crypto/aes/aes_ctr.c

/* crypto/aes/aes_ctr.c -*- mode:C; c-file-style: "eay" -*- */
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#ifndef AES_DEBUG
# ifndef NDEBUG
#  define NDEBUG
# endif
#endif
#include <assert.h>

#include <openssl/aes.h>
#include "aes_locl.h"

/* NOTE: the IV/counter CTR mode is big-endian.  The rest of the AES code
 * is endian-neutral. */

/* increment counter (128-bit int) by 1 */
static void AES_ctr128_inc(unsigned char *counter) {
	unsigned long c;

	/* 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 */
	c = GETU32(counter +  0);
	c++;	c &= 0xFFFFFFFF;
	PUTU32(counter +  0, c);
}

/* 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().
 *
 * 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 ivec[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(ivec, ecount_buf, key);
 			AES_ctr128_inc(ivec);
		}
		*(out++) = *(in++) ^ ecount_buf[n];
		n = (n+1) % AES_BLOCK_SIZE;
	}

	*num=n;
}