Adding a Digital Signature to a PDF Using a PFX File And iTextSharp in C#

using System;
using System.IO;
using System.Linq;
using iTextSharp.text.pdf;
using iTextSharp.text.pdf.security;
using Org.BouncyCastle.Crypto;
using Org.BouncyCastle.Pkcs;

namespace DigitalSignPDF
{
    class Program
    {
        static void Main(string[] args)
        {
            // Step 2: Load PDF Document
            string pdfFilePath = "C:\\Users\\Admin\\Documents\\MyPDF.pdf";
            PdfReader pdfReader = new PdfReader(pdfFilePath);

            // Step 3: Load PFX Certificate
            string pfxFilePath = "D:\\Uday Dodiya\\Digital_Sign\\Uday Dodiya.pfx";
            string pfxPassword = "uday1234";
            Pkcs12Store pfxKeyStore = new Pkcs12Store(new FileStream(pfxFilePath, FileMode.Open, FileAccess.Read), pfxPassword.ToCharArray());

            // Step 4: Initialize the PDF Stamper And Creating the Signature Appearance
            PdfStamper pdfStamper = PdfStamper.CreateSignature(pdfReader, new FileStream("C:\\Users\\Admin\\Documents\\MyPDF_Signed.pdf", FileMode.Create), '\0', null, true);
            PdfSignatureAppearance signatureAppearance = pdfStamper.SignatureAppearance;
            signatureAppearance.Reason = "Digital Signature Reason";
            signatureAppearance.Location = "Digital Signature Location";

            // Set the signature appearance location (in points)
            float x = 360;
            float y = 130;
            signatureAppearance.Acro6Layers = false;
            signatureAppearance.Layer4Text = PdfSignatureAppearance.questionMark;
            signatureAppearance.SetVisibleSignature(new iTextSharp.text.Rectangle(x, y, x + 150, y + 50), 1, "signature");

            // Step 5: Sign the Document
            string alias = pfxKeyStore.Aliases.Cast<string>().FirstOrDefault(entryAlias => pfxKeyStore.IsKeyEntry(entryAlias));

            if (alias != null)
            {
                ICipherParameters privateKey = pfxKeyStore.GetKey(alias).Key;
                IExternalSignature pks = new PrivateKeySignature(privateKey, DigestAlgorithms.SHA256);
                MakeSignature.SignDetached(signatureAppearance, pks, new Org.BouncyCastle.X509.X509Certificate[] { pfxKeyStore.GetCertificate(alias).Certificate }, null, null, null, 0, CryptoStandard.CMS);
            }
            else
            {
                Console.WriteLine("Private key not found in the PFX certificate.");
            }

            // Step 6: Save the Signed PDF
            pdfStamper.Close();

            Console.WriteLine("PDF signed successfully!");
        }
    }
}

Output

Console Window

Original PDF File (MyPDF.pdf)

Signed PDF File (MyPDF_Signed.pdf)

Add a Digital Signature to Multiple Pages of a PDF Using PFX File And iTextSharp in C#

To add a digital signature to all pages of a PDF using iTextSharp in C#, you need to loop through each page of the PDF and apply the digital signature to each page. Here's how you can modify your code to achieve this.

using System;
using System.IO;
using System.Linq;
using iTextSharp.text.pdf;
using iTextSharp.text.pdf.security;
using Microsoft.VisualBasic;
using Org.BouncyCastle.Crypto;
using Org.BouncyCastle.Pkcs;

namespace DigitalSignPDF
{
    class Program
    {
        static void Main(string[] args)
        {
            // Step 2: Load PDF Document
            string pdfFilePath = "C:\\Users\\Admin\\Documents\\MyPDF1.pdf";
            string destinationPath = "C:\\Users\\Admin\\Documents\\MyPDF_Signed1.pdf";
            PdfReader pdfReader = new PdfReader(pdfFilePath);

            // Step 3: Load PFX Certificate
            string pfxFilePath = "D:\\Uday Dodiya\\Digital_Sign\\Uday Dodiya.pfx";
            string pfxPassword = "uday1234";
            Pkcs12Store pfxKeyStore = new Pkcs12Store(new FileStream(pfxFilePath, FileMode.Open, FileAccess.Read), pfxPassword.ToCharArray());

            // Step 4: Count the pages and add signature in each page
            int page = pdfReader.NumberOfPages;
            for(int  i=1; i<=page; i++)
            {
                if(i > 1)
                {
                    FileStream stremfile = new FileStream(destinationPath, FileMode.Open, FileAccess.Read);
                    pdfReader = new PdfReader(stremfile);
                    File.Delete(destinationPath);
                }
                
                // Step 4.1: Initialize the PDF Stamper And Creating the Signature Appearance
                FileStream signedPdf = new FileStream(destinationPath, FileMode.Create, FileAccess.ReadWrite);
                PdfStamper pdfStamper = PdfStamper.CreateSignature(pdfReader, signedPdf, '\0', null, true);
                PdfSignatureAppearance signatureAppearance = pdfStamper.SignatureAppearance;

                signatureAppearance.Reason = "Digital Signature Reason";
                signatureAppearance.Location = "Digital Signature Location";
                signatureAppearance.Acro6Layers = false;

                // Set the signature appearance location (in points)
                float x = 360;
                float y = 130;
                signatureAppearance.Acro6Layers = false;
                signatureAppearance.Layer4Text = PdfSignatureAppearance.questionMark;
                signatureAppearance.SetVisibleSignature(new iTextSharp.text.Rectangle(x, y, x + 150, y + 50), i, null);


                // Step 4.2: Sign the Document
                string alias = pfxKeyStore.Aliases.Cast<string>().FirstOrDefault(entryAlias => pfxKeyStore.IsKeyEntry(entryAlias));
                ICipherParameters privateKey = pfxKeyStore.GetKey(alias).Key;
                IExternalSignature pks = new PrivateKeySignature(privateKey, DigestAlgorithms.SHA256);

                MakeSignature.SignDetached(signatureAppearance, pks, new Org.BouncyCastle.X509.X509Certificate[] { pfxKeyStore.GetCertificate(alias).Certificate }, null, null, null, 0, CryptoStandard.CMS);

                // Step 4.3: Save the Signed PDF
                pdfReader.Close();
                pdfStamper.Close();
            }
            Console.WriteLine("PDF signed successfully!");
        }
    }
}

Summary

So you see, adding a digital signature to a PDF is very easy.

In this article, we will learn How to add a digital signature to your PDF using iTextSharp in C#.

If you like my article, please like it and comment with your views on it.

Thank you. Enjoy Coding.