttributes is a collection of authenticated
    attributes.  The authenticatedAttributes structure is optional,
    but it must be present if the content type of the
    EncapsulatedContentInfo value being authenticated is not id-data.
    If the authenticatedAttributes field is present, then the
    digestAlgorithm field must also be present.  Each
    AuthenticatedAttribute in the SET must be DER encoded.  Useful
    attribute types are defined in Section 11.  If the
    authenticatedAttributes 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.
    unauthenticatedAttributes 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 message being authenticated or a message digest
 of message being authenticated together with the originator's
 authenticated attributes.
 If authenticatedAttributes 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 authenticatedAttributes 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

Housley Standards Track [Page 25] RFC 2630 Cryptographic Message Syntax June 1999

 authenticatedAttributes.  A separate encoding of the
 authenticatedAttributes field is performed for message digest
 calculation.  The IMPLICIT [2] tag in the authenticatedAttributes
 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
 authenticatedAttributes 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 messages of any length that have
 the same message digest.
 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
 authenticatedAttributes 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 may 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 authenticatedAttributes is
 authenticated as described in section 9.2.  For authentication to
 succeed, the message MAC value calculated by the recipient must be

Housley Standards Track [Page 26] RFC 2630 Cryptographic Message Syntax June 1999

 the same as the value of the mac field.  Similarly, for
 authentication to succeed when the authenticatedAttributes field is
 present, the content message digest value calculated by the recipient
 must be the same as the message digest value included in the
 authenticatedAttributes message-digest attribute.

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
 imported from X.509 [X.509-88].
 There are many alternatives for each type of algorithm listed.  For
 each of these five types, Section 12 lists the algorithms that must
 be included in a CMS implementation.

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 message) to another octet string
 (the message digest).
    DigestAlgorithmIdentifier ::= AlgorithmIdentifier

10.1.2 SignatureAlgorithmIdentifier

 The SignatureAlgorithmIdentifier type identifies a signature
 algorithm.  Examples include DSS and RSA.  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

Housley Standards Track [Page 27] RFC 2630 Cryptographic Message Syntax June 1999

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, and previously
 distributed symmetric key-encrypting keys 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
 message) to another octet string (the ciphertext) under control of 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.  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

10.2 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 CertificateRevocationLists

 The CertificateRevocationLists type gives a set of certificate
 revocation lists (CRLs). It is intended that the set contain
 information sufficient to determine whether the certificates and

Housley Standards Track [Page 28] RFC 2630 Cryptographic Message Syntax June 1999

 attribute certificates with which the set is associated are revoked
 or not.  However, there may be more CRLs than necessary or there may
 be fewer CRLs than necessary.
 The CertificateList may contain a CRL, an Authority Revocation List
 (ARL), a Delta Revocation List, or an Attribute Certificate
 Revocation List.  All of these lists share a common syntax.
 CRLs are specified in X.509 [X.509-97], and they are profiled for use
 in the Internet in RFC 2459 [PROFILE].
 The definition of CertificateList is imported from X.509.
    CertificateRevocationLists ::= SET OF CertificateList

10.2.2 CertificateChoices

 The CertificateChoices type gives either a PKCS #6 extended
 certificate [PKCS#6], an X.509 certificate, or an X.509 attribute
 certificate [X.509-97].  The PKCS #6 extended certificate is
 obsolete.  PKCS #6 certificates are included for backward
 compatibility, and their use should be avoided.  The Internet profile
 of X.509 certificates is specified in the "Internet X.509 Public Key
 Infrastructure: Certificate and CRL Profile" [PROFILE].
 The definitions of Certificate and AttributeCertificate are imported
 from X.509.
    CertificateChoices ::= CHOICE {
       certificate Certificate,                 -- See X.509
       extendedCertificate [0] IMPLICIT ExtendedCertificate,
                                                -- Obsolete
       attrCert [1] IMPLICIT AttributeCertificate }
                                                -- See X.509 and X9.57

10.2.3 CertificateSet

 The CertificateSet type provides a set of certificates.  It is
 intended that the set be sufficient to contain chains 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 "chain" is outside the scope of this
 document.  Some applications may impose upper limits on the length of
 a chain; others may enforce certain relationships between the
 subjects and issuers of certificates within a chain.

Housley Standards Track [Page 29] RFC 2630 Cryptographic Message Syntax June 1999

    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 imported from X.501 [X.501-88], and the
 definition of CertificateSerialNumber is imported from X.509
 [X.509-97].
    IssuerAndSerialNumber ::= SEQUENCE {
      issuer Name,
      serialNumber CertificateSerialNumber }
    CertificateSerialNumber ::= INTEGER

10.2.5 CMSVersion

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

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 may impede interoperability.
    OtherKeyAttribute ::= SEQUENCE {
      keyAttrId OBJECT IDENTIFIER,
      keyAttr ANY DEFINED BY keyAttrId OPTIONAL }

Housley Standards Track [Page 30] RFC 2630 Cryptographic Message Syntax June 1999

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 2459 [PROFILE].
 Some of the attributes defined in this section were originally
 defined in PKCS #9 [PKCS#9], others were not previously defined.  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.

11.1 Content Type

 The content-type attribute type specifies the content type of the
 ContentInfo value being signed in signed-data.  The content-type
 attribute type is required if there are any authenticated attributes
 present.
 The content-type attribute must be a signed attribute or an
 authenticated attribute; it cannot be an unsigned attribute, an
 unauthenticated attribute, or an unprotectedAttribute.
 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
 A content-type 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 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.

Housley Standards Track [Page 31] RFC 2630 Cryptographic Message Syntax June 1999

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 is
 required if there are any attributes present.  Within authenticated-
 data, the message-digest authenticated attribute type is required if
 there are any attributes present.
 The message-digest attribute must be a signed attribute or an
 authenticated attribute; it cannot be an unsigned attribute or an
 unauthenticated 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 is defined as a SET OF Attributes.  The
 SignedAttributes in a signerInfo must not include multiple instances
 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 may be a signed attribute; it cannot be an
 unsigned attribute, an authenticated attribute, or an unauthenticated
 attribute.

Housley Standards Track [Page 32] RFC 2630 Cryptographic Message Syntax June 1999

 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 }
 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 Greenwich Mean Time (Zulu) and
 must include seconds (i.e., times are YYMMDDHHMMSSZ), even where the
 number of seconds is zero.  Midnight (GMT) 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 shall be
    interpreted as 19YY; and
    Where YY is less than 50, the year shall be interpreted as 20YY.
 GeneralizedTime values shall be expressed in Greenwich Mean Time
 (Zulu) 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 is defined as a SET OF Attributes.  The
 SignedAttributes in a signerInfo 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,

Housley Standards Track [Page 33] RFC 2630 Cryptographic Message Syntax June 1999

 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 DER encoding of the signatureValue
 field of a SignerInfo value in signed-data.  Thus, the
 countersignature attribute type countersigns (signs in serial)
 another signature.
 The countersignature attribute must be an unsigned attribute; it
 cannot be a signed attribute, an authenticated attribute, or an
 unauthenticated 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 contain a message-digest
    attribute if it contains any other attributes, but need not
    contain a content-type attribute, as there is no content type for
    countersignatures.
    2.  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
 arbitrarily long series of countersignatures.

Housley Standards Track [Page 34] RFC 2630 Cryptographic Message Syntax June 1999

12 Supported Algorithms

 This section lists the algorithms that must be implemented.
 Additional algorithms that should be implemented are also included.

12.1 Digest Algorithms

 CMS implementations must include SHA-1.  CMS implementations should
 include MD5.
 Digest algorithm identifiers are located in the SignedData
 digestAlgorithms field, the SignerInfo digestAlgorithm field, the
 DigestedData digestAlgorithm field, and the AuthenticatedData
 digestAlgorithm field.
 Digest values are located in the DigestedData digest field, and
 digest values are located in the Message Digest authenticated
 attribute.  In addition, digest values are input to signature
 algorithms.

12.1.1 SHA-1

 The SHA-1 digest algorithm is defined in FIPS Pub 180-1 [SHA1]. The
 algorithm identifier for SHA-1 is:
    sha-1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
        oiw(14) secsig(3) algorithm(2) 26 }
 The AlgorithmIdentifier parameters field is optional.  If present,
 the parameters field must contain an ASN.1 NULL.  Implementations
 should accept SHA-1 AlgorithmIdentifiers with absent parameters as
 well as NULL parameters.  Implementations should generate SHA-1
 AlgorithmIdentifiers with NULL parameters.

12.1.2 MD5

 The MD5 digest algorithm is defined in RFC 1321 [MD5].  The algorithm
 identifier for MD5 is:
    md5 OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
        rsadsi(113549) digestAlgorithm(2) 5 }
 The AlgorithmIdentifier parameters field must be present, and the
 parameters field must contain NULL.  Implementations may accept the
 MD5 AlgorithmIdentifiers with absent parameters as well as NULL
 parameters.

Housley Standards Track [Page 35] RFC 2630 Cryptographic Message Syntax June 1999

12.2 Signature Algorithms

 CMS implementations must include DSA.  CMS implementations may
 include RSA.
 Signature algorithm identifiers are located in the SignerInfo
 signatureAlgorithm field.  Also, signature algorithm identifiers are
 located in the SignerInfo signatureAlgorithm field of
 countersignature attributes.
 Signature values are located in the SignerInfo signature field.
 Also, signature values are located in the SignerInfo signature field
 of countersignature attributes.

12.2.1 DSA

 The DSA signature algorithm is defined in FIPS Pub 186 [DSS].  DSA is
 always used with the SHA-1 message digest algorithm.  The algorithm
 identifier for DSA is:
    id-dsa-with-sha1 OBJECT IDENTIFIER ::=  { iso(1) member-body(2)
        us(840) x9-57 (10040) x9cm(4) 3 }
 The AlgorithmIdentifier parameters field must not be present.

12.2.2 RSA

 The RSA signature algorithm is defined in RFC 2347 [NEWPKCS#1]. RFC
 2347 specifies the use of the RSA signature algorithm with the SHA-1
 and MD5 message digest algorithms.  The algorithm identifier for RSA
 is:
    rsaEncryption OBJECT IDENTIFIER ::= { iso(1) member-body(2)
        us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 1 }

12.3 Key Management Algorithms

 CMS accommodates three general key management techniques: key
 agreement, key transport, and previously distributed symmetric key-
 encryption keys.

12.3.1 Key Agreement Algorithms

 CMS implementations must include key agreement using X9.42
 Ephemeral-Static Diffie-Hellman.
 Any symmetric encryption algorithm that a CMS implementation includes
 as a content-encryption algorithm must also be included as a key-

