X-Git-Url: https://git.distorted.org.uk/u/mdw/catacomb/blobdiff_plain/898a4e2555438ff8adb08b4d82690d08715e1048..b817bfc642225b8c3c0b6a7e42d1fb949b61a606:/mp-gcd.c

diff --git a/mp-gcd.c b/mp-gcd.c
index 6135e54..5e663a1 100644
--- a/mp-gcd.c
+++ b/mp-gcd.c
@@ -1,6 +1,6 @@
 /* -*-c-*-
  *
- * $Id: mp-gcd.c,v 1.6 2004/03/21 22:52:06 mdw Exp $
+ * $Id: mp-gcd.c,v 1.7 2004/04/08 01:36:15 mdw Exp $
  *
  * Extended GCD calculation
  *
@@ -27,33 +27,6 @@
  * MA 02111-1307, USA.
  */
 
-/*----- Revision history --------------------------------------------------* 
- *
- * $Log: mp-gcd.c,v $
- * Revision 1.6  2004/03/21 22:52:06  mdw
- * Merge and close elliptic curve branch.
- *
- * Revision 1.5.4.1  2004/03/21 22:39:46  mdw
- * Elliptic curves on binary fields work.
- *
- * Revision 1.5  2000/10/08 12:02:41  mdw
- * Use Euclid's algorithm rather than the binary one.
- *
- * Revision 1.4  2000/06/17 11:34:46  mdw
- * More hacking for the signs of the outputs.
- *
- * Revision 1.3  1999/12/10 23:18:39  mdw
- * Change interface for suggested destinations.
- *
- * Revision 1.2  1999/11/22 20:49:56  mdw
- * Fix bug which failed to favour `x' when `y' wasn't wanted and the two
- * arguments needed swapping.
- *
- * Revision 1.1  1999/11/17 18:02:16  mdw
- * New multiprecision integer arithmetic suite.
- *
- */
-
 /*----- Header files ------------------------------------------------------*/
 
 #include "mp.h"
@@ -256,6 +229,27 @@ void mp_gcd(mp **gcd, mp **xx, mp **yy, mp *a, mp *b)
   MP_DROP(a); MP_DROP(b);
 }
 
+/* -- @mp_modinv@ --- *
+ *
+ * Arguments:	@mp *d@ = destination
+ *		@mp *x@ = argument
+ *		@mp *p@ = modulus
+ *
+ * Returns:	The inverse %$x^{-1} \bmod p$%.
+ *
+ * Use:		Computes a modular inverse.    An assertion fails if %$p$%
+ *		has no inverse.
+ */
+
+mp *mp_modinv(mp *d, mp *x, mp *p)
+{
+  mp *g = MP_NEW;
+  mp_gcd(&g, 0, &d, p, x);
+  assert(MP_EQ(g, MP_ONE));
+  mp_drop(g);
+  return (d);
+}
+
 /*----- Test rig ----------------------------------------------------------*/
 
 #ifdef TEST_RIG