Working with Digital Signature

5 Dec 202424 minutes to read

To quickly get started with digitally signing a PDF document and validating signatures using the Blazor PDF Library, check this video:

Adding a digital signature

The Essential® PDF allows you to add a digital signature to the PDF document. To add a digital signature, you need a certificate with private keys. The Essential® PDF provides support for digital signature by using the PFX files, Hardware Security Module (HSM), Online Certificate Status Protocol (OCSP), Certificate Revocation List (CRL), Windows Certificate Store, and supports signatures using the Elliptic Curve Digital Signature Algorithm (ECDSA).

The following code example explains how to add a digital signature to the PDF document by using PdfSignature class.

C# [Cross-platform]

//Creates a new PDF document.
PdfDocument document = new PdfDocument();
//Adds a new page.
PdfPageBase page = document.Pages.Add();
//Create PDF graphics for the page.
PdfGraphics graphics = page.Graphics;

//Creates a certificate instance from PFX file with private key.
FileStream certificateStream = new FileStream("PDF.pfx", FileMode.Open, FileAccess.Read);
PdfCertificate pdfCert = new PdfCertificate(certificateStream, "password123");
//Creates a digital signature.
PdfSignature signature = new PdfSignature(document, page, pdfCert, "Signature");
//Sets an image for signature field.
FileStream imageStream = new FileStream("signature.jpg", FileMode.Open, FileAccess.Read);
//Sets an image for signature field.
PdfBitmap signatureImage = new PdfBitmap(imageStream);
//Sets signature information.
signature.Bounds = new RectangleF(new PointF(0, 0), signatureImage.PhysicalDimension);
signature.ContactInfo = "[e-mail]";
signature.LocationInfo = "Honolulu, Hawaii";
signature.Reason = "I am author of this document.";
//Draws the signature image.
signature.Appearance.Normal.Graphics.DrawImage(signatureImage, 0, 0);

//Save the document into stream.
MemoryStream stream = new MemoryStream();
document.Save(stream);
stream.Position = 0;
//Close the document.
document.Close(true);
//Defining the ContentType for PDF file.
string contentType = "application/pdf";
//Define the file name.
string fileName = "Output.pdf";
//Creates a FileContentResult object by using the file contents, content type, and file name.
return File(stream, contentType, fileName);

C# [Windows-specific]

//Creates a new PDF document
PdfDocument document = new PdfDocument();
//Adds a new page
PdfPageBase page = document.Pages.Add();
//Create PDF graphics for the page
PdfGraphics graphics = page.Graphics;

//Creates a certificate instance from PFX file with private key
PdfCertificate pdfCert = new PdfCertificate(@"PDF.pfx", "password123");
//Creates a digital signature
PdfSignature signature = new PdfSignature(document, page, pdfCert, "Signature");
//Sets an image for signature field
PdfBitmap signatureImage = new PdfBitmap(@"signature.jpg");
//Sets signature information
signature.Bounds = new RectangleF(new PointF(0, 0), signatureImage.PhysicalDimension);
signature.ContactInfo = "[e-mail]";
signature.LocationInfo = "Honolulu, Hawaii";
signature.Reason = "I am author of this document.";
//Draws the signature image
signature.Appearance.Normal.Graphics.DrawImage(signatureImage, 0, 0);

//Saves and closes the document
document.Save("Output.pdf");
document.Close(true);

VB.NET [Windows-specific]

'Creates a new PDF document
Dim document As New PdfDocument()
'Adds a new page
Dim page As PdfPageBase = document.Pages.Add()
'Create PDF graphics for the page
Dim graphics As PdfGraphics = page.Graphics

'Creates a certificate instance from PFX file with private key
Dim pdfCert As New PdfCertificate("PDF.pfx", "password123")
'Creates a digital signature
Dim signature As New PdfSignature(document, page, pdfCert, "Signature")
'Sets an image for signature field
Dim signatureImage As New PdfBitmap("signature.jpg")
'Sets signature information
signature.Bounds = New RectangleF(New PointF(0, 0), signatureImage.PhysicalDimension)
signature.ContactInfo = "[e-mail]"
signature.LocationInfo = "Honolulu, Hawaii"
signature.Reason = "I am author of this document."
'Draws the signature image
signature.Appearance.Normal.Graphics.DrawImage(signatureImage, 0, 0)

'Saves and closes the document
document.Save("Output.pdf")
document.Close(True)

You can download a complete working sample from GitHub.

Adding a digital signature using stream

The following code example illustrates how to add a digital signature in the PDF document as stream using the PdfSignature class as follows.

C# [Cross-platform]

//Creates a new PDF document
PdfDocument document = new PdfDocument();
//Adds a new page
PdfPageBase page = document.Pages.Add();
//Create PDF graphics for the page
PdfGraphics graphics = page.Graphics;

//Creates a certificate instance from PFX file with private key
FileStream certificateStream = new FileStream("PDF.pfx", FileMode.Open, FileAccess.Read);
PdfCertificate pdfCert = new PdfCertificate(certificateStream, "password123");
//Creates a digital signature
PdfSignature signature = new PdfSignature(document, page, pdfCert, "Signature");
//Sets an image for signature field
FileStream imageStream = new FileStream("signature.jpg", FileMode.Open, FileAccess.Read);
//Sets an image for signature field
PdfBitmap signatureImage = new PdfBitmap(imageStream);
//Sets signature information
signature.Bounds = new RectangleF(new PointF(0, 0), signatureImage.PhysicalDimension);
signature.ContactInfo = "[e-mail]";
signature.LocationInfo = "Honolulu, Hawaii";
signature.Reason = "I am author of this document.";
//Draws the signature image
signature.Appearance.Normal.Graphics.DrawImage(signatureImage, 0, 0);

//Save the document into stream
MemoryStream stream = new MemoryStream();
document.Save(stream);
stream.Position = 0;
//Close the document
document.Close(true);
//Defining the ContentType for pdf file
string contentType = "application/pdf";
//Define the file name
string fileName = "Output.pdf";
//Creates a FileContentResult object by using the file contents, content type, and file name
return File(stream, contentType, fileName);

C# [Windows-specific]

//Creates a new PDF document
PdfDocument document = new PdfDocument();
//Adds a new page
PdfPageBase page = document.Pages.Add();
//Create PDF graphics for the page
PdfGraphics graphics = page.Graphics;

//Gets a stream from .pfx file
Stream pfxStream = File.OpenRead("PDF.pfx");
//Creates a certificate instance from PFX file stream with private key
PdfCertificate pdfCert = new PdfCertificate(pfxStream, "password123");
//Creates a digital signature
PdfSignature signature = new PdfSignature(document, page, pdfCert, "Signature");
//Sets an image for signature field
PdfBitmap signatureImage = new PdfBitmap(@"signature.jpg");
//Sets signature information
signature.Bounds = new RectangleF(new PointF(0, 0), signatureImage.PhysicalDimension);
signature.ContactInfo = "[e-mail]";
signature.LocationInfo = "Honolulu, Hawaii";
signature.Reason = "I am author of this document.";
//Draws the signature image
signature.Appearance.Normal.Graphics.DrawImage(signatureImage, 0, 0);

//Saves and closes the document
document.Save("Output.pdf");
document.Close(true);

VB.NET [Windows-specific]

'Creates a new PDF document
Dim document As New PdfDocument()
'Adds a new page
Dim page As PdfPageBase = document.Pages.Add()
'Create PDF graphics for the page
Dim graphics As PdfGraphics = page.Graphics

'Gets stream from .pfx file
Dim pfxStream As Stream = File.OpenRead("PDF.pfx")
'Creates a certificate instance from PFX file stream with private key
Dim pdfCert As New PdfCertificate(pfxStream, "password123")
'Creates a digital signature
Dim signature As New PdfSignature(document, page, pdfCert, "Signature")
'Sets an image for signature field
Dim signatureImage As New PdfBitmap("signature.jpg")
'Sets signature information
signature.Bounds = New RectangleF(New PointF(0, 0), signatureImage.PhysicalDimension)
signature.ContactInfo = "[e-mail]"
signature.LocationInfo = "Honolulu, Hawaii"
signature.Reason = "I am author of this document."
'Draws the signature image
signature.Appearance.Normal.Graphics.DrawImage(signatureImage, 0, 0)

'Saves and closes the document
document.Save("Output.pdf")
document.Close(True)

You can add a digital signature to an existing document as follows.

C# [Cross-platform]

//Load the PDF document
FileStream docStream = new FileStream("Input.pdf", FileMode.Open, FileAccess.Read);
PdfLoadedDocument loadedDocument = new PdfLoadedDocument(docStream);
//Gets the page
PdfLoadedPage page = loadedDocument.Pages[0] as PdfLoadedPage;

//Creates a signature field
PdfSignatureField signatureField = new PdfSignatureField(page, "SignatureField");
signatureField.Bounds = new RectangleF(0, 0, 100, 100);
signatureField.Signature = new PdfSignature();
//Adds certificate to the signature field
FileStream certificateStream = new FileStream("PDF.pfx", FileMode.Open, FileAccess.Read);
signatureField.Signature.Certificate = new PdfCertificate(certificateStream, "password123");
signatureField.Signature.Reason = "I am author of this document";
//Adds the field
loadedDocument.Form.Fields.Add(signatureField);

//Save the document into stream
MemoryStream stream = new MemoryStream();
loadedDocument.Save(stream);
stream.Position = 0;
//Close the document
loadedDocument.Close(true);
//Defining the ContentType for pdf file
string contentType = "application/pdf";
//Define the file name
string fileName = "Output.pdf";
//Creates a FileContentResult object by using the file contents, content type, and file name
return File(stream, contentType, fileName);

C# [Windows-specific]

//Loads the PDF document with signature field
PdfLoadedDocument loadedDocument = new PdfLoadedDocument("Input.pdf");         
//Gets the page
PdfLoadedPage page = loadedDocument.Pages[0] as PdfLoadedPage;

//Creates a signature field
PdfSignatureField signatureField = new PdfSignatureField(page, "SignatureField");
signatureField.Bounds = new RectangleF(0, 0, 100, 100);
signatureField.Signature = new PdfSignature();
//Adds certificate to the signature field
signatureField.Signature.Certificate = new PdfCertificate(@"PDF.pfx", "password123");
signatureField.Signature.Reason = "I am author of this document";
//Adds the field
loadedDocument.Form.Fields.Add(signatureField);

//Saves the certified PDF document
loadedDocument.Save(@"Output.pdf");
loadedDocument.Close(true);

VB.NET [Windows-specific]

'Loads the PDF document with signature field
Dim loadedDocument As New PdfLoadedDocument("Input.pdf")
'Gets the page
Dim page As PdfLoadedPage = TryCast(loadedDocument.Pages(0), PdfLoadedPage)

'Creates a signature field
Dim signatureField As New PdfSignatureField(page, "SignatureField")
signatureField.Bounds = New RectangleF(0, 0, 100, 100)
signatureField.Signature = New PdfSignature()
'Adds certificate to the signature field
signatureField.Signature.Certificate = New PdfCertificate("PDF.pfx", "password123")
signatureField.Signature.Reason = "I am author of this document"
'Adds the field
loadedDocument.Form.Fields.Add(signatureField)

'Saves the certified PDF document
loadedDocument.Save("Output.pdf")
loadedDocument.Close(True)

You can download a complete working sample from GitHub.

Adding a digital signature using X509Certificate2

The following code example illustrates how to add digital signature in a PDF document using X509Certificate2 as follows.

C# [Cross-platform]

//Creates a new PDF document
PdfDocument document = new PdfDocument();
//Adds a new page  
PdfPage page = document.Pages.Add();
//Create PDF graphics for the page
PdfGraphics graphics = page.Graphics;

//Creates a certificate instance from PFX file with private key  
X509Certificate2 certificate = new X509Certificate2("PDF.pfx", "password123");
PdfCertificate pdfCertificate = new PdfCertificate(certificate);
//Creates a digital signature  
PdfSignature signature = new PdfSignature(document, page, pdfCertificate, "Signature");
//Sets an image for signature field
FileStream imageStream = new FileStream("signature.jpg", FileMode.Open, FileAccess.Read);
PdfBitmap signatureImage = new PdfBitmap(imageStream);
//Sets signature information
signature.Bounds = new RectangleF(new PointF(0, 0), signatureImage.PhysicalDimension);
signature.ContactInfo = "[e-mail]";
signature.LocationInfo = "Honolulu, Hawaii";
signature.Reason = "I am author of this document.";
//Draws the signature image
signature.Appearance.Normal.Graphics.DrawImage(signatureImage, 0, 0);

//Creating the stream object
MemoryStream stream = new MemoryStream();
//Save the document as stream
document.Save(stream);
//If the position is not set to '0', then the PDF will be empty
stream.Position = 0;
//Close the document
document.Close(true);
//Defining the ContentType for pdf file
string contentType = "application/pdf";
//Define the file name
string fileName = "Output.pdf";
//Creates a FileContentResult object by using the file contents, content type, and file name
return File(stream, contentType, fileName);

C# [Windows-specific]

//Creates a new PDF document
PdfDocument document = new PdfDocument();
//Adds a new page  
PdfPage page = document.Pages.Add();
//Create PDF graphics for the page
PdfGraphics graphics = page.Graphics;

//Creates a certificate instance from PFX file with private key  
X509Certificate2 certificate = new X509Certificate2("PDF.pfx", "password123");
PdfCertificate pdfCertificate = new PdfCertificate(certificate);
//Creates a digital signature  
PdfSignature signature = new PdfSignature(document, page, pdfCertificate, "Signature");
//Sets an image for signature field
PdfBitmap signatureImage = new PdfBitmap(@"signature.jpg");
//Sets signature information
signature.Bounds = new RectangleF(new PointF(0, 0), signatureImage.PhysicalDimension);
signature.ContactInfo = "[e-mail]";
signature.LocationInfo = "Honolulu, Hawaii";
signature.Reason = "I am author of this document.";
//Draws the signature image
signature.Appearance.Normal.Graphics.DrawImage(signatureImage, 0, 0);

//Save the document
document.Save("Output.pdf");
//Close the document
document.Close(true);

VB.NET [Windows-specific]

'Creates a new PDF document
Dim document As New PdfDocument()
'Adds a new page  
Dim page As PdfPage = document.Pages.Add()
'Create PDF graphics for the page
Dim graphics As PdfGraphics = page.Graphics

'Creates a certificate instance from PFX file with private key  
Dim certificate As New X509Certificate2("PDF.pfx", "password123")
Dim pdfCertificate As New PdfCertificate(certificate)
'Creates a digital signature 
Dim signature As New PdfSignature(document, page, pdfCertificate, "Signature")
'Sets an image for signature field
Dim signatureImage As New PdfBitmap("signature.jpg")
'Sets signature information
signature.Bounds = New RectangleF(New PointF(0, 0), signatureImage.PhysicalDimension)
signature.ContactInfo = "[e-mail]"
signature.LocationInfo = "Honolulu, Hawaii"
signature.Reason = "I am author of this document."
'Draws the signature image
signature.Appearance.Normal.Graphics.DrawImage(signatureImage, 0, 0)

'Save the document
document.Save("Output.pdf")
'Close the document
document.Close(True)

You can download a complete working sample from GitHub.

Signing an existing document

You can load the signature field from the existing PDF document using PdfLoadedSignatureField class and add certificate to the document using PdfCertificate class.

C# [Cross-platform]

//Load the PDF document
FileStream docStream = new FileStream("Input.pdf", FileMode.Open, FileAccess.Read);
PdfLoadedDocument loadedDocument = new PdfLoadedDocument(docStream);
//Gets the first page of the document
PdfLoadedPage page = loadedDocument.Pages[0] as PdfLoadedPage;
//Gets the first signature field of the PDF document
PdfLoadedSignatureField field = loadedDocument.Form.Fields[0] as PdfLoadedSignatureField;

//Creates a certificate
FileStream certificateStream = new FileStream("PDF.pfx", FileMode.Open, FileAccess.Read);
PdfCertificate certificate = new PdfCertificate(certificateStream, "password123");
field.Signature = new PdfSignature(loadedDocument, page, certificate, "Signature", field);

//Save the document into stream
MemoryStream stream = new MemoryStream();
loadedDocument.Save(stream);
stream.Position = 0;
//Close the document
loadedDocument.Close(true);
//Defining the ContentType for pdf file
string contentType = "application/pdf";
//Define the file name
string fileName = "Output.pdf";
//Creates a FileContentResult object by using the file contents, content type, and file name
return File(stream, contentType, fileName);

C# [Windows-specific]

//Loads a PDF document
PdfLoadedDocument loadedDocument = new PdfLoadedDocument("Input.pdf");
//Gets the first page of the document
PdfLoadedPage page = loadedDocument.Pages[0] as PdfLoadedPage;
//Gets the first signature field of the PDF document
PdfLoadedSignatureField field = loadedDocument.Form.Fields[0] as PdfLoadedSignatureField;

//Creates a certificate
PdfCertificate certificate = new PdfCertificate(@"PDF.pfx", "password123");
field.Signature = new PdfSignature(loadedDocument,page,certificate,"Signature",field);

//Saves the document
loadedDocument.Save("Output.pdf");
//Close the document
loadedDocument.Close(true);

VB.NET [Windows-specific]

'Loads a PDF document
Dim loadedDocument As New PdfLoadedDocument("Input.pdf")
'Gets the first page of the document
Dim page As PdfLoadedPage = TryCast(loadedDocument.Pages(0), PdfLoadedPage)
'Gets the first signature field of the PDF document
Dim field As PdfLoadedSignatureField = TryCast(loadedDocument.Form.Fields(0), PdfLoadedSignatureField)

'Creates a certificate
Dim certificate As New PdfCertificate("PDF.pfx", "password123")
field.Signature = New PdfSignature(loadedDocument, page, certificate, "Signature", field)

'Saves the document
loadedDocument.Save("Output.pdf")
'Close the document
loadedDocument.Close(True)

You can download a complete working sample from GitHub.

Sign an existing document using stream

You can load the signature field from an existing PDF document using PdfLoadedSignatureField class and add certificate to the document as stream using PdfCertificate as follows.

C# [Cross-platform]

//Load the PDF document
FileStream docStream = new FileStream("Input.pdf", FileMode.Open, FileAccess.Read);
PdfLoadedDocument loadedDocument = new PdfLoadedDocument(docStream);
//Gets the first page of the document
PdfLoadedPage page = loadedDocument.Pages[0] as PdfLoadedPage;
//Gets the first signature field of the PDF document
PdfLoadedSignatureField field = loadedDocument.Form.Fields[0] as PdfLoadedSignatureField;

//Creates a certificate
FileStream certificateStream = new FileStream("PDF.pfx", FileMode.Open, FileAccess.Read);
PdfCertificate certificate = new PdfCertificate(certificateStream, "password123");
field.Signature = new PdfSignature(loadedDocument, page, certificate, "Signature", field);

//Save the document into stream
MemoryStream stream = new MemoryStream();
loadedDocument.Save(stream);
stream.Position = 0;
//Close the document
loadedDocument.Close(true);
//Defining the ContentType for pdf file
string contentType = "application/pdf";
//Define the file name
string fileName = "Output.pdf";
//Creates a FileContentResult object by using the file contents, content type, and file name
return File(stream, contentType, fileName);

C# [Windows-specific]

//Loads a PDF document
PdfLoadedDocument loadedDocument = new PdfLoadedDocument("Input.pdf");
//Gets the first page of the document
PdfLoadedPage page = loadedDocument.Pages[0] as PdfLoadedPage;
//Gets the first signature field of the PDF document
PdfLoadedSignatureField field = loadedDocument.Form.Fields[0] as PdfLoadedSignatureField;

//Gets the stream from .pfx file
Stream pfxStream = File.OpenRead("PDF.pfx");
//Creates a certificate instance from PFX file stream with private key
PdfCertificate certificate = new PdfCertificate(pfxStream, "password123");
field.Signature = new PdfSignature(loadedDocument, page, certificate, "Signature", field);

//Save the document
loadedDocument.Save("Output.pdf");
//Close the document
loadedDocument.Close(true);

VB.NET [Windows-specific]

'Loads a PDF document
Dim loadedDocument As New PdfLoadedDocument("Input.pdf")
'Gets the first page of the document
Dim page As PdfLoadedPage = TryCast(loadedDocument.Pages(0), PdfLoadedPage)
'Gets the first signature field of the PDF document
Dim field As PdfLoadedSignatureField = TryCast(loadedDocument.Form.Fields(0), PdfLoadedSignatureField)

'Gets the stream from .pfx file
Dim pfxStream As Stream = File.OpenRead("PDF.pfx")
'Creates a certificate instance from PFX file stream with private key
Dim certificate As New PdfCertificate(pfxStream, "password123")
field.Signature = New PdfSignature(loadedDocument, page, certificate, "Signature", field)

'Save the document
loadedDocument.Save("Output.pdf")
'Close the document
loadedDocument.Close(True)

Externally sign a PDF document

You can sign the PDF document with an external digital signature created from various sources, such as an HSM, USB token, smart card, or other cloud services such as DigitalSign. The following approaches are available for externally signing the PDF documents.

• ComputeHash event
• IPdfExternalSigner interface

Externally sign the PDF document using ComputeHash event

The following code example shows how to sign the PDF document from an external signature using ComputeHash event.

