/* 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.

 */

#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 initialization 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;

   }

   /* ctrlen == counter width */

   ctr->ctrlen   = (ctr_mode & 255) ? (ctr_mode & 255) : cipher_descriptor[cipher].block_length;

   if (ctr->ctrlen > cipher_descriptor[cipher].block_length) {

      return CRYPT_INVALID_ARG;

   }

   if ((ctr_mode & 0x1000) == CTR_COUNTER_BIG_ENDIAN) {

      ctr->ctrlen = cipher_descriptor[cipher].block_length - ctr->ctrlen;

   }

   /* 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 & 0x1000;

   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->ctrlen; 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 >= ctr->ctrlen; 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

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/* git commit:  $Format:%H$ */

/* commit time: $Format:%ai$ */