RFC 3852
Cryptographic Message Syntax (CMS)
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 MUST be 0; however, if
the encapsulated content type is other than id-data, then the
value of version MUST 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 derived from 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 MUST be 2. If unprotectedAttrs is absent,
then version MUST 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,
authAttrs [2] IMPLICIT AuthAttributes OPTIONAL,
mac MessageAuthenticationCode,
unauthAttrs [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. The version MUST be
assigned as follows:
IF (originatorInfo is present) AND
((any certificates with a type of other are present) OR
(any crls with a type of other are present))
THEN version is 3
ELSE
IF ((originatorInfo is present) AND
(any version 2 attribute certificates are present))
THEN version is 1
ELSE version is 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
authAttrs field MUST also be present.
encapContentInfo is the content that is authenticated, as defined
in section 5.2.
authAttrs is a collection of authenticated attributes. The
authAttrs structure is optional, but it MUST be present if the
content type of the EncapsulatedContentInfo value being
authenticated is not id-data. If the authAttrs field is present,
then the digestAlgorithm field MUST also be present. The
AuthAttributes structure MUST be DER encoded, even if the rest of
the structure is BER encoded. Useful attribute types are defined
in Section 11. If the authAttrs 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.
unauthAttrs 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 content being authenticated or a message digest
of content being authenticated together with the originator's
authenticated attributes.
If authAttrs 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 authAttrs 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 authAttrs. A separate
encoding of the authAttrs field is performed for message digest
calculation. The IMPLICIT [2] tag in the authAttrs 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 authAttrs 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 contents 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 authAttrs 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 MUST 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 authAttrs is authenticated as described in section 9.2. For authentication to succeed, the MAC value calculated by the recipient MUST be the same as the value of the mac field. Similarly, for authentication to succeed when the authAttrs field is present, the content message digest value calculated by the recipient MUST be the same as the message digest value included in the authAttrs message-digest attribute. If the AuthenticatedData includes authAttrs, then the content-type attribute value MUST match the AuthenticatedData encapContentInfo eContentType value.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 taken from X.509 [X.509-88]. There are many alternatives for each algorithm type.
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 content) to another octet string (the message digest). DigestAlgorithmIdentifier ::= AlgorithmIdentifier10.1.2. SignatureAlgorithmIdentifier
The SignatureAlgorithmIdentifier type identifies a signature algorithm. Examples include RSA, DSA, and ECDSA. 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 ::= AlgorithmIdentifier10.1.3. 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, previously distributed symmetric key-encrypting keys, and symmetric key-encrypting keys derived from passwords are supported. KeyEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier10.1.4. 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 plaintext) 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-SHA-1. 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
10.1.6. KeyDerivationAlgorithmIdentifier
The KeyDerivationAlgorithmIdentifier type is specified in RFC 3211
[PWRI]. The KeyDerivationAlgorithmIdentifier definition is repeated
here for completeness.
Key derivation algorithms convert a password or shared secret value
into a key-encryption key.
KeyDerivationAlgorithmIdentifier ::= 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. RevocationInfoChoices
The RevocationInfoChoices type gives a set of revocation status
information alternatives. 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.
However, there MAY be more revocation status information than
necessary or there MAY be less revocation status information than
necessary. X.509 Certificate revocation lists (CRLs) [X.509-97] are
the primary source of revocation status information, but any other
revocation information format can be supported. The
OtherRevocationInfoFormat alternative is provided to support any
other revocation information format without further modifications to
the CMS. For example, Online Certificate Status Protocol (OCSP)
Responses [OCSP] can be supported using the
OtherRevocationInfoFormat.
