catcrypt: Implement symmetric key-encapsulation and signature schemes.

[u/mdw/catacomb] / mpmont-exp.c

diff --git a/mpmont-exp.c b/mpmont-exp.c
index f67a8ec..0caf118 100644 (file)
--- a/mpmont-exp.c
+++ b/mpmont-exp.c
@@ -1,6 +1,6 @@
 /* -*-c-*-
  *
- * $Id: mpmont-exp.c,v 1.1 2004/04/01 12:50:09 mdw Exp $
+ * $Id$
  *
  * Modular exponentiation with Montgomery reduction
  *
@@ -27,18 +27,6 @@
  * MA 02111-1307, USA.
  */
 
-/*----- Revision history --------------------------------------------------* 
- *
- * $Log: mpmont-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"
@@ -64,20 +52,19 @@ mp *mpmont_expr(mpmont *mm, mp *d, mp *a, mp *e)
 
   MP_COPY(a);
   MP_SHRINK(e);
-  if (e->f & MP_NEG) {
-    mp *g = MP_NEW;
-    a = mpmont_reduce(mm, a, a);
-    mp_gcd(&g, 0, &a, mm->m, a);
-    assert(MP_EQ(g, MP_ONE));
-    a = mpmont_mul(mm, a, a, mm->r2);
-    mp_drop(g);
-  }
-  if (MP_LEN(e) == 0)
+  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 = mpmont_reduce(mm, a, a);
+      a = mp_modinv(a, a, mm->m);
+      a = mpmont_mul(mm, a, a, mm->r2);
+    }
+    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);