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S/MIME Working Group                                         R. Housley
Internet Draft                                                   SPYRUS
expires in six months                                     December 1997


                      Cryptographic Message Syntax

                     <draft-ietf-smime-cms-02.txt>


Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
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   ftp.isi.edu (US West Coast).


Abstract

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

   The Cryptographic Message Syntax is derived from PKCS #7 version 1.5.
   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
   subscribe, send a message to:
        ietf-smime-request@imc.org
   with the single ''subscribe'' word 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 authenticated 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 is general enough to support many
   different content types. This document defines three: data, signed
   data, and enveloped data. Other content types may be added in the
   future, and additional content types can be defined outside this
   document.

   The Cryptographic Message Syntax exports one content type,
   ContentInfo, as well as the various object identifiers.

   As a general design philosophy, content types permit single pass
   processing using indefinite-length Basic Encoding Rules (BER)
   encoding. Single-pass operation is especially helpful if content is
   large, stored on tapes, or is "piped" from another process. Single-
   pass operation has one significant drawback; it is difficult to
   perform encode operations using the Distinguished Encoding Rules
   (DER) encoding in a single pass since the lengths of the various
   components may not be known in advance. Since DER encoding is
   required by the signed-data content type, an extra pass may be
   necessary when a content type other than data is encapsulated.




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3  General Syntax

   The Cryptographic Message Syntax associates a content type with a
   content. The syntax shall have ASN.1 type ContentInfo:

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

      ContentType ::= OBJECT IDENTIFIER

   The fields of ContentInfo have the following meanings:

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

      content is the content. The field is optional, although it is
      generally present. In the rare cases where it is absent, the
      intended value must be supplied by other means.

   The type of content can be determined uniquely by contentType, so the
   type defined along with the object identifier should not be a CHOICE
   type.

   The optional omission of the content field makes it possible to
   construct "external signatures." In the case of external signatures,
   the content being signed would be absent from the encapsulated
   ContentInfo value included in the signed-data content type. If the
   ContentInfo value is absent, the signatureValue is calculated as
   though the ContentInfo value were present. The presumed ContentInfo
   must have the content type set to id-data and the content omitted.

4  Data Content Type

   The data content type is identified by the following object
   identifier:

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

   The data content type is just an octet string. It shall have ASN.1
   type Data:

      Data ::= OCTET STRING

   The data content type is intended to refer to arbitrary octet
   strings, such as ASCII text files; the interpretation is left to the



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   application. Such strings need not have any internal structure
   (although they may; they could even be DER encoded).

5  Signed-data Content Type

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

   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 authenticating any information other than the content
      (see Section 5.2), the message digest of the content and the other
      information are digested with the signer's message digest
      algorithm, 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 5.2. 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 it 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 four parts. The first part describes the
   top-level type SignedData, the second part describes the per-signer
   information type SignerInfo, and the third and fourth parts describe



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   the message digest calculation and signature generation processes.

5.1  SignedData Type

   The signed-data content type is identified by the following object
   identifier:

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

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

      SignedData ::= SEQUENCE {
        version Version,
        digestAlgorithms DigestAlgorithmIdentifiers,
        contentInfo ContentInfo,
        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, then the value of version
      shall be 1; however, if attribute certificates are present, 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 5.3.

      contentInfo is the content that is signed. It can have any type.

      certificates is a collection of certificates. It is intended that
      the set of certificates be sufficient to contain chains from a
      recognized "root" or "top-level certification authority" to all of
      the signers in the signerInfos field. There may be more
      certificates than necessary, and there may be certificates
      sufficient to contain chains from two or more independent top-



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      level certification authorities. There may also be fewer
      certificates than necessary, if it is expected recipients have an
      alternate means of obtaining necessary certificates (e.g., from a
      previous set of certificates). If no attribute certificates are
      present in the collection, then the value of version shall be 1;
      however, if attribute certificates are present, then the value of
      version shall be 3.

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

      signerInfos is a collection of per-signer information. There may
      be any number of elements in the collection, including zero.

