*/int xts_decrypt(
const unsigned char *ct, unsigned long ptlen,
unsigned char *pt,
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;
}
}
/* Decrypt the tweak back */
if ((err = cipher_descriptor[xts->cipher].ecb_decrypt(T, tweak, &xts->key2)) != CRYPT_OK) {
return err;
}
return CRYPT_OK;
}
int xts_decrypt(
const uint8_t *ct, unsigned long ptlen,
uint8_t *pt,
const uint8_t *tweak,
symmetric_xts *xts)
{
aesedp_decrypt_ctx *decrypt_ctx = &xts->key1.decrypt;
uint8_t PP[16], CC[16], T[16];
uint32_t i, m, mo, lim;
uint32_t err;
/* check inputs */
if((pt == 0) || (ct == 0)|| (tweak == 0) || (xts == 0)) return 1;
/* 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 , yes - encrypt */
if ((err = aes_encrypt(tweak, T, &xts->key2.encrypt)) != 0) {
return CRYPT_INVALID_KEYSIZE;
}
/* 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, decrypt_ctx);
ct += 16;
pt += 16;
}
/* if ptlen not divide 16 then */
if (mo > 0) {
memcpy(CC, T, 16);
xts_mult_x(CC);
/* PP = tweak decrypt block m-1 */
if ((err = tweak_uncrypt(ct, PP, CC, decrypt_ctx)) != 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, decrypt_ctx)) != CRYPT_OK) {
return err;
}
}
return CRYPT_OK;
}
