/// <summary>
/// Encodes hash using DER.
/// </summary>
/// <param name="hashData">The hash data.</param>
/// <returns>DER Encoded byte array</returns>
protected byte[] DerEncode(byte[] hashData)
{
var alg = new DerData();
alg.Write(_oid);
alg.WriteNull();
var data = new DerData();
data.Write(alg);
data.Write(hashData);
return data.Encode();
}