How to: Sign a PDF Document with a Certificate Stored on a Hardware Device

• Sep 19, 2023
• 3 minutes to read

The PDF Document API allows you to retrieve a certificate from a hardware device (such as the Windows Certificate Store, SmartCard, USB Token). This example demonstrates how to use a certificate stored on a user’s machine. You can also adapt this solution to sign documents with certificates from any physical store.

View Example: PDF Document API - Sign a PDF document with a certificate stored on a hardware device

Obtain a certificate from a Windows certificate store. In this example, the X509Certificate2UI class object displays a system dialog. This dialog allows you to select an X.509 certificate installed on the current machine.

Tip

You can adapt this code to read a certificate from a SmartCard or USB Token: How to enter a PIN for an X509Certificate2 certificate programmatically when signing a PDF (in C#)

Pass the retrieved certificate to the Pkcs7Signer object constructor to create a PKCS#7 signature with the selected certificate. Call the PdfDocumentSigner.SaveDocument method to sign and save a document.

C#

using DevExpress.Pdf;
using System;
using System.Diagnostics;
using System.IO;
using System.Security.Cryptography.X509Certificates;
using DevExpress.Office.DigitalSignatures;

namespace SignPDFWithHardwareCertificate
{
  class Program
  {
    static void Main(string[] args)
    {
        X509Certificate2 cert = GetCertificate();
        if (cert != null)
        {
          SignPDF(cert);
        }
        else
          Console.WriteLine("There are no installed certificates on this machine.");
    }

    static X509Certificate2 GetCertificate()
    {
      // Get a certificate from a Windows Store
      X509Store store = new X509Store(StoreLocation.CurrentUser);
      store.Open(OpenFlags.ReadOnly | OpenFlags.OpenExistingOnly);

      // Display a dialog box to select a certificate from the Windows Store
      X509Certificate2Collection selectedCertificates =
              X509Certificate2UI.SelectFromCollection(store.Certificates, null, null, X509SelectionFlag.SingleSelection);

      // Get the first certificate that has a primary key
      foreach (var certificate in selectedCertificates)
      {
        if (certificate.HasPrivateKey)
             return certificate;
      }

      return null;
    }

   static void SignPDF(X509Certificate2 cert)
   {
     using (var signer = new PdfDocumentSigner(File.OpenRead("Demo.pdf")))
     {
       // Create a PKCS#7 signature
       Pkcs7Signer pkcs7Signature = new Pkcs7Signer(cert, HashAlgorithmType.SHA256);

       // Create a signature field on the first page
       var signatureFieldInfo = new PdfSignatureFieldInfo(1);

       // Specify the field's name and location
       signatureFieldInfo.Name = "SignatureField";
       signatureFieldInfo.SignatureBounds = new PdfRectangle(20, 20, 150, 150);

       // Apply a signature to a newly created signature field
       var cooperSignature = new PdfSignatureBuilder(pkcs7Signature, signatureFieldInfo);
       cooperSignature.SetImageData(System.IO.File.ReadAllBytes("JaneCooper.jpg"));

       // Sign and save the document
       signer.SaveDocument("SignedDocument.pdf", cooperSignature);
     }
          Process.Start("SignedDocument.pdf");
   }
 }
}

VB.NET

Imports DevExpress.Pdf
Imports System
Imports System.Diagnostics
Imports System.IO
Imports System.Security.Cryptography.X509Certificates
Imports DevExpress.Office.DigitalSignatures

Namespace SignPDFWithHardwareCertificate

    Friend Class Program

        Shared Sub Main(ByVal args As String())
            Dim cert As X509Certificate2 = GetCertificate()
            If cert IsNot Nothing Then
                SignPDF(cert)
            Else
                Console.WriteLine("There are no installed certificates on this machine.")
            End If
        End Sub

        Private Shared Function GetCertificate() As X509Certificate2
            ' Get a certificate from a Windows Store
            Dim store As X509Store = New X509Store(StoreLocation.CurrentUser)
            store.Open(OpenFlags.ReadOnly Or OpenFlags.OpenExistingOnly)
            ' Display a dialog box to select a certificate from the Windows Store
            Dim selectedCertificates As X509Certificate2Collection = X509Certificate2UI.SelectFromCollection(store.Certificates, Nothing, Nothing, X509SelectionFlag.SingleSelection)
            ' Get the first certificate that has a primary key
            For Each certificate In selectedCertificates
                If certificate.HasPrivateKey Then Return certificate
            Next

            Return Nothing
        End Function

        Private Shared Sub SignPDF(ByVal cert As X509Certificate2)
            Using signer = New PdfDocumentSigner(File.OpenRead("Demo.pdf"))
                ' Create a PKCS#7 signature
                Dim pkcs7Signature As Pkcs7Signer = New Pkcs7Signer(cert, HashAlgorithmType.SHA256)
                ' Create a signature field on the first page
                Dim signatureFieldInfo = New PdfSignatureFieldInfo(1)
                ' Specify the field's name and location
                signatureFieldInfo.Name = "SignatureField"
                signatureFieldInfo.SignatureBounds = New PdfRectangle(20, 20, 150, 150)
                ' Apply a signature to a newly created signature field
                Dim cooperSignature = New PdfSignatureBuilder(pkcs7Signature, signatureFieldInfo)
                cooperSignature.SetImageData(File.ReadAllBytes("JaneCooper.jpg"))
                ' Sign and save the document
                signer.SaveDocument("SignedDocument.pdf", cooperSignature)
            End Using

            Call Process.Start("SignedDocument.pdf")
        End Sub
    End Class
End Namespace