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/* rsa-blind.c
*
* RSA blinding. It is used for timing resistant decryption or signing.
*/
/* nettle, low-level cryptographics library
*
* Copyright (C) 2001, 2012 Niels Möller, Nikos Mavrogiannopoulos
*
* 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., 51 Franklin Street, Fifth Floor, Boston,
* MA 02111-1301, USA.
*/
#if HAVE_CONFIG_H
# include "config.h"
#endif
#include "rsa.h"
#include "bignum.h"
/* Blinds the c, by computing c *= r^e (mod n), for a random r. Also
returns the inverse (ri), for use by rsa_unblind. */
void
_rsa_blind (const struct rsa_public_key *pub,
void *random_ctx, nettle_random_func *random,
mpz_t c, mpz_t ri)
{
mpz_t r;
mpz_init(r);
/* c = c*(r^e)
* ri = r^(-1)
*/
do
{
nettle_mpz_random(r, random_ctx, random, pub->n);
/* invert r */
}
while (!mpz_invert (ri, r, pub->n));
/* c = c*(r^e) mod n */
mpz_powm(r, r, pub->e, pub->n);
mpz_mul(c, c, r);
mpz_fdiv_r(c, c, pub->n);
mpz_clear(r);
}
/* c *= ri mod n */
void
_rsa_unblind (const struct rsa_public_key *pub, mpz_t c, const mpz_t ri)
{
mpz_mul(c, c, ri);
mpz_fdiv_r(c, c, pub->n);
}
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