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


                      Cryptographic Message Syntax

                     <draft-ietf-smime-cms-08.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,
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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/>.






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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 content
   types.  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 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



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   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 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 signed-
      data, enveloped-data, digested-data, encrypted-data, and
      authenticated-data are defined in this document.  If additional
      content types are defined in other documents, the ASN.1 type
      defined should not be a CHOICE type.

4  Data Content Type

   The following object identifier identifies the data content type:

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

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

   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



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   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
      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



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   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 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 and the
      encapsulated content type is id-data, then the value of version
      shall be 1; however, if attribute certificates are present or the
      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



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      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.

   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.

5.2 EncapsulatedContentInfo Type

   The content is represented in the type EncapsulatedContentInfo:

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

      ContentType ::= OBJECT IDENTIFIER




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   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.

5.3  SignerInfo Type

   Per-signer information is represented in the type SignerInfo:

      SignerInfo ::= SEQUENCE {
        version CMSVersion,
        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

   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.



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      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 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.

   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



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   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.

   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.






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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 using any of the three key
   management techniques for each recipient.

   The typical application of the enveloped-data content type will
   represent one or more recipients' digital envelopes on content of the
   data or signed-data content types.

   Enveloped-data is constructed by the following steps:

      1.  A content-encryption key for a particular content-encryption
      algorithm is generated at random.

      2.  The content-encryption key is encrypted for each recipient.
      The details of this encryption depend on the key management
      algorithm used, but 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






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         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.

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 }

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

      RecipientInfos ::= SET OF RecipientInfo





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      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.  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.

   The fields of type EncryptedContentInfo have the following meanings:

      contentType indicates the type of content.




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      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
   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 }






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   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 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 }



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      RecipientEncryptedKeys ::= SEQUENCE OF RecipientEncryptedKey

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

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

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

      SubjectKeyIdentifier ::= OCTET STRING

   The fields of type KeyAgreeRecipientInfo have the following meanings:

      version is the syntax version number.  It 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



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      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.

      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.




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      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.

      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







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   trailing end with one of the following strings:

                     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.

   Any of the three key management techniques can be used for each
   recipient of the same encrypted content.

7  Digested-data Content Type

   The digested-data content type consists of content of any type and a
   message digest of the content.

   Typically, the digested-data content type is used to provide content
   integrity, and the result generally becomes an input to the
   enveloped-data content type.

   The following steps construct digested-data:

      1.  A message digest is computed on the content with a message-
      digest algorithm.

      2.  The message-digest algorithm and the message digest are
      collected together with the content into a DigestedData value.

   A recipient verifies the message digest by comparing the message
   digest to an independently computed message digest.

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

      id-digestedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
          us(840) rsadsi(113549) pkcs(1) pkcs7(7) 5 }



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   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.

      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 CMSVersion,
        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 CMSVersion,
        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, 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 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
   (MAC) on either the message being authenticated or the 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.  Although the encapContentInfo eContent OCTET
   STRING 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.

   If authenticatedAttributes field is present, the content-type
   attribute (as described in Section 11.1) and the mac-value attribute
   (as described in section 11.5) must be included, and the input to the
   MAC calculation process is the DER encoding of
   authenticatedAttributes.  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.  The DER



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   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
   content octets of the authenticatedAttributes value.

   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.

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.





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10.1.1  DigestAlgorithmIdentifier

   The DigestAlgorithmIdentifier type identifies a message-digest
   algorithm.  Examples include SHA-1, MD2, and MD5.  A message-digest
   algorithm maps an octet string (the 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

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






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   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
   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, and they are profiled for use in the
   Internet in RFC TBD.

   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.  PKCS #5
   certificates are included for backward compatibility, and their use
   should be avoided.  The Internet profile of X.509 certificates is
   specified in RFC TBD.



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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  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) }






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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 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 [RFC TBD2] and the Enhanced Security
   Services for S/MIME [RFC TBD3], 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 or
   unauthenticated attribute.

   The following object identifier identifies the content-type
   attribute:



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      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.

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, 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.



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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:

      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.

   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



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   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,
   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.



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   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.

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, even though
   the syntax is defined as a SET OF AttributeValue.  There must not be
   zero or multiple instances of AttributeValue present.

   The AuthAttributes syntax is defined as a SET OF Attributes.  The
   AuthAttributes in an AuthenticatedData must not include multiple
   instances of the MAC-value attribute.

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 may
   include MD5.



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   Digest algorithm identifiers are located in the SignedData
   digestAlgorithms field, the SignerInfo digestAlgorithm field, and the
   DigestedData 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 [RFC 1321].  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.

