Я новичок в iTextSharp (и StackOverFlow). Я'я пытаюсь подписать PDF в C # с помощью внешнего USB-токена. Я пытаюсь использовать следующий код, который явыкопал из интернета.

Org.BouncyCastle.X509.X509CertificateParser cp = new Org.BouncyCastle.X509.X509CertificateParser();

//Get Sertifiacte
X509Certificate2 certClient = null;
X509Store st = new X509Store(StoreName.My, StoreLocation.CurrentUser);
st.Open(OpenFlags.MaxAllowed);
X509Certificate2Collection collection = X509Certificate2UI.SelectFromCollection(st.Certificates, "Please choose certificate:", "", X509SelectionFlag.SingleSelection);
if (collection.Count > 0){
   certClient = collection[0];
}
st.Close();
//Get Cert Chain
IList chain = new List();
X509Chain x509chain = new X509Chain();
x509chain.Build(certClient );
foreach (X509ChainElement x509ChainElement in x509chain.ChainElements){
    chain.Add(DotNetUtilities.FromX509Certificate(x509ChainElement.Certificate));
}

PdfReader reader = new PdfReader(sourceDocument);
FileStream resStream = new FileStream(resultDocument, FileMode.Create, FileAccess.ReadWrite);

PdfStamper stamper = PdfStamper.CreateSignature(reader, resStream , '\0', null, true);

PdfSignatureAppearance appearance = stamper.SignatureAppearance;
appearance.Reason = reason;
appearance.Location = location;
appearance.SetVisibleSignature(new iTextSharp.text.Rectangle(20, 10, 170, 60), 1, "Signed");

X509Certificate2Signature es = new X509Certificate2Signature(certClient, "SHA-1");
MakeSignature.SignDetached(appearance, es, chain, null, null, null, 0, CryptoStandard.CMS);

Проблема в том, что я получаю исключение:

System.Security.Cryptography.CryptographicException was unhandled
  Message=Invalid type specified.

  Source=mscorlib
  StackTrace:
       at System.Security.Cryptography.CryptographicException.ThrowCryptographicException(Int32 hr)
       at System.Security.Cryptography.Utils._GetKeyParameter(SafeKeyHandle hKey, UInt32 paramID)
       at System.Security.Cryptography.Utils.GetKeyPairHelper(CspAlgorithmType keyType, CspParameters parameters, Boolean randomKeyContainer, Int32 dwKeySize, SafeProvHandle& safeProvHandle, SafeKeyHandle& safeKeyHandle)
       at System.Security.Cryptography.RSACryptoServiceProvider.GetKeyPair()
       at System.Security.Cryptography.RSACryptoServiceProvider..ctor(Int32 dwKeySize, CspParameters parameters, Boolean useDefaultKeySize)
       at System.Security.Cryptography.X509Certificates.X509Certificate2.get_PrivateKey()
       at iTextSharp.text.pdf.security.X509Certificate2Signature..ctor(X509Certificate2 certificate, String hashAlgorithm)
       at WindowsFormsApplication1.PDFSignerHelper.signPdfFile(String sourceDocument, String resultDocument, X509Certificate2 certClient, String reason, String location)
  InnerException: