Digital Signature issue. [Itext] - itext7

Question

I'm using itext7 for digitally signing the PDF document. below is my scenario:
when a sender send's a PDF document for review (or) signing. we're digitally signing the certificate from our end for security purpose.
when the document from (1) is sent to another sender for signing, the first signer signature is becoming invalid and second signer signature is becoming valid.
we're not showing the digitally signed image.
Code we've written:
public static ConversionJob AddDigitalSignature(string messageId, string keyFilename, string keyFilePassword, string pdfUserPassword, string pdfOwnerPassword, string signatureCreator, string pdfFilename, string outputFilename, bool restrictPdf = false)
{
var encrypt = !string.IsNullOrEmpty(pdfUserPassword) || !string.IsNullOrEmpty(pdfOwnerPassword) || restrictPdf;
outputFilename = !string.IsNullOrEmpty(outputFilename) ? outputFilename : pdfFilename;
var convertJob = new ConversionJob(messageId, "DigitalSignManager.AddDigitalSignature", pdfFilename, outputFilename);
var flattenFile = pdfFilename + ".flatten.pdf";
var encryptedFile = pdfFilename + ".encrypt.pdf";
try
{
_Log.Info("Adding digital signature to file {0} :: [id={1}]", pdfFilename, messageId);
// Remove existing signatures
if (File.Exists(flattenFile))
File.Delete(flattenFile);
ReaderProperties readerOptions = new ReaderProperties();
if (!string.IsNullOrEmpty(pdfOwnerPassword) || restrictPdf)
{
readerOptions.SetPassword(Encoding.ASCII.GetBytes(pdfOwnerPassword));
if (File.Exists(encryptedFile))
File.Delete(encryptedFile);
}
PdfReader reader = new PdfReader(pdfFilename, readerOptions);
PdfDocument pdfDoc = new PdfDocument(reader, new PdfWriter(flattenFile));
PdfAcroForm form = PdfAcroForm.GetAcroForm(pdfDoc, true);
bool hasSignature = form.GetSignatureFlags() > 0;
if (hasSignature)
{
List<string> sigFields = new List<string>();
var fields = form.GetFormFields();
foreach (var field in fields)
{
var formType = field.Value.GetFormType();
if (formType.Equals(PdfName.Sig)) // here's a sig item
{
form.PartialFormFlattening(field.Key);
}
}
form.FlattenFields();
}
//var signFields = form.GetFormFields();
//if (signFields.Keys.Count > 1)
//{
// foreach (var field in signFields)
// {
// var formType = field.Value.GetFormType();
// if (formType.Equals(PdfName.Sig)) // here's a sig item
// {
// form.PartialFormFlattening(field.Key);
// }
// }
// form.FlattenFields();
//}
SignatureUtil signUtil = new SignatureUtil(pdfDoc);
IList<String> names = signUtil.GetSignatureNames();
if (names.Count >= 1)
{
foreach (String name in names)
{
PdfPKCS7 pkcs7 = signUtil.ReadSignatureData(name);
var validity = pkcs7.VerifySignatureIntegrityAndAuthenticity();
var test = signUtil.SignatureCoversWholeDocument(name);
form.PartialFormFlattening(name);
//form.RemoveField(name);
}
form.FlattenFields();
}
if (!pdfDoc.IsClosed())
pdfDoc.Close();
reader.Close();
// reapply password if encrypted
// Sprint S11-119 S11-166 PDF Encryption Start
if (encrypt)
{
var encrypted = Encrypt(messageId, pdfUserPassword, pdfOwnerPassword, flattenFile, encryptedFile, restrictPdf);
convertJob.Encrypted = encrypted.ConvertResult == ConvertResult.Success;
}
// Sprint S11-119 S11-166 PDF Encryption End
// digitally sign the file with certificate
string reason = _Configuration.DigitalSignatureReason; //"Test Electronic Signature";
string location = _Configuration.DigitalSignatureLocation; //"US North America West";
var digitalCert = new DigitalCert(keyFilename, keyFilePassword, true);
var presignPdf = encrypt ? encryptedFile : flattenFile;
PdfReader finalReader = new PdfReader(presignPdf, readerOptions);
StampingProperties sp = new StampingProperties();
sp.UseAppendMode();
PdfSigner signer = new PdfSigner(finalReader, new FileStream(outputFilename, FileMode.OpenOrCreate, FileAccess.ReadWrite), sp);
// Creating the appearance
Rectangle rect = new Rectangle(0, 0, 0, 0);
PdfSignatureAppearance appearance = signer.GetSignatureAppearance()
//.SetCertificate()
.SetSignatureCreator(signatureCreator)
.SetReason(reason)
.SetLocation(location)
.SetReuseAppearance(false)
.SetPageRect(rect)
.SetPageNumber(1);
//signer.SetFieldName("sig");
signer.SetCertificationLevel(PdfSigner.CERTIFIED_FORM_FILLING);
signer.SetSignDate(DateTime.Now);
// Creating the signature
IExternalSignature pks = new PrivateKeySignature(digitalCert.Akp, digitalCert.DigestAlgorithm);
signer.SignDetached(pks, digitalCert.Chain, null, null, null, 0, PdfSigner.CryptoStandard.CADES);
finalReader.Close();
// cleanup
if (File.Exists(flattenFile))
File.Delete(flattenFile);
if (File.Exists(encryptedFile))
File.Delete(encryptedFile);
//PdfDocument pdfDoc1 = new PdfDocument(new PdfWriter(outputFilename),sp.UseAppendMode());
//PdfAcroForm form1 = PdfAcroForm.GetAcroForm(pdfDoc1, true);
//SignatureUtil signUtil1 = new SignatureUtil(pdfDoc1);
//IList<String> names1 = signUtil1.GetSignatureNames();
//if (names1.Count >= 1)
//{
// foreach (String name in names1)
// {
// PdfPKCS7 pkcs7 = signUtil1.ReadSignatureData(name);
// var validity = pkcs7.VerifySignatureIntegrityAndAuthenticity();
// var test = signUtil1.SignatureCoversWholeDocument(name);
// if (!validity)
// form1.PartialFormFlattening(name);
// }
// form1.FlattenFields();
// if (!pdfDoc1.IsClosed())
// pdfDoc1.Close();
//}
convertJob.SetComplete(ConvertResult.Success);
}
catch (Exception ex)
{
var errorMessage = string.Format("Error adding digital signature to {0}: {1} :: [id={2}]", pdfFilename, ex.Message, messageId);
_Log.Error(ex, FailureType.Conversion, errorMessage);
convertJob.SetComplete(ConvertResult.Failed, errorMessage, ex);
}
return convertJob;
}
the commented code is for removing the not verified signature and adding only the valid signature. But same issue is coming.