X-Git-Url: https://git.distorted.org.uk/u/mdw/catacomb/blobdiff_plain/77d943105c69c4dc44a2e8d882a1fe8a69a94a2c..391faf428ac513f9031c773016d79fdbe8b4653c:/ec-prime.c

diff --git a/ec-prime.c b/ec-prime.c
index bdc6368..827c0f2 100644
--- a/ec-prime.c
+++ b/ec-prime.c
@@ -1,6 +1,6 @@
 /* -*-c-*-
  *
- * $Id: ec-prime.c,v 1.4 2004/03/21 22:52:06 mdw Exp $
+ * $Id: ec-prime.c,v 1.5 2004/03/22 02:19:10 mdw Exp $
  *
  * Elliptic curves over prime fields
  *
@@ -30,6 +30,11 @@
 /*----- Revision history --------------------------------------------------* 
  *
  * $Log: ec-prime.c,v $
+ * Revision 1.5  2004/03/22 02:19:10  mdw
+ * Rationalise the sliding-window threshold.  Drop guarantee that right
+ * arguments to EC @add@ are canonical, and fix up projective implementations
+ * to cope.
+ *
  * Revision 1.4  2004/03/21 22:52:06  mdw
  * Merge and close elliptic curve branch.
  *
@@ -282,19 +287,26 @@ static ec *ecprojadd(ec_curve *c, ec *d, const ec *a, const ec *b)
     EC_COPY(d, a);
   else {
     field *f = c->f;
-    mp *p, *q, *r, *w, *u, *s, *dx, *dy, *dz;
+    mp *p, *q, *r, *w, *u, *uu, *s, *ss, *dx, *dy, *dz;
 
     q = F_SQR(f, MP_NEW, a->z);		/* %$z_0^2$% */
     u = F_MUL(f, MP_NEW, q, b->x);	/* %$u = x_1 z_0^2$% */
     p = F_MUL(f, MP_NEW, q, b->y);	/* %$y_1 z_0^2$% */
     s = F_MUL(f, q, p, a->z);		/* %$s = y_1 z_0^3$% */
 
-    w = F_SUB(f, p, a->x, u);		/* %$w = x_0 - u$% */
-    r = F_SUB(f, MP_NEW, a->y, s);	/* %$r = y_0 - s$% */
+    q = F_SQR(f, MP_NEW, b->z);		/* %$z_1^2$% */
+    uu = F_MUL(f, MP_NEW, q, a->x);	/* %$uu = x_0 z_1^2$%*/
+    p = F_MUL(f, p, q, a->y);		/* %$y_0 z_1^2$% */
+    ss = F_MUL(f, q, p, b->z);		/* %$ss = y_0 z_1^3$% */
+
+    w = F_SUB(f, p, uu, u);		/* %$w = uu - u$% */
+    r = F_SUB(f, MP_NEW, ss, s);	/* %$r = ss - s$% */
     if (F_ZEROP(f, w)) {
       MP_DROP(w);
       MP_DROP(u);
       MP_DROP(s);
+      MP_DROP(uu);
+      MP_DROP(ss);
       if (F_ZEROP(f, r)) {
 	MP_DROP(r);
 	return (c->ops->dbl(c, d, a));
@@ -304,12 +316,13 @@ static ec *ecprojadd(ec_curve *c, ec *d, const ec *a, const ec *b)
 	return (d);
       }
     }
-    u = F_ADD(f, u, u, a->x);		/* %$t = x_0 + u$% */
-    s = F_ADD(f, s, s, a->y);		/* %$m = y_0 + r$% */
+    u = F_ADD(f, u, u, uu);		/* %$t = uu + u$% */
+    s = F_ADD(f, s, s, ss);		/* %$m = ss + r$% */
 
-    dz = F_MUL(f, MP_NEW, a->z, w);	/* %$z' = z_0 w$% */
+    uu = F_MUL(f, uu, a->z, w);		/* %$z_0 w$% */
+    dz = F_MUL(f, ss, uu, b->z);	/* %$z' = z_0 z_1 w$% */
 
-    p = F_SQR(f, MP_NEW, w);		/* %$w^2$% */
+    p = F_SQR(f, uu, w);		/* %$w^2$% */
     q = F_MUL(f, MP_NEW, p, u);		/* %$t w^2$% */
     u = F_MUL(f, u, p, w);		/* %$w^3$% */
     p = F_MUL(f, p, u, s);		/* %$m w^3$% */