Problem - Kevin Tafuro

SPC_HMAC_Reset(&ctx);
SPC_HMAC_Update(&ctx, ctr, sizeof(ctr));
SPC_HMAC_Final(out, &ctx);
for (i = 0; i < len % MAC_OUT_SZ; i++) *out++ = *in++ ^ keystream[i];
}
return out;
}
Note that this code is not optimized; it works on individual characters to avoid
potential endian-ness problems.
See Also
Recipes 6.4, 6.10
6.21 Securely Authenticating a MAC (Thwarting
Capture Replay Attacks)
Problem
You are using a MAC, and you need to make sure that when you get a message, you
properly validate the MAC.
Solution
If you’re using an ever-increasing nonce (which we strongly recommend), check to
make sure that the nonce associated with the message is indeed larger than the last
one. Then, of course, recalculate the MAC and check against the transmitted MAC.
Discussion
The following is an example of validating a MAC using the OMAC1 implementation
in Recipe 6.11, along with AES-128. We nonce the MAC by using a 16-byte nonce as
the first block of input, as discussed in Recipe 6.12. Note that we expect you to be
MAC’ing the ciphertext, as discussed in Recipe 6.18.
#include
#include
/* last_nonce must be a pointer to a NULL on first invocation. */
int spc_omac1_validate(unsigned char *ct, size_t ctlen, unsigned char sent_nonce[16],
unsigned char *sent_tag, unsigned char *k,
unsigned char **last_nonce) {
int i;
SPC_OMAC_CTX c;
unsigned char calc_tag[16]; /* Maximum tag size for OMAC. */
Securely Authenticating a MAC (Thwarting Capture Replay Attacks) | 303
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