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

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