.NET+iText移动端签名问题：PDF提示文档已篡改

我使用.NET结合iText开发移动端签名应用时遇到问题：将签名添加至PDF文件后，文档提示“已被篡改或损坏”。我可以从响应XML中解析出用户证书与Base64格式的签名值，但通过iText将它们添加到PDF时，仍会出现该错误提示。

public Envelope Imzala() {

        Byte[] bytes = null;
        PdfReader pdfReader = null;
        FileInfo fInfo = new FileInfo(@"C:\mobilimza\imzasiz.pdf");
        byte[] hashValue = null;
        using (SHA256 mySHA256 = SHA256.Create())
        {
            using (FileStream fileStream = fInfo.Open(FileMode.Open))
            {
                pdfReader = new PdfReader(fileStream);
                try
                {
                    fileStream.Position = 0;
                    hashValue = mySHA256.ComputeHash(fileStream);


                }
                catch (IOException e)
                {
                    Console.WriteLine($"I/O Exception: {e.Message}");
                }
            }
        }
        bytes = hashValue;
        DataToBeSigned = Convert.ToBase64String(bytes);

        string soapResult = string.Empty;
        XmlDocument responseXML = new XmlDocument();
        try
        {
            WebRequest webRequest = CreateWebRequest(GlobalParameters.imzaServisLink);
            XmlDocument soapEnvelopeXml = new XmlDocument();



            string xml1 = @""// my sign request;


            soapEnvelopeXml.LoadXml(xml1);
            using (Stream stream = webRequest.GetRequestStream())
            {
                soapEnvelopeXml.Save(stream);
            }
            using (WebResponse response = webRequest.GetResponse())
            {


                using (StreamReader rd = new StreamReader(response.GetResponseStream()))
                {
                    soapResult = rd.ReadToEnd();
                    Envelope envelope;
                    var mySerializer = new XmlSerializer(typeof(Envelope));
                    using (TextReader reader = new StringReader(soapResult))
                    {
                        envelope = (Envelope)mySerializer.Deserialize(reader);

                        string base64sign = envelope.Body.MSS_SignatureResponse.MSS_SignatureResp.MSS_Signature.Base64Signature;

                        byte[] data = Convert.FromBase64String(base64sign);

                        string decodedText = "<Root>" + Encoding.UTF8.GetString(data) + "</Root>";

                        XmlDocument xmlDoc = new XmlDocument();
                        xmlDoc.LoadXml(decodedText);


                        XmlElement keyInfoElement = (XmlElement)xmlDoc.SelectSingleNode("//KeyInfo");
                        XmlElement x509CertificateElement = (XmlElement)keyInfoElement.SelectSingleNode(".//X509Certificate");

                        XmlElement signvalue = (XmlElement)xmlDoc.SelectSingleNode(".//SignatureValue");

                        if (x509CertificateElement != null)
                        {
                            string x509CertificateText = x509CertificateElement.InnerText;
                            string signText = signvalue.InnerText;
                            byte[] x509data = Convert.FromBase64String(x509CertificateText);
                            X509Certificate2 x509 = new(x509data);

                            PdfWriter pdfWriter = new PdfWriter(@"c:\mobilimza\imzali.pdf");

                            StampingProperties sp = new StampingProperties();
                            sp.UseAppendMode();
                            PdfSigner pdfSigner = new PdfSigner(pdfReader, pdfWriter, sp);

                            PdfSignatureAppearance appearance = pdfSigner.GetSignatureAppearance();

                            //pdfSigner.SetFieldName("Signature");
                            //appearance.SetPageNumber(1);

                            // Set the X.509 certificate chain
                            X509CertificateParser parser = new X509CertificateParser();
                            Org.BouncyCastle.X509.X509Certificate[] chain = new Org.BouncyCastle.X509.X509Certificate[] { DotNetUtilities.FromX509Certificate(x509) };

                            pdfSigner.SignDetached(new CustomExternalSignature1(Convert.FromBase64String(signText)), chain, null, null, null, 0, PdfSigner.CryptoStandard.CMS);

                        }

                        return envelope;
                    }


                }


            }
        }
        catch (Exception ee)
        {
            throw;
        }
    }

PDF文档打开后提示“已被篡改或损坏”

内容的提问来源于stack exchange，提问作者tolgahan özden