PDF Document has been altered

I am getting "Document has been altered or corrupted since it was
signed" error when opening signed pdf in Reader using first method.
I created 2 PDF's. First using opensc-pkcs11.dll and second using MS API
ComputeSignature.

Method I
------------
1) calulate SHA1 hash from ss.ToArray() in function SmartCardSign - see
below
2) this SHA1 hash is enrypted on SmartCard using SHA1_RSA, output is
digest which is 128 bytes long

Method II
------------
For testing purposes I used Paulo's SignDetached example described here:
http://itextpdf.sourceforge.net/howtosign.html#signextitextsharp2

There are 3 differences in both PDFs:

1) timestamp field /M(D:20090703162907+03'00')
2) the digest of 128 bytes.
3) trailer in the end of PDF: trailer
<</Info 40 0 R/Root 27 0 R/Size 41/ID
[<115a158e7e38fc64ce62f3efd16f19b4><6f0db7989f0096942ed2c06fa2bba064>]>>


First and second differences are Ok, aren't they ? What aboud trailer
???
Can someone please explain me what am I doing wrong...



public void SignDetached(string filename, string outfile)
{
    X509Certificate2 card = GetCertificate();
    if (card == null)
        throw new ArgumentNullException("Missing certificate");

    Org.BouncyCastle.X509.X509CertificateParser cp = new
Org.BouncyCastle.X509.X509CertificateParser();
    Org.BouncyCastle.X509.X509Certificate[] chain = new
Org.BouncyCastle.X509.X509Certificate[] {
cp.ReadCertificate(card.RawData) };

    PdfReader reader = new PdfReader(filename);
    PdfStamper stp = PdfStamper.CreateSignature(reader, new
FileStream(outfile, FileMode.Create), '\0');
    if (metadata != null)
        stp.XmpMetadata = metadata.getStreamedMetaData();
    PdfSignatureAppearance sap = stp.SignatureAppearance;
    if (appearance.Visible)
        sap.SetVisibleSignature(new Rectangle(100, 100, 300, 200), 1,
null);
    sap.SignDate = DateTime.Now;
    sap.SetCrypto(null, chain, null, null);
    sap.Reason = appearance.Reason;
    sap.Location = appearance.Location;
    sap.Contact = appearance.Contact;
    sap.Acro6Layers = true;
    sap.Render =
PdfSignatureAppearance.SignatureRender.NameAndDescription;            
    PdfSignature dic = new PdfSignature(PdfName.ADOBE_PPKLITE,
PdfName.ADBE_PKCS7_DETACHED);            
    dic.Date = new PdfDate(sap.SignDate);
    dic.Name = PdfPKCS7.GetSubjectFields(chain[0]).GetField("CN");
    if (sap.Reason != null)
        dic.Reason = sap.Reason;
    if (sap.Location != null)
        dic.Location = sap.Location;
    if (sap.Contact != null)
        dic.Contact = sap.Contact;
    sap.CryptoDictionary = dic;
   
    int csize = 1536;
    Hashtable exc = new Hashtable();
    exc[PdfName.CONTENTS] = csize * 2 + 2;
    sap.PreClose(exc);

    Stream s = sap.RangeStream;
    MemoryStream ss = new MemoryStream();
    int read = 0;
    byte[] buff = new byte[8192];
    while ((read = s.Read(buff, 0, 8192)) > 0)
    {
        ss.Write(buff, 0, read);
    }

    bool success = SmartCardSign(ss.ToArray(), ref rsadata);
    if (success == false)
        throw new ArgumentNullException("Failed to sign using
SmartCard");

    PdfPKCS7 pk7 = new PdfPKCS7(null, chain, null, "SHA1", true);
    pk7.SetExternalDigest(rsadata, null, "RSA");
    byte[] pk = pk7.GetEncodedPKCS7();

    byte[] outc = new byte[csize];

    PdfDictionary dic2 = new PdfDictionary();

    Array.Copy(pk, 0, outc, 0, pk.Length);

    dic2.Put(PdfName.CONTENTS, new
PdfString(outc).SetHexWriting(true));
    sap.Close(dic2);
}

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