--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/bn_mp_dr_reduce.c	Mon May 31 18:25:22 2004 +0000
@@ -0,0 +1,88 @@
+/* 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>
+
+/* reduce "x" in place modulo "n" using the Diminished Radix algorithm.
+ *
+ * Based on algorithm from the paper
+ *
+ * "Generating Efficient Primes for Discrete Log Cryptosystems"
+ *                 Chae Hoon Lim, Pil Loong Lee,
+ *          POSTECH Information Research Laboratories
+ *
+ * The modulus must be of a special format [see manual]
+ *
+ * Has been modified to use algorithm 7.10 from the LTM book instead
+ *
+ * Input x must be in the range 0 <= x <= (n-1)**2
+ */
+int
+mp_dr_reduce (mp_int * x, mp_int * n, mp_digit k)
+{
+  int      err, i, m;
+  mp_word  r;
+  mp_digit mu, *tmpx1, *tmpx2;
+
+  /* m = digits in modulus */
+  m = n->used;
+
+  /* ensure that "x" has at least 2m digits */
+  if (x->alloc < m + m) {
+    if ((err = mp_grow (x, m + m)) != MP_OKAY) {
+      return err;
+    }
+  }
+
+/* top of loop, this is where the code resumes if
+ * another reduction pass is required.
+ */
+top:
+  /* aliases for digits */
+  /* alias for lower half of x */
+  tmpx1 = x->dp;
+
+  /* alias for upper half of x, or x/B**m */
+  tmpx2 = x->dp + m;
+
+  /* set carry to zero */
+  mu = 0;
+
+  /* compute (x mod B**m) + k * [x/B**m] inline and inplace */
+  for (i = 0; i < m; i++) {
+      r         = ((mp_word)*tmpx2++) * ((mp_word)k) + *tmpx1 + mu;
+      *tmpx1++  = (mp_digit)(r & MP_MASK);
+      mu        = (mp_digit)(r >> ((mp_word)DIGIT_BIT));
+  }
+
+  /* set final carry */
+  *tmpx1++ = mu;
+
+  /* zero words above m */
+  for (i = m + 1; i < x->used; i++) {
+      *tmpx1++ = 0;
+  }
+
+  /* clamp, sub and return */
+  mp_clamp (x);
+
+  /* if x >= n then subtract and reduce again
+   * Each successive "recursion" makes the input smaller and smaller.
+   */
+  if (mp_cmp_mag (x, n) != MP_LT) {
+    s_mp_sub(x, n, x);
+    goto top;
+  }
+  return MP_OKAY;
+}