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S/MIME Working Group                                         R. Housley
Internet Draft                                                   SPYRUS
expires in six months                                         June 1998


                      Cryptographic Message Syntax

                     <draft-ietf-smime-cms-06.txt>


Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
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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
   [RFC 2315].  Wherever possible, backward compatibility is preserved;
   however, changes were necessary to accommodate attribute certificate
   transfer and key agreement techniques for key management.

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






Housley                                                         [Page 1]


INTERNET DRAFT                                                 June 1998


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, Paul Hoffman, Scott Hollenbeck, Burt Kaliski, John Pawling,
   Blake Ramsdell, Jim Schaad, and Dave Solo for their efforts and
   support.


1  Introduction

   This document describes the Cryptographic Message Syntax.  This
   syntax is used to digitally sign or encrypt arbitrary messages.

   The Cryptographic Message Syntax describes an encapsulation syntax
   for data protection.  It supports digital signatures 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,
   using BER-encoding.  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 one or more
   protection content type.  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 type and the data, signed-data, and enveloped-



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   data content types.  The other content types may be implemented if
   desired.

   As a general design philosophy, content types permit single pass
   processing using indefinite-length Basic Encoding Rules (BER)
   encoding.  Single-pass operation is especially helpful if content is
   large, stored on tapes, or is "piped" from another process.  Single-
   pass operation has one significant drawback: it is difficult to
   perform encode operations using the Distinguished Encoding Rules
   (DER) 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 validate a content that
   contains an unrecognized attribute.

3  General Syntax

   The Cryptographic Message Syntax (CMS) associates a protection
   content type with a protection 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 protection 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 protection content.  The type of protection content
      can be determined uniquely by contentType.  Protection content
      types for signed-data, enveloped-data, digested-data, encrypted-
      data, and authenticated-data are defined in this document.  If
      additional protection content types are defined in other
      documents, the ASN.1 type defined along with the object identifier
      should not be a CHOICE type.









Housley                                                         [Page 3]


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4  Data Content Type

   The following object identifier identifies the data content type:

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

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

   The data content type is generally encapsulated in the signed-data,
   enveloped-data, digested-data, encrypted-data, or authenticated-data
   content type.  Object identifiers other than id-data may be used to
   identify the specific type of encapsulated content, but such usage is
   outside the scope of this specification.

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 two signers employ the same message digest algorithm, then the
      message digest need be computed for only one of them.  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



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      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 validate the signature
   value.  The signer's public key is referenced by an issuer
   distinguished name and an issuer-specific serial number that uniquely
   identify the certificate containing the public key.  The signer's
   certificate may be included in the SignedData certificates field.

   This section is divided into six parts.  The first part describes the
   top-level type SignedData, the second part describes
   EncapsulatedContentInfo, the third part describes the per-signer
   information type SignerInfo, and the fourth, fifth, and sixth parts
   describe the message digest calculation, signature generation, and
   signature validation 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 Version,
        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 and the
      encapsulated content type is id-data, then the value of version
      shall be 1; however, if attribute certificates are present or the



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      encapsulated content type is other than id-data, 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.

      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).  If no attribute
      certificates are present in the collection, then the value of
      version shall be 1; however, 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.

   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



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   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.

5.2 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 uniquely specifies the
      content type.

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

5.3  SignerInfo Type

   Per-signer information is represented in the type SignerInfo:

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

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

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

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




Housley                                                         [Page 7]


INTERNET DRAFT                                                 June 1998


      AttributeValue ::= ANY

      SignatureValue ::= OCTET STRING

   The fields of type SignerInfo have the following meanings:

      version is the syntax version number; it shall be 1.

      issuerAndSerialNumber specifies the signer's certificate (and
      thereby the signer's public key) by issuer distinguished name and
      issuer-specific serial number.

      digestAlgorithm identifies the message digest algorithm, and any
      associated parameters, used by the signer.  The message digest is
      computed over the encapsulated content and signed attributes, if
      present.  The message digest algorithm should be among those
      listed in the digestAlgorithms field of the associated SignerData.
      The message digesting process is described in Section 5.4.

