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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
*/
/**
@file ocb_decrypt.c
OCB implementation, decrypt data, by Tom St Denis
*/
#include "tomcrypt.h"
#ifdef LTC_OCB_MODE
/**
Decrypt a block with OCB.
@param ocb The OCB state
@param ct The ciphertext (length of the block size of the block cipher)
@param pt [out] The plaintext (length of ct)
@return CRYPT_OK if successful
*/
int ocb_decrypt(ocb_state *ocb, const unsigned char *ct, unsigned char *pt)
{
unsigned char Z[MAXBLOCKSIZE], tmp[MAXBLOCKSIZE];
int err, x;
LTC_ARGCHK(ocb != NULL);
LTC_ARGCHK(pt != NULL);
LTC_ARGCHK(ct != NULL);
/* check if valid cipher */
if ((err = cipher_is_valid(ocb->cipher)) != CRYPT_OK) {
return err;
}
LTC_ARGCHK(cipher_descriptor[ocb->cipher].ecb_decrypt != NULL);
/* check length */
if (ocb->block_len != cipher_descriptor[ocb->cipher].block_length) {
return CRYPT_INVALID_ARG;
}
/* Get Z[i] value */
ocb_shift_xor(ocb, Z);
/* xor ct in, encrypt, xor Z out */
for (x = 0; x < ocb->block_len; x++) {
tmp[x] = ct[x] ^ Z[x];
}
if ((err = cipher_descriptor[ocb->cipher].ecb_decrypt(tmp, pt, &ocb->key)) != CRYPT_OK) {
return err;
}
for (x = 0; x < ocb->block_len; x++) {
pt[x] ^= Z[x];
}
/* compute checksum */
for (x = 0; x < ocb->block_len; x++) {
ocb->checksum[x] ^= pt[x];
}
#ifdef LTC_CLEAN_STACK
zeromem(Z, sizeof(Z));
zeromem(tmp, sizeof(tmp));
#endif
return CRYPT_OK;
}
#endif
/* $Source$ */
/* $Revision$ */
/* $Date$ */
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