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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://libtomcrypt.com
*/
#include "tomcrypt.h"
/**
@file ctr_start.c
CTR implementation, start chain, Tom St Denis
*/
#ifdef LTC_CTR_MODE
/**
Initialize a CTR context
@param cipher The index of the cipher desired
@param IV The initial vector
@param key The secret key
@param keylen The length of the secret key (octets)
@param num_rounds Number of rounds in the cipher desired (0 for default)
@param ctr_mode The counter mode (CTR_COUNTER_LITTLE_ENDIAN or CTR_COUNTER_BIG_ENDIAN)
@param ctr The CTR state to initialize
@return CRYPT_OK if successful
*/
int ctr_start( int cipher,
const unsigned char *IV,
const unsigned char *key, int keylen,
int num_rounds, int ctr_mode,
symmetric_CTR *ctr)
{
int x, err;
LTC_ARGCHK(IV != NULL);
LTC_ARGCHK(key != NULL);
LTC_ARGCHK(ctr != NULL);
/* bad param? */
if ((err = cipher_is_valid(cipher)) != CRYPT_OK) {
return err;
}
/* setup cipher */
if ((err = cipher_descriptor[cipher].setup(key, keylen, num_rounds, &ctr->key)) != CRYPT_OK) {
return err;
}
/* copy ctr */
ctr->blocklen = cipher_descriptor[cipher].block_length;
ctr->cipher = cipher;
ctr->padlen = 0;
ctr->mode = ctr_mode & 1;
for (x = 0; x < ctr->blocklen; x++) {
ctr->ctr[x] = IV[x];
}
if (ctr_mode & LTC_CTR_RFC3686) {
/* increment the IV as per RFC 3686 */
if (ctr->mode == CTR_COUNTER_LITTLE_ENDIAN) {
/* little-endian */
for (x = 0; x < ctr->blocklen; x++) {
ctr->ctr[x] = (ctr->ctr[x] + (unsigned char)1) & (unsigned char)255;
if (ctr->ctr[x] != (unsigned char)0) {
break;
}
}
} else {
/* big-endian */
for (x = ctr->blocklen-1; x >= 0; x--) {
ctr->ctr[x] = (ctr->ctr[x] + (unsigned char)1) & (unsigned char)255;
if (ctr->ctr[x] != (unsigned char)0) {
break;
}
}
}
}
return cipher_descriptor[ctr->cipher].ecb_encrypt(ctr->ctr, ctr->pad, &ctr->key);
}
#endif
/* $Source: /cvs/libtom/libtomcrypt/src/modes/ctr/ctr_start.c,v $ */
/* $Revision: 1.11 $ */
/* $Date: 2006/11/05 01:46:35 $ */
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