Housley Standards Track [Page 36] RFC 2630 Cryptographic Message Syntax June 1999

 encryption algorithm.  CMS implementations must include key agreement
 of Triple-DES pairwise key-encryption keys and Triple-DES wrapping of
 Triple-DES content-encryption keys.  CMS implementations should
 include key agreement of RC2 pairwise key-encryption keys and RC2
 wrapping of RC2 content-encryption keys.  The key wrap algorithm for
 Triple-DES and RC2 is described in section 12.3.3.
 A CMS implementation may support mixed key-encryption and content-
 encryption algorithms.  For example, a 128-bit RC2 content-encryption
 key may be wrapped with 168-bit Triple-DES key-encryption key.
 Similarly, a 40-bit RC2 content-encryption key may be wrapped with
 128-bit RC2 key-encryption key.
 For key agreement of RC2 key-encryption keys, 128 bits must be
 generated as input to the key expansion process used to compute the
 RC2 effective key [RC2].
 Key agreement algorithm identifiers are located in the EnvelopedData
 RecipientInfos KeyAgreeRecipientInfo keyEncryptionAlgorithm and
 AuthenticatedData RecipientInfos KeyAgreeRecipientInfo
 keyEncryptionAlgorithm fields.
 Key wrap algorithm identifiers are located in the KeyWrapAlgorithm
 parameters within the EnvelopedData RecipientInfos
 KeyAgreeRecipientInfo keyEncryptionAlgorithm and AuthenticatedData
 RecipientInfos KeyAgreeRecipientInfo keyEncryptionAlgorithm fields.
 Wrapped content-encryption keys are located in the EnvelopedData
 RecipientInfos KeyAgreeRecipientInfo RecipientEncryptedKeys
 encryptedKey field.  Wrapped message-authentication keys are located
 in the AuthenticatedData RecipientInfos KeyAgreeRecipientInfo
 RecipientEncryptedKeys encryptedKey field.

12.3.1.1 X9.42 Ephemeral-Static Diffie-Hellman

 Ephemeral-Static Diffie-Hellman key agreement is defined in RFC 2631
 [DH-X9.42].  When using Ephemeral-Static Diffie-Hellman, the
 EnvelopedData RecipientInfos KeyAgreeRecipientInfo and
 AuthenticatedData RecipientInfos KeyAgreeRecipientInfo fields are
 used as follows:
    version must be 3.
    originator must be the originatorKey alternative.  The
    originatorKey algorithm fields must contain the dh-public-number
    object identifier with absent parameters.  The originatorKey
    publicKey field must contain the sender's ephemeral public key.
    The dh-public-number object identifier is:

Housley Standards Track [Page 37] RFC 2630 Cryptographic Message Syntax June 1999

       dh-public-number OBJECT IDENTIFIER ::= { iso(1) member-body(2)
           us(840) ansi-x942(10046) number-type(2) 1 }
    ukm may be absent.  When present, the ukm is used to ensure that a
    different key-encryption key is generated when the ephemeral
    private key might be used more than once.
    keyEncryptionAlgorithm must be the id-alg-ESDH algorithm
    identifier.  The algorithm identifier parameter field for id-alg-
    ESDH is KeyWrapAlgorihtm, and this parameter must be present.  The
    KeyWrapAlgorithm denotes the symmetric encryption algorithm used
    to encrypt the content-encryption key with the pairwise key-
    encryption key generated using the Ephemeral-Static Diffie-Hellman
    key agreement algorithm.  Triple-DES and RC2 key wrap algorithms
    are discussed in section 12.3.3.  The id-alg-ESDH algorithm
    identifier and parameter syntax is:
     id-alg-ESDH OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
         rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 5 }
     KeyWrapAlgorithm ::= AlgorithmIdentifier
    recipientEncryptedKeys contains an identifier and an encrypted key
    for each recipient.  The RecipientEncryptedKey
    KeyAgreeRecipientIdentifier must contain either the
    issuerAndSerialNumber identifying the recipient's certificate or
    the RecipientKeyIdentifier containing the subject key identifier
    from the recipient's certificate.  In both cases, the recipient's
    certificate contains the recipient's static public key.
    RecipientEncryptedKey EncryptedKey must contain the content-
    encryption key encrypted with the Ephemeral-Static Diffie-Hellman
    generated pairwise key-encryption key using the algorithm
    specified by the KeyWrapAlgortihm.

12.3.2 Key Transport Algorithms

 CMS implementations should include key transport using RSA.  RSA
 implementations must include key transport of Triple-DES content-
 encryption keys.  RSA implementations should include key transport of
 RC2 content-encryption keys.
 Key transport algorithm identifiers are located in the EnvelopedData
 RecipientInfos KeyTransRecipientInfo keyEncryptionAlgorithm and
 AuthenticatedData RecipientInfos KeyTransRecipientInfo
 keyEncryptionAlgorithm fields.
 Key transport encrypted content-encryption keys are located in the
 EnvelopedData RecipientInfos KeyTransRecipientInfo encryptedKey

Housley Standards Track [Page 38] RFC 2630 Cryptographic Message Syntax June 1999

 field.  Key transport encrypted message-authentication keys are
 located in the AuthenticatedData RecipientInfos KeyTransRecipientInfo
 encryptedKey field.

12.3.2.1 RSA

 The RSA key transport algorithm is the RSA encryption scheme defined
 in RFC 2313 [PKCS#1], block type is 02, where the message to be
 encrypted is the content-encryption key.  The algorithm identifier
 for RSA is:
    rsaEncryption OBJECT IDENTIFIER ::= { iso(1) member-body(2)
        us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 1 }
 The AlgorithmIdentifier parameters field must be present, and the
 parameters field must contain NULL.
 When using a Triple-DES content-encryption key, adjust the parity
 bits for each DES key comprising the Triple-DES key prior to RSA
 encryption.
 The use of RSA encryption, as defined in RFC 2313 [PKCS#1], to
 provide confidentiality has a known vulnerability concerns.  The
 vulnerability is primarily relevant to usage in interactive
 applications rather than to store-and-forward environments.  Further
 information and proposed countermeasures are discussed in the
 Security Considerations section of this document.
 Note that the same encryption scheme is also defined in RFC 2437
 [NEWPKCS#1].  Within RFC 2437, this scheme is called
 RSAES-PKCS1-v1_5.

