From: mdw <mdw>
Date: Fri, 19 Nov 1999 13:17:26 +0000 (+0000)
Subject: Add extra interface to exponentiation which returns a Montgomerized
X-Git-Url: https://git.distorted.org.uk/u/mdw/catacomb/commitdiff_plain/17ad212e749b467593355532eeb475374277463e

Add extra interface to exponentiation which returns a Montgomerized
result.
---

diff --git a/mpmont.c b/mpmont.c
index d924009..4885151 100644
--- a/mpmont.c
+++ b/mpmont.c
@@ -1,6 +1,6 @@
 /* -*-c-*-
  *
- * $Id: mpmont.c,v 1.1 1999/11/17 18:02:16 mdw Exp $
+ * $Id: mpmont.c,v 1.2 1999/11/19 13:17:26 mdw Exp $
  *
  * Montgomery reduction
  *
@@ -30,6 +30,10 @@
 /*----- Revision history --------------------------------------------------* 
  *
  * $Log: mpmont.c,v $
+ * Revision 1.2  1999/11/19 13:17:26  mdw
+ * Add extra interface to exponentiation which returns a Montgomerized
+ * result.
+ *
  * Revision 1.1  1999/11/17 18:02:16  mdw
  * New multiprecision integer arithmetic suite.
  *
@@ -158,6 +162,8 @@ mp *mpmont_reduce(mpmont *mm, mp *d, const mp *a)
   d->vl -= dv - d->v;
   MP_SHRINK(d);
   d->f = a->f & MP_BURN;
+  if (MP_CMP(d, >=, mm->m))
+    d = mp_sub(d, d, mm->m);
   return (d);
 }
 
@@ -221,43 +227,67 @@ mp *mpmont_mul(mpmont *mm, mp *d, const mp *a, const mp *b)
   d->vl -= dv - d->v;
   MP_SHRINK(d);
   d->f = (a->f | b->f) & MP_BURN;
+  if (MP_CMP(d, >=, mm->m))
+    d = mp_sub(d, d, mm->m);
   return (d);
 }
 
-/* --- @mpmont_exp@ --- *
+/* --- @mpmont_expr@ --- *
  *
  * Arguments:	@mpmont *mm@ = pointer to Montgomery reduction context
  *		@const mp *a@ = base
  *		@const mp *e@ = exponent
  *
- * Returns:	Result, %$a^e \bmod m$%.
+ * Returns:	Result, %$a^e R \bmod m$%.
  */
 
-mp *mpmont_exp(mpmont *mm, const mp *a, const mp *e)
+mp *mpmont_expr(mpmont *mm, const mp *a, const mp *e)
 {
   mpscan sc;
   mp *ar = mpmont_mul(mm, MP_NEW, a, mm->r2);
   mp *d = MP_COPY(mm->r);
+  mp *spare = MP_NEW;
 
   mp_scan(&sc, e);
 
   if (MP_STEP(&sc)) {
+    size_t sq = 0;
     for (;;) {
       mp *dd;
       if (MP_BIT(&sc)) {
-	dd = mpmont_mul(mm, MP_NEW, d, ar);
-	MP_DROP(d);
-	d = dd;
+	while (sq) {
+	  dd = mpmont_mul(mm, spare, ar, ar);	  
+	  spare = ar; ar = dd;
+	  sq--;
+	}
+	dd = mpmont_mul(mm, spare, d, ar);
+	spare = d; d = dd;
       }
+      sq++;
       if (!MP_STEP(&sc))
 	break;
-      dd = mpmont_mul(mm, MP_NEW, ar, ar);
-      MP_DROP(ar);
-      ar = dd;
     }
   }
   MP_DROP(ar);
-  return (mpmont_reduce(mm, d, d));
+  if (spare != MP_NEW)
+    MP_DROP(spare);
+  return (d);
+}
+
+/* --- @mpmont_exp@ --- *
+ *
+ * Arguments:	@mpmont *mm@ = pointer to Montgomery reduction context
+ *		@const mp *a@ = base
+ *		@const mp *e@ = exponent
+ *
+ * Returns:	Result, %$a^e \bmod m$%.
+ */
+
+mp *mpmont_exp(mpmont *mm, const mp *a, const mp *e)
+{
+  mp *d = mpmont_expr(mm, a, e);
+  d = mpmont_reduce(mm, d, d);
+  return (d);
 }
 
 /*----- Test rig ----------------------------------------------------------*/
@@ -288,7 +318,7 @@ static int tcreate(dstr *v)
     fputs("\n*** bad r", stderr);
     fputs("\nm = ", stderr); mp_writefile(m, stderr, 10);
     fputs("\nexpected ", stderr); mp_writefile(r, stderr, 10);
-    fputs("\n   found ", stderr); mp_writefile(r, stderr, 10);
+    fputs("\n   found ", stderr); mp_writefile(mm.r, stderr, 10);
     fputc('\n', stderr);
     ok = 0;
   }
@@ -297,7 +327,7 @@ static int tcreate(dstr *v)
     fputs("\n*** bad r2", stderr);
     fputs("\nm = ", stderr); mp_writefile(m, stderr, 10);
     fputs("\nexpected ", stderr); mp_writefile(r2, stderr, 10);
-    fputs("\n   found ", stderr); mp_writefile(r2, stderr, 10);
+    fputs("\n   found ", stderr); mp_writefile(mm.r2, stderr, 10);
     fputc('\n', stderr);
     ok = 0;
   }