The CertificateList may contain a CRL, an Authority Revocation List (ARL), a Delta CRL, or an Attribute Certificate Revocation List. All of these lists share a common syntax. The CertificateList type gives a certificate revocation list (CRL). CRLs are specified in X.509 [X.509-97], and they are profiled for use in the Internet in RFC 3280 [PROFILE]. The definition of CertificateList is taken from X.509. RevocationInfoChoices ::= SET OF RevocationInfoChoice RevocationInfoChoice ::= CHOICE { crl CertificateList, other [1] IMPLICIT OtherRevocationInfoFormat } OtherRevocationInfoFormat ::= SEQUENCE { otherRevInfoFormat OBJECT IDENTIFIER, otherRevInfo ANY DEFINED BY otherRevInfoFormat }10.2.2. CertificateChoices
The CertificateChoices type gives either a PKCS #6 extended certificate [PKCS#6], an X.509 certificate, a version 1 X.509 attribute certificate (ACv1) [X.509-97], a version 2 X.509 attribute certificate (ACv2) [X.509-00], or any other certificate format. The PKCS #6 extended certificate is obsolete. The PKCS #6 certificate is included for backward compatibility, and PKCS #6 certificates SHOULD NOT be used. The ACv1 is also obsolete. ACv1 is included for backward compatibility, and ACv1 SHOULD NOT be used. The Internet profile of X.509 certificates is specified in the "Internet X.509 Public Key Infrastructure: Certificate and CRL Profile" [PROFILE]. The Internet profile of ACv2 is specified in the "An Internet Attribute Certificate Profile for Authorization" [ACPROFILE]. The OtherCertificateFormat alternative is provided to support any other certificate format without further modifications to the CMS. The definition of Certificate is taken from X.509. The definitions of AttributeCertificate are taken from X.509-1997 and X.509-2000. The definition from X.509-1997 is assigned to AttributeCertificateV1 (see section 12.2), and the definition from X.509-2000 is assigned to AttributeCertificateV2.
CertificateChoices ::= CHOICE {
certificate Certificate,
extendedCertificate [0] IMPLICIT ExtendedCertificate, -- Obsolete
v1AttrCert [1] IMPLICIT AttributeCertificateV1, -- Obsolete
v2AttrCert [2] IMPLICIT AttributeCertificateV2,
other [3] IMPLICIT OtherCertificateFormat }
OtherCertificateFormat ::= SEQUENCE {
otherCertFormat OBJECT IDENTIFIER,
otherCert ANY DEFINED BY otherCertFormat }
10.2.3. CertificateSet
The CertificateSet type provides a set of certificates. It is
intended that the set be sufficient to contain certification paths
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 "certification path" is outside the scope of
this document. However, [PROFILE] provides a definition for X.509
certificates. Some applications may impose upper limits on the
length of a certification path; others may enforce certain
relationships between the subjects and issuers of certificates within
a certification path.
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 taken from X.501 [X.501-88], and the
definition of CertificateSerialNumber is taken from X.509 [X.509-97].
IssuerAndSerialNumber ::= SEQUENCE {
issuer Name,
serialNumber CertificateSerialNumber }
CertificateSerialNumber ::= INTEGER
10.2.5. CMSVersion
The CMSVersion type gives a syntax version number, for compatibility with future revisions of this specification. CMSVersion ::= INTEGER { v0(0), v1(1), v2(2), v3(3), v4(4), v5(5) }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 STRING10.2.7. 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 might 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 RFC 3280 [PROFILE]. Some of the attributes defined in this section were originally defined in PKCS #9 [PKCS#9]; others were originally defined in a previous version of this specification [CMS1]. 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 within signed-data or authenticated-data. The content- type attribute type MUST be present whenever signed attributes are present in signed-data or authenticated attributes present in authenticated-data. The content-type attribute value MUST match the encapContentInfo eContentType value in the signed-data or authenticated-data. The content-type attribute MUST be a signed attribute or an authenticated attribute; it MUST NOT be an unsigned attribute, unauthenticated attribute, or unprotected attribute. The following object identifier identifies the content-type attribute: id-contentType OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) 3 } Content-type attribute values have ASN.1 type ContentType: ContentType ::= OBJECT IDENTIFIER Even though the syntax is defined as a SET OF AttributeValue, a content-type attribute MUST have a single attribute value; zero or multiple instances of AttributeValue are not permitted. 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 MUST be present when there are any signed attributes present. Within authenticated-data, the message-digest authenticated attribute type MUST be present when there are any authenticated attributes present.