12.3.3 Symmetric Key-Encryption Key Algorithms

 CMS implementations may include symmetric key-encryption key
 management.  Such CMS implementations must include Triple-DES key-
 encryption keys wrapping Triple-DES content-encryption keys, and such
 CMS implementations should include RC2 key-encryption keys wrapping
 RC2 content-encryption keys.  Only 128-bit RC2 keys may be used as
 key-encryption keys, and they must be used with the
 RC2ParameterVersion parameter set to 58.  A CMS implementation may
 support mixed key-encryption and content-encryption algorithms.  For
 example, a 40-bit RC2 content-encryption key may be wrapped with
 168-bit Triple-DES key-encryption key or with a 128-bit RC2 key-
 encryption key.

Housley Standards Track [Page 39] RFC 2630 Cryptographic Message Syntax June 1999

 Key wrap algorithm identifiers are located in the EnvelopedData
 RecipientInfos KEKRecipientInfo keyEncryptionAlgorithm and
 AuthenticatedData RecipientInfos KEKRecipientInfo
 keyEncryptionAlgorithm fields.
 Wrapped content-encryption keys are located in the EnvelopedData
 RecipientInfos KEKRecipientInfo encryptedKey field.  Wrapped
 message-authentication keys are located in the AuthenticatedData
 RecipientInfos KEKRecipientInfo encryptedKey field.
 The output of a key agreement algorithm is a key-encryption key, and
 this key-encryption key is used to encrypt the content-encryption
 key.  In conjunction with key agreement algorithms, CMS
 implementations must include encryption of content-encryption keys
 with the pairwise key-encryption key generated using a key agreement
 algorithm.  To support key agreement, key wrap algorithm identifiers
 are located in the KeyWrapAlgorithm parameter of the EnvelopedData
 RecipientInfos KeyAgreeRecipientInfo keyEncryptionAlgorithm and
 AuthenticatedData RecipientInfos KeyAgreeRecipientInfo
 keyEncryptionAlgorithm fields.  Wrapped content-encryption keys are
 located in the EnvelopedData RecipientInfos KeyAgreeRecipientInfo
 RecipientEncryptedKeys encryptedKey field, wrapped message-
 authentication keys are located in the AuthenticatedData
 RecipientInfos KeyAgreeRecipientInfo RecipientEncryptedKeys
 encryptedKey field.

12.3.3.1 Triple-DES Key Wrap

 Triple-DES key encryption has the algorithm identifier:
    id-alg-CMS3DESwrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)
        us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 6 }
 The AlgorithmIdentifier parameter field must be NULL.
 The key wrap algorithm used to encrypt a Triple-DES content-
 encryption key with a Triple-DES key-encryption key is specified in
 section 12.6.
 Out-of-band distribution of the Triple-DES key-encryption key used to
 encrypt the Triple-DES content-encryption key is beyond of the scope
 of this document.

Housley Standards Track [Page 40] RFC 2630 Cryptographic Message Syntax June 1999

12.3.3.2 RC2 Key Wrap

 RC2 key encryption has the algorithm identifier:
    id-alg-CMSRC2wrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)
        us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 7 }
 The AlgorithmIdentifier parameter field must be RC2wrapParameter:
    RC2wrapParameter ::= RC2ParameterVersion
    RC2ParameterVersion ::= INTEGER
 The RC2 effective-key-bits (key size) greater than 32 and less than
 256 is encoded in the RC2ParameterVersion.  For the effective-key-
 bits of 40, 64, and 128, the rc2ParameterVersion values are 160, 120,
 and 58 respectively.  These values are not simply the RC2 key length.
 Note that the value 160 must be encoded as two octets (00 A0),
 because the one octet (A0) encoding represents a negative number.
 Only 128-bit RC2 keys may be used as key-encryption keys, and they
 must be used with the RC2ParameterVersion parameter set to 58.
 The key wrap algorithm used to encrypt a RC2 content-encryption key
 with a RC2 key-encryption key is specified in section 12.6.
 Out-of-band distribution of the RC2 key-encryption key used to
 encrypt the RC2 content-encryption key is beyond of the scope of this
 document.

12.4 Content Encryption Algorithms

 CMS implementations must include Triple-DES in CBC mode.  CMS
 implementations should include RC2 in CBC mode.
 Content encryption algorithms identifiers are located in the
 EnvelopedData EncryptedContentInfo contentEncryptionAlgorithm and the
 EncryptedData EncryptedContentInfo contentEncryptionAlgorithm fields.
 Content encryption algorithms are used to encipher the content
 located in the EnvelopedData EncryptedContentInfo encryptedContent
 field and the EncryptedData EncryptedContentInfo encryptedContent
 field.

Housley Standards Track [Page 41] RFC 2630 Cryptographic Message Syntax June 1999

12.4.1 Triple-DES CBC

 The Triple-DES algorithm is described in ANSI X9.52 [3DES].  The
 Triple-DES is composed from three sequential DES [DES] operations:
 encrypt, decrypt, and encrypt.  Three-Key Triple-DES uses a different
 key for each DES operation.  Two-Key Triple-DES uses one key for the
 two encrypt operations and different key for the decrypt operation.
 The same algorithm identifiers are used for Three-Key Triple-DES and
 Two-Key Triple-DES.  The algorithm identifier for Triple-DES in
 Cipher Block Chaining (CBC) mode is:
    des-ede3-cbc OBJECT IDENTIFIER ::= { iso(1) member-body(2)
        us(840) rsadsi(113549) encryptionAlgorithm(3) 7 }
 The AlgorithmIdentifier parameters field must be present, and the
 parameters field must contain a CBCParameter:
    CBCParameter ::= IV
    IV ::= OCTET STRING  -- exactly 8 octets

12.4.2 RC2 CBC

 The RC2 algorithm is described in RFC 2268 [RC2].  The algorithm
 identifier for RC2 in CBC mode is:
    rc2-cbc OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
        rsadsi(113549) encryptionAlgorithm(3) 2 }
 The AlgorithmIdentifier parameters field must be present, and the
 parameters field must contain a RC2CBCParameter:
    RC2CBCParameter ::= SEQUENCE {
      rc2ParameterVersion INTEGER,
      iv OCTET STRING  }  -- exactly 8 octets
 The RC2 effective-key-bits (key size) greater than 32 and less than
 256 is encoded in the rc2ParameterVersion.  For the effective-key-
 bits of 40, 64, and 128, the rc2ParameterVersion values are 160, 120,
 and 58 respectively.  These values are not simply the RC2 key length.
 Note that the value 160 must be encoded as two octets (00 A0), since
 the one octet (A0) encoding represents a negative number.

