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/* rsa_decrypt.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_decrypt(struct rsa_private_key *key,
unsigned *length, uint8_t *message,
const mpz_t gibberish)
{
uint8_t *em;
uint8_t *terminator;
unsigned padding;
unsigned message_length;
mpz_t m;
mpz_init(m);
rsa_compute_root(key, m, gibberish);
em = alloca(key->size);
nettle_mpz_get_str_256(key->size, em, m);
mpz_clear(m);
/* Check format */
if (em[0] || em[1] != 2)
return 0;
terminator = memchr(em + 2, 0, key->size - 2);
if (!terminator)
return 0;
padding = terminator - (em + 2);
if (padding < 8)
return 0;
message_length = key->size - 3 - padding;
if (*length < message_length)
return 0;
memcpy(message, terminator + 1, message_length);
*length = message_length;
return 1;
}