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RFC 2630

Cryptographic Message Syntax

Pages: 60
Obsoleted by:  33693370
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Network Working Group                                        R. Housley
Request for Comments: 2630                                       SPYRUS
Category: Standards Track                                     June 1999


                      Cryptographic Message Syntax

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

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

1 Introduction ................................................. 4 2 General Overview ............................................. 4 3 General Syntax ............................................... 5 4 Data Content Type ............................................ 5 5 Signed-data Content Type ..................................... 6 5.1 SignedData Type ......................................... 7 5.2 EncapsulatedContentInfo Type ............................ 8 5.3 SignerInfo Type ......................................... 9 5.4 Message Digest Calculation Process ...................... 11 5.5 Message Signature Generation Process .................... 12 5.6 Message Signature Verification Process .................. 12 6 Enveloped-data Content Type .................................. 12 6.1 EnvelopedData Type ...................................... 14 6.2 RecipientInfo Type ...................................... 15 6.2.1 KeyTransRecipientInfo Type ....................... 16 6.2.2 KeyAgreeRecipientInfo Type ....................... 17 6.2.3 KEKRecipientInfo Type ............................ 19 6.3 Content-encryption Process .............................. 20 6.4 Key-encryption Process .................................. 20 7 Digested-data Content Type ................................... 21 8 Encrypted-data Content Type .................................. 22 9 Authenticated-data Content Type .............................. 23 9.1 AuthenticatedData Type .................................. 23 9.2 MAC Generation .......................................... 25 9.3 MAC Verification ........................................ 26 10 Useful Types ................................................. 27 10.1 Algorithm Identifier Types ............................. 27 10.1.1 DigestAlgorithmIdentifier ...................... 27 10.1.2 SignatureAlgorithmIdentifier ................... 27 10.1.3 KeyEncryptionAlgorithmIdentifier ............... 28 10.1.4 ContentEncryptionAlgorithmIdentifier ........... 28 10.1.5 MessageAuthenticationCodeAlgorithm ............. 28 10.2 Other Useful Types ..................................... 28 10.2.1 CertificateRevocationLists ..................... 28 10.2.2 CertificateChoices ............................. 29 10.2.3 CertificateSet ................................. 29 10.2.4 IssuerAndSerialNumber .......................... 30 10.2.5 CMSVersion ..................................... 30 10.2.6 UserKeyingMaterial ............................. 30 10.2.7 OtherKeyAttribute .............................. 30
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   11  Useful Attributes ............................................ 31
       11.1  Content Type ........................................... 31
       11.2  Message Digest ......................................... 32
       11.3  Signing Time ........................................... 32
       11.4  Countersignature ....................................... 34
   12  Supported Algorithms ......................................... 35
       12.1  Digest Algorithms ...................................... 35
             12.1.1  SHA-1 .......................................... 35
             12.1.2  MD5 ............................................ 35
       12.2  Signature Algorithms ................................... 36
             12.2.1  DSA ............................................ 36
             12.2.2  RSA ............................................ 36
       12.3  Key Management Algorithms .............................. 36
             12.3.1  Key Agreement Algorithms ....................... 36
                     12.3.1.1  X9.42 Ephemeral-Static Diffie-Hellman. 37
             12.3.2  Key Transport Algorithms ....................... 38
                     12.3.2.1  RSA .................................. 39
             12.3.3  Symmetric Key-Encryption Key Algorithms ........ 39
                     12.3.3.1  Triple-DES Key Wrap .................. 40
                     12.3.3.2  RC2 Key Wrap ......................... 41
      12.4  Content Encryption Algorithms ........................... 41
            12.4.1  Triple-DES CBC .................................. 42
            12.4.2  RC2 CBC ......................................... 42
      12.5  Message Authentication Code Algorithms .................. 42
            12.5.1  HMAC with SHA-1 ................................. 43
      12.6  Triple-DES and RC2 Key Wrap Algorithms .................. 43
            12.6.1  Key Checksum .................................... 44
            12.6.2  Triple-DES Key Wrap ............................. 44
            12.6.3  Triple-DES Key Unwrap ........................... 44
            12.6.4  RC2 Key Wrap .................................... 45
            12.6.5  RC2 Key Unwrap .................................. 46
   Appendix A:  ASN.1 Module ........................................ 47
   References ....................................................... 55
   Security Considerations .......................................... 56
   Acknowledgments .................................................. 58
   Author's Address ................................................. 59
   Full Copyright Statement ......................................... 60
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1 Introduction