12.5 Message Authentication Code Algorithms

 CMS implementations that support authenticatedData must include HMAC
 with SHA-1.

Housley Standards Track [Page 42] RFC 2630 Cryptographic Message Syntax June 1999

 MAC algorithm identifiers are located in the AuthenticatedData
 macAlgorithm field.
 MAC values are located in the AuthenticatedData mac field.

12.5.1 HMAC with SHA-1

 The HMAC with SHA-1 algorithm is described in RFC 2104 [HMAC].  The
 algorithm identifier for HMAC with SHA-1 is:
    hMAC-SHA1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
        dod(6) internet(1) security(5) mechanisms(5) 8 1 2 }
 The AlgorithmIdentifier parameters field must be absent.

12.6 Triple-DES and RC2 Key Wrap Algorithms

 CMS implementations must include encryption of a Triple-DES content-
 encryption key with a Triple-DES key-encryption key using the
 algorithm specified in Sections 12.6.2 and 12.6.3.  CMS
 implementations should include encryption of a RC2 content-encryption
 key with a RC2 key-encryption key using the algorithm specified in
 Sections 12.6.4 and 12.6.5.  Triple-DES and RC2 content-encryption
 keys are encrypted in Cipher Block Chaining (CBC) mode [MODES].
 Key Transport algorithms allow for the content-encryption key to be
 directly encrypted; however, key agreement and symmetric key-
 encryption key algorithms encrypt the content-encryption key with a
 second symmetric encryption algorithm.  This section describes how
 the Triple-DES or RC2 content-encryption key is formatted and
 encrypted.
 Key agreement algorithms generate a pairwise key-encryption key, and
 a key wrap algorithm is used to encrypt the content-encryption key
 with the pairwise key-encryption key.  Similarly, a key wrap
 algorithm is used to encrypt the content-encryption key in a
 previously distributed key-encryption key.
 The key-encryption key is generated by the key agreement algorithm or
 distributed out of band.  For key agreement of RC2 key-encryption
 keys, 128 bits must be generated as input to the key expansion
 process used to compute the RC2 effective key [RC2].
 The same algorithm identifier is used for both 2-key and 3-key
 Triple-DES.  When the length of the content-encryption key to be
 wrapped is a 2-key Triple-DES key, a third key with the same value as
 the first key is created.  Thus, all Triple-DES content-encryption
 keys are wrapped like 3-key Triple-DES keys.

Housley Standards Track [Page 43] RFC 2630 Cryptographic Message Syntax June 1999

12.6.1 Key Checksum

 The CMS Checksum Algorithm is used to provide a content-encryption
 key integrity check value.  The algorithm is:
 1.  Compute a 20 octet SHA-1 [SHA1] message digest on the
     content-encryption key.
 2.  Use the most significant (first) eight octets of the message
     digest value as the checksum value.

12.6.2 Triple-DES Key Wrap

 The Triple-DES key wrap algorithm encrypts a Triple-DES content-
 encryption key with a Triple-DES key-encryption key.  The Triple-DES
 key wrap algorithm is:
 1.  Set odd parity for each of the DES key octets comprising
     the content-encryption key, call the result CEK.
 2.  Compute an 8 octet key checksum value on CEK as described above
     in Section 12.6.1, call the result ICV.
 3.  Let CEKICV = CEK || ICV.
 4.  Generate 8 octets at random, call the result IV.
 5.  Encrypt CEKICV in CBC mode using the key-encryption key.  Use
     the random value generated in the previous step as the
     initialization vector (IV).  Call the ciphertext TEMP1.
 6.  Let TEMP2 = IV || TEMP1.
 7.  Reverse the order of the octets in TEMP2.  That is, the most
     significant (first) octet is swapped with the least significant
     (last) octet, and so on.  Call the result TEMP3.
 8.  Encrypt TEMP3 in CBC mode using the key-encryption key.  Use
     an initialization vector (IV) of 0x4adda22c79e82105.
     The ciphertext is 40 octets long.
 Note:  When the same content-encryption key is wrapped in different
 key-encryption keys, a fresh initialization vector (IV) must be
 generated for each invocation of the key wrap algorithm.

12.6.3 Triple-DES Key Unwrap

 The Triple-DES key unwrap algorithm decrypts a Triple-DES content-
 encryption key using a Triple-DES key-encryption key.  The Triple-DES
 key unwrap algorithm is:
 1.  If the wrapped content-encryption key is not 40 octets, then
     error.
 2.  Decrypt the wrapped content-encryption key in CBC mode using
     the key-encryption key.  Use an initialization vector (IV)
     of 0x4adda22c79e82105.  Call the output TEMP3.

