Problem - Kevin Tafuro

spc_omac1_init(&c, k, 16);
if (*last_nonce) {
for (i = 0; i < 16; i++)
if (sent_nonce[i] > (*last_nonce)[i]) goto nonce_okay;
return 0; /* Nonce is equal to or less than last nonce. */
}
nonce_okay:
spc_omac_update(&c, sent_nonce, 16);
spc_omac_update(&c, ct, ctlen);
spc_omac_final(&c, calc_tag);
for (i = 0; i < 16; i++)
if (calc_tag[i] != sent_tag[i]) return 0;
if (sent_nonce) {
if (!*last_nonce) *last_nonce = (unsigned char *)malloc(16);
if (!*last_nonce) abort(); /* Consider an exception instead. */
memcpy(*last_nonce, sent_nonce, 16);
}
return 1;
}
This code requires you to pass in a char ** to track the last nonce that was received.
You’re expected to allocate your own char *, set it to NULL, and pass in the address of
that char *. The validate function will update that memory with the last valid nonce
it saw, so that it can check the new nonce against the last nonce to make sure it got
bigger. The function will return 1 if the MAC validates; otherwise, it will return 0.
See Also
Recipes 6.11, 6.12, 6.18
6.22 Parallelizing MACs
Problem
You want to use a MAC, but parallelize the computation.
Solution
Run multiple MACs at the same time, then MAC the resulting tags together (and in
order) to yield one tag.
Discussion
If you want to perform message authentication in parallel, you can do so with a variation
of interleaving (which we discussed for block ciphers in Recipes 5.12 through
5.14) Basically, you can run multiple MACs keyed separately at the same time and
304 | Chapter 6: Hashes and Message Authentication
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