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



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   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 2313 [RFC 2313].  RFC
   2313 specifies the use of the RSA signature algorithm with the MD5
   message digest algorithm.  That definition is extended here to
   include support for the SHA-1 message digest algorithm as well.  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.

   This specification modifies RFC 2313 to include SHA-1 as an
   additional message digest algorithm.  Section 10.1.2 of RFC 2313 is
   modified to list SHA-1 in the bullet item about digestAlgorithm.  The
   following object identifier is added to the list in section 10.1.2 of
   RFC 2313:

      sha-1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
          oiw(14) secsig(3) algorithm(2) 26 }

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.  CMS implementations must include
   key agreement of Triple-DES pairwise key-encryption keys and Triple-
   DES wrapping Triple-DES content-encryption keys.  CMS implementations



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   should include key agreement of RC2 pairwise key-encryption keys and
   RC2 wrapping RC2 content-encryption keys.  The key wrap algorithm is
   described in section 12.6.

   Key agreement algorithm identifiers are located in the EnvelopedData
   RecipientInfo KeyAgreeRecipientInfo keyEncryptionAlgorithm field.

   Wrapped content-encryption keys are located in the EnvelopedData
   RecipientInfo KeyAgreeRecipientInfo recipientEncryptedKeys
   encryptedKey field.

12.3.1.1  X9.42 Ephemeral-Static Diffie-Hellman with Triple-DES

   Ephemeral-Static Diffie-Hellman key agreement is defined in RFC TBD1
   [RFC TBD1].  When using Ephemeral-Static Diffie-Hellman with Triple-
   DES, the EnvelopedData RecipientInfo 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:

         dh-public-number OBJECT IDENTIFIER ::= { iso(1) member-body(2)
             us(840) ansi-x942(10046) number-type(2) 1 }

      ukm may be absent.  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-ESDHwith3DES algorithm
      identifier with absent parameters.  The id-alg-ESDHwith3DES
      algorithm identifier is:

         id-alg-ESDHwith3DES OBJECT IDENTIFIER ::= { iso(1) member-body(2)
             us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 1 }

      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-



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      encryption Triple-DES key wrapped in the pairwise key agreement
      Triple-DES key.

12.3.1.2  X9.42 Ephemeral-Static Diffie-Hellman with RC2

   Ephemeral-Static Diffie-Hellman key agreement is defined in RFC TBD1
   [RFC TBD1].  When using Ephemeral-Static Diffie-Hellman with RC2, the
   EnvelopedData RecipientInfo 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:

         dh-public-number OBJECT IDENTIFIER ::= { iso(1) member-body(2)
             us(840) ansi-x942(10046) number-type(2) 1 }

      ukm may be absent.  The ukm is used to ensure that a different
      key-encryption key is generated if the ephemeral private key might
      be used for more than once.

      keyEncryptionAlgorithm must be the id-alg-ESDHwithRC2 algorithm
      identifier with absent parameters.  The id-alg-ESDHwithRC2
      algorithm identifier is:

         id-alg-ESDHwithRC2 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
             us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 2 }

      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 RC2 key wrapped in the pairwise key agreement RC2 key.

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.



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   Key transport algorithm identifiers are located in the EnvelopedData
   RecipientInfo KeyTransRecipientInfo keyEncryptionAlgorithm field.

   Key transport encrypted content-encryption keys are located in the
   EnvelopedData RecipientInfo KeyTransRecipientInfo EncryptedKey field.

12.3.2.1  RSA

   The RSA key transport algorithm is defined in RFC 2313 [RFC 2313].
   RFC 2313 specifies the transport of content-encryption keys,
   including Triple-DES and RC2 keys.  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.

   The use of RSA encryption, as defined in RFC 2313, 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.

12.3.3  Symmetric Key-Encryption Key Algorithms

   CMS implementations may include symmetric key-encryption key
   management.  Such implementations must include Triple-DES key-
   encryption keys wrapping Triple-DES content-encryption keys, and such
   implementations should include Triple-DES key-encryption keys
   wrapping RC2 content-encryption keys.  The key wrap algorithm is
   specified in section 12.6.

   Symmetric key-encryption key algorithm identifiers are located in the
   EnvelopedData RecipientInfo KEKRecipientInfo keyEncryptionAlgorithm
   field.

   Wrapped content-encryption keys are located in the EnvelopedData
   RecipientInfo KEKRecipientInfo encryptedKey field.