Housley Standards Track [Page 44] RFC 2630 Cryptographic Message Syntax June 1999

 3.  Reverse the order of the octets in TEMP3.  That is, the most
     significant (first) octet is swapped with the least significant
     (last) octet, and so on.  Call the result TEMP2.
 4.  Decompose the TEMP2 into IV and TEMP1.  IV is the most
     significant (first) 8 octets, and TEMP1 is the least significant
     (last) 32 octets.
 5.  Decrypt TEMP1 in CBC mode using the key-encryption key.  Use
     the IV value from the previous step as the initialization vector.
     Call the ciphertext CEKICV.
 6.  Decompose the CEKICV into CEK and ICV. CEK is the most significant
     (first) 24 octets, and ICV is the least significant (last) 8 octets.
 7.  Compute an 8 octet key checksum value on CEK as described above
     in Section 12.6.1.  If the computed key checksum value does not
     match the decrypted key checksum value, ICV, then error.
 8.  Check for odd parity each of the DES key octets comprising CEK.
     If parity is incorrect, then there is an error.
 9.  Use CEK as the content-encryption key.

12.6.4 RC2 Key Wrap

 The RC2 key wrap algorithm encrypts a RC2 content-encryption key with
 a RC2 key-encryption key.  The RC2 key wrap algorithm is:
 1.  Let the content-encryption key be called CEK, and let the length
     of the content-encryption key in octets be called LENGTH.  LENGTH
     is a single octet.
 2.  Let LCEK = LENGTH || CEK.
 3.  Let LCEKPAD = LCEK || PAD.  If the length of LCEK is a multiple
     of 8, the PAD has a length of zero.  If the length of LCEK is
     not a multiple of 8, then PAD contains the fewest number of
     random octets to make the length of LCEKPAD a multiple of 8.
 4.  Compute an 8 octet key checksum value on LCEKPAD as described
     above in Section 12.6.1, call the result ICV.
 5.  Let LCEKPADICV = LCEKPAD || ICV.
 6.  Generate 8 octets at random, call the result IV.
 7.  Encrypt LCEKPADICV in CBC mode using the key-encryption key.
     Use the random value generated in the previous step as the
     initialization vector (IV).  Call the ciphertext TEMP1.
 8.  Let TEMP2 = IV || TEMP1.
 9.  Reverse the order of the octets in TEMP2.  That is, the most
     significant (first) octet is swapped with the least significant
     (last) octet, and so on.  Call the result TEMP3.
 10. Encrypt TEMP3 in CBC mode using the key-encryption key.  Use
     an initialization vector (IV) of 0x4adda22c79e82105.
 Note:  When the same content-encryption key is wrapped in different
 key-encryption keys, a fresh initialization vector (IV) must be
 generated for each invocation of the key wrap algorithm.

Housley Standards Track [Page 45] RFC 2630 Cryptographic Message Syntax June 1999

12.6.5 RC2 Key Unwrap

 The RC2 key unwrap algorithm decrypts a RC2 content-encryption key
 using a RC2 key-encryption key.  The RC2 key unwrap algorithm is:
 1.  If the wrapped content-encryption key is not a multiple of 8
     octets, then error.
 2.  Decrypt the wrapped content-encryption key in CBC mode using
     the key-encryption key.  Use an initialization vector (IV)
     of 0x4adda22c79e82105.  Call the output TEMP3.
 3.  Reverse the order of the octets in TEMP3.  That is, the most
     significant (first) octet is swapped with the least significant
     (last) octet, and so on.  Call the result TEMP2.
 4.  Decompose the TEMP2 into IV and TEMP1.  IV is the most
     significant (first) 8 octets, and TEMP1 is the remaining octets.
 5.  Decrypt TEMP1 in CBC mode using the key-encryption key.  Use
     the IV value from the previous step as the initialization vector.
     Call the plaintext LCEKPADICV.
 6.  Decompose the LCEKPADICV into LCEKPAD, and ICV.  ICV is the
     least significant (last) octet 8 octets.  LCEKPAD is the
     remaining octets.
 7.  Compute an 8 octet key checksum value on LCEKPAD as described
     above in Section 12.6.1.  If the computed key checksum value
     does not match the decrypted key checksum value, ICV, then error.
 8.  Decompose the LCEKPAD into LENGTH, CEK, and PAD.  LENGTH is the
     most significant (first) octet.  CEK is the following LENGTH
     octets.  PAD is the remaining octets, if any.
 9.  If the length of PAD is more than 7 octets, then error.
 10. Use CEK as the content-encryption key.

Housley Standards Track [Page 46] RFC 2630 Cryptographic Message Syntax June 1999

Appendix A: ASN.1 Module

CryptographicMessageSyntax

  { iso(1) member-body(2) us(840) rsadsi(113549)
    pkcs(1) pkcs-9(9) smime(16) modules(0) cms(1) }

DEFINITIONS IMPLICIT TAGS ::= BEGIN

– EXPORTS All – The types and values defined in this module are exported for use in – the other ASN.1 modules. Other applications may use them for their – own purposes.

IMPORTS

  1. - Directory Information Framework (X.501)

Name

         FROM InformationFramework { joint-iso-itu-t ds(5) modules(1)
              informationFramework(1) 3 }
  1. - Directory Authentication Framework (X.509)

AlgorithmIdentifier, AttributeCertificate, Certificate,

      CertificateList, CertificateSerialNumber
         FROM AuthenticationFramework { joint-iso-itu-t ds(5)
              module(1) authenticationFramework(7) 3 } ;

– Cryptographic Message Syntax

ContentInfo ::= SEQUENCE {

contentType ContentType,
content [0] EXPLICIT ANY DEFINED BY contentType }

ContentType ::= OBJECT IDENTIFIER

SignedData ::= SEQUENCE {

version CMSVersion,
digestAlgorithms DigestAlgorithmIdentifiers,
encapContentInfo EncapsulatedContentInfo,
certificates [0] IMPLICIT CertificateSet OPTIONAL,
crls [1] IMPLICIT CertificateRevocationLists OPTIONAL,
signerInfos SignerInfos }

