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