Digitally Sign a PDF File in C#

static void DigitallySignPDFFile(string unsignedDocument, string signedDocument)
{
    PdfSignature ps = new PdfSignature("");

    //load the PDF document
    ps.LoadPdfDocument(unsignedDocument);
    ps.SignaturePosition = SignaturePosition.TopRight;
    ps.SigningReason = "I approve this document";
    ps.SigningLocation = "Accounting department";
    ps.SignaturePosition = SignaturePosition.TopLeft;

    //the signature will be timestamped.
    ps.TimeStamping.ServerUrl = new Uri("https://ca.signfiles.com/TSAServer.aspx");
    //set the digital signature hash algorithm
    ps.HashAlgorithm = SignLib.HashAlgorithm.SHA256;

    //Digital signature certificate can be loaded from various sources
    //Load the signature certificate from a PFX or P12 file
    ps.DigitalSignatureCertificate = DigitalCertificate.LoadCertificate("cert.pfx", "123456");

    //Load the certificate from Microsoft Store. 
    //The smart card or USB token certificates are usually available on Microsoft Certificate Store (start - run - certmgr.msc).
    //If the smart card certificate not appears on Microsoft Certificate Store it cannot be used by the library
    //ps.DigitalSignatureCertificate = DigitalCertificate.LoadCertificate(false, string.Empty, "Select Certificate", "Select the certificate for digital signature");

    //write the signed file
    File.WriteAllBytes(signedDocument, ps.ApplyDigitalSignature());
}

static void VerifyPDFSignature(string signedDocument)
{
    PdfSignature ps = new PdfSignature("");

    ps.LoadPdfDocument(signedDocument);

    Console.WriteLine("Number of signatures: " + ps.DocumentProperties.DigitalSignatures.Count.ToString());

    //verify every digital signature form the PDF document
    foreach (PdfSignatureInfo csi in ps.DocumentProperties.DigitalSignatures)
    {
      Console.WriteLine("Signature name: " + csi.SignatureName);
      Console.WriteLine("Hash Algorithm: " + csi.HashAlgorithm.ToString());
      Console.WriteLine("Signature Certificate Information");
      ExtractCertificateInformation(csi.SignatureCertificate);
      Console.WriteLine("Signature Is Valid: " + csi.SignatureIsValid.ToString());
      Console.WriteLine("Signature Time: " + csi.SignatureTime.ToLocalTime().ToString());
      Console.WriteLine("Is Timestamped: " + csi.SignatureIsTimestamped);

      if (csi.SignatureIsTimestamped == true)
      {
        Console.WriteLine("Hash Algorithm: " + csi.TimestampInfo.HashAlgorithm.FriendlyName);
        Console.WriteLine("Is TimestampAltered: " + csi.TimestampInfo.IsTimestampAltered.ToString());
        Console.WriteLine("TimestampSerial Number: " + csi.TimestampInfo.SerialNumber);
        Console.WriteLine("TSA Certificate: " + csi.TimestampInfo.TsaCertificate.Subject);
      }
    }
}

static void ExtractCertificateInformation(X509Certificate2 cert)
{
    Console.WriteLine("Certificate subject:" + cert.Subject);
    Console.WriteLine("Certificate issued by:" + cert.GetNameInfo(X509NameType.SimpleName, true));
    Console.WriteLine("Certificate will expire on: " + cert.NotAfter.ToString());
    Console.WriteLine("Certificate is time valid: " + DigitalCertificate.VerifyDigitalCertificate(cert, VerificationType.LocalTime).ToString());
}

static void Main(string[] args)
{
    DigitallySignPDFFile("source.pdf", "source[signed].pdf");
    VerifyPDFSignature("source[signed].pdf");
}

See also: