Problem - Kevin Tafuro

ing the all-zero data block. Each precomputed value is computed by using a 128-bit
shift and a conditional XOR. The last block of data is padded, if necessary, and
XOR’d with one of these two values, depending on its length.
Here is an all-in-one wrapper to OMAC, exporting both OMAC1 and OMAC2:
int SPC_OMAC1(unsigned char key[ ], int keylen, unsigned char in[ ], size_t l,
unsigned char out[16]) {
SPC_OMAC_CTX c;
if (!spc_omac1_init(&c, key, keylen)) return 0;
spc_omac_update(&c, in, l);
spc_omac_final(&c, out);
return 1;
}
int SPC_OMAC2(unsigned char key[ ], int keylen, unsigned char in[ ], size_t l,
unsigned char out[16]) {
SPC_OMAC_CTX c;
if (!spc_omac2_init(&c, key, keylen)) return 0;
spc_omac_update(&c, in, l);
spc_omac_final(&c, out);
return 1;
}
See Also
Recipes 5.5, 6.7, 6.9, 13.2
6.12 Using HMAC or OMAC with a Nonce
Problem
You want to use HMAC or OMAC, but improve its resistance to birthday attacks
and capture replay attacks.
Solution
Use an ever-incrementing nonce that is concatenated to your message.
Discussion
Be sure to actually test the nonce when validating the nonce value, so
as to thwart capture replay attacks. (See Recipe 6.21.)
Using HMAC or OMAC with a Nonce | 285
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