This document describes the Cryptographic Message Syntax. This syntax is used to digitally sign, digest, authenticate, or encrypt arbitrary messages. The Cryptographic Message Syntax describes an encapsulation syntax for data protection. It supports digital signatures, message authentication codes, and encryption. The syntax allows multiple encapsulation, so one encapsulation envelope can be nested inside another. Likewise, one party can digitally sign some previously encapsulated data. It also allows arbitrary attributes, such as signing time, to be signed along with the message content, and provides for other attributes such as countersignatures to be associated with a signature. The Cryptographic Message Syntax can support a variety of architectures for certificate-based key management, such as the one defined by the PKIX working group. The Cryptographic Message Syntax values are generated using ASN.1 [X.208-88], using BER-encoding [X.209-88]. Values are typically represented as octet strings. While many systems are capable of transmitting arbitrary octet strings reliably, it is well known that many electronic-mail systems are not. This document does not address mechanisms for encoding octet strings for reliable transmission in such environments.

2 General Overview

The Cryptographic Message Syntax (CMS) is general enough to support many different content types. This document defines one protection content, ContentInfo. ContentInfo encapsulates a single identified content type, and the identified type may provide further encapsulation. This document defines six content types: data, signed-data, enveloped-data, digested-data, encrypted-data, and authenticated-data. Additional content types can be defined outside this document. An implementation that conforms to this specification must implement the protection content, ContentInfo, and must implement the data, signed-data, and enveloped-data content types. The other content types may be implemented if desired. As a general design philosophy, each content type permits single pass processing using indefinite-length Basic Encoding Rules (BER) encoding. Single-pass operation is especially helpful if content is large, stored on tapes, or is "piped" from another process. Single-
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   pass operation has one significant drawback: it is difficult to
   perform encode operations using the Distinguished Encoding Rules
   (DER) [X.509-88] encoding in a single pass since the lengths of the
   various components may not be known in advance.  However, signed
   attributes within the signed-data content type and authenticated
   attributes within the authenticated-data content type require DER
   encoding.  Signed attributes and authenticated attributes must be
   transmitted in DER form to ensure that recipients can verify a
   content that contains one or more unrecognized attributes.  Signed
   attributes and authenticated attributes are the only CMS data types
   that require DER encoding.

3 General Syntax

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

4 Data Content Type

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

   The data content type is generally encapsulated in the signed-data,
   enveloped-data, digested-data, encrypted-data, or authenticated-data
   content type.

5 Signed-data Content Type

The signed-data content type consists of a content of any type and zero or more signature values. Any number of signers in parallel can sign any type of content. The typical application of the signed-data content type represents one signer's digital signature on content of the data content type. Another typical application disseminates certificates and certificate revocation lists (CRLs). The process by which signed-data is constructed involves the following steps: 1. For each signer, a message digest, or hash value, is computed on the content with a signer-specific message-digest algorithm. If the signer is signing any information other than the content, the message digest of the content and the other information are digested with the signer's message digest algorithm (see Section 5.4), and the result becomes the "message digest." 2. For each signer, the message digest is digitally signed using the signer's private key. 3. For each signer, the signature value and other signer-specific information are collected into a SignerInfo value, as defined in Section 5.3. Certificates and CRLs for each signer, and those not corresponding to any signer, are collected in this step. 4. The message digest algorithms for all the signers and the SignerInfo values for all the signers are collected together with the content into a SignedData value, as defined in Section 5.1. A recipient independently computes the message digest. This message digest and the signer's public key are used to verify the signature value. The signer's public key is referenced either by an issuer distinguished name along with an issuer-specific serial number or by a subject key identifier that uniquely identifies the certificate containing the public key. The signer's certificate may be included in the SignedData certificates field.
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   This section is divided into six parts.  The first part describes the
   top-level type SignedData, the second part describes
   EncapsulatedContentInfo, the third part describes the per-signer
   information type SignerInfo, and the fourth, fifth, and sixth parts
   describe the message digest calculation, signature generation, and
   signature verification processes, respectively.