12.3.3.1  Triple-DES Key Wrap

   Triple-DES key encryption has the algorithm identifier:

      id-alg-3DESwrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)
          us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 3 }



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   The AlgorithmIdentifier parameter field must be NULL.

   Distribution of the Triple-DES key-encryption key used to encrypt the
   Triple-DES content-encryption key is out of the scope of this
   document.

12.3.3.2  RC2 Key Wrap

   RC2 key encryption has the algorithm identifier:

      id-alg-RC2wrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)
          us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 4 }

   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.

   Distribution of the RC2 key-encryption key used to encrypt the RC2
   content-encryption key is out 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 field
   and the EncryptedData EncryptedContentInfo contentEncryptionAlgorithm
   field.

   Content encryption algorithms are used to encipher the content
   located in the EnvelopedData EncryptedContentInfo encryptedContent
   field and the EncryptedData EncryptedContentInfo encryptedContent
   field.

12.4.1  Triple-DES CBC

   The Triple-DES algorithm is described in [3DES].  The algorithm
   identifier for Triple-DES is:



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      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 contain
   a CBCParameter:

      CBCParameter ::= IV

      IV ::= OCTET STRING  -- exactly 8 octets

12.4.2  RC2 CBC

   The RC2 algorithm is described in RFC 2268 [RFC 2268].  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 contain
   a RC2-CBC:

      RC2-CBC parameter ::= 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.  CMS implementations may also include DES MAC.

   MAC algorithm identifiers are located in the AuthenticatedData
   macAlgorithm field.

   MAC values are located in the AuthenticatedData mac field.  MAC
   values are also located in the mac-value authenticated attribute.

12.5.1  HMAC with SHA-1

   The HMAC with SHA-1 algorithm is described in RFC 2104 [RFC 2104].
   The algorithm identifier for HMAC with SHA-1 is:




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      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.

12.5.2  DES MAC

   The DES MAC algorithm is described in FIPS Pub 113 [DES MAC].  CMS
   implementations choosing to implement DES MAC must support 32 bit MAC
   values. CMS implementations should also support 64 bit MAC values.
   The algorithm identifier for DES MAC is:

      DES-MAC OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
          oiw(14) secsig(3) algorithm(2) 10 }

   The AlgorithmIdentifier parameters field must be present.  The
   parameters contain an INTEGER identifying the length in bits of the
   MAC value, constrained to multiples of eight between 16 and 64:

      DESMACLength ::= INTEGER  -- may be 16, 24, 32, 40, 48, 56, or 64

12.6  CMS Key Wrap Algorithm

   CMS implementations must implement the key wrap algorithm specified
   in this section.

   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 content-encryption key is formatted and encrypted.

   Key agreement algorithms generate a pairwise key-encryption key, and
   this key wrap algorithm is used to encrypt the content-encryption key
   with the pairwise key-encryption key.  Similarly, this 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 [RFC 2268].

   The block size of the key-encryption algorithm must be implicitly
   determined from the KeyEncryptionAlgorithmIdentifier field.
   Likewise, the size of the content-encryption key must be implicitly
   determined from the ContentEncryptionAlgorithmIdentifier field.




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   Since the same algorithm identifier is used for both 2-key and 3-key
   Triple-DES, three keys are always wrapped for Triple-DES.  Thus, 2-
   key Triple-DES provides three keys where the first and third keys are
   the same.

12.6.1  Sum of Sums Key Checksum

   The Sum of Sums [SUM] key checksum algorithm is:

   1.  Initialize two 16 bit integers, sum1 and sum2, to zero.
   2.  Loop through the octets of the content-encryption key, most
       significant octet to least significant octet.
       2a.  Create a 16 bit integer, called temp, by concatenating
            eight zero bits and the key octet.
       2b.  sum1 = sum1 + temp.
       2c.  sum2 = sum2 + sum1.
   3.  Use sum2 as the checksum value.

12.6.2  Key Wrap

   1.  Modify the content-encryption key to meet any restrictions on the key.
       For example, adjust the parity bits for each DES key comprising a
       Triple-DES key.
   2.  Compute a 16-bit key checksum value on the content-encryption key as
       described above.
   3.  Generate a 32-bit random salt value.
   4.  Concatenate the salt, content-encryption key, and key checksum value.
   5.  Randomly generate the number of pad octets necessary to make the result
       a multiple of block size of the key-encryption algorithm (the Triple-DES
       block size is 8 bytes), then append them to the result.
   6.  Encrypt the result with the key-encryption algorithm key.  Use an IV
       with each octet equal to 'A5' hexadecimal.