DigestAlgorithmIdentifiers ::= SET OF DigestAlgorithmIdentifier

SignerInfos ::= SET OF SignerInfo

Housley Standards Track [Page 47] RFC 2630 Cryptographic Message Syntax June 1999

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

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 CertificateRevocationLists OPTIONAL }

RecipientInfos ::= SET OF RecipientInfo

EncryptedContentInfo ::= SEQUENCE {

contentType ContentType,
contentEncryptionAlgorithm ContentEncryptionAlgorithmIdentifier,
encryptedContent [0] IMPLICIT EncryptedContent OPTIONAL }

EncryptedContent ::= OCTET STRING

Housley Standards Track [Page 48] RFC 2630 Cryptographic Message Syntax June 1999

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

RecipientInfo ::= CHOICE {

ktri KeyTransRecipientInfo,
kari [1] KeyAgreeRecipientInfo,
kekri [2] KEKRecipientInfo }

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 }

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 }

Housley Standards Track [Page 49] RFC 2630 Cryptographic Message Syntax June 1999

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 }

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,
authenticatedAttributes [2] IMPLICIT AuthAttributes OPTIONAL,
mac MessageAuthenticationCode,
unauthenticatedAttributes [3] IMPLICIT UnauthAttributes OPTIONAL }

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

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

MessageAuthenticationCode ::= OCTET STRING

Housley Standards Track [Page 50] RFC 2630 Cryptographic Message Syntax June 1999

DigestAlgorithmIdentifier ::= AlgorithmIdentifier

SignatureAlgorithmIdentifier ::= AlgorithmIdentifier

KeyEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier

ContentEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier

MessageAuthenticationCodeAlgorithm ::= AlgorithmIdentifier

CertificateRevocationLists ::= SET OF CertificateList

CertificateChoices ::= CHOICE {

certificate Certificate,  -- See X.509
extendedCertificate [0] IMPLICIT ExtendedCertificate,  -- Obsolete
attrCert [1] IMPLICIT AttributeCertificate }  -- See X.509 & X9.57

CertificateSet ::= SET OF CertificateChoices

IssuerAndSerialNumber ::= SEQUENCE {

issuer Name,
serialNumber CertificateSerialNumber }

CMSVersion ::= INTEGER { v0(0), v1(1), v2(2), v3(3), v4(4) }

UserKeyingMaterial ::= OCTET STRING

OtherKeyAttribute ::= SEQUENCE {

keyAttrId OBJECT IDENTIFIER,
keyAttr ANY DEFINED BY keyAttrId OPTIONAL }

CMS Attributes

MessageDigest ::= OCTET STRING

SigningTime ::= Time

Time ::= CHOICE {

utcTime UTCTime,
generalTime GeneralizedTime }

Countersignature ::= SignerInfo

Housley Standards Track [Page 51] RFC 2630 Cryptographic Message Syntax June 1999

– Algorithm Identifiers

sha-1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)

  oiw(14) secsig(3) algorithm(2) 26 }

md5 OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)

  rsadsi(113549) digestAlgorithm(2) 5 }

id-dsa-with-sha1 OBJECT IDENTIFIER ::= { iso(1) member-body(2)

  us(840) x9-57 (10040) x9cm(4) 3 }

rsaEncryption OBJECT IDENTIFIER ::= { iso(1) member-body(2)

  us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 1 }

dh-public-number OBJECT IDENTIFIER ::= { iso(1) member-body(2)

  us(840) ansi-x942(10046) number-type(2) 1 }

id-alg-ESDH OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)

  rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 5 }

id-alg-CMS3DESwrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)

  us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 6 }

id-alg-CMSRC2wrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)

  us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 7 }

des-ede3-cbc OBJECT IDENTIFIER ::= { iso(1) member-body(2)

  us(840) rsadsi(113549) encryptionAlgorithm(3) 7 }

rc2-cbc OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)

  rsadsi(113549) encryptionAlgorithm(3) 2 }

hMAC-SHA1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)

  dod(6) internet(1) security(5) mechanisms(5) 8 1 2 }

– Algorithm Parameters

KeyWrapAlgorithm ::= AlgorithmIdentifier

RC2wrapParameter ::= RC2ParameterVersion

RC2ParameterVersion ::= INTEGER

CBCParameter ::= IV

IV ::= OCTET STRING – exactly 8 octets

Housley Standards Track [Page 52] RFC 2630 Cryptographic Message Syntax June 1999

RC2CBCParameter ::= SEQUENCE {

rc2ParameterVersion INTEGER,
iv OCTET STRING  }  -- exactly 8 octets

– Content Type Object Identifiers

id-ct-contentInfo OBJECT IDENTIFIER ::= { iso(1) member-body(2)

  us(840) rsadsi(113549) pkcs(1) pkcs-9(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 }

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 }

– 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 }

Housley Standards Track [Page 53] RFC 2630 Cryptographic Message Syntax June 1999

– Obsolete Extended Certificate syntax from PKCS#6

ExtendedCertificate ::= SEQUENCE {

extendedCertificateInfo ExtendedCertificateInfo,
signatureAlgorithm SignatureAlgorithmIdentifier,
signature Signature }

ExtendedCertificateInfo ::= SEQUENCE {

version CMSVersion,
certificate Certificate,
attributes UnauthAttributes }

Signature ::= BIT STRING

END – of CryptographicMessageSyntax

Housley Standards Track [Page 54] RFC 2630 Cryptographic Message Syntax June 1999