5.1 SignedData Type

The following object identifier identifies the signed-data content type: id-signedData OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs7(7) 2 } The signed-data content type shall have ASN.1 type SignedData: SignedData ::= SEQUENCE { version CMSVersion, digestAlgorithms DigestAlgorithmIdentifiers, encapContentInfo EncapsulatedContentInfo, certificates [0] IMPLICIT CertificateSet OPTIONAL, crls [1] IMPLICIT CertificateRevocationLists OPTIONAL, signerInfos SignerInfos } DigestAlgorithmIdentifiers ::= SET OF DigestAlgorithmIdentifier SignerInfos ::= SET OF SignerInfo The fields of type SignedData have the following meanings: version is the syntax version number. If no attribute certificates are present in the certificates field, the encapsulated content type is id-data, and all of the elements of SignerInfos are version 1, then the value of version shall be 1. Alternatively, if attribute certificates are present, the encapsulated content type is other than id-data, or any of the elements of SignerInfos are version 3, then the value of version shall be 3. digestAlgorithms is a collection of message digest algorithm identifiers. There may be any number of elements in the collection, including zero. Each element identifies the message digest algorithm, along with any associated parameters, used by one or more signer. The collection is intended to list the message digest algorithms employed by all of the signers, in any order, to facilitate one-pass signature verification. The message digesting process is described in Section 5.4.
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      encapContentInfo is the signed content, consisting of a content
      type identifier and the content itself.  Details of the
      EncapsulatedContentInfo type are discussed in section 5.2.

      certificates is a collection of certificates.  It is intended that
      the set of certificates be sufficient to contain chains from a
      recognized "root" or "top-level certification authority" to all of
      the signers in the signerInfos field.  There may be more
      certificates than necessary, and there may be certificates
      sufficient to contain chains from two or more independent top-
      level certification authorities.  There may also be fewer
      certificates than necessary, if it is expected that recipients
      have an alternate means of obtaining necessary certificates (e.g.,
      from a previous set of certificates).  As discussed above, if
      attribute certificates are present, then the value of version
      shall be 3.

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

      signerInfos is a collection of per-signer information.  There may
      be any number of elements in the collection, including zero.  The
      details of the SignerInfo type are discussed in section 5.3.

5.2 EncapsulatedContentInfo Type

The content is represented in the type EncapsulatedContentInfo: EncapsulatedContentInfo ::= SEQUENCE { eContentType ContentType, eContent [0] EXPLICIT OCTET STRING OPTIONAL } ContentType ::= OBJECT IDENTIFIER The fields of type EncapsulatedContentInfo have the following meanings: eContentType is an object identifier that uniquely specifies the content type. eContent is the content itself, carried as an octet string. The eContent need not be DER encoded.
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   The optional omission of the eContent within the
   EncapsulatedContentInfo field makes it possible to construct
   "external signatures."  In the case of external signatures, the
   content being signed is absent from the EncapsulatedContentInfo value
   included in the signed-data content type.  If the eContent value
   within EncapsulatedContentInfo is absent, then the signatureValue is
   calculated and the eContentType is assigned as though the eContent
   value was present.

   In the degenerate case where there are no signers, the
   EncapsulatedContentInfo value being "signed" is irrelevant.  In this
   case, the content type within the EncapsulatedContentInfo value being
   "signed" should be id-data (as defined in section 4), and the content
   field of the EncapsulatedContentInfo value should be omitted.

5.3 SignerInfo Type

Per-signer information is represented in the type SignerInfo: SignerInfo ::= SEQUENCE { version CMSVersion, sid SignerIdentifier, digestAlgorithm DigestAlgorithmIdentifier, signedAttrs [0] IMPLICIT SignedAttributes OPTIONAL, signatureAlgorithm SignatureAlgorithmIdentifier, signature SignatureValue, unsignedAttrs [1] IMPLICIT UnsignedAttributes OPTIONAL } SignerIdentifier ::= CHOICE { issuerAndSerialNumber IssuerAndSerialNumber, subjectKeyIdentifier [0] SubjectKeyIdentifier } SignedAttributes ::= SET SIZE (1..MAX) OF Attribute UnsignedAttributes ::= SET SIZE (1..MAX) OF Attribute Attribute ::= SEQUENCE { attrType OBJECT IDENTIFIER, attrValues SET OF AttributeValue } AttributeValue ::= ANY SignatureValue ::= OCTET STRING The fields of type SignerInfo have the following meanings: version is the syntax version number. If the SignerIdentifier is the CHOICE issuerAndSerialNumber, then the version shall be 1. If
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      the SignerIdentifier is subjectKeyIdentifier, then the version
      shall be 3.

      sid specifies the signer's certificate (and thereby the signer's
      public key).  The signer's public key is needed by the recipient
      to verify the signature.  SignerIdentifier provides two
      alternatives for specifying the signer's public key.  The
      issuerAndSerialNumber alternative identifies the signer's
      certificate by the issuer's distinguished name and the certificate
      serial number; the subjectKeyIdentifier identifies the signer's
      certificate by the X.509 subjectKeyIdentifier extension value.

