/* -*-c-*-
*
- * $Id: mp-gcd.c,v 1.5.4.1 2004/03/21 22:39:46 mdw Exp $
+ * $Id$
*
* Extended GCD calculation
*
* MA 02111-1307, USA.
*/
-/*----- Revision history --------------------------------------------------*
- *
- * $Log: mp-gcd.c,v $
- * 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"
if (xx || yy)
f |= f_ext;
- if (a->f & MP_NEG)
+ if (MP_NEGP(a))
f |= f_aneg;
- if (b->f & MP_NEG)
+ if (MP_NEGP(b))
f |= f_bneg;
/* --- Ensure that @a@ is larger than @b@ --- *
if (gcd) {
if (*gcd) MP_DROP(*gcd);
a = MP_COPY(a);
- if (a->f & MP_NEG) {
+ if (MP_NEGP(a)) {
MP_SPLIT(a);
a->f &= ~MP_NEG;
f |= f_aneg;
u = MP_COPY(a);
v = MP_COPY(b);
- while (MP_LEN(v)) {
+ while (!MP_ZEROP(v)) {
mp *t;
mp_div(&q, &u, u, v);
if (f & f_ext) {
*/
if (yy) {
- if (y->f & MP_NEG) {
+ if (MP_NEGP(y)) {
do {
y = mp_add(y, y, a);
x = mp_sub(x, x, b);
- } while (y->f & MP_NEG);
+ } while (MP_NEGP(y));
} else {
while (MP_CMP(y, >=, a)) {
y = mp_sub(y, y, a);
}
}
} else {
- if (x->f & MP_NEG) {
+ if (MP_NEGP(x)) {
do
x = mp_add(x, x, b);
- while (x->f & MP_NEG);
+ while (MP_NEGP(x));
} else {
while (MP_CMP(x, >=, b))
x = mp_sub(x, x, 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