C# [Cross-platform]

//Get the stream from the document
FileStream documentStream = new FileStream("PDF_Succinctly.pdf ", FileMode.Open, FileAccess.Read);
//Load the existing PDF document
PdfLoadedDocument document = new PdfLoadedDocument(documentStream);

//Creates a digital signature
PdfSignature signature = new PdfSignature(document, document.Pages[0], null, "DigitalSignature");
signature.ComputeHash += Signature_ComputeHash;

//Save the PDF document to stream
MemoryStream ms = new MemoryStream();
document.Save(ms);
//Close the PDF document 
document.Close(true);

private static void Signature_ComputeHash1(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
    FileStream certificateStream = new FileStream("DigitalSignatureTest.pfx", FileMode.Open, FileAccess.Read);
    X509Certificate2 certificate = new X509Certificate2(certificateStream, "DigitalPass123");
    var 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();
}

C# [Windows-specific]

//Load existing PDF document
PdfLoadedDocument document = new PdfLoadedDocument("PDF_Succinctly.pdf");
//Creates a digital signature
PdfSignature signature = new PdfSignature(document, document.Pages[0], null, "DigitalSignature");
signature.ComputeHash += Signature_ComputeHash;

//Save the PDF document
document.Save("ExternalSignature.pdf");
//Close the document
document.Close(true);

void Signature_ComputeHash(object sender, PdfSignatureEventArgs arguments)
{
    //Get the document bytes
    byte[] documentBytes = arguments.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("DigitalSignatureTest.pfx", "DigitalPass123");
    var 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
    arguments.SignedData = signedCms.Encode();
}

VB.NET [Windows-specific]

'Load existing PDF document
Dim document As PdfLoadedDocument = New PdfLoadedDocument("PDF_Succinctly.pdf")
'Creates a digital signature
Dim signature As PdfSignature = New PdfSignature(document, document.Pages(0), Nothing, "DigitalSignature")
AddHandler signature.ComputeHash, AddressOf Signature_ComputeHash

'Save the PDF document
document.Save("ExternalSignature.pdf")
'Close the document
document.Close(True)

Private Sub Signature_ComputeHash(ByVal sender As Object, ByVal arguments As PdfSignatureEventArgs)
    'Get the document bytes
    Dim documentBytes As Byte() = arguments.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("DigitalSignatureTest.pfx", "DigitalPass123")
    Dim 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
    arguments.SignedData = signedCms.Encode()
End Sub

You can download a complete working sample from GitHub.

Externally sign the PDF document using IPdfExternalSigner

The following code example shows how to sign the PDF document from external signature using IPdfExternalSigner.

C# [Cross-platform]

//Get the stream from the document
FileStream documentStream = new FileStream("Input.pdf ", FileMode.Open, FileAccess.Read);
//Load the existing PDF document
PdfLoadedDocument loadedDocument = new PdfLoadedDocument(documentStream);

//Creates a digital signature.
PdfSignature signature = new PdfSignature(loadedDocument, loadedDocument.Pages[0], null, "Signature");
//Sets the signature information.
signature.Bounds = new RectangleF(new PointF(0, 0), new SizeF(100, 30));
signature.Settings.CryptographicStandard = CryptographicStandard.CADES;
signature.Settings.DigestAlgorithm = DigestAlgorithm.SHA1;

//Create an external signer.
IPdfExternalSigner externalSignature = new ExternalSigner("SHA1");

//Add public certificates.
List<X509Certificate2> certificates = new List<X509Certificate2>();
certificates.Add(new X509Certificate2(Convert.FromBase64String(PublicCert)));
signature.AddExternalSigner(externalSignature, certificates, null);

//Save the document into stream
MemoryStream stream = new MemoryStream();
loadedDocument.Save(stream);
stream.Position = 0;
//Close the document
loadedDocument.Close(true);
//Defining the ContentType for pdf file
string contentType = "application/pdf";
//Define the file name
string fileName = "Output.pdf";
//Creates a FileContentResult object by using the file contents, content type, and file name
return File(stream, contentType, fileName);

//Create the external signer class and sign the document hash.
class ExternalSigner : IPdfExternalSigner
{
    private string _hashAlgorithm;
    public string HashAlgorithm
    {
        get { return _hashAlgorithm; }
    }

    public ExternalSigner(string hashAlgorithm)
    {
        _hashAlgorithm = hashAlgorithm;
    }
    public byte[] Sign(byte[] message, out byte[] timeStampResponse)
    {
        timeStampResponse = null;
        FileStream certificateStream = new FileStream("PDF.pfx", FileMode.Open, FileAccess.Read);
        X509Certificate2 digitalID = new X509Certificate2(new X509Certificate2(certificateStream, "password123"));
        if (digitalID.PrivateKey is System.Security.Cryptography.RSACryptoServiceProvider)
        {
            System.Security.Cryptography.RSACryptoServiceProvider rsa = (System.Security.Cryptography.RSACryptoServiceProvider)digitalID.PrivateKey;
            return rsa.SignData(message, HashAlgorithm);
        }
        else if (digitalID.PrivateKey is RSACng)
        {
            RSACng rsa = (RSACng)digitalID.PrivateKey;
            return rsa.SignData(message, System.Security.Cryptography.HashAlgorithmName.SHA1, RSASignaturePadding.Pkcs1);
        }
        return null;
    }
}

C# [Windows-specific]

//Load an existing PDF document.
PdfLoadedDocument loadedDocument = new PdfLoadedDocument("Input.pdf");

//Creates a digital signature.
PdfSignature signature = new PdfSignature(loadedDocument, loadedDocument.Pages[0], null, "Signature");
//Sets the signature information.
signature.Bounds = new RectangleF(new PointF(0, 0), new SizeF(100, 30));
signature.Settings.CryptographicStandard = CryptographicStandard.CADES;
signature.Settings.DigestAlgorithm = DigestAlgorithm.SHA1;

//Create an external signer.
IPdfExternalSigner externalSignature = new ExternalSigner("SHA1");

//Add public certificates.
List<X509Certificate2> certificates = new List<X509Certificate2>();
certificates.Add(new X509Certificate2(Convert.FromBase64String(PublicCert)));
signature.AddExternalSigner(externalSignature, certificates, null);

//Saves the document.
loadedDocument.Save("Output.pdf");
//Closes the document.
loadedDocument.Close(true);

//Create the external signer class and sign the document hash.
class ExternalSigner : IPdfExternalSigner
{
    private string _hashAlgorithm;
    public string HashAlgorithm
    {
        get { return _hashAlgorithm; }
    }

    public ExternalSigner(string hashAlgorithm)
    {
        _hashAlgorithm = hashAlgorithm;
    }
    public byte[] Sign(byte[] message, out byte[] timeStampResponse)
    {
        timeStampResponse = null;

        X509Certificate2 digitalID = new X509Certificate2(new X509Certificate2(Path.GetFullPath("PDF.pfx"), "password123"));
        if (digitalID.PrivateKey is System.Security.Cryptography.RSACryptoServiceProvider)
        {
            System.Security.Cryptography.RSACryptoServiceProvider rsa = (System.Security.Cryptography.RSACryptoServiceProvider)digitalID.PrivateKey;
            return rsa.SignData(message, HashAlgorithm);
        }
        else if (digitalID.PrivateKey is RSACng)
        {
            RSACng rsa = (RSACng)digitalID.PrivateKey;
            return rsa.SignData(message, System.Security.Cryptography.HashAlgorithmName.SHA1, RSASignaturePadding.Pkcs1);
        }
        return null;
    }
}

VB.NET [Windows-specific]

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

'Creates a digital signature. 
Dim signature As PdfSignature = New PdfSignature(loadedDocument, loadedDocument.Pages(0), Nothing, "Signature")
'Sets the signature information.
signature.Bounds = New RectangleF(New PointF(0, 0), New SizeF(100, 30))
signature.Settings.CryptographicStandard = CryptographicStandard.CADES
signature.Settings.DigestAlgorithm = DigestAlgorithm.SHA1

'Create an external signer.
Dim externalSignature As IPdfExternalSigner = New ExternalSigner("SHA1")
Dim certificates As List(Of X509Certificate2) = New List(Of X509Certificate2)()
certificates.Add(New X509Certificate2(Convert.FromBase64String(PublicCert)))
signature.AddExternalSigner(externalSignature, certificates, Nothing)

'Saves the document.
loadedDocument.Save("Output.pdf")
'Closes the document.
loadedDocument.Close(True)

Class ExternalSigner
    Implements IPdfExternalSigner

    Private _hashAlgorithm As String

    Public ReadOnly Property HashAlgorithm As String Implements IPdfExternalSigner.HashAlgorithm
        Get
            Return _hashAlgorithm
        End Get
    End Property

    Public Sub New(ByVal hashAlgorithm As String)
        _hashAlgorithm = hashAlgorithm
    End Sub

    Public Function Sign(message() As Byte, ByRef timeStampResponse() As Byte) As Byte() Implements IPdfExternalSigner.Sign
        timeStampResponse = Nothing
        Dim digitalID As X509Certificate2 = New X509Certificate2(New X509Certificate2("PDF.pfx", "password123"))

        If TypeOf digitalID.PrivateKey Is System.Security.Cryptography.RSACryptoServiceProvider Then
            Dim rsa As System.Security.Cryptography.RSACryptoServiceProvider = CType(digitalID.PrivateKey, System.Security.Cryptography.RSACryptoServiceProvider)
            Return rsa.SignData(message, HashAlgorithm)
        ElseIf TypeOf digitalID.PrivateKey Is RSACng Then
            Dim rsa As RSACng = CType(digitalID.PrivateKey, RSACng)
            Return rsa.SignData(message, System.Security.Cryptography.HashAlgorithmName.SHA1, RSASignaturePadding.Pkcs1)
        End If

        Return Nothing
    End Function
End Class

You can download a complete working sample from GitHub.

Adding Timestamps to a PDF document using external signing

This section explains how to add a timestamp to a PDF document during the external signing process. We have implemented a solution that generates a timestamp response from a timestamp server URI using the BouncyCastle library. The accepted timestamp token follows the RFC3161 standard.

NOTE

In this example, we have used the open-source BouncyCastle library. Ensure you review its licensing before including it in your production environment. Alternatively, you can directly use the timestamp token provided by your service provider.

The following example illustrates the process of adding timestamps to a PDF document during the external signing process.

C# [Cross-platform]

// Get the stream from the input PDF document.
FileStream documentStream = new FileStream("Input.pdf", FileMode.Open, FileAccess.Read);

// Load the existing PDF document.
PdfLoadedDocument loadedDocument = new PdfLoadedDocument(documentStream);

// Create a digital signature for the first page of the document.
PdfSignature signature = new PdfSignature(loadedDocument, loadedDocument.Pages[0], null, "Signature");

// Set the signature bounds and cryptographic settings.
signature.Bounds = new RectangleF(new PointF(0, 0), new SizeF(100, 30));
signature.Settings.CryptographicStandard = CryptographicStandard.CADES;
signature.Settings.DigestAlgorithm = DigestAlgorithm.SHA1;

// Create an external signer using the SHA1 hash algorithm.
IPdfExternalSigner externalSignature = new ExternalSigner("SHA1");

// Add the public certificates for the external signer.
List<X509Certificate2> certificates = new List<X509Certificate2>();
certificates.Add(new X509Certificate2(Convert.FromBase64String(PublicCert)));
signature.AddExternalSigner(externalSignature, certificates, null);

//Create file stream.
using (FileStream outputFileStream = new FileStream(Path.GetFullPath(@"Output.pdf"), FileMode.Create, FileAccess.ReadWrite))
{
    //Save the PDF document to file stream.
    loadedDocument.Save(outputFileStream);
}
//Close the document.
loadedDocument.Close(true);

C# [Windows-specific]

//Load an existing PDF document.
PdfLoadedDocument loadedDocument = new PdfLoadedDocument("Input.pdf");

//Creates a digital signature.
PdfSignature signature = new PdfSignature(loadedDocument, loadedDocument.Pages[0], null, "Signature");
//Sets the signature information.
signature.Bounds = new RectangleF(new PointF(0, 0), new SizeF(100, 30));
signature.Settings.CryptographicStandard = CryptographicStandard.CADES;
signature.Settings.DigestAlgorithm = DigestAlgorithm.SHA1;

//Create an external signer.
IPdfExternalSigner externalSignature = new ExternalSigner("SHA1");

//Add public certificates.
List<X509Certificate2> certificates = new List<X509Certificate2>();
certificates.Add(new X509Certificate2(Convert.FromBase64String(PublicCert)));
signature.AddExternalSigner(externalSignature, certificates, null);

//Saves the document.
loadedDocument.Save("Output.pdf");
//Closes the document.
loadedDocument.Close(true);

VB.NET [Windows-specific]

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

'Creates a digital signature. 
Dim signature As PdfSignature = New PdfSignature(loadedDocument, loadedDocument.Pages(0), Nothing, "Signature")
'Sets the signature information.
signature.Bounds = New RectangleF(New PointF(0, 0), New SizeF(100, 30))
signature.Settings.CryptographicStandard = CryptographicStandard.CADES
signature.Settings.DigestAlgorithm = DigestAlgorithm.SHA1

'Create an external signer.
Dim externalSignature As IPdfExternalSigner = New ExternalSigner("SHA1")
Dim certificates As List(Of X509Certificate2) = New List(Of X509Certificate2)()
certificates.Add(New X509Certificate2(Convert.FromBase64String(PublicCert)))
signature.AddExternalSigner(externalSignature, certificates, Nothing)

'Saves the document.
loadedDocument.Save("Output.pdf")
'Closes the document.
loadedDocument.Close(True)

To sign the document using the X509Certificate2 API and generate an RFC3161-compliant timestamp token with the BouncyCastle library, add the following code:

C# [Cross-platform]

// External signer class that implements IPdfExternalSigner for signing the document hash.
class ExternalSigner : IPdfExternalSigner
{
  private string _hashAlgorithm;

  // Gets the hash algorithm used for signing.
  public string HashAlgorithm
  {
      get { return _hashAlgorithm; }
  }

  // Constructor that sets the hash algorithm for the signer.
  public ExternalSigner(string hashAlgorithm)
  {
      _hashAlgorithm = hashAlgorithm;
  }

  // Sign the document hash and return the timestamp response.
  public byte[] Sign(byte[] message, out byte[] timeStampResponse)
  {
      byte[] signedBytes = null;
      X509Certificate2 digitalID = new X509Certificate2(new X509Certificate2(Path.GetFullPath(@"Data/PDF.pfx"), "password123"));

      // Use the appropriate signing algorithm based on the private key type.
      if (digitalID.PrivateKey is System.Security.Cryptography.RSACryptoServiceProvider)
      {
          System.Security.Cryptography.RSACryptoServiceProvider rsa = (System.Security.Cryptography.RSACryptoServiceProvider)digitalID.PrivateKey;
          signedBytes = rsa.SignData(message, HashAlgorithm);
      }
      else if (digitalID.PrivateKey is RSACng)
      {
          RSACng rsa = (RSACng)digitalID.PrivateKey;
          signedBytes = rsa.SignData(message, System.Security.Cryptography.HashAlgorithmName.SHA1, RSASignaturePadding.Pkcs1);
      }

      // Generate an RFC3161 timestamp token for the signed data.
      timeStampResponse = GetRFC3161TimeStampToken(signedBytes);
      return signedBytes;
  }

  // Generate the RFC3161 timestamp token using the provided signed data.
  public byte[] GetRFC3161TimeStampToken(byte[] bytes)
  {
      SHA1 sha1 = SHA1CryptoServiceProvider.Create();
      byte[] hash = sha1.ComputeHash(bytes);

      // Create a timestamp request using the SHA1 hash.
      TimeStampRequestGenerator reqGen = new TimeStampRequestGenerator();
      reqGen.SetCertReq(true);
      TimeStampRequest tsReq = reqGen.Generate(TspAlgorithms.Sha1, hash, BigInteger.ValueOf(100));
      byte[] tsData = tsReq.GetEncoded();

      // Send the timestamp request to the server.
      HttpWebRequest req = (HttpWebRequest)WebRequest.Create("https://rfc3161.ai.moda"); // Update your timestamp URI here
      req.Method = "POST";
      req.ContentType = "application/timestamp-query";
      req.Headers.Add("Authorization", "Basic " + Convert.ToBase64String(Encoding.ASCII.GetBytes("9024:yourPass")));
      req.ContentLength = tsData.Length;

      // Write the request data to the stream.
      Stream reqStream = req.GetRequestStream();
      reqStream.Write(tsData, 0, tsData.Length);
      reqStream.Close();

      // Get the timestamp response from the server.
      HttpWebResponse res = (HttpWebResponse)req.GetResponse();
      if (res != null)
      {
          Stream resStream = new BufferedStream(res.GetResponseStream());
          TimeStampResponse tsRes = new TimeStampResponse(resStream);
          return tsRes.TimeStampToken.GetEncoded();
      }

      return null;
  }
}

C# [Windows-specific]

// External signer class that implements IPdfExternalSigner for signing the document hash.
class ExternalSigner : IPdfExternalSigner
{
  private string _hashAlgorithm;

  // Gets the hash algorithm used for signing.
  public string HashAlgorithm
  {
      get { return _hashAlgorithm; }
  }

  // Constructor that sets the hash algorithm for the signer.
  public ExternalSigner(string hashAlgorithm)
  {
      _hashAlgorithm = hashAlgorithm;
  }

  // Sign the document hash and return the timestamp response.
  public byte[] Sign(byte[] message, out byte[] timeStampResponse)
  {
      byte[] signedBytes = null;
      X509Certificate2 digitalID = new X509Certificate2(new X509Certificate2(Path.GetFullPath(@"Data/PDF.pfx"), "password123"));

      // Use the appropriate signing algorithm based on the private key type.
      if (digitalID.PrivateKey is System.Security.Cryptography.RSACryptoServiceProvider)
      {
          System.Security.Cryptography.RSACryptoServiceProvider rsa = (System.Security.Cryptography.RSACryptoServiceProvider)digitalID.PrivateKey;
          signedBytes = rsa.SignData(message, HashAlgorithm);
      }
      else if (digitalID.PrivateKey is RSACng)
      {
          RSACng rsa = (RSACng)digitalID.PrivateKey;
          signedBytes = rsa.SignData(message, System.Security.Cryptography.HashAlgorithmName.SHA1, RSASignaturePadding.Pkcs1);
      }

      // Generate an RFC3161 timestamp token for the signed data.
      timeStampResponse = GetRFC3161TimeStampToken(signedBytes);
      return signedBytes;
  }

  // Generate the RFC3161 timestamp token using the provided signed data.
  public byte[] GetRFC3161TimeStampToken(byte[] bytes)
  {
      SHA1 sha1 = SHA1CryptoServiceProvider.Create();
      byte[] hash = sha1.ComputeHash(bytes);

      // Create a timestamp request using the SHA1 hash.
      TimeStampRequestGenerator reqGen = new TimeStampRequestGenerator();
      reqGen.SetCertReq(true);
      TimeStampRequest tsReq = reqGen.Generate(TspAlgorithms.Sha1, hash, BigInteger.ValueOf(100));
      byte[] tsData = tsReq.GetEncoded();

      // Send the timestamp request to the server.
      HttpWebRequest req = (HttpWebRequest)WebRequest.Create("https://rfc3161.ai.moda"); // Update your timestamp URI here
      req.Method = "POST";
      req.ContentType = "application/timestamp-query";
      req.Headers.Add("Authorization", "Basic " + Convert.ToBase64String(Encoding.ASCII.GetBytes("9024:yourPass")));
      req.ContentLength = tsData.Length;

      // Write the request data to the stream.
      Stream reqStream = req.GetRequestStream();
      reqStream.Write(tsData, 0, tsData.Length);
      reqStream.Close();

      // Get the timestamp response from the server.
      HttpWebResponse res = (HttpWebResponse)req.GetResponse();
      if (res != null)
      {
          Stream resStream = new BufferedStream(res.GetResponseStream());
          TimeStampResponse tsRes = new TimeStampResponse(resStream);
          return tsRes.TimeStampToken.GetEncoded();
      }

      return null;
  }
}

VB.NET [Windows-specific]

' External signer class that implements IPdfExternalSigner for signing the document hash.
Public Class ExternalSigner
    Implements IPdfExternalSigner

    Private _hashAlgorithm As String

    ' Gets the hash algorithm used for signing.
    Public ReadOnly Property HashAlgorithm() As String Implements IPdfExternalSigner.HashAlgorithm
        Get
            Return _hashAlgorithm
        End Get
    End Property

    ' Constructor that sets the hash algorithm for the signer.
    Public Sub New(hashAlgorithm As String)
        _hashAlgorithm = hashAlgorithm
    End Sub

    ' Sign the document hash and return the timestamp response.
    Public Function Sign(message As Byte(), ByRef timeStampResponse As Byte()) As Byte() Implements IPdfExternalSigner.Sign
        Dim signedBytes As Byte() = Nothing

        ' Load the digital ID (certificate) from the file system.
        Dim digitalID As New X509Certificate2(Path.GetFullPath("Data/PDF.pfx"), "password123")

        ' Use the appropriate signing algorithm based on the private key type.
        If TypeOf digitalID.PrivateKey Is RSACryptoServiceProvider Then
            ' Use RSACryptoServiceProvider to sign the data.
            Dim rsa As RSACryptoServiceProvider = CType(digitalID.PrivateKey, RSACryptoServiceProvider)
            signedBytes = rsa.SignData(message, HashAlgorithm)
        ElseIf TypeOf digitalID.PrivateKey Is RSACng Then
            ' Use RSACng to sign the data with SHA1 and PKCS1 padding.
            Dim rsa As RSACng = CType(digitalID.PrivateKey, RSACng)
            signedBytes = rsa.SignData(message, HashAlgorithmName.SHA1, RSASignaturePadding.Pkcs1)
        End If

        ' Generate an RFC3161 timestamp token for the signed data.
        timeStampResponse = GetRFC3161TimeStampToken(signedBytes)

        ' Return the signed bytes.
        Return signedBytes
    End Function

    ' Generate the RFC3161 timestamp token using the provided signed data.
    Public Function GetRFC3161TimeStampToken(bytes As Byte()) As Byte()
        ' Create a SHA1 hash of the provided signed data.
        Dim sha1 As SHA1 = SHA1CryptoServiceProvider.Create()
        Dim hash As Byte() = sha1.ComputeHash(bytes)

        ' Create a timestamp request using the SHA1 hash.
        Dim reqGen As New TimeStampRequestGenerator()
        reqGen.SetCertReq(True)
        Dim tsReq As TimeStampRequest = reqGen.Generate(TspAlgorithms.Sha1, hash, BigInteger.ValueOf(100))
        Dim tsData As Byte() = tsReq.GetEncoded()

        ' Send the timestamp request to the server.
        Dim req As HttpWebRequest = CType(WebRequest.Create("https://rfc3161.ai.moda"), HttpWebRequest) ' Update your timestamp URI here
        req.Method = "POST"
        req.ContentType = "application/timestamp-query"
        req.Headers.Add("Authorization", "Basic " & Convert.ToBase64String(Encoding.ASCII.GetBytes("9024:yourPass")))
        req.ContentLength = tsData.Length

        ' Write the request data to the request stream.
        Using reqStream As Stream = req.GetRequestStream()
            reqStream.Write(tsData, 0, tsData.Length)
        End Using

        ' Get the response from the timestamp server.
        Dim res As HttpWebResponse = CType(req.GetResponse(), HttpWebResponse)

        ' If a response is received, read and return the timestamp token.
        If res IsNot Nothing Then
            Using resStream As Stream = New BufferedStream(res.GetResponseStream())
                Dim tsRes As New TimeStampResponse(resStream)
                Return tsRes.TimeStampToken.GetEncoded()
            End Using
        End If

        ' Return null if no valid response is received.
        Return Nothing
    End Function
End Class

You can download a complete working sample from GitHub.

