rsa-encrypt.c

/* rsa_encrypt.c
 *
 * The RSA publickey algorithm. PKCS#1 encryption.
 */

/* nettle, low-level cryptographics library
 *
 * Copyright (C) 2001 Niels Mller
 *  
 * 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.
 */

#include "rsa.h"

#include "bignum.h"

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

int
rsa_encrypt(struct rsa_public_key *key,
	    /* For padding */
	    void *random_ctx, nettle_random_func random,
	    unsigned length, const uint8_t *message,
	    mpz_t gibbberish)
{
  uint8_t *em;
  unsigned padding;
  unsigned i;
  
  /* The message is encoded as a string of the same length as the
   * modulo n, of the form
   *
   *   00 02 pad 00 message
   *
   * where padding should be at least 8 pseudorandomly generated
   * *non-zero* octets. */
     
  if (length + 11 < key->size)
    /* Message too long for this key. */
    return 0;

  /* At least 8 bits of random padding */
  padding = key->size - length - 3;
  assert(padding >= 8);
  
  em = alloca(key->size - 1);
  em[0] = 2;

  random(random_ctx, padding, em + 1);

  /* Replace 0-octets with 1 */
  for (i = 0; i<padding; i++)
    if (!em[i+1])
      em[i+1] = 1;
  
  memcpy(em + 1 + padding, message, length);

  nettle_mpz_set_str_256(gibbberish, key->size - 1, em);
  mpz_powm(gibbberish, gibbberish, key->e, key->n);

  return 1;  
}