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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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tree47f8ad0d7291237c7f9af988c5e05275ed9286ee /libtommath/bn_mp_montgomery_reduce.c
parentb23d942a1e86ddee18c2309afd7fa7e9afa79ef8 (diff)
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Import of libtommath 0.33
Diffstat (limited to 'libtommath/bn_mp_montgomery_reduce.c')
-rw-r--r--libtommath/bn_mp_montgomery_reduce.c114
1 files changed, 114 insertions, 0 deletions
diff --git a/libtommath/bn_mp_montgomery_reduce.c b/libtommath/bn_mp_montgomery_reduce.c
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+#include <tommath.h>
+#ifdef BN_MP_MONTGOMERY_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
+ */
+
+/* computes xR**-1 == x (mod N) via Montgomery Reduction */
+int
+mp_montgomery_reduce (mp_int * x, mp_int * n, mp_digit rho)
+{
+ int ix, res, digs;
+ mp_digit mu;
+
+ /* can the fast reduction [comba] method be used?
+ *
+ * Note that unlike in mul you're safely allowed *less*
+ * than the available columns [255 per default] since carries
+ * are fixed up in the inner loop.
+ */
+ digs = n->used * 2 + 1;
+ if ((digs < MP_WARRAY) &&
+ n->used <
+ (1 << ((CHAR_BIT * sizeof (mp_word)) - (2 * DIGIT_BIT)))) {
+ return fast_mp_montgomery_reduce (x, n, rho);
+ }
+
+ /* grow the input as required */
+ if (x->alloc < digs) {
+ if ((res = mp_grow (x, digs)) != MP_OKAY) {
+ return res;
+ }
+ }
+ x->used = digs;
+
+ for (ix = 0; ix < n->used; ix++) {
+ /* mu = ai * rho mod b
+ *
+ * The value of rho must be precalculated via
+ * montgomery_setup() such that
+ * it equals -1/n0 mod b this allows the
+ * following inner loop to reduce the
+ * input one digit at a time
+ */
+ mu = (mp_digit) (((mp_word)x->dp[ix]) * ((mp_word)rho) & MP_MASK);
+
+ /* a = a + mu * m * b**i */
+ {
+ register int iy;
+ register mp_digit *tmpn, *tmpx, u;
+ register mp_word r;
+
+ /* alias for digits of the modulus */
+ tmpn = n->dp;
+
+ /* alias for the digits of x [the input] */
+ tmpx = x->dp + ix;
+
+ /* set the carry to zero */
+ u = 0;
+
+ /* Multiply and add in place */
+ for (iy = 0; iy < n->used; iy++) {
+ /* compute product and sum */
+ r = ((mp_word)mu) * ((mp_word)*tmpn++) +
+ ((mp_word) u) + ((mp_word) * tmpx);
+
+ /* get carry */
+ u = (mp_digit)(r >> ((mp_word) DIGIT_BIT));
+
+ /* fix digit */
+ *tmpx++ = (mp_digit)(r & ((mp_word) MP_MASK));
+ }
+ /* At this point the ix'th digit of x should be zero */
+
+
+ /* propagate carries upwards as required*/
+ while (u) {
+ *tmpx += u;
+ u = *tmpx >> DIGIT_BIT;
+ *tmpx++ &= MP_MASK;
+ }
+ }
+ }
+
+ /* at this point the n.used'th least
+ * significant digits of x are all zero
+ * which means we can shift x to the
+ * right by n.used digits and the
+ * residue is unchanged.
+ */
+
+ /* x = x/b**n.used */
+ mp_clamp(x);
+ mp_rshd (x, n->used);
+
+ /* if x >= n then x = x - n */
+ if (mp_cmp_mag (x, n) != MP_LT) {
+ return s_mp_sub (x, n, x);
+ }
+
+ return MP_OKAY;
+}
+#endif