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/* 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, initialize state, by Tom St Denis
*/
#include "tomcrypt.h"
#ifdef GCM_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);
}
/**
GCM multiply by H
@param gcm The GCM state which holds the H value
@param I The value to multiply H by
*/
void gcm_mult_h(gcm_state *gcm, unsigned char *I)
{
unsigned char T[16];
#ifdef GCM_TABLES
int x, y;
XMEMCPY(T, &gcm->PC[0][I[0]][0], 16);
for (x = 1; x < 16; x++) {
#ifdef LTC_FAST
for (y = 0; y < 16; y += sizeof(LTC_FAST_TYPE)) {
*((LTC_FAST_TYPE *)(T + y)) ^= *((LTC_FAST_TYPE *)(&gcm->PC[x][I[x]][y]));
}
#else
for (y = 0; y < 16; y++) {
T[y] ^= gcm->PC[x][I[x]][y];
}
#endif
}
#else
gcm_gf_mult(gcm->H, I, T);
#endif
XMEMCPY(I, T, 16);
}
#endif
/* $Source: /cvs/libtom/libtomcrypt/src/encauth/gcm/gcm_gf_mult.c,v $ */
/* $Revision: 1.16 $ */
/* $Date: 2005/05/21 14:33:42 $ */
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