We're trying to reach closure on this, so please look at it as soon as you can
an comment.
--Paul Hoffman, Director
--Internet Mail Consortium
12 Supported Algorithms
This section lists the algorithms that must be implemented.
Additional algorithms that may be implemented are also included.
12.1 Digest Algorithms
CMS implementations must include SHA-1. CMS implementations may include MD5.
Digest algorithm identifiers are used in the SignedData digestAlgorithms
field, the SignerInfo digestAlgorithm field, and the DigestedData
digestAlgorithm field.
Digest values are located in the DigestedData digest field. In addition,
digest values are located in the Message Digest authenticated attribute.
12.1.1 SHA-1
The SHA-1 digest algorithm is defined in FIPS Pub 180-1 [FIPS 180-1]. 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 must be present and must contain
an ASN.1 NULL. Implementations should accept this AlgorithmIdentifier with
a parameters field that contains null, as well as an absent parameters
field.
12.1.2 MD5
The MD5 digest algorithm is defined in RFC 1321. 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 must contain
an ASN.1 NULL. Implementations should accept this AlgorithmIdentifier with
a parameters field that contains null, as well as an absent parameters
field.
12.2 Signature Algorithms
CMS implementations must include DSA. CMS implementations may include RSA.
Signature algorithm identifiers are used in the SignerInfo
signatureAlgorithm field.
12.2.1 DSA
The DSA signature algorithm is defined in FIPS Pub 186 [FIPS 186]. 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 parameter field must not be present.
12.2.2 RSA
The RSA signature algorithm is defined in RFC 2313 as modified below. 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 must contain
an ASN.1 NULL. Implementations should accept this AlgorithmIdentifier with
a parameters field that contains null, as well as an absent parameters
field.
This specification modifies RFC 2313 to include SHA-1 as an additional
digest algorithm. Section 10.1.2 of RFC 2313 is modified to list SHA-1
in the bullet item about digestAlgorithm. The following OID is added to
the list in section 10.1.2 of RFC 2313:
sha-1 OBJECT IDENTIFIER ::= {iso(1) identified-organization(3) oiw(14)
secsig(3) algorithm(2) 26}
12.3 Key Encryption Algorithms
CMS implementations must include Static Diffie-Hellman with tripleDES. CMS
implementations may include RSA. CMS implementations may include
Static Diffie-Hellman with RC2.
Key encryption algorithms are used in the KeyTransRecipientInfo
keyEncryptionAlgorithm field, in the KeyAgreeRecipientInfo
keyEncryptionAlgorithm field, and in the MailListRecipientInfo
keyEncryptionAlgorithm field. These are all part of the RecipientInfo
type, which is part of the EnvelopedData type.
12.3.1 Key Agreement Key Encryption Algorithms Used in
KeyAgreeRecipientInfo
12.3.1.1 Static Diffie-Hellman with tripleDES
Static Diffie-Hellman key encryption is defined in RFC TBD (Diffie-Hellman
Key Agreement Method, currently draft-ietf-smime-x942). The algorithm
identifier for static Diffie-Hellman with tripleDES is
id-smime-cms-dh-with-tripleDES ::= { TBD }
The AlgorithmIdentifier parameters field must be present and must contain
an ASN.1 NULL. Implementations should accept this AlgorithmIdentifier with
a parameters field that contains null, as well as an absent parameters
field.
12.3.1.2 Static Diffie-Hellman with RC2
Diffie-Hellman key encryption is defined in RFC TBD (Diffie-Hellman Key
Agreement Method, currently draft-ietf-smime-x942). The algorithm
identifier for static Diffie-Hellman with RC2 is
id-smime-cms-dh-with-rC2 ::= { TBD }
For the effective-key-bits (key size) greater than 32 and less than
256, the algorithm parameters are encoded as:
id-smime-cms-dh-with-rC2 parameter ::= SEQUENCE {
rc2ParameterVersion INTEGER,
iv OCTET STRING (8)}
For the effective-key-bits of 40, 64, and 128, the rc2ParameterVersion
values are 160, 120, 58 respectively. It is very important to note that
these values are not simply the RC2 keylength. Also note that the value 160
must be encoded as two octets (00 A0), because encoding as one octet (A0)
is a negative number in ASN.1.
12.3.2 Key Transport Key Encryption Algorithms Used in
KeyTransRecipientInfo
12.3.2.1 RSA
The RSA encryption algorithm is defined in RFC 2313 as modified below. 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 must contain
an ASN.1 NULL. Implementations should accept this AlgorithmIdentifier with
a parameters field that contains null, as well as an absent parameters
field.
