--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/bn_mp_reduce.c	Mon May 31 18:25:22 2004 +0000
@@ -0,0 +1,84 @@
+/* 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.org
+ */
+#include <tommath.h>
+
+/* reduces x mod m, assumes 0 < x < m**2, mu is 
+ * precomputed via mp_reduce_setup.
+ * From HAC pp.604 Algorithm 14.42
+ */
+int
+mp_reduce (mp_int * x, mp_int * m, mp_int * mu)
+{
+  mp_int  q;
+  int     res, um = m->used;
+
+  /* q = x */
+  if ((res = mp_init_copy (&q, x)) != MP_OKAY) {
+    return res;
+  }
+
+  /* q1 = x / b**(k-1)  */
+  mp_rshd (&q, um - 1);         
+
+  /* according to HAC this optimization is ok */
+  if (((unsigned long) um) > (((mp_digit)1) << (DIGIT_BIT - 1))) {
+    if ((res = mp_mul (&q, mu, &q)) != MP_OKAY) {
+      goto CLEANUP;
+    }
+  } else {
+    if ((res = s_mp_mul_high_digs (&q, mu, &q, um - 1)) != MP_OKAY) {
+      goto CLEANUP;
+    }
+  }
+
+  /* q3 = q2 / b**(k+1) */
+  mp_rshd (&q, um + 1);         
+
+  /* x = x mod b**(k+1), quick (no division) */
+  if ((res = mp_mod_2d (x, DIGIT_BIT * (um + 1), x)) != MP_OKAY) {
+    goto CLEANUP;
+  }
+
+  /* q = q * m mod b**(k+1), quick (no division) */
+  if ((res = s_mp_mul_digs (&q, m, &q, um + 1)) != MP_OKAY) {
+    goto CLEANUP;
+  }
+
+  /* x = x - q */
+  if ((res = mp_sub (x, &q, x)) != MP_OKAY) {
+    goto CLEANUP;
+  }
+
+  /* If x < 0, add b**(k+1) to it */
+  if (mp_cmp_d (x, 0) == MP_LT) {
+    mp_set (&q, 1);
+    if ((res = mp_lshd (&q, um + 1)) != MP_OKAY)
+      goto CLEANUP;
+    if ((res = mp_add (x, &q, x)) != MP_OKAY)
+      goto CLEANUP;
+  }
+
+  /* Back off if it's too big */
+  while (mp_cmp (x, m) != MP_LT) {
+    if ((res = s_mp_sub (x, m, x)) != MP_OKAY) {
+      goto CLEANUP;
+    }
+  }
+  
+CLEANUP:
+  mp_clear (&q);
+
+  return res;
+}