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  • 快速注册永信彩票218tcom:RFC2630 - Cryptographic Message Syntax

    来源:本网整理

      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 RFC2315 [PKCS#7]. Wherever possible, backward
    compatibility is preserved; however, changes were necessary to
    accommodate attribute certificate transfer and key agreement
    techniques for key management.

    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

    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

    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-

    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

    (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.

    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.

    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.

    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

    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.

    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.

    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.

    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.

    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

    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

    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.

    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

    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.

    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.

    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.

    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.

    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.

    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 }

    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.

    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

    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

    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

    10.1.3 KeyEncryptionAlgorithmIdentifier

    The KeyEncryptionAlgorithmIdentifier type identifies a key-encryption
    algorithm used to encrypt a content-encryption key. The encryption
    operation maps an octet string (the key) to another octet string (the
    encrypted key) under control of a key-encryption key. The decryption
    operation is the inverse of the encryption operation. Context
    determines which operation is intended.

    The details of encryption and decryption depend on the key management
    algorithm used. Key transport, key agreement, and previously
    distributed symmetric key-encrypting keys are supported.

    KeyEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier

    10.1.4 ContentEncryptionAlgorithmIdentifier

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

    ContentEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier

    10.1.5 MessageAuthenticationCodeAlgorithm

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

    MessageAuthenticationCodeAlgorithm ::= AlgorithmIdentifier

    10.2 Other Useful Types

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

    10.2.1 CertificateRevocationLists

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

    attribute certificates with which the set is associated are revoked
    or not. However, there may be more CRLs than necessary or there may
    be fewer CRLs than necessary.

    The CertificateList may contain a CRL, an Authority Revocation List
    (ARL), a Delta Revocation List, or an Attribute Certificate
    Revocation List. All of these lists share a common syntax.

    CRLs are specified in X.509 [X.509-97], and they are profiled for use
    in the Internet in RFC2459 [PROFILE].

    The definition of CertificateList is imported from X.509.

    CertificateRevocationLists ::= SET OF CertificateList

    10.2.2 CertificateChoices

    The CertificateChoices type gives either a PKCS #6 extended
    certificate [PKCS#6], an X.509 certificate, or an X.509 attribute
    certificate [X.509-97]. The PKCS #6 extended certificate is
    obsolete. PKCS #6 certificates are included for backward
    compatibility, and their use should be avoided. The Internet profile
    of X.509 certificates is specified in the "Internet X.509 Public Key
    Infrastructure: Certificate and CRL Profile" [PROFILE].

    The definitions of Certificate and AttributeCertificate are imported
    from X.509.

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

    10.2.3 CertificateSet

    The CertificateSet type provides a set of certificates. It is
    intended that the set be sufficient to contain chains from a
    recognized "root" or "top-level certification authority" to all of
    the sender certificates with which the set is associated. However,
    there may be more certificates than necessary, or there may be fewer
    than necessary.

    The precise meaning of a "chain" is outside the scope of this
    document. Some applications may impose upper limits on the length of
    a chain; others may enforce certain relationships between the
    subjects and issuers of certificates within a chain.

    CertificateSet ::= SET OF CertificateChoices

    10.2.4 IssuerAndSerialNumber

    The IssuerAndSerialNumber type identifies a certificate, and thereby
    an entity and a public key, by the distinguished name of the
    certificate issuer and an issuer-specific certificate serial number.

    The definition of Name is imported from X.501 [X.501-88], and the
    definition of CertificateSerialNumber is imported from X.509
    [X.509-97].

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

    CertificateSerialNumber ::= INTEGER

    10.2.5 CMSVersion

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

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

    10.2.6 UserKeyingMaterial

    The UserKeyingMaterial type gives a syntax for user keying material
    (UKM). Some key agreement algorithms require UKMs to ensure that a
    different key is generated each time the same two parties generate a
    pairwise key. The sender provides a UKM for use with a specific key
    agreement algorithm.

    UserKeyingMaterial ::= OCTET STRING

    10.2.7 OtherKeyAttribute

    The OtherKeyAttribute type gives a syntax for the inclusion of other
    key attributes that permit the recipient to select the key used by
    the sender. The attribute object identifier must be registered along
    with the syntax of the attribute itself. Use of this structure
    should be avoided since it may impede interoperability.

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

    11 Useful Attributes

    This section defines attributes that may be used with signed-data,
    enveloped-data, encrypted-data, or authenticated-data. The syntax of
    Attribute is compatible with X.501 [X.501-88] and RFC2459 [PROFILE].
    Some of the attributes defined in this section were originally
    defined in PKCS #9 [PKCS#9], others were not previously defined. The
    attributes are not listed in any particular order.

    Additional attributes are defined in many places, notably the S/MIME
    Version 3 Message Specification [MSG] and the Enhanced Security
    Services for S/MIME [ESS], which also include recommendations on the
    placement of these attributes.

    11.1 Content Type

    The content-type attribute type specifies the content type of the
    ContentInfo value being signed in signed-data. The content-type
    attribute type is required if there are any authenticated attributes
    present.

    The content-type attribute must be a signed attribute or an
    authenticated attribute; it cannot be an unsigned attribute, an
    unauthenticated attribute, or an unprotectedAttribute.

