Problem - Kevin Tafuro

if (!(vfy = (unsigned char *)malloc(dlen))) return 0;
EVP_DigestFinal(&ctx, vfy, &outlen);
in += dlen;
for (i = 0; i < dlen; i++)
if (vfy[i] != toverify[i + dlen]) {
free(vfy);
return 0;
}
free(vfy);
return 1;
}
The first function, spc_create_nonced_digest( ), automatically selects a nonce from
the OpenSSLrandom number generator and returns twice the digest size in output,
where the first digest-sized block is the nonce and the second is the hash. The second
function, spc_verify_nonced_digest( ), takes data consisting of a nonce concatenated
with a hash value, and returns 1 if the hash validates, and 0 otherwise.
Two macros can make extracting the nonce and the hash easier:
#include
#include
#include
/* Here, l is the output length of spc_create_nonced_digest( ) */
#define spc_extract_nonce(l, s) (s)
#define spc_extract_digest(l, s) ((s)+((l) / 2))
Here’s a sample program using this API:
int main(int argc, char *argv[ ]) {
unsigned int i, ol;
unsigned char *s = "Testing hashes with nonces.";
unsigned char *dgst, *nonce, *ret;
ret = spc_create_nonced_digest(EVP_sha1( ), s, strlen(s), &ol);
nonce = spc_extract_nonce(ol, ret);
dgst = spc_extract_digest(ol, ret);
printf("Nonce = ");
for(i = 0; i < ol / 2; i++)
printf("%02x", nonce[i]);
printf("\nSHA1-Nonced(Nonce, \"%s\") = \n\t", s);
for(i = 0; i < ol / 2; i++)
printf("%02x", dgst[i]);
printf("\n");
if (spc_verify_nonced_digest(EVP_sha1( ), s, strlen(s), ret))
printf("Recalculation verified integrity.\n");
else
printf("Recalculation FAILED to match.\n");
return 0;
}
See Also
Recipe 6.7
Using a Nonce to Protect Against Birthday Attacks | 273
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