References

 3DES       American National Standards Institute.  ANSI X9.52-1998,
            Triple Data Encryption Algorithm Modes of Operation. 1998.
 DES        American National Standards Institute.  ANSI X3.106,
            "American National Standard for Information Systems - Data
            Link Encryption".  1983.
 DH-X9.42   Rescorla, E., "Diffie-Hellman Key Agreement Method",
            RFC 2631, June 1999.
 DSS        National Institute of Standards and Technology.
            FIPS Pub 186: Digital Signature Standard.  19 May 1994.
 ESS        Hoffman, P., Editor, "Enhanced Security Services for
            S/MIME", RFC 2634, June 1999.
 HMAC       Krawczyk, H., "HMAC: Keyed-Hashing for Message
            Authentication", RFC 2104, February 1997.
 MD5        Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
            April 1992.
 MODES      National Institute of Standards and Technology.
            FIPS Pub 81: DES Modes of Operation.  2 December 1980.
 MSG        Ramsdell, B., Editor, "S/MIME Version 3 Message
            Specification", RFC 2633, June 1999.
 NEWPKCS#1  Kaliski, B., "PKCS #1: RSA Encryption, Version 2.0",
            RFC 2347, October 1998.
 PROFILE    Housley, R., Ford, W., Polk, W. and D. Solo, "Internet
            X.509 Public Key Infrastructure: Certificate and CRL
            Profile", RFC 2459, January 1999.
 PKCS#1     Kaliski, B., "PKCS #1: RSA Encryption, Version 1.5.",
            RFC 2313, 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.
 PKCS#9     RSA Laboratories.  PKCS #9: Selected Attribute Types,
            Version 1.1.  November 1993.

Housley Standards Track [Page 55] RFC 2630 Cryptographic Message Syntax June 1999

 RANDOM     Eastlake, D., Crocker, S. and J. Schiller, "Randomness
            Recommendations for Security", RFC 1750, December 1994.
 RC2        Rivest, R., "A Description of the RC2 (r) Encryption
            Algorithm", RFC 2268, March 1998.
 SHA1       National Institute of Standards and Technology.
            FIPS Pub 180-1: Secure Hash Standard.  17 April 1995.
 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.
 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.

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 messages 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.

Housley Standards Track [Page 56] RFC 2630 Cryptographic Message Syntax June 1999

 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, and Appendix 3 of FIPS Pub 186 [DSS] provides one quality
 PRNG technique.
 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,
 a message content is encrypted with 168-bit Triple-DES and the
 Triple-DES content-encryption key is wrapped with a 40-bit RC2 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 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.
 Section 12.6 specifies key wrap algorithms used to encrypt a Triple-
 DES [3DES] content-encryption key with a Triple-DES key-encryption
 key or to encrypt a RC2 [RC2] content-encryption key with a RC2 key-
 encryption key.  The key wrap algorithms make use of CBC mode
 [MODES].  These key wrap algorithms have been reviewed for use with
 Triple and RC2.  They have not been reviewed for use with other
 cryptographic modes or other encryption algorithms.  Therefore, if a
 CMS implementation wishes to support ciphers in addition to Triple-
 DES or RC2, then additional key wrap algorithms need to be defined to
 support the additional ciphers.
 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 reduce.  Therefore, cryptographic
 algorithm implementations should be modular allowing new algorithms
 to be readily inserted.  That is, implementers should be prepared for
 the set of mandatory to implement algorithms to change over time.
 The countersignature unauthenticated attribute includes a digital
 signature that is computed on the content signature value, thus the
 countersigning process need not know the original signed content.

Housley Standards Track [Page 57] RFC 2630 Cryptographic Message Syntax June 1999

 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.
 Users of CMS, particularly those employing CMS to support interactive
 applications, should be aware that PKCS #1 Version 1.5 as specified
 in RFC 2313 [PKCS#1] is vulnerable to adaptive chosen ciphertext
 attacks when applied for encryption purposes.  Exploitation of this
 identified vulnerability, revealing the result of a particular RSA
 decryption, requires access to an oracle which will respond to a
 large number of ciphertexts (based on currently available results,
 hundreds of thousands or more), which are constructed adaptively in
 response to previously-received replies providing information on the
 successes or failures of attempted decryption operations.  As a
 result, the attack appears significantly less feasible to perpetrate
 for store-and-forward S/MIME environments than for directly
 interactive protocols.  Where CMS constructs are applied as an
 intermediate encryption layer within an interactive request-response
 communications environment, exploitation could be more feasible.
 An updated version of PKCS #1 has been published, PKCS #1 Version 2.0
 [NEWPKCS#1].  This new document will supersede RFC 2313.  PKCS #1
 Version 2.0 preserves support for the encryption padding format
 defined in PKCS #1 Version 1.5 [PKCS#1], and it also defines a new
 alternative.  To resolve the adaptive chosen ciphertext
 vulnerability, the PKCS #1 Version 2.0 specifies and recommends use
 of Optimal Asymmetric Encryption Padding (OAEP) when RSA encryption
 is used to provide confidentiality.  Designers of protocols and
 systems employing CMS for interactive environments should either
 consider usage of OAEP, or should ensure that information which could
 reveal the success or failure of attempted PKCS #1 Version 1.5
 decryption operations is not provided.  Support for OAEP will likely
 be added to a future version of the CMS specification.

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, 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, and Dave
 Solo for their efforts and support.

Housley Standards Track [Page 58] RFC 2630 Cryptographic Message Syntax June 1999

Author's Address

 Russell Housley
 SPYRUS
 381 Elden Street
 Suite 1120
 Herndon, VA 20170
 USA
 EMail: housley@spyrus.com

Housley Standards Track [Page 59] RFC 2630 Cryptographic Message Syntax June 1999

Full Copyright Statement

 Copyright (C) The Internet Society (1999).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS 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.

Acknowledgement

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

Housley Standards Track [Page 60]

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