X-Git-Url: https://git.distorted.org.uk/u/mdw/catacomb/blobdiff_plain/c3caa2face1cda7002eb58245ad75865bf437455..20fa0f6976d598481208c0583d72b2ccef637be9:/gf-gcd.c

diff --git a/gf-gcd.c b/gf-gcd.c
index 7c09d3a..03cee47 100644
--- a/gf-gcd.c
+++ b/gf-gcd.c
@@ -1,13 +1,13 @@
 /* -*-c-*-
  *
- * $Id: gf-gcd.c,v 1.2 2004/03/21 22:52:06 mdw Exp $
+ * $Id$
  *
  * Euclidian algorithm on binary polynomials
  *
  * (c) 2004 Straylight/Edgeware
  */
 
-/*----- Licensing notice --------------------------------------------------* 
+/*----- Licensing notice --------------------------------------------------*
  *
  * This file is part of Catacomb.
  *
@@ -15,29 +15,18 @@
  * it under the terms of the GNU Library General Public License as
  * published by the Free Software Foundation; either version 2 of the
  * License, or (at your option) any later version.
- * 
+ *
  * Catacomb is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  * GNU Library General Public License for more details.
- * 
+ *
  * You should have received a copy of the GNU Library General Public
  * License along with Catacomb; if not, write to the Free
  * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
  * MA 02111-1307, USA.
  */
 
-/*----- Revision history --------------------------------------------------* 
- *
- * $Log: gf-gcd.c,v $
- * Revision 1.2  2004/03/21 22:52:06  mdw
- * Merge and close elliptic curve branch.
- *
- * Revision 1.1.2.1  2004/03/21 22:39:46  mdw
- * Elliptic curves on binary fields work.
- *
- */
-
 /*----- Header files ------------------------------------------------------*/
 
 #include "gf.h"
@@ -62,7 +51,7 @@ void gf_gcd(mp **gcd, mp **xx, mp **yy, mp *a, mp *b)
   mp *x = MP_ONE, *X = MP_ZERO;
   mp *y = MP_ZERO, *Y = MP_ONE;
   mp *u, *v;
-  mp *q = MP_NEW;
+  mp *q = MP_NEW, *t, *spare = MP_NEW;
   unsigned f = 0;
 
 #define f_swap 1u
@@ -80,7 +69,7 @@ void gf_gcd(mp **gcd, mp **xx, mp **yy, mp *a, mp *b)
    */
 
   if (MPX_UCMP(a->v, a->vl, <, b->v, b->vl)) {
-    { mp *t = a; a = b; b = t; }
+    t = a; a = b; b = t;
     f |= f_swap;
   }
 
@@ -117,34 +106,25 @@ void gf_gcd(mp **gcd, mp **xx, mp **yy, mp *a, mp *b)
     return;
   }
 
-  /* --- Take a reference to the arguments --- */
-
-  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;
+  /* --- Main extended Euclidean algorithm --- */
 
   u = MP_COPY(a);
   v = MP_COPY(b);
 
-  while (MP_LEN(v)) {
-    mp *t;
+  while (!MP_ZEROP(v)) {
     gf_div(&q, &u, u, v);
     if (f & f_ext) {
-      t = gf_mul(MP_NEW, X, q);
-      t = gf_add(t, x, t);
-      MP_DROP(x); x = X; X = t;
-      t = gf_mul(MP_NEW, Y, q);
-      t = gf_add(t, y, t);
-      MP_DROP(y); y = Y; Y = t;
+      t = gf_mul(spare, X, q);
+      t = gf_add(t, t, x);
+      spare = x; x = X; X = t;
+      t = gf_mul(spare, Y, q);
+      t = gf_add(t, t, y);
+      spare = y; y = Y; Y = t;
     }
     t = u; u = v; v = t;
   }
 
