This project displays an error."CryptographicException was unhandled by user code. Bad Data"

public static string Decrypt(string cipherString, bool useHashing)
    {
        byte[] keyArray;
        byte[] toEncryptArray = Convert.FromBase64String(cipherString);
        System.Configuration.AppSettingsReader settingsReader = new AppSettingsReader();
        string key = (string)settingsReader.GetValue("connection", typeof(String));
        if (useHashing)
        {
            MD5CryptoServiceProvider hashmd5 = new MD5CryptoServiceProvider();
            keyArray = hashmd5.ComputeHash(UTF8Encoding.UTF8.GetBytes(key));
            hashmd5.Clear();
        }
        else
            keyArray = UTF8Encoding.UTF8.GetBytes(key);

        TripleDESCryptoServiceProvider tdes = new TripleDESCryptoServiceProvider();
        tdes.Key = keyArray;
        tdes.Mode = CipherMode.ECB;
        tdes.Padding = PaddingMode.PKCS7;

        ICryptoTransform cTransform = tdes.CreateDecryptor();
        byte[] resultArray = cTransform.TransformFinalBlock(toEncryptArray, 0, toEncryptArray.Length);
        tdes.Clear();
        return UTF8Encoding.UTF8.GetString(resultArray);
    }
}