RFC 2630
Cryptographic Message Syntax
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 RFC 2459 [PROFILE]. The definition of CertificateList is imported from X.509. CertificateRevocationLists ::= SET OF CertificateList10.2.2 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 The PKCS #6 extended certificate is obsolete.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 X9.5710.2.3 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 RFC 2459 [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 RFC 1321 [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 RFC 2347 [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 RFC 2631 [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 RFC 2313 [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 RFC 2313 [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 RFC 2437 [NEWPKCS#1]. Within RFC 2437, 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 octets12.4.2 RC2 CBC
The RC2 algorithm is described in RFC 2268 [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 RFC 2104 [HMAC]. The algorithm identifier for HMAC with SHA-1 is: hMAC-SHA1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3) dod(6) internet(1) security(5) mechanisms(5) 8 1 2 } The AlgorithmIdentifier parameters field must be absent.12.6 Triple-DES and RC2 Key Wrap Algorithms
CMS implementations must include encryption of a Triple-DES content- encryption key with a Triple-DES key-encryption key using the algorithm specified in Sections 12.6.2 and 12.6.3. CMS implementations should include encryption of a RC2 content-encryption key with a RC2 key-encryption key using the algorithm specified in Sections 12.6.4 and 12.6.5. Triple-DES and RC2 content-encryption keys are encrypted in Cipher Block Chaining (CBC) mode [MODES]. Key Transport algorithms allow for the content-encryption key to be directly encrypted; however, key agreement and symmetric key- encryption key algorithms encrypt the content-encryption key with a second symmetric encryption algorithm. This section describes how the Triple-DES or RC2 content-encryption key is formatted and encrypted. Key agreement algorithms generate a pairwise key-encryption key, and a key wrap algorithm is used to encrypt the content-encryption key with the pairwise key-encryption key. Similarly, a key wrap algorithm is used to encrypt the content-encryption key in a previously distributed key-encryption key. The key-encryption key is generated by the key agreement algorithm or distributed out of band. For key agreement of RC2 key-encryption keys, 128 bits must be generated as input to the key expansion process used to compute the RC2 effective key [RC2]. The same algorithm identifier is used for both 2-key and 3-key Triple-DES. When the length of the content-encryption key to be wrapped is a 2-key Triple-DES key, a third key with the same value as the first key is created. Thus, all Triple-DES content-encryption keys are wrapped like 3-key Triple-DES keys.
12.6.1 Key Checksum
The CMS Checksum Algorithm is used to provide a content-encryption key integrity check value. The algorithm is: 1. Compute a 20 octet SHA-1 [SHA1] message digest on the content-encryption key. 2. Use the most significant (first) eight octets of the message digest value as the checksum value.12.6.2 Triple-DES Key Wrap
The Triple-DES key wrap algorithm encrypts a Triple-DES content- encryption key with a Triple-DES key-encryption key. The Triple-DES key wrap algorithm is: 1. Set odd parity for each of the DES key octets comprising the content-encryption key, call the result CEK. 2. Compute an 8 octet key checksum value on CEK as described above in Section 12.6.1, call the result ICV. 3. Let CEKICV = CEK || ICV. 4. Generate 8 octets at random, call the result IV. 5. Encrypt CEKICV in CBC mode using the key-encryption key. Use the random value generated in the previous step as the initialization vector (IV). Call the ciphertext TEMP1. 6. Let TEMP2 = IV || TEMP1. 7. Reverse the order of the octets in TEMP2. That is, the most significant (first) octet is swapped with the least significant (last) octet, and so on. Call the result TEMP3. 8. Encrypt TEMP3 in CBC mode using the key-encryption key. Use an initialization vector (IV) of 0x4adda22c79e82105. The ciphertext is 40 octets long. Note: When the same content-encryption key is wrapped in different key-encryption keys, a fresh initialization vector (IV) must be generated for each invocation of the key wrap algorithm.12.6.3 Triple-DES Key Unwrap
The Triple-DES key unwrap algorithm decrypts a Triple-DES content- encryption key using a Triple-DES key-encryption key. The Triple-DES key unwrap algorithm is: 1. If the wrapped content-encryption key is not 40 octets, then error. 2. Decrypt the wrapped content-encryption key in CBC mode using the key-encryption key. Use an initialization vector (IV) of 0x4adda22c79e82105. Call the output TEMP3.
3. Reverse the order of the octets in TEMP3. That is, the most
significant (first) octet is swapped with the least significant
(last) octet, and so on. Call the result TEMP2.
