X-Git-Url: https://git.distorted.org.uk/u/mdw/catacomb/blobdiff_plain/ba6e6b64033b1f9de49feccb5c9cd438354481f7..0f00dc4c8eb47e67bc0f148c2dd109f73a451e0a:/base/keysz-conv.c?ds=sidebyside

diff --git a/base/keysz-conv.c b/base/keysz-conv.c
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+/* -*-c-*-
+ *
+ * Key length conversions
+ *
+ * (c) 2007 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 <math.h>
+
+#include "keysz.h"
+
+/*----- Main code ---------------------------------------------------------*/
+
+/* --- Primitive conversions --- */
+
+static double fromnfs(double nbits)
+{
+  /* --- Magic taken from P1363 --- */
+#define LOG2 0.6931471805599453094172321214581765680755
+#define MAGIC 8.567936			/* = 17.135872 / 2 */
+  double z = log(nbits * LOG2)/LOG2;
+  return 4.0/3.0 * pow(3 * nbits * z*z, 1.0/3.0) - MAGIC;
+}
+
+static double tonfs(double nbits)
+{
+  double hi, lo, mid, val;
+
+  /* --- Find upper and lower bounds --- */
+
+  for (hi = nbits; fromnfs(hi) < nbits; hi *= 2);
+  for (lo = nbits; fromnfs(lo) > nbits; lo /= 2);
+
+  /* --- Bisect the interval until we get an answer --- */
+
+#define THRESH 0.01
+  for (;;) {
+    mid = (hi + lo)/2;
+    val = fromnfs(mid);
+    if (fabs(val - nbits) < THRESH)
+      return (mid);
+    else if (val < nbits)
+      lo = mid;
+    else
+      hi = mid;
+  }
+}
+
+static double torho(double nbits) { return 2*nbits; }
+static double fromrho(double nbits) { return nbits/2; }
+
+/* --- @keysz_fromdl@, @_fromschnorr@, @_fromif@, @_fromec@ --- *
+ *
+ * Arguments:	@double nbits@ = key size
+ *
+ * Returns:	Equivalent symmetric key size.
+ *
+ * Use:		Converts key lengths of various kinds of reference problems
+ *		to (roughly) equivalent symmetric key sizes.
+ *
+ *		  * Given the bit length of %$p$%, @keysz_fromdl@ returns a
+ *		    key size representing the difficulty of computing
+ *		    discrete logarithms in %$\gf{p}$%, for %$p$% prime or a
+ *		    small power of a prime.
+ *
+ *		  * Given the bit length of %$r$%, @keysz_fromschnorr@
+ *		    returns a key size representing the difficulty of
+ *		    computing discrete logarithms in a subgroup of %$\gf{q}$%
+ *		    of order %$r$%.
+ *
+ *		  * Given the bit length of %$n$%, @keysz_fromif@ returns a
+ *		    key size representing the difficulty of factoring a
+ *		    `hard' number %$n = p q$%, where %$p$% and %$q$% are
+ *		    primes of (near enough) the same length.
+ *
+ *		  * Given the bit length of %$r$%, @keysz_fromec@ returns a
+ *		    key size representing the difficulty of computing
+ *		    discrete logarithms in a subgroup of order-%$r$% of an
+ *		    elliptic curve over a finite field.
+ *
+ *		These functions take and return @double@ rather than an
+ *		integer type in order to preserve precision between
+ *		conversions.
+ */
+
+#define CONVERSIONS(_)							\
+  _(dl, nfs)								\
+  _(schnorr, rho)							\
+  _(if, nfs)								\
+  _(ec, rho)
+
+#define DEFINE_FROM(what, how)						\
+  double keysz_from##what(double nbits) { return from##how(nbits); }
+
+CONVERSIONS(DEFINE_FROM)
+
+/* --- @keysz_todl@, @_toschnorr@, @_toif@, @_toec@ --- *
+ *
+ * Arguments:	@unsigned long nbits@ = symmetric key size
+ *
+ * Returns:	Equivalent key size.
+ *
+ * Use:		Converts symmetric key sizes to (roughly) equivalent key
+ *		sizes for various kinds of reference problems.  These are the
+ *		approximate inverses of the functions above.
+ */
+
+#define DEFINE_TO(what, how)						\
+  double keysz_to##what(double nbits) { return to##how(nbits); }
+
+CONVERSIONS(DEFINE_TO)
+
+/*----- Interactive mode for testing --------------------------------------*/
+
+#ifdef TEST_RIG
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+static const struct entry {
+  const char *name;
+  double (*func)(double);
+} tab[] = {
+#define ENTRY_FROM(what, how) { "from" #what, keysz_from##what },
+#define ENTRY_TO(what, how) { "to" #what, keysz_to##what },
+  CONVERSIONS(ENTRY_FROM)
+  CONVERSIONS(ENTRY_TO)
+  { 0 }
+};
+
+int main(int argc, char *argv[])
+{
+  const struct entry *e;
+  double d;
+
+  if (!argv[1]) {
+    fprintf(stderr, "conversions:");
+    for (e = tab; e->name; e++) fprintf(stderr, " %s", e->name);
+    putc('\n', stderr);
+    return (1);
+  }
+  for (e = tab; e->name && strcmp(e->name, argv[1]); e++);
+  if (!e) {
+    fprintf(stderr, "unknown conversion `%s'\n", argv[1]);
+    return (1);
+  }
+  for (argv += 2; *argv; argv++) {
+    d = strtod(*argv, 0);
+    printf("%s(%g) = %g\n", e->name, d, e->func(d));
+  }
+  return (0);
+}
+
+#endif
+
+/*----- That's all, folks -------------------------------------------------*/