From 419d33ed2f44bd5f00b0c78a81aaa1abc37f2056 Mon Sep 17 00:00:00 2001
From: mdw <mdw>
Date: Sat, 29 Jul 2000 17:04:33 +0000
Subject: [PATCH] Change to use left-to-right bitwise exponentiation.  This
 will improve performance when the base is small.

---
 mpbarrett.c | 66 +++++++++++++++++++++++++++++++++++++++++--------------------
 1 file changed, 45 insertions(+), 21 deletions(-)

diff --git a/mpbarrett.c b/mpbarrett.c
index aaa1bb7..affbc65 100644
--- a/mpbarrett.c
+++ b/mpbarrett.c
@@ -1,6 +1,6 @@
 /* -*-c-*-
  *
- * $Id: mpbarrett.c,v 1.4 2000/06/17 11:45:09 mdw Exp $
+ * $Id: mpbarrett.c,v 1.5 2000/07/29 17:04:33 mdw Exp $
  *
  * Barrett modular reduction
  *
@@ -30,6 +30,10 @@
 /*----- Revision history --------------------------------------------------* 
  *
  * $Log: mpbarrett.c,v $
+ * Revision 1.5  2000/07/29 17:04:33  mdw
+ * Change to use left-to-right bitwise exponentiation.  This will improve
+ * performance when the base is small.
+ *
  * Revision 1.4  2000/06/17 11:45:09  mdw
  * Major memory management overhaul.  Added arena support.  Use the secure
  * arena for secret integers.  Replace and improve the MP management macros
@@ -187,33 +191,53 @@ mp *mpbarrett_exp(mpbarrett *mb, mp *d, mp *a, mp *e)
 {
   mpscan sc;
   mp *x = MP_ONE;
-  mp *spare = MP_NEW;
+  mp *spare = (e->f & MP_BURN) ? MP_NEWSEC : MP_NEW;
+  unsigned sq = 0;
 
   a = MP_COPY(a);
-  mp_scan(&sc, e);
-  if (e->f & MP_BURN)
-    spare = MP_NEWSEC;
-  if (MP_STEP(&sc)) {
-    size_t sq = 0;
+  mp_rscan(&sc, e);
+  if (!MP_RSTEP(&sc))
+    goto exit;
+  while (!MP_RBIT(&sc))
+    MP_RSTEP(&sc);
+
+  /* --- Do the main body of the work --- */
+
+  for (;;) {
+    sq++;
+    while (sq) {
+      mp *y;
+      y = mp_sqr(spare, x);
+      y = mpbarrett_reduce(mb, y, y);
+      spare = x; x = y;
+      sq--;
+    }
+    {
+      mp *y = mp_mul(spare, x, a);
+      y = mpbarrett_reduce(mb, y, y);
+      spare = x; x = y;
+    }
     for (;;) {
-      mp *dd;
-      if (MP_BIT(&sc)) {
-        while (sq) {
-          dd = mp_sqr(spare, a);
-          dd = mpbarrett_reduce(mb, dd, dd);
-          spare = a; a = dd;
-          sq--;
-        }
-        dd = mp_mul(spare, x, a);
-	dd = mpbarrett_reduce(mb, dd, dd);
-        spare = x; x = dd;
-      }
+      if (!MP_RSTEP(&sc))
+	goto done;
+      if (MP_RBIT(&sc))
+	break;
       sq++;
-      if (!MP_STEP(&sc))
-        break;
     }
   }
 
+  /* --- Do a final round of squaring --- */
+
+done:
+  while (sq) {
+    mp *y;
+    y = mp_sqr(spare, x);
+    y = mpbarrett_reduce(mb, y, y);
+    spare = x; x = y;
+    sq--;
+  }  
+
+exit:
   MP_DROP(a);
   if (spare != MP_NEW)
     MP_DROP(spare);
-- 
2.11.0