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/*
* Copyright (c) 2001-2007, Tom St Denis
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
* 1. Redistributions of source code must retain the above copyright notice,
* this list of conditions and the following disclaimer.
*
* 2. Redistributions in binary form must reproduce the above copyright notice,
* this list of conditions and the following disclaimer in the documentation
* and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
* LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE.
*/
/* 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://libtom.org
*/
#include "tomcrypt.h"
/**
Source donated by Elliptic Semiconductor Inc (www.ellipticsemi.com) to the LibTom Projects
*/
#ifdef LTC_XTS_MODE
static int tweak_uncrypt(const unsigned char *C, unsigned char *P, unsigned char *T, symmetric_xts *xts)
{
unsigned long x;
int err;
/* tweak encrypt block i */
#ifdef LTC_FAST
for (x = 0; x < 16; x += sizeof(LTC_FAST_TYPE)) {
*((LTC_FAST_TYPE*)&P[x]) = *((LTC_FAST_TYPE*)&C[x]) ^ *((LTC_FAST_TYPE*)&T[x]);
}
#else
for (x = 0; x < 16; x++) {
P[x] = C[x] ^ T[x];
}
#endif
err = cipher_descriptor[xts->cipher]->ecb_decrypt(P, P, &xts->key1);
#ifdef LTC_FAST
for (x = 0; x < 16; x += sizeof(LTC_FAST_TYPE)) {
*((LTC_FAST_TYPE*)&P[x]) ^= *((LTC_FAST_TYPE*)&T[x]);
}
#else
for (x = 0; x < 16; x++) {
P[x] = P[x] ^ T[x];
}
#endif
/* LFSR the tweak */
xts_mult_x(T);
return err;
}
/** XTS Decryption
@param ct [in] Ciphertext
@param ptlen Length of plaintext (and ciphertext)
@param pt [out] Plaintext
@param tweak [in] The 128--bit encryption tweak (e.g. sector number)
@param xts The XTS structure
Returns CRYPT_OK upon success
*/
int xts_decrypt(const unsigned char *ct, unsigned long ptlen, unsigned char *pt, unsigned char *tweak,
symmetric_xts *xts)
{
const struct ltc_cipher_descriptor *desc;
unsigned char PP[16], CC[16], T[16];
unsigned long i, m, mo, lim;
int err;
/* check inputs */
LTC_ARGCHK(pt != NULL);
LTC_ARGCHK(ct != NULL);
LTC_ARGCHK(tweak != NULL);
LTC_ARGCHK(xts != NULL);
/* check if valid */
if ((err = cipher_is_valid(xts->cipher)) != CRYPT_OK) {
return err;
}
/* get number of blocks */
m = ptlen >> 4;
mo = ptlen & 15;
/* must have at least one full block */
if (m == 0) {
return CRYPT_INVALID_ARG;
}
if (mo == 0) {
lim = m;
} else {
lim = m - 1;
}
desc = cipher_descriptor[xts->cipher];
if (desc->accel_xts_decrypt && lim > 0) {
/* use accelerated decryption for whole blocks */
if ((err = desc->accel_xts_decrypt(ct, pt, lim, tweak, &xts->key1,
&xts->key2) != CRYPT_OK)) {
return err;
}
ct += lim * 16;
pt += lim * 16;
/* tweak is encrypted on output */
XMEMCPY(T, tweak, sizeof(T));
} else {
/* encrypt the tweak */
if ((err = desc->ecb_encrypt(tweak, T, &xts->key2)) != CRYPT_OK) {
return err;
}
for (i = 0; i < lim; i++) {
if ((err = tweak_uncrypt(ct, pt, T, xts)) != CRYPT_OK) {
return err;
}
ct += 16;
pt += 16;
}
}
/* if ptlen not divide 16 then */
if (mo > 0) {
XMEMCPY(CC, T, 16);
xts_mult_x(CC);
/* PP = tweak decrypt block m-1 */
if ((err = tweak_uncrypt(ct, PP, CC, xts)) != CRYPT_OK) {
return err;
}
/* Pm = first ptlen % 16 bytes of PP */
for (i = 0; i < mo; i++) {
CC[i] = ct[16+i];
pt[16+i] = PP[i];
}
for (; i < 16; i++) {
CC[i] = PP[i];
}
/* Pm-1 = Tweak uncrypt CC */
if ((err = tweak_uncrypt(CC, pt, T, xts)) != CRYPT_OK) {
return err;
}
}
/* Decrypt the tweak back */
if ((err = desc->ecb_decrypt(T, tweak, &xts->key2)) != CRYPT_OK) {
return err;
}
return CRYPT_OK;
}
#endif
/* $Source: /cvs/libtom/libtomcrypt/src/modes/xts/xts_decrypt.c,v $ */
/* $Revision: 1.5 $ */
/* $Date: 2007/05/12 14:05:56 $ */
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