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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)
commitb9cf65a08e6a59e434685e894e3189c201ac6791 (patch)
tree1f0868ef44c9f17d83d10dc94343df7b8cfe1842 /libtommath/bn_mp_reduce.c
parentc6a259aeeca4814a97cf6694814c63e74e4e18fa (diff)
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Import of libtommath 0.33
Diffstat (limited to 'libtommath/bn_mp_reduce.c')
-rw-r--r--libtommath/bn_mp_reduce.c97
1 files changed, 97 insertions, 0 deletions
diff --git a/libtommath/bn_mp_reduce.c b/libtommath/bn_mp_reduce.c
new file mode 100644
index 0000000..cfcb55a
--- /dev/null
+++ b/libtommath/bn_mp_reduce.c
@@ -0,0 +1,97 @@
+#include <tommath.h>
+#ifdef BN_MP_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
+ */
+
+/* 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 {
+#ifdef BN_S_MP_MUL_HIGH_DIGS_C
+ if ((res = s_mp_mul_high_digs (&q, mu, &q, um - 1)) != MP_OKAY) {
+ goto CLEANUP;
+ }
+#elif defined(BN_FAST_S_MP_MUL_HIGH_DIGS_C)
+ if ((res = fast_s_mp_mul_high_digs (&q, mu, &q, um - 1)) != MP_OKAY) {
+ goto CLEANUP;
+ }
+#else
+ {
+ res = MP_VAL;
+ goto CLEANUP;
+ }
+#endif
+ }
+
+ /* 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;
+}
+#endif