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/*
* Copyright (C) 2000, 2001, 2002, 2003, 2004, 2005 Free Software Foundation
*
* Author: Nikos Mavroyanopoulos
*
* This file is part of GNUTLS.
*
* The GNUTLS 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.
*
* This 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 this library; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307
* USA
*
*/
#include <gnutls_int.h>
#include <gnutls_errors.h>
/*
--Example--
you: X = g ^ x mod p;
peer:Y = g ^ y mod p;
your_key = Y ^ x mod p;
his_key = X ^ y mod p;
// generate our secret and the public value (X) for it
X = gnutls_calc_dh_secret(&x, g, p);
// now we can calculate the shared secret
key = gnutls_calc_dh_key(Y, x, g, p);
_gnutls_mpi_release(x);
_gnutls_mpi_release(g);
*/
#define MAX_BITS 18000
/* returns the public value (X), and the secret (ret_x).
*/
mpi_t gnutls_calc_dh_secret(mpi_t * ret_x, mpi_t g, mpi_t prime)
{
mpi_t e, x;
int x_size = _gnutls_mpi_get_nbits(prime) - 1;
/* The size of the secret key is less than
* prime/2
*/
if (x_size > MAX_BITS || x_size <= 0) {
gnutls_assert();
return NULL;
}
x = _gnutls_mpi_new(x_size);
if (x == NULL) {
gnutls_assert();
if (ret_x)
*ret_x = NULL;
return NULL;
}
/* FIXME: (x_size/8)*8 is there to overcome a bug in libgcrypt
* which does not really check the bits given but the bytes.
*/
do {
_gnutls_mpi_randomize(x, (x_size / 8) * 8, GCRY_STRONG_RANDOM);
/* Check whether x is zero.
*/
} while( _gnutls_mpi_cmp_ui( x, 0)==0);
e = _gnutls_mpi_alloc_like(prime);
if (e == NULL) {
gnutls_assert();
if (ret_x)
*ret_x = NULL;
_gnutls_mpi_release(&x);
return NULL;
}
_gnutls_mpi_powm(e, g, x, prime);
if (ret_x)
*ret_x = x;
else
_gnutls_mpi_release(&x);
return e;
}
mpi_t gnutls_calc_dh_key(mpi_t f, mpi_t x, mpi_t prime)
{
mpi_t k;
int bits;
bits = _gnutls_mpi_get_nbits(prime);
if (bits <= 0 || bits > MAX_BITS) {
gnutls_assert();
return NULL;
}
k = _gnutls_mpi_alloc_like(prime);
if (k == NULL)
return NULL;
_gnutls_mpi_powm(k, f, x, prime);
return k;
}
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