From cd1b35daa4b2db076ed1613fc16b7341ca0f2c52 Mon Sep 17 00:00:00 2001
From: Sergey Matveev <stargrave@stargrave.org>
Date: Mon, 12 Jun 2023 15:41:15 +0300
Subject: [PATCH] gost3410.Curve.Contains

---
 gogost.go         |  2 +-
 gost3410/curve.go | 27 ++++++++++++++++-----------
 news.texi         |  5 +++++
 3 files changed, 22 insertions(+), 12 deletions(-)

diff --git a/gogost.go b/gogost.go
index 2aad3a2..d34624f 100644
--- a/gogost.go
+++ b/gogost.go
@@ -1,4 +1,4 @@
 // Pure Go GOST cryptographic functions library.
 package gogost
 
-const Version = "5.10.0"
+const Version = "5.11.0"
diff --git a/gost3410/curve.go b/gost3410/curve.go
index 6cc1135..50d0071 100644
--- a/gost3410/curve.go
+++ b/gost3410/curve.go
@@ -63,17 +63,7 @@ func NewCurve(p, q, a, b, x, y, e, d, co *big.Int) (*Curve, error) {
 		X:    x,
 		Y:    y,
 	}
-	r1 := big.NewInt(0)
-	r2 := big.NewInt(0)
-	r1.Mul(c.Y, c.Y)
-	r1.Mod(r1, c.P)
-	r2.Mul(c.X, c.X)
-	r2.Add(r2, c.A)
-	r2.Mul(r2, c.X)
-	r2.Add(r2, c.B)
-	r2.Mod(r2, c.P)
-	c.pos(r2)
-	if r1.Cmp(r2) != 0 {
+	if !c.Contains(c.X, c.Y) {
 		return nil, errors.New("gogost/gost3410: invalid curve parameters")
 	}
 	if e != nil && d != nil {
@@ -88,6 +78,21 @@ func NewCurve(p, q, a, b, x, y, e, d, co *big.Int) (*Curve, error) {
 	return &c, nil
 }
 
+// Is point on curve?
+func (c *Curve) Contains(x, y *big.Int) bool {
+	r1 := big.NewInt(0)
+	r2 := big.NewInt(0)
+	r1.Mul(y, y)
+	r1.Mod(r1, c.P)
+	r2.Mul(x, x)
+	r2.Add(r2, c.A)
+	r2.Mul(r2, x)
+	r2.Add(r2, c.B)
+	r2.Mod(r2, c.P)
+	c.pos(r2)
+	return r1.Cmp(r2) == 0
+}
+
 // Get the size of the point's coordinate in bytes.
 // 32 for 256-bit curves, 64 for 512-bit ones.
 func (c *Curve) PointSize() int {
diff --git a/news.texi b/news.texi
index 92f34a1..3b0a8af 100644
--- a/news.texi
+++ b/news.texi
@@ -3,6 +3,11 @@
 
 @table @strong
 
+@anchor{Release 5.11.0}
+@item 5.11.0
+You can check if public key is on curve with
+@code{gost3410.Curve.Contains} method now.
+
 @anchor{Release 5.10.0}
 @item 5.10.0
     @itemize
-- 
2.44.0