      digestAlgorithm identifies the message digest algorithm, and any
      associated parameters, used by the signer.  The message digest is
      computed on either the content being signed or the content
      together with the signed attributes using the process described in
      section 5.4.  The message digest algorithm should be among those
      listed in the digestAlgorithms field of the associated SignerData.

      signedAttributes is a collection of attributes that are signed.
      The field is optional, but it must be present if the content type
      of the EncapsulatedContentInfo value being signed is not id-data.
      Each SignedAttribute in the SET must be DER encoded.  Useful
      attribute types, such as signing time, are defined in Section 11.
      If the field is present, it must contain, at a minimum, the
      following two attributes:

         A content-type attribute having as its value the content type
         of the EncapsulatedContentInfo value being signed.  Section
         11.1 defines the content-type attribute.  The content-type
         attribute is not required when used as part of a
         countersignature unsigned attribute as defined in section 11.4.

         A message-digest attribute, having as its value the message
         digest of the content.  Section 11.2 defines the message-digest
         attribute.

      signatureAlgorithm identifies the signature algorithm, and any
      associated parameters, used by the signer to generate the digital
      signature.

      signature is the result of digital signature generation, using the
      message digest and the signer's private key.

      unsignedAttributes is a collection of attributes that are not
      signed.  The field is optional.  Useful attribute types, such as
      countersignatures, are defined in Section 11.
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   The fields of type SignedAttribute and UnsignedAttribute have the
   following meanings:

      attrType indicates the type of attribute.  It is an object
      identifier.

      attrValues is a set of values that comprise the attribute.  The
      type of each value in the set can be determined uniquely by
      attrType.

5.4 Message Digest Calculation Process

The message digest calculation process computes a message digest on either the content being signed or the content together with the signed attributes. In either case, the initial input to the message digest calculation process is the "value" of the encapsulated content being signed. Specifically, the initial input is the encapContentInfo eContent OCTET STRING to which the signing process is applied. Only the octets comprising the value of the eContent OCTET STRING are input to the message digest algorithm, not the tag or the length octets. The result of the message digest calculation process depends on whether the signedAttributes field is present. When the field is absent, the result is just the message digest of the content as described above. When the field is present, however, the result is the message digest of the complete DER encoding of the SignedAttributes value contained in the signedAttributes field. Since the SignedAttributes value, when present, must contain the content type and the content message digest attributes, those values are indirectly included in the result. The content type attribute is not required when used as part of a countersignature unsigned attribute as defined in section 11.4. A separate encoding of the signedAttributes field is performed for message digest calculation. The IMPLICIT [0] tag in the signedAttributes field is not used for the DER encoding, rather an EXPLICIT SET OF tag is used. That is, the DER encoding of the SET OF tag, rather than of the IMPLICIT [0] tag, is to be included in the message digest calculation along with the length and content octets of the SignedAttributes value. When the signedAttributes field is absent, then only the octets comprising the value of the signedData encapContentInfo eContent OCTET STRING (e.g., the contents of a file) are input to the message digest calculation. This has the advantage that the length of the content being signed need not be known in advance of the signature generation process.
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   Although the encapContentInfo eContent OCTET STRING tag and length
   octets are not included in the message digest calculation, they are
   still protected by other means.  The length octets are protected by
   the nature of the message digest algorithm since it is
   computationally infeasible to find any two distinct messages of any
   length that have the same message digest.

5.5 Message Signature Generation Process

The input to the signature generation process includes the result of the message digest calculation process and the signer's private key. The details of the signature generation depend on the signature algorithm employed. The object identifier, along with any parameters, that specifies the signature algorithm employed by the signer is carried in the signatureAlgorithm field. The signature value generated by the signer is encoded as an OCTET STRING and carried in the signature field.

5.6 Message Signature Verification Process

The input to the signature verification process includes the result of the message digest calculation process and the signer's public key. The recipient may obtain the correct public key for the signer by any means, but the preferred method is from a certificate obtained from the SignedData certificates field. The selection and validation of the signer's public key may be based on certification path validation (see [PROFILE]) as well as other external context, but is beyond the scope of this document. The details of the signature verification depend on the signature algorithm employed. The recipient may not rely on any message digest values computed by the originator. If the signedData signerInfo includes signedAttributes, then the content message digest must be calculated as described in section 5.4. For the signature to be valid, the message digest value calculated by the recipient must be the same as the value of the messageDigest attribute included in the signedAttributes of the signedData signerInfo.

