From d827b2c862fd305d9d8d4bae762ae0e23bb339db Mon Sep 17 00:00:00 2001
From: Ondrej Mikle <ondrej.mikle@nic.cz>
Date: Fri, 4 Jul 2014 17:40:07 +0200
Subject: [PATCH] Account for case when point.y == 0 when doubling.

---
 ecdsa.c | 21 +++++++++++++++------
 ecdsa.h |  1 +
 2 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/ecdsa.c b/ecdsa.c
index 7a98fcb5b..20ef26c25 100644
--- a/ecdsa.c
+++ b/ecdsa.c
@@ -54,14 +54,12 @@ void point_add(const curve_point *cp1, curve_point *cp2)
 		point_copy(cp1, cp2);
 		return;
 	}
-	if (point_is_equal(cp1, cp2)) {
-		point_double(cp2);
+	if (point_is_negative_of(cp1, cp2)) {
+		point_set_infinity(cp2);
 		return;
 	}
-	if (point_is_negative_of(cp1, cp2)) {
-		// set to point at infinity
-		bn_zero(&(cp2->x));
-		bn_zero(&(cp2->y));
+	if (point_is_equal(cp1, cp2)) {
+		point_double(cp2);
 		return;
 	}
 
@@ -98,6 +96,10 @@ void point_double(curve_point *cp)
 	if (point_is_infinity(cp)) {
 		return;
 	}
+	if (bn_is_zero(&(cp->y))) {
+		point_set_infinity(cp);
+		return;
+	}
 
 	memcpy(&inverse_y, &(cp->y), sizeof(bignum256));
 	bn_inverse(&inverse_y, &prime256k1);
@@ -151,6 +153,13 @@ void point_multiply(const bignum256 *k, const curve_point *p, curve_point *res)
 	bn_mod(&(res->y), &prime256k1);
 }
 
+// set point to internal representation of point at infinity
+void point_set_infinity(curve_point *p)
+{
+	bn_zero(&(p->x));
+	bn_zero(&(p->y));
+}
+
 // return true iff p represent point at infinity
 // both coords are zero in internal representation
 int point_is_infinity(const curve_point *p)
diff --git a/ecdsa.h b/ecdsa.h
index 40eef61cb..5aaf7d633 100644
--- a/ecdsa.h
+++ b/ecdsa.h
@@ -36,6 +36,7 @@ void point_copy(const curve_point *cp1, curve_point *cp2);
 void point_add(const curve_point *cp1, curve_point *cp2);
 void point_double(curve_point *cp);
 void point_multiply(const bignum256 *k, const curve_point *p, curve_point *res);
+void point_set_infinity(curve_point *p);
 int point_is_infinity(const curve_point *p);
 int point_is_equal(const curve_point *p, const curve_point *q);
 int point_is_negative_of(const curve_point *p, const curve_point *q);