1 files changed, 86 insertions, 0 deletions
diff --git a/bn_s_mp_mul_digs.c b/bn_s_mp_mul_digs.c
new file mode 100644
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--- /dev/null
+++ b/bn_s_mp_mul_digs.c
@@ -0,0 +1,86 @@
+#include <tommath.h>
+#ifdef BN_S_MP_MUL_DIGS_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
+ */
+
+/* multiplies |a| * |b| and only computes upto digs digits of result
+ * HAC pp. 595, Algorithm 14.12  Modified so you can control how 
+ * many digits of output are created.
+ */
+int s_mp_mul_digs (mp_int * a, mp_int * b, mp_int * c, int digs)
+{
+  mp_int  t;
+  int     res, pa, pb, ix, iy;
+  mp_digit u;
+  mp_word r;
+  mp_digit tmpx, *tmpt, *tmpy;
+
+  /* can we use the fast multiplier? */
+  if (((digs) < MP_WARRAY) &&
+      MIN (a->used, b->used) < 
+          (1 << ((CHAR_BIT * sizeof (mp_word)) - (2 * DIGIT_BIT)))) {
+    return fast_s_mp_mul_digs (a, b, c, digs);
+  }
+
+  if ((res = mp_init_size (&t, digs)) != MP_OKAY) {
+    return res;
+  }
+  t.used = digs;
+
+  /* compute the digits of the product directly */
+  pa = a->used;
+  for (ix = 0; ix < pa; ix++) {
+    /* set the carry to zero */
+    u = 0;
+
+    /* limit ourselves to making digs digits of output */
+    pb = MIN (b->used, digs - ix);
+
+    /* setup some aliases */
+    /* copy of the digit from a used within the nested loop */
+    tmpx = a->dp[ix];
+    
+    /* an alias for the destination shifted ix places */
+    tmpt = t.dp + ix;
+    
+    /* an alias for the digits of b */
+    tmpy = b->dp;
+
+    /* compute the columns of the output and propagate the carry */
+    for (iy = 0; iy < pb; iy++) {
+      /* compute the column as a mp_word */
+      r       = ((mp_word)*tmpt) +
+                ((mp_word)tmpx) * ((mp_word)*tmpy++) +
+                ((mp_word) u);
+
+      /* the new column is the lower part of the result */
+      *tmpt++ = (mp_digit) (r & ((mp_word) MP_MASK));
+
+      /* get the carry word from the result */
+      u       = (mp_digit) (r >> ((mp_word) DIGIT_BIT));
+    }
+    /* set carry if it is placed below digs */
+    if (ix + iy < digs) {
+      *tmpt = u;
+    }
+  }
+
+  mp_clamp (&t);
+  mp_exch (&t, c);
+
+  mp_clear (&t);
+  return MP_OKAY;
+}
+#endif