X-Git-Url: https://git.distorted.org.uk/u/mdw/catacomb/blobdiff_plain/4edc47b89bc56cd4041fdb0f4e8e892acd589ed8..c65df27983057ec76ed0e72bb370f9a5ae7dad28:/f-prime.c

diff --git a/f-prime.c b/f-prime.c
index 088032b..f58bf63 100644
--- a/f-prime.c
+++ b/f-prime.c
@@ -1,6 +1,6 @@
 /* -*-c-*-
  *
- * $Id: f-prime.c,v 1.9 2004/04/01 21:28:41 mdw Exp $
+ * $Id: f-prime.c,v 1.12 2004/04/08 01:36:15 mdw Exp $
  *
  * Prime fields with Montgomery arithmetic
  *
@@ -27,53 +27,6 @@
  * MA 02111-1307, USA.
  */
 
-/*----- Revision history --------------------------------------------------* 
- *
- * $Log: f-prime.c,v $
- * Revision 1.9  2004/04/01 21:28:41  mdw
- * Normal basis support (translates to poly basis internally).  Rewrite
- * EC and prime group table generators in awk, so that they can reuse data
- * for repeated constants.
- *
- * Revision 1.8  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.)
- *
- * Revision 1.7  2004/03/27 17:54:11  mdw
- * Standard curves and curve checking.
- *
- * Revision 1.6  2004/03/23 15:19:32  mdw
- * Test elliptic curves more thoroughly.
- *
- * Revision 1.5  2004/03/23 12:08:26  mdw
- * Random field-element selection.
- *
- * Revision 1.4  2004/03/21 22:52:06  mdw
- * Merge and close elliptic curve branch.
- *
- * Revision 1.3.4.3  2004/03/21 22:39:46  mdw
- * Elliptic curves on binary fields work.
- *
- * Revision 1.3.4.2  2004/03/20 00:13:31  mdw
- * Projective coordinates for prime curves
- *
- * Revision 1.3.4.1  2003/06/10 13:43:53  mdw
- * Simple (non-projective) curves over prime fields now seem to work.
- *
- * Revision 1.3  2003/05/15 23:25:59  mdw
- * Make elliptic curve stuff build.
- *
- * Revision 1.2  2002/01/13 13:48:44  mdw
- * Further progress.
- *
- * Revision 1.1  2001/04/29 18:12:33  mdw
- * Prototype version.
- *
- */
-
 /*----- Header files ------------------------------------------------------*/
 
 #include <mLib/sub.h>
@@ -135,7 +88,7 @@ static mp *fsqr(field *ff, mp *d, mp *x) {
 
 static mp *finv(field *ff, mp *d, mp *x) {
   fctx *f = (fctx *)ff; d = mpmont_reduce(&f->mm, d, x);
-  mp_gcd(0, 0, &d, f->mm.m, d); return (mpmont_mul(&f->mm, d, d, f->mm.r2));
+  d = mp_modinv(d, d, f->mm.m); return (mpmont_mul(&f->mm, d, d, f->mm.r2));
 }
 
 static mp *freduce(field *ff, mp *d, mp *x)
@@ -175,7 +128,7 @@ static mp *fhlv(field *ff, mp *d, mp *x) {
 
 /* --- Field operations table --- */
 
-static field_ops fops = {
+static const field_ops fops = {
   FTY_PRIME, "prime",
   fdestroy, frand, field_stdsamep,
   fin, fout,
@@ -188,7 +141,7 @@ static field_ops fops = {
  *
  * Arguments:	@mp *p@ = the characteristic of the field
  *
- * Returns:	A pointer to the field.
+ * Returns:	A pointer to the field or null.
  *
  * Use:		Creates a field structure for a prime field of size %$p$%,
  *		using Montgomery reduction for arithmetic.
@@ -196,7 +149,11 @@ static field_ops fops = {
 
 field *field_prime(mp *p)
 {
-  fctx *f = CREATE(fctx);
+  fctx *f;
+
+  if (!MP_ISPOS(p) || !MP_ISODD(p))
+    return (0);
+  f = CREATE(fctx);
   f->f.ops = &fops;
   mpmont_create(&f->mm, p);
   f->f.zero = MP_ZERO;