tomcrypt/src/encauth/gcm/gcm_gf_mult.c

/* LibTomCrypt, modular cryptographic library -- Tom St Denis
 *
 * LibTomCrypt is a library that provides various cryptographic
 * algorithms in a highly modular and flexible manner.
 *
 * The library is free for all purposes without any express
 * guarantee it works.
 *
 * Tom St Denis, tomstdenis@gmail.com, http://libtomcrypt.org
 */

/**
   @file gcm_gf_mult.c
   GCM implementation, do the GF mult, by Tom St Denis
*/
#include "tomcrypt.h"

#if defined(GCM_MODE) || defined(LRW_MODE)

/* right shift */
static void gcm_rightshift(unsigned char *a)
{
   int x;
   for (x = 15; x > 0; x--) {
       a[x] = (a[x]>>1) | ((a[x-1]<<7)&0x80);
   }
   a[0] >>= 1;
}

/* c = b*a */
static const unsigned char mask[] = { 0x80, 0x40, 0x20, 0x10, 0x08, 0x04, 0x02, 0x01 };
static const unsigned char poly[] = { 0x00, 0xE1 };

     
/**
  GCM GF multiplier (internal use only) 
  @param a   First value
  @param b   Second value
  @param c   Destination for a * b
 */  
void gcm_gf_mult(const unsigned char *a, const unsigned char *b, unsigned char *c)
{
   unsigned char Z[16], V[16];
   unsigned x, y, z;

   zeromem(Z, 16);
   XMEMCPY(V, a, 16);
   for (x = 0; x < 128; x++) {
       if (b[x>>3] & mask[x&7]) {
          for (y = 0; y < 16; y++) {
              Z[y] ^= V[y]; 
          }
       }
       z     = V[15] & 0x01;
       gcm_rightshift(V);
       V[0] ^= poly[z];
   }
   XMEMCPY(c, Z, 16);
}
#endif

/* $Source$ */
/* $Revision$ */
/* $Date$ */