This specification modifies RFC 2313 to include SHA-1 as an additional
digest algorithm. Section 10.1.2 of RFC 2313 is modified to list SHA-1
in the bullet item about digestAlgorithm. The following OID is added to
the list in section 10.1.2 of RFC 2313:
sha-1 OBJECT IDENTIFIER ::= {iso(1) identified-organization(3) oiw(14)
secsig(3) algorithm(2) 26}
12.3.3 Mail List Key Encryption Algorithms Used in MailListRecipientInfo
Mail list key encryption is defined in this section. The key material used
to encrypt the message encryption key is distributed by an independent
mechanism. Two different mail list key encryption algorithms are defined by
this draft. CMS implementations must include mail list key encryption with
tripleDES if mail list key encryption is implemented. CMS implementations may
also implement mail list key encryption with RC2.
12.3.3.1 Mail List Key Encryption with tripleDES
Mail list key encryption with triple DES has the algorithm identifier
id-smime-cms-mlk-with-tripleDES ::= {TBD}
The AlgorithmIdentifier parameter field must be encoded as ASN.1 NULL.
Distribution of the key material used to encryption the message encryption
key is out of the scope of this document.
12.3.3.2 Mail List Key Encryption with RC2
Mail list key encryption with RC2 has the algorithm identifier
id-smime-cms-mlk-with-rC2 ::= {TBD}
For the effective-key-bits (key size) greater than 32 and less than 256, the
algorithm parameters are encoded as:
id-smime-cms-dh-with-rC2 parameter ::= SEQUENCE {
rc2ParameterVersion INTEGER,
iv OCTET STRING (8)}
For the effective-key-bits of 40, 64, and 128, the rc2ParameterVersion
values are 160, 120, 58 respectively. It is very important to note that
these values are not simply the RC2 keylength. Also note that the value 160
must be encoded as two octets (00 A0), because encoding as one octet (A0) is
a negative number in ASN.1.
12.4 Key Wrapping Algorithm
Key Transport algorithms allow for the content-encryption key to be directly
encrypted. Key agreement and mail list key exchange algorithms encrypt the
content-encryption key with a second (possibly different) block encryption
algorithm. This section describes how the content-encryption key is
formatted and encrypted.
The bits used for the key-encryption key are generated by the key agreement
algorithm or are distributed as the mail list key. When obtaining the bits
of the key-encryption key the minimal number of bits needed to form the
key-encryption key must be used. As an example only the first 40 bits of
key material would be used when using RC2/40-bit as a key-encryption
algorithm even though one may use more as RC2 starts by compressing the
input bits down to the algorithm size.
The block size of the key-encryption algorithm can be determined from the
CMS KeyEncryptionAlgorithmIdentifier protocol field. Likewise, the size of
the content-encryption key can be determined from the CMS
ContentEncryptionAlgorithmIdentifier protocol field. Triple-DES may be an
exception here; the same identifier is used for both 2-key and 3-key Triple
DES. This is probably easily handled by always wrapping three keys, even
if the first and third keys match.
The key checksum algorithm is:
1. Initialize two 16 bit integers, sum1 and sum2, to zero.
2. Loop through the octets of the content-encryption key, most
significant octet to least significant octet.
2a. Create a 16 bit integer, called temp, by concatenating
8 zero bits and the key octet.
2b. sum1 = sum1 + temp.
2c. sum2 = sum2 + sum1.
3. Use sum2 as the checksum value.
The key wrap algorithm is:
1. Modify the content-encryption key to meet any restrictions on the key.
An example of this would be modifying parity bits for DES and tripleDES
keys.
2. Compute a 16-bit key checksum value on the content-encryption key as
described above.
3. Generate a 4-octet random salt value.
4. Concatenate the salt, content-encryption key, and key checksum value.
5. Randomly generate the number of pad octets necessary to make the result
a multiple of block size of the key-encryption algorithm (the Triple-DES
algorithm block size is 8 bytes), then append them to the result.
{{{I am going to assume that this was suppose to be a random padding.}}}
6. Encrypt the result with the key-encryption algorithm key. Use an IV
with each octet equal to 'A5' hexadecimal. (Some key-encryption algorithms
encode the IV as part of the parameters. The IV must still be the constant
octet string.)
The key unwrap algorithm is:
1. Decrypt the wrapped ciphertext using the key-encryption key. Use an IV
with each octet equal to 'A5' hexadecimal.
