How to create Long Term Validation (LTV) when signing PDF documents externally

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Syncfusion Essential® PDF is a .NET PDF library used to create, read, and edit PDF documents. Using this library, you can create a Long-Term Validation when signing a PDF document externally in C# and VB.

Steps to create an LTV programmatically:

Create a new C# console application project.

Create a console application in Visual Studio

Install the Syncfusion.Pdf.WinForms NuGet package as a reference to your .NET Framework application from NuGet.org.

Refer Syncfusion.Pdf.WinForms NuGet to the project

Include the following namespace in the Program.cs file.

using System.Collections.Generic;
using Syncfusion.Pdf.Parsing;
using Syncfusion.Pdf;
using Syncfusion.Pdf.Security;
using System.Security.Cryptography.X509Certificates;
using System.Security.Cryptography.Pkcs;
using System.Security.Cryptography;

VB.NET

Imports System.Collections.Generic
Imports Syncfusion.Pdf.Parsing
Imports Syncfusion.Pdf
Imports Syncfusion.Pdf.Security
Imports System.Security.Cryptography.X509Certificates
Imports System.Security.Cryptography.Pkcs
Imports System.Security.Cryptography

Use the following code snippet to create Long Term Validation (LTV).

// Load an existing PDF document
PdfLoadedDocument loadedDocument = new PdfLoadedDocument("../../Input.pdf");
 
// Get the page of the existing PDF document
PdfLoadedPage loadedPage = loadedDocument.Pages[0] as PdfLoadedPage;
 
// Create a new PDF signature without PdfCertificate instance
PdfSignature signature = new PdfSignature(loadedDocument, loadedPage, null, "Signature1");
 
// Hook up the ComputeHash event
signature.ComputeHash += Signature_ComputeHash;
 
// Create X509Certificate2 from your certificate to create a long-term validity
X509Certificate2 x509 = new X509Certificate2("../../PDF.pfx", "password123");
 
// Create LTV with your public certificates
signature.CreateLongTermValidity(new List<X509Certificate2> { x509 });
 
// Save and close the PDF document
loadedDocument.Save("SignedDocument.pdf");
 
loadedDocument.Close(true);

VB.NET

'Load an existing PDF document
Dim loadedDocument As PdfLoadedDocument = New PdfLoadedDocument("../../Input.pdf")

'Get the page of the existing PDF document
Dim loadedPage As PdfLoadedPage = TryCast(loadedDocument.Pages(0), PdfLoadedPage)

'Create a new PDF signature without PdfCertificate instance
Dim signature As PdfSignature = New PdfSignature(loadedDocument, loadedPage, Nothing, "Signature1")

'Hook up the ComputeHash event
AddHandler signature.ComputeHash, AddressOf Signature_ComputeHash

'Create X509Certificate2 from your certificate to create long-term validity
Dim x509 As X509Certificate2 = New X509Certificate2("../../PDF.pfx", "password123")

'Create LTV with your public certificates
signature.CreateLongTermValidity(New List(Of X509Certificate2) From { x509 })

'Save and close the PDF document
loadedDocument.Save("SignedDocument.pdf")			  

loadedDocument.Close(True);

Add the following code to sign a PDF document externally.

private static void Signature_ComputeHash(object sender, PdfSignatureEventArgs ars)
{
    // Get the document bytes
    byte[] documentBytes = ars.Data;

    SignedCms signedCms = new SignedCms(new ContentInfo(documentBytes), detached: true);

    // Compute the signature using the specified digital ID file and the password
    X509Certificate2 certificate = new X509Certificate2("../../PDF.pfx", "password123");

    CmsSigner cmsSigner = new CmsSigner(certificate);

    // Set the digest algorithm SHA256
    cmsSigner.DigestAlgorithm = new Oid("2.16.840.1.101.3.4.2.1");			

    signedCms.ComputeSignature(cmsSigner);

    // Embed the encoded digital signature to the PDF document
    ars.SignedData = signedCms.Encode();
}

VB.NET

Private Shared Sub Signature_ComputeHash(ByVal sender As Object, ByVal ars As PdfSignatureEventArgs)

    'Get the document bytes
    Dim documentBytes As Byte() = ars.Data

    Dim signedCms As SignedCms = New SignedCms(New ContentInfo(documentBytes), detached:=True)

    'Compute the signature using the specified digital ID file and the password
    Dim certificate As X509Certificate2 = New X509Certificate2("../../PDF.pfx", "password123")

    Dim cmsSigner As CmsSigner = New CmsSigner(certificate)

    'Set the digest algorithm SHA256
    cmsSigner.DigestAlgorithm = New Oid("2.16.840.1.101.3.4.2.1")

    signedCms.ComputeSignature(cmsSigner);
  
    'Embed the encoded digital signature to the PDF document
    ars.SignedData = signedCms.Encode();

End Sub

You can download the working sample from CreateLongTermValidity.

By executing the program, you will get the PDF document as follows.

Output document

Take a moment to peruse the documentation, where you will find other options like digitally sign a PDF file, digitally sign an existing PDF document, remove the digital signature from an existing PDF document, and more with code examples.

Click here to explore the rich set of Syncfusion Essential® PDF features.

How to create Long Term Validation (LTV) when signing PDF documents externally

Steps to create an LTV programmatically:

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