Create Long Term Validation (LTV) when signing PDF documents externally

You can create a Long Term validation (LTV) when signing PDF documents externally using your private/public certificates.The following code example shows how to create an LTV when signing a PDF document from external signature using CreateLongTermValidity method in PdfSignature class.

C# [Cross-platform]

//Get the stream from the document.
    FileStream documentStream = new FileStream(Path.GetFullPath(@"Data/Input.pdf"), FileMode.Open, FileAccess.Read);

    //Load an existing PDF document.
    PdfLoadedDocument document = new PdfLoadedDocument(documentStream);

    //Get the page of the existing PDF document.
    PdfLoadedPage loadedPage = document.Pages[0] as PdfLoadedPage;

    //Create a new PDF signature without PdfCertificate instance.
    PdfSignature signature = new PdfSignature(document, loadedPage, null, "Signature1");

    //Hook up the ComputeHash event.
    signature.ComputeHash += Signature_ComputeHash;

    //Save the document into stream
    MemoryStream stream = new MemoryStream();
    document.Save(stream);
    //Close the document
    document.Close(true);

    //Load an existing PDF stream..
    PdfLoadedDocument loadedDocument = new PdfLoadedDocument(stream);

    //Gets the first signature field of the PDF document
    PdfLoadedSignatureField signatureField = loadedDocument.Form.Fields[0] as PdfLoadedSignatureField;
    PdfSignature pdfSignature = signatureField.Signature;

    //Create X509Certificate2 from your certificate to create a long-term validity.
    X509Certificate2 x509 = new X509Certificate2(Path.GetFullPath(@"Data/PDF.pfx"), "syncfusion");

    //Create LTV with your public certificates.
    pdfSignature.CreateLongTermValidity(new List<X509Certificate2> { x509 });

    //Save the PDF document
    MemoryStream ms = new MemoryStream();
    loadedDocument.Save(ms);
    //If the position is not set to '0' then the PDF will be empty
    ms.Position = 0;
    //Close the document
    loadedDocument.Close(true);

    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(Path.GetFullPath(@"Data/PDF.pfx"), "syncfusion");
        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();
    }

C# [Windows-specific]

//Load an existing PDF document.
    PdfLoadedDocument document = new PdfLoadedDocument("Input.pdf");

    //Get the page of the existing PDF document.
    PdfLoadedPage loadedPage = document.Pages[0] as PdfLoadedPage;

    //Create a new PDF signature without PdfCertificate instance.
    PdfSignature signature = new PdfSignature(document, loadedPage, null, "Signature1");

    //Hook up the ComputeHash event.
    signature.ComputeHash += Signature_ComputeHash;

    //Save the document.
    document.Save("SignedDocument");
    //Close the document
    document.Close(true);

    //Load an existing PDF stream..
    PdfLoadedDocument loadedDocument = new PdfLoadedDocument("SignedDocument");

    //Gets the first signature field of the PDF document
    PdfLoadedSignatureField signatureField = loadedDocument.Form.Fields[0] as PdfLoadedSignatureField;
    PdfSignature pdfSignature = signatureField.Signature;

    //Create X509Certificate2 from your certificate to create a long-term validity.
    X509Certificate2 x509 = new X509Certificate2(Path.GetFullPath(@"Data/PDF.pfx"), "syncfusion");

    //Create LTV with your public certificates.
    pdfSignature.CreateLongTermValidity(new List<X509Certificate2> { x509 });
    
    //Save and close the PDF document
    loadedDocument.Save("Output.pdf");
    loadedDocument.Close(true);

    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(Path.GetFullPath(@"Data/PDF.pfx"), "syncfusion");
        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 [Windows-specific]

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

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

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

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

    ' Save the document.
    document.Save("SignedDocument")
    ' Close the document
    document.Close(True)

    ' Load an existing PDF stream.
    Dim loadedDocument As New PdfLoadedDocument("SignedDocument")

    ' Gets the first signature field of the PDF document
    Dim signatureField As PdfLoadedSignatureField = TryCast(loadedDocument.Form.Fields(0), PdfLoadedSignatureField)
    Dim pdfSignature As PdfSignature = signatureField.Signature

    ' Create X509Certificate2 from your certificate to create a long-term validity.
    Dim x509 As New X509Certificate2(Path.GetFullPath("Data/PDF.pfx"), "syncfusion")

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

    ' Save and close the PDF document
    loadedDocument.Save("Output.pdf")
    loadedDocument.Close(True)

    Private Sub Signature_ComputeHash(sender As Object, ars As PdfSignatureEventArgs)
        ' Get the document bytes
        Dim documentBytes As Byte() = ars.Data

        Dim signedCms As New SignedCms(New ContentInfo(documentBytes), True)

        ' Compute the signature using the specified digital ID file and the password
        Dim certificate As New X509Certificate2(Path.GetFullPath("Data/PDF.pfx"), "syncfusion")
        Dim cmsSigner As 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 a complete working sample from GitHub.

Digitally sign a PDF document using long-term archive timestamps (LTA)

The PDF LTA signature is the next level of the LTV signature. It follows the standard PAdES B-LTA. According to the standard, the validation-related information of the timestamp is added to the DSS along with other signature information mentioned in the LTV signature.

The document timestamp is also applied to the PDF document, so it provides more viability to the signature. This level is recommended for qualified electronic signatures.

The following code example shows how to sign a PDF document with LTA using TimeStampServer property and EnableLtv property in PdfSignature class.

C# [Cross-platform]

//Load existing PDF document.
FileStream documentStream1 = new FileStream("PDF_Succinctly.pdf", FileMode.Open, FileAccess.Read);
PdfLoadedDocument loadedDocument = new PdfLoadedDocument(documentStream1);
//Load digital ID with password.
FileStream documentStream2 = new FileStream("DigitalSignatureTest.pfx", FileMode.Open, FileAccess.Read);
PdfCertificate certificate = new PdfCertificate(documentStream2, "DigitalPass123");

//Create a signature with loaded digital ID.
PdfSignature signature = new PdfSignature(loadedDocument, loadedDocument.Pages[0], certificate, "DigitalSignature");
signature.Settings.CryptographicStandard = CryptographicStandard.CADES;
signature.Settings.DigestAlgorithm = DigestAlgorithm.SHA256;
signature.TimeStampServer = new TimeStampServer(new Uri("http://timestamping.ensuredca.com"));
//Enable LTV document.
signature.EnableLtv = true;

//Save the PDF document.
MemoryStream stream = new MemoryStream();
loadedDocument.Save(stream);
//Close the document.
loadedDocument.Close(true);

//Load existing PDF document.
PdfLoadedDocument ltDocument = new PdfLoadedDocument(stream);
//Load the existing PDF page.
PdfLoadedPage lpage = ltDocument.Pages[0] as PdfLoadedPage;

//Create PDF signature with empty certificate.
PdfSignature timeStamp = new PdfSignature(lpage, "timestamp");
timeStamp.TimeStampServer = new TimeStampServer(new Uri("http://timestamping.ensuredca.com"));

//Save and close the PDF document.
MemoryStream stream1 = new MemoryStream();
ltDocument.Save(stream1);
//Close the document.
ltDocument.Close(true);
//Defining the ContentType for pdf file.
string contentType = "application/pdf";
//Define the file name.
string fileName = "Output.pdf";
//Creates a FileContentResult object by using the file contents, content type, and file name.
return File(stream1, contentType, fileName);

C# [Windows-specific]

//Load existing PDF document.
PdfLoadedDocument loadedDocument = new PdfLoadedDocument("PDF_Succinctly.pdf");
//Load digital ID with password.
PdfCertificate certificate = new PdfCertificate("DigitalSignatureTest.pfx", "DigitalPass123");

//Create a signature with loaded digital ID.
PdfSignature signature = new PdfSignature(loadedDocument, loadedDocument.Pages[0], certificate, "DigitalSignature");
signature.Settings.CryptographicStandard = CryptographicStandard.CADES;
signature.Settings.DigestAlgorithm = DigestAlgorithm.SHA256;
signature.TimeStampServer = new TimeStampServer(new Uri("http://timestamping.ensuredca.com"));
//Enable LTV document.
signature.EnableLtv = true;

//Save the PDF document.
loadedDocument.Save("LTV_document.pdf");
//Close the document.
loadedDocument.Close(true);

//Load existing PDF document.
PdfLoadedDocument ltDocument = new PdfLoadedDocument("LTV_document.pdf");
//Load the existing PDF page.
PdfLoadedPage lpage = ltDocument.Pages[0] as PdfLoadedPage;

//Create PDF signature with empty certificate.
PdfSignature timeStamp = new PdfSignature(lpage, "timestamp");
timeStamp.TimeStampServer = new TimeStampServer(new Uri("http://timestamping.ensuredca.com"));

//Save and close the PDF document.
ltDocument.Save("PAdES B-LTA.pdf");
ltDocument.Close(true);

VB.NET [Windows-specific]

'Loads a PDF document.
Dim loadedDocument As PdfLoadedDocument = New PdfLoadedDocument("PDF_Succinctly.pdf")
'Load digital ID with password.
Dim certificate As PdfCertificate = New PdfCertificate("DigitalSignatureTest.pfx", "DigitalPass123")

'Create a signature with loaded digital ID.
Dim signature As PdfSignature = New PdfSignature(loadedDocument, loadedDocument.Pages(0), certificate, "DigitalSignature")
signature.Settings.CryptographicStandard = CryptographicStandard.CADES
signature.Settings.DigestAlgorithm = DigestAlgorithm.SHA256
signature.TimeStampServer = New TimeStampServer(New Uri("http://timestamping.ensuredca.com"))
'Enable LTV document.
signature.EnableLtv = True

'Save the document.
loadedDocument.Save("LTV_document.pdf")
'Close the document.
loadedDocument.Close(True)

'Loads a PDF document.
Dim ltDocument As PdfLoadedDocument = New PdfLoadedDocument("LTV_document.pdf")
'Load the existing PDF page.
Dim lpage As PdfLoadedPage = TryCast(ltDocument.Pages(0), PdfLoadedPage)

'Create PDF signature with empty certificate.
Dim timeStamp As PdfSignature = New PdfSignature(lpage, "timestamp")
timeStamp.TimeStampServer = New TimeStampServer(New Uri("http://timestamping.ensuredca.com"))

'Save the document.
ltDocument.Save("PAdES B-LTA.pdf")
'Close the document.
ltDocument.Close(True)

You can download a complete working sample from GitHub.

Digitally sign a PDF document using the Windows certificate store

A Windows certificate store is a secure way to store the digital ID. If a root certificate is added to the Windows certificate store, you do not need to manually add and trust each of the certificates that are already present in the Windows certificate store.

You can retrieve the digital ID X509Certificate2 from the Windows certificate store X509Store and use it to add a digital signature to a PDF document using PdfSignature class.

The following code example shows how to create a PDF digital signature using the Windows certificate store.

C#

//Initialize the Windows store.
X509Store store = new X509Store("MY", StoreLocation.CurrentUser);
store.Open(OpenFlags.ReadOnly | OpenFlags.OpenExistingOnly);
X509Certificate2Collection collection = (X509Certificate2Collection)store.Certificates;
//Find the certificate using thumb print.
X509Certificate2Collection fcollection = (X509Certificate2Collection)collection.Find(X509FindType.FindByThumbprint, "F85E1C5D93115CA3F969DA3ABC8E0E9547FCCF5A", true);
X509Certificate2 digitalID = collection[0];

//Load existing PDF document.
FileStream documentStream = new FileStream("PDF_Succinctly.pdf", FileMode.Open, FileAccess.Read);
PdfLoadedDocument loadedDocument = new PdfLoadedDocument(documentStream);
//Load X509Certificate2.
PdfCertificate certificate = new PdfCertificate(digitalID);

//Create a Revision 2 signature with loaded digital ID.
PdfSignature signature = new PdfSignature(loadedDocument, loadedDocument.Pages[0], certificate, "DigitalSignature");
//Changing the digital signature standard and hashing algorithm.
signature.Settings.CryptographicStandard = CryptographicStandard.CADES;
signature.Settings.DigestAlgorithm = DigestAlgorithm.SHA512;

//Save the PDF document.
MemoryStream stream = new MemoryStream();
loadedDocument.Save(stream);
//Close the document.
loadedDocument.Close(true);
//Defining the ContentType for pdf file.
string contentType = "application/pdf";
//Define the file name.
string fileName = "Output.pdf";
//Creates a FileContentResult object by using the file contents, content type, and file name.
return File(stream, contentType, fileName);

C#

//Initialize the Windows store.
X509Store store = new X509Store("MY", StoreLocation.CurrentUser);
store.Open(OpenFlags.ReadOnly | OpenFlags.OpenExistingOnly);
X509Certificate2Collection collection = (X509Certificate2Collection)store.Certificates;
//Find the certificate using thumb print.
X509Certificate2Collection fcollection = (X509Certificate2Collection)collection.Find(X509FindType.FindByThumbprint, "F85E1C5D93115CA3F969DA3ABC8E0E9547FCCF5A", true);
X509Certificate2 digitalID = fcollection[0];

//Load existing PDF document.
PdfLoadedDocument loadedDocument = new PdfLoadedDocument("PDF_Succinctly.pdf");
//Load X509Certificate2.
PdfCertificate certificate = new PdfCertificate(digitalID);

//Create a Revision 2 signature with loaded digital ID.
PdfSignature signature = new PdfSignature(loadedDocument, loadedDocument.Pages[0], certificate, "DigitalSignature");
//Changing the digital signature standard and hashing algorithm.
signature.Settings.CryptographicStandard = CryptographicStandard.CADES;
signature.Settings.DigestAlgorithm = DigestAlgorithm.SHA512;

//Save the PDF document.
loadedDocument.Save("WindowsStore.pdf");
//Close the document.
loadedDocument.Close(true);

VB.NET

'Initialize the Windows store.
Dim store As X509Store = New X509Store("MY", StoreLocation.CurrentUser)
store.Open(OpenFlags.[ReadOnly] Or OpenFlags.OpenExistingOnly)
'Find the certificate using thumb print.
Dim collection As X509Certificate2Collection = CType(store.Certificates, X509Certificate2Collection)
Dim fcollection As X509Certificate2Collection = CType(collection.Find(X509FindType.FindByThumbprint, "F85E1C5D93115CA3F969DA3ABC8E0E9547FCCF5A", True), X509Certificate2Collection)
Dim digitalID As X509Certificate2 = fcollection(0)

'Load existing PDF document.
Dim loadedDocument As PdfLoadedDocument = New PdfLoadedDocument("PDF_Succinctly.pdf")
'Load X509Certificate2.
Dim certificate As PdfCertificate = New PdfCertificate(digitalID)
'Create a Revision 2 signature with loaded digital ID.
Dim signature As PdfSignature = New PdfSignature(loadedDocument, loadedDocument.Pages(0), certificate, "DigitalSignature")
'Changing the digital signature standard and hashing algorithm.
signature.Settings.CryptographicStandard = CryptographicStandard.CADES
signature.Settings.DigestAlgorithm = DigestAlgorithm.SHA512

'Save the PDF document.
loadedDocument.Save("WindowsStore.pdf")
'Close the document.
loadedDocument.Close(True)

You can download a complete working sample from GitHub.

Adding a signature validation appearance based on the signature

You can add the dynamic signature validation appearance to the signature field by enabling the EnableValidationAppearance property available in the PdfSignature class, the appearance will change based on the PDF reader validation.

Refer to the following code sample.

C# [Cross-platform]

//Creates a new PDF document
PdfDocument document = new PdfDocument();
//Adds a new page
PdfPageBase page = document.Pages.Add();
//Create PDF graphics for the page.
PdfGraphics graphics = page.Graphics;

//Creates a certificate instance from the PFX file with private key
FileStream certificateStream = new FileStream("PDF.pfx", FileMode.Open, FileAccess.Read);
PdfCertificate pdfCert = new PdfCertificate(certificateStream, "password123");
//Creates a digital signature
PdfSignature signature = new PdfSignature(document, page, pdfCert, "Signature");
//Sets an image for the signature field
FileStream imageStream = new FileStream("signature.jpg", FileMode.Open, FileAccess.Read);
//Sets an image for signature field
PdfBitmap signatureImage = new PdfBitmap(imageStream);
//Sets enable signature validation appearance
signature.EnableValidationAppearance = true;
//Sets signature information
signature.Bounds = new RectangleF(new PointF(0, 0), signatureImage.PhysicalDimension);
signature.ContactInfo = "[e-mail]";
signature.LocationInfo = "Honolulu, Hawaii";
signature.Reason = "I am author of this document.";
//Draws the signature image
signature.Appearance.Normal.Graphics.DrawImage(signatureImage, 0, 0);

//Save the document into stream
MemoryStream stream = new MemoryStream();
document.Save(stream);
stream.Position = 0;
//Close the document
document.Close(true);
//Defining the ContentType for PDF file
string contentType = "application/pdf";
//Define the file name
string fileName = "Output.pdf";
//Creates a FileContentResult object by using the file contents, content type, and file name
return File(stream, contentType, fileName);

C# [Windows-specific]

//Creates a new PDF document
PdfDocument document = new PdfDocument();         
//Add a new page
PdfPageBase page = document.Pages.Add();
//Create PDF graphics for the page
PdfGraphics graphics=page.Graphics;

//Creates a certificate instance from the PFX file with private key         
PdfCertificate pdfCert = new PdfCertificate(@"PDF.pfx", "password123");
//Creates a digital signature
PdfSignature signature = new PdfSignature(document, page, pdfCert, "Signature");
//Sets an image for the signature field
PdfBitmap signatureImage = new PdfBitmap(@"signature.jpg");
//Sets enable signature validation appearance
signature.EnableValidationAppearance = true;
//Sets signature information
signature.ContactInfo = "[e-mail]";
signature.LocationInfo = "Honolulu, Hawaii";
signature.Reason = "I am author of this document.";
//Draw the signature image  
signature.Appearance.Normal.Graphics.DrawImage(signatureImage, 0, 0);
 
//Save and close document
document.Save("Output.pdf");
document.Close( true );

VB.NET [Windows-specific]

'Creates a new PDF document
Dim document As New PdfDocument()
'Adds a new page
Dim page As PdfPageBase = document.Pages.Add()
'Create PDF graphics for the page
Dim graphics As PdfGraphics = page.Graphics

'Creates a certificate instance from the PFX file with private key
Dim pdfCert As New PdfCertificate("PDF.pfx", "password123")
'Creates a digital signature
Dim signature As New PdfSignature(document, page, pdfCert, "Signature")
'Sets an image for the signature field
Dim signatureImage As New PdfBitmap("signature.jpg")
'Sets enable signature validation appearance    
signature.EnableValidationAppearance = True
'Sets signature info
signature.Bounds = New RectangleF(New PointF(0, 0), signatureImage.PhysicalDimension)
signature.ContactInfo = "[e-mail]"
signature.LocationInfo = "Honolulu, Hawaii"
signature.Reason = "I am author of this document."
'Draws the signature image
signature.Appearance.Normal.Graphics.DrawImage(signatureImage, 0, 0)

'Save and close the document
document.Save("Output.pdf")
document.Close(True)

You can download a complete working sample from GitHub.