4. Decompose the TEMP2 into IV and TEMP1. IV is the most
significant (first) 8 octets, and TEMP1 is the least significant
(last) 32 octets.
5. Decrypt TEMP1 in CBC mode using the key-encryption key. Use
the IV value from the previous step as the initialization vector.
Call the ciphertext CEKICV.
6. Decompose the CEKICV into CEK and ICV. CEK is the most significant
(first) 24 octets, and ICV is the least significant (last) 8 octets.
7. Compute an 8 octet key checksum value on CEK as described above
in Section 12.6.1. If the computed key checksum value does not
match the decrypted key checksum value, ICV, then error.
8. Check for odd parity each of the DES key octets comprising CEK.
If parity is incorrect, then there is an error.
9. Use CEK as the content-encryption key.
12.6.4 RC2 Key Wrap
The RC2 key wrap algorithm encrypts a RC2 content-encryption key with
a RC2 key-encryption key. The RC2 key wrap algorithm is:
1. Let the content-encryption key be called CEK, and let the length
of the content-encryption key in octets be called LENGTH. LENGTH
is a single octet.
2. Let LCEK = LENGTH || CEK.
3. Let LCEKPAD = LCEK || PAD. If the length of LCEK is a multiple
of 8, the PAD has a length of zero. If the length of LCEK is
not a multiple of 8, then PAD contains the fewest number of
random octets to make the length of LCEKPAD a multiple of 8.
4. Compute an 8 octet key checksum value on LCEKPAD as described
above in Section 12.6.1, call the result ICV.
5. Let LCEKPADICV = LCEKPAD || ICV.
6. Generate 8 octets at random, call the result IV.
7. Encrypt LCEKPADICV in CBC mode using the key-encryption key.
Use the random value generated in the previous step as the
initialization vector (IV). Call the ciphertext TEMP1.
8. Let TEMP2 = IV || TEMP1.
9. Reverse the order of the octets in TEMP2. That is, the most
significant (first) octet is swapped with the least significant
(last) octet, and so on. Call the result TEMP3.
10. Encrypt TEMP3 in CBC mode using the key-encryption key. Use
an initialization vector (IV) of 0x4adda22c79e82105.
Note: When the same content-encryption key is wrapped in different
key-encryption keys, a fresh initialization vector (IV) must be
generated for each invocation of the key wrap algorithm.
12.6.5 RC2 Key Unwrap
The RC2 key unwrap algorithm decrypts a RC2 content-encryption key using a RC2 key-encryption key. The RC2 key unwrap algorithm is: 1. If the wrapped content-encryption key is not a multiple of 8 octets, then error. 2. Decrypt the wrapped content-encryption key in CBC mode using the key-encryption key. Use an initialization vector (IV) of 0x4adda22c79e82105. Call the output TEMP3. 3. Reverse the order of the octets in TEMP3. That is, the most significant (first) octet is swapped with the least significant (last) octet, and so on. Call the result TEMP2. 4. Decompose the TEMP2 into IV and TEMP1. IV is the most significant (first) 8 octets, and TEMP1 is the remaining octets. 5. Decrypt TEMP1 in CBC mode using the key-encryption key. Use the IV value from the previous step as the initialization vector. Call the plaintext LCEKPADICV. 6. Decompose the LCEKPADICV into LCEKPAD, and ICV. ICV is the least significant (last) octet 8 octets. LCEKPAD is the remaining octets. 7. Compute an 8 octet key checksum value on LCEKPAD as described above in Section 12.6.1. If the computed key checksum value does not match the decrypted key checksum value, ICV, then error. 8. Decompose the LCEKPAD into LENGTH, CEK, and PAD. LENGTH is the most significant (first) octet. CEK is the following LENGTH octets. PAD is the remaining octets, if any. 9. If the length of PAD is more than 7 octets, then error. 10. Use CEK as the content-encryption key.