The message-digest attribute MUST be a signed attribute or an
authenticated attribute; it MUST NOT be an unsigned attribute,
unauthenticated attribute, or unprotected 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 and AuthAttributes syntax are each
defined as a SET OF Attributes. The SignedAttributes in a signerInfo
MUST include only one instance of the message-digest attribute.
Similarly, the AuthAttributes in an AuthenticatedData MUST include
only one instance 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 MUST be a signed attribute or an
authenticated attribute; it MUST NOT be an unsigned attribute,
unauthenticated attribute, or unprotected 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 Coordinated Universal Time (formerly known as Greenwich Mean Time (GMT) and Zulu clock time) and MUST include seconds (i.e., times are YYMMDDHHMMSSZ), even where the number of seconds is zero. Midnight 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 MUST be interpreted as 19YY; and Where YY is less than 50, the year MUST be interpreted as 20YY. GeneralizedTime values MUST be expressed in Coordinated Universal Time 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 and the AuthAttributes syntax are each defined as a SET OF Attributes. The SignedAttributes in a signerInfo MUST NOT include multiple instances of the signing-time attribute. Similarly, the AuthAttributes in an AuthenticatedData 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 signature OCTET STRING in a SignerInfo value of the signed-data. That is, the message digest is computed over the octets comprising the value of the OCTET STRING, neither the tag nor length octets are included. Thus, the countersignature attribute type countersigns (signs in serial) another signature.
The countersignature attribute MUST be an unsigned attribute; it MUST
NOT be a signed attribute, an authenticated attribute, an
unauthenticated attribute, or an unprotected 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 NOT contain a content-type
attribute; there is no content type for countersignatures.
2. The signedAttributes field MUST contain a message-digest
attribute if it contains any other attributes.
3. 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 an
arbitrarily long series of countersignatures.
12. ASN.1 Modules
Section 12.1 contains the ASN.1 module for the CMS, and section 12.2
contains the ASN.1 module for the Version 1 Attribute Certificate.
12.1. CMS ASN.1 Module
CryptographicMessageSyntax2004 { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) modules(0) cms-2004(24) } 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 -- Imports from RFC 3280 [PROFILE], Appendix A.1 AlgorithmIdentifier, Certificate, CertificateList, CertificateSerialNumber, Name FROM PKIX1Explicit88 { iso(1) identified-organization(3) dod(6) internet(1) security(5) mechanisms(5) pkix(7) mod(0) pkix1-explicit(18) } -- Imports from RFC 3281 [ACPROFILE], Appendix B AttributeCertificate FROM PKIXAttributeCertificate { iso(1) identified-organization(3) dod(6) internet(1) security(5) mechanisms(5) pkix(7) mod(0) attribute-cert(12) } -- Imports from Appendix B of this document AttributeCertificateV1 FROM AttributeCertificateVersion1 { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) modules(0) v1AttrCert(15) } ; -- 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 RevocationInfoChoices 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 RevocationInfoChoices OPTIONAL }
RecipientInfos ::= SET SIZE (1..MAX) 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,