    The following object identifier identifies the content-type
    attribute:

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

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

    ContentType ::= OBJECT IDENTIFIER

    A content-type attribute must have a single attribute value, even
    though the syntax is defined as a SET OF AttributeValue. There must
    not be zero or multiple instances of AttributeValue present.

    The SignedAttributes and AuthAttributes syntaxes are each defined as
    a SET OF Attributes. The SignedAttributes in a signerInfo must not
    include multiple instances of the content-type attribute. Similarly,
    the AuthAttributes in an AuthenticatedData must not include multiple
    instances of the content-type attribute.

    11.2 Message Digest

    The message-digest attribute type specifies the message digest of the
    encapContentInfo eContent OCTET STRING being signed in signed-data
    (see section 5.4) or authenticated in authenticated-data (see section
    9.2). For signed-data, the message digest is computed using the
    signer's message digest algorithm. For authenticated-data, the
    message digest is computed using the originator's message digest
    algorithm.

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

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

    The following object identifier identifies the message-digest
    attribute:

    id-messageDigest OBJECT IDENTIFIER ::= { iso(1) member-body(2)
    us(840) rsadsi(113549) pkcs(1) pkcs9(9) 4 }

    Message-digest attribute values have ASN.1 type MessageDigest:

    MessageDigest ::= OCTET STRING

    A message-digest attribute must have a single attribute value, even
    though the syntax is defined as a SET OF AttributeValue. There must
    not be zero or multiple instances of AttributeValue present.

    The SignedAttributes syntax is defined as a SET OF Attributes. The
    SignedAttributes in a signerInfo must not include multiple instances
    of the message-digest attribute.

    11.3 Signing Time

    The signing-time attribute type specifies the time at which the
    signer (purportedly) performed the signing process. The signing-time
    attribute type is intended for use in signed-data.

    The signing-time attribute may be a signed attribute; it cannot be an
    unsigned attribute, an authenticated attribute, or an unauthenticated
    attribute.

    The following object identifier identifies the signing-time
    attribute:

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

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

    SigningTime ::= Time

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

    Note: The definition of Time matches the one specified in the 1997
    version of X.509 [X.509-97].

    Dates between 1 January 1950 and 31 December 2049 (inclusive) must be
    encoded as UTCTime. Any dates with year values before 1950 or after
    2049 must be encoded as GeneralizedTime.

    UTCTime values must be expressed in Greenwich Mean Time (Zulu) and
    must include seconds (i.e., times are YYMMDDHHMMSSZ), even where the
    number of seconds is zero. Midnight (GMT) must be represented as
    "YYMMDD000000Z". Century information is implicit, and the century
    must be determined as follows:

    Where YY is greater than or equal to 50, the year shall be
    interpreted as 19YY; and

    Where YY is less than 50, the year shall be interpreted as 20YY.

    GeneralizedTime values shall be expressed in Greenwich Mean Time
    (Zulu) and must include seconds (i.e., times are YYYYMMDDHHMMSSZ),
    even where the number of seconds is zero. GeneralizedTime values
    must not include fractional seconds.

    A signing-time attribute must have a single attribute value, even
    though the syntax is defined as a SET OF AttributeValue. There must
    not be zero or multiple instances of AttributeValue present.

    The SignedAttributes syntax is defined as a SET OF Attributes. The
    SignedAttributes in a signerInfo must not include multiple instances
    of the signing-time attribute.

    No requirement is imposed concerning the correctness of the signing
    time, and acceptance of a purported signing time is a matter of a
    recipient's discretion. It is expected, however, that some signers,

    such as time-stamp servers, will be trusted implicitly.

    11.4 Countersignature

    The countersignature attribute type specifies one or more signatures
    on the contents octets of the DER encoding of the signatureValue
    field of a SignerInfo value in signed-data. Thus, the
    countersignature attribute type countersigns (signs in serial)
    another signature.

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

    The following object identifier identifies the countersignature
    attribute:

    id-countersignature OBJECT IDENTIFIER ::= { iso(1) member-body(2)
    us(840) rsadsi(113549) pkcs(1) pkcs9(9) 6 }

    Countersignature attribute values have ASN.1 type Countersignature:

    Countersignature ::= SignerInfo

    Countersignature values have the same meaning as SignerInfo values
    for ordinary signatures, except that:

    1. The signedAttributes field must contain a message-digest
    attribute if it contains any other attributes, but need not
    contain a content-type attribute, as there is no content type for
    countersignatures.

    2. The input to the message-digesting process is the contents
    octets of the DER encoding of the signatureValue field of the
    SignerInfo value with which the attribute is associated.

    A countersignature attribute can have multiple attribute values. The
    syntax is defined as a SET OF AttributeValue, and there must be one
    or more instances of AttributeValue present.

    The UnsignedAttributes syntax is defined as a SET OF Attributes. The
    UnsignedAttributes in a signerInfo may include multiple instances of
    the countersignature attribute.

    A countersignature, since it has type SignerInfo, can itself contain
    a countersignature attribute. Thus it is possible to construct
    arbitrarily long series of countersignatures.

    12 Supported Algorithms

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

    12.1 Digest Algorithms

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

    Digest algorithm identifiers are located in the SignedData
    digestAlgorithms field, the SignerInfo digestAlgorithm field, the
    DigestedData digestAlgorithm field, and the AuthenticatedData
    digestAlgorithm field.

    Digest values are located in the DigestedData digest field, and
    digest values are located in the Message Digest authenticated
    attribute. In addition, digest values are input to signature
    algorithms.

