142 lines
3.1 KiB
C
142 lines
3.1 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://libtom.org
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*/
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#include "tomcrypt.h"
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/**
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Source donated by Elliptic Semiconductor Inc (www.ellipticsemi.com) to the LibTom Projects
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*/
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#ifdef LTC_XTS_MODE
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static int tweak_uncrypt(const unsigned char *C, unsigned char *P, unsigned char *T, symmetric_xts *xts)
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{
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unsigned long x;
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int err;
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/* tweak encrypt block i */
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#ifdef LTC_FAST
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for (x = 0; x < 16; x += sizeof(LTC_FAST_TYPE)) {
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*((LTC_FAST_TYPE*)&P[x]) = *((LTC_FAST_TYPE*)&C[x]) ^ *((LTC_FAST_TYPE*)&T[x]);
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}
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#else
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for (x = 0; x < 16; x++) {
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P[x] = C[x] ^ T[x];
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}
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#endif
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err = cipher_descriptor[xts->cipher].ecb_decrypt(P, P, &xts->key1);
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#ifdef LTC_FAST
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for (x = 0; x < 16; x += sizeof(LTC_FAST_TYPE)) {
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*((LTC_FAST_TYPE*)&P[x]) ^= *((LTC_FAST_TYPE*)&T[x]);
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}
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#else
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for (x = 0; x < 16; x++) {
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P[x] = P[x] ^ T[x];
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}
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#endif
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/* LFSR the tweak */
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xts_mult_x(T);
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return err;
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}
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/** XTS Decryption
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@param ct [in] Ciphertext
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@param ptlen Length of plaintext (and ciphertext)
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@param pt [out] Plaintext
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@param tweak [in] The 128--bit encryption tweak (e.g. sector number)
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@param xts The XTS structure
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Returns CRYPT_OK upon success
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*/int xts_decrypt(
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const unsigned char *ct, unsigned long ptlen,
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unsigned char *pt,
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const unsigned char *tweak,
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symmetric_xts *xts)
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{
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unsigned char PP[16], CC[16], T[16];
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unsigned long i, m, mo, lim;
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int err;
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/* check inputs */
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LTC_ARGCHK(pt != NULL);
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LTC_ARGCHK(ct != NULL);
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LTC_ARGCHK(tweak != NULL);
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LTC_ARGCHK(xts != NULL);
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/* check if valid */
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if ((err = cipher_is_valid(xts->cipher)) != CRYPT_OK) {
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return err;
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}
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/* get number of blocks */
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m = ptlen >> 4;
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mo = ptlen & 15;
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/* must have at least one full block */
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if (m == 0) {
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return CRYPT_INVALID_ARG;
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}
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/* encrypt the tweak */
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if ((err = cipher_descriptor[xts->cipher].ecb_encrypt(tweak, T, &xts->key2)) != CRYPT_OK) {
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return err;
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}
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/* for i = 0 to m-2 do */
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if (mo == 0) {
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lim = m;
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} else {
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lim = m - 1;
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}
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for (i = 0; i < lim; i++) {
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err = tweak_uncrypt(ct, pt, T, xts);
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ct += 16;
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pt += 16;
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}
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/* if ptlen not divide 16 then */
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if (mo > 0) {
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XMEMCPY(CC, T, 16);
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xts_mult_x(CC);
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/* PP = tweak decrypt block m-1 */
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if ((err = tweak_uncrypt(ct, PP, CC, xts)) != CRYPT_OK) {
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return err;
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}
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/* Pm = first ptlen % 16 bytes of PP */
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for (i = 0; i < mo; i++) {
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CC[i] = ct[16+i];
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pt[16+i] = PP[i];
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}
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for (; i < 16; i++) {
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CC[i] = PP[i];
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}
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/* Pm-1 = Tweak uncrypt CC */
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if ((err = tweak_uncrypt(CC, pt, T, xts)) != CRYPT_OK) {
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return err;
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}
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}
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return CRYPT_OK;
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}
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#endif
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/* $Source$ */
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/* $Revision$ */
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/* $Date$ */
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