X-Git-Url: https://pd.if.org/git/?a=blobdiff_plain;f=tomsfastmath%2Fsrc%2Fmont%2Ffp_montgomery_calc_normalization.c;fp=tomsfastmath%2Fsrc%2Fmont%2Ffp_montgomery_calc_normalization.c;h=c21c8d04a9799e2772d21d7c383845d1f8bc505d;hb=66bc25938679f1d6a1d1200f329093d82a5e99b4;hp=0000000000000000000000000000000000000000;hpb=a52ee0733f420ca20224049260d6fc5cf7d8f621;p=zpackage

diff --git a/tomsfastmath/src/mont/fp_montgomery_calc_normalization.c b/tomsfastmath/src/mont/fp_montgomery_calc_normalization.c
new file mode 100644
index 0000000..c21c8d0
--- /dev/null
+++ b/tomsfastmath/src/mont/fp_montgomery_calc_normalization.c
@@ -0,0 +1,43 @@
+/* TomsFastMath, a fast ISO C bignum library.
+ * 
+ * This project is meant to fill in where LibTomMath
+ * falls short.  That is speed ;-)
+ *
+ * This project is public domain and free for all purposes.
+ * 
+ * Tom St Denis, tomstdenis@gmail.com
+ */
+#include <tfm_private.h>
+
+/* computes a = B**n mod b without division or multiplication useful for
+ * normalizing numbers in a Montgomery system.
+ */
+void fp_montgomery_calc_normalization(fp_int *a, fp_int *b)
+{
+  int     x, bits;
+
+  /* how many bits of last digit does b use */
+  bits = fp_count_bits (b) % DIGIT_BIT;
+  if (!bits) bits = DIGIT_BIT;
+
+  /* compute A = B^(n-1) * 2^(bits-1) */
+  if (b->used > 1) {
+     fp_2expt (a, (b->used - 1) * DIGIT_BIT + bits - 1);
+  } else {
+     fp_set(a, 1);
+     bits = 1;
+  }
+
+  /* now compute C = A * B mod b */
+  for (x = bits - 1; x < (int)DIGIT_BIT; x++) {
+    fp_mul_2 (a, a);
+    if (fp_cmp_mag (a, b) != FP_LT) {
+      s_fp_sub (a, b, a);
+    }
+  }
+}
+
+
+/* $Source$ */
+/* $Revision$ */
+/* $Date$ */