ctr.c

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/**
 * @file ctr.c
 * @brief Counter(CTR) mode
 *
 * @section License
 *
 * SPDX-License-Identifier: GPL-2.0-or-later
 *
 * Copyright (C) 2010-2026 Oryx Embedded SARL. All rights reserved.
 *
 * This file is part of CycloneCRYPTO Open.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2
 * of the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 *
 * @section Description
 *
 * The Counter (CTR) mode is a confidentiality mode that features the application
 * of the forward cipher to a set of input blocks, called counters, to produce
 * a sequence of output blocks that are exclusive-ORed with the plaintext to
 * produce the ciphertext, and vice versa. Refer to SP 800-38A for more details
 *
 * @author Oryx Embedded SARL (www.oryx-embedded.com)
 * @version 2.6.6
 **/
 
//Switch to the appropriate trace level
#define TRACE_LEVEL CRYPTO_TRACE_LEVEL
 
//Dependencies
#include "core/crypto.h"
#include "cipher_modes/ctr.h"
#include "debug.h"
 
//Check crypto library configuration
#if (CTR_SUPPORT == ENABLED)
 
 
/**
 * @brief CTR encryption
 * @param[in] cipher Cipher algorithm
 * @param[in] context Cipher algorithm context
 * @param[in] m Size in bits of the specific part of the block to be incremented
 * @param[in,out] t Initial counter block
 * @param[in] p Plaintext to be encrypted
 * @param[out] c Ciphertext resulting from the encryption
 * @param[in] length Total number of data bytes to be encrypted
 * @return Error code
 **/
 
__weak_func error_t ctrEncrypt(const CipherAlgo *cipher, void *context, uint_t m,
   uint8_t *t, const uint8_t *p, uint8_t *c, size_t length)
{
   size_t i;
   size_t n;
   uint8_t o[16];
 
   //The parameter must be a multiple of 8
   if((m % 8) != 0)
      return ERROR_INVALID_PARAMETER;
 
   //Determine the size, in bytes, of the specific part of the block
   //to be incremented
   m = m / 8;
 
   //Check the resulting value
   if(m > cipher->blockSize)
      return ERROR_INVALID_PARAMETER;
 
   //Process plaintext
   while(length > 0)
   {
      //CTR mode operates in a block-by-block fashion
      n = MIN(length, cipher->blockSize);
 
      //Compute O(j) = CIPH(T(j))
      cipher->encryptBlock(context, t, o);
 
      //Compute C(j) = P(j) XOR T(j)
      for(i = 0; i < n; i++)
      {
         c[i] = p[i] ^ o[i];
      }
 
      //Complete block?
      if(n == cipher->blockSize)
      {
         //Standard incrementing function
         ctrIncBlock(t, 1, cipher->blockSize, m);
      }
 
      //Next block
      p += n;
      c += n;
      length -= n;
   }
 
   //Successful encryption
   return NO_ERROR;
}
 
 
/**
 * @brief CTR decryption
 * @param[in] cipher Cipher algorithm
 * @param[in] context Cipher algorithm context
 * @param[in] m Size in bits of the specific part of the block to be incremented
 * @param[in,out] t Initial counter block
 * @param[in] c Ciphertext to be decrypted
 * @param[out] p Plaintext resulting from the decryption
 * @param[in] length Total number of data bytes to be decrypted
 * @return Error code
 **/
 
error_t ctrDecrypt(const CipherAlgo *cipher, void *context, uint_t m,
   uint8_t *t, const uint8_t *c, uint8_t *p, size_t length)
{
   //Decryption is the same the as encryption with P and C interchanged
   return ctrEncrypt(cipher, context, m, t, c, p, length);
}
 
 
/**
 * @brief Increment counter block
 * @param[in,out] ctr Pointer to the counter block
 * @param[in] inc Value of the increment
 * @param[in] blockSize Size of the block, in bytes
 * @param[in] m Size of the specific part of the block to be incremented
 **/
 
void ctrIncBlock(uint8_t *ctr, uint32_t inc, size_t blockSize, size_t m)
{
   size_t i;
   uint32_t temp;
 
   //The function increments the right-most bytes of the block. The remaining
   //left-most bytes remain unchanged
   for(temp = inc, i = 1; i <= m; i++)
   {
      //Increment the current byte and propagate the carry
      temp += ctr[blockSize - i];
      ctr[blockSize - i] = temp & 0xFF;
      temp >>= 8;
   }
}
 
#endif