All:
S/MIME 3 requires a function to encrypt one Triple-DES key in another.
Last March, I proposed an algorithm to wrap a Triple-DES content-encryption
key in a Triple-DES key-encryption key. With a few minor modifications,
that algorithm was incorporated into the Cryptographic Message Syntax (CMS)
Internet-Draft. Burt Kaliski recently discovered a flaw in that algorithm.
In this message I propose a new algorithm.
Here is a summary of how S/MIME v3 will use the function. The S/MIME v3
originator locally generates a random content-encryption Triple-DES key.
As the name implies, this key will be used to encrypt the message content.
Then, Diffie-Hellman is used to generate a pairwise Triple-DES key with
each message recipient, and the content-encryption key is encrypted under
the pairwise key.
The wrap algorithm is also used to support mail lists. As setup, a
key-encryption key is distributed to a group of recipients. Then, the
originator locally generates a random content-encryption Triple-DES key,
and the content-encryption key is encrypted under the pairwise key. The
key-encryption key could be used for many messages to the group.
Please review the revised algorithm. Once a key wrap algorithm is
selected, the S/MIME v3 specifications will be ready for review by the
Internet Engineering Steering Group (IESG). Please do not propose
improvements that are covered by any issued or pending patents.
Thanks for your assistance,
Russ
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The Triple-DES key wrap algorithm uses techniques similar to
Bellare-Rogaway Optimal Asymmetric Encryption Padding [OAEP].
X.1 Key Checksum
The Fletcher checksum algorithm [FLETCHER] is used to provide an
integrity check value. The 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 (first) octet to least significant (last) octet.
2a. Create a 16 bit integer, called TEMP, by concatenating
eight zero bits and the key octet.
2b. SUM1 = SUM1 + TEMP.
2c. SUM2 = SUM2 + SUM1.
3. Use SUM2 as the 16-bit checksum value.
X.2 Triple-DES Key Wrap
1. Adjust the parity bits for each of the three DES keys comprising the
Triple-DES content-encryption key, call the parity adjusted key CEK.
2. Compute an integrity check value on CEK, called ICV, as described above.
3. Let CEKICV = 0x18 || CEK || 0x00 || ICV.
4. Generate a 160-bit random value, called RAND.
5. Let H1 = SHA-1 ( RAND || 0x01 ).
6. Let H2 = SHA-1 ( RAND || 0x02 ).
7. Let MASK1 equal the most significant (first) 224 bits of H1 || H2.
8. Let MKEY = CEKICV XOR MASK1.
9. Let MASK2 = SHA-1 ( MKEY ).
10. Let MRAND = MASK2 XOR RAND.
11. Let MDATA = MRAND || MKEY.
12. Encrypt MDATA with the key-encryption key and CBC mode. Use an IV
with each octet equal to 0xA5. The ciphertext is 384 bits long.
X.3 Triple-DES Key Unwrap
The key unwrap algorithm is:
1. Decrypt the ciphertext using the key-encryption key and CBC mode. Use
an IV with each octet equal to 0xA5.
2. Decompose the plaintext result. The most significant (first) 160 bits
are MRAND, and the remaining 224 bits are MKEY.
3. Let MASK2 = SHA-1 ( MKEY ).
4. Let RAND = MASK2 XOR MRAND.
5. Let H1 = SHA-1 ( RAND || 0x01 ).
6. Let H2 = SHA-1 ( RAND || 0x02 ).
7. Let MASK1 equal the most significant (first) 224 bits of H1 || H2.
8. Let CEKICV = MASK1 XOR MKEY.
9. Decompose CEKICV. The most significant (first) 8 bits are LENGTH, the
following 192 bits are CEK, the following 8 bits are PAD, and the
remaining 16 bits are ICV1.
10. If LENGTH is not 0x18, then error.
11. If PAD is not 0x00, then error.
12. Compute an integrity check value on CEK, called ICV2, as described
above.
13. If ICV1 and ICV2 are not equal, then error.
14. Check for odd parity of each octet in each DES key that makes up the
Triple-DES content-encryption key. If any parity bit is incorrect,
then error.
REFERENCES
FLETCHER Fletcher, J. "An Arithmetic Checksum for Serial
Transmissions", IEEE Transactions on Communication,
Vol. COM-30, No. 1, pp. 247-252, January 1982.
OAEP Bellare-Rogaway Optimal Asymmetric Encryption Padding
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