Abstract & expose CompressedPointToSignAndY

Add scanner/valuer interface to babyjub.PublicKeyComp

babyjub/babyjub.go

@@ -222,13 +222,26 @@ func (p *Point) Compress() [32]byte {
 // Decompress a compressed Point into p, and also returns the decompressed
 // Point.  Returns error if the compressed Point is invalid.
 func (p *Point) Decompress(leBuf [32]byte) (*Point, error) {
+	var sign bool
+	sign, p.Y = CompressedPointToSignAndY(leBuf)
+	return PointFromSignAndY(sign, p.Y)
+}
+
+// CompressedPointToSignAndY returns the sign and coordinate Y from a given
+// compressed point. This method does not check that the Point belongs to the
+// BabyJubJub curve, thus does not return error in such case. This method is
+// intended to obtain the sign and the Y coordinate without checking if the
+// point belongs to the curve, if the objective is to uncompress a point
+// Decompress method should be used instead.
+func CompressedPointToSignAndY(leBuf [32]byte) (bool, *big.Int) {
 	sign := false
+	y := big.NewInt(0)
 	if (leBuf[31] & 0x80) != 0x00 { //nolint:gomnd
 		sign = true
 		leBuf[31] = leBuf[31] & 0x7F //nolint:gomnd
 	}
-	utils.SetBigIntFromLEBytes(p.Y, leBuf[:])
-	return PointFromSignAndY(sign, p.Y)
+	utils.SetBigIntFromLEBytes(y, leBuf[:])
+	return sign, y
 }
 
 // PointFromSignAndY returns a Point from a Sign and the Y coordinate