    12.1.1 SHA-1

    The SHA-1 digest algorithm is defined in FIPS Pub 180-1 [SHA1]. The
    algorithm identifier for SHA-1 is:

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

    The AlgorithmIdentifier parameters field is optional. If present,
    the parameters field must contain an ASN.1 NULL. Implementations
    should accept SHA-1 AlgorithmIdentifiers with absent parameters as
    well as NULL parameters. Implementations should generate SHA-1
    AlgorithmIdentifiers with NULL parameters.

    12.1.2 MD5

    The MD5 digest algorithm is defined in RFC1321 [MD5]. The algorithm
    identifier for MD5 is:

    md5 OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
    rsadsi(113549) digestAlgorithm(2) 5 }

    The AlgorithmIdentifier parameters field must be present, and the
    parameters field must contain NULL. Implementations may accept the
    MD5 AlgorithmIdentifiers with absent parameters as well as NULL
    parameters.

    12.2 Signature Algorithms

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

    Signature algorithm identifiers are located in the SignerInfo
    signatureAlgorithm field. Also, signature algorithm identifiers are
    located in the SignerInfo signatureAlgorithm field of
    countersignature attributes.

    Signature values are located in the SignerInfo signature field.
    Also, signature values are located in the SignerInfo signature field
    of countersignature attributes.

    12.2.1 DSA

    The DSA signature algorithm is defined in FIPS Pub 186 [DSS]. DSA is
    always used with the SHA-1 message digest algorithm. The algorithm
    identifier for DSA is:

    id-dsa-with-sha1 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
    us(840) x9-57 (10040) x9cm(4) 3 }

    The AlgorithmIdentifier parameters field must not be present.

    12.2.2 RSA

    The RSA signature algorithm is defined in RFC2347 [NEWPKCS#1]. RFC
    2347 specifies the use of the RSA signature algorithm with the SHA-1
    and MD5 message digest algorithms. The algorithm identifier for RSA
    is:

    rsaEncryption OBJECT IDENTIFIER ::= { iso(1) member-body(2)
    us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 1 }

    12.3 Key Management Algorithms

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

    12.3.1 Key Agreement Algorithms

    CMS implementations must include key agreement using X9.42
    Ephemeral-Static Diffie-Hellman.

    Any symmetric encryption algorithm that a CMS implementation includes
    as a content-encryption algorithm must also be included as a key-

    encryption algorithm. CMS implementations must include key agreement
    of Triple-DES pairwise key-encryption keys and Triple-DES wrapping of
    Triple-DES content-encryption keys. CMS implementations should
    include key agreement of RC2 pairwise key-encryption keys and RC2
    wrapping of RC2 content-encryption keys. The key wrap algorithm for
    Triple-DES and RC2 is described in section 12.3.3.

    A CMS implementation may support mixed key-encryption and content-
    encryption algorithms. For example, a 128-bit RC2 content-encryption
    key may be wrapped with 168-bit Triple-DES key-encryption key.
    Similarly, a 40-bit RC2 content-encryption key may be wrapped with
    128-bit RC2 key-encryption key.

    For key agreement of RC2 key-encryption keys, 128 bits must be
    generated as input to the key expansion process used to compute the
    RC2 effective key [RC2].

    Key agreement algorithm identifiers are located in the EnvelopedData
    RecipientInfos KeyAgreeRecipientInfo keyEncryptionAlgorithm and
    AuthenticatedData RecipientInfos KeyAgreeRecipientInfo
    keyEncryptionAlgorithm fields.

    Key wrap algorithm identifiers are located in the KeyWrapAlgorithm
    parameters within the EnvelopedData RecipientInfos
    KeyAgreeRecipientInfo keyEncryptionAlgorithm and AuthenticatedData
    RecipientInfos KeyAgreeRecipientInfo keyEncryptionAlgorithm fields.

    Wrapped content-encryption keys are located in the EnvelopedData
    RecipientInfos KeyAgreeRecipientInfo RecipientEncryptedKeys
    encryptedKey field. Wrapped message-authentication keys are located
    in the AuthenticatedData RecipientInfos KeyAgreeRecipientInfo
    RecipientEncryptedKeys encryptedKey field.

    12.3.1.1 X9.42 Ephemeral-Static Diffie-Hellman

    Ephemeral-Static Diffie-Hellman key agreement is defined in RFC2631
    [DH-X9.42]. When using Ephemeral-Static Diffie-Hellman, the
    EnvelopedData RecipientInfos KeyAgreeRecipientInfo and
    AuthenticatedData RecipientInfos KeyAgreeRecipientInfo fields are
    used as follows:

    version must be 3.

    originator must be the originatorKey alternative. The
    originatorKey algorithm fields must contain the dh-public-number
    object identifier with absent parameters. The originatorKey
    publicKey field must contain the sender's ephemeral public key.
    The dh-public-number object identifier is:

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

    ukm may be absent. When present, the ukm is used to ensure that a
    different key-encryption key is generated when the ephemeral
    private key might be used more than once.

