File: cbc.c

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/* cbc.c

   Cipher block chaining mode.

   Copyright (C) 2001, 2011 Niels Möller

   This file is part of GNU Nettle.

   GNU Nettle is free software: you can redistribute it and/or
   modify it under the terms of either:

     * the GNU Lesser General Public License as published by the Free
       Software Foundation; either version 3 of the License, or (at your
       option) any later version.

   or

     * 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.

   or both in parallel, as here.

   GNU Nettle 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 copies of the GNU General Public License and
   the GNU Lesser General Public License along with this program.  If
   not, see http://www.gnu.org/licenses/.
*/

#if HAVE_CONFIG_H
# include "config.h"
#endif

#include <assert.h>
#include <stdlib.h>
#include <string.h>

#include "cbc.h"

#include "memxor.h"
#include "nettle-internal.h"

void
cbc_encrypt(const void *ctx, nettle_cipher_func *f,
	    size_t block_size, uint8_t *iv,
	    size_t length, uint8_t *dst,
	    const uint8_t *src)
{
  assert(!(length % block_size));

  for ( ; length; length -= block_size, src += block_size, dst += block_size)
    {
      memxor(iv, src, block_size);
      f(ctx, block_size, dst, iv);
      memcpy(iv, dst, block_size);
    }
}

/* Don't allocate any more space than this on the stack */
#define CBC_BUFFER_LIMIT 512

void
cbc_decrypt(const void *ctx, nettle_cipher_func *f,
	    size_t block_size, uint8_t *iv,
	    size_t length, uint8_t *dst,
	    const uint8_t *src)
{
  assert(!(length % block_size));

  if (!length)
    return;

  if (src != dst)
    {
      /* Decrypt in ECB mode */
      f(ctx, length, dst, src);

      /* XOR the cryptotext, shifted one block */
      memxor(dst, iv, block_size);
      memxor(dst + block_size, src, length - block_size);
      memcpy(iv, src + length - block_size, block_size);
    }

  else
    {
      /* For in-place CBC, we decrypt into a temporary buffer of size
       * at most CBC_BUFFER_LIMIT, and process that amount of data at
       * a time. */
      
      /* NOTE: We assume that block_size <= CBC_BUFFER_LIMIT, and we
	 depend on memxor3 working from the end of the area, allowing
	 certain overlapping operands. */ 

      TMP_DECL(buffer, uint8_t, CBC_BUFFER_LIMIT);
      TMP_DECL(initial_iv, uint8_t, NETTLE_MAX_CIPHER_BLOCK_SIZE);

      size_t buffer_size;

      if (length <= CBC_BUFFER_LIMIT)
	buffer_size = length;
      else
	buffer_size
	  = CBC_BUFFER_LIMIT - (CBC_BUFFER_LIMIT % block_size);

      TMP_ALLOC(buffer, buffer_size);
      TMP_ALLOC(initial_iv, block_size);

      for ( ; length > buffer_size;
	    length -= buffer_size, src += buffer_size, dst += buffer_size)
	{
	  f(ctx, buffer_size, buffer, src);
	  memcpy(initial_iv, iv, block_size);
	  memcpy(iv, src + buffer_size - block_size, block_size);
	  memxor3(dst + block_size, buffer + block_size, src,
		  buffer_size - block_size);
	  memxor3(dst, buffer, initial_iv, block_size);
	}

      f(ctx, length, buffer, src);
      memcpy(initial_iv, iv, block_size);
      /* Copies last block */
      memcpy(iv, src + length - block_size, block_size);
      /* Writes all but first block, reads all but last block. */
      memxor3(dst + block_size, buffer + block_size, src,
	      length - block_size);
      /* Writes first block. */
      memxor3(dst, buffer, initial_iv, block_size);
    }
}

#if 0
#include "twofish.h"
#include "aes.h"

static void foo(void)
{
  struct CBC_CTX(struct twofish_ctx, TWOFISH_BLOCK_SIZE) ctx;
  uint8_t src[TWOFISH_BLOCK_SIZE];
  uint8_t dst[TWOFISH_BLOCK_SIZE];
  
  CBC_ENCRYPT(&ctx, twofish_encrypt, TWOFISH_BLOCK_SIZE, dst, src);

  /* Should result in a warning */
  CBC_ENCRYPT(&ctx, aes_encrypt, TWOFISH_BLOCK_SIZE, dst, src);
  
}

static void foo2(void)
{
  struct twofish_ctx ctx;
  uint8_t iv[TWOFISH_BLOCK_SIZE];
  uint8_t src[TWOFISH_BLOCK_SIZE];
  uint8_t dst[TWOFISH_BLOCK_SIZE];
  
  CBC_ENCRYPT2(&ctx, twofish_encrypt, TWOFISH_BLOCK_SIZE, iv, TWOFISH_BLOCK_SIZE, dst, src);
  /* Should result in a warning */
  CBC_ENCRYPT2(&ctx, aes_encrypt, TWOFISH_BLOCK_SIZE, iv, TWOFISH_BLOCK_SIZE, dst, src);
}

#endif