      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.

         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.

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



Housley                                                         [Page 8]


INTERNET DRAFT                                                 June 1998


      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.  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.

   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.



Housley                                                         [Page 9]


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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 Validation Process

   The input to the signature validation process includes the result of
   the message digest calculation process and the signer's public key.
   The details of the signature validation 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.

   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:




Housley                                                        [Page 10]


INTERNET DRAFT                                                 June 1998


         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

         mail list keys:  the content-encryption key is encrypted in a
         previously distributed symmetric 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.

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 Version,
        originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL,
        recipientInfos RecipientInfos,
        encryptedContentInfo EncryptedContentInfo }




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      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

   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 originatorInfo is absent and all of the
      RecipientInfo structures are version 0, 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
         management algorithms.  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.



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   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 algorithm 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
   techniques that are supported: key transport, key agreement, and
   previously distributed mail list keys.  In all cases, the content-
   encryption key is transferred to one or more recipient in encrypted
   form.

      RecipientInfo ::= CHOICE {
        ktri KeyTransRecipientInfo,
        kari [1] KeyAgreeRecipientInfo,
        mlri [2] MailListRecipientInfo }

      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 Version,  -- always set to 0 or 2
        rid RecipientIdentifier,
        keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
        encryptedKey EncryptedKey }

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




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INTERNET DRAFT                                                 June 1998


   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.

      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.

      KeyAgreeRecipientInfo ::= SEQUENCE {
        version Version,  -- 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 }



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      RecipientEncryptedKeys ::= SEQUENCE OF RecipientEncryptedKey

      RecipientEncryptedKey ::= SEQUENCE {
        rid RecipientIdentifier,
        encryptedKey EncryptedKey }

      RecipientIdentifier ::= CHOICE {
        issuerAndSerialNumber IssuerAndSerialNumber,
        rKeyId [0] IMPLICIT RecipientKeyIdentifier }

      RecipientKeyIdentifier ::= SEQUENCE {
        subjectKeyIdentifier SubjectKeyIdentifier,
        date GeneralizedTime OPTIONAL,
        other OtherKeyAttribute OPTIONAL }

      SubjectKeyIdentifier ::= OCTET STRING

   The fields of type KeyAgreeRecipientInfo have the following meanings:

      version is the syntax version number.  It 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 the
      encrypted key for one or more recipients.  The RecipientIdentifier
      is a CHOICE with two alternatives specifying the recipient's



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      certificate, and thereby the recipient's public key, that was used
      by the sender to generate a pairwise key.  The recipient's
      certificate must contain a key agreement public key.  The content-
      encryption key is encrypted in the pairwise 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 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.

      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  MailListRecipientInfo Type

   Recipient information using previously distributed symmetric keys is
   represented in the type MailListRecipientInfo.  Each instance of
   MailListRecipientInfo will transfer the content-encryption key to one
   or more recipients who have the previously distributed key-encryption
   key.

      MailListRecipientInfo ::= SEQUENCE {
        version Version,  -- always set to 4
        mlkid MailListKeyIdentifier,
        keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
        encryptedKey EncryptedKey }

      MailListKeyIdentifier ::= SEQUENCE {
        kekIdentifier OCTET STRING,
        date GeneralizedTime OPTIONAL,
        other OtherKeyAttribute OPTIONAL }

   The fields of type MailListRecipientInfo have the following meanings:

      version is the syntax version number.  It shall always be 4.





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      mlkid 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 in 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 MailListKeyIdentifier have the following meanings:

      kekIdentifier identifies the key-encryption key that was
      previously distributed to the sender and one or more recipients.

      date is optional.  When present, the date specifies a single key-
      encryption key from a set that was previously distributed.

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

6.3  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-(l mod k) octets all having value k-(l mod k), where l is the
   length of the input.  In other words, the input is padded at the
   trailing end with one of the following strings:






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                     01 -- if l mod k = k-1
                  02 02 -- if l mod k = k-2
                      .
                      .
                      .
            k k ... k k -- if l 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.

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 }








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   The digested-data content type shall have ASN.1 type DigestedData:

      DigestedData ::= SEQUENCE {
        version Version,
        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.

