lib/gnutls_dh.c

   1 /*
   2  * Copyright (C) 2000, 2001, 2002, 2003, 2004, 2005, 2008, 2010 Free
   3  * Software Foundation, Inc.
   4  *
   5  * Author: Nikos Mavrogiannopoulos
   6  *
   7  * This file is part of GnuTLS.
   8  *
   9  * The GnuTLS is free software; you can redistribute it and/or
  10  * modify it under the terms of the GNU Lesser General Public License
  11  * as published by the Free Software Foundation; either version 2.1 of
  12  * the License, or (at your option) any later version.
  13  *
  14  * This library is distributed in the hope that it will be useful, but
  15  * WITHOUT ANY WARRANTY; without even the implied warranty of
  16  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  17  * Lesser General Public License for more details.
  18  *
  19  * You should have received a copy of the GNU Lesser General Public
  20  * License along with this library; if not, write to the Free Software
  21  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301,
  22  * USA
  23  *
  24  */
  25
  26 #include <gnutls_int.h>
  27 #include <gnutls_errors.h>
  28 #include <gnutls_dh.h>
  29
  30
  31 /*
  32         --Example--
  33         you: X = g ^ x mod p;
  34         peer:Y = g ^ y mod p;
  35
  36         your_key = Y ^ x mod p;
  37         his_key  = X ^ y mod p;
  38
  39 //      generate our secret and the public value (X) for it
  40         X = gnutls_calc_dh_secret(&x, g, p);
  41 //      now we can calculate the shared secret
  42         key = gnutls_calc_dh_key(Y, x, g, p);
  43         _gnutls_mpi_release(x);
  44         _gnutls_mpi_release(g);
  45 */
  46
  47 #define MAX_BITS 18000
  48
  49 /* returns the public value (X), and the secret (ret_x).
  50  */
  51 bigint_t
  52 gnutls_calc_dh_secret (bigint_t * ret_x, bigint_t g, bigint_t prime)
  53 {
  54   bigint_t e, x = NULL;
  55   int x_size = _gnutls_mpi_get_nbits (prime) - 1;
  56   /* The size of the secret key is less than
  57    * prime/2
  58    */
  59
  60   if (x_size > MAX_BITS || x_size <= 0)
  61     {
  62       gnutls_assert ();
  63       return NULL;
  64     }
  65
  66   x = _gnutls_mpi_randomize (NULL, x_size, GNUTLS_RND_RANDOM);
  67   if (x == NULL)
  68     {
  69       gnutls_assert ();
  70       return NULL;
  71     }
  72
  73   e = _gnutls_mpi_alloc_like (prime);
  74   if (e == NULL)
  75     {
  76       gnutls_assert ();
  77       if (ret_x)
  78         *ret_x = NULL;
  79
  80       _gnutls_mpi_release (&x);
  81       return NULL;
  82     }
  83
  84   _gnutls_mpi_powm (e, g, x, prime);
  85
  86   if (ret_x)
  87     *ret_x = x;
  88   else
  89     _gnutls_mpi_release (&x);
  90   return e;
  91 }
  92
  93
  94 bigint_t
  95 gnutls_calc_dh_key (bigint_t f, bigint_t x, bigint_t prime)
  96 {
  97   bigint_t k;
  98   int bits;
  99
 100   bits = _gnutls_mpi_get_nbits (prime);
 101   if (bits <= 0 || bits > MAX_BITS)
 102     {
 103       gnutls_assert ();
 104       return NULL;
 105     }
 106
 107   k = _gnutls_mpi_alloc_like (prime);
 108   if (k == NULL)
 109     return NULL;
 110   _gnutls_mpi_powm (k, f, x, prime);
 111   return k;
 112 }
 113
 114 /*-
 115  * _gnutls_get_dh_params - Returns the DH parameters pointer
 116  * @dh_params: is an DH parameters structure, or NULL.
 117  * @func: is a callback function to receive the parameters or NULL.
 118  * @session: a gnutls session.
 119  *
 120  * This function will return the dh parameters pointer.
 121  -*/
 122 gnutls_dh_params_t
 123 _gnutls_get_dh_params (gnutls_dh_params_t dh_params,
 124                        gnutls_params_function * func,
 125                        gnutls_session_t session)
 126 {
 127   gnutls_params_st params;
 128   int ret;
 129
 130   /* if cached return the cached */
 131   if (session->internals.params.dh_params)
 132     return session->internals.params.dh_params;
 133
 134   if (dh_params)
 135     {
 136       session->internals.params.dh_params = dh_params;
 137     }
 138   else if (func)
 139     {
 140       ret = func (session, GNUTLS_PARAMS_DH, &params);
 141       if (ret == 0 && params.type == GNUTLS_PARAMS_DH)
 142         {
 143           session->internals.params.dh_params = params.params.dh;
 144           session->internals.params.free_dh_params = params.deinit;
 145         }
 146     }
 147
 148   return session->internals.params.dh_params;
 149 }