   In the degenerate case where there are no signers, the ContentInfo
   value being "signed" is irrelevant. In this case, the content type
   within the ContentInfo value being "signed" should be data, and the
   content field of the ContentInfo value should be omitted.

5.2  SignerInfo Type

   Per-signer information is represented in the type SignerInfo:

      SignerInfo ::= SEQUENCE {
        version Version,
        issuerAndSerialNumber IssuerAndSerialNumber,
        digestAlgorithm DigestAlgorithmIdentifier,
        authenticatedAttributes [0] IMPLICIT Attributes OPTIONAL,
        signatureAlgorithm SignatureAlgorithmIdentifier,
        signature SignatureValue,
        unauthenticatedAttributes [1] IMPLICIT Attributes OPTIONAL }

      Attributes ::= SET OF Attribute

      SignatureValue ::= OCTET STRING

   The fields of type SignerInfo have the following meanings:

      version is the syntax version number. It shall always be 1.

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




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      digestAlgorithm identifies the message digest algorithm, and any
      associated parameters, used by the signer. The message digest is
      computed over the content and authenticated attributes, if
      present. The message digest algorithm should be among those listed
      in the digestAlgorithms field of the superior SignerInfo value.
      The message digesting process is described in Section 5.3.

      authenticatedAttributes is a collection of attributes that are
      signed. The field is optional, but it must be present if the
      content type of the ContentInfo value being signed is not data. If
      the field is present, it must contain, at a minimum, two
      attributes:

         A content-type attribute having as its value the content type
         of the ContentInfo value being signed. Section 8.1 defines the
         content-type attribute.

         A message-digest attribute, having as its value the message
         digest of the content. Section 8.2 defines the message-digest
         attribute.

         Other attribute types that might be useful here, such as
         signing time, are defined in Section 8.

      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.

      unauthenticatedAttributes is a collection of attributes that are
      not signed. The field is optional. Attribute types that might be
      useful here, such as countersignatures, are defined in Section 10.

5.3  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
   signer's authenticated attributes. In either case, the initial input
   to the message digest calculation process is the "value" of the
   content being signed. Specifically, the initial input is the content
   octets of the DER encoding of the content field of the ContentInfo
   value to which the signing process is applied. Only the contents
   octets of the DER encoding of that field are input to the message
   digest algorithm, not the identifier octets or the length octets.

   The result of the message digest calculation process depends on



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   whether the authenticatedAttributes 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 Attributes
   value contained in the authenticatedAttributes field. Since the
   Attributes value, when the field is present, must contain as
   attributes the content type and the message digest of the content,
   those values are indirectly included in the result. A separate
   encoding of the authenticatedAttributes field is performed for
   message digest calculation. The IMPLICIT [0] tag in the
   authenticatedAttributes field is not used for the DER encoding,
   rather an EXPLICIT SET OF tag is used. That is, the DER encoding of
   the SET OF tag, rather than of the IMPLICIT [0] tag, is to be
   included in the message digest calculation along with the length and
   contents octets of the Attributes value.

   When the content being signed has content type data and the
   authenticatedAttributes field is absent, then just the value of the
   data (e.g., the contents of a file) is 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 encryption process.

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

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

5.4  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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5.5  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 generation 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
   authenticatedAttributes, then the ASN.1 DER encoded content of the
   signedData object must be digested as described in section 5.3. 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 authenticatedAttributes 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 any number of recipients.

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

   Enveloped-data is constructed by the following steps:

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

      2.  The content-encryption key is encrypted for each recipient.
      The details of this encryption depend on the key management
      algorithm used, but three general techniques are supported:

         key transport:  the content-encryption key is encrypted in the
         recipient's public key;

         key agreement:  the recipient's public key and the sender's
         private key are used to generate a pairwise symmetric key, then
         the content-encryption key is encrypted in the pairwise
         symmetric key; and

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



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      3.  For the originator and 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 into a EnvelopedData value as
      defined in Section 6.1.

