/* rsa-blind.c

RSA blinding. Used for resistance to timing-attacks.

Copyright (C) 2001, 2012 Niels Möller, Nikos Mavrogiannopoulos

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 "rsa.h"

#include "rsa-internal.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_sec(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);

}