Adding a timestamp in digital signature

Essential® PDF allows you to add timestamp in the digital signature of the PDF document using TimeStampServer property in PdfSignature class. The following code example explains the same.

C# [Cross-platform]

//Creates a new PDF document
PdfDocument document = new PdfDocument();
//Adds a new page
PdfPageBase page = document.Pages.Add();
//Create PDF graphics for the page
PdfGraphics graphics = page.Graphics;

//Creates a certificate instance from PFX file with private key
FileStream certificateStream = new FileStream("PDF.pfx", FileMode.Open, FileAccess.Read);
PdfCertificate pdfCert = new PdfCertificate(certificateStream, "password123");
//Creates a digital signature
PdfSignature signature = new PdfSignature(document, page, pdfCert, "Signature");
//Sets an image for signature field
FileStream imageStream = new FileStream("syncfusion_logo.jpeg", FileMode.Open, FileAccess.Read);
//Sets an image for signature field
PdfBitmap image = new PdfBitmap(imageStream);
//Adds time stamp by using the server URI and credentials
signature.TimeStampServer = new TimeStampServer(new Uri("http://syncfusion.digistamp.com"), "user", "123456");
//Sets signature info
signature.Bounds = new RectangleF(new PointF(0, 0), image.PhysicalDimension);
signature.ContactInfo = "[e-mail]";
signature.LocationInfo = "Honolulu, Hawaii";
signature.Reason = "I am author of this document.";
//Draws the signature image
signature.Appearance.Normal.Graphics.DrawImage(image, 0, 0);

//Save the document into stream
MemoryStream stream = new MemoryStream();
document.Save(stream);
stream.Position = 0;
//Close the document
document.Close(true);
//Defining the ContentType for pdf file
string contentType = "application/pdf";
//Define the file name
string fileName = "Output.pdf";
//Creates a FileContentResult object by using the file contents, content type, and file name
return File(stream, contentType, fileName);

C# [Windows-specific]

//Creates a new PDF document
PdfDocument document = new PdfDocument();
//Adds a new page
PdfPage page = document.Pages.Add();
//Create PDF graphics for the page
PdfGraphics graphics = page.Graphics;

//Creates a certificate instance from PFX file with private key
PdfCertificate pdfCert = new PdfCertificate(@"PDF.pfx", "password123");
//Creates a digital signature
PdfSignature signature = new PdfSignature(page, pdfCert, "Signature");
//Change the digital signature standard and hashing algorithm
signature.Settings.CryptographicStandard = CryptographicStandard.CADES;
signature.Settings.DigestAlgorithm = DigestAlgorithm.SHA512;
//Sets an image for signature field
PdfBitmap image = new PdfBitmap(@"syncfusion_logo.jpeg");
//Adds time stamp by using the server URI and credentials
signature.TimeStampServer = new TimeStampServer(new Uri("http://syncfusion.digistamp.com"), "user", "123456");
//Sets signature info
signature.Bounds = new RectangleF(new PointF(0, 0), image.PhysicalDimension);
signature.ContactInfo = "[e-mail]";
signature.LocationInfo = "Honolulu, Hawaii";
signature.Reason = "I am author of this document.";
//Draws the signature image
signature.Appearance.Normal.Graphics.DrawImage(image, 0, 0);

//Save and close the document
document.Save("Output.pdf");
document.Close(true);

VB.NET [Windows-specific]

'Creates a new PDF document
Dim document As New PdfDocument()
'Adds a new page
Dim page As PdfPage = document.Pages.Add()
'Create PDF graphics for the page
Dim graphics As PdfGraphics = page.Graphics

'Creates a certificate instance from PFX file with private key
Dim pdfCert As New PdfCertificate("PDF.pfx", "password123")
'Creates a digital signature
Dim signature As New PdfSignature(page, pdfCert, "Signature")
'Sets an image for signature field
Dim image As New PdfBitmap("syncfusion_logo.jpeg")
'Adds time stamp by using the server URI and credentials
signature.TimeStampServer = New TimeStampServer(New Uri("http://syncfusion.digistamp.com"), "user", "123456")
'Sets signature info
signature.Bounds = New RectangleF(New PointF(0, 0), image.PhysicalDimension)
signature.ContactInfo = "[e-mail]"
signature.LocationInfo = "Honolulu, Hawaii"
signature.Reason = "I am author of this document."
'Draws the signature image
signature.Appearance.Normal.Graphics.DrawImage(image, 0, 0)

'Save and close the document
document.Save("Output.pdf")
document.Close(True)

You can download a complete working sample from GitHub.

Adding a timestamp to PDF document

You can add timestamp to the PDF document using TimeStampServer property in PdfSignature class. The following code example explains the same.

C# [Cross-platform]

//Create a new pdf document
PdfDocument document = new PdfDocument();
//Adds a new page
PdfPage page = document.Pages.Add();

//Creates a digital signature
PdfSignature signature = new PdfSignature(page, "Signature");
//Add the time stamp by using the server URI
signature.TimeStampServer = new TimeStampServer(new Uri("http://syncfusion.digistamp.com"), "user", "123456");

//Save the document into stream
MemoryStream stream = new MemoryStream();
document.Save(stream);
stream.Position = 0;
//Close the document
document.Close(true);
//Defining the ContentType for pdf file
string contentType = "application/pdf";
//Define the file name
string fileName = "Output.pdf";
//Creates a FileContentResult object by using the file contents, content type, and file name
return File(stream, contentType, fileName);

C# [Windows-specific]

//Create a new PDF document
PdfDocument document = new PdfDocument();
//Add a new page
PdfPage page = document.Pages.Add();

//Creates a digital signature
PdfSignature signature = new PdfSignature(page, "Signature");
//Add the time stamp by using the server URI
signature.TimeStampServer = new TimeStampServer(new Uri("http://syncfusion.digistamp.com"), "user", "123456");

//Save and close the document
document.Save("Output.pdf");
document.Close(true);

VB.NET [Windows-specific]

'Creates a new PDF document
Dim document As New PdfDocument()
'Adds a new page
Dim page As PdfPage = document.Pages.Add()

'Creates a digital signature
Dim signature As New PdfSignature(page, "Signature")
'Adds time stamp by using the server URI
signature.TimeStampServer = New TimeStampServer(New Uri("http://syncfusion.digistamp.com"), "user", "123456")

'Save and close the document
document.Save("Output.pdf")
document.Close(True)

You can download a complete working sample from GitHub.

Adding a timestamp to existing PDF document

You can add timestamp to the existing PDF document using TimeStampServer property in PdfSignature class. The following code example explains the same.

C# [Cross-platform]

//Load the PDF document
FileStream docStream = new FileStream("Input.pdf", FileMode.Open, FileAccess.Read);
PdfLoadedDocument loadedDocument = new PdfLoadedDocument(docStream);
//Gets the page
PdfLoadedPage page = loadedDocument.Pages[0] as PdfLoadedPage;

//Creates a digital signature
PdfSignature signature = new PdfSignature(page, "Signature");
//Add the time stamp by using the server URI
signature.TimeStampServer = new TimeStampServer(new Uri("http://syncfusion.digistamp.com"), "user", "123456");

//Save the document into stream
MemoryStream stream = new MemoryStream();
loadedDocument.Save(stream);
stream.Position = 0;
//Close the document
loadedDocument.Close(true);
//Defining the ContentType for pdf file
string contentType = "application/pdf";
//Define the file name
string fileName = "Output.pdf";
//Creates a FileContentResult object by using the file contents, content type, and file name
return File(stream, contentType, fileName);

C# [Windows-specific]

//Load the existing PDF document
PdfLoadedDocument loadedDocument = new PdfLoadedDocument("Input.pdf");
//Add a new page
PdfLoadedPage page = loadedDocument.Pages[0] as PdfLoadedPage;

//Creates a digital signature
PdfSignature signature = new PdfSignature(page, "Signature");
//Add the time stamp by using the server URI
signature.TimeStampServer = new TimeStampServer(new Uri("http://syncfusion.digistamp.com"), "user", "123456");

//Save the PDF document
loadedDocument.Save("Output.pdf");
//Close the document
loadedDocument.Close(true);

VB.NET [Windows-specific]

'Load the PDF document
Dim document As New PdfLoadedDocument("Input.pdf")
'Gets the first page of the document
Dim page As PdfLoadedPage = TryCast(document.Pages(0), PdfLoadedPage)

'Creates a digital signature
Dim signature As New PdfSignature(page, "Signature")
'Adds time stamp by using the server URI
signature.TimeStampServer = New TimeStampServer(New Uri("http://syncfusion.digistamp.com"), "user", "123456")

'Save and close the document
document.Save("Output.pdf")
document.Close(True)

You can download a complete working sample from GitHub.

Retrieve certificate details from an existing signed PDF document

The Essential® PDF provides support to get the certificate details from an existing signed PDF document such as,

• Signed date
• Expiry date
• Signed name
• Subject name
• Issuer name
• Certificate distinguished names (country, state, street, email, organization, organization unit, locality, and more).

You can get the above certificate details from an existing signed PDF document using ValidFrom and ValidTo property.The following code example explains the same.

C# [Cross-platform]

//Load the PDF document
FileStream docStream = new FileStream("Input.pdf", FileMode.Open, FileAccess.Read);
PdfLoadedDocument loadedDocument = new PdfLoadedDocument(docStream);
//Get the signature field
PdfLoadedSignatureField signatureField = loadedDocument.Form.Fields[0] as PdfLoadedSignatureField;

//Get the certificate
PdfCertificate certificate = signatureField.Signature.Certificate;
//Get the signed date
DateTime date = signatureField.Signature.SignedDate;
//Get the signed name
string name = signatureField.Signature.SignedName;
//Gets the certificate subject's name
string subjectName = certificate.SubjectName;
//Gets the certificate issuer's name
string issuerName = certificate.IssuerName;
//Get certificate validation date information
DateTime validFrom = certificate.ValidFrom;
DateTime validTo = certificate.ValidTo;

//Close the document
loadedDocument.Close(true);

C# [Windows-specific]

//Load the existing signed PDF
PdfLoadedDocument loadedDocument = new PdfLoadedDocument("SignedDocument.pdf");
//Load the PDF form
PdfLoadedForm loadedForm = loadedDocument.Form as PdfLoadedForm;
//Get the signature field
PdfLoadedSignatureField signatureField = loadedForm.Fields[0] as PdfLoadedSignatureField;

//Get the certificate
PdfCertificate certificate = signatureField.Signature.Certificate;
//Get the signed date
DateTime date = signatureField.Signature.SignedDate;
//Get the signed name
string name = signatureField.Signature.SignedName;
//Get the certificate names based on their distinguished name.
string subjectName = certificate.SubjectName
string subjectCountry = certificate.GetValue(PdfCertificateDistinguishedName.Country, PdfCertificateField.Subject);
string subjectOrganization = certificate.GetValue(PdfCertificateDistinguishedName.Organization, PdfCertificateField.Subject);
//Issuer details
string issuerName = certificate.IssuerName;
string issuerOrganization = certificate.GetValue(PdfCertificateDistinguishedName.Organization, PdfCertificateField.Issuer);
string issuerCountry = certificate.GetValue(PdfCertificateDistinguishedName.Country, PdfCertificateField.Issuer);
//Get certificate validation date information
DateTime validFrom = certificate.ValidFrom;
DateTime validTo = certificate.ValidTo;

//Close the document
loadedDocument.Close(true);

VB.NET [Windows-specific]

'Load the existing signed PDF
Dim loadedDocument As PdfLoadedDocument = New PdfLoadedDocument("../../Signed.pdf")
'Load the PDF form
Dim loadedForm As PdfLoadedForm = TryCast(loadedDocument.Form, PdfLoadedForm)
'Get the signature field
Dim signatureField As PdfLoadedSignatureField = TryCast(loadedForm.Fields(0), PdfLoadedSignatureField)

'Get the certificate
Dim certificate As PdfCertificate = signatureField.Signature.Certificate
'Get the signed date
Dim signedDate As DateTime = signatureField.Signature.SignedDate
'Get the signed name
Dim name As String = signatureField.Signature.SignedName
'Get the certificate names based on their distinguished name
Dim subjectName As String = certificate.SubjectName
Dim subjectCountry As String = certificate.GetValue(PdfCertificateDistinguishedName.Country, PdfCertificateField.Subject)
Dim subjectOrganization As String = certificate.GetValue(PdfCertificateDistinguishedName.Organization, PdfCertificateField.Subject)
'Issuer details
Dim issuerName As String = certificate.IssuerName
Dim issuerOrganization As String = certificate.GetValue(PdfCertificateDistinguishedName.Organization, PdfCertificateField.Issuer)
Dim issuerCountry As String = certificate.GetValue(PdfCertificateDistinguishedName.Country, PdfCertificateField.Issuer)
'Get certificate validation date information
Dim validFrom As DateTime = certificate.ValidFrom
Dim validTo As DateTime = certificate.ValidTo

'Close the PDF document
loadedDocument.Close(True)

You can download a complete working sample from GitHub.

Enable Long Term Validation (LTV) PDF signature

The Essential® PDF supports creating long term signature validation when signing the PDF document. The LTV signature allows you to check the validity of a signature long after the document was signed. To achieve long term validation, all the required elements for signature validation must be embedded in the signed PDF.

NOTE

The resulted PDF document size will be large since all the necessary signature information, Certificate Revocation List (CRL), and Online Certificate Status Protocol (OCSP) are embedded.

The following code example explains how to create LTV PDF using EnableLtv property in PdfSignature class when signing the PDF document.

C# [Cross-platform]

//Creates a new PDF document.
    PdfDocument document = new PdfDocument();

    //Adds a new page.
    PdfPageBase page = document.Pages.Add();

    //Create graphics for the page. 
    PdfGraphics graphics = page.Graphics;

    //Creates a certificate instance from PFX file with private key.
    FileStream certificateStream = new FileStream(Path.GetFullPath(@"Data/PDF.pfx"), FileMode.Open, FileAccess.Read);
    PdfCertificate pdfCert = new PdfCertificate(certificateStream, "DigitalPass123");

    //Creates a digital signature.
    PdfSignature signature = new PdfSignature(document, page, pdfCert, "Signature");
    signature.Settings.CryptographicStandard = CryptographicStandard.CADES;
    signature.Settings.DigestAlgorithm = DigestAlgorithm.SHA256;

    //Save the document into stream
    MemoryStream stream = new MemoryStream();
    document.Save(stream);
    //Close the document
    document.Close(true);

    //Load an existing PDF stream.
    PdfLoadedDocument loadedDocument = new PdfLoadedDocument(stream);

    //Gets the first signature field of the PDF document
    PdfLoadedSignatureField signatureField = loadedDocument.Form.Fields[0] as PdfLoadedSignatureField;
    PdfSignature pdfSignature = signatureField.Signature;

    //Enable LTV on Signature.
    pdfSignature.EnableLtv = true;

    //Save the document into stream
    MemoryStream ms = new MemoryStream();
    loadedDocument.Save(ms);
    stream.ms = 0;
    //Close the document
    loadedDocument.Close(true);

C# [Windows-specific]

//Creates a new PDF document.
    PdfDocument document = new PdfDocument();

    //Adds a new page.
    PdfPageBase page = document.Pages.Add();

    //Create graphics for the page. 
    PdfGraphics graphics = page.Graphics;

    //Creates a certificate instance from PFX file with private key.
    PdfCertificate pdfCert = new PdfCertificate(Path.GetFullPath(@"Data/PDF.pfx"), "DigitalPass123");

    //Creates a digital signature.
    PdfSignature signature = new PdfSignature(document, page, pdfCert, "Signature");
    signature.Settings.CryptographicStandard = CryptographicStandard.CADES;
    signature.Settings.DigestAlgorithm = DigestAlgorithm.SHA256;

    //Save the document.

    document.Save("SignedDocument.pdf");
    //Close the document
    document.Close(true);

    //Load an existing PDF stream.
    PdfLoadedDocument loadedDocument = new PdfLoadedDocument("SignedDocument.pdf");

    //Gets the first signature field of the PDF document
    PdfLoadedSignatureField signatureField = loadedDocument.Form.Fields[0] as PdfLoadedSignatureField;
    PdfSignature pdfSignature = signatureField.Signature;

    //Enable LTV on Signature.
    pdfSignature.EnableLtv = true;

    //Save and close the document
    loadedDocument.Save("Output.pdf");
    loadedDocument.Close(true);

VB.NET [Windows-specific]

' Creates a new PDF document.
    Dim document As New PdfDocument()

    ' Adds a new page.
    Dim page As PdfPageBase = document.Pages.Add()

    ' Create graphics for the page.
    Dim graphics As PdfGraphics = page.Graphics

    ' Creates a certificate instance from PFX file with private key.
    Dim pdfCert As New PdfCertificate(Path.GetFullPath("Data/PDF.pfx"), "DigitalPass123")

    ' Creates a digital signature.
    Dim signature As New PdfSignature(document, page, pdfCert, "Signature")
    signature.Settings.CryptographicStandard = CryptographicStandard.CADES
    signature.Settings.DigestAlgorithm = DigestAlgorithm.SHA256

    ' Save the document.
    document.Save("SignedDocument.pdf")

    ' Close the document.
    document.Close(True)

    ' Load an existing PDF stream.
    Dim loadedDocument As New PdfLoadedDocument("SignedDocument.pdf")

    ' Gets the first signature field of the PDF document.
    Dim signatureField As PdfLoadedSignatureField = TryCast(loadedDocument.Form.Fields(0), PdfLoadedSignatureField)
    Dim pdfSignature As PdfSignature = signatureField.Signature

    ' Enable LTV on Signature.
    pdfSignature.EnableLtv = True

    ' Save and close the document.
    loadedDocument.Save("Output.pdf")
    loadedDocument.Close(True)

You can download a complete working sample from GitHub.

Adding a digital signature with customization

The PdfSignatureSettings allows you to add customized digital signatures to the PDF document.

Adding a digital signature with CAdES format

As per the PDF specification 2.0, now Syncfusion PDF library supports digital signature based on CAdES (CMS Advanced Electronics Signature). The CAdES based digital signature can remain valid for long periods, even if underlying cryptographic algorithms are broken. Using the API CryptographicStandard, you can change the standard between CMS (Cryptographic Message Syntax) and CAdES.

The following code example explains how to add a digital signature with cryptographic standard as CAdES through CryptographicStandard Enum when creating the PDF document.

C# [Cross-platform]

//Create a new PDF document
PdfDocument document = new PdfDocument();
//Add a new page
PdfPageBase page = document.Pages.Add();
//Create PDF graphics for the page
PdfGraphics graphics = page.Graphics;

//Creates a certificate instance from PFX file with private key
FileStream certificateStream = new FileStream("PDF.pfx", FileMode.Open, FileAccess.Read);
PdfCertificate pdfCert = new PdfCertificate(certificateStream, "password123");
//Creates a digital signature
PdfSignature signature = new PdfSignature(document, page, pdfCert, "Signature");
//Sets signature settings to customize cryptographic standard specified
PdfSignatureSettings settings = signature.Settings;
settings.CryptographicStandard  = CryptographicStandard.CADES;

//Sets an image for signature field
FileStream imageStream = new FileStream("signature.jpg", FileMode.Open, FileAccess.Read);
//Sets an image for signature field
PdfBitmap signatureImage = new PdfBitmap(imageStream);
//Sets signature information
signature.Bounds = new RectangleF(new PointF(0, 0), signatureImage.PhysicalDimension);
signature.ContactInfo = "[e-mail]";
signature.LocationInfo = "Honolulu, Hawaii";
signature.Reason = "I am author of this document.";
//Draws the signature image
signature.Appearance.Normal.Graphics.DrawImage(signatureImage, 0, 0);

//Save the document into stream
MemoryStream stream = new MemoryStream();
document.Save(stream);
stream.Position = 0;
//Close the document
document.Close(true);
//Defining the ContentType for pdf file
string contentType = "application/pdf";
//Define the file name
string fileName = "Output.pdf";
//Creates a FileContentResult object by using the file contents, content type, and file name
return File(stream, contentType, fileName);

C# [Windows-specific]

//Creates a new PDF document
PdfDocument document = new PdfDocument();
//Adds a new page
PdfPageBase page = document.Pages.Add();
//Create PDF graphics for the page
PdfGraphics graphics = page.Graphics;

//Creates a certificate instance from PFX file with private key
PdfCertificate pdfCert = new PdfCertificate(@"PDF.pfx", "password123");
//Creates a digital signature
PdfSignature signature = new PdfSignature(document, page, pdfCert, "Signature");
//Sets signature settings to customize cryptographic standard specified
PdfSignatureSettings settings = signature.Settings;
settings.CryptographicStandard  = CryptographicStandard.CADES;

//Sets an image for signature field
PdfBitmap signatureImage = new PdfBitmap(@"signature.jpg");
//Sets signature information
signature.Bounds = new RectangleF(new PointF(0, 0), signatureImage.PhysicalDimension);
signature.ContactInfo = "[e-mail]";
signature.LocationInfo = "Honolulu, Hawaii";
signature.Reason = "I am author of this document.";
//Draws the signature image
signature.Appearance.Normal.Graphics.DrawImage(signatureImage, 0, 0);

//Save and close the document
document.Save("Output.pdf");
document.Close(true);

VB.NET [Windows-specific]

'Creates a new PDF document
Dim document As New PdfDocument()
'Adds a new page
Dim page As PdfPageBase = document.Pages.Add()
'Create PDF graphics for the page
Dim graphics As PdfGraphics = page.Graphics

'Creates a certificate instance from PFX file with private key
Dim pdfCert As New PdfCertificate("PDF.pfx", "password123")
'Creates a digital signature
Dim signature As New PdfSignature(document, page, pdfCert, "Signature")
'Sets signature settings to customize cryptographic standard specified
Dim settings As PdfSignatureSettings = signature.Settings
settings.CryptographicStandard  = CryptographicStandard.CADES

'Sets an image for signature field
Dim signatureImage As New PdfBitmap("signature.jpg")
'Sets signature info
signature.Bounds = New RectangleF(New PointF(0, 0), signatureImage.PhysicalDimension)
signature.ContactInfo = "[e-mail]"
signature.LocationInfo = "Honolulu, Hawaii"
signature.Reason = "I am author of this document."
'Draws the signature image
signature.Appearance.Normal.Graphics.DrawImage(signatureImage, 0, 0)

'Save and close the document
document.Save("Output.pdf")
document.Close(True)

You can download a complete working sample from GitHub.