   A recipient opens the envelope by decrypting one of the encrypted
   content-encryption keys and 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 enveloped-data content type is identified by the following object
   identifier:

      id-envelopedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
          us(840) rsadsi(113549) pkcs(1) pkcs7(7) 3 }

   The enveloped-data content type shall have ASN.1 type EnvelopedData:

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

      OriginatorInfo ::= SEQUENCE {
        certs [0] IMPLICIT CertificateSet OPTIONAL,
        crls [1] IMPLICIT CertificateRevocationLists OPTIONAL,
        ukms [2] IMPLICIT UserKeyingMaterials OPTIONAL }

      RecipientInfos ::= SET OF RecipientInfo

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



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      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 of 2, 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, CRLs, and user keying
      material (UKMs):

         certs is a collection of certificates. certs may contain
         originator certificates associated with several different key
         management algorithms. The certificates contained in certs are
         intended to be sufficient to make chains from a recognized
         "root" or "top-level certification authority" to all
         recipients. However, certs may contain more certificates than
         necessary, and there may be certificates sufficient to make
         chains from two or more independent top-level certification
         authorities. Alternatively, certs may contain fewer
         certificates than necessary, if it is expected that recipients
         have an alternate means of obtaining necessary certificates
         (e.g., from a previous set of certificates).

         crls is a collection of CRLs. It is intended that the set
         contain information sufficient to determine whether or not the
         certificates in the certs field are valid, but such
         correspondence is not necessary. There may be more CRLs than
         necessary, and there may also be fewer CRLs than necessary.

         ukms is a collection of UKMs. The set includes a member for
         each key management algorithm employed by the originator that
         requires a UKM. In general, several recipients will use each
         UKM in the set.

      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 algorithm is used for all recipients.

      encryptedContent is the result of encrypting the content. The
      field is optional, and if the field is not present, its intended
      value must be supplied by other means.

   The recipientInfos field comes before the encryptedContentInfo field
   so that an EnvelopedData value may be processed in a single pass.

6.2  RecipientInfo Type

   Per-recipient information is represented in the type RecipientInfo:

      RecipientInfo ::= SEQUENCE {
        version Version,
        rid RecipientIdentifier,
        originatorCert [0] EXPLICIT EntityIdentifier OPTIONAL,
        keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
        encryptedKey EncryptedKey }

      RecipientIdentifier ::= CHOICE {
        issuerAndSerialNumber IssuerAndSerialNumber,
        rKeyId [0] IMPLICIT RecipientKeyIdentifier,
        mlKeyId [1] IMPLICIT MailListKeyIdentifier }

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

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

      EntityIdentifier ::= CHOICE {
        issuerAndSerialNumber IssuerAndSerialNumber,
        subjectKeyIdentifier SubjectKeyIdentifier }

      SubjectKeyIdentifier ::= OCTET STRING

      EncryptedKey ::= OCTET STRING

   The fields of type RecipientInfo have the following meanings:




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      version is the syntax version number. If the OriginatorCert is
      absent and the RecipientIdentifier is the CHOICE
      issuerAndSerialNumber, then the version shall be 0. If the
      OriginatorCert is present or the RecipientIdentifier is either the
      CHOICE rKeyId or mlKeyId, 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.

      originatorCert optionally specifies the originator's certificate
      to be used by this recipient.  This field should be included when
      the originator has more than one certificate containing a public
      key associated with the key management algorithm used for this
      recipient.

      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.

   The RecipientIdentifier is a CHOICE with three alternatives.  The
   first two alternatives, issuerAndSerialNumber and rKeyId, specifies
   the recipient's certificate, and thereby the recipient's public key.
   The rKeyId alternative may optionally specify other parameters
   needed, such as the date.  If the recipient's certificate contains a
   key transport public key, then the content-encryption key is
   encrypted with the recipient's public key. If the recipient's
   certificate contains a key agreement public key, then a pairwise
   symmetric key is established and used to encrypt the content-
   encryption key.  The third alternative, mlKeyId, specifies a
   symmetric key encryption key that was previously distributed to the
   sender and recipient.

   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 UKMs was used by the sender.

      other is optional.  When present, this field contains additional
      information used by the recipient to locate the keying material
      used by the sender.