6 Enveloped-data Content Type

The enveloped-data content type consists of an encrypted content of any type and encrypted content-encryption keys for one or more recipients. The combination of the encrypted content and one encrypted content-encryption key for a recipient is a "digital envelope" for that recipient. Any type of content can be enveloped for an arbitrary number of recipients using any of the three key management techniques for each recipient.
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   The typical application of the enveloped-data content type will
   represent one or more recipients' digital envelopes on content of the
   data or signed-data content types.

   Enveloped-data is constructed by the following steps:

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

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

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

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

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

      3.  For each recipient, the encrypted content-encryption key and
      other recipient-specific information are collected into a
      RecipientInfo value, defined in Section 6.2.

      4.  The content is encrypted with the content-encryption key.
      Content encryption may require that the content be padded to a
      multiple of some block size; see Section 6.3.

      5.  The RecipientInfo values for all the recipients are collected
      together with the encrypted content to form an EnvelopedData value
      as defined in Section 6.1.

   A recipient opens the digital envelope by decrypting one of the
   encrypted content-encryption keys and then decrypting the encrypted
   content with the recovered content-encryption key.

   This section is divided into four parts.  The first part describes
   the top-level type EnvelopedData, the second part describes the per-
   recipient information type RecipientInfo, and the third and fourth
   parts describe the content-encryption and key-encryption processes.
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6.1 EnvelopedData Type

The following object identifier identifies the enveloped-data content type: id-envelopedData OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs7(7) 3 } The enveloped-data content type shall have ASN.1 type EnvelopedData: EnvelopedData ::= SEQUENCE { version CMSVersion, originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL, recipientInfos RecipientInfos, encryptedContentInfo EncryptedContentInfo, unprotectedAttrs [1] IMPLICIT UnprotectedAttributes OPTIONAL } OriginatorInfo ::= SEQUENCE { certs [0] IMPLICIT CertificateSet OPTIONAL, crls [1] IMPLICIT CertificateRevocationLists OPTIONAL } RecipientInfos ::= SET OF RecipientInfo EncryptedContentInfo ::= SEQUENCE { contentType ContentType, contentEncryptionAlgorithm ContentEncryptionAlgorithmIdentifier, encryptedContent [0] IMPLICIT EncryptedContent OPTIONAL } EncryptedContent ::= OCTET STRING UnprotectedAttributes ::= SET SIZE (1..MAX) OF Attribute The fields of type EnvelopedData have the following meanings: version is the syntax version number. If originatorInfo is present, then version shall be 2. If any of the RecipientInfo structures included have a version other than 0, then the version shall be 2. If unprotectedAttrs is present, then version shall be 2. If originatorInfo is absent, all of the RecipientInfo structures are version 0, and unprotectedAttrs is absent, then version shall be 0. originatorInfo optionally provides information about the originator. It is present only if required by the key management algorithm. It may contain certificates and CRLs: certs is a collection of certificates. certs may contain originator certificates associated with several different key
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         management algorithms.  certs may also contain attribute
         certificates associated with the originator.  The certificates
         contained in certs are intended to be sufficient to make chains
         from a recognized "root" or "top-level certification authority"
         to all recipients.  However, certs may contain more
         certificates than necessary, and there may be certificates
         sufficient to make chains from two or more independent top-
         level certification authorities.  Alternatively, certs may
         contain fewer certificates than necessary, if it is expected
         that recipients have an alternate means of obtaining necessary
         certificates (e.g., from a previous set of certificates).

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

      recipientInfos is a collection of per-recipient information.
      There must be at least one element in the collection.

      encryptedContentInfo is the encrypted content information.

      unprotectedAttrs is a collection of attributes that are not
      encrypted.  The field is optional.  Useful attribute types are
      defined in Section 11.

   The fields of type EncryptedContentInfo have the following meanings:

      contentType indicates the type of content.

      contentEncryptionAlgorithm identifies the content-encryption
      algorithm, and any associated parameters, used to encrypt the
      content.  The content-encryption process is described in Section
      6.3.  The same content-encryption algorithm and content-encryption
      key is used for all recipients.

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

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

6.2 RecipientInfo Type

Per-recipient information is represented in the type RecipientInfo. RecipientInfo has a different format for the three key management
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   techniques that are supported: key transport, key agreement, and
   previously distributed symmetric key-encryption keys.  Any of the
   three key management techniques can be used for each recipient of the
   same encrypted content.  In all cases, the content-encryption key is
   transferred to one or more recipient in encrypted form.

