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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://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,
const unsigned char *tweak,
symmetric_xts *xts)
{
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;
}
/* encrypt the tweak */
if ((err = cipher_descriptor[xts->cipher].ecb_encrypt(tweak, T, &xts->key2)) != CRYPT_OK) {
return err;
}
/* for i = 0 to m-2 do */
if (mo == 0) {
lim = m;
} else {
lim = m - 1;
}
for (i = 0; i < lim; i++) {
err = tweak_uncrypt(ct, pt, T, xts);
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;
}
}
return CRYPT_OK;
}
#endif
/* $Source$ */
/* $Revision$ */
/* $Date$ */
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