using System;
using System.IO;
using System.Text;
using System.Security.Cryptography.X509Certificates;
using iTextSharp.text;
using iTextSharp.text.pdf;
using iTextSharp.text.pdf.security;

namespace Test
{
    class Program
    {
        static void Main(string[] args)
        {
            System.IO.Stream stream = new MemoryStream();
            Document document = new Document();
            document.SetPageSize(PageSize.A4);
            PdfWriter writer = PdfWriter.GetInstance(document, stream);
            writer.CloseStream = false;
            document.Open();
            document.Add(new Paragraph("Hello World"));
            document.Close();
            writer.Close();

            string destPdfFileName = @"D:out.pdf";
            string pfxFileName = @"D:cert.pfx";
            string pfxPassword = "password";
            var cert = new X509Certificate2(pfxFileName, pfxPassword);

            stream.Position = 0;
            Org.BouncyCastle.X509.X509CertificateParser cp = new Org.BouncyCastle.X509.X509CertificateParser();
            Org.BouncyCastle.X509.X509Certificate[] chain = new Org.BouncyCastle.X509.X509Certificate[] { cp.ReadCertificate(cert.RawData) };
            IExternalSignature externalSignature = new X509Certificate2Signature(cert, "SHA-1");
            PdfReader pdfReader = new PdfReader(stream);
            FileStream signedPdf = new FileStream(destPdfFileName, FileMode.Create);  // the output pdf file
            PdfStamper pdfStamper = PdfStamper.CreateSignature(pdfReader, signedPdf, '');
            PdfSignatureAppearance signatureAppearance = pdfStamper.SignatureAppearance;
            signatureAppearance.Reason = "Reason";
            signatureAppearance.Location = "Location";
            signatureAppearance.SetVisibleSignature(new iTextSharp.text.Rectangle(20, 10, 170, 60), 1, "Signature");
            MakeSignature.SignDetached(signatureAppearance, externalSignature, chain, null, null, null, 0, CryptoStandard.CMS);

            Console.ReadKey();
        }
    }
}