+/*----- 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*/);
+