Appendix A: ASN.1 Module
CryptographicMessageSyntax { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) modules(0) cms(1) } DEFINITIONS IMPLICIT TAGS ::= BEGIN -- EXPORTS All -- The types and values defined in this module are exported for use in -- the other ASN.1 modules. Other applications may use them for their -- own purposes. IMPORTS -- Directory Information Framework (X.501) Name FROM InformationFramework { joint-iso-itu-t ds(5) modules(1) informationFramework(1) 3 } -- Directory Authentication Framework (X.509) AlgorithmIdentifier, AttributeCertificate, Certificate, CertificateList, CertificateSerialNumber FROM AuthenticationFramework { joint-iso-itu-t ds(5) module(1) authenticationFramework(7) 3 } ; -- Cryptographic Message Syntax ContentInfo ::= SEQUENCE { contentType ContentType, content [0] EXPLICIT ANY DEFINED BY contentType } ContentType ::= OBJECT IDENTIFIER SignedData ::= SEQUENCE { version CMSVersion, digestAlgorithms DigestAlgorithmIdentifiers, encapContentInfo EncapsulatedContentInfo, certificates [0] IMPLICIT CertificateSet OPTIONAL, crls [1] IMPLICIT CertificateRevocationLists OPTIONAL, signerInfos SignerInfos } DigestAlgorithmIdentifiers ::= SET OF DigestAlgorithmIdentifier SignerInfos ::= SET OF SignerInfo
EncapsulatedContentInfo ::= SEQUENCE {
eContentType ContentType,
eContent [0] EXPLICIT OCTET STRING OPTIONAL }
SignerInfo ::= SEQUENCE {
version CMSVersion,
sid SignerIdentifier,
digestAlgorithm DigestAlgorithmIdentifier,
signedAttrs [0] IMPLICIT SignedAttributes OPTIONAL,
signatureAlgorithm SignatureAlgorithmIdentifier,
signature SignatureValue,
unsignedAttrs [1] IMPLICIT UnsignedAttributes OPTIONAL }
SignerIdentifier ::= CHOICE {
issuerAndSerialNumber IssuerAndSerialNumber,
subjectKeyIdentifier [0] SubjectKeyIdentifier }
SignedAttributes ::= SET SIZE (1..MAX) OF Attribute
UnsignedAttributes ::= SET SIZE (1..MAX) OF Attribute
Attribute ::= SEQUENCE {
attrType OBJECT IDENTIFIER,
attrValues SET OF AttributeValue }
AttributeValue ::= ANY
SignatureValue ::= OCTET STRING
EnvelopedData ::= SEQUENCE {
version CMSVersion,
originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL,
recipientInfos RecipientInfos,
encryptedContentInfo EncryptedContentInfo,
unprotectedAttrs [1] IMPLICIT UnprotectedAttributes OPTIONAL }
OriginatorInfo ::= SEQUENCE {
certs [0] IMPLICIT CertificateSet OPTIONAL,
crls [1] IMPLICIT CertificateRevocationLists OPTIONAL }
RecipientInfos ::= SET OF RecipientInfo
EncryptedContentInfo ::= SEQUENCE {
contentType ContentType,
contentEncryptionAlgorithm ContentEncryptionAlgorithmIdentifier,
encryptedContent [0] IMPLICIT EncryptedContent OPTIONAL }
EncryptedContent ::= OCTET STRING
UnprotectedAttributes ::= SET SIZE (1..MAX) OF Attribute
RecipientInfo ::= CHOICE {
ktri KeyTransRecipientInfo,
kari [1] KeyAgreeRecipientInfo,
kekri [2] KEKRecipientInfo }
EncryptedKey ::= OCTET STRING
KeyTransRecipientInfo ::= SEQUENCE {
version CMSVersion, -- always set to 0 or 2
rid RecipientIdentifier,
keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
encryptedKey EncryptedKey }
RecipientIdentifier ::= CHOICE {
issuerAndSerialNumber IssuerAndSerialNumber,
subjectKeyIdentifier [0] SubjectKeyIdentifier }
KeyAgreeRecipientInfo ::= SEQUENCE {
version CMSVersion, -- always set to 3
originator [0] EXPLICIT OriginatorIdentifierOrKey,
ukm [1] EXPLICIT UserKeyingMaterial OPTIONAL,
keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
recipientEncryptedKeys RecipientEncryptedKeys }
OriginatorIdentifierOrKey ::= CHOICE {
issuerAndSerialNumber IssuerAndSerialNumber,
subjectKeyIdentifier [0] SubjectKeyIdentifier,
originatorKey [1] OriginatorPublicKey }
OriginatorPublicKey ::= SEQUENCE {
algorithm AlgorithmIdentifier,
publicKey BIT STRING }
RecipientEncryptedKeys ::= SEQUENCE OF RecipientEncryptedKey
RecipientEncryptedKey ::= SEQUENCE {
rid KeyAgreeRecipientIdentifier,
encryptedKey EncryptedKey }
KeyAgreeRecipientIdentifier ::= CHOICE {
issuerAndSerialNumber IssuerAndSerialNumber,
rKeyId [0] IMPLICIT RecipientKeyIdentifier }
