General robustification.

[u/mdw/catacomb] / f-prime.c

diff --git a/f-prime.c b/f-prime.c
index 66db76a..43d6da3 100644 (file)
--- a/f-prime.c
+++ b/f-prime.c
@@ -1,6 +1,6 @@
 /* -*-c-*-
  *
- * $Id: f-prime.c,v 1.10 2004/04/02 01:03:49 mdw Exp $
+ * $Id: f-prime.c,v 1.11 2004/04/03 03:32:05 mdw Exp $
  *
  * Prime fields with Montgomery arithmetic
  *
@@ -30,6 +30,9 @@
 /*----- Revision history --------------------------------------------------* 
  *
  * $Log: f-prime.c,v $
+ * Revision 1.11  2004/04/03 03:32:05  mdw
+ * General robustification.
+ *
  * Revision 1.10  2004/04/02 01:03:49  mdw
  * Miscellaneous constification.
  *
@@ -191,7 +194,7 @@ static const 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.
@@ -199,7 +202,11 @@ static const 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;