Customize digestion algorithm

In addition, you can now set the different message digest algorithm to sign PDF document using the DigestAlgorithm enum available in the class PdfSignatureSettings.

The following message digest algorithms are now supported:

• SHA1
• SHA256
• SHA384
• SHA512
• RIPEMD160

The following code example explains how to add a digital signature with various digest algorithms to the PDF document by specifying the DigestAlgorithm property as SHA256 through DigestAlgorithm enum in PdfSignatureSettings class.

C# [Cross-platform]

//Create a new PDF document
PdfDocument document = new PdfDocument();
//Add a new page
PdfPageBase page = document.Pages.Add();
//Create PDF graphics for the page
PdfGraphics graphics = page.Graphics;

//Creates a certificate instance from PFX file with private key
FileStream certificateStream = new FileStream("PDF.pfx", FileMode.Open, FileAccess.Read);
PdfCertificate pdfCert = new PdfCertificate(certificateStream, "password123");
//Creates a digital signature
PdfSignature signature = new PdfSignature(document, page, pdfCert, "Signature");
//Sets signature settings to customize digest algorithm specified
PdfSignatureSettings settings = signature.Settings;
settings.DigestAlgorithm = DigestAlgorithm.SHA256;

//Sets an image for signature field
FileStream imageStream = new FileStream("signature.jpg", FileMode.Open, FileAccess.Read);
//Sets an image for signature field
PdfBitmap signatureImage = new PdfBitmap(imageStream);
//Sets signature information
signature.Bounds = new RectangleF(new PointF(0, 0), signatureImage.PhysicalDimension);
signature.ContactInfo = "[e-mail]";
signature.LocationInfo = "Honolulu, Hawaii";
signature.Reason = "I am author of this document.";
//Draws the signature image
signature.Appearance.Normal.Graphics.DrawImage(signatureImage, 0, 0);

//Save the document into stream
MemoryStream stream = new MemoryStream();
document.Save(stream);
stream.Position = 0;
//Close the document
document.Close(true);
//Defining the ContentType for pdf file
string contentType = "application/pdf";
//Define the file name
string fileName = "Output.pdf";
//Creates a FileContentResult object by using the file contents, content type, and file name
return File(stream, contentType, fileName);

C# [Windows-specific]

//Create a new PDF document
PdfDocument document = new PdfDocument();
//Add a new page
PdfPageBase page = document.Pages.Add();
//Create PDF graphics for the page
PdfGraphics graphics = page.Graphics;

//Creates a certificate instance from PFX file with private key
PdfCertificate pdfCert = new PdfCertificate(@"PDF.pfx", "password123");
//Creates a digital signature
PdfSignature signature = new PdfSignature(document, page, pdfCert, "Signature");
//Sets signature settings to customize digest algorithm specified
PdfSignatureSettings settings = signature.Settings;
settings.DigestAlgorithm = DigestAlgorithm.SHA256;

//Sets an image for signature field
PdfBitmap signatureImage = new PdfBitmap(@"signature.jpg");
//Sets signature information
signature.Bounds = new RectangleF(new PointF(0, 0), signatureImage.PhysicalDimension);
signature.ContactInfo = "[e-mail]";
signature.LocationInfo = "Honolulu, Hawaii";
signature.Reason = "I am author of this document.";
//Draws the signature image
signature.Appearance.Normal.Graphics.DrawImage(signatureImage, 0, 0);

//Save and close the document
document.Save("Output.pdf");
document.Close(true);

VB.NET [Windows-specific]

'Creates a new PDF document
Dim document As New PdfDocument()
'Adds a new page
Dim page As PdfPageBase = document.Pages.Add()
'Create PDF graphics for the page
Dim graphics As PdfGraphics = page.Graphics

'Creates a certificate instance from PFX file with private key
Dim pdfCert As New PdfCertificate("PDF.pfx", "password123")
'Creates a digital signature
Dim signature As New PdfSignature(document, page, pdfCert, "Signature")
'Sets signature settings to customize digest algorithm specified
Dim settings As PdfSignatureSettings = signature.Settings
settings.DigestAlgorithm = DigestAlgorithm.SHA256

'Sets an image for signature field
Dim signatureImage As New PdfBitmap("signature.jpg")
'Sets signature info
signature.Bounds = New RectangleF(New PointF(0, 0), signatureImage.PhysicalDimension)
signature.ContactInfo = "[e-mail]"
signature.LocationInfo = "Honolulu, Hawaii"
signature.Reason = "I am author of this document."
'Draws the signature image
signature.Appearance.Normal.Graphics.DrawImage(signatureImage, 0, 0)

'Save and close the document
document.Save("Output.pdf")
document.Close(True)

You can download a complete working sample from GitHub.

Digital signature validation

Added the support to validate the digital signatures in an existing PDF document. Digital signature validation covers the following steps to ensure the validity of the signatures:

• Validate the document modification.
• Validate the certificate chain.
• Ensure the signature with timestamp time.
• Check the revocation status of the certificate with OCSP and CRL.
• Ensure the multiple digital signatures.

You can use the ValidateSignature method available in the PdfLoadedSignatureField class to validate the digital signature.

You can get the overall status from the IsSignatureValid property available in the PdfSignatureValidationResult class.

The following code example explains how to validate the digitally signed PDF document signature.

C# [Cross-platform]

//Get the stream from the document.
FileStream documentStream = new FileStream(Path.GetFullPath(@"Data/Input.pdf"), FileMode.Open, FileAccess.Read);

//Load an existing signed PDF document.
PdfLoadedDocument loadedDocument = new PdfLoadedDocument(documentStream);

//Get signature field.
PdfLoadedSignatureField signatureField = loadedDocument.Form.Fields[0] as PdfLoadedSignatureField;

//X509Certificate2Collection to check the signer's identity using root certificates.
X509CertificateCollection collection = new X509CertificateCollection();

//Creates a certificate instance from PFX file with private key.
FileStream certificateStream = new FileStream(Path.GetFullPath(@"Data/PDF.pfx"), FileMode.Open, FileAccess.Read);
byte[] data = new byte[certificateStream.Length];
certificateStream.Read(data, 0, data.Length);

//Create new X509Certificate2 with the root certificate.
X509Certificate2 certificate = new X509Certificate2(data, "syncfusion");

//Add the certificate to the collection.
collection.Add(certificate);

//Validate signature and get the validation result.
PdfSignatureValidationResult result = signatureField.ValidateSignature(collection);

//Checks whether the signature is valid or not.
SignatureStatus status = result.SignatureStatus;

//Checks whether the document is modified or not.
bool isModified = result.IsDocumentModified;

Console.WriteLine("Document modified: " + isModified);

//Signature details.
string issuerName = signatureField.Signature.Certificate.IssuerName;
DateTime validFrom = signatureField.Signature.Certificate.ValidFrom;
DateTime validTo = signatureField.Signature.Certificate.ValidTo;
string signatureAlgorithm = result.SignatureAlgorithm;
DigestAlgorithm digestAlgorithm = result.DigestAlgorithm;

Console.WriteLine("Issuer Name: " + issuerName);
Console.WriteLine("Valid From: " + validFrom);
Console.WriteLine("Valid To: " + validTo);
Console.WriteLine("Signature Algorithm: " + signatureAlgorithm);
Console.WriteLine("Digest Algorithm: " + digestAlgorithm);

//Revocation validation details.
RevocationResult revocationDetails = result.RevocationResult;
RevocationStatus revocationStatus = revocationDetails.OcspRevocationStatus;
bool isRevokedCRL = revocationDetails.IsRevokedCRL;

Console.WriteLine("Revocation Status: " + revocationStatus);
Console.WriteLine("Is Revoked CRL: " + isRevokedCRL);

//Close the document.
loadedDocument.Close(true);

C# [Windows-specific]

//Get the stream from the document.
FileStream documentStream = new FileStream(Path.GetFullPath(@"Data/Input.pdf"), FileMode.Open, FileAccess.Read);

//Load an existing signed PDF document.
PdfLoadedDocument loadedDocument = new PdfLoadedDocument(documentStream);

//Get signature field.
PdfLoadedSignatureField signatureField = loadedDocument.Form.Fields[0] as PdfLoadedSignatureField;

//X509Certificate2Collection to check the signer's identity using root certificates.
X509CertificateCollection collection = new X509CertificateCollection();

//Creates a certificate instance from PFX file with private key.
FileStream certificateStream = new FileStream(Path.GetFullPath(@"Data/PDF.pfx"), FileMode.Open, FileAccess.Read);
byte[] data = new byte[certificateStream.Length];
certificateStream.Read(data, 0, data.Length);

//Create new X509Certificate2 with the root certificate.
X509Certificate2 certificate = new X509Certificate2(data, "syncfusion");

//Add the certificate to the collection.
collection.Add(certificate);

//Validate signature and get the validation result.
PdfSignatureValidationResult result = signatureField.ValidateSignature(collection);

//Checks whether the signature is valid or not.
SignatureStatus status = result.SignatureStatus;

//Checks whether the document is modified or not.
bool isModified = result.IsDocumentModified;

Console.WriteLine("Document modified: " + isModified);

//Signature details.
string issuerName = signatureField.Signature.Certificate.IssuerName;
DateTime validFrom = signatureField.Signature.Certificate.ValidFrom;
DateTime validTo = signatureField.Signature.Certificate.ValidTo;
string signatureAlgorithm = result.SignatureAlgorithm;
DigestAlgorithm digestAlgorithm = result.DigestAlgorithm;

Console.WriteLine("Issuer Name: " + issuerName);
Console.WriteLine("Valid From: " + validFrom);
Console.WriteLine("Valid To: " + validTo);
Console.WriteLine("Signature Algorithm: " + signatureAlgorithm);
Console.WriteLine("Digest Algorithm: " + digestAlgorithm);

//Revocation validation details.
RevocationResult revocationDetails = result.RevocationResult;
RevocationStatus revocationStatus = revocationDetails.OcspRevocationStatus;
bool isRevokedCRL = revocationDetails.IsRevokedCRL;

Console.WriteLine("Revocation Status: " + revocationStatus);
Console.WriteLine("Is Revoked CRL: " + isRevokedCRL);

//Close the document.
loadedDocument.Close(true);

VB.NET [Windows-specific]

' Get the stream from the document.
Dim documentStream As New FileStream(Path.GetFullPath("Data/Input.pdf"), FileMode.Open, FileAccess.Read)

' Load an existing signed PDF document.
Dim loadedDocument As New PdfLoadedDocument(documentStream)

' Get signature field.
Dim signatureField As PdfLoadedSignatureField = TryCast(loadedDocument.Form.Fields(0), PdfLoadedSignatureField)

' X509Certificate2Collection to check the signer's identity using root certificates.
Dim collection As New X509CertificateCollection()

' Creates a certificate instance from PFX file with private key.
Dim certificateStream As New FileStream(Path.GetFullPath("Data/PDF.pfx"), FileMode.Open, FileAccess.Read)
Dim data(CInt(certificateStream.Length) - 1) As Byte
certificateStream.Read(data, 0, data.Length)

' Create new X509Certificate2 with the root certificate.
Dim certificate As New X509Certificate2(data, "syncfusion")

' Add the certificate to the collection.
collection.Add(certificate)

' Validate signature and get the validation result.
Dim result As PdfSignatureValidationResult = signatureField.ValidateSignature(collection)

' Checks whether the signature is valid or not.
Dim status As SignatureStatus = result.SignatureStatus

' Checks whether the document is modified or not.
Dim isModified As Boolean = result.IsDocumentModified

Console.WriteLine("Document modified: " & isModified)

' Signature details.
Dim issuerName As String = signatureField.Signature.Certificate.IssuerName
Dim validFrom As DateTime = signatureField.Signature.Certificate.ValidFrom
Dim validTo As DateTime = signatureField.Signature.Certificate.ValidTo
Dim signatureAlgorithm As String = result.SignatureAlgorithm
Dim digestAlgorithm As DigestAlgorithm = result.DigestAlgorithm

Console.WriteLine("Issuer Name: " & issuerName)
Console.WriteLine("Valid From: " & validFrom)
Console.WriteLine("Valid To: " & validTo)
Console.WriteLine("Signature Algorithm: " & signatureAlgorithm)
Console.WriteLine("Digest Algorithm: " & digestAlgorithm)

' Revocation validation details.
Dim revocationDetails As RevocationResult = result.RevocationResult
Dim revocationStatus As RevocationStatus = revocationDetails.OcspRevocationStatus
Dim isRevokedCRL As Boolean = revocationDetails.IsRevokedCRL

Console.WriteLine("Revocation Status: " & revocationStatus)
Console.WriteLine("Is Revoked CRL: " & isRevokedCRL)

' Close the document.
loadedDocument.Close(True)

You can download a complete working sample from GitHub.

Validate all signatures in PDF document

Added the support to validate all the digital signatures in an existing PDF document.

You can use the ValidateSignatures method available in the PdfLoadedFormFieldCollection class to validate all the digital signatures. You can get the list of PdfSignatureValidationResult from the ValidateSignatures method.

The following code example explains how to validate all the signatures in digitally signed PDF document.

C# [Cross-platform]

//Get the stream from the document
FileStream documentStream = new FileStream("Input.pdf", FileMode.Open, FileAccess.Read);
//Load an existing signed PDF document
PdfLoadedDocument loadedDocument = new PdfLoadedDocument(documentStream);

//X509Certificate2Collection to check the signer's identity using root certificates
X509Certificate2Collection collection = new X509Certificate2Collection();
//Creates a certificate instance from PFX file with private key
FileStream certificateStream = new FileStream("PDF.pfx", FileMode.Open, FileAccess.Read);
byte[] data = new byte[certificateStream.Length];
certificateStream.Read(data, 0, data.Length);
//Create new X509Certificate2 with the root certificate
X509Certificate2 certificate = new X509Certificate2(data, "password123");
//Add the certificate to the collection
collection.Add(certificate);
//Validate all signatures in loaded PDF document and get the list of validation result
List<PdfSignatureValidationResult> results;
bool isValid = loadedDocument.Form.Fields.ValidateSignatures(collection, out results);
Console.WriteLine("All signatures in the document are valid: " + isValid);
//Close the document
loadedDocument.Close(true);

C# [Windows-specific]

//Load an existing signed PDF document
PdfLoadedDocument loadedDocument = new PdfLoadedDocument("Input.pdf");

//X509Certificate2Collection to check the signer's identity using root certificates
X509Certificate2Collection collection = new X509Certificate2Collection();
//Create new X509Certificate2 with the root certificate
X509Certificate2 certificate = new X509Certificate2("PDF.pfx", "password123");
//Add the certificate to the collection
collection.Add(certificate);
//Validate all signatures in loaded PDF document and get the list of validation result
List<PdfSignatureValidationResult> results;
bool isValid = loadedDocument.Form.Fields.ValidateSignatures(collection, out results);
Console.WriteLine("All signatures in the document are valid: " + isValid);
//Close the document
loadedDocument.Close(true);

VB.NET [Windows-specific]

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

'X509Certificate2Collection to check the signer's identity using root certificates
Dim collection As X509Certificate2Collection = New X509Certificate2Collection()
'Create new X509Certificate2 with the root certificate
Dim certificate As X509Certificate2 = New X509Certificate2("PDF.pfx", "password123")
'Add the certificate to the collection
collection.Add(certificate)
'Validate all signatures in loaded PDF document and get the list of validation result
Dim results As List(Of PdfSignatureValidationResult)
Dim isValid As Boolean = loadedDocument.Form.Fields.ValidateSignatures(collection, results)
Console.WriteLine("All signatures in the document are valid: " + isValid);
'Close the document
loadedDocument.Close(true)

You can download a complete working sample from GitHub.

Deferred signing in PDF document

The following code sample shows how to be deferred signing in a PDF document from an external signature using IPdfExternalSigner class and AddExternalSigner method in PdfSignature.

Steps for deferred signing:

1. Create a PDF document with an empty signature.
2. Users will sign the document hash using the external services.
3. Replace the empty signature with a signed hash from the external services using ReplaceEmptySignature method.

C# [Cross-platform]

//Get the stream from the document.
FileStream documentStream = new FileStream("PDF_Succinctly.pdf ", FileMode.Open, FileAccess.Read);
//Load an existing PDF document.
PdfLoadedDocument loadedDocument = new PdfLoadedDocument(documentStream);

//Creates a digital signature.
PdfSignature signature = new PdfSignature(loadedDocument, loadedDocument.Pages[0], null, "Signature");
//Sets the signature information.
signature.Bounds = new RectangleF(new PointF(0, 0), new SizeF(100, 30));
signature.Settings.CryptographicStandard = CryptographicStandard.CADES;
signature.Settings.DigestAlgorithm = DigestAlgorithm.SHA1;

//Create an external signer.
IPdfExternalSigner externalSignature = new SignEmpty("SHA1");
//Add public certificates.
System.Collections.Generic.List<X509Certificate2> certificates = new System.Collections.Generic.List<X509Certificate2>();
certificates.Add(new X509Certificate2(Convert.FromBase64String(PublicCert)));
signature.AddExternalSigner(externalSignature, certificates, null);

//Save the document.
MemoryStream stream = new MemoryStream();
loadedDocument.Save(stream);
//Close the PDF document.
loadedDocument.Close(true);
//Defining the ContentType for a PDF file.
string contentType = "application/pdf";
//Define the file name.
string fileName = "Output.pdf";
//Creates a FileContentResult object by using the file contents, content type, and file name.
return File(stream, contentType, fileName);

void DeferredSign()
{
    //Create an external signer with a signed hash message.
    IPdfExternalSigner externalSigner = new ExternalSigner("SHA1", signedHash);
    //Add public certificates.
    System.Collections.Generic.List<X509Certificate2> publicCertificates = new System.Collections.Generic.List<X509Certificate2>();
    publicCertificates.Add(new X509Certificate2(Convert.FromBase64String(PublicCert)));

    //Create an output file stream.
    MemoryStream outputFileStream = new MemoryStream();
    //Get the stream from the document.
    FileStream inputFileStream = new FileStream("EmptySignature.pdf", FileMode.Open, FileAccess.Read);
    string pdfPassword = string.Empty;
    //Replace an empty signature.
    PdfSignature.ReplaceEmptySignature(inputFileStream, pdfPassword, outputFileStream, signatureName, externalSigner, publicCertificates);
}

/// <summary>
/// Represents to sign an empty signature from the external signer.
/// </summary>
class SignEmpty : IPdfExternalSigner
{
    private string _hashAlgorithm;

    public string HashAlgorithm
    {
        get { return _hashAlgorithm; }
    }

    public SignEmpty(string hashAlgorithm)
    {
        _hashAlgorithm = hashAlgorithm;
    }

    public byte[] Sign(byte[] message, out byte[] timeStampResponse)
    {
        //Send document hash for signing using the external services.
        SignDocumentHash(message);
        //Set a null value to create an empty signed document.
        byte[] signedBytes = null;
        timeStampResponse = null;
        return signedBytes;
    }

    private void SignDocumentHash(byte[] documentHash)
    {
      X509Certificate2 digitalID = new X509Certificate2(new X509Certificate2(Path.GetFullPath("../../../PDF.pfx"), "password123"));
      if (digitalID.PrivateKey is System.Security.Cryptography.RSACryptoServiceProvider)
      {
        System.Security.Cryptography.RSACryptoServiceProvider rsa = (System.Security.Cryptography.RSACryptoServiceProvider)digitalID.PrivateKey;
        Program.SignedHash = rsa.SignData(documentHash, HashAlgorithm);
      }
      else if (digitalID.PrivateKey is RSACng)
      {
        RSACng rsa = (RSACng)digitalID.PrivateKey;
        Program.SignedHash = rsa.SignData(documentHash, System.Security.Cryptography.HashAlgorithmName.SHA1, RSASignaturePadding.Pkcs1);
      }
    }
}