    keyEncryptionAlgorithm must be the id-alg-ESDH algorithm
    identifier. The algorithm identifier parameter field for id-alg-
    ESDH is KeyWrapAlgorihtm, and this parameter must be present. The
    KeyWrapAlgorithm denotes the symmetric encryption algorithm used
    to encrypt the content-encryption key with the pairwise key-
    encryption key generated using the Ephemeral-Static Diffie-Hellman
    key agreement algorithm. Triple-DES and RC2 key wrap algorithms
    are discussed in section 12.3.3. The id-alg-ESDH algorithm
    identifier and parameter syntax is:

    id-alg-ESDH OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
    rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 5 }

    KeyWrapAlgorithm ::= AlgorithmIdentifier

    recipientEncryptedKeys contains an identifier and an encrypted key
    for each recipient. The RecipientEncryptedKey
    KeyAgreeRecipientIdentifier must contain either the
    issuerAndSerialNumber identifying the recipient's certificate or
    the RecipientKeyIdentifier containing the subject key identifier
    from the recipient's certificate. In both cases, the recipient's
    certificate contains the recipient's static public key.
    RecipientEncryptedKey EncryptedKey must contain the content-
    encryption key encrypted with the Ephemeral-Static Diffie-Hellman
    generated pairwise key-encryption key using the algorithm
    specified by the KeyWrapAlgortihm.

    12.3.2 Key Transport Algorithms

    CMS implementations should include key transport using RSA. RSA
    implementations must include key transport of Triple-DES content-
    encryption keys. RSA implementations should include key transport of
    RC2 content-encryption keys.

    Key transport algorithm identifiers are located in the EnvelopedData
    RecipientInfos KeyTransRecipientInfo keyEncryptionAlgorithm and
    AuthenticatedData RecipientInfos KeyTransRecipientInfo
    keyEncryptionAlgorithm fields.

    Key transport encrypted content-encryption keys are located in the
    EnvelopedData RecipientInfos KeyTransRecipientInfo encryptedKey

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

    12.3.2.1 RSA

    The RSA key transport algorithm is the RSA encryption scheme defined
    in RFC2313 [PKCS#1], block type is 02, where the message to be
    encrypted is the content-encryption key. The algorithm identifier
    for RSA is:

    rsaEncryption OBJECT IDENTIFIER ::= { iso(1) member-body(2)
    us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 1 }

    The AlgorithmIdentifier parameters field must be present, and the
    parameters field must contain NULL.

    When using a Triple-DES content-encryption key, adjust the parity
    bits for each DES key comprising the Triple-DES key prior to RSA
    encryption.

    The use of RSA encryption, as defined in RFC2313 [PKCS#1], to
    provide confidentiality has a known vulnerability concerns. The
    vulnerability is primarily relevant to usage in interactive
    applications rather than to store-and-forward environments. Further
    information and proposed countermeasures are discussed in the
    Security Considerations section of this document.

    Note that the same encryption scheme is also defined in RFC2437
    [NEWPKCS#1]. Within RFC2437, this scheme is called
    RSAES-PKCS1-v1_5.

    12.3.3 Symmetric Key-Encryption Key Algorithms

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

    Key wrap algorithm identifiers are located in the EnvelopedData
    RecipientInfos KEKRecipientInfo keyEncryptionAlgorithm and
    AuthenticatedData RecipientInfos KEKRecipientInfo
    keyEncryptionAlgorithm fields.

    Wrapped content-encryption keys are located in the EnvelopedData
    RecipientInfos KEKRecipientInfo encryptedKey field. Wrapped
    message-authentication keys are located in the AuthenticatedData
    RecipientInfos KEKRecipientInfo encryptedKey field.

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

    12.3.3.1 Triple-DES Key Wrap

    Triple-DES key encryption has the algorithm identifier:

    id-alg-CMS3DESwrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)
    us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 6 }

    The AlgorithmIdentifier parameter field must be NULL.

    The key wrap algorithm used to encrypt a Triple-DES content-
    encryption key with a Triple-DES key-encryption key is specified in
    section 12.6.

    Out-of-band distribution of the Triple-DES key-encryption key used to
    encrypt the Triple-DES content-encryption key is beyond of the scope
    of this document.

    12.3.3.2 RC2 Key Wrap

    RC2 key encryption has the algorithm identifier:

    id-alg-CMSRC2wrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)
    us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 7 }

    The AlgorithmIdentifier parameter field must be RC2wrapParameter:

    RC2wrapParameter ::= RC2ParameterVersion

    RC2ParameterVersion ::= INTEGER

    The RC2 effective-key-bits (key size) greater than 32 and less than
    256 is encoded in the RC2ParameterVersion. For the effective-key-
    bits of 40, 64, and 128, the rc2ParameterVersion values are 160, 120,
    and 58 respectively. These values are not simply the RC2 key length.
    Note that the value 160 must be encoded as two octets (00 A0),
    because the one octet (A0) encoding represents a negative number.

    Only 128-bit RC2 keys may be used as key-encryption keys, and they
    must be used with the RC2ParameterVersion parameter set to 58.

    The key wrap algorithm used to encrypt a RC2 content-encryption key
    with a RC2 key-encryption key is specified in section 12.6.

    Out-of-band distribution of the RC2 key-encryption key used to
    encrypt the RC2 content-encryption key is beyond of the scope of this
    document.

    12.4 Content Encryption Algorithms

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

    Content encryption algorithms identifiers are located in the
    EnvelopedData EncryptedContentInfo contentEncryptionAlgorithm and the
    EncryptedData EncryptedContentInfo contentEncryptionAlgorithm fields.

