### diff libtommath/bn_mp_montgomery_setup.c @ 389:5ff8218bcee9

author Matt Johnston Thu, 11 Jan 2007 03:14:55 +0000 eed26cff980b 60fc6476e044
line wrap: on
line diff
```--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libtommath/bn_mp_montgomery_setup.c	Thu Jan 11 03:14:55 2007 +0000
@@ -0,0 +1,59 @@
+#include <tommath.h>
+#ifdef BN_MP_MONTGOMERY_SETUP_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis
+ *
+ * LibTomMath is a library that provides multiple-precision
+ * integer arithmetic as well as number theoretic functionality.
+ *
+ * The library was designed directly after the MPI library by
+ * Michael Fromberger but has been written from scratch with
+ * additional optimizations in place.
+ *
+ * The library is free for all purposes without any express
+ * guarantee it works.
+ *
+ * Tom St Denis, [email protected], http://math.libtomcrypt.com
+ */
+
+/* setups the montgomery reduction stuff */
+int
+mp_montgomery_setup (mp_int * n, mp_digit * rho)
+{
+  mp_digit x, b;
+
+/* fast inversion mod 2**k
+ *
+ * Based on the fact that
+ *
+ * XA = 1 (mod 2**n)  =>  (X(2-XA)) A = 1 (mod 2**2n)
+ *                    =>  2*X*A - X*X*A*A = 1
+ *                    =>  2*(1) - (1)     = 1
+ */
+  b = n->dp[0];
+
+  if ((b & 1) == 0) {
+    return MP_VAL;
+  }
+
+  x = (((b + 2) & 4) << 1) + b; /* here x*a==1 mod 2**4 */
+  x *= 2 - b * x;               /* here x*a==1 mod 2**8 */
+#if !defined(MP_8BIT)
+  x *= 2 - b * x;               /* here x*a==1 mod 2**16 */
+#endif
+#if defined(MP_64BIT) || !(defined(MP_8BIT) || defined(MP_16BIT))
+  x *= 2 - b * x;               /* here x*a==1 mod 2**32 */
+#endif
+#ifdef MP_64BIT
+  x *= 2 - b * x;               /* here x*a==1 mod 2**64 */
+#endif
+
+  /* rho = -1/m mod b */
+  *rho = (unsigned long)(((mp_word)1 << ((mp_word) DIGIT_BIT)) - x) & MP_MASK;
+
+  return MP_OKAY;
+}
+#endif
+
+/* \$Source: /cvs/libtom/libtommath/bn_mp_montgomery_setup.c,v \$ */
+/* \$Revision: 1.4 \$ */
+/* \$Date: 2006/12/04 21:34:03 \$ */```