X-Git-Url: https://git.distorted.org.uk/u/mdw/catacomb/blobdiff_plain/c3caa2face1cda7002eb58245ad75865bf437455..298182ad446aaced14dea7ed0e7c968946787288:/mp-gcd.c

diff --git a/mp-gcd.c b/mp-gcd.c
index 6135e54..c78181f 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 $
+ * $Id$
  *
  * 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"
@@ -90,9 +63,9 @@ void mp_gcd(mp **gcd, mp **xx, mp **yy, mp *a, mp *b)
   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@ --- *
@@ -114,7 +87,7 @@ void mp_gcd(mp **gcd, mp **xx, mp **yy, mp *a, mp *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;
@@ -145,20 +118,20 @@ void mp_gcd(mp **gcd, mp **xx, mp **yy, mp *a, mp *b)
     return;
   }
 
-  /* --- Take a reference to the arguments --- */
+  /* --- Force the signs on the arguments and take copies --- */
 
   a = MP_COPY(a);
   b = MP_COPY(b);
 
-  /* --- Make sure @a@ and @b@ are not both even --- */
-
   MP_SPLIT(a); a->f &= ~MP_NEG;
   MP_SPLIT(b); b->f &= ~MP_NEG;
 
   u = MP_COPY(a);
   v = MP_COPY(b);
 
-  while (MP_LEN(v)) {
+  /* --- Main extended Euclidean algorithm --- */
+
+  while (!MP_ZEROP(v)) {
     mp *t;
     mp_div(&q, &u, u, v);
     if (f & f_ext) {
@@ -205,11 +178,11 @@ void mp_gcd(mp **gcd, mp **xx, mp **yy, mp *a, mp *b)
      */
 
     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);
@@ -217,10 +190,10 @@ void mp_gcd(mp **gcd, mp **xx, mp **yy, mp *a, mp *b)
 	}
       }
     } 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);
@@ -256,10 +229,52 @@ 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
 
+static int modinv(dstr *v)
+{
+  int ok = 1;
+  mp *x = *(mp **)v[0].buf;
+  mp *m = *(mp **)v[1].buf;
+  mp *r = *(mp **)v[2].buf;
+
+  mp *y = mp_modinv(MP_NEW, x, m);
+  if (!MP_EQ(y, r)) {
+    fputs("\n*** mp_modinv failed", stderr);
+    fputs("\nx      = ", stderr); mp_writefile(x, stderr, 10);
+    fputs("\nm      = ", stderr); mp_writefile(m, stderr, 10);
+    fputs("\nexpect = ", stderr); mp_writefile(r, stderr, 10);
+    fputs("\nresult = ", stderr); mp_writefile(y, stderr, 10);
+    ok = 0;
+  }
+  MP_DROP(x); MP_DROP(m); MP_DROP(r); MP_DROP(y);
+  assert(mparena_count(MPARENA_GLOBAL) == 0);
+  return (ok);  
+}
+
 static int gcd(dstr *v)
 {
   int ok = 1;
@@ -317,6 +332,7 @@ static int gcd(dstr *v)
 
 static test_chunk tests[] = {
   { "gcd", gcd, { &type_mp, &type_mp, &type_mp, &type_mp, &type_mp, 0 } },
+  { "modinv", modinv, { &type_mp, &type_mp, &type_mp, 0 } },
   { 0, 0, { 0 } }
 };