Greetings again. Russ decided that summer was for vacations, so he's gone
this week. Because we want to get everything ready for last call ASAP
(modulo stupid patent issues), he asked Blake and I to try to clean up the
last big empty part of the CMS draft, which is section 12. Below is my
suggested cut at it. Please read it over carefully and comment, since there
are likely to be things that are wrong with it. If we can do significant
bashing this week, Russ said he'd rev the draft as soon as he is back.
--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 parameter field is optional. If present, the
parameter field must contain an ASN.1 NULL. Implementations should
generate SHA-1 AlgorithmIdentifiers with the parameter field absent.
Implementations should accept SHA-1 AlgorithmIdentifiers with NULL
parameters as well as absent parameters. Note that current CMS
implementations encode SHA-1 both with and without the NULL attributes.
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 parameter field is optional. If present, the
parameter field must contain an ASN.1 NULL. Implementations should
generate MD5 AlgorithmIdentifiers with the parameter field absent.
Implementations should accept MD5 AlgorithmIdentifiers with NULL parameters
as well as absent parameters. Note that current CMS implementations encode
MD5 both with and without the NULL attributes.
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 RFC TBD (Diffie-Hellman Key
Agreement Method, currently draft-ietf-smime-x942). The algorithm
identifier for DSA is:
id-dsa OBJECT IDENTIFIER ::= {iso(1) member-body(2) us(840) x9-57(10040)
x9cm(4) 1}
The id-dsa algorithm syntax includes an optional sequence of parameters.
These parameters are commonly referred to as p, q, and g. If the DSA
algorithm parameters are present, they are included using the following
ASN.1 structure:
Dss-Parms ::= SEQUENCE {
p INTEGER,
q INTEGER,
g INTEGER }
If the parameters are omitted, the parameters component must be omitted
entirely, so the AlgorithmIdentifier in this case is a SEQUENCE of only one
component, the OBJECT IDENTIFIER id-dsa.
12.2.2 RSA
The RSA signature algorithm is defined in RFC 2313. 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 parameter field is optional. If present, the
parameter field must contain an ASN.1 NULL. Implementations should
generate RSA AlgorithmIdentifiers with the parameter field absent.
Implementations should accept RSA AlgorithmIdentifiers with NULL parameters
as well as absent parameters. Note that current CMS implementations encode
RSA both with and without the NULL attributes.
12.3 Key Encryption Algorithms
CMS implementations must include X9.42 Static Diffie-Hellman. CMS
implementations may include RSA.
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 X9.42 Static Diffie-Hellman with desCBC
Diffie-Hellman key encryption is defined in X9.42 [X9.42]. The algorithm
identifier for static Diffie-Hellman is
id-smime-cms-x942-with-desCBC ::= { TBD }
The AlgorithmIdentifier parameter field is optional. If present, the
parameter field must contain an ASN.1 NULL. Implementations should accept
static Diffie-Hellman AlgorithmIdentifiers with NULL parameters as well as
absent parameters. Implementations should generate static Diffie-Hellman
AlgorithmIdentifiers with the parameter field absent.
12.3.2 RSA
The RSA encryption algorithm is defined in RFC 2313 [RFC 2313]. 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 parameter field is optional. If present, the
parameter field must contain an ASN.1 NULL. Implementations should
generate RSA AlgorithmIdentifiers with the parameter field absent.
Implementations should accept RSA AlgorithmIdentifiers with NULL parameters
as well as absent parameters. Note that current CMS implementations encode
RSA both with and without the NULL attributes.
12.4 Triple-DES Key Wrap
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. Adjust the parity bits on the Triple-DES 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. Generate the number of pad octets necessary to make the
result a multiple of 8 bytes (the Triple-DES algorithm block
size), then append them to the result.
6. Encrypt the result with the key-encryption key. Use an IV
with each octet equal to 'A5' hexadecimal.
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. Check the parity of the Triple-DES keys. If any is
incorrect then there is an error.
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.
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}
{{{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.