lib/gnutls_dh.c

   1 /*
   2  * Copyright (C) 2000, 2001, 2002, 2003, 2004, 2005 Free Software Foundation
   3  *
   4  * Author: Nikos Mavrogiannopoulos
   5  *
   6  * This file is part of GNUTLS.
   7  *
   8  * The GNUTLS library is free software; you can redistribute it and/or
   9  * modify it under the terms of the GNU Lesser General Public License
  10  * as published by the Free Software Foundation; either version 2.1 of
  11  * the License, or (at your option) any later version.
  12  *
  13  * This library is distributed in the hope that it will be useful, but
  14  * WITHOUT ANY WARRANTY; without even the implied warranty of
  15  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  16  * Lesser General Public License for more details.
  17  *
  18  * You should have received a copy of the GNU Lesser General Public
  19  * License along with this library; if not, write to the Free Software
  20  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301,
  21  * USA
  22  *
  23  */
  24
  25 #include <gnutls_int.h>
  26 #include <gnutls_errors.h>
  27
  28
  29 /*
  30         --Example--
  31         you: X = g ^ x mod p;
  32         peer:Y = g ^ y mod p;
  33
  34         your_key = Y ^ x mod p;
  35         his_key  = X ^ y mod p;
  36
  37 //      generate our secret and the public value (X) for it
  38         X = gnutls_calc_dh_secret(&x, g, p);
  39 //      now we can calculate the shared secret
  40         key = gnutls_calc_dh_key(Y, x, g, p);
  41         _gnutls_mpi_release(x);
  42         _gnutls_mpi_release(g);
  43 */
  44
  45 #define MAX_BITS 18000
  46
  47 /* returns the public value (X), and the secret (ret_x).
  48  */
  49 mpi_t
  50 gnutls_calc_dh_secret (mpi_t * ret_x, mpi_t g, mpi_t prime)
  51 {
  52   mpi_t e, x;
  53   int x_size = _gnutls_mpi_get_nbits (prime) - 1;
  54   /* The size of the secret key is less than
  55    * prime/2
  56    */
  57
  58   if (x_size > MAX_BITS || x_size <= 0)
  59     {
  60       gnutls_assert ();
  61       return NULL;
  62     }
  63
  64   x = _gnutls_mpi_new (x_size);
  65   if (x == NULL)
  66     {
  67       gnutls_assert ();
  68       if (ret_x)
  69         *ret_x = NULL;
  70
  71       return NULL;
  72     }
  73
  74   /* FIXME: (x_size/8)*8 is there to overcome a bug in libgcrypt
  75    * which does not really check the bits given but the bytes.
  76    */
  77   do
  78     {
  79       _gnutls_mpi_randomize (x, (x_size / 8) * 8, GCRY_STRONG_RANDOM);
  80       /* Check whether x is zero.
  81        */
  82     }
  83   while (_gnutls_mpi_cmp_ui (x, 0) == 0);
  84
  85   e = _gnutls_mpi_alloc_like (prime);
  86   if (e == NULL)
  87     {
  88       gnutls_assert ();
  89       if (ret_x)
  90         *ret_x = NULL;
  91
  92       _gnutls_mpi_release (&x);
  93       return NULL;
  94     }
  95
  96   _gnutls_mpi_powm (e, g, x, prime);
  97
  98   if (ret_x)
  99     *ret_x = x;
 100   else
 101     _gnutls_mpi_release (&x);
 102   return e;
 103 }
 104
 105
 106 mpi_t
 107 gnutls_calc_dh_key (mpi_t f, mpi_t x, mpi_t prime)
 108 {
 109   mpi_t k;
 110   int bits;
 111
 112   bits = _gnutls_mpi_get_nbits (prime);
 113   if (bits <= 0 || bits > MAX_BITS)
 114     {
 115       gnutls_assert ();
 116       return NULL;
 117     }
 118
 119   k = _gnutls_mpi_alloc_like (prime);
 120   if (k == NULL)
 121     return NULL;
 122   _gnutls_mpi_powm (k, f, x, prime);
 123   return k;
 124 }
 125
 126 /*-
 127   * _gnutls_get_dh_params - Returns the DH parameters pointer
 128   * @dh_params: is an DH parameters structure, or NULL.
 129   * @func: is a callback function to receive the parameters or NULL.
 130   * @session: a gnutls session.
 131   *
 132   * This function will return the dh parameters pointer.
 133   *
 134   -*/
 135 gnutls_dh_params_t
 136 _gnutls_get_dh_params (gnutls_dh_params_t dh_params,
 137                        gnutls_params_function * func,
 138                        gnutls_session_t session)
 139 {
 140   gnutls_params_st params;
 141   int ret;
 142
 143   /* if cached return the cached */
 144   if (session->internals.params.dh_params)
 145     return session->internals.params.dh_params;
 146
 147   if (dh_params)
 148     {
 149       session->internals.params.dh_params = dh_params;
 150     }
 151   else if (func)
 152     {
 153       ret = func (session, GNUTLS_PARAMS_DH, &params);
 154       if (ret == 0 && params.type == GNUTLS_PARAMS_DH)
 155         {
 156           session->internals.params.dh_params = params.params.dh;
 157           session->internals.params.free_dh_params = params.deinit;
 158         }
 159     }
 160
 161   return session->internals.params.dh_params;
 162 }