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

      EncryptedKey ::= OCTET STRING

6.2.1 KeyTransRecipientInfo Type

Per-recipient information using key transport is represented in the type KeyTransRecipientInfo. Each instance of KeyTransRecipientInfo transfers the content-encryption key to one recipient. KeyTransRecipientInfo ::= SEQUENCE { version CMSVersion, -- always set to 0 or 2 rid RecipientIdentifier, keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier, encryptedKey EncryptedKey } RecipientIdentifier ::= CHOICE { issuerAndSerialNumber IssuerAndSerialNumber, subjectKeyIdentifier [0] SubjectKeyIdentifier } The fields of type KeyTransRecipientInfo have the following meanings: version is the syntax version number. If the RecipientIdentifier is the CHOICE issuerAndSerialNumber, then the version shall be 0. If the RecipientIdentifier is subjectKeyIdentifier, then the version shall be 2. rid specifies the recipient's certificate or key that was used by the sender to protect the content-encryption key. The RecipientIdentifier provides two alternatives for specifying the recipient's certificate, and thereby the recipient's public key. The recipient's certificate must contain a key transport public key. The content-encryption key is encrypted with the recipient's public key. The issuerAndSerialNumber alternative identifies the recipient's certificate by the issuer's distinguished name and the certificate serial number; the subjectKeyIdentifier identifies the recipient's certificate by the X.509 subjectKeyIdentifier extension value.
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      keyEncryptionAlgorithm identifies the key-encryption algorithm,
      and any associated parameters, used to encrypt the content-
      encryption key for the recipient.  The key-encryption process is
      described in Section 6.4.

      encryptedKey is the result of encrypting the content-encryption
      key for the recipient.

6.2.2 KeyAgreeRecipientInfo Type

Recipient information using key agreement is represented in the type KeyAgreeRecipientInfo. Each instance of KeyAgreeRecipientInfo will transfer the content-encryption key to one or more recipient that uses the same key agreement algorithm and domain parameters for that algorithm. KeyAgreeRecipientInfo ::= SEQUENCE { version CMSVersion, -- always set to 3 originator [0] EXPLICIT OriginatorIdentifierOrKey, ukm [1] EXPLICIT UserKeyingMaterial OPTIONAL, keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier, recipientEncryptedKeys RecipientEncryptedKeys } OriginatorIdentifierOrKey ::= CHOICE { issuerAndSerialNumber IssuerAndSerialNumber, subjectKeyIdentifier [0] SubjectKeyIdentifier, originatorKey [1] OriginatorPublicKey } OriginatorPublicKey ::= SEQUENCE { algorithm AlgorithmIdentifier, publicKey BIT STRING } RecipientEncryptedKeys ::= SEQUENCE OF RecipientEncryptedKey RecipientEncryptedKey ::= SEQUENCE { rid KeyAgreeRecipientIdentifier, encryptedKey EncryptedKey } KeyAgreeRecipientIdentifier ::= CHOICE { issuerAndSerialNumber IssuerAndSerialNumber, rKeyId [0] IMPLICIT RecipientKeyIdentifier } RecipientKeyIdentifier ::= SEQUENCE { subjectKeyIdentifier SubjectKeyIdentifier, date GeneralizedTime OPTIONAL, other OtherKeyAttribute OPTIONAL } SubjectKeyIdentifier ::= OCTET STRING
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   The fields of type KeyAgreeRecipientInfo have the following meanings:

      version is the syntax version number.  It shall always be 3.

      originator is a CHOICE with three alternatives specifying the
      sender's key agreement public key.  The sender uses the
      corresponding private key and the recipient's public key to
      generate a pairwise key.  The content-encryption key is encrypted
      in the pairwise key.  The issuerAndSerialNumber alternative
      identifies the sender's certificate, and thereby the sender's
      public key, by the issuer's distinguished name and the certificate
      serial number.  The subjectKeyIdentifier alternative identifies
      the sender's certificate, and thereby the sender's public key, by
      the X.509 subjectKeyIdentifier extension value.  The originatorKey
      alternative includes the algorithm identifier and sender's key
      agreement public key. Permitting originator anonymity since the
      public key is not certified.

      ukm is optional.  With some key agreement algorithms, the sender
      provides a User Keying Material (UKM) to ensure that a different
      key is generated each time the same two parties generate a
      pairwise key.

      keyEncryptionAlgorithm identifies the key-encryption algorithm,
      and any associated parameters, used to encrypt the content-
      encryption key in the key-encryption key.  The key-encryption
      process is described in Section 6.4.

      recipientEncryptedKeys includes a recipient identifier and
      encrypted key for one or more recipients.  The
      KeyAgreeRecipientIdentifier is a CHOICE with two alternatives
      specifying the recipient's certificate, and thereby the
      recipient's public key, that was used by the sender to generate a
      pairwise key-encryption key.  The recipient's certificate must
      contain a key agreement public key.  The content-encryption key is
      encrypted in the pairwise key-encryption key.  The
      issuerAndSerialNumber alternative identifies the recipient's
      certificate by the issuer's distinguished name and the certificate
      serial number; the RecipientKeyIdentifier is described below.  The
      encryptedKey is the result of encrypting the content-encryption
      key in the pairwise key-encryption key generated using the key
      agreement algorithm.