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   The fields of type MailListKeyIdentifier have the following meanings:

      kekIdentifier identifies the key-encryption key that was
      previously distributed to the sender and the recipient.

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

      other is optional.  When present, this field contains additional
      information used by the recipient to locate the keying material
      used by the sender.

6.3  Content-encryption Process

   The input to the content-encryption process is the "value" of the
   content being enveloped. Specifically, the input is the content
   octets of a definite-length BER encoding of the content field of the
   ContentInfo value. Only the content octets of the BER encoding are
   encrypted, not the identifier octets or length octets; those other
   octets are not included.

   When the content being enveloped has content type data, then just the
   value of the data (e.g., the contents of a file) is encrypted. This
   has the advantage that the length of the content being encrypted need
   not be known in advance of the encryption process.

   The identifier octets and the length octets are not encrypted. The
   length octets may be protected implicitly by the encryption process,
   depending on the encryption algorithm. The identifier octets are not
   protected at all, although they can be recovered from the content
   type, assuming that the content type uniquely determines the
   identifier octets. Explicit protection of the identifier and length
   octets requires that the signed-data content type be employed prior
   to enveloping.

   A definite-length BER encoding is used to ensure that the bit
   indicating whether the length is definite or indefinite is not
   recorded in the enveloped-data content type. Definite-length encoding
   is more appropriate for simple types such as octet strings, so
   definite-length encoding is chosen.

   Some content-encryption algorithms assume the input length is a
   multiple of k octets, where k is greater than one. For such
   algorithms, the input shall be padded at the trailing end with
   k-(l mod k) octets all having value k-(l mod k), where l is the
   length of the input. In other words, the input is padded at the
   trailing end with one of the following strings:



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                     01 -- if l mod k = k-1
                  02 02 -- if l mod k = k-2
                      .
                      .
                      .
            k k ... k k -- if l mod k = 0

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

6.4  Key-encryption Process

   The input to the key-encryption process -- the value supplied to the
   recipient's key-encryption algorithm --is just the "value" of the
   content-encryption key.

7  Digested-data Content Type

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

   The typical application of the digested-data content type will be to
   provide content integrity, and that the result generally becomes the
   input to the enveloped-data content type.

   Digested-data is constructed by the following steps:

      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 digested-data content type is identified by the following object
   identifier:

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








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

      DigestedData ::= SEQUENCE {
        version Version,
        digestAlgorithm DigestAlgorithmIdentifier,
        contentInfo ContentInfo,
        digest Digest }

      Digest ::= OCTET STRING

   The fields of type DigestedData have the following meanings:

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

      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 5.3 in the
      case when there are no authenticated attributes.

      contentInfo is the content that is digested. It may have any of
      the defined content types.

      digest is the result of the message-digesting process.

   The ordering of the digestAlgorithm field, the contentInfo 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 are assumed to 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 encrypted-data content type is identified by the following object
   identifier:

      id-encryptedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
          us(840) rsadsi(113549) pkcs(1) pkcs7(7) 6 }







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

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

   The fields of type EncryptedData have the following meanings:

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

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

9  Useful Types

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

9.1  CertificateRevocationLists

   The CertificateRevocationLists type gives a set of certificate
   revocation lists (CRLs). It is intended that the set contain
   information sufficient to determine whether the certificates with
   which the set is associated are revoked or not.  However, there may
   be more CRLs than necessary, or there may be fewer than necessary.

   The definition of CertificateList is imported from X.509.

      CertificateRevocationLists ::= SET OF CertificateList

9.2  ContentEncryptionAlgorithmIdentifier

   The ContentEncryptionAlgorithmIdentifier type identifies a content-
   encryption algorithm such as DES. A content-encryption algorithm
   supports encryption and decryption operations. The encryption
   operation maps an octet string (the message) to another octet string
   (the ciphertext) under control of a content-encryption key. The
   decryption operation is the inverse of the encryption operation.
   Context determines which operation is intended.

   The definition of AlgorithmIdentifier is imported from X.509.

      ContentEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier

9.3  DigestAlgorithmIdentifier

   The DigestAlgorithmIdentifier type identifies a message-digest
   algorithm. Examples include SHA-1, MD2, and MD5. A message-digest



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   algorithm maps an octet string (the message) to another octet string
   (the message digest).

   The definition of AlgorithmIdentifier is imported from X.509.

      DigestAlgorithmIdentifier ::= AlgorithmIdentifier

9.4  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.

   The definition of AlgorithmIdentifier is imported from X.509.

      SignatureAlgorithmIdentifier ::= AlgorithmIdentifier

9.5  CertificateChoices

   The CertificateChoices type gives either a PKCS #6 extended
   certificate, an X.509 certificate, or an X.509 attribute certificate.
   The PKCS #6 extended certificate is obsolete.  It is included for
   backwards compatibility, and its use should be avoided.

   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

9.6  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



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   a chain; others may enforce certain relationships between the
   subjects and issuers of certificates within a chain.

      CertificateSet ::= SET OF CertificateChoices

9.7  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
   SerialNumber is imported from X.509.


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

      SerialNumber ::= INTEGER

9.8  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.

   The definition of AlgorithmIdentifier is imported from X.509.

      KeyEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier

9.9  Version

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

      Version ::= INTEGER







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9.10  UserKeyingMaterial

   The UserKeyingMaterial type gives a syntax user keying material
   (UKM). Some key management algorithms require UKMs. The sender
   provides a UKM for the specific key management algorithm.

   The definition of AlgorithmIdentifier is imported from X.509.

      UserKeyingMaterial ::= SEQUENCE {
        algorithm AlgorithmIdentifier,
        ukm OCTET STRING }

9.11  UserKeyingMaterials

   The UserKeyingMaterial type provides a set of user keying materials
   (UKMs). This allows the sender to provide a UKM for each key
   management algorithm that requires one.

      UserKeyingMaterials ::= SET OF UserKeyingMaterial

9.12  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 }

10  Useful Attributes

   This section defines attributes that may used with signed-data. All
   of these attributes ware originally defined in PKCS #9, and they are
   included here for easy reference. The attributes are not listed in
   any particular order.

10.1  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 is identified by the following object
   identifier:



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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.

10.2  Message Digest

   The message-digest attribute type specifies the message digest of the
   contents octets of the DER encoding of the content field of the
   ContentInfo value being signed in signed-data, where the message
   digest is computed using the signer's message digest algorithm. The
   message-digest attribute type is required if there are any
   authenticated attributes present.

   The message-digest attribute is identified by the following object
   identifier:

      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.

10.3  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 is identified by the following object
   identifier:

      id-signingTime OBJECT IDENTIFIER ::= { iso(1) member-body(2)
          us(840) rsadsi(113549) pkcs(1) pkcs9(9) 5 }

   Signing-time attribute values have ASN.1 type SigningTime:








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      SigningTime ::= Time

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

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

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

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

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

10.4  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
   unauthenticated attribute; it cannot be an authenticated attribute.
   The signing-time attribute is identified by the following object
   identifier:

      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 authenticatedAttributes 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.



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   A countersignature attribute can have multiple attribute values.

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

11  Supported Algorithms

   To be supplied.  However, this section will list the must implement
   algorithms and other algorithms that may be implemented.  Here are my
   current thoughts...

   MUST implement: DSS, SHA-1, Diffie-Hellman (X9.42), and Triple-DES
   CBC (with three keys).

   MAY implement: RSA (signature and key management), MD5, KEA, RC2, DES
   CBC, and SKIPJACK.






