-  MP_DROP(q);
+  MP_DROP(q); if (spare) MP_DROP(spare);
   if (!gcd)
     MP_DROP(u);
   else {
@@ -160,7 +140,7 @@ void gf_gcd(mp **gcd, mp **xx, mp **yy, mp *a, mp *b)
     /* --- If @a@ and @b@ got swapped, swap the coefficients back --- */
 
     if (f & f_swap) {
-      mp *t = x; x = y; y = t;
+      t = x; x = y; y = t;
       t = a; a = b; b = t;
     }
 
@@ -183,7 +163,28 @@ void gf_gcd(mp **gcd, mp **xx, mp **yy, mp *a, mp *b)
 
   MP_DROP(v);
   MP_DROP(X); MP_DROP(Y);
-  MP_DROP(a); MP_DROP(b);
+}
+
+/* -- @gf_modinv@ --- *
+ *
+ * Arguments:	@mp *d@ = destination
+ *		@mp *x@ = argument
+ *		@mp *p@ = modulus
+ *
+ * Returns:	The inverse %$x^{-1} \bmod p$%.
+ *
+ * Use:		Computes a modular inverse, the catch being that the
+ *		arguments and results are binary polynomials.  An assertion
+ *		fails if %$p$% has no inverse.
+ */
+
+mp *gf_modinv(mp *d, mp *x, mp *p)
+{
+  mp *g = MP_NEW;
+  gf_gcd(&g, 0, &d, p, x);
+  assert(MP_EQ(g, MP_ONE));
+  mp_drop(g);
+  return (d);
 }
 
 /*----- Test rig ----------------------------------------------------------*/
@@ -202,18 +203,18 @@ static int gcd(dstr *v)
   mp *gg = MP_NEW, *xx = MP_NEW, *yy = MP_NEW;
   gf_gcd(&gg, &xx, &yy, a, b);
   if (!MP_EQ(x, xx)) {
-    fputs("\n*** mp_gcd(x) failed", stderr);
-    fputs("\na      = ", stderr); mp_writefile(a, stderr, 16);
-    fputs("\nb      = ", stderr); mp_writefile(b, stderr, 16);
+    fputs("\n*** gf_gcd(x) failed", stderr);
+    fputs("\na	    = ", stderr); mp_writefile(a, stderr, 16);
+    fputs("\nb	    = ", stderr); mp_writefile(b, stderr, 16);
     fputs("\nexpect = ", stderr); mp_writefile(x, stderr, 16);
     fputs("\nresult = ", stderr); mp_writefile(xx, stderr, 16);
     fputc('\n', stderr);
     ok = 0;
   }
   if (!MP_EQ(y, yy)) {
-    fputs("\n*** mp_gcd(y) failed", stderr);
-    fputs("\na      = ", stderr); mp_writefile(a, stderr, 16);
-    fputs("\nb      = ", stderr); mp_writefile(b, stderr, 16);
+    fputs("\n*** gf_gcd(y) failed", stderr);
+    fputs("\na	    = ", stderr); mp_writefile(a, stderr, 16);
+    fputs("\nb	    = ", stderr); mp_writefile(b, stderr, 16);
     fputs("\nexpect = ", stderr); mp_writefile(y, stderr, 16);
     fputs("\nresult = ", stderr); mp_writefile(yy, stderr, 16);
     fputc('\n', stderr);
@@ -231,9 +232,9 @@ static int gcd(dstr *v)
   }
 
   if (!MP_EQ(g, gg)) {
-    fputs("\n*** mp_gcd(gcd) failed", stderr);
-    fputs("\na      = ", stderr); mp_writefile(a, stderr, 16);
-    fputs("\nb      = ", stderr); mp_writefile(b, stderr, 16);
+    fputs("\n*** gf_gcd(gcd) failed", stderr);
+    fputs("\na	    = ", stderr); mp_writefile(a, stderr, 16);
+    fputs("\nb	    = ", stderr); mp_writefile(b, stderr, 16);
     fputs("\nexpect = ", stderr); mp_writefile(g, stderr, 16);
     fputs("\nresult = ", stderr); mp_writefile(gg, stderr, 16);
     fputc('\n', stderr);