openssl/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;
}