pkcs1.c 1.67 KB
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/* pkcs1.c
 *
 * PKCS1 embedding.
 */

/* nettle, low-level cryptographics library
 *
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 * Copyright (C) 2003 Niels Möller
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 *  
 * The nettle library is free software; you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as published by
 * the Free Software Foundation; either version 2.1 of the License, or (at your
 * option) any later version.
 * 
 * The nettle library 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 Lesser General Public
 * License for more details.
 * 
 * You should have received a copy of the GNU Lesser General Public License
 * along with the nettle library; see the file COPYING.LIB.  If not, write to
 * the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
 * MA 02111-1307, USA.
 */

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

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

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#include "pkcs1.h"

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/* Formats the PKCS#1 padding, of the form
 *
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 *   0x00 0x01 0xff ... 0xff 0x00 id ...digest...
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 *
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 * where the 0xff ... 0xff part consists of at least 8 octets. The 
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 * total size equals the octet size of n.
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 */
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uint8_t *
pkcs1_signature_prefix(unsigned key_size,
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		       uint8_t *buffer,
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		       unsigned id_size,
		       const uint8_t *id,
		       unsigned digest_size)
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{
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  unsigned j;
  
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  if (key_size < 11 + id_size + digest_size)
    return NULL;
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  j = key_size - digest_size - id_size;
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  memcpy (buffer + j, id, id_size);
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  buffer[0] = 0;
  buffer[1] = 1;
  buffer[j-1] = 0;
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  assert(j >= 11);
  memset(buffer + 2, 0xff, j - 3);
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  return buffer + j + id_size;
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}