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

    12.4.1 Triple-DES CBC

    The Triple-DES algorithm is described in ANSI X9.52 [3DES]. The
    Triple-DES is composed from three sequential DES [DES] operations:
    encrypt, decrypt, and encrypt. Three-Key Triple-DES uses a different
    key for each DES operation. Two-Key Triple-DES uses one key for the
    two encrypt operations and different key for the decrypt operation.
    The same algorithm identifiers are used for Three-Key Triple-DES and
    Two-Key Triple-DES. The algorithm identifier for Triple-DES in
    Cipher Block Chaining (CBC) mode is:

    des-ede3-cbc OBJECT IDENTIFIER ::= { iso(1) member-body(2)
    us(840) rsadsi(113549) encryptionAlgorithm(3) 7 }

    The AlgorithmIdentifier parameters field must be present, and the
    parameters field must contain a CBCParameter:

    CBCParameter ::= IV

    IV ::= OCTET STRING -- exactly 8 octets

    12.4.2 RC2 CBC

    The RC2 algorithm is described in RFC2268 [RC2]. The algorithm
    identifier for RC2 in CBC mode is:

    rc2-cbc OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
    rsadsi(113549) encryptionAlgorithm(3) 2 }

    The AlgorithmIdentifier parameters field must be present, and the
    parameters field must contain a RC2CBCParameter:

    RC2CBCParameter ::= SEQUENCE {
    rc2ParameterVersion INTEGER,
    iv OCTET STRING } -- exactly 8 octets

    The RC2 effective-key-bits (key size) greater than 32 and less than
    256 is encoded in the rc2ParameterVersion. For the effective-key-
    bits of 40, 64, and 128, the rc2ParameterVersion values are 160, 120,
    and 58 respectively. These values are not simply the RC2 key length.
    Note that the value 160 must be encoded as two octets (00 A0), since
    the one octet (A0) encoding represents a negative number.

    12.5 Message Authentication Code Algorithms

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

    MAC algorithm identifiers are located in the AuthenticatedData
    macAlgorithm field.

    MAC values are located in the AuthenticatedData mac field.

    12.5.1 HMAC with SHA-1

    The HMAC with SHA-1 algorithm is described in RFC2104 [HMAC]. The
    algorithm identifier for HMAC with SHA-1 is:

    hMAC-SHA1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
    dod(6) internet(1) security(5) mechanisms(5) 8 1 2 }

    The AlgorithmIdentifier parameters field must be absent.

    12.6 Triple-DES and RC2 Key Wrap Algorithms

    CMS implementations must include encryption of a Triple-DES content-
    encryption key with a Triple-DES key-encryption key using the
    algorithm specified in Sections 12.6.2 and 12.6.3. CMS
    implementations should include encryption of a RC2 content-encryption
    key with a RC2 key-encryption key using the algorithm specified in
    Sections 12.6.4 and 12.6.5. Triple-DES and RC2 content-encryption
    keys are encrypted in Cipher Block Chaining (CBC) mode [MODES].

    Key Transport algorithms allow for the content-encryption key to be
    directly encrypted; however, key agreement and symmetric key-
    encryption key algorithms encrypt the content-encryption key with a
    second symmetric encryption algorithm. This section describes how
    the Triple-DES or RC2 content-encryption key is formatted and
    encrypted.

    Key agreement algorithms generate a pairwise key-encryption key, and
    a key wrap algorithm is used to encrypt the content-encryption key
    with the pairwise key-encryption key. Similarly, a key wrap
    algorithm is used to encrypt the content-encryption key in a
    previously distributed key-encryption key.

    The key-encryption key is generated by the key agreement algorithm or
    distributed out of band. For key agreement of RC2 key-encryption
    keys, 128 bits must be generated as input to the key expansion
    process used to compute the RC2 effective key [RC2].

    The same algorithm identifier is used for both 2-key and 3-key
    Triple-DES. When the length of the content-encryption key to be
    wrapped is a 2-key Triple-DES key, a third key with the same value as
    the first key is created. Thus, all Triple-DES content-encryption
    keys are wrapped like 3-key Triple-DES keys.

    12.6.1 Key Checksum

    The CMS Checksum Algorithm is used to provide a content-encryption
    key integrity check value. The algorithm is:

    1. Compute a 20 octet SHA-1 [SHA1] message digest on the
    content-encryption key.
    2. Use the most significant (first) eight octets of the message
    digest value as the checksum value.

    12.6.2 Triple-DES Key Wrap

    The Triple-DES key wrap algorithm encrypts a Triple-DES content-
    encryption key with a Triple-DES key-encryption key. The Triple-DES
    key wrap algorithm is:

    1. Set odd parity for each of the DES key octets comprising
    the content-encryption key, call the result CEK.
    2. Compute an 8 octet key checksum value on CEK as described above
    in Section 12.6.1, call the result ICV.
    3. Let CEKICV = CEK ICV.
    4. Generate 8 octets at random, call the result IV.
    5. Encrypt CEKICV in CBC mode using the key-encryption key. Use
    the random value generated in the previous step as the
    initialization vector (IV). Call the ciphertext TEMP1.
    6. Let TEMP2 = IV TEMP1.
    7. Reverse the order of the octets in TEMP2. That is, the most
    significant (first) octet is swapped with the least significant
    (last) octet, and so on. Call the result TEmp3.
    8. Encrypt TEMP3 in CBC mode using the key-encryption key. Use
    an initialization vector (IV) of 0x4adda22c79e82105.
    The ciphertext is 40 octets long.