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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

   IMPORTS

     -- Directory Information Framework (X.501)
           Attribute, 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

   Data ::= OCTET STRING

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

   DigestAlgorithmIdentifiers ::= SET OF DigestAlgorithmIdentifier








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   SignerInfos ::= SET OF SignerInfo

   SignerInfo ::= SEQUENCE {
     version Version,
     issuerAndSerialNumber IssuerAndSerialNumber,
     digestAlgorithm DigestAlgorithmIdentifier,
     authenticatedAttributes [0] IMPLICIT Attributes OPTIONAL,
     signatureAlgorithm SignatureAlgorithmIdentifier,
     signature SignatureValue,
     unauthenticatedAttributes [1] IMPLICIT Attributes OPTIONAL }

   Attributes ::= SET OF Attribute

   SignatureValue ::= OCTET STRING

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

   OriginatorInfo ::= SEQUENCE {
     certs [0] IMPLICIT CertificateSet OPTIONAL,
     crls [1] IMPLICIT CertificateRevocationLists OPTIONAL,
     ukms [2] IMPLICIT UserKeyingMaterials OPTIONAL }

   RecipientInfos ::= SET OF RecipientInfo

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

   EncryptedContent ::= OCTET STRING

   RecipientInfo ::= SEQUENCE {
     version Version,
     rid RecipientIdentifier,
     originatorCert [0] EXPLICIT EntityIdentifier OPTIONAL,
     keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
     encryptedKey EncryptedKey }

   RecipientIdentifier ::= CHOICE {
     issuerAndSerialNumber IssuerAndSerialNumber,
     rKeyId [0] IMPLICIT RecipientKeyIdentifier,
     mlKeyId [1] IMPLICIT MailListKeyIdentifier }





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

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

   EntityIdentifier ::= CHOICE {
     issuerAndSerialNumber IssuerAndSerialNumber,
     subjectKeyIdentifier SubjectKeyIdentifier }

   SubjectKeyIdentifier ::= OCTET STRING

   EncryptedKey ::= OCTET STRING

   DigestedData ::= SEQUENCE {
     version Version,
     digestAlgorithm DigestAlgorithmIdentifier,
     contentInfo ContentInfo,
     digest Digest }

   Digest ::= OCTET STRING

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

   CertificateRevocationLists ::= SET OF CertificateList

   ContentEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier

   DigestAlgorithmIdentifier ::= AlgorithmIdentifier

   SignatureAlgorithmIdentifier ::= AlgorithmIdentifier

   CertificateChoices ::= CHOICE {
     certificate Certificate,  -- See X.509
     extendedCertificate [0] IMPLICIT ExtendedCertificate,  -- Obsolete
     attrCert [1] IMPLICIT AttributeCertificate }  -- See X.509 and X9.57

   CertificateSet ::= SET OF CertificateChoices

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



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   SerialNumber ::= INTEGER

   KeyEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier

   Version ::= INTEGER

   UserKeyingMaterial ::= SEQUENCE {
     algorithm AlgorithmIdentifier,
     ukm OCTET STRING }

   UserKeyingMaterials ::= SET OF UserKeyingMaterial

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



   -- Attributes

   MessageDigest ::= OCTET STRING

   SigningTime  ::= Time

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

   Countersignature ::= SignerInfo



   -- Object Identifiers

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

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

   id-envelopedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
       us(840) rsadsi(113549) pkcs(1) pkcs7(7) 3 }

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

   id-encryptedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
       us(840) rsadsi(113549) pkcs(1) pkcs7(7) 6 }



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   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 }



   -- 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 Version,
     certificate Certificate,
     attributes Attributes }

   Signature ::= BIT STRING


   END -- of CryptographicMessageSyntax

















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References

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

   PKCS #7    RSA Laboratories. PKCS #7: Cryptographic Message Syntax
              Standard. Version 1.5, November 1993.

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

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

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

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

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

Security Considerations

   The Cryptographic Message Syntax provides a method for digitally
   signing data, digesting data, and encrypting data.

   Implementations must protect the signer's private key.  Compromise of
   the signer's private key permits masquerade.

   Implementations must protect the key management private key and the
   content-encryption key.  Compromise of the key management private key
   may result in the disclosure of all messages protected with that key.
   Similarly, compromise of the content-encryption key may result in
   disclosure of the encrypted content.

Author Address

   Russell Housley
   SPYRUS
   PO Box 1198
   Herndon, VA 20172
   USA
   housley@spyrus.com







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