dsig.c: Allow precomputed hashes to be read from a file.

[u/mdw/catacomb] / dh-check.c

/* -*-c-*-
 *
 * $Id: dh-check.c,v 1.3 2004/04/08 01:36:15 mdw Exp $
 *
 * Checks Diffie-Hellman group parameters
 *
 * (c) 2001 Straylight/Edgeware
 */

/*----- Licensing notice --------------------------------------------------*
 *
 * This file is part of Catacomb.
 *
 * Catacomb is free software; you can redistribute it and/or modify
 * it under the terms of the GNU Library General Public License as
 * published by the Free Software Foundation; either version 2 of the
 * License, or (at your option) any later version.
 *
 * Catacomb 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 Library General Public License for more details.
 *
 * You should have received a copy of the GNU Library General Public
 * License along with Catacomb; if not, write to the Free
 * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
 * MA 02111-1307, USA.
 */

/*----- Header files ------------------------------------------------------*/

#include <mLib/dstr.h>

#include "dh.h"
#include "keycheck.h"
#include "mp.h"
#include "mpmont.h"
#include "mpmul.h"

/*----- Main code ---------------------------------------------------------*/

/* --- @dh_checkparam@ --- *
 *
 * Arguments:	@keycheck *kc@ = keycheck state
 *		@const dh_param *dp@ = pointer to the parameter set
 *		@mp **v@ = optional vector of factors
 *		@size_t n@ = size of vector
 *
 * Returns:	Zero if all OK, or return status from function.
 *
 * Use:		Checks a set of Diffie-Hellman parameters for consistency and
 *		security.
 */

int dh_checkparam(keycheck *kc, const dh_param *dp, mp **v, size_t n)
{
  int rc = 0;
  mpmont mm;
  mp *pm1 = MP_NEW;
  mp *q = MP_NEW;
  mp *x;
  mpmul mu;
  size_t i;

  /* --- Check that the numbers which are supposed to be prime are --- */

  if ((!v && keycheck_prime(kc, KCSEV_WARN, dp->q, "q")) ||
      keycheck_prime(kc, KCSEV_ERR, dp->p, "p"))
    goto fail;

  /* --- Ensure that %$q$% is a sensible choice of number --- */

  pm1 = mp_sub(pm1, dp->p, MP_ONE);
  mp_div(0, &q, pm1, dp->q);
  if (!mp_eq(q, MP_ZERO) &&
      keycheck_report(kc, KCSEV_ERR, "q not a factor of p - 1"))
    goto fail;

  /* --- Check that %$g$% is actually right --- *
   *
   * This isn't perfect.  If %$q$% is composite and we don't have the factors
   * of %$p - 1$% then the order of %$g$% may be some factor of %$q$% which
   * we can't find.  (If we do have the factors, we check them all lower
   * down.)  We do strip out powers of two from %$q$% before testing, though.
   */

  if ((mp_eq(dp->g, MP_ONE) || mp_eq(dp->g, pm1)) &&
      keycheck_report(kc, KCSEV_ERR, "g is degenerate (+/-1 mod p)"))
    goto fail;
  q = mp_odd(q, dp->q, &i);
  mpmont_create(&mm, dp->p);
  x = mpmont_mul(&mm, MP_NEW, dp->g, mm.r2);
  q = mpmont_expr(&mm, q, x, q);
  mp_drop(x);
  do {
    if (mp_eq(q, mm.r) != !i) {
      if (keycheck_report(kc, KCSEV_ERR, "order of g != q")) {
	mpmont_destroy(&mm);
	goto fail;
      }
      break;
    }
    if (i) {
      q = mp_sqr(q, q);
      q = mpmont_reduce(&mm, q, q);
    }
  } while (i--);

  /* --- Check Lim-Lee primes more carefully --- *
   *
   * In this case, we really can be sure whether the order of %$g$% is
   * actually %$q$% as advertised.  Also ensure that the individual primes
   * are really prime, and that their product is correct.
   */

  if (!v)
    mpmont_destroy(&mm);
  else {
    dstr d = DSTR_INIT;
    mp *r = MP_NEW;

    mpmul_init(&mu);
    for (i = 0; i < n; i++) {
      DRESET(&d);
      dstr_putf(&d, "factor f_%lu of p", (unsigned long)i);
      if ((rc = keycheck_prime(kc, KCSEV_ERR, v[i], d.buf)) != 0)
	break;
      mp_div(&q, &r, dp->q, v[i]);
      if (mp_eq(r, MP_ZERO) && !mp_eq(q, MP_ONE)) {
	q = mpmont_exp(&mm, q, dp->g, q);
	if (mp_eq(q, MP_ONE) &&
	    (rc = keycheck_report(kc, KCSEV_ERR,
				  "order of g is proper divisor of q")) != 0)
	  break;
      }
      mpmul_add(&mu, v[i]);
    }
    mp_drop(q);
    mp_drop(r);
    q = mpmul_done(&mu);
    mpmont_destroy(&mm);
    dstr_destroy(&d);
    if (rc)
      goto fail;
    q = mp_lsl(q, q, 1);
    if (!mp_eq(q, pm1) &&
	keycheck_report(kc, KCSEV_ERR, "product of f_i != (p - 1)/2"))
      goto fail;
  }

  /* --- Finally, check the key sizes --- */

  if ((mp_bits(dp->p) < 1024 &&
       keycheck_report(kc, KCSEV_WARN,
		       "p too small to resist index calculus attacks")) ||
      (mp_bits(dp->q) < 160 &&
       keycheck_report(kc, KCSEV_WARN,
		       "q too small to resist collision-finding attacks")))
    goto fail;

  /* --- Done --- */

tidy:
  mp_drop(q);
  mp_drop(pm1);
  return (rc);
fail:
  rc = -1;
  goto tidy;
}

/*----- That's all, folks -------------------------------------------------*/