X-Git-Url: https://git.distorted.org.uk/u/mdw/catacomb/blobdiff_plain/ba6e6b64033b1f9de49feccb5c9cd438354481f7..0f00dc4c8eb47e67bc0f148c2dd109f73a451e0a:/pub/dh-check.c

diff --git a/pub/dh-check.c b/pub/dh-check.c
new file mode 100644
index 0000000..06f02ae
--- /dev/null
+++ b/pub/dh-check.c
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+/* -*-c-*-
+ *
+ * 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 -------------------------------------------------*/