Problem - Kevin Tafuro

techniques from Chapter 8), the key derivation step is unnecessary, because reusing
{key, nonce} pairs is already incredibly unlikely in such a situation.
Both communicating parties must initialize their queue with identical parameters.
When you’re done with a queue, you should deallocate internally allocated memory
by calling spc_cipherq_cleanup( ):
void spc_cipherq_cleanup(SPC_CIPHERQ *q) {
spc_memset(q, 0, sizeof(SPC_CIPHERQ));
}
Here are implementations of the encryption and decryption operations (including a
helper function), both of which return a newly allocated buffer containing the results
of the appropriate operation:
static void increment_counter(SPC_CIPHERQ *q) {
if (!++q->nonce[10]) if (!++q->nonce[9]) if (!++q->nonce[8]) if (!++q->nonce[7])
if (!++q->nonce[6]) ++q->nonce[5];
}
unsigned char *spc_cipherq_encrypt(SPC_CIPHERQ *q, unsigned char *m, size_t mlen,
size_t *ol) {
unsigned char *ret;
if (!(ret = (unsigned char *)malloc(mlen + 16))) {
if (ol) *ol = 0;
return 0;
}
cwc_encrypt(&(q->ctx), 0, 0, m, mlen, q->nonce, ret);
increment_counter(q);
if (ol) *ol = mlen + 16;
return ret;
}
unsigned char *spc_cipherq_decrypt(SPC_CIPHERQ *q, unsigned char *m, size_t mlen,
size_t *ol) {
unsigned char *ret;
if (!(ret = (unsigned char *)malloc(mlen - 16))) {
if (ol) *ol = 0;
return 0;
}
if (!cwc_decrypt(&(q->ctx), 0, 0, m, mlen, q->nonce, ret)) {
free(ret);
if (ol) *ol = 0;
return 0;
}
increment_counter(q);
if (ol) *ol = mlen - 16;
return ret;
}
Using a High-Level, Error-Resistant Encryption and Decryption API | 219
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