// Curve is not supported by crypto/ecdh.
func (k *PublicKey) ECDH() (*ecdh.PublicKey, error) {
	c := curveToECDH(k.Curve)
	if c == nil {
		return nil, errors.New("ecdsa: unsupported curve by crypto/ecdh")
	}
	if !k.Curve.IsOnCurve(k.X, k.Y) {
		return nil, errors.New("ecdsa: invalid public key")
	}
	return c.NewPublicKey(elliptic.Marshal(k.Curve, k.X, k.Y))
}

// Equal reports whether pub and x have the same value.
//