From 18ba492d74cdc5ac1a9a2de4e67a344da46a7175 Mon Sep 17 00:00:00 2001
From: Abd ar-Rahman Hamidi <bakhtiyor.h@gmail.com>
Date: Wed, 9 Jan 2019 17:02:28 +0500
Subject: [PATCH] Add checking z sign in affineFromJacobian

---
 crypto/secp256k1/curve.go | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/crypto/secp256k1/curve.go b/crypto/secp256k1/curve.go
index 5409ee1d2c..8f83cccad9 100644
--- a/crypto/secp256k1/curve.go
+++ b/crypto/secp256k1/curve.go
@@ -116,6 +116,10 @@ func (BitCurve *BitCurve) IsOnCurve(x, y *big.Int) bool {
 // affineFromJacobian reverses the Jacobian transform. See the comment at the
 // top of the file.
 func (BitCurve *BitCurve) affineFromJacobian(x, y, z *big.Int) (xOut, yOut *big.Int) {
+	if z.Sign() == 0 {
+		return new(big.Int), new(big.Int)
+	}
+
 	zinv := new(big.Int).ModInverse(z, BitCurve.P)
 	zinvsq := new(big.Int).Mul(zinv, zinv)