Problem - Kevin Tafuro

Using a MAC in CTR mode is easy. As illustrated in Figure 6-7, key it, then use it to
“MAC” a nonce concatenated with a counter. XOR the results with the plaintext.
Figure 6-7. Encrypting with a MAC in counter mode
For example, here’s a function that encrypts a stream of data using the HMAC-SHA1
implementation from Recipe 6.10:
#include
#include
#define NONCE_LEN 16
#define CTR_LEN 16
#define MAC_OUT_SZ 20
unsigned char *spc_MAC_encrypt(unsigned char *in, size_t len, unsigned char *key,
int keylen, unsigned char *nonce) {
/* We're using a 128-bit nonce and a 128-bit counter, packed into one variable */
int i;
size_t blks;
SPC_HMAC_CTX ctx;
unsigned char ctr[NONCE_LEN + CTR_LEN];
unsigned char keystream[MAC_OUT_SZ];
unsigned char *out;
if (!(out = (unsigned char *)malloc(len))) abort( );
SPC_HMAC_Init(&ctx, key, keylen);
memcpy(ctr, nonce, NONCE_LEN);
memset(ctr + NONCE_LEN, 0, CTR_LEN);
blks = len / MAC_OUT_SZ;
while (blks--) {
SPC_HMAC_Reset(&ctx);
SPC_HMAC_Update(&ctx, ctr, sizeof(ctr));
SPC_HMAC_Final(out, &ctx);
i = NONCE_LEN + CTR_LEN;
/* Increment the counter. */
while (i-- != NONCE_LEN)
if (++ctr[i]) break;
for (i = 0; i < MAC_OUT_SZ; i++) *out++ = *in++ ^ keystream[i];
}
if (len % MAC_OUT_SZ) {
302 | Chapter 6: Hashes and Message Authentication
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