90 lines
2.0 KiB
C
90 lines
2.0 KiB
C
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/* LibTomCrypt, modular cryptographic library -- Tom St Denis
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*
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* LibTomCrypt is a library that provides various cryptographic
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* algorithms in a highly modular and flexible manner.
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*
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* The library is free for all purposes without any express
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* guarantee it works.
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*
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* Tom St Denis, tomstdenis@gmail.com, http://libtomcrypt.org
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*/
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/**
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@file gcm_gf_mult.c
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GCM implementation, initialize state, by Tom St Denis
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*/
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#include "tomcrypt.h"
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#ifdef GCM_MODE
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/* right shift */
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static void gcm_rightshift(unsigned char *a)
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{
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int x;
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for (x = 15; x > 0; x--) {
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a[x] = (a[x]>>1) | ((a[x-1]<<7)&0x80);
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}
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a[0] >>= 1;
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}
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/* c = b*a */
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static const unsigned char mask[] = { 0x80, 0x40, 0x20, 0x10, 0x08, 0x04, 0x02, 0x01 };
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static const unsigned char poly[] = { 0x00, 0xE1 };
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/**
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GCM GF multiplier (internal use only)
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@param a First value
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@param b Second value
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@param c Destination for a * b
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*/
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void gcm_gf_mult(const unsigned char *a, const unsigned char *b, unsigned char *c)
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{
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unsigned char Z[16], V[16];
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unsigned x, y, z;
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zeromem(Z, 16);
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XMEMCPY(V, a, 16);
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for (x = 0; x < 128; x++) {
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if (b[x>>3] & mask[x&7]) {
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for (y = 0; y < 16; y++) {
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Z[y] ^= V[y];
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}
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}
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z = V[15] & 0x01;
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gcm_rightshift(V);
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V[0] ^= poly[z];
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}
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XMEMCPY(c, Z, 16);
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}
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/**
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GCM multiply by H
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@param gcm The GCM state which holds the H value
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@param I The value to multiply H by
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*/
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void gcm_mult_h(gcm_state *gcm, unsigned char *I)
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{
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unsigned char T[16];
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#ifdef GCM_TABLES
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int x, y;
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XMEMCPY(T, &gcm->PC[0][I[0]][0], 16);
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for (x = 1; x < 16; x++) {
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#ifdef LTC_FAST
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for (y = 0; y < 16; y += sizeof(LTC_FAST_TYPE)) {
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*((LTC_FAST_TYPE *)(T + y)) ^= *((LTC_FAST_TYPE *)(&gcm->PC[x][I[x]][y]));
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}
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#else
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for (y = 0; y < 16; y++) {
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T[y] ^= gcm->PC[x][I[x]][y];
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}
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#endif
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}
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#else
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gcm_gf_mult(gcm->H, I, T);
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#endif
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XMEMCPY(I, T, 16);
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}
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#endif
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