A signature is not valid in iTextSharp

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itextsharp

Hello,
I got a VB.NET script which makes a PDF document signed. It means - insert a signature and a timestamp. But the final document is not verified and it shows that the signature is not valid: "this document has been altered or corrupted"

Here is the code:

certificateData = podepsat(8)
Dim bytes as Byte()
If String.IsNullOrEmpty(podepsat(2)) Then
   bytes = certificateData.Export(X509ContentType.Pfx)
Else
   bytes = certificateData.Export(X509ContentType.Pfx, podepsat(2))
End If

                processCert(bytes, podepsat(2))

                Dim tsc As ITSAClient = New TSAClientBouncyCastle(podepsat(9), podepsat(10), podepsat(11))

                Dim reader As New PdfReader(fileName)
                fileName = "signed-" & fileName
                Dim fout As New FileStream(fileName, FileMode.Create, FileAccess.Write)
                Dim stp As PdfStamper = PdfStamper.CreateSignature(reader, fout, "\0", Nothing, False)
                Dim sap As PdfSignatureAppearance = stp.SignatureAppearance

                sap.SetCrypto(Nothing, chain, Nothing, PdfSignatureAppearance.SELF_SIGNED)
                sap.Reason = podepsat(3)
                sap.Contact = podepsat(4)
                sap.Location = podepsat(5)
                sap.SignDate = podepsat(6)
                sap.SetVisibleSignature(New iTextSharp.text.Rectangle(100, 100, 300, 200), 1, "Signature")
                Dim dic As New PdfSignature(PdfName.ADOBE_PPKLITE, New PdfName("adbe.pkcs7.detached"))
                dic.Reason = sap.Reason
                dic.Location = sap.Location
                dic.Contact = sap.Contact
                dic.Date = New PdfDate(sap.SignDate)
                sap.CryptoDictionary = dic

                Dim contentEstimated As Integer = 15000
                Dim exc As New Dictionary(Of PdfName, Integer)
                exc(PdfName.CONTENTS) = contentEstimated * 2 + 2
                sap.PreClose(exc)

                Dim sgn As New PdfPKCS7(akp, chain, Nothing, "SHA1", False)
                Dim messageDigest As IDigest = DigestUtilities.GetDigest("SHA1")
                Dim data As Stream = sap.GetRangeStream()
                Dim buf As Byte() = New Byte(8191) {}
                Dim n As Integer
                While (n = data.Read(buf, 0, buf.Length)) > 0
                    messageDigest.BlockUpdate(buf, 0, n)
                End While
                Dim hash As Byte() = New Byte(messageDigest.GetDigestSize() - 1) {}
                messageDigest.DoFinal(hash, 0)
                Dim cal As DateTime = DateTime.Now
                Dim ocsp As Byte() = Nothing
                If chain.Length >= 2 Then
                    Dim url As [String] = PdfPKCS7.GetOCSPURL(chain(0))
                    If url IsNot Nothing AndAlso url.Length > 0 Then
                        ocsp = New OcspClientBouncyCastle().GetEncoded(chain(0), chain(1), url)
                    End If
                End If
                Dim sh As Byte() = sgn.GetAuthenticatedAttributeBytes(hash, cal, ocsp)
                sgn.Update(sh, 0, sh.Length)

                Dim encodedSig As Byte() = sgn.GetEncodedPKCS7(hash, cal, tsc, ocsp)
                If contentEstimated + 2 < encodedSig.Length Then
                    Throw New Exception("Not enough space")
                End If

                Dim paddedSig As Byte() = New Byte(contentEstimated - 1) {}
                System.Array.Copy(encodedSig, 0, paddedSig, 0, encodedSig.Length)

                Dim dic2 As New PdfDictionary()
                dic2.Put(PdfName.CONTENTS, New PdfString(paddedSig).SetHexWriting(True))

                sap.Close(dic2)

Can you help me, please, where is the problem?
processCert fill the akp and chain which are defined up in the code.
certificateData is also declared up.

I can`t find the problem.. Help me solve it, please.

Thank you for your help!