pwri [3] PasswordRecipientInfo,
ori [4] OtherRecipientInfo }
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 }
PasswordRecipientInfo ::= SEQUENCE {
version CMSVersion, -- always set to 0
keyDerivationAlgorithm [0] KeyDerivationAlgorithmIdentifier
OPTIONAL,
keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
encryptedKey EncryptedKey }
OtherRecipientInfo ::= SEQUENCE {
oriType OBJECT IDENTIFIER,
oriValue ANY DEFINED BY oriType }
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,
authAttrs [2] IMPLICIT AuthAttributes OPTIONAL,
mac MessageAuthenticationCode,
unauthAttrs [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
KeyDerivationAlgorithmIdentifier ::= AlgorithmIdentifier
RevocationInfoChoices ::= SET OF RevocationInfoChoice
RevocationInfoChoice ::= CHOICE {
crl CertificateList,
other [1] IMPLICIT OtherRevocationInfoFormat }
OtherRevocationInfoFormat ::= SEQUENCE {
otherRevInfoFormat OBJECT IDENTIFIER,
otherRevInfo ANY DEFINED BY otherRevInfoFormat }
CertificateChoices ::= CHOICE {
certificate Certificate,
extendedCertificate [0] IMPLICIT ExtendedCertificate, -- Obsolete
v1AttrCert [1] IMPLICIT AttributeCertificateV1, -- Obsolete
v2AttrCert [2] IMPLICIT AttributeCertificateV2,
other [3] IMPLICIT OtherCertificateFormat }
AttributeCertificateV2 ::= AttributeCertificate
OtherCertificateFormat ::= SEQUENCE {
otherCertFormat OBJECT IDENTIFIER,
otherCert ANY DEFINED BY otherCertFormat }
CertificateSet ::= SET OF CertificateChoices
IssuerAndSerialNumber ::= SEQUENCE {
issuer Name,
serialNumber CertificateSerialNumber }
CMSVersion ::= INTEGER { v0(0), v1(1), v2(2), v3(3), v4(4), v5(5) }
UserKeyingMaterial ::= OCTET STRING
OtherKeyAttribute ::= SEQUENCE {
keyAttrId OBJECT IDENTIFIER,
keyAttr ANY DEFINED BY keyAttrId OPTIONAL }
-- Content Type Object Identifiers
id-ct-contentInfo OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(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 }
-- The CMS Attributes
MessageDigest ::= OCTET STRING
SigningTime ::= Time
Time ::= CHOICE {
utcTime UTCTime,
generalTime GeneralizedTime }
Countersignature ::= SignerInfo
-- 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
ExtendedCertificateOrCertificate ::= CHOICE {
certificate Certificate,
extendedCertificate [0] IMPLICIT ExtendedCertificate }
ExtendedCertificate ::= SEQUENCE {
extendedCertificateInfo ExtendedCertificateInfo,
signatureAlgorithm SignatureAlgorithmIdentifier,
signature Signature }
ExtendedCertificateInfo ::= SEQUENCE {
version CMSVersion,
certificate Certificate,
attributes UnauthAttributes }
Signature ::= BIT STRING
END -- of CryptographicMessageSyntax2004
12.2. Version 1 Attribute Certificate ASN.1 Module
AttributeCertificateVersion1
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) v1AttrCert(15) }
DEFINITIONS EXPLICIT TAGS ::=
BEGIN
-- EXPORTS All
IMPORTS
-- Imports from RFC 3280 [PROFILE], Appendix A.1
AlgorithmIdentifier, Attribute, CertificateSerialNumber,
Extensions, UniqueIdentifier
FROM PKIX1Explicit88
{ iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7)
mod(0) pkix1-explicit(18) }
-- Imports from RFC 3280 [PROFILE], Appendix A.2
GeneralNames
FROM PKIX1Implicit88
{ iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7)
mod(0) pkix1-implicit(19) }
-- Imports from RFC 3281 [ACPROFILE], Appendix B
AttCertValidityPeriod, IssuerSerial
FROM PKIXAttributeCertificate
{ iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7)
mod(0) attribute-cert(12) } ;
-- Definition extracted from X.509-1997 [X.509-97], but
-- different type names are used to avoid collisions.
AttributeCertificateV1 ::= SEQUENCE {
acInfo AttributeCertificateInfoV1,
signatureAlgorithm AlgorithmIdentifier,
signature BIT STRING }
AttributeCertificateInfoV1 ::= SEQUENCE {
version AttCertVersionV1 DEFAULT v1,
subject CHOICE {
baseCertificateID [0] IssuerSerial,
-- associated with a Public Key Certificate
subjectName [1] GeneralNames },
-- associated with a name
issuer GeneralNames,
signature AlgorithmIdentifier,
serialNumber CertificateSerialNumber,
attCertValidityPeriod AttCertValidityPeriod,
attributes SEQUENCE OF Attribute,
issuerUniqueID UniqueIdentifier OPTIONAL,
extensions Extensions OPTIONAL }
AttCertVersionV1 ::= INTEGER { v1(0) }
END -- of AttributeCertificateVersion1
13. References
13.1. Normative References
[ACPROFILE] Farrell, S. and R. Housley, "An Internet Attribute
Certificate Profile for Authorization", RFC 3281, April
2002.
[PROFILE] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002.
[STDWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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. [X.509-00] ITU-T. Recommendation X.509: The Directory - Authentication Framework. 2000.13.2. Informative References
[CMS1] Housley, R., "Cryptographic Message Syntax", RFC 2630, June 1999. [CMS2] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3369, August 2002. [CMSALG] Housley, R., "Cryptographic Message Syntax (CMS) Algorithms", RFC 3370, August 2002. [ESS] Hoffman, P., "Enhanced Security Services for S/MIME", RFC 2634, June 1999. [MSAC] Microsoft Development Network (MSDN) Library, "Authenticode", April 2004 Release. [MSG] Ramsdell, B., "S/MIME Version 3.1 Message Specification", RFC 3851, July 2004. [OCSP] Myers, M., Ankney, R., Malpani, A., Galperin, S. and C. Adams, "X.509 Internet Public Key Infrastructure Online Certificate Status Protocol - OCSP", RFC 2560, June 1999. [OLDMSG] Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L., and L. Repka, "S/MIME Version 2 Message Specification", RFC 2311, 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", RFC 2315, March 1998.
[PKCS#9] RSA Laboratories. PKCS #9: Selected Attribute Types, Version 1.1. November 1993. [PWRI] Gutmann, P., "Password-based Encryption for CMS", RFC 3211, December 2001. [RANDOM] Eastlake 3rd, D., Crocker, S., and J. Schiller, "Randomness Recommendations for Security", RFC 1750, December 1994.14. 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 contents 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. The key management technique employed to distribute message- authentication keys must itself provide data origin authentication, otherwise the contents are delivered with integrity from an unknown source. Neither RSA [PKCS#1, NEWPKCS#1] nor Ephemeral-Static Diffie-Hellman [DH-X9.42] provide the necessary data origin authentication. Static-Static Diffie-Hellman [DH-X9.42] does provide the necessary data origin authentication when both the originator and recipient public keys are bound to appropriate identities in X.509 certificates. When more than two parties share the same message-authentication key, data origin authentication is not provided. Any party that knows the message-authentication key can compute a valid MAC, therefore the contents could originate from any one of the parties.
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. RFC 1750 [RANDOM] offers important guidance in this area. 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, content is encrypted with Triple-DES using a 168-bit Triple-DES content-encryption key, and the content-encryption key is wrapped with RC2 using a 40-bit RC2 key-encryption 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 the 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. 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 be reduced. Therefore, cryptographic algorithm implementations should be modular, allowing new algorithms to be readily inserted. That is, implementors should be prepared for the set of algorithms that must be supported to change over time. The countersignature unsigned 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.
15. 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, Simon Blake-Wilson, 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, Dave Solo, Paul Timmel, and Sean Turner for their efforts and support.16. Author's Address
Russell Housley Vigil Security, LLC 918 Spring Knoll Drive Herndon, VA 20170 USA EMail: housley@vigilsec.com
17. Full Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf- ipr@ietf.org. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society.