/// <summary>
/// Sign a Transaction object
/// </summary>
/// <param name="txn">Transaction to sign</param>
/// <returns>String of signature</returns>
private string SignTransaction(Transaction txn)
{
var signedTxn = _txnProcessor.GetSignedTransaction(txn);
var serializedTxn = signedTxn.Serialize(false);
var signature = _security.SignTransaction(serializedTxn, _neoWallet);
return(signature);
}