      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 }




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   The encrypted-data content type shall have ASN.1 type EncryptedData:

      EncryptedData ::= SEQUENCE {
        version Version,
        encryptedContentInfo EncryptedContentInfo }

   The fields of type EncryptedData have the following meanings:

      version is the syntax version number.  It shall be 0.

      encryptedContentInfo is the encrypted content information, as
      defined in Section 6.1.

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 validate 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), the
      MAC value of the content and the other information are generated
      using the same message authentication code algorithm and key, and
      the result becomes the "MAC value."

9.1  AuthenticatedData Type

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



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

   The authenticated-data content type shall have ASN.1 type
   AuthenticatedData:

      AuthenticatedData ::= SEQUENCE {
        version Version,
        originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL,
        recipientInfos RecipientInfos,
        macAlgorithm MessageAuthenticationCodeAlgorithm,
        encapContentInfo EncapsulatedContentInfo,
        authenticatedAttributes [1] IMPLICIT AuthAttributes OPTIONAL,
        mac MessageAuthenticationCode,
        unauthenticatedAttributes [2] 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 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 algorithm
      identifier.  It identifies the message authentication code
      algorithm, along with any associated parameters, used by the
      originator.  Placement of the macAlgorithm field facilitates one-
      pass processing by the recipient.

      encapContentInfo is the content that is authenticated, as defined
      in section 5.2.

      authenticatedAttributes is a collection of attributes that are
      authenticated.  The field is optional, but it must be present if
      the content type of the EncapsulatedContentInfo value being



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      authenticated is not id-data.  Each AuthenticatedAttribute in the
      SET must be DER encoded.  Useful attribute types 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.

         A mac-value attribute, having as its value the message
         authentication code of the content.  Section 11.5 defines the
         mac-value 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 on
   either the message content or the content together with the
   originator's authenticated attributes.

   If there are no authenticated attributes, the MAC input data is the
   content octets of the DER encoding of the content field of the
   ContentInfo value to which the MAC process is applied.  Only the
   contents octets of the DER encoding of that field are input to the
   MAC algorithm, not the identifier octets or the length octets.

   If authenticated attributes are present, they must include the
   content-type attribute (as described in Section 11.1) and mac-value
   attribute (as described in section 11.5).  The MAC input data is the
   complete DER encoding of the Attributes value contained in the
   authenticatedAttributes field.  Since the Attributes value, when the
   field is present, must contain as attributes the content type and the
   mac value of the content, those values are indirectly included in the
   result.  A separate encoding of the authenticatedAttributes field is
   performed for MAC calculation.  The IMPLICIT [0] 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 [0] tag, is to be
   included in the MAC calculation along with the length and contents
   octets of the AuthAttributes value.

   If the content has content type id-data and the



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   authenticatedAttributes field is absent, then just the value of the
   data (e.g., the contents of a file) is input to the MAC calculation.
   This has the advantage that the length of the content need not be
   known in advance of the MAC calculation process.  Although the tag
   and length octets are not included in the MAC calculation, they are
   still protected by other means.  The length octets are protected by
   the nature of the MAC algorithm since it is computationally
   infeasible to find any two distinct messages of any length that have
   the same MAC.

   The fact that the MAC is computed on part of a DER encoding does not
   mean that DER is the required method of representing that part for
   data transfer.  Indeed, it is expected that some implementations will
   store objects in forms other than their DER encodings, but such
   practices do not affect MAC computation.

   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., DES-MAC and 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 Validation

   The input to the MAC validation process includes the input data
   (determined based on the presence or absence of authenticated
   attributes, as defined in 9.2), and the authentication key conveyed
   in recipientInfo.  The details of the MAC validation process depend
   on the MAC algorithm employed.

   The recipient may not rely on any MAC values computed by the
   originator.  If the originator includes authenticated attributes,
   then the content of the authenticatedAttributes must be authenticated
   as described in section 9.2.  For the MAC to be valid, the message
   MAC value calculated by the recipient must be the same as the value
   of the macValue attribute included in the authenticatedAttributes.
   Likewise, the attribute MAC value calculated by the recipient must be
   the same as the value of the mac field included in the
   authenticatedData.

