X-Git-Url: https://git.distorted.org.uk/u/mdw/catacomb/blobdiff_plain/d34decd2b2b88240cf4ca68a2a5feb7bf36de6e7..22bab86c9df047bdd258283c6567821319ba7a6f:/mpbarrett.c

diff --git a/mpbarrett.c b/mpbarrett.c
index aaa1bb7..0af753e 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 1.6 2000/10/08 12:03:44 mdw Exp $
  *
  * Barrett modular reduction
  *
@@ -30,6 +30,13 @@
 /*----- Revision history --------------------------------------------------* 
  *
  * $Log: mpbarrett.c,v $
+ * Revision 1.6  2000/10/08 12:03:44  mdw
+ * (mpbarrett_reduce): Cope with negative numbers.
+ *
+ * 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
@@ -110,8 +117,7 @@ void mpbarrett_destroy(mpbarrett *mb)
  * Returns:	The residue of @m@ modulo the number in the reduction
  *		context.
  *
- * Use:		Performs an efficient modular reduction.  The argument is
- *		assumed to be positive.
+ * Use:		Performs an efficient modular reduction.
  */
 
 mp *mpbarrett_reduce(mpbarrett *mb, mp *d, mp *m)
@@ -157,7 +163,7 @@ mp *mpbarrett_reduce(mpbarrett *mb, mp *d, mp *m)
     mpx_umul(r->v, r->vl, q->v + k + 1, q->vl, mb->m->v, mb->m->vl);
     MP_DEST(d, k + 1, r->f);
     mpx_usub(d->v, d->vl, m->v, mvl, r->v, r->vl);
-    d->f = (m->f | r->f) & MP_BURN;
+    d->f = (m->f | r->f) & (MP_BURN | MP_NEG);
     MP_DROP(r);
     MP_DROP(q);
     MP_DROP(m);
@@ -169,6 +175,13 @@ mp *mpbarrett_reduce(mpbarrett *mb, mp *d, mp *m)
   while (MPX_UCMP(d->v, d->vl, >=, mb->m->v, mb->m->vl))
     mpx_usub(d->v, d->vl, d->v, d->vl, mb->m->v, mb->m->vl);
 
+  /* --- Fix up the sign --- */
+
+  if (d->f & MP_NEG) {
+    mpx_usub(d->v, d->vl, mb->m->v, mb->m->vl, d->v, d->vl);
+    d->f &= ~MP_NEG;
+  }
+
   MP_SHRINK(d);
   return (d);
 }
@@ -187,33 +200,53 @@ 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);
@@ -238,7 +271,7 @@ static int vmod(dstr *v)
   mpbarrett_create(&mb, n);
   s = mpbarrett_reduce(&mb, MP_NEW, x);
 
-  if (MP_CMP(s, !=, r)) {
+  if (!MP_EQ(s, r)) {
     fputs("\n*** barrett reduction failure\n", stderr);
     fputs("x = ", stderr); mp_writefile(x, stderr, 10); fputc('\n', stderr);
     fputs("n = ", stderr); mp_writefile(n, stderr, 10); fputc('\n', stderr);
@@ -270,7 +303,7 @@ static int vexp(dstr *v)
 
   mr = mpbarrett_exp(&mb, MP_NEW, a, b);
 
-  if (MP_CMP(mr, !=, r)) {
+  if (!MP_EQ(mr, r)) {
     fputs("\n*** barrett modexp failed", stderr);
     fputs("\n m = ", stderr); mp_writefile(m, stderr, 10);
     fputs("\n a = ", stderr); mp_writefile(a, stderr, 10);