#include <openssl/aes.h>
#include "aes_locl.h"
-/* NOTE: CTR mode is big-endian. The rest of the AES code
+/* NOTE: the IV/counter CTR mode is big-endian. The rest of the AES code
* is endian-neutral. */
-/* increment counter (128-bit int) by 2^64 */
+/* increment counter (128-bit int) by 1 */
static void AES_ctr128_inc(unsigned char *counter) {
unsigned long c;
- /* Grab 3rd dword of counter and increment */
-#ifdef L_ENDIAN
- c = GETU32(counter + 8);
- c++;
- PUTU32(counter + 8, c);
-#else
- c = GETU32(counter + 4);
- c++;
- PUTU32(counter + 4, c);
-#endif
+ /* Grab bottom dword of counter and increment */
+ c = GETU32(counter + 12);
+ c++; c &= 0xFFFFFFFF;
+ PUTU32(counter + 12, c);
+
+ /* if no overflow, we're done */
+ if (c)
+ return;
+
+ /* Grab 1st dword of counter and increment */
+ c = GETU32(counter + 8);
+ c++; c &= 0xFFFFFFFF;
+ PUTU32(counter + 8, c);
+
+ /* if no overflow, we're done */
+ if (c)
+ return;
+
+ /* Grab 2nd dword of counter and increment */
+ c = GETU32(counter + 4);
+ c++; c &= 0xFFFFFFFF;
+ PUTU32(counter + 4, c);
/* if no overflow, we're done */
if (c)
return;
/* Grab top dword of counter and increment */
-#ifdef L_ENDIAN
- c = GETU32(counter + 12);
- c++;
- PUTU32(counter + 12, c);
-#else
c = GETU32(counter + 0);
- c++;
+ c++; c &= 0xFFFFFFFF;
PUTU32(counter + 0, c);
-#endif
-
}
/* The input encrypted as though 128bit counter mode is being
* 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().
+ *
+ * This algorithm assumes that the counter is in the x lower bits
+ * of the IV (ivec), and that the application has full control over
+ * overflow and the rest of the IV. This implementation takes NO
+ * responsability for checking that the counter doesn't overflow
+ * into the rest of the IV when incremented.
*/
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 ivec[AES_BLOCK_SIZE],
unsigned char ecount_buf[AES_BLOCK_SIZE],
unsigned int *num) {
while (l--) {
if (n == 0) {
- AES_encrypt(counter, ecount_buf, key);
- AES_ctr128_inc(counter);
+ AES_encrypt(ivec, ecount_buf, key);
+ AES_ctr128_inc(ivec);
}
*(out++) = *(in++) ^ ecount_buf[n];
n = (n+1) % AES_BLOCK_SIZE;