RecipientKeyIdentifier ::= SEQUENCE {
subjectKeyIdentifier SubjectKeyIdentifier,
date GeneralizedTime OPTIONAL,
other OtherKeyAttribute OPTIONAL }
SubjectKeyIdentifier ::= OCTET STRING
KEKRecipientInfo ::= SEQUENCE {
version CMSVersion, -- always set to 4
kekid KEKIdentifier,
keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
encryptedKey EncryptedKey }
KEKIdentifier ::= SEQUENCE {
keyIdentifier OCTET STRING,
date GeneralizedTime OPTIONAL,
other OtherKeyAttribute OPTIONAL }
DigestedData ::= SEQUENCE {
version CMSVersion,
digestAlgorithm DigestAlgorithmIdentifier,
encapContentInfo EncapsulatedContentInfo,
digest Digest }
Digest ::= OCTET STRING
EncryptedData ::= SEQUENCE {
version CMSVersion,
encryptedContentInfo EncryptedContentInfo,
unprotectedAttrs [1] IMPLICIT UnprotectedAttributes OPTIONAL }
AuthenticatedData ::= SEQUENCE {
version CMSVersion,
originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL,
recipientInfos RecipientInfos,
macAlgorithm MessageAuthenticationCodeAlgorithm,
digestAlgorithm [1] DigestAlgorithmIdentifier OPTIONAL,
encapContentInfo EncapsulatedContentInfo,
authenticatedAttributes [2] IMPLICIT AuthAttributes OPTIONAL,
mac MessageAuthenticationCode,
unauthenticatedAttributes [3] IMPLICIT UnauthAttributes OPTIONAL }
AuthAttributes ::= SET SIZE (1..MAX) OF Attribute
UnauthAttributes ::= SET SIZE (1..MAX) OF Attribute
MessageAuthenticationCode ::= OCTET STRING
DigestAlgorithmIdentifier ::= AlgorithmIdentifier
SignatureAlgorithmIdentifier ::= AlgorithmIdentifier
KeyEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier
ContentEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier
MessageAuthenticationCodeAlgorithm ::= AlgorithmIdentifier
CertificateRevocationLists ::= SET OF CertificateList
CertificateChoices ::= CHOICE {
certificate Certificate, -- See X.509
extendedCertificate [0] IMPLICIT ExtendedCertificate, -- Obsolete
attrCert [1] IMPLICIT AttributeCertificate } -- See X.509 & X9.57
CertificateSet ::= SET OF CertificateChoices
IssuerAndSerialNumber ::= SEQUENCE {
issuer Name,
serialNumber CertificateSerialNumber }
CMSVersion ::= INTEGER { v0(0), v1(1), v2(2), v3(3), v4(4) }
UserKeyingMaterial ::= OCTET STRING
OtherKeyAttribute ::= SEQUENCE {
keyAttrId OBJECT IDENTIFIER,
keyAttr ANY DEFINED BY keyAttrId OPTIONAL }
-- CMS Attributes
MessageDigest ::= OCTET STRING
SigningTime ::= Time
Time ::= CHOICE {
utcTime UTCTime,
generalTime GeneralizedTime }
Countersignature ::= SignerInfo
-- Algorithm Identifiers
sha-1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
oiw(14) secsig(3) algorithm(2) 26 }
md5 OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) digestAlgorithm(2) 5 }
id-dsa-with-sha1 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) x9-57 (10040) x9cm(4) 3 }
rsaEncryption OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 1 }
dh-public-number OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) ansi-x942(10046) number-type(2) 1 }
id-alg-ESDH OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 5 }
id-alg-CMS3DESwrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 6 }
id-alg-CMSRC2wrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 7 }
des-ede3-cbc OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) encryptionAlgorithm(3) 7 }
rc2-cbc OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) encryptionAlgorithm(3) 2 }
hMAC-SHA1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
dod(6) internet(1) security(5) mechanisms(5) 8 1 2 }
-- Algorithm Parameters
KeyWrapAlgorithm ::= AlgorithmIdentifier
RC2wrapParameter ::= RC2ParameterVersion
RC2ParameterVersion ::= INTEGER
CBCParameter ::= IV
IV ::= OCTET STRING -- exactly 8 octets
RC2CBCParameter ::= SEQUENCE {
rc2ParameterVersion INTEGER,
iv OCTET STRING } -- exactly 8 octets
-- Content Type Object Identifiers
id-ct-contentInfo OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16)
ct(1) 6 }
id-data OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 1 }
id-signedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 2 }