    Note: When the same content-encryption key is wrapped in different
    key-encryption keys, a fresh initialization vector (IV) must be
    generated for each invocation of the key wrap algorithm.

    12.6.3 Triple-DES Key Unwrap

    The Triple-DES key unwrap algorithm decrypts a Triple-DES content-
    encryption key using a Triple-DES key-encryption key. The Triple-DES
    key unwrap algorithm is:

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

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

    12.6.4 RC2 Key Wrap

    The RC2 key wrap algorithm encrypts a RC2 content-encryption key with
    a RC2 key-encryption key. The RC2 key wrap algorithm is:

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

    Note: When the same content-encryption key is wrapped in different
    key-encryption keys, a fresh initialization vector (IV) must be
    generated for each invocation of the key wrap algorithm.

    12.6.5 RC2 Key Unwrap

    The RC2 key unwrap algorithm decrypts a RC2 content-encryption key
    using a RC2 key-encryption key. The RC2 key unwrap algorithm is:

    1. If the wrapped content-encryption key is not a multiple of 8
    octets, then error.
    2. Decrypt the wrapped content-encryption key in CBC mode using
    the key-encryption key. Use an initialization vector (IV)
    of 0x4adda22c79e82105. Call the output TEMP3.
    3. Reverse the order of the octets in TEMP3. That is, the most
    significant (first) octet is swapped with the least significant
    (last) octet, and so on. Call the result TEMP2.
    4. Decompose the TEMP2 into IV and TEMP1. IV is the most
    significant (first) 8 octets, and TEMP1 is the remaining octets.

    5. Decrypt TEMP1 in CBC mode using the key-encryption key. Use
    the IV value from the previous step as the initialization vector.
    Call the plaintext LCEKPADICV.
    6. Decompose the LCEKPADICV into LCEKPAD, and ICV. ICV is the
    least significant (last) octet 8 octets. LCEKPAD is the
    remaining octets.
    7. Compute an 8 octet key checksum value on LCEKPAD as described
    above in Section 12.6.1. If the computed key checksum value
    does not match the decrypted key checksum value, ICV, then error.
    8. Decompose the LCEKPAD into LENGTH, CEK, and PAD. LENGTH is the
    most significant (first) octet. CEK is the following LENGTH
    octets. PAD is the remaining octets, if any.
    9. If the length of PAD is more than 7 octets, then error.
    10. Use CEK as the content-encryption key.

    Appendix A: ASN.1 Module

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

    DEFINITIONS IMPLICIT TAGS ::=
    BEGIN

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

    IMPORTS

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

    -- Directory Authentication Framework (X.509)
    AlgorithmIdentifier, AttributeCertificate, Certificate,
    CertificateList, CertificateSerialNumber
    FROM AuthenticationFramework { joint-iso-itu-t ds(5)
    module(1) authenticationFramework(7) 3 } ;

    -- Cryptographic Message Syntax

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

    ContentType ::= OBJECT IDENTIFIER

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

    DigestAlgorithmIdentifiers ::= SET OF DigestAlgorithmIdentifier

    SignerInfos ::= SET OF SignerInfo

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

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

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

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

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

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

    AttributeValue ::= ANY

    SignatureValue ::= OCTET STRING

    EnvelopedData ::= SEQUENCE {
    version CMSVersion,
    originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL,
    recipientInfos RecipientInfos,
    encryptedContentInfo EncryptedContentInfo,
    unprotectedAttrs [1] IMPLICIT UnprotectedAttributes OPTIONAL }

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

    RecipientInfos ::= SET OF RecipientInfo

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

    EncryptedContent ::= OCTET STRING

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

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

    EncryptedKey ::= OCTET STRING

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

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

    KeyAgreeRecipientInfo ::= SEQUENCE {
    version CMSVersion, -- always set to 3
    originator [0] EXPLICIT OriginatorIdentifierOrKey,
    ukm [1] EXPLICIT UserKeyingMaterial OPTIONAL,
    keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
    recipientEncryptedKeys RecipientEncryptedKeys }

    OriginatorIdentifierOrKey ::= CHOICE {
    issuerAndSerialNumber IssuerAndSerialNumber,
    subjectKeyIdentifier [0] SubjectKeyIdentifier,
    originatorKey [1] OriginatorPublicKey }

    OriginatorPublicKey ::= SEQUENCE {
    algorithm AlgorithmIdentifier,
    publicKey BIT STRING }

    RecipientEncryptedKeys ::= SEQUENCE OF RecipientEncryptedKey

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

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

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

    SubjectKeyIdentifier ::= OCTET STRING

    KEKRecipientInfo ::= SEQUENCE {
    version CMSVersion, -- always set to 4
    kekid KEKIdentifier,
    keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
    encryptedKey EncryptedKey }

    KEKIdentifier ::= SEQUENCE {
    keyIdentifier OCTET STRING,
    date GeneralizedTime OPTIONAL,
    other OtherKeyAttribute OPTIONAL }

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

    Digest ::= OCTET STRING

    EncryptedData ::= SEQUENCE {
    version CMSVersion,
    encryptedContentInfo EncryptedContentInfo,
    unprotectedAttrs [1] IMPLICIT UnprotectedAttributes OPTIONAL }