10  Useful Types

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



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10.1  Algorithm Identifier Types

   All of the algorithm identifiers have the same type:
   AlgorithmIdentifier.  The definition of AlgorithmIdentifier is
   imported from X.509.

   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

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





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10.1.4  ContentEncryptionAlgorithmIdentifier

   The ContentEncryptionAlgorithmIdentifier type identifies a content-
   encryption algorithm.  Examples include DES, 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 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 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.  The PKCS #6 extended certificate is obsolete.  It is
   included for backward compatibility, and its use should be avoided.




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   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.

      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, and the definition of
   CertificateSerialNumber is imported from X.509.

      IssuerAndSerialNumber ::= SEQUENCE {
        issuer Name,
        serialNumber CertificateSerialNumber }

      CertificateSerialNumber ::= INTEGER

10.2.5  Version

   The Version type gives a syntax version number, for compatibility
   with future revisions of this document.

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






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10.2.6  UserKeyingMaterial

   The UserKeyingMaterial type gives a syntax 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 }

11  Useful Attributes

   This section defines attributes that may used with signed-data or
   authenticated-data.  Some of these attributes were originally defined
   in PKCS #9 [PKCS #9], others are defined and specified here.  The
   attributes are not listed in any particular order.

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 or
   unauthenticated attribute.

   The following object identifier identifies the content-type
   attribute:

      id-contentType OBJECT IDENTIFIER ::= { iso(1) member-body(2)
          us(840) rsadsi(113549) pkcs(1) pkcs9(9) 3 }

   Content-type attribute values have ASN.1 type ContentType:




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      ContentType ::= OBJECT IDENTIFIER

   A content-type attribute must have a single attribute value.

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), where the message digest is computed using the
   signer's message digest algorithm.

   Within signed-data, the message-digest signed attribute type is
   required if there are any attributes present.

   The message-digest attribute must be a signed attribute; it cannot be
   an unsigned attribute, an authenticated attribute, or 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.

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.

   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:




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      SigningTime ::= Time

      Time ::= CHOICE {
        utcTime          UTCTime,
        generalizedTime  GeneralizedTime }

   Note: The definition of Time matches the one specified in the 1997
   version of X.509.

   Dates through the year 2049 must be encoded as UTCTime, and dates in
   the year 2050 or later must be encoded as GeneralizedTime.

   A signing-time attribute must have a single attribute value.

   No requirement is imposed concerning the correctness of the signing
   time, and acceptance of a purported signing time is a matter of a
   recipient's discretion.  It is expected, however, that some signers,
   such as time-stamp servers, will be trusted implicitly.

11.4  Countersignature

   The countersignature attribute type specifies one or more signatures
   on the contents octets of the 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.



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      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 fact that a countersignature is computed on a signature value
   means that the countersigning process need not know the original
   content input to the signing process.  This has advantages both in
   efficiency and in confidentiality.  A countersignature, since it has
   type SignerInfo, can itself contain a countersignature attribute.
   Thus it is possible to construct arbitrarily long series of
   countersignatures.

11.5  Message Authentication Code (MAC) Value

   The MAC-value attribute type specifies the MAC of the
   encapContentInfo eContent OCTET STRING being authenticated in
   authenticated-data (see section 9), where the MAC value is computed
   using the originator's MAC algorithm and the data-authentication key.

   Within authenticated-data, the MAC-value attribute type is required
   if there are any authenticated attributes present.

   The MAC-value attribute must be a authenticated attribute; it cannot
   be an signed attribute, an unsigned attribute, or unauthenticated
   attribute.

   The following object identifier identifies the MAC-value attribute:

      id-macValue OBJECT IDENTIFIER ::= { iso(1) member-body(2)
          us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) aa(2) 8 }

   MAC-value attribute values have ASN.1 type MACValue:

      MACValue ::= OCTET STRING

   A MAC-value attribute must have a single attribute value.

12  Supported Algorithms

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

12.1  Digest Algorithms

   CMS implementations must include SHA-1.  CMS implementations may
   include MD5.