   The fields of type RecipientKeyIdentifier have the following
   meanings:

      subjectKeyIdentifier identifies the recipient's certificate by the
      X.509 subjectKeyIdentifier extension value.
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      date is optional.  When present, the date specifies which of the
      recipient's previously distributed UKMs was used by the sender.

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

6.2.3 KEKRecipientInfo Type

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

6.3 Content-encryption Process

The content-encryption key for the desired content-encryption algorithm is randomly generated. The data to be protected is padded as described below, then the padded data is encrypted using the content-encryption key. The encryption operation maps an arbitrary string of octets (the data) to another string of octets (the ciphertext) under control of a content-encryption key. The encrypted data is included in the envelopedData encryptedContentInfo encryptedContent OCTET STRING. The input to the content-encryption process is the "value" of the content being enveloped. Only the value octets of the envelopedData encryptedContentInfo encryptedContent OCTET STRING are encrypted; the OCTET STRING tag and length octets are not encrypted. Some content-encryption algorithms assume the input length is a multiple of k octets, where k is greater than one. For such algorithms, the input shall be padded at the trailing end with k-(lth mod k) octets all having value k-(lth mod k), where lth is the length of the input. In other words, the input is padded at the trailing end with one of the following strings: 01 -- if lth mod k = k-1 02 02 -- if lth mod k = k-2 . . . k k ... k k -- if lth mod k = 0 The padding can be removed unambiguously since all input is padded, including input values that are already a multiple of the block size, and no padding string is a suffix of another. This padding method is well defined if and only if k is less than 256.

6.4 Key-encryption Process

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

The digested-data content type consists of content of any type and a message digest of the content. Typically, the digested-data content type is used to provide content integrity, and the result generally becomes an input to the enveloped-data content type. The following steps construct digested-data: 1. A message digest is computed on the content with a message- digest algorithm. 2. The message-digest algorithm and the message digest are collected together with the content into a DigestedData value. A recipient verifies the message digest by comparing the message digest to an independently computed message digest. The following object identifier identifies the digested-data content type: id-digestedData OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs7(7) 5 } The digested-data content type shall have ASN.1 type DigestedData: DigestedData ::= SEQUENCE { version CMSVersion, digestAlgorithm DigestAlgorithmIdentifier, encapContentInfo EncapsulatedContentInfo, digest Digest } Digest ::= OCTET STRING The fields of type DigestedData have the following meanings: version is the syntax version number. If the encapsulated content type is id-data, then the value of version shall be 0; however, if the encapsulated content type is other than id-data, then the value of version shall be 2. digestAlgorithm identifies the message digest algorithm, and any associated parameters, under which the content is digested. The message-digesting process is the same as in Section 5.4 in the case when there are no signed attributes.
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      encapContentInfo is the content that is digested, as defined in
      section 5.2.

      digest is the result of the message-digesting process.

   The ordering of the digestAlgorithm field, the encapContentInfo
   field, and the digest field makes it possible to process a
   DigestedData value in a single pass.

8 Encrypted-data Content Type

The encrypted-data content type consists of encrypted content of any type. Unlike the enveloped-data content type, the encrypted-data content type has neither recipients nor encrypted content-encryption keys. Keys must be managed by other means. The typical application of the encrypted-data content type will be to encrypt the content of the data content type for local storage, perhaps where the encryption key is a password. The following object identifier identifies the encrypted-data content type: id-encryptedData OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs7(7) 6 } The encrypted-data content type shall have ASN.1 type EncryptedData: EncryptedData ::= SEQUENCE { version CMSVersion, encryptedContentInfo EncryptedContentInfo, unprotectedAttrs [1] IMPLICIT UnprotectedAttributes OPTIONAL } The fields of type EncryptedData have the following meanings: version is the syntax version number. If unprotectedAttrs is present, then version shall be 2. If unprotectedAttrs is absent, then version shall be 0. encryptedContentInfo is the encrypted content information, as defined in Section 6.1. unprotectedAttrs is a collection of attributes that are not encrypted. The field is optional. Useful attribute types are defined in Section 11.
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9 Authenticated-data Content Type

