banderwagon: check point in curve in SetBytesUncompressed (#69)

* banderwagon: check if point is in the curve in untrusted calls for SetBytesUncompressed

Signed-off-by: Ignacio Hagopian <jsign.uy@gmail.com>

* add tests for SetBytesUncompressed border cases

Signed-off-by: Ignacio Hagopian <jsign.uy@gmail.com>

* fix typo

Signed-off-by: Ignacio Hagopian <jsign.uy@gmail.com>

---------

Signed-off-by: Ignacio Hagopian <jsign.uy@gmail.com>

banderwagon/element.go

@@ -1,6 +1,7 @@
 package banderwagon
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
 	"math/big"
@@ -243,17 +244,27 @@ func (p *Element) SetBytesUncompressed(buf []byte, trusted bool) error {
 	var x fp.Element
 	x.SetBytes(buf[:coordinateSize])
 
-	// subgroup check
+	var y fp.Element
+	// point in curve & subgroup check
 	if !trusted {
+		point := bandersnatch.GetPointFromX(&x, true)
+		if point == nil {
+			return fmt.Errorf("point not in the curve")
+		}
+		calculatedYBytes := point.Y.Bytes()
+		if !bytes.Equal(calculatedYBytes[:], buf[coordinateSize:]) {
+			return fmt.Errorf("provided Y coordinate doesn't correspond to X")
+		}
+		y = point.Y
+
 		err := subgroupCheck(x)
 		if err != nil {
 			return err
 		}
+	} else {
+		y.SetBytes(buf[coordinateSize:])
 	}
 
-	var y fp.Element
-	y.SetBytes(buf[coordinateSize:])
-
 	*p = Element{inner: bandersnatch.PointProj{
 		X: x,
 		Y: y,

banderwagon/element_test.go

@@ -327,6 +327,48 @@ func TestBatchNormalize(t *testing.T) {
 	})
 }
 
+func TestSetUncompressedFail(t *testing.T) {
+	t.Parallel()
+	one := fp.One()
+
+	t.Run("X not in curve", func(t *testing.T) {
+		startX := one
+		// Find in startX a point that isn't in the curve
+		for {
+			point := bandersnatch.GetPointFromX(&startX, true)
+			if point == nil {
+				break
+			}
+			startX.Add(&startX, &one)
+			continue
+		}
+		var serializedPoint [UncompressedSize]byte
+		xBytes := startX.Bytes()
+		yBytes := Generator.inner.Y.Bytes() // Use some valid-ish Y, but this shouldn't matter much.
+		copy(serializedPoint[:], xBytes[:])
+		copy(serializedPoint[CompressedSize:], yBytes[:])
+
+		var point2 Element
+		if err := point2.SetBytesUncompressed(serializedPoint[:], false); err == nil {
+			t.Fatalf("the point must be rejected")
+		}
+	})
+
+	t.Run("wrong Y", func(t *testing.T) {
+		gen := Generator
+		// Despite X would lead to a point in the curve,
+		// we modify Y+1 to check the provided (serialized) Y
+		// coordinate isn't trusted blindly.
+		gen.inner.Y.Add(&gen.inner.Y, &one)
+
+		pointBytes := gen.BytesUncompressed()
+		var point2 Element
+		if err := point2.SetBytesUncompressed(pointBytes[:], false); err == nil {
+			t.Fatalf("the point must be rejected")
+		}
+	})
+}
+
 func FuzzDeserializationCompressed(f *testing.F) {
 	f.Fuzz(func(t *testing.T, serializedpoint []byte) {
 		var point Element