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12.1.1  SHA-1

   [*** Add pointer to algorithm specification. Provide OID. ***]

12.1.2  MD5

   [*** Add pointer to algorithm specification. Provide OID. ***]

12.2  Signature Algorithms

   CMS implementations must include DSA.  CMS implementations may
   include RSA.

12.2.1  DSA

   [*** Add pointer to algorithm specification. Provide OID. Provide
   ASN.1 for parameters and signature value. ***]

12.2.2  RSA

   [*** Add pointer to algorithm specification. Provide OID. Provide
   ASN.1 for parameters and signature value. ***]

12.3  Key Encryption Algorithms

   CMS implementations must include X9.42 Static Diffie-Hellman.  CMS
   implementations may include RSA and Triple-DES.

12.3.1  X9.42 Static Diffie-Hellman

   [*** Add pointer to algorithm specification. Provide OID. Provide
   ASN.1 for parameters. ***]

12.3.2  RSA

   [*** Add pointer to algorithm specification. Provide OID. Provide
   ASN.1 for parameters. ***]

12.3.3  Triple-DES Key Wrap

   [*** Add pointer to algorithm specification. Provide OID. ***]

12.4  Content Encryption Algorithms

   CMS implementations must include Triple-DES in CBC mode.  CMS
   implementations may include DES in CBC mode and RC2 in CBC mode.





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12.4.1  Triple-DES CBC

   [*** Add pointer to algorithm specification. Provide OID. ***]

12.4.2  DES CBC

   [*** Add pointer to algorithm specification. Provide OID. ***]

12.4.3  RC2 CBC

   [*** Add pointer to algorithm specification. Provide OID. ***]

12.5  Message Authentication Code Algorithms

   No MAC algorithms are mandatory.  CMS implementations may include DES
   MAC and HMAC.

12.5.1  DES MAC

   [*** Add pointer to algorithm specification. Provide OID. ***]

12.5.2  HMAC

   [*** Add pointer to algorithm specification. Provide OID. ***]



























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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

     -- Directory Information Framework (X.501)
           Name
              FROM InformationFramework { joint-iso-itu-t ds(5) modules(1)
                   informationFramework(1) 3 }

     -- 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 OPTIONAL }

   ContentType ::= OBJECT IDENTIFIER

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

   DigestAlgorithmIdentifiers ::= SET OF DigestAlgorithmIdentifier

   SignerInfos ::= SET OF SignerInfo




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

   ContentType ::= OBJECT IDENTIFIER

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

   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 Version,
     originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL,
     recipientInfos RecipientInfos,
     encryptedContentInfo EncryptedContentInfo }

   OriginatorInfo ::= SEQUENCE {
     certs [0] IMPLICIT CertificateSet OPTIONAL,
     crls [1] IMPLICIT CertificateRevocationLists OPTIONAL }















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   RecipientInfos ::= SET OF RecipientInfo

   EncryptedContentInfo ::= SEQUENCE {
     contentType ContentType,
     contentEncryptionAlgorithm ContentEncryptionAlgorithmIdentifier,
     encryptedContent [0] IMPLICIT EncryptedContent OPTIONAL }

   EncryptedContent ::= OCTET STRING

   RecipientInfo ::= CHOICE {
     ktri KeyTransRecipientInfo,
     kari [1] KeyAgreeRecipientInfo,
     mlri [2] MailListRecipientInfo }

   EncryptedKey ::= OCTET STRING

   KeyTransRecipientInfo ::= SEQUENCE {
     version Version,  -- always set to 0 or 2
     rid RecipientIdentifier,
     keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
     encryptedKey EncryptedKey }

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

   KeyAgreeRecipientInfo ::= SEQUENCE {
     version Version,  -- 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 RecipientIdentifier,
     encryptedKey EncryptedKey }




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   RecipientIdentifier ::= CHOICE {
     issuerAndSerialNumber IssuerAndSerialNumber,
     rKeyId [0] IMPLICIT RecipientKeyIdentifier }

