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

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+/* -*-c-*-
+ *
+ * Key size management
+ *
+ * (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.
+ */
+
+#ifndef CATACOMB_KEYSZ_H
+#define CATACOMB_KEYSZ_H
+
+#ifdef __cplusplus
+  extern "C" {
+#endif
+
+/*----- Header files ------------------------------------------------------*/
+
+#include <stddef.h>
+
+#include <mLib/bits.h>
+
+/*----- Data structures ---------------------------------------------------*/
+
+/* --- Key size type constants --- *
+ *
+ * A key size limitation is an array of bytes.  The first byte describes the
+ * kind of limitation on the key size %$k$%; the rest are argument bytes
+ * %$a_i$%, for %$i \ge 0$%.  In all cases, %$a_0$% is the `recommended' key
+ * size.
+ *
+ *   * @KSZ_ANY@ means there is no restriction.
+ *
+ *   * @KSZ_RANGE@ requires that %$k \ge a_1$%, %$k \equiv 0 \pmod{a_3}$%,
+ *     and, if %$a_2 \ne 0$%, %$k \le a_2$%.
+ *
+ *   * @KSZ_SET@ requires that %$k \in {\,a_i\,}$%.
+ */
+
+enum {
+  KSZ_ANY,				/* Allows any key at all */
+  KSZ_RANGE,				/* Allows keys within a range */
+  KSZ_SET				/* Allows specific sizes of keys */
+};
+
+/*----- Key sizes for symmetric algorithms --------------------------------*/
+
+/* --- @keysz@ --- *
+ *
+ * Arguments:	@size_t sz@ = a proposed key size, or zero
+ *		@const octet *ksz@ = pointer to key size table
+ *
+ * Returns:	See below.
+ *
+ * Use:		Returns a sensible key size.  If @sz@ is nonzero, it is
+ *		interpreted as an amount (in bytes) of key material which the
+ *		caller has available, and the return value is either the
+ *		largest allowable key size less than or equal to the caller's
+ *		size, or zero if there is no valid key length small enough.
+ *		If @sz@ is zero, the function returns a `recommended' key
+ *		size.
+ */
+
+extern size_t keysz(size_t /*sz*/, const octet */*ksz*/);
+
+#define KSZ_CHECK(pre, sz) (keysz((sz), pre##_keysz) == (sz))
+#define KSZ_ASSERT(pre, sz)						\
+  assert(((void)"Bad key size for " #pre, KSZ_CHECK(pre, sz)))
+
+/*----- Key size conversions ----------------------------------------------*/
+
+/* --- @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.
+ */
+
+extern double keysz_fromdl(double /*nbits*/);
+extern double keysz_fromschnorr(double /*nbits*/);
+extern double keysz_fromif(double /*nbits*/);
+extern double keysz_fromec(double /*nbits*/);
+
+/* --- @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.
+ */
+
+extern double keysz_todl(double /*nbits*/);
+extern double keysz_toschnorr(double /*nbits*/);
+extern double keysz_toif(double /*nbits*/);
+extern double keysz_toec(double /*nbits*/);
+
+/*----- That's all, folks -------------------------------------------------*/
+
+#ifdef __cplusplus
+  }
+#endif
+
+#endif