2. Decompose the result into the content-encryption key and key checksum
values
3. Compute a 16-bit key checksum value on the content-encryption key as
described above.
4. If computed key checksum value does not match the decrypted key checksum
value, then there is an error.
5. If there are restrictions on keys, then check if the content encryption
key meets these restrictions. An example of this would be to check the
parity bits for DES and tripleDES keys. If any restriction is incorrect
then there is an error.
12.5 Content Encryption Algorithms
CMS implementations must include Triple-DES in CBC mode. CMS
implementations may include DES in CBC mode and RC2 in CBC mode.
Content encryption algorithms appear in the EncryptedContentInfo
contentEncryptionAlgorithm field, which is part of the EnvelopedData type
and the EncryptedData type.
12.5.1 Triple-DES CBC
The Triple-DES algorithm is described in [3DES]. The algorithm identifier
for Triple-DES is:
DES-EDE3-CBC OBJECT IDENTIFIER ::= {iso(1) member-body(2) us(840)
rsadsi(113549) encryptionAlgorithm(3) 7}
The AlgorithmIdentifier parameter field is required and has the structure:
CBCParameter ::= IV
IV ::= OCTET STRING -- 8 octets.
12.5.2 DES CBC
The DES algorithm in CBC mode is described in [DES]. The algorithm
identifier for DES in CBC mode is:
DES-CBC OBJECT IDENTIFIER ::= {iso(1) identified-organization(3)
oiw(14) secsig(3) algorithm(2) 7}
The AlgorithmIdentifier parameter field is required and has the structure:
CBCParameter ::= IV
IV ::= OCTET STRING -- 8 octets.
12.5.3 RC2 CBC
The RC2 algorithm is described in RFC 2268. 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}
For the effective-key-bits (key size) greater than 32 and less than
256, the RC2-CBC algorithm parameters are encoded as:
RC2-CBC parameter ::= SEQUENCE {
rc2ParameterVersion INTEGER,
iv OCTET STRING (8)}
For the effective-key-bits of 40, 64, and 128, the rc2ParameterVersion
values are 160, 120, 58 respectively. It is very important to note that
these values are not simply the RC2 keylength. Also note that the value 160
must be encoded as two octets (00 A0), because encoding as one octet (A0)
is a negative number in ASN.1.
12.6 Message Authentication Code Algorithms
No MAC algorithms are mandatory. CMS implementations may include DES
MAC and HMAC.
MAC algorithms appear in the AuthenticatedData mac field.
12.6.1 DES MAC
The DES MAC algorithm is described in FIPS Pub 113 [FIPS-113]. The
algorithm identifier for DES MAC is:
DES-MAC OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
oiw(14) secsig(3) algorithm(2) 10 }
The AlgorithmIdentifier parameter field is required. It is an INTEGER, and
carries the length in bits of the MAC, constrained to multiples of eight
from 16 to 64.
12.6.2 HMAC-SHA1
The HMAC-SHA1 algorithm is described in RFC 2104. The algorithm identifier for
HMAC-SHA1 is:
HMAC-SHA1 OBJECT IDENTIFIER ::= {iso(1) identified-organization(3)
dod(6) internet(1) security(5) mechanisms(5) 8 1 2}
The AlgorithmIdentifier parameter field must not be present.
{{{References added}}}
3DES W. Tuchman. "Hellman Presents No Shortcut Solutions To DES". IEEE
Spectrum, v. 16, n. 7, pp40-41. July 1979.
DES American National Standards Institute. ANSI X3.106, "American National
Standard for Information Systems- Data Link Encryption". 1983.
FIPS 113 National Institute of Standards and Technology. FIPS Pub 113:
Computer Data Authentication. May 1985.
FIPS 180-1 National Institute of Standards and Technology. FIPS Pub 180-1:
Secure Hash Standard. April 1995.
FIPS 186 National Institute of Standards and Technology. FIPS Pub 186:
Digital Signature Standard. May 1994.
RFC 1321 Rivest, R. The MD5 Message Digest Algorithm. April 1992.
RFC 2104 Krawczyk, H. HMAC: Keyed-Hashing for Message Authentication.
February 1997.
RFC 2268 Rivest, R. A Description of the RC2 (r) Encryption Algorithm.
March 1998.
X9.42 ANSI X9.42-199x. Public Key Cryptography for The Financial Services
Industry: Agreement of Symmetric Algorithm Keys Using Diffie-Hellman
(Working Draft). December 1997.