Sagepay报告API错误无法验证签名价值

我是。net的新手，需要一些帮助。我试图创建一个后台系统与使用sagepay管理和报告API sagepay集成。我在网上找到了一个非常有用的示例代码，但是当我试图调用API时，我得到了上述错误。下面是执行该调用的示例代码。我想知道有没有人能告诉我哪里出错了?

       private string BuildCommandString(string command, string vendor, string user, string xmldata, string password = null, string signature = null)
    {
        return string.Format("<command>{0}</command><vendor>{1}</vendor><user>{2}</user>{3}{4}{5}",
            command,
            Vendor,
            User,
            xmldata ?? string.Empty,
            (string.IsNullOrEmpty(password) == false ? "<password>" + password + "</password>" : string.Empty),
            (string.IsNullOrEmpty(signature) == false ? "<signature>" + signature + "</signature>" : string.Empty));
    }
    /// <summary>
    /// Perform the main call for the API and collect the response
    /// </summary>
    /// <param name="command">api command name</param>
    /// <param name="xmldata">optional extra data for api</param>
    /// <returns>new SagePayResponse or null if communication error</returns>
    protected SagePayResponse ProcessAPI(string command, string xmldata)
    {
        // get the requiest
        HttpWebRequest httpRequest = (HttpWebRequest)WebRequest.Create(Url);
        httpRequest.Method = "POST";
        // build data
        string data = BuildCommandString(command, Vendor, User, xmldata, Password);
        // apply signature
        MD5 md5 = new MD5CryptoServiceProvider();
        byte[] hash = md5.ComputeHash(Encoding.UTF8.GetBytes(data));
        string sig = BitConverter.ToString(hash).Replace("-", string.Empty);
        // rebuild with signature
        data = "XML=<vspaccess>" + BuildCommandString(command, Vendor, User, xmldata, null, sig) + "</vspaccess>";
        // get the data
        byte[] bytes = Encoding.UTF8.GetBytes(data);
        httpRequest.ContentType = "application/x-www-form-urlencoded";
        httpRequest.ContentLength = data.Length;
        // get the request stream
        Stream requestStream = httpRequest.GetRequestStream();
        requestStream.Write(bytes, 0, bytes.Length);
        requestStream.Close();
        // call the sagepay url and get response
        SagePayResponse sagePayResponse = null;
        HttpWebResponse response = (HttpWebResponse)httpRequest.GetResponse();
        try
        {
            if (response.StatusCode == HttpStatusCode.OK)
            {
                Stream responseStream = response.GetResponseStream();
                //string contentType = response.ContentType;
                StreamReader reader = new StreamReader(responseStream, Encoding.UTF8);
                try
                {
                    //Console.WriteLine("response ok");
                    sagePayResponse = new SagePayResponse(reader.ReadToEnd());
                }
                finally
                {
                    reader.Close();
                }
            }
        }
        finally
        {
            response.Close();
        }
        return sagePayResponse;
    }