Problem - Kevin Tafuro

If you’re using an off-the-shelf HMAC implementation, such as OpenSSL’s or CryptoAPI’s,
you can easily concatenate your nonce to the beginning of your message.
You should use a nonce that’s at least half as large as your key size, if not larger. Ultimately,
we would recommend that any nonce contain a message counter that is 64
bits (it can be smaller if you’re 100% sure you’ll never use every counter value) and a
random portion that is at least 64 bits. The random portion can generally be chosen
per session instead of per message.
Here’s a simple wrapper that provides a nonced all-in-one version of OMAC1, using
the implementation from Recipe 6.11 and a 16-byte nonce:
void spc_OMAC1_nonced(unsigned char key[ ], int keylen, unsigned char in[ ],
size_t l, unsigned char nonce[16], unsigned char out[16]) {
SPC_OMAC_CTX c;
if (!spc_omac1_init(&c, key, keylen)) abort( );
spc_omac_update(&c, nonce, 16);
spc_omac_update(&c, in, l);
spc_omac_final(&c, out);
}
See Also
Recipes 6.11, 6.21
6.13 Using a MAC That’s Reasonably Fast in
Software and Hardware
Problem
You want to use a MAC that is fast in both software and hardware.
Solution
Use CMAC. It is available from http://www.zork.org/cmac/.
Discussion
Be sure to look at our generic recommendations for using a MAC (see
Recipe 6.9).
286 | Chapter 6: Hashes and Message Authentication
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