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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@iahu.ca, http://libtomcrypt.org
*/
#include "mycrypt.h"
#ifdef CBC
int cbc_decrypt(const unsigned char *ct, unsigned char *pt, symmetric_CBC *cbc)
{
int x, err;
unsigned char tmp[MAXBLOCKSIZE], tmp2[MAXBLOCKSIZE];
_ARGCHK(pt != NULL);
_ARGCHK(ct != NULL);
_ARGCHK(cbc != NULL);
/* decrypt the block from ct into tmp */
if ((err = cipher_is_valid(cbc->cipher)) != CRYPT_OK) {
return err;
}
_ARGCHK(cipher_descriptor[cbc->cipher].ecb_decrypt != NULL);
/* is blocklen valid? */
if (cbc->blocklen < 0 || cbc->blocklen > (int)sizeof(cbc->IV)) {
return CRYPT_INVALID_ARG;
}
/* decrypt and xor IV against the plaintext of the previous step */
cipher_descriptor[cbc->cipher].ecb_decrypt(ct, tmp, &cbc->key);
for (x = 0; x < cbc->blocklen; x++) {
/* copy CT in case ct == pt */
tmp2[x] = ct[x];
/* actually decrypt the byte */
pt[x] = tmp[x] ^ cbc->IV[x];
}
/* replace IV with this current ciphertext */
for (x = 0; x < cbc->blocklen; x++) {
cbc->IV[x] = tmp2[x];
}
#ifdef CLEAN_STACK
zeromem(tmp, sizeof(tmp));
zeromem(tmp2, sizeof(tmp2));
#endif
return CRYPT_OK;
}
#endif
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