id-envelopedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 3 }
id-digestedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 5 }
id-encryptedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 6 }
id-ct-authData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16)
ct(1) 2 }
-- Attribute Object Identifiers
id-contentType OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) 3 }
id-messageDigest OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) 4 }
id-signingTime OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) 5 }
id-countersignature OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) 6 }
-- Obsolete Extended Certificate syntax from PKCS#6
ExtendedCertificate ::= SEQUENCE {
extendedCertificateInfo ExtendedCertificateInfo,
signatureAlgorithm SignatureAlgorithmIdentifier,
signature Signature }
ExtendedCertificateInfo ::= SEQUENCE {
version CMSVersion,
certificate Certificate,
attributes UnauthAttributes }
Signature ::= BIT STRING
END -- of CryptographicMessageSyntax
References
3DES American National Standards Institute. ANSI X9.52-1998, Triple Data Encryption Algorithm Modes of Operation. 1998. DES American National Standards Institute. ANSI X3.106, "American National Standard for Information Systems - Data Link Encryption". 1983. DH-X9.42 Rescorla, E., "Diffie-Hellman Key Agreement Method", RFC 2631, June 1999. DSS National Institute of Standards and Technology. FIPS Pub 186: Digital Signature Standard. 19 May 1994. ESS Hoffman, P., Editor, "Enhanced Security Services for S/MIME", RFC 2634, June 1999. HMAC Krawczyk, H., "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, February 1997. MD5 Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April 1992. MODES National Institute of Standards and Technology. FIPS Pub 81: DES Modes of Operation. 2 December 1980. MSG Ramsdell, B., Editor, "S/MIME Version 3 Message Specification", RFC 2633, June 1999. NEWPKCS#1 Kaliski, B., "PKCS #1: RSA Encryption, Version 2.0", RFC 2347, October 1998. PROFILE Housley, R., Ford, W., Polk, W. and D. Solo, "Internet X.509 Public Key Infrastructure: Certificate and CRL Profile", RFC 2459, January 1999. PKCS#1 Kaliski, B., "PKCS #1: RSA Encryption, Version 1.5.", RFC 2313, 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.
RANDOM Eastlake, D., Crocker, S. and J. Schiller, "Randomness
Recommendations for Security", RFC 1750, December 1994.
RC2 Rivest, R., "A Description of the RC2 (r) Encryption
Algorithm", RFC 2268, March 1998.
SHA1 National Institute of Standards and Technology.
FIPS Pub 180-1: Secure Hash Standard. 17 April 1995.
X.208-88 CCITT. Recommendation X.208: Specification of Abstract
Syntax Notation One (ASN.1). 1988.
X.209-88 CCITT. Recommendation X.209: Specification of Basic
Encoding Rules for Abstract Syntax Notation One (ASN.1).
1988.
X.501-88 CCITT. Recommendation X.501: The Directory - Models.
1988.
X.509-88 CCITT. Recommendation X.509: The Directory -
Authentication Framework. 1988.
X.509-97 ITU-T. Recommendation X.509: The Directory -
Authentication Framework. 1997.
Security Considerations
The Cryptographic Message Syntax provides a method for digitally
signing data, digesting data, encrypting data, and authenticating
data.
Implementations must protect the signer's private key. Compromise of
the signer's private key permits masquerade.
Implementations must protect the key management private key, the
key-encryption key, and the content-encryption key. Compromise of
the key management private key or the key-encryption key may result
in the disclosure of all messages protected with that key.
Similarly, compromise of the content-encryption key may result in
disclosure of the associated encrypted content.
Implementations must protect the key management private key and the
message-authentication key. Compromise of the key management private
key permits masquerade of authenticated data. Similarly, compromise
of the message-authentication key may result in undetectable
modification of the authenticated content.
