git.distorted.org.uk Git - u/mdw/catacomb/blob - dh-check.c

   1 /* -*-c-*-
   2  *
   3  * $Id: dh-check.c,v 1.2 2001/06/16 12:56:38 mdw Exp $
   4  *
   5  * Checks Diffie-Hellman group parameters
   6  *
   7  * (c) 2001 Straylight/Edgeware
   8  */
   9
  10 /*----- Licensing notice --------------------------------------------------*
  11  *
  12  * This file is part of Catacomb.
  13  *
  14  * Catacomb is free software; you can redistribute it and/or modify
  15  * it under the terms of the GNU Library General Public License as
  16  * published by the Free Software Foundation; either version 2 of the
  17  * License, or (at your option) any later version.
  18  *
  19  * Catacomb is distributed in the hope that it will be useful,
  20  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  21  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  22  * GNU Library General Public License for more details.
  23  *
  24  * You should have received a copy of the GNU Library General Public
  25  * License along with Catacomb; if not, write to the Free
  26  * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
  27  * MA 02111-1307, USA.
  28  */
  29
  30 /*----- Revision history --------------------------------------------------*
  31  *
  32  * $Log: dh-check.c,v $
  33  * Revision 1.2  2001/06/16 12:56:38  mdw
  34  * Fixes for interface change to @mpmont_expr@ and @mpmont_mexpr@.
  35  *
  36  * Revision 1.1  2001/02/03 16:08:24  mdw
  37  * Add consistency checking for public keys.
  38  *
  39  */
  40
  41 /*----- Header files ------------------------------------------------------*/
  42
  43 #include <mLib/dstr.h>
  44
  45 #include "dh.h"
  46 #include "keycheck.h"
  47 #include "mp.h"
  48 #include "mpmont.h"
  49 #include "mpmul.h"
  50
  51 /*----- Main code ---------------------------------------------------------*/
  52
  53 /* --- @dh_checkparam@ --- *
  54  *
  55  * Arguments:   @keycheck *kc@ = keycheck state
  56  *              @const dh_param *dp@ = pointer to the parameter set
  57  *              @mp **v@ = optional vector of factors
  58  *              @size_t n@ = size of vector
  59  *
  60  * Returns:     Zero if all OK, or return status from function.
  61  *
  62  * Use:         Checks a set of Diffie-Hellman parameters for consistency and
  63  *              security.
  64  */
  65
  66 int dh_checkparam(keycheck *kc, const dh_param *dp, mp **v, size_t n)
  67 {
  68   int rc = 0;
  69   mpmont mm;
  70   mp *pm1 = MP_NEW;
  71   mp *q = MP_NEW;
  72   mp *x;
  73   mpmul mu;
  74   size_t i;
  75
  76   /* --- Check that the numbers which are supposed to be prime are --- */
  77
  78   if ((!v && keycheck_prime(kc, KCSEV_WARN, dp->q, "q")) ||
  79       keycheck_prime(kc, KCSEV_ERR, dp->p, "p"))
  80     goto fail;
  81
  82   /* --- Ensure that %$q$% is a sensible choice of number --- */
  83
  84   pm1 = mp_sub(pm1, dp->p, MP_ONE);
  85   mp_div(0, &q, pm1, dp->q);
  86   if (!mp_eq(q, MP_ZERO) &&
  87       keycheck_report(kc, KCSEV_ERR, "q not a factor of p - 1"))
  88     goto fail;
  89
  90   /* --- Check that %$g$% is actually right --- *
  91    *
  92    * This isn't perfect.  If %$q$% is composite and we don't have the factors
  93    * of %$p - 1$% then the order of %$g$% may be some factor of %$q$% which
  94    * we can't find.  (If we do have the factors, we check them all lower
  95    * down.)  We do strip out powers of two from %$q$% before testing, though.
  96    */
  97
  98   if ((mp_eq(dp->g, MP_ONE) || mp_eq(dp->g, pm1)) &&
  99       keycheck_report(kc, KCSEV_ERR, "g is degenerate (+/-1 mod p)"))
 100     goto fail;
 101   q = mp_odd(q, dp->q, &i);
 102   mpmont_create(&mm, dp->p);
 103   x = mpmont_mul(&mm, MP_NEW, dp->g, mm.r2);
 104   q = mpmont_expr(&mm, q, x, q);
 105   mp_drop(x);
 106   do {
 107     if (mp_eq(q, mm.r) != !i) {
 108       if (keycheck_report(kc, KCSEV_ERR, "order of g != q")) {
 109         mpmont_destroy(&mm);
 110         goto fail;
 111       }
 112       break;
 113     }
 114     if (i) {
 115       q = mp_sqr(q, q);
 116       q = mpmont_reduce(&mm, q, q);
 117     }
 118   } while (i--);
 119
 120   /* --- Check Lim-Lee primes more carefully --- *
 121    *
 122    * In this case, we really can be sure whether the order of %$g$% is
 123    * actually %$q$% as advertised.  Also ensure that the individual primes
 124    * are really prime, and that their product is correct.
 125    */
 126
 127   if (!v)
 128     mpmont_destroy(&mm);
 129   else {
 130     dstr d = DSTR_INIT;
 131     mp *r = MP_NEW;
 132
 133     mpmul_init(&mu);
 134     for (i = 0; i < n; i++) {
 135       DRESET(&d);
 136       dstr_putf(&d, "factor f_%lu of p", (unsigned long)i);
 137       if ((rc = keycheck_prime(kc, KCSEV_ERR, v[i], d.buf)) != 0)
 138         break;
 139       mp_div(&q, &r, dp->q, v[i]);
 140       if (mp_eq(r, MP_ZERO) && !mp_eq(q, MP_ONE)) {
 141         q = mpmont_exp(&mm, q, dp->g, q);
 142         if (mp_eq(q, MP_ONE) &&
 143             (rc = keycheck_report(kc, KCSEV_ERR,
 144                                   "order of g is proper divisor of q")) != 0)
 145           break;
 146       }
 147       mpmul_add(&mu, v[i]);
 148     }
 149     mp_drop(q);
 150     mp_drop(r);
 151     q = mpmul_done(&mu);
 152     mpmont_destroy(&mm);
 153     dstr_destroy(&d);
 154     if (rc)
 155       goto fail;
 156     q = mp_lsl(q, q, 1);
 157     if (!mp_eq(q, pm1) &&
 158         keycheck_report(kc, KCSEV_ERR, "product of f_i != (p - 1)/2"))
 159       goto fail;
 160   }
 161
 162   /* --- Finally, check the key sizes --- */
 163
 164   if ((mp_bits(dp->p) < 1024 &&
 165        keycheck_report(kc, KCSEV_WARN,
 166                        "p too small to resist index calculus attacks")) ||
 167       (mp_bits(dp->q) < 160 &&
 168        keycheck_report(kc, KCSEV_WARN,
 169                        "q too small to resist collision-finding attacks")))
 170     goto fail;
 171
 172   /* --- Done --- */
 173
 174 tidy:
 175   mp_drop(q);
 176   mp_drop(pm1);
 177   return (rc);
 178 fail:
 179   rc = -1;
 180   goto tidy;
 181 }
 182
 183 /*----- That's all, folks -------------------------------------------------*/