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authorKevin B Kenny <kennykb@acm.org>2005-01-19 22:41:26 (GMT)
committerKevin B Kenny <kennykb@acm.org>2005-01-19 22:41:26 (GMT)
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parentb23d942a1e86ddee18c2309afd7fa7e9afa79ef8 (diff)
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Import of libtommath 0.33
Diffstat (limited to 'libtommath/bn_mp_dr_reduce.c')
-rw-r--r--libtommath/bn_mp_dr_reduce.c90
1 files changed, 90 insertions, 0 deletions
diff --git a/libtommath/bn_mp_dr_reduce.c b/libtommath/bn_mp_dr_reduce.c
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+#include <tommath.h>
+#ifdef BN_MP_DR_REDUCE_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, tomstdenis@iahu.ca, http://math.libtomcrypt.org
+ */
+
+/* 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 Joong 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;
+}
+#endif