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

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

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

    MessageAuthenticationCode ::= OCTET STRING

    DigestAlgorithmIdentifier ::= AlgorithmIdentifier

    SignatureAlgorithmIdentifier ::= AlgorithmIdentifier

    KeyEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier

    ContentEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier

    MessageAuthenticationCodeAlgorithm ::= AlgorithmIdentifier

    CertificateRevocationLists ::= SET OF CertificateList

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

    CertificateSet ::= SET OF CertificateChoices

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

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

    UserKeyingMaterial ::= OCTET STRING

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

    -- CMS Attributes

    MessageDigest ::= OCTET STRING

    SigningTime ::= Time

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

    Countersignature ::= SignerInfo

    -- Algorithm Identifiers

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

    md5 OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
    rsadsi(113549) digestAlgorithm(2) 5 }

    id-dsa-with-sha1 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
    us(840) x9-57 (10040) x9cm(4) 3 }

    rsaEncryption OBJECT IDENTIFIER ::= { iso(1) member-body(2)
    us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 1 }

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

    id-alg-ESDH OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
    rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 5 }

    id-alg-CMS3DESwrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)
    us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 6 }

    id-alg-CMSRC2wrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)
    us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 7 }

    des-ede3-cbc OBJECT IDENTIFIER ::= { iso(1) member-body(2)
    us(840) rsadsi(113549) encryptionAlgorithm(3) 7 }

    rc2-cbc OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
    rsadsi(113549) encryptionAlgorithm(3) 2 }

    hMAC-SHA1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
    dod(6) internet(1) security(5) mechanisms(5) 8 1 2 }

    -- Algorithm Parameters

    KeyWrapAlgorithm ::= AlgorithmIdentifier

    RC2wrapParameter ::= RC2ParameterVersion

    RC2ParameterVersion ::= INTEGER

    CBCParameter ::= IV

    IV ::= OCTET STRING -- exactly 8 octets

    RC2CBCParameter ::= SEQUENCE {
    rc2ParameterVersion INTEGER,
    iv OCTET STRING } -- exactly 8 octets

    -- Content Type Object Identifiers

    id-ct-contentInfo OBJECT IDENTIFIER ::= { iso(1) member-body(2)
    us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16)
    ct(1) 6 }

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

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

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

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

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

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

    -- Attribute Object Identifiers

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

    id-messageDigest OBJECT IDENTIFIER ::= { iso(1) member-body(2)
    us(840) rsadsi(113549) pkcs(1) pkcs9(9) 4 }

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

    id-countersignature OBJECT IDENTIFIER ::= { iso(1) member-body(2)
    us(840) rsadsi(113549) pkcs(1) pkcs9(9) 6 }

    -- Obsolete Extended Certificate syntax from PKCS#6

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

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

    Signature ::= BIT STRING

    END -- of CryptographicMessageSyntax

    References

    3DES American National Standards Institute. ANSI X9.52-1998,
    Triple Data Encryption Algorithm Modes of Operation. 1998.

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

    DH-X9.42 Rescorla, E., "Diffie-Hellman Key Agreement Method",
    RFC2631, June 1999.

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

    ESS Hoffman, P., Editor, "Enhanced Security Services for
    S/MIME", RFC2634, June 1999.

    HMAC Krawczyk, H., "HMAC: Keyed-Hashing for Message
    Authentication", RFC2104, February 1997.

    MD5 Rivest, R., "The MD5 Message-Digest Algorithm", RFC1321,
    April 1992.

    MODES National Institute of Standards and Technology.
    FIPS Pub 81: DES Modes of Operation. 2 December 1980.

    MSG Ramsdell, B., Editor, "S/MIME Version 3 Message
    Specification", RFC2633, June 1999.

    NEWPKCS#1 Kaliski, B., "PKCS #1: RSA Encryption, Version 2.0",
    RFC2347, October 1998.

    PROFILE Housley, R., Ford, W., Polk, W. and D. Solo, "Internet
    X.509 Public Key Infrastructure: Certificate and CRL
    Profile", RFC2459, January 1999.

    PKCS#1 Kaliski, B., "PKCS #1: RSA Encryption, Version 1.5.",
    RFC2313, March 1998.

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

    PKCS#7 Kaliski, B., "PKCS #7: Cryptographic Message Syntax,
    Version 1.5.", RFC2315, March 1998.

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

    RANDOM Eastlake, D., Crocker, S. and J. Schiller, "Randomness
    Recommendations for Security", RFC1750, December 1994.

    RC2 Rivest, R., "A Description of the RC2 (r) Encryption
    Algorithm", RFC2268, March 1998.

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

    X.208-88 CCITT. Recommendation X.208: Specification of Abstract
    Syntax Notation One (ASN.1). 1988.

    X.209-88 CCITT. Recommendation X.209: Specification of Basic
    Encoding Rules for Abstract Syntax Notation One (ASN.1).
    1988.

    X.501-88 CCITT. Recommendation X.501: The Directory - Models.
    1988.

    X.509-88 CCITT. Recommendation X.509: The Directory -
    Authentication Framework. 1988.

    X.509-97 ITU-T. Recommendation X.509: The Directory -
    Authentication Framework. 1997.