   Some key-encryption algorithm identifiers include an IV as part of
   the parameters.  The IV must still be the constant above.

12.6.3  Key Unwrap

   The key unwrap algorithm is:

   1.  Decrypt the ciphertext using the key-encryption key.  Use an IV
       with each octet equal to 'A5' hexadecimal.
   2.  Decompose the result into the content-encryption key and key checksum
       values.  The salt and pad values are discarded.
   3.  Compute a 16-bit key checksum value on the content-encryption key
       as described above.
   4.  If computed key checksum value does not match the decrypted key
       checksum value, then there is an error.



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   5.  If there are restrictions on keys, then check if the
       content-encryption key meets these restrictions.  For example,
       check for odd parity of each octet in each DES key that makes up
       a Triple-DES key.  If any restriction is incorrect, then there is
       an error.














































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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 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




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

   SignerInfo ::= SEQUENCE {
     version CMSVersion,
     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 CMSVersion,
     originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL,
     recipientInfos RecipientInfos,
     encryptedContentInfo EncryptedContentInfo }

   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

   RecipientInfo ::= CHOICE {
     ktri KeyTransRecipientInfo,
     kari [1] KeyAgreeRecipientInfo,
     kekri [2] KEKRecipientInfo }



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   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 }

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

   SubjectKeyIdentifier ::= OCTET STRING







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   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 }

   AuthenticatedData ::= SEQUENCE {
     version CMSVersion,
     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

   DigestAlgorithmIdentifier ::= AlgorithmIdentifier

   SignatureAlgorithmIdentifier ::= AlgorithmIdentifier

   KeyEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier

   ContentEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier

   MessageAuthenticationCodeAlgorithm ::= AlgorithmIdentifier



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   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

   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 }

   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 }





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   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 }

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

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

   Signature ::= BIT STRING

   END -- of CryptographicMessageSyntax



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References

   3DES       Tuchman, W.  "Hellman Presents No Shortcut Solutions To DES".
              IEEE Spectrum, v. 16, n. 7, pp40-41.  July 1979.

   DES        American National Standards Institute.  ANSI X3.106,
              "American National Standard for Information Systems - Data
              Link Encryption".  1983.

   DES MAC    National Institute of Standards and Technology.  FIPS Pub 113:
              Computer Data Authentication.  May 1985.

   DSS        National Institute of Standards and Technology.
              FIPS Pub 186: Digital Signature Standard.  19 May 1994.

   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.

   RFC 1321   Rivest, R.  The MD5 Message-Digest Algorithm.  April 1992.

   RFC 1750   Eastlake, D.; S. Crocker; J. Schiller.  Randomness
              Recommendations for Security.  December 1994.

   RFC 2104   Krawczyk, H.  HMAC: Keyed-Hashing for Message Authentication.
              February 1997.

   RFC 2268   Rivest, R.  A Description of the RC2 (r) Encryption Algorithm.
              March 1998.

   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.

   RFC 2437   Kaliski, B.  PKCS #1: RSA Encryption, Version 2.0.
              October 1998.

   RFC TBD    Housley, R., W. Ford, W. Polk, D. Solo.  Internet
              X.509 Public Key Infrastructure: Certificate and CRL
              Profile.  (currently draft-ietf-pkix-ipki-part1-*.txt)

   RFC TBD1   Rescorla, E.  Ephemeral-Static Diffie-Hellman Key
              Agreement Method.  (currently draft-ietf-smime-x942-*.txt)




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   RFC TBD2   Ramsdell, B.  S/MIME Version 3 Message Specification.
              (currently draft-ietf-smime-msg-*.txt)

   RFC TBD3   Hoffman, P.  Enhanced Security Services for S/MIME.
              (currently draft-ietf-smime-ess-*.txt)

   SHA1       National Institute of Standards and Technology.
              FIPS Pub 180-1: Secure Hash Standard.  17 April 1995.

   SUM        Fletcher, J.  An Arithmetic Checksum for Serial
              Transmissions.  Reprint UCRL-82569, Lawrence Livermore
              Laboraory, University of California.  May  1979.

   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, 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.

   Implementations must randomly generate content-encryption keys,
   message-authentication keys, initialization vectors (IVs), salt



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   values, 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 offers important guidance in this
   area, and Appendix 3 of FIPS Pub 186 [DSS] provides one quality PRNG
   technique.

   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.
   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 validate the original signature value
   prior to countersigning it (this validation 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 [RFC 2313] 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
   [RFC 2437].  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 [RFC 2313], 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



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

Author Address

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

   housley@spyrus.com



























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