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

9.1 AuthenticatedData Type

The following object identifier identifies the authenticated-data content type: id-ct-authData OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) ct(1) 2 }
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   The authenticated-data content type shall have ASN.1 type
   AuthenticatedData:

      AuthenticatedData ::= SEQUENCE {
        version CMSVersion,
        originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL,
        recipientInfos RecipientInfos,
        macAlgorithm MessageAuthenticationCodeAlgorithm,
        digestAlgorithm [1] DigestAlgorithmIdentifier OPTIONAL,
        encapContentInfo EncapsulatedContentInfo,
        authenticatedAttributes [2] IMPLICIT AuthAttributes OPTIONAL,
        mac MessageAuthenticationCode,
        unauthenticatedAttributes [3] IMPLICIT UnauthAttributes OPTIONAL }

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

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

      MessageAuthenticationCode ::= OCTET STRING

   The fields of type AuthenticatedData have the following meanings:

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

      originatorInfo optionally provides information about the
      originator.  It is present only if required by the key management
      algorithm.  It may contain certificates, attribute certificates,
      and CRLs, as defined in Section 6.1.

      recipientInfos is a collection of per-recipient information, as
      defined in Section 6.1.  There must be at least one element in the
      collection.

      macAlgorithm is a message authentication code (MAC) algorithm
      identifier.  It identifies the MAC algorithm, along with any
      associated parameters, used by the originator.  Placement of the
      macAlgorithm field facilitates one-pass processing by the
      recipient.

      digestAlgorithm identifies the message digest algorithm, and any
      associated parameters, used to compute a message digest on the
      encapsulated content if authenticated attributes are present.  The
      message digesting process is described in Section 9.2.  Placement
      of the digestAlgorithm field facilitates one-pass processing by
      the recipient.  If the digestAlgorithm field is present, then the
      authenticatedAttributes field must also be present.
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      encapContentInfo is the content that is authenticated, as defined
      in section 5.2.

      authenticatedAttributes is a collection of authenticated
      attributes.  The authenticatedAttributes structure is optional,
      but it must be present if the content type of the
      EncapsulatedContentInfo value being authenticated is not id-data.
      If the authenticatedAttributes field is present, then the
      digestAlgorithm field must also be present.  Each
      AuthenticatedAttribute in the SET must be DER encoded.  Useful
      attribute types are defined in Section 11.  If the
      authenticatedAttributes field is present, it must contain, at a
      minimum, the following two attributes:

         A content-type attribute having as its value the content type
         of the EncapsulatedContentInfo value being authenticated.
         Section 11.1 defines the content-type attribute.

         A message-digest attribute, having as its value the message
         digest of the content.  Section 11.2 defines the message-digest
         attribute.

      mac is the message authentication code.

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

9.2 MAC Generation

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

   The message digest calculation process computes a message digest on
   the content being authenticated.  The initial input to the message
   digest calculation process is the "value" of the encapsulated content
   being authenticated.  Specifically, the input is the encapContentInfo
   eContent OCTET STRING to which the authentication process is applied.
   Only the octets comprising the value of the encapContentInfo eContent
   OCTET STRING are input to the message digest algorithm, not the tag
   or the length octets.  This has the advantage that the length of the
   content being authenticated need not be known in advance.  Although
   the encapContentInfo eContent OCTET STRING tag and length octets are
   not included in the message digest calculation, they are still
   protected by other means.  The length octets are protected by the
   nature of the message digest algorithm since it is computationally
   infeasible to find any two distinct messages of any length that have
   the same message digest.

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

9.3 MAC Verification

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

10 Useful Types

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

10.1 Algorithm Identifier Types

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

10.1.1 DigestAlgorithmIdentifier

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

10.1.2 SignatureAlgorithmIdentifier

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

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

10.1.4 ContentEncryptionAlgorithmIdentifier

The ContentEncryptionAlgorithmIdentifier type identifies a content- encryption algorithm. Examples include Triple-DES and RC2. A content-encryption algorithm supports encryption and decryption operations. The encryption operation maps an octet string (the message) to another octet string (the ciphertext) under control of a content-encryption key. The decryption operation is the inverse of the encryption operation. Context determines which operation is intended. ContentEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier

10.1.5 MessageAuthenticationCodeAlgorithm

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


(page 28 continued on part 2)

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