    Security Considerations

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

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

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

    Implementations must protect the key management private key and the
    message-authentication key. Compromise of the key management private
    key permits masquerade of authenticated data. Similarly, compromise
    of the message-authentication key may result in undetectable
    modification of the authenticated content.

    Implementations must randomly generate content-encryption keys,
    message-authentication keys, initialization vectors (IVs), and
    padding. Also, the generation of public/private key pairs relies on
    a random numbers. The use of inadequate pseudo-random number
    generators (PRNGs) to generate cryptographic keys can result in
    little or no security. An attacker may find it much easier to
    reproduce the PRNG environment that produced the keys, searching the
    resulting small set of possibilities, rather than brute force
    searching the whole key space. The generation of quality random
    numbers is difficult. RFC1750 [RANDOM] offers important guidance in
    this area, and Appendix 3 of FIPS Pub 186 [DSS] provides one quality
    PRNG technique.

    When using key agreement algorithms or previously distributed
    symmetric key-encryption keys, a key-encryption key is used to
    encrypt the content-encryption key. If the key-encryption and
    content-encryption algorithms are different, the effective security
    is determined by the weaker of the two algorithms. If, for example,
    a message content is encrypted with 168-bit Triple-DES and the
    Triple-DES content-encryption key is wrapped with a 40-bit RC2 key,
    then at most 40 bits of protection is provided. A trivial search to
    determine the value of the 40-bit RC2 key can recover Triple-DES key,
    and then the Triple-DES key can be used to decrypt the content.
    Therefore, implementers must ensure that key-encryption algorithms
    are as strong or stronger than content-encryption algorithms.

    Section 12.6 specifies key wrap algorithms used to encrypt a Triple-
    DES [3DES] content-encryption key with a Triple-DES key-encryption
    key or to encrypt a RC2 [RC2] content-encryption key with a RC2 key-
    encryption key. The key wrap algorithms make use of CBC mode
    [MODES]. These key wrap algorithms have been reviewed for use with
    Triple and RC2. They have not been reviewed for use with other
    cryptographic modes or other encryption algorithms. Therefore, if a
    CMS implementation wishes to support ciphers in addition to Triple-
    DES or RC2, then additional key wrap algorithms need to be defined to
    support the additional ciphers.

    Implementers should be aware that cryptographic algorithms become
    weaker with time. As new cryptoanalysis techniques are developed and
    computing performance improves, the work factor to break a particular
    cryptographic algorithm will reduce. Therefore, cryptographic
    algorithm implementations should be modular allowing new algorithms
    to be readily inserted. That is, implementers should be prepared for
    the set of mandatory to implement algorithms to change over time.

    The countersignature unauthenticated attribute includes a digital
    signature that is computed on the content signature value, thus the
    countersigning process need not know the original signed content.

    This structure permits implementation efficiency advantages; however,
    this structure may also permit the countersigning of an inappropriate
    signature value. Therefore, implementations that perform
    countersignatures should either verify the original signature value
    prior to countersigning it (this verification requires processing of
    the original content), or implementations should perform
    countersigning in a context that ensures that only appropriate
    signature values are countersigned.

    Users of CMS, particularly those employing CMS to support interactive
    applications, should be aware that PKCS #1 Version 1.5 as specified
    in RFC2313 [PKCS#1] is vulnerable to adaptive chosen ciphertext
    attacks when applied for encryption purposes. Exploitation of this
    identified vulnerability, revealing the result of a particular RSA
    decryption, requires access to an Oracle which will respond to a
    large number of ciphertexts (based on currently available results,
    hundreds of thousands or more), which are constructed adaptively in
    response to previously-received replies providing information on the
    successes or failures of attempted decryption operations. As a
    result, the attack appears significantly less feasible to perpetrate
    for store-and-forward S/MIME environments than for directly
    interactive protocols. Where CMS constructs are applied as an
    intermediate encryption layer within an interactive request-response
    communications environment, exploitation could be more feasible.

    An updated version of PKCS #1 has been published, PKCS #1 Version 2.0
    [NEWPKCS#1]. This new document will supersede RFC2313. PKCS #1
    Version 2.0 preserves support for the encryption padding format
    defined in PKCS #1 Version 1.5 [PKCS#1], and it also defines a new
    alternative. To resolve the adaptive chosen ciphertext
    vulnerability, the PKCS #1 Version 2.0 specifies and recommends use
    of Optimal Asymmetric Encryption Padding (OAEP) when RSA encryption
    is used to provide confidentiality. Designers of protocols and
    systems employing CMS for interactive environments should either
    consider usage of OAEP, or should ensure that information which could
    reveal the success or failure of attempted PKCS #1 Version 1.5
    decryption operations is not provided. Support for OAEP will likely
    be added to a future version of the CMS specification.

    Acknowledgments

    This document is the result of contributions from many professionals.
    I appreciate the hard work of all members of the IETF S/MIME Working
    Group. I extend a special thanks to Rich Ankney, Tim Dean, Steve
    Dusse, Carl Ellison, Peter Gutmann, Bob Jueneman, Stephen Henson,
    Paul Hoffman, Scott Hollenbeck, Don Johnson, Burt Kaliski, John Linn,
    John Pawling, Blake Ramsdell, Francois Rousseau, Jim Schaad, and Dave
    Solo for their efforts and support.

    Author's Address

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

    EMail: [email protected]

    Full Copyright Statement

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

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

    Funding for the RFCEditor function is currently provided by the
    Internet Society.

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