+/* -*-c-*-
+ *
+ * $Id: dh-check.c,v 1.1 2001/02/03 16:08:24 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.
+ */
+
+/*----- Revision history --------------------------------------------------*
+ *
+ * $Log: dh-check.c,v $
+ * Revision 1.1 2001/02/03 16:08:24 mdw
+ * Add consistency checking for public keys.
+ *
+ */
+
+/*----- 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;
+ 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);
+ q = mpmont_expr(&mm, q, dp->g, q);
+ 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 -------------------------------------------------*/
projects / u / mdw / catacomb / commitdiff