   RecipientKeyIdentifier ::= SEQUENCE {
     subjectKeyIdentifier SubjectKeyIdentifier,
     date GeneralizedTime OPTIONAL,
     other OtherKeyAttribute OPTIONAL }

   SubjectKeyIdentifier ::= OCTET STRING

   MailListRecipientInfo ::= SEQUENCE {
     version Version,  -- always set to 4
     mlkid MailListKeyIdentifier,
     keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
     encryptedKey EncryptedKey }

   MailListKeyIdentifier ::= SEQUENCE {
     kekIdentifier OCTET STRING,
     date GeneralizedTime OPTIONAL,
     other OtherKeyAttribute OPTIONAL }

   DigestedData ::= SEQUENCE {
     version Version,
     digestAlgorithm DigestAlgorithmIdentifier,
     encapContentInfo EncapsulatedContentInfo,
     digest Digest }

   Digest ::= OCTET STRING

   EncryptedData ::= SEQUENCE {
     version Version,
     encryptedContentInfo EncryptedContentInfo }

   AuthenticatedData ::= SEQUENCE {
     version Version,
     originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL,
     recipientInfos RecipientInfos,
     macAlgorithm MessageAuthenticationCodeAlgorithm,
     encapContentInfo EncapsulatedContentInfo,
     authenticatedAttributes [1] IMPLICIT AuthAttributes OPTIONAL,
     mac MessageAuthenticationCode,
     unauthenticatedAttributes [2] IMPLICIT UnauthAttributes OPTIONAL }

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

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




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   MessageAuthenticationCode ::= OCTET STRING

   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 }

   KeyEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier

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

   UserKeyingMaterial ::= OCTET STRING

   UserKeyingMaterials ::= SET SIZE (1..MAX) OF UserKeyingMaterial

   OtherKeyAttribute ::= SEQUENCE {
     keyAttrId OBJECT IDENTIFIER,
     keyAttr ANY DEFINED BY keyAttrId OPTIONAL }


   -- CMS Attributes

   MessageDigest ::= OCTET STRING

   SigningTime  ::= Time

   Time ::= CHOICE {
     utcTime UTCTime,
     generalTime GeneralizedTime }




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   Countersignature ::= SignerInfo

   MACValue ::= OCTET STRING


   -- Object Identifiers

   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 }

   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 }

   id-macValue OBJECT IDENTIFIER ::= { iso(1) member-body(2)
       us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) aa(2) 8 }


   -- Obsolete Extended Certificate syntax from PKCS#6

   ExtendedCertificateOrCertificate ::= CHOICE {
     certificate Certificate,
     extendedCertificate [0] IMPLICIT ExtendedCertificate }




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   ExtendedCertificate ::= SEQUENCE {
     extendedCertificateInfo ExtendedCertificateInfo,
     signatureAlgorithm SignatureAlgorithmIdentifier,
     signature Signature }

   ExtendedCertificateInfo ::= SEQUENCE {
     version Version,
     certificate Certificate,
     attributes UnauthAttributes }

   Signature ::= BIT STRING


   END -- of CryptographicMessageSyntax





































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References

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

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

   PKCS #6    RSA Laboratories.  PKCS #6: Extended-Certificate Syntax
              Standard, Version 1.5.  November 1993.

   PKCS #9    RSA Laboratories.  PKCS #9: Selected Attribute Types,
              Version 1.1.  November 1993.

   X.208      CCITT.  Recommendation X.208: Specification of Abstract
              Syntax Notation One (ASN.1).  1988.

   X.209      CCITT.  Recommendation X.209: Specification of Basic Encoding
              Rules for Abstract Syntax Notation One (ASN.1).  1988.

   X.501      CCITT.  Recommendation X.501: The Directory - Models.  1988.

   X.509      CCITT.  Recommendation X.509: The Directory - Authentication
              Framework.  1988.

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 and the
   content-encryption key.  Compromise of the key management private 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 encrypted content.












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Author Address

   Russell Housley
   SPYRUS
   381 Elden Street
   Suite 1120
   Herndon, VA 20170
   USA

   housley@spyrus.com









































Housley                                                        [Page 41]


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