int crypt_ctr_decrypt(cipher_context *cctx, uint8 *in, uint8 *out, ulong length, int64 offset)
{
int64 alignment;
unsigned char ivec[MAX_CIPHER_BLOCK_SIZE];
unsigned char ecount_buf[MAX_CIPHER_BLOCK_SIZE];
unsigned int n;
unsigned long l=length;
if(offset%MAX_CIPHER_BLOCK_SIZE)
{
alignment = offset/MAX_CIPHER_BLOCK_SIZE;
ctr_init_iv(ivec, alignment);
crypt_enc_block(cctx, ivec, ecount_buf, MAX_CIPHER_BLOCK_SIZE);
AES_ctr128_inc(ivec);
n = offset%MAX_CIPHER_BLOCK_SIZE;
}
else
{
ctr_init_iv(ivec, offset/MAX_CIPHER_BLOCK_SIZE);
n = 0;
}
while (l) {
if (n == 0) {
crypt_enc_block(cctx, ivec, ecount_buf, MAX_CIPHER_BLOCK_SIZE);
AES_ctr128_inc(ivec);
}
*(out++) = *(in++) ^ ecount_buf[n];
n = (n+1) % MAX_CIPHER_BLOCK_SIZE;
l--;
}
return 1;
}
/* The input encrypted as though 128bit counter mode is being
* used. The extra state information to record how much of the
* 128bit block we have used is contained in *num, and the
* encrypted counter is kept in ecount_buf. Both *num and
* ecount_buf must be initialised with zeros before the first
* call to AES_ctr128_encrypt().
*/
void AES_ctr128_encrypt(const unsigned char *in, unsigned char *out,
const unsigned long length, const AES_KEY *key,
unsigned char counter[AES_BLOCK_SIZE],
unsigned char ecount_buf[AES_BLOCK_SIZE],
unsigned int *num) {
unsigned int n;
unsigned long l=length;
assert(in && out && key && counter && num);
assert(*num < AES_BLOCK_SIZE);
n = *num;
while (l--) {
if (n == 0) {
AES_encrypt(counter, ecount_buf, key);
AES_ctr128_inc(counter);
}
*(out++) = *(in++) ^ ecount_buf[n];
n = (n+1) % AES_BLOCK_SIZE;
}
*num=n;
}
