X-Git-Url: https://git.distorted.org.uk/u/mdw/catacomb/blobdiff_plain/34e4f738bcba58e6d8c4cabbb0b3232a65b42a9d..f52f2db067dc1388b16ab00ddb53e26a381a6e3e:/mpbarrett-exp.c

diff --git a/mpbarrett-exp.c b/mpbarrett-exp.c
index 87d8af2..ef8fa59 100644
--- a/mpbarrett-exp.c
+++ b/mpbarrett-exp.c
@@ -1,13 +1,13 @@
 /* -*-c-*-
  *
- * $Id: mpbarrett-exp.c,v 1.1 2004/04/01 12:50:09 mdw Exp $
+ * $Id$
  *
  * Modular exponentiation using Barrett reduction
  *
  * (c) 2004 Straylight/Edgeware
  */
 
-/*----- Licensing notice --------------------------------------------------* 
+/*----- Licensing notice --------------------------------------------------*
  *
  * This file is part of Catacomb.
  *
@@ -15,30 +15,18 @@
  * it under the terms of the GNU Library General Public License as
  * published by the Free Software Foundation; either version 2 of the
  * License, or (at your option) any later version.
- * 
+ *
  * Catacomb is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  * GNU Library General Public License for more details.
- * 
+ *
  * You should have received a copy of the GNU Library General Public
  * License along with Catacomb; if not, write to the Free
  * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
  * MA 02111-1307, USA.
  */
 
-/*----- Revision history --------------------------------------------------* 
- *
- * $Log: mpbarrett-exp.c,v $
- * Revision 1.1  2004/04/01 12:50:09  mdw
- * Add cyclic group abstraction, with test code.  Separate off exponentation
- * functions for better static linking.  Fix a buttload of bugs on the way.
- * Generally ensure that negative exponents do inversion correctly.  Add
- * table of standard prime-field subgroups.  (Binary field subgroups are
- * currently unimplemented but easy to add if anyone ever finds a good one.)
- *
- */
-
 /*----- Header files ------------------------------------------------------*/
 
 #include "mp.h"
@@ -50,11 +38,11 @@
 /* --- @mpbarrett_exp@ --- *
  *
  * Arguments:	@mpbarrett *mb@ = pointer to Barrett reduction context
- *              @mp *d@ = fake destination
- *              @mp *a@ = base
- *              @mp *e@ = exponent
+ *		@mp *d@ = fake destination
+ *		@mp *a@ = base
+ *		@mp *e@ = exponent
  *
- * Returns:     Result, %$a^e \bmod m$%.
+ * Returns:	Result, %$a^e \bmod m$%.
  */
 
 mp *mpbarrett_exp(mpbarrett *mb, mp *d, mp *a, mp *e)
@@ -64,18 +52,16 @@ mp *mpbarrett_exp(mpbarrett *mb, mp *d, mp *a, mp *e)
 
   MP_COPY(a);
   MP_SHRINK(e);
-  if (e->f & MP_NEG) {
-    mp *g = MP_NEW;
-    mp_gcd(&g, 0, &a, mb->m, a);
-    assert(MP_EQ(g, MP_ONE));
-    mp_drop(g);
-  }
-  if (!MP_LEN(e))
+  if (MP_ZEROP(e))
     ;
-  else if (MP_LEN(e) < EXP_THRESH)
-    EXP_SIMPLE(x, a, e);
-  else
-    EXP_WINDOW(x, a, e);
+  else {
+    if (MP_NEGP(e))
+      a = mp_modinv(a, a, mb->m);
+    if (MP_LEN(e) < EXP_THRESH)
+      EXP_SIMPLE(x, a, e);
+    else
+      EXP_WINDOW(x, a, e);
+  }
   mp_drop(d);
   mp_drop(spare);
   mp_drop(a);