--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libtommath/bn_s_mp_mul_high_digs_fast.c	Thu Oct 15 19:55:15 2020 +0800
@@ -0,0 +1,81 @@
+#include "tommath_private.h"
+#ifdef BN_S_MP_MUL_HIGH_DIGS_FAST_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* this is a modified version of fast_s_mul_digs that only produces
+ * output digits *above* digs.  See the comments for fast_s_mul_digs
+ * to see how it works.
+ *
+ * This is used in the Barrett reduction since for one of the multiplications
+ * only the higher digits were needed.  This essentially halves the work.
+ *
+ * Based on Algorithm 14.12 on pp.595 of HAC.
+ */
+mp_err s_mp_mul_high_digs_fast(const mp_int *a, const mp_int *b, mp_int *c, int digs)
+{
+   int     olduse, pa, ix, iz;
+   mp_err   err;
+   mp_digit W[MP_WARRAY];
+   mp_word  _W;
+
+   /* grow the destination as required */
+   pa = a->used + b->used;
+   if (c->alloc < pa) {
+      if ((err = mp_grow(c, pa)) != MP_OKAY) {
+         return err;
+      }
+   }
+
+   /* number of output digits to produce */
+   pa = a->used + b->used;
+   _W = 0;
+   for (ix = digs; ix < pa; ix++) {
+      int      tx, ty, iy;
+      mp_digit *tmpx, *tmpy;
+
+      /* get offsets into the two bignums */
+      ty = MP_MIN(b->used-1, ix);
+      tx = ix - ty;
+
+      /* setup temp aliases */
+      tmpx = a->dp + tx;
+      tmpy = b->dp + ty;
+
+      /* this is the number of times the loop will iterrate, essentially its
+         while (tx++ < a->used && ty-- >= 0) { ... }
+       */
+      iy = MP_MIN(a->used-tx, ty+1);
+
+      /* execute loop */
+      for (iz = 0; iz < iy; iz++) {
+         _W += (mp_word)*tmpx++ * (mp_word)*tmpy--;
+      }
+
+      /* store term */
+      W[ix] = (mp_digit)_W & MP_MASK;
+
+      /* make next carry */
+      _W = _W >> (mp_word)MP_DIGIT_BIT;
+   }
+
+   /* setup dest */
+   olduse  = c->used;
+   c->used = pa;
+
+   {
+      mp_digit *tmpc;
+
+      tmpc = c->dp + digs;
+      for (ix = digs; ix < pa; ix++) {
+         /* now extract the previous digit [below the carry] */
+         *tmpc++ = W[ix];
+      }
+
+      /* clear unused digits [that existed in the old copy of c] */
+      MP_ZERO_DIGITS(tmpc, olduse - ix);
+   }
+   mp_clamp(c);
+   return MP_OKAY;
+}
+#endif