Implementations must randomly generate content-encryption keys, message-authentication keys, initialization vectors (IVs), and padding. Also, the generation of public/private key pairs relies on a random numbers. The use of inadequate pseudo-random number generators (PRNGs) to generate cryptographic keys can result in little or no security. An attacker may find it much easier to reproduce the PRNG environment that produced the keys, searching the resulting small set of possibilities, rather than brute force searching the whole key space. The generation of quality random numbers is difficult. RFC 1750 [RANDOM] offers important guidance in this area, and Appendix 3 of FIPS Pub 186 [DSS] provides one quality PRNG technique. When using key agreement algorithms or previously distributed symmetric key-encryption keys, a key-encryption key is used to encrypt the content-encryption key. If the key-encryption and content-encryption algorithms are different, the effective security is determined by the weaker of the two algorithms. If, for example, a message content is encrypted with 168-bit Triple-DES and the Triple-DES content-encryption key is wrapped with a 40-bit RC2 key, then at most 40 bits of protection is provided. A trivial search to determine the value of the 40-bit RC2 key can recover Triple-DES key, and then the Triple-DES key can be used to decrypt the content. Therefore, implementers must ensure that key-encryption algorithms are as strong or stronger than content-encryption algorithms. Section 12.6 specifies key wrap algorithms used to encrypt a Triple- DES [3DES] content-encryption key with a Triple-DES key-encryption key or to encrypt a RC2 [RC2] content-encryption key with a RC2 key- encryption key. The key wrap algorithms make use of CBC mode [MODES]. These key wrap algorithms have been reviewed for use with Triple and RC2. They have not been reviewed for use with other cryptographic modes or other encryption algorithms. Therefore, if a CMS implementation wishes to support ciphers in addition to Triple- DES or RC2, then additional key wrap algorithms need to be defined to support the additional ciphers. Implementers should be aware that cryptographic algorithms become weaker with time. As new cryptoanalysis techniques are developed and computing performance improves, the work factor to break a particular cryptographic algorithm will reduce. Therefore, cryptographic algorithm implementations should be modular allowing new algorithms to be readily inserted. That is, implementers should be prepared for the set of mandatory to implement algorithms to change over time. The countersignature unauthenticated attribute includes a digital signature that is computed on the content signature value, thus the countersigning process need not know the original signed content.
This structure permits implementation efficiency advantages; however, this structure may also permit the countersigning of an inappropriate signature value. Therefore, implementations that perform countersignatures should either verify the original signature value prior to countersigning it (this verification requires processing of the original content), or implementations should perform countersigning in a context that ensures that only appropriate signature values are countersigned. Users of CMS, particularly those employing CMS to support interactive applications, should be aware that PKCS #1 Version 1.5 as specified in RFC 2313 [PKCS#1] is vulnerable to adaptive chosen ciphertext attacks when applied for encryption purposes. Exploitation of this identified vulnerability, revealing the result of a particular RSA decryption, requires access to an oracle which will respond to a large number of ciphertexts (based on currently available results, hundreds of thousands or more), which are constructed adaptively in response to previously-received replies providing information on the successes or failures of attempted decryption operations. As a result, the attack appears significantly less feasible to perpetrate for store-and-forward S/MIME environments than for directly interactive protocols. Where CMS constructs are applied as an intermediate encryption layer within an interactive request-response communications environment, exploitation could be more feasible. An updated version of PKCS #1 has been published, PKCS #1 Version 2.0 [NEWPKCS#1]. This new document will supersede RFC 2313. PKCS #1 Version 2.0 preserves support for the encryption padding format defined in PKCS #1 Version 1.5 [PKCS#1], and it also defines a new alternative. To resolve the adaptive chosen ciphertext vulnerability, the PKCS #1 Version 2.0 specifies and recommends use of Optimal Asymmetric Encryption Padding (OAEP) when RSA encryption is used to provide confidentiality. Designers of protocols and systems employing CMS for interactive environments should either consider usage of OAEP, or should ensure that information which could reveal the success or failure of attempted PKCS #1 Version 1.5 decryption operations is not provided. Support for OAEP will likely be added to a future version of the CMS specification.Acknowledgments
This document is the result of contributions from many professionals. I appreciate the hard work of all members of the IETF S/MIME Working Group. I extend a special thanks to Rich Ankney, Tim Dean, Steve Dusse, Carl Ellison, Peter Gutmann, Bob Jueneman, Stephen Henson, Paul Hoffman, Scott Hollenbeck, Don Johnson, Burt Kaliski, John Linn, John Pawling, Blake Ramsdell, Francois Rousseau, Jim Schaad, and Dave Solo for their efforts and support.
Author's Address
Russell Housley SPYRUS 381 Elden Street Suite 1120 Herndon, VA 20170 USA EMail: housley@spyrus.com
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