/// <summary>
/// Represents to replace an empty signature from an external signer.
/// </summary>
class ExternalSigner : IPdfExternalSigner
{
    private string _hashAlgorithm;
    private byte[] _signedHash;
    public string HashAlgorithm
    {
        get { return _hashAlgorithm; }
    }

    public ExternalSigner(string hashAlgorithm, byte[] hash)
    {
        _hashAlgorithm = hashAlgorithm;
        _signedHash = hash;
    }

    public byte[] Sign(byte[] message, out byte[] timeStampResponse)
    {
        //Set the signed hash message to replace an empty signature.
        byte[] signedBytes = _signedHash;
        timeStampResponse = null;
        return signedBytes;
    }
}

C# [Windows-specific]

//Load an existing PDF document.
PdfLoadedDocument loadedDocument = new PdfLoadedDocument("PDF_Succinctly.pdf");

//Creates a digital signature.
PdfSignature signature = new PdfSignature(loadedDocument, loadedDocument.Pages[0], null, "Signature");
//Sets the signature information.
signature.Bounds = new RectangleF(new PointF(0, 0), new SizeF(100, 30));
signature.Settings.CryptographicStandard = CryptographicStandard.CADES;
signature.Settings.DigestAlgorithm = DigestAlgorithm.SHA1;

//Create an external signer.
IPdfExternalSigner externalSignature = new SignEmpty("SHA1");
//Add public certificates.
System.Collections.Generic.List<X509Certificate2> certificates = new System.Collections.Generic.List<X509Certificate2>();
certificates.Add(new X509Certificate2(Convert.FromBase64String(PublicCert)));
signature.AddExternalSigner(externalSignature, certificates, null);

//Saves the document.
loadedDocument.Save("EmptySignature.pdf");
//Closes the document.
loadedDocument.Close(true);

void DeferredSign()
{
    //Create an external signer with a signed hash message.
    IPdfExternalSigner externalSigner = new ExternalSigner("SHA1", signedHash);
    //Add public certificates.
    System.Collections.Generic.List<X509Certificate2> publicCertificates = new System.Collections.Generic.List<X509Certificate2>();
    publicCertificates.Add(new X509Certificate2(Convert.FromBase64String(PublicCert)));

    //Create an output file stream.
    MemoryStream outputFileStream = new MemoryStream();
    //Get the stream from the document.
    FileStream inputFileStream = new FileStream("EmptySignature.pdf", FileMode.Open, FileAccess.Read);
    string pdfPassword = string.Empty;
    //Replace an empty signature.
    PdfSignature.ReplaceEmptySignature(inputFileStream, pdfPassword, outputFileStream, signatureName, externalSigner, publicCertificates);
}

/// <summary>
/// Represents to sign an empty signature from an external signer.
/// </summary>
class SignEmpty : IPdfExternalSigner
{
    private string _hashAlgorithm;
    public string HashAlgorithm
    {
        get { return _hashAlgorithm; }
    }

    public SignEmpty(string hashAlgorithm)
    {
        _hashAlgorithm = hashAlgorithm;
    }

    public byte[] Sign(byte[] message, out byte[] timeStampResponse)
    {
        //Send the document hash for signing using the external services.
        SignDocumentHash(message);
        //Set a null value to create an empty signed document.
        byte[] signedBytes = null;
        timeStampResponse = null;
        return signedBytes;
    }

    private void SignDocumentHash(byte[] documentHash)
    {
      X509Certificate2 digitalID = new X509Certificate2(new X509Certificate2(Path.GetFullPath("../../../PDF.pfx"), "password123"));
      if (digitalID.PrivateKey is System.Security.Cryptography.RSACryptoServiceProvider)
      {
        System.Security.Cryptography.RSACryptoServiceProvider rsa = (System.Security.Cryptography.RSACryptoServiceProvider)digitalID.PrivateKey;
        Program.SignedHash = rsa.SignData(documentHash, HashAlgorithm);
      }
      else if (digitalID.PrivateKey is RSACng)
      {
        RSACng rsa = (RSACng)digitalID.PrivateKey;
        Program.SignedHash = rsa.SignData(documentHash, System.Security.Cryptography.HashAlgorithmName.SHA1, RSASignaturePadding.Pkcs1);
      }
    }
}


/// <summary>
/// Represents to replace an empty signature from an external signer.
/// </summary>
class ExternalSigner : IPdfExternalSigner
{
    private string _hashAlgorithm;
    private byte[] _signedHash;
    public string HashAlgorithm
    {
        get { return _hashAlgorithm; }
    }

    public ExternalSigner(string hashAlgorithm, byte[] hash)
    {
        _hashAlgorithm = hashAlgorithm;
        _signedHash = hash;
    }

    public byte[] Sign(byte[] message, out byte[] timeStampResponse)
    {
        //Set the signed hash message to replace an empty signature.
        byte[] signedBytes = _signedHash;
        timeStampResponse = null;
        return signedBytes;
    }
}

VB.NET [Windows-specific]

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

'Creates a digital signature.
Dim signature As PdfSignature = New PdfSignature(loadedDocument, loadedDocument.Pages(0), Nothing, "Signature")
'Sets the signature information.
signature.Bounds = New RectangleF(New PointF(0, 0), New SizeF(100, 30))
signature.Settings.CryptographicStandard = CryptographicStandard.CADES
signature.Settings.DigestAlgorithm = DigestAlgorithm.SHA1

'Create an external signer.
Dim externalSignature As IPdfExternalSigner = New SignEmpty("SHA1")
'Add public certificates.
Dim certificates As System.Collections.Generic.List(Of X509Certificate2) = New System.Collections.Generic.List(Of X509Certificate2)
certificates.Add(New X509Certificate2(Convert.FromBase64String(PublicCert)))
signature.AddExternalSigner(externalSignature, certificates, Nothing)

'Save the document.
loadedDocument.Save("EmptySignature.pdf")
'Close the document.
loadedDocument.Close(True)

Private Sub DeferredSign()
    Dim externalSigner As IPdfExternalSigner = New ExternalSigner("SHA1", signedHash)
    Dim publicCertificates As System.Collections.Generic.List(Of X509Certificate2) = New System.Collections.Generic.List(Of X509Certificate2)()
    publicCertificates.Add(New X509Certificate2(Convert.FromBase64String(PublicCert)))
    Dim outputFileStream As FileStream = New FileStream("DeferredSign.pdf", FileMode.Create, FileAccess.ReadWrite)
    Dim inputFileStream As FileStream = New FileStream("EmptySignature.pdf", FileMode.Open, FileAccess.Read)
    Dim pdfPassword As String = String.Empty
    PdfSignature.ReplaceEmptySignature(inputFileStream, pdfPassword, outputFileStream, "Signature", externalSigner, publicCertificates)
End Sub

''' <summary>
''' Represents to sign an empty signature. 
''' </summary>
Class SignEmpty
    Implements IPdfExternalSigner

    Private _hashAlgorithm As String

    Public ReadOnly Property HashAlgorithm As String Implements IPdfExternalSigner.HashAlgorithm
        Get
            Return Me._hashAlgorithm
        End Get
    End Property

    Public Sub New(ByVal hashAlgorithm As String)
        MyBase.New
        Me._hashAlgorithm = hashAlgorithm
    End Sub

    Private Function IPdfExternalSigner_Sign(message() As Byte, ByRef timeStampResponse() As Byte) As Byte() Implements IPdfExternalSigner.Sign
        'Send document hash for signing using the external services.
        Me.SignDocumentHash(message)
        'Set a null value to create an empty signed document.
        Dim signedBytes() As Byte = Nothing
        timeStampResponse = Nothing
        Return signedBytes
    End Function

    Private Sub SignDocumentHash(ByVal documentHash As Byte())
    Dim digitalID As X509Certificate2 = New X509Certificate2(New X509Certificate2("PDF.pfx", "password123"))

    If TypeOf digitalID.PrivateKey Is System.Security.Cryptography.RSACryptoServiceProvider Then
        Dim rsa As System.Security.Cryptography.RSACryptoServiceProvider = CType(digitalID.PrivateKey, System.Security.Cryptography.RSACryptoServiceProvider)
        SignedHash = rsa.SignData(documentHash, HashAlgorithm)
    ElseIf TypeOf digitalID.PrivateKey Is RSACng Then
        Dim rsa As RSACng = CType(digitalID.PrivateKey, RSACng)
        SignedHash = rsa.SignData(documentHash, System.Security.Cryptography.HashAlgorithmName.SHA1, RSASignaturePadding.Pkcs1)
    End If
    End Sub
End Class

''' <summary>
''' Represents to replace the empty signature.
''' </summary>
Class ExternalSigner
    Implements IPdfExternalSigner
    Private _hashAlgorithm As String
    Private _signedHash() As Byte

    Public ReadOnly Property HashAlgorithm As String Implements IPdfExternalSigner.HashAlgorithm
        Get
            Return Me._hashAlgorithm
        End Get
    End Property

    Public Sub New(ByVal hashAlgorithm As String, ByVal hash() As Byte)
        MyBase.New
        Me._hashAlgorithm = hashAlgorithm
        Me._signedHash = hash
    End Sub

    Private Function IPdfExternalSigner_Sign(message() As Byte, ByRef timeStampResponse() As Byte) As Byte() Implements IPdfExternalSigner.Sign
        'Set the signed hash message to replace an empty signature.
        Dim signedBytes() As Byte = Me._signedHash
        timeStampResponse = Nothing
        Return signedBytes
    End Function
End Class

You can download a complete working sample from GitHub.

Adding the estimated size of the signature

The following code sample shows how to add the estimated size of the signature in the PDF document using EstimatedSignatureSize property.

C# [Cross-platform]

//Creating a new PDF Document. 
PdfDocument document = new PdfDocument();
//Adding a new page to the PDF document.
PdfPageBase page = document.Pages.Add();

//Creates a certificate instance from the PFX file with a private key.
FileStream certificateStream = new FileStream("PDF.pfx", FileMode.Open, FileAccess.Read);
PdfCertificate pdfCert = new PdfCertificate(certificateStream, "password123"); 
//Add a new signature to the PDF page.
PdfSignature signature = new PdfSignature(document, page, pdfCert, "Signature");
signature.Bounds = new Rectangle(10, 20, 400, 200);
//Sets the estimated size of the signature.
signature.EstimatedSignatureSize = 20000;

//Creating the stream object.
MemoryStream stream = new MemoryStream();
//Save the document into stream.
document.Save(stream);
//If the position is not set to '0,' then the PDF will be empty.
stream.Position = 0;
//Close the document.
document.Close(true);
//Defining the ContentType for a PDF file.
string contentType = "application/pdf";
//Define the file name.
string fileName = "Output.pdf";
//Creates a FileContentResult object by using the file contents, content type, and file name.
return File(stream, contentType, fileName);

C# [Windows-specific]

//Creating a new PDF Document. 
PdfDocument document = new PdfDocument();
//Adding a new page to the PDF document. 
PdfPageBase page = document.Pages.Add();

//Create a PDF certificate.
PdfCertificate pdfCert = new PdfCertificate(@"PDF.pfx", "password123");
//Add a new signature to the PDF page.
PdfSignature signature = new PdfSignature(document, page, pdfCert, "Signature");
signature.Bounds = new Rectangle(10, 20, 400, 200);
//Sets the estimated size of the signature.
signature.EstimatedSignatureSize = 20000;

//Save the PDF document.
document.Save("Output.pdf");
//Close the PDF document.
document.Close(true);

VB.NET [Windows-specific]

'Creating a new PDF Document. 
Dim document As PdfDocument =  New PdfDocument()  
'Adding a new page to the PDF document. 
Dim page As PdfPageBase =  document.Pages.Add() 
 
'Create a PDF certificate.
Dim pdfCert As PdfCertificate =  New PdfCertificate("PDF.pfx","password123") 
'Add a new signature to the PDF page.
Dim signature As PdfSignature =  New PdfSignature(document,page,pdfCert,"Signature") 
signature.Bounds = New Rectangle(10, 20, 400, 200)
'Sets the estimated size of the signature.
signature.EstimatedSignatureSize = 20000
 
'Save the PDF document.
document.Save("Output.pdf")
'Close the PDF document.
document.Close(True)

You can download a complete working sample from GitHub.

Deferred signing without PKCS7 encoding

The following code sample shows deferred signing in a PDF document without PKCS7 encoding from an external signature using IPdfExternalSigner class and AddExternalSigner method.

Steps for deferred signing:

1. Create a PDF document with an empty signature.
2. Users will sign the document hash using the external services.
3. Replace the empty signature with a PKCS7 encoded signed hash from the external services using ReplaceEmptySignature method.

C# [Cross-platform]

//Get the stream from a document.
FileStream documentStream = new FileStream("PDF_Succinctly.pdf ", FileMode.Open, FileAccess.Read);
//Load an existing PDF document.
PdfLoadedDocument loadedDocument = new PdfLoadedDocument(documentStream);

//Creates a digital signature.
PdfSignature signature = new PdfSignature(loadedDocument, loadedDocument.Pages[0], null, "Signature");
//Sets the signature information.
signature.Bounds = new RectangleF(new PointF(0, 0), new SizeF(100, 30));
signature.Settings.CryptographicStandard = CryptographicStandard.CADES;
signature.Settings.DigestAlgorithm = DigestAlgorithm.SHA1;

//Create an external signer.
IPdfExternalSigner externalSignature = new SignEmpty("SHA1");
//Add public certificates.
System.Collections.Generic.List<X509Certificate2> certificates = new System.Collections.Generic.List<X509Certificate2>();
signature.AddExternalSigner(externalSignature, certificates, null);

//Save a document.
MemoryStream stream = new MemoryStream();
loadedDocument.Save(stream);
//Close a PDF document.
loadedDocument.Close(true);
//Defining the ContentType for a PDF file.
string contentType = "application/pdf";
//Define the file name.
string fileName = "Output.pdf";
//Creates a FileContentResult object by using the file contents, content type, and file name.
return File(stream, contentType, fileName);

//Create an external signer with a signed hash message.
IPdfExternalSigner externalSigner = new ExternalSigner("SHA1");
//Add public certificates.
System.Collections.Generic.List<X509Certificate2> publicCertificates = new System.Collections.Generic.List<X509Certificate2>();
publicCertificates.Add(new X509Certificate2(Convert.FromBase64String(PublicCert)));

//Create an output file stream.
MemoryStream outputFileStream = new MemoryStream();    
// Get the stream from a document.
FileStream inputFileStream = new FileStream("EmptySignature.pdf", FileMode.Open, FileAccess.Read);
string pdfPassword = string.Empty;
//Deferred signing without PKCS7 encoding.
PdfSignature.ReplaceEmptySignature(inputFileStream, pdfPassword, outputFileStream, signatureName, externalSigner, publicCertificates, false);

/// <summary>
/// Represents to create an empty signature.
/// </summary>
class SignEmpty : IPdfExternalSigner
{
    private string _hashAlgorithm;
    public string HashAlgorithm
    {
        get { return _hashAlgorithm; }
    }

    public SignEmpty(string hashAlgorithm)
    {
        _hashAlgorithm = hashAlgorithm;
    }

    public byte[] Sign(byte[] message, out byte[] timeStampResponse)
    {
        //Set a null value to create an empty signed document.
        byte[] signedBytes = null;
        timeStampResponse = null;
        return signedBytes;
    }
}

/// <summary>
/// Represents to replace an empty signature from an external signer.
/// </summary>
class ExternalSigner : IPdfExternalSigner
{
    private string _hashAlgorithm;
    public string HashAlgorithm
    {
        get { return _hashAlgorithm; }
    }

    public ExternalSigner(string hashAlgorithm)
    {
        _hashAlgorithm = hashAlgorithm;
    }

    public byte[] Sign(byte[] message, out byte[] timeStampResponse)
    {
        //Set the signed encoded PKCS7 hash message to replace an empty signature.
        byte[] signedBytes = EncodedPKCS7Hash;
        timeStampResponse = null;
        return signedBytes;
    }
}

C# [Windows-specific]

//Load an existing PDF document.
PdfLoadedDocument loadedDocument = new PdfLoadedDocument("PDF_Succinctly.pdf");

//Creates a digital signature.
PdfSignature signature = new PdfSignature(loadedDocument, loadedDocument.Pages[0], null, "Signature");
//Sets the signature information.
signature.Bounds = new RectangleF(new PointF(0, 0), new SizeF(100, 30));
signature.Settings.CryptographicStandard = CryptographicStandard.CADES;
signature.Settings.DigestAlgorithm = DigestAlgorithm.SHA1;

//Create an external signer.
IPdfExternalSigner externalSignature = new SignEmpty("SHA1");
//Add public certificates.
System.Collections.Generic.List<X509Certificate2> certificates = new System.Collections.Generic.List<X509Certificate2>();
signature.AddExternalSigner(externalSignature, certificates, null);

//Save a document.
loadedDocument.Save("EmptySignature.pdf");
//Close a document.
loadedDocument.Close(true);

//Create an external signer with a signed hash message.
IPdfExternalSigner externalSigner = new ExternalSigner("SHA1");
//Add public certificates.
System.Collections.Generic.List<X509Certificate2> publicCertificates = new System.Collections.Generic.List<X509Certificate2>();
publicCertificates.Add(new X509Certificate2(Convert.FromBase64String(PublicCert)));

//Create an output file stream.
MemoryStream outputFileStream = new MemoryStream();
//Get the stream from a document.
FileStream inputFileStream = new FileStream("EmptySignature.pdf", FileMode.Open, FileAccess.Read);
string pdfPassword = string.Empty;
//Deferred signing without PKCS7 encoding.
PdfSignature.ReplaceEmptySignature(inputFileStream, pdfPassword, outputFileStream, signatureName, externalSigner, publicCertificates, false);

/// <summary>
/// Represents to create an empty signature.
/// </summary>
class SignEmpty : IPdfExternalSigner
{
    private string _hashAlgorithm;
    public string HashAlgorithm
    {
        get { return _hashAlgorithm; }
    }

    public SignEmpty(string hashAlgorithm)
    {
        _hashAlgorithm = hashAlgorithm;
    }

    public byte[] Sign(byte[] message, out byte[] timeStampResponse)
    {
        //Set a null value to create an empty signed document.
        byte[] signedBytes = null;
        timeStampResponse = null;
        return signedBytes;
    }
}

/// <summary>
/// Represents to replace an empty signature from an external signer.
/// </summary>
class ExternalSigner : IPdfExternalSigner
{
    private string _hashAlgorithm;
    public string HashAlgorithm
    {
        get { return _hashAlgorithm; }
    }

    public ExternalSigner(string hashAlgorithm)
    {
        _hashAlgorithm = hashAlgorithm;
    }

    public byte[] Sign(byte[] message, out byte[] timeStampResponse)
    {
        //Set the signed encoded PKCS7 hash message to replace an empty signature.
        byte[] signedBytes = EncodedPKCS7Hash;
        timeStampResponse = null;
        return signedBytes;
    }
}

VB.NET [Windows-specific]

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

'Creates a digital signature.
Dim signature As PdfSignature = New PdfSignature(loadedDocument, loadedDocument.Pages(0), Nothing, "Signature")
'Sets the signature information.
signature.Bounds = New RectangleF(New PointF(0, 0), New SizeF(100, 30))
signature.Settings.CryptographicStandard = CryptographicStandard.CADES
signature.Settings.DigestAlgorithm = DigestAlgorithm.SHA1

'Create an external signer.
Dim externalSignature As IPdfExternalSigner = New SignEmpty("SHA1")
'Add public certificates.
Dim certificates As System.Collections.Generic.List(Of X509Certificate2) = New System.Collections.Generic.List(Of X509Certificate2)
signature.AddExternalSigner(externalSignature, certificates, Nothing)

'Save the document.
loadedDocument.Save("EmptySignature.pdf")
'Close the document.
loadedDocument.Close(True)

'Create an external signer with a signed hash message.
Dim externalSigner As IPdfExternalSigner = New ExternalSigner("SHA1")
'Add public certificates.
Dim publicCertificates As System.Collections.Generic.List(Of X509Certificate2) = New System.Collections.Generic.List(Of X509Certificate2)
publicCertificates.Add(New X509Certificate2(Convert.FromBase64String(PublicCert)))

'Create an output file stream.
Dim outputFileStream As MemoryStream = New MemoryStream
'Get the stream from the document.
Dim documentStream As FileStream = New FileStream("EmptySignature.pdf ", FileMode.Open, FileAccess.Read)
Dim pdfPassword As String = String.Empty
'Deferred signing without PKCS7 encoding 
PdfSignature.ReplaceEmptySignature(documentStream, pdfPassword, outputFileStream, "Signature", externalSigner, publicCertificates, False)

''' <summary>
''' Represents to create an empty signature.
''' </summary>
Class SignEmpty
    Implements IPdfExternalSigner
    Private _hashAlgorithm As String

    Public ReadOnly Property HashAlgorithm As String Implements IPdfExternalSigner.HashAlgorithm
        Get
            Return Me._hashAlgorithm
        End Get
    End Property

    Public Sub New(ByVal hashAlgorithm As String)
        MyBase.New
        Me._hashAlgorithm = hashAlgorithm
    End Sub

    Private Function IPdfExternalSigner_Sign(message() As Byte, ByRef timeStampResponse() As Byte) As Byte() Implements IPdfExternalSigner.Sign
        'Set a null value to create an empty signed document.
        Dim signedBytes() As Byte = Nothing
        timeStampResponse = Nothing
        Return signedBytes
    End Function
End Class

''' <summary>
''' Represents to replace the empty signature.
''' </summary>
Class ExternalSigner
    Implements IPdfExternalSigner
    Private _hashAlgorithm As String
    Public ReadOnly Property HashAlgorithm As String Implements IPdfExternalSigner.HashAlgorithm
        Get
            Return Me._hashAlgorithm
        End Get
    End Property

    Public Sub New(ByVal hashAlgorithm As String)
        MyBase.New
        Me._hashAlgorithm = hashAlgorithm
    End Sub

    Private Function IPdfExternalSigner_Sign(message() As Byte, ByRef timeStampResponse() As Byte) As Byte() Implements IPdfExternalSigner.Sign
        'Set the signed encoded PKCS7 hash message to replace an empty signature.
        Dim signedBytes() As Byte = EncodedPKCS7Hash
        timeStampResponse = Nothing
        Return signedBytes
    End Function
End Class

Drawing text/image in the Signature Appearance

The following code example illustrates how to draw text/images in a digital appearance using Appearance property and DrawImage method as follows.

C# [Cross-platform]

//Create a new PDF document.
PdfDocument document = new PdfDocument();
//Add a new page.
PdfPageBase page = document.Pages.Add();
//Create PDF graphics for the page.
PdfGraphics graphics = page.Graphics;

//Create a certificate instance from a PFX file with a private key.
FileStream certificateStream = new FileStream("PDF.pfx", FileMode.Open, FileAccess.Read);
PdfCertificate pdfCert = new PdfCertificate(certificateStream, "password123");
//Create a digital signature.
PdfSignature signature = new PdfSignature(document, page, pdfCert, "Signature");
//Set an image for signature field.
FileStream imageStream = new FileStream("signature.jpg", FileMode.Open, FileAccess.Read);
//Set an image for signature field.
PdfBitmap signatureImage = new PdfBitmap(imageStream);
//Set the signature information.
signature.Bounds = new RectangleF(new PointF(0, 0), signatureImage.PhysicalDimension);
signature.ContactInfo = "[e-mail]";
signature.LocationInfo = "Honolulu, Hawaii";
signature.Reason = "I am author of this document.";
//Create appearance for the digital signature.
signature.Appearance.Normal.Graphics.DrawImage(signatureImage, signature.Bounds);

//Save the document into stream.
MemoryStream stream = new MemoryStream();
document.Save(stream);
stream.Position = 0;
//Close the document.
document.Close(true);
//Define the ContentType for pdf file.
string contentType = "application/pdf";
//Define the file name.
string fileName = "Output.pdf";
//Create a FileContentResult object by using the file contents, content type, and file name.
return File(stream, contentType, fileName);

C# [Windows-specific]

//Creates a new PDF document.
PdfDocument document = new PdfDocument();
//Adds a new page.
PdfPageBase page = document.Pages.Add();
//Create PDF graphics for the page.
PdfGraphics graphics = page.Graphics;

//Creates a certificate instance from PFX file with private key.
PdfCertificate pdfCert = new PdfCertificate(@"PDF.pfx", "password123");
//Creates a digital signature.
PdfSignature signature = new PdfSignature(document, page, pdfCert, "Signature");
//Sets an image for signature field.
PdfBitmap signatureImage = new PdfBitmap(@"signature.png");
//Sets signature information.
signature.Bounds = new RectangleF(0,0,200,100);
signature.ContactInfo = "[e-mail]";
signature.LocationInfo = "Honolulu, Hawaii";
signature.Reason = "I am author of this document.";
//Create appearance for the digital siganture.
signature.Appearance.Normal.Graphics.DrawImage(signatureImage, signature.Bounds);

//Save the document.
document.Save("DigitalSignature.pdf");
//Close the document.
document.Close(true);

VB.NET [Windows-specific]

'Create a new PDF document.
Dim document As New PdfDocument()
'Add a new page.
Dim page As PdfPageBase = document.Pages.Add()
'Create PDF graphics for the page.
Dim graphics As PdfGraphics = page.Graphics

'Create a certificate instance from a PFX file with a private key.
Dim pdfCert As New PdfCertificate("PDF.pfx", "password123")
'Create a digital signature.
Dim signature As New PdfSignature(document, page, pdfCert, "Signature")
'Set an image for signature field.
Dim signatureImage As New PdfBitmap("signature.jpg")
'Set the signature information.
signature.Bounds = New RectangleF(New PointF(0, 0), signatureImage.PhysicalDimension)
signature.ContactInfo = "[e-mail]"
signature.LocationInfo = "Honolulu, Hawaii"
signature.Reason = "I am author of this document."
'Create appearance for the digital signature.
signature.Appearance.Normal.Graphics.DrawImage(signatureImage, signature.Bounds)

'Save and close the document.
document.Save("Output.pdf")
document.Close(True)

You can download a complete working sample from GitHub.

Long Term Validation (LTV) information

Added support for LTV validation and getting CRL and OCSP embedded details from the digital signature using LtvVerificationInfo class.

C# [Cross-platform]

//Gets the stream from the document
FileStream documentStream = new FileStream(Path.GetFullPath(@"Data/Input.pdf"), FileMode.Open, FileAccess.Read);

//Loads an existing signed PDF document
PdfLoadedDocument document = new PdfLoadedDocument(documentStream);

// Gets the signature field
PdfLoadedSignatureField signatureField = document.Form.Fields[0] as PdfLoadedSignatureField;

// Validates signature and get validation result
PdfSignatureValidationResult result = signatureField.ValidateSignature();

// Gets the LTV verification Information.
LtvVerificationInfo ltvVerificationInfo = result.LtvVerificationInfo;

// Checks whether the signature document LTV is enabled.
bool isLtvEnabled = ltvVerificationInfo.IsLtvEnabled;

// Checks whether the signature document has CRL embedded.
bool isCrlEmbedded = ltvVerificationInfo.IsCrlEmbedded;

// Checks whether the signature document has OCSP embedded.
bool isOcspEmbedded = ltvVerificationInfo.IsOcspEmbedded;

Console.WriteLine("LTV enabled: " + isLtvEnabled);
Console.WriteLine("CRL embedded: " + isCrlEmbedded);
Console.WriteLine("OCSP embedded: " + isOcspEmbedded);

// Closes the document
document.Close(true);

C# [Windows-specific]

//Gets the stream from the document
FileStream documentStream = new FileStream(Path.GetFullPath(@"Data/Input.pdf"), FileMode.Open, FileAccess.Read);

//Loads an existing signed PDF document
PdfLoadedDocument document = new PdfLoadedDocument(documentStream);

// Gets the signature field
PdfLoadedSignatureField signatureField = document.Form.Fields[0] as PdfLoadedSignatureField;

// Validates signature and get validation result
PdfSignatureValidationResult result = signatureField.ValidateSignature();

// Gets the LTV verification Information.
LtvVerificationInfo ltvVerificationInfo = result.LtvVerificationInfo;

// Checks whether the signature document LTV is enabled.
bool isLtvEnabled = ltvVerificationInfo.IsLtvEnabled;

// Checks whether the signature document has CRL embedded.
bool isCrlEmbedded = ltvVerificationInfo.IsCrlEmbedded;

// Checks whether the signature document has OCSP embedded.
bool isOcspEmbedded = ltvVerificationInfo.IsOcspEmbedded;

Console.WriteLine("LTV enabled: " + isLtvEnabled);
Console.WriteLine("CRL embedded: " + isCrlEmbedded);
Console.WriteLine("OCSP embedded: " + isOcspEmbedded);

// Closes the document
document.Close(true);

VB.NET [Windows-specific]

' Gets the stream from the document
Dim documentStream As New FileStream(Path.GetFullPath("Data/Input.pdf"), FileMode.Open, FileAccess.Read)

' Loads an existing signed PDF document
Dim document As New PdfLoadedDocument(documentStream)

' Gets the signature field
Dim signatureField As PdfLoadedSignatureField = TryCast(document.Form.Fields(0), PdfLoadedSignatureField)

' Validates signature and get validation result
Dim result As PdfSignatureValidationResult = signatureField.ValidateSignature()

' Gets the LTV verification Information.
Dim ltvVerificationInfo As LtvVerificationInfo = result.LtvVerificationInfo

' Checks whether the signature document LTV is enabled.
Dim isLtvEnabled As Boolean = ltvVerificationInfo.IsLtvEnabled

' Checks whether the signature document has CRL embedded.
Dim isCrlEmbedded As Boolean = ltvVerificationInfo.IsCrlEmbedded

' Checks whether the signature document has OCSP embedded.
Dim isOcspEmbedded As Boolean = ltvVerificationInfo.IsOcspEmbedded

Console.WriteLine("LTV enabled: " & isLtvEnabled)
Console.WriteLine("CRL embedded: " & isCrlEmbedded)
Console.WriteLine("OCSP embedded: " & isOcspEmbedded)

' Closes the document
document.Close(True)

You can download a complete working sample from GitHub.

Customized revocation validation

Added support to customize revocation validation using PdfSignatureValidationOptions.

C# [Cross-platform]

//Gets the stream from the document
FileStream documentStream = new FileStream(Path.GetFullPath(@"Data/Input.pdf"), FileMode.Open, FileAccess.Read);

//Loads an existing signed PDF document
PdfLoadedDocument document = new PdfLoadedDocument(documentStream);

// Gets the signature field
PdfLoadedSignatureField signatureField = document.Form.Fields[0] as PdfLoadedSignatureField;

// Signature validation options
PdfSignatureValidationOptions options = new PdfSignatureValidationOptions();

// Sets the revocation type
options.RevocationValidationType = RevocationValidationType.Crl;

// Validate signature and get validation result
PdfSignatureValidationResult result = signatureField.ValidateSignature(options);

//Check whether the CRL is revoked
if (result.RevocationResult.IsRevokedCRL)
{
    Console.WriteLine("CRL is revoked");
}
else
{
    Console.WriteLine("CRL is not revoked");
}

// Closes the document
document.Close(true);

C# [Windows-specific]

//Gets the stream from the document
FileStream documentStream = new FileStream(Path.GetFullPath(@"Data/Input.pdf"), FileMode.Open, FileAccess.Read);

//Loads an existing signed PDF document
PdfLoadedDocument document = new PdfLoadedDocument(documentStream);

// Gets the signature field
PdfLoadedSignatureField signatureField = document.Form.Fields[0] as PdfLoadedSignatureField;

// Signature validation options
PdfSignatureValidationOptions options = new PdfSignatureValidationOptions();

// Sets the revocation type
options.RevocationValidationType = RevocationValidationType.Crl;

// Validate signature and get validation result
PdfSignatureValidationResult result = signatureField.ValidateSignature(options);

//Check whether the CRL is revoked
if (result.RevocationResult.IsRevokedCRL)
{
    Console.WriteLine("CRL is revoked");
}
else
{
    Console.WriteLine("CRL is not revoked");
}

// Closes the document
document.Close(true);

VB.NET [Windows-specific]

' Gets the stream from the document
Dim documentStream As New FileStream(Path.GetFullPath("Data/Input.pdf"), FileMode.Open, FileAccess.Read)

' Loads an existing signed PDF document
Dim document As New PdfLoadedDocument(documentStream)

' Gets the signature field
Dim signatureField As PdfLoadedSignatureField = TryCast(document.Form.Fields(0), PdfLoadedSignatureField)

' Signature validation options
Dim options As New PdfSignatureValidationOptions()

' Sets the revocation type
options.RevocationValidationType = RevocationValidationType.Crl

' Validate signature and get validation result
Dim result As PdfSignatureValidationResult = signatureField.ValidateSignature(options)

' Check whether the CRL is revoked
If result.RevocationResult.IsRevokedCRL Then
    Console.WriteLine("CRL is revoked")
Else
    Console.WriteLine("CRL is not revoked")
End If

' Closes the document
document.Close(True)

You can download a complete working sample from GitHub.

Remove existing digital signatures from a PDF document

The following code example illustrates how to remove existing digital signatures from a PDF document using Remove method in PdfLoadedFormFieldCollection class.

C# [Cross-platform]

//Load an existing PDF document.
FileStream docStream = new FileStream("Input.pdf", FileMode.Open, FileAccess.Read);
PdfLoadedDocument pdfLoadedDocument = new PdfLoadedDocument(docStream);

//Get the signature field from PDF form field collection.
PdfLoadedSignatureField signatureField = pdfLoadedDocument.Form.Fields[0] as PdfLoadedSignatureField;
//Remove signature field from form field collection.
pdfLoadedDocument.Form.Fields.Remove(signatureField);

//Save the document into stream.
MemoryStream stream = new MemoryStream();
pdfLoadedDocument.Save(stream);
stream.Position = 0;
//Close the document.
pdfLoadedDocument.Close(true);
//Defining the ContentType for PDF file.
string contentType = "application/pdf";
//Define the file name.
string fileName = "Output.pdf";
//Creates a FileContentResult object by using the file contents, content type, and file name.
return File(stream, contentType, fileName);

C# [Windows-specific]

//Load an existing PDF document.
PdfLoadedDocument pdfLoadedDocument = new PdfLoadedDocument("Input.pdf");

//Get the signature field from PDF form field collection.
PdfLoadedSignatureField signatureField = pdfLoadedDocument.Form.Fields[0] as PdfLoadedSignatureField;
//Remove signature field from form field collection.
pdfLoadedDocument.Form.Fields.Remove(signatureField);

//Save and close the PDF document.
pdfLoadedDocument.Save("RemoveDigital.pdf");
pdfLoadedDocument.Close(true);

VB.NET [Windows-specific]

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

'Get the signature field from PDF form field collection.
Dim signatureField As PdfLoadedSignatureField = TryCast(pdfLoadedDocument.Form.Fields(0), PdfLoadedSignatureField)
'Remove signature field from form field collection.
pdfLoadedDocument.Form.Fields.Remove(signatureField)

'Save and close the PDF document.
pdfLoadedDocument.Save("RemoveDigital.pdf")
pdfLoadedDocument.Close(True)

You can download a complete working sample from GitHub.

Certified signature

The following code snippet illustrates how to sign a PDF document without showing the digital signature using Certificated property in PdfSignature class.

C# [Cross-platform]

//Get stream from an existing PDF document. 
FileStream docStream = new FileStream("Input.pdf", FileMode.Open, FileAccess.Read);
PdfLoadedDocument loadedDocument = new PdfLoadedDocument(docStream);
//Load digital ID with password.
FileStream certificateStream = new FileStream("PDF.pfx", FileMode.Open, FileAccess.Read);
PdfCertificate certificate = new PdfCertificate(certificateStream, "password123");

//Create a signature with loaded digital ID.
PdfSignature signature = new PdfSignature(loadedDocument, loadedDocument.Pages[0], certificate, "DigitalSignature");
signature.Settings.CryptographicStandard = CryptographicStandard.CADES;
signature.Settings.DigestAlgorithm = DigestAlgorithm.SHA256;
//This property enables the author or certifying signature.
signature.Certificated = true;
//Allow the form fill and and comments.
signature.DocumentPermissions = PdfCertificationFlags.AllowFormFill | PdfCertificationFlags.AllowComments;

//Save the document into stream.
MemoryStream stream = new MemoryStream();
document.Save(stream);
stream.Position = 0;
//Close the document.
document.Close(true);
//Defining the ContentType for pdf file.
string contentType = "application/pdf";
//Define the file name.
string fileName = "Output.pdf";
//Creates a FileContentResult object by using the file contents, content type, and file name.
return File(stream, contentType, fileName);

C# [Windows-specific]

//Load an existing PDF document.
PdfLoadedDocument loadedDocument = new PdfLoadedDocument("Input.pdf");
//Load digital ID with password.
PdfCertificate certificate = new PdfCertificate(@"PDF.pfx", "password123");

//Create a signature with loaded digital ID.
PdfSignature signature = new PdfSignature(loadedDocument, loadedDocument.Pages[0], certificate, "DigitalSignature");
signature.Settings.CryptographicStandard = CryptographicStandard.CADES;
signature.Settings.DigestAlgorithm = DigestAlgorithm.SHA256;
//This property enables the author or certifying signature.
signature.Certificated = true;
//Allow the form fill and and comments.
signature.DocumentPermissions = PdfCertificationFlags.AllowFormFill | PdfCertificationFlags.AllowComments;

//Save and close the PDF document.
loadedDocument.Save("Certifying.pdf");
loadedDocument.Close(true);

VB.NET [Windows-specific]

'Load an existing PDF document.
Dim loadedDocument As PdfLoadedDocument = New PdfLoadedDocument("Input.pdf")
'Load digital ID with password. 
Dim certificate As PdfCertificate = New PdfCertificate("PDF.pfx", "password123")

'Create a signature with loaded digital ID. 
Dim signature As PdfSignature = New PdfSignature(loadedDocument, loadedDocument.Pages(0), certificate, "DigitalSignature")
signature.Settings.CryptographicStandard = CryptographicStandard.CADES
signature.Settings.DigestAlgorithm = DigestAlgorithm.SHA256
'This property enables the author or certifying signature.
signature.Certificated = True
'Allow the form fill and and comments. 
signature.DocumentPermissions = PdfCertificationFlags.AllowFormFill Or PdfCertificationFlags.AllowComments

'Save and close the PDF document.
loadedDocument.Save("Certifying.pdf")
loadedDocument.Close(True)

You can download a complete working sample from GitHub.

Retrieve digital signature information from an existing PDF document

The following code snippet illustrates how to retrieve digital signature information from an existing PDF document using IssuerName, ValidFrom, and ValidTo property.

C# [Cross-platform]

//Get stream from an existing PDF document. 
FileStream docStream = new FileStream("Input.pdf", FileMode.Open, FileAccess.Read);
PdfLoadedDocument loadedDocument = new PdfLoadedDocument(docStream);

//Get the signature field from PdfLoadedDocument form field collection.
PdfLoadedSignatureField signatureField = loadedDocument.Form.Fields[0] as PdfLoadedSignatureField;
PdfSignature signature = signatureField.Signature;

//Extract the signature information.
Console.WriteLine("Digitally Signed by: " + signature.Certificate.IssuerName);
Console.WriteLine("Valid From: " + signature.Certificate.ValidFrom);
Console.WriteLine("Valid To: " + signature.Certificate.ValidTo);
Console.WriteLine("Hash Algorithm : " + signature.Settings.DigestAlgorithm);
Console.WriteLine("Cryptographics Standard : " + signature.Settings.CryptographicStandard);

//Close the document.
loadedDocument.Close(true);

C# [Windows-specific]

//Load an existing PDF document.
PdfLoadedDocument loadedDocument = new PdfLoadedDocument("Input.pdf");

//Get the signature field from PdfLoadedDocument form field collection.
PdfLoadedSignatureField signatureField = loadedDocument.Form.Fields[0] as PdfLoadedSignatureField;
PdfSignature signature = signatureField.Signature;

//Extract the signature information.
Console.WriteLine("Digitally Signed by: " + signature.Certificate.IssuerName);
Console.WriteLine("Valid From: " + signature.Certificate.ValidFrom);
Console.WriteLine("Valid To: " + signature.Certificate.ValidTo);
Console.WriteLine("Hash Algorithm : " + signature.Settings.DigestAlgorithm);
Console.WriteLine("Cryptographics Standard : " + signature.Settings.CryptographicStandard);

//Close the document.
loadedDocument.Close(true);

VB.NET [Windows-specific]

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

'Get the signature field from PdfLoadedDocument form field collection.
Dim signatureField As PdfLoadedSignatureField = TryCast(loadedDocument.Form.Fields(0), PdfLoadedSignatureField)
Dim signature As PdfSignature = signatureField.Signature

'Extract the signature information.
Console.WriteLine("Digitally Signed by: " & signature.Certificate.IssuerName)
Console.WriteLine("Valid From: " & signature.Certificate.ValidFrom)
Console.WriteLine("Valid To: " & signature.Certificate.ValidTo)
Console.WriteLine("Hash Algorithm : " & signature.Settings.DigestAlgorithm)
Console.WriteLine("Cryptographics Standard : " & signature.Settings.CryptographicStandard)

'Close the document.
loadedDocument.Close(True)

You can download a complete working sample from GitHub.

Adding multiple signatures to a PDF document

The following code example illustrates how to add multiple signatures to a PDF document without invalidating the previous signature.

NOTE

It is recommended to use licensed assemblies or registered license keys in your respective applications to add multiple digital signatures to the PDF documents without invalidating the previous signatures.

C# [Cross-platform]

//Load the PDF document.
FileStream docStream = new FileStream("SignatureFields.pdf", FileMode.Open, FileAccess.Read);
PdfLoadedDocument loadedDocument = new PdfLoadedDocument(docStream);
//Get the first page of the document.
PdfLoadedPage page = loadedDocument.Pages[0] as PdfLoadedPage;

//Get the first signature field of the PDF document.
PdfLoadedSignatureField signatureField1 = loadedDocument.Form.Fields[0] as PdfLoadedSignatureField;
//Creates a certificate.
FileStream certificateStream1 = new FileStream("PDF.pfx", FileMode.Open, FileAccess.Read);
PdfCertificate certificate1 = new PdfCertificate(certificateStream1, "password123");
//Add signature to the signature field.
signatureField1.Signature = new PdfSignature(loadedDocument, page, certificate1, "Signature", signatureField1);
//Get the image as a stream. 
FileStream imageStream = new FileStream("Student Signature.jpg", FileMode.Open, FileAccess.Read);
PdfBitmap signatureImage = new PdfBitmap(imageStream);
//Draw an image in signature appearance. 
signatureField1.Signature.Appearance.Normal.Graphics.DrawImage(signatureImage, 0, 0, 90, 20);

//Save the document into the stream.
MemoryStream stream = new MemoryStream();
loadedDocument.Save(stream);

//Load the signed PDF document.
PdfLoadedDocument signedDocument = new PdfLoadedDocument(stream);
//Load the PDF page.
PdfLoadedPage loadedPage = signedDocument.Pages[0] as PdfLoadedPage;

//Get the first signature field of the PDF document.
PdfLoadedSignatureField signatureField2 = signedDocument.Form.Fields[1] as PdfLoadedSignatureField;
//Add the signature to the signature field. 
signatureField1.Signature = new PdfSignature(signedDocument, loadedPage, certificate1, "Signature", signatureField2);
//Load the image as a stream. 
FileStream imageStream1 = new FileStream("Teacher Signature.png", FileMode.Open, FileAccess.Read);
PdfBitmap signatureImage1 = new PdfBitmap(imageStream1);
//Draw the image in signature appearance. 
signatureField1.Signature.Appearance.Normal.Graphics.DrawImage(signatureImage1, 0, 0, 90, 20);

//Saving the PDF to the MemoryStream.
MemoryStream signedStream = new MemoryStream();
signedDocument.Save(signedStream);
//Set the position as '0'.
signedStream.Position = 0;
//Close the documents. 
signedDocument.Close(true);
loadedDocument.Close(true);
//Defining the ContentType for a pdf file.
string contentType = "application/pdf";
//Define the file name.
string fileName = "Multiple_Signature.pdf";
//Create the FileContentResult object by using the file contents, content type, and file name. 
return File(signedStream, contentType, fileName);

C# [Windows-specific]

//Load an existing PDF document. 
PdfLoadedDocument loadedDocument = new PdfLoadedDocument("Input.pdf");
//Get the first page of the document.
PdfLoadedPage page = loadedDocument.Pages[0] as PdfLoadedPage;

//Get the first signature field of the PDF document.
PdfLoadedSignatureField signatureField1 = loadedDocument.Form.Fields[0] as PdfLoadedSignatureField;
//Create a certificate instance from a PFX file with a private key.
PdfCertificate certificate1 = new PdfCertificate("PDF.pfx", "password123");
//Add a signature to the signature field.
signatureField1.Signature = new PdfSignature(loadedDocument, page, certificate1, "Signature", signatureField1);
//Set an image for the signature field.
PdfBitmap signatureImage = new PdfBitmap(@"Student Signature.jpg");
//Insert an image in the signature appearance. 
signatureField1.Signature.Appearance.Normal.Graphics.DrawImage(signatureImage, 0, 0, 90, 20);

//Save the document into the stream.
MemoryStream stream = new MemoryStream();
loadedDocument.Save(stream);

//Load the signed PDF document.
PdfLoadedDocument signedDocument = new PdfLoadedDocument(stream);
//Load the PDF page.
PdfLoadedPage loadedPage = signedDocument.Pages[0] as PdfLoadedPage;

//Get the first signature field of the PDF document.
PdfLoadedSignatureField signatureField2 = signedDocument.Form.Fields[1] as PdfLoadedSignatureField;
//Add a signature to the signature field. 
signatureField2.Signature = new PdfSignature(signedDocument, loadedPage, certificate1, "Signature", signatureField2);
//Set an image for the signature field.
PdfBitmap signatureImage1 = new PdfBitmap(@"Teacher Signature.png");
//Draw an image in the signature appearance. 
signatureField2.Signature.Appearance.Normal.Graphics.DrawImage(signatureImage1, 0, 0, 90, 20);

//Save the PDF document. 
signedDocument.Save("Multiple_signature.pdf");
//Close the PDF documents. 
signedDocument.Close(true);
loadedDocument.Close(true);

VB.NET [Windows-specific]

'Load an existing PDF document. 
Dim loadedDocument As PdfLoadedDocument = New PdfLoadedDocument("Input.pdf")
'Get the first page of the document.
Dim page As PdfLoadedPage = TryCast(loadedDocument.Pages(0), PdfLoadedPage)

'Get the first signature field of the PDF document.
Dim signatureField1 As PdfLoadedSignatureField = TryCast(loadedDocument.Form.Fields(0), PdfLoadedSignatureField)
'Create a certificate instance from the PFX file with a private key.
Dim certificate1 As PdfCertificate = New PdfCertificate("PDF.pfx", "password123")
'Add a signature to the signature field. 
signatureField1.Signature = New PdfSignature(loadedDocument, page, certificate1, "Signature", signatureField1)
'Set an image for the signature field.
Dim signatureImage As PdfBitmap = New PdfBitmap("Student Signature.jpg")
'Draw an image in the signature appearance. 
signatureField1.Signature.Appearance.Normal.Graphics.DrawImage(signatureImage, 0, 0, 90, 20)

'Save the document into the stream.
Dim stream As MemoryStream = New MemoryStream()
loadedDocument.Save(stream)

'Load the signed PDF document.
Dim signedDocument As PdfLoadedDocument = New PdfLoadedDocument(stream)
'Load the PDF page.
Dim loadedPage As PdfLoadedPage = TryCast(signedDocument.Pages(0), PdfLoadedPage)

'Get the first signature field of the PDF document.
Dim signatureField2 As PdfLoadedSignatureField = TryCast(signedDocument.Form.Fields(1), PdfLoadedSignatureField)
signatureField2.Signature = New PdfSignature(signedDocument, loadedPage, certificate1, "Signature", signatureField2)
'Set an image for the signature field.
Dim signatureImage1 As PdfBitmap = New PdfBitmap("Teacher Signature.png")
'Draw an image in the signature appearance. 
signatureField2.Signature.Appearance.Normal.Graphics.DrawImage(signatureImage1, 0, 0, 90, 20)

'Save the PDF document. 
signedDocument.Save("Multiple_signature.pdf")
'Close the PDF documents. 
signedDocument.Close(True)
loadedDocument.Close(True)

You can download a complete working sample from GitHub.

Retrieve revocation certificate information from digital signature

The following code example illustrates how to retrieve revocation certificate information from digital signature using PdfSignerCertificate.

C# [Cross-platform]

//Load an existing signed PDF document.
FileStream documentStream = new FileStream(@"Input.pdf", FileMode.Open, FileAccess.Read);
PdfLoadedDocument loadedDocument = new PdfLoadedDocument(documentStream);

//Get signature field.
PdfLoadedSignatureField loadedSignatureField = loadedDocument.Form.Fields[0] as PdfLoadedSignatureField;
//X509Certificate2Collection to check the signer's identity using root certificates.
X509CertificateCollection collection = new X509CertificateCollection();
//Create new X509Certificate2 with the root certificate.
X509Certificate2 certificate = new X509Certificate2(@"Root.cer");
//Add the certificate to the collection.
collection.Add(certificate);
//Create new X509Certificate2 with the intermediate certificate.
certificate = new X509Certificate2(@"Intermediate.cer");
//Add the certificate to the collection.
collection.Add(certificate);
//Validate signature and get the validation result.
PdfSignatureValidationResult result = loadedSignatureField.ValidateSignature(collection);

foreach (PdfSignerCertificate signerCertificate in result.SignerCertificates)
{
    if (signerCertificate.OcspCertificate != null)
    {
        foreach (X509Certificate2 item in signerCertificate.OcspCertificate.Certificates)
        {
            string subjectName = "The OCSP Response was signed by " + item.SubjectName.Name;
        }
        bool isEmbbed = signerCertificate.OcspCertificate.IsEmbedded;
        DateTime validForm = signerCertificate.OcspCertificate.ValidFrom;
        DateTime validTo = signerCertificate.OcspCertificate.ValidTo;

    }
    if (signerCertificate.CrlCertificate != null)
    {
        foreach (X509Certificate2 item in signerCertificate.CrlCertificate.Certificates)
        {
            string subjectName = "The CRL was signed by " + item.SubjectName.Name;
        }
        bool isEmbbed = signerCertificate.CrlCertificate.IsEmbedded;
        DateTime validForm = signerCertificate.CrlCertificate.ValidFrom;
        DateTime validTo = signerCertificate.CrlCertificate.ValidTo;
    }
}

//Close the document.
loadedDocument.Close(true);

C# [Windows-specific]

//Load an existing signed PDF document.
PdfLoadedDocument loadedDocument = new PdfLoadedDocument("Input.pdf");

//Get signature field.
PdfLoadedSignatureField loadedSignatureField = loadedDocument.Form.Fields[0] as PdfLoadedSignatureField;
//X509Certificate2Collection to check the signer's identity using root certificates.
X509CertificateCollection collection = new X509CertificateCollection();
//Create new X509Certificate2 with the root certificate.
X509Certificate2 certificate = new X509Certificate2(@"Root.cer");
//Add the certificate to the collection.
collection.Add(certificate);
//Create new X509Certificate2 with the intermediate certificate.
certificate = new X509Certificate2(@"Intermediate.cer");
//Add the certificate to the collection.
collection.Add(certificate);

//Validate signature and get the validation result.
PdfSignatureValidationResult result = loadedSignatureField.ValidateSignature(collection);

foreach (PdfSignerCertificate signerCertificate in result.SignerCertificates)
{
    if (signerCertificate.OcspCertificate != null)
    {
        foreach (X509Certificate2 item in signerCertificate.OcspCertificate.Certificates)
        {
            string subjectName = "The OCSP Response was signed by " + item.SubjectName.Name;
        }
        bool isEmbbed = signerCertificate.OcspCertificate.IsEmbedded;
        DateTime validForm = signerCertificate.OcspCertificate.ValidFrom;
        DateTime validTo = signerCertificate.OcspCertificate.ValidTo;

    }
    if (signerCertificate.CrlCertificate != null)
    {
        foreach (X509Certificate2 item in signerCertificate.CrlCertificate.Certificates)
        {
            string subjectName = "The CRL was signed by " + item.SubjectName.Name;
        }
        bool isEmbbed = signerCertificate.CrlCertificate.IsEmbedded;
        DateTime validForm = signerCertificate.CrlCertificate.ValidFrom;
        DateTime validTo = signerCertificate.CrlCertificate.ValidTo;
    }
}
//Close the document.
loadedDocument.Close(true);

VB.NET [Windows-specific]

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

'Get signature field.
Dim loadedSignatureField As PdfLoadedSignatureField = TryCast(loadedDocument.Form.Fields(0), PdfLoadedSignatureField)
'X509Certificate2Collection to check the signer's identity using root certificates.
Dim collection As X509CertificateCollection = New X509CertificateCollection()
'Create new X509Certificate2 with the root certificate.
Dim certificate As X509Certificate2 = New X509Certificate2("Root.cer")
'Add the certificate to the collection.
collection.Add(certificate)
'Create new X509Certificate2 with the intermediate certificate.
certificate = New X509Certificate2("Intermediate.cer")
'Add the certificate to the collection.
collection.Add(certificate)

'Validate signature and get the validation result.
Dim result As PdfSignatureValidationResult = loadedSignatureField.ValidateSignature(collection)

For Each signerCertificate As PdfSignerCertificate In result.SignerCertificates

    If signerCertificate.OcspCertificate IsNot Nothing Then

        For Each item As X509Certificate2 In signerCertificate.OcspCertificate.Certificates
            Dim subjectName As String = "The OCSP Response was signed by " & item.SubjectName.Name
        Next

        Dim isEmbbed As Boolean = signerCertificate.OcspCertificate.IsEmbedded
        Dim validForm As DateTime = signerCertificate.OcspCertificate.ValidFrom
        Dim validTo As DateTime = signerCertificate.OcspCertificate.ValidTo
    End If

    If signerCertificate.CrlCertificate IsNot Nothing Then

        For Each item As X509Certificate2 In signerCertificate.CrlCertificate.Certificates
            Dim subjectName As String = "The CRL was signed by " & item.SubjectName.Name
        Next

        Dim isEmbbed As Boolean = signerCertificate.CrlCertificate.IsEmbedded
        Dim validForm As DateTime = signerCertificate.CrlCertificate.ValidFrom
        Dim validTo As DateTime = signerCertificate.CrlCertificate.ValidTo
    End If
Next
'Close the document.
loadedDocument.Close(True)

You can download a complete working sample from GitHub.

Retrieve signed revision information from digital signature

The following code example illustrates how to retrieve signed revision information from digital signature using Revision property of PdfLoadedSignatureField.

C# [Cross-platform]

//Load an existing PDF document.
FileStream inputStream = new FileStream(@"Input.pdf", FileMode.Open, FileAccess.Read);
PdfLoadedDocument document = new PdfLoadedDocument(inputStream);
//Get the document revisions. 
PdfRevision[] revisions = document.Revisions;
foreach (PdfRevision rev in revisions)
{
    //Get revision start position.
    long startPosition = rev.StartPosition;
}
//Load the existing signature field.
PdfLoadedSignatureField field = document.Form.Fields[0] as PdfLoadedSignatureField;
//Get the revision of the signature.
int revisionIndex = field.Revision;
//Close the document.
document.Close(true);

C# [Windows-specific]

//Load an existing PDF document
PdfLoadedDocument document = new PdfLoadedDocument("Input.pdf");
//Get the document revisions. 
PdfRevision[] revisions = document.Revisions;
foreach (PdfRevision rev in revisions)
{
    //Get revision start position.
    long startPosition = rev.StartPosition;
}
//Load the existing signature field.
PdfLoadedSignatureField field = document.Form.Fields[0] as PdfLoadedSignatureField;
//Get the revision of the signature.
int revisionIndex = field.Revision;
//Close the document.
document.Close(true);

VB.NET [Windows-specific]

'Load an existing PDF document.
Dim document As PdfLoadedDocument = New PdfLoadedDocument("Input.pdf")
'Get the document revisions. 
Dim revisions As PdfRevision() = document.Revisions
For Each rev As PdfRevision In revisions
    'Get revision start position.
    Dim startPosition As Long = rev.StartPosition
Next
'Load the existing signature field.
Dim field As PdfLoadedSignatureField = TryCast(document.Form.Fields(0), PdfLoadedSignatureField)
'Get the revision of the signature.
Dim revisionIndex As Integer = field.Revision
'Close the document.
document.Close(True)

You can download a complete working sample from GitHub.

Retrieve revocation certificate information from digital signature embed timestamp

The following code example illustrates how to retrieve revocation certificate information from digital signature embed timestamp using PdfSignerCertificate.

C# [Cross-platform]

//Load an existing PDF document.
FileStream inputStream = new FileStream(@"Input.pdf", FileMode.Open, FileAccess.Read);
PdfLoadedDocument document = new PdfLoadedDocument(inputStream);
//Gets the signature field.
PdfLoadedSignatureField signatureField = document.Form.Fields[0] as PdfLoadedSignatureField;
//Validates signature and gets the validation result.
PdfSignatureValidationResult result = signatureField.ValidateSignature();
//Gets signer certificates
PdfSignerCertificate[] certifcate = result.TimeStampInformation.SignerCertificates;
//Close the document.
document.Close(true);

C# [Windows-specific]

//Load an existing PDF document
PdfLoadedDocument document = new PdfLoadedDocument("Input.pdf");
//Gets the signature field.
PdfLoadedSignatureField signatureField = document.Form.Fields[0] as PdfLoadedSignatureField;
//Validates signature and gets the validation result.
PdfSignatureValidationResult result = signatureField.ValidateSignature();
//Gets signer certificates
PdfSignerCertificate[] certifcate = result.TimeStampInformation.SignerCertificates;
//Close the document.
document.Close(true);

VB.NET [Windows-specific]

'Load an existing PDF document.
Dim loadedDocument As PdfLoadedDocument = New PdfLoadedDocument("Input.pdf")
'Gets the signature field.
Dim loadedSignatureField As PdfLoadedSignatureField = TryCast(loadedDocument.Form.Fields(0), PdfLoadedSignatureField)
'Validates signature and gets the validation result.
Dim result As PdfSignatureValidationResult = loadedSignatureField.ValidateSignature()
'Gets signer certificates
Dim certifcate As PdfSignerCertificate() = result.TimeStampInformation.SignerCertificates
'Close the document.
loadedDocument.Close(True)

You can download a complete working sample from GitHub.

Get images from the existing signed signature field

Utilize the GetImages method within the PdfLoadedSignatureField class to obtain images from the appearance of an existing signed signature field. The following code example provides a clear demonstration of this functionality.

C# [Cross-platform]

//Load an existing PDF file.
FileStream fileStream = new FileStream("Input.pdf", FileMode.Open, FileAccess.Read);
PdfLoadedDocument ldoc = new PdfLoadedDocument(fileStream);
//Get the existing signed signature field.
PdfLoadedSignatureField loadedSignature = ldoc.Form.Fields[0] as PdfLoadedSignatureField;
//Get the image streams.
Stream[] imageStreams = loadedSignature.GetImages();
for (int i = 0; i < imageStreams.Length; i++) {
    File.WriteAllBytes("Output" + i.ToString() + ".jpg", (imageStreams[i] as MemoryStream).ToArray());               
}
//Close a PDF document.
ldoc.Close(true);

C# [Windows-specific]

//Load an existing PDF file.
PdfLoadedDocument ldoc = new PdfLoadedDocument("Input.pdf");
//Get the existing signed signature field.
PdfLoadedSignatureField loadedSignature = ldoc.Form.Fields[0] as PdfLoadedSignatureField;
//Get the image.
Image[] images = loadedSignature.GetImages();
for (int i = 0; i < images.Length; i++) {
    images[i].Save("Image" + i.ToString() + ".jpg", ImageFormat.Png);
}
//Close a PDF document.
ldoc.Close(true);

VB.NET [Windows-specific]

'Load an existing PDF file.
Dim fileStream As FileStream = New FileStream("Input.pdf", FileMode.Open, FileAccess.Read)
Dim ldoc As PdfLoadedDocument = New PdfLoadedDocument(fileStream)
'Get the existing signed signature field.
Dim loadedSignature As PdfLoadedSignatureField = TryCast(ldoc.Form.Fields(0), PdfLoadedSignatureField)
'Get the image.
Dim images As Image() = loadedSignature.GetImages()
For i As Integer = 0 To images.Length - 1
    images(i).Save("Image.jpg", Imaging.ImageFormat.Png)
Next
'Close a PDF document.
ldoc.Close(True)

You can download a complete working sample from GitHub.

NOTE

This method retrieves the images when rendered on the signed signature field appearance; otherwise, it will return null.

Integrating signature and timestamp certificates into the Document Secure Store (DSS).

Effortlessly Integrate signature and timestamp certificates into the Document Security Store (DSS) with the Essential® PDF Library. This streamlined process enhances certificate management, ensuring robust validation for your PDF documents. Below is a code example demonstrating how to include certificates when creating Long-Term Validity (LTV) from an external signature, utilizing the CreateLongTermValidity method in the PdfSignature class.

C# [Cross-platform]

//Loads an existing document
PdfLoadedDocument document = new PdfLoadedDocument("Input.pdf");
//Gets the signature field
PdfLoadedSignatureField signatureField = document.Form.Fields[0] as PdfLoadedSignatureField;
//Add public Certificates
List<X509Certificate2> x509Certificate2s = new List<X509Certificate2>();
//Create long term validation of the signature.
signatureField.Signature.CreateLongTermValidity(x509Certificate2s , true);
//Save the document into stream
MemoryStream stream = new MemoryStream();
document.Save(stream);
//Close the document
document.Close(true);
//Loads the stream from the document
PdfLoadedDocument loadedDocument = new PdfLoadedDocument(stream);
// Access the Document Security Store details
PdfDocumentSecureStore pdfDocumentSecureStore = loadedDocument.DocumentSecureStore;
// Store the DSS certificates on X509Certificate2 certificates.
X509Certificate2[] cert2 = pdfDocumentSecureStore.Certificates;
foreach(X509Certificate2 cert in cert2)
{
    PdfCertificate certificate = new PdfCertificate(cert);
}
// Close the document
loadedDocument.Close(true);

C# [Windows-specific]

//Gets the stream from the document
FileStream documentStream = new FileStream("Input.pdf", FileMode.Open, FileAccess.Read);
//Loads an existing signed PDF document
PdfLoadedDocument document = new PdfLoadedDocument(documentStream);
//Gets the signature field
PdfLoadedSignatureField signatureField = document.Form.Fields[0] as PdfLoadedSignatureField;
//Add public Certificates
List<X509Certificate2> x509Certificate2s = new List<X509Certificate2>();
//Create long term validation of the signature.
signatureField.Signature.CreateLongTermValidity(x509Certificate2s, true);
//Save the document into stream
MemoryStream stream = new MemoryStream();
document.Save(stream);
//Close the document
document.Close(true);