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 ::= AlgorithmIdentifier
10.1.2. 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 ::= 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, previously distributed
symmetric key-encrypting keys, and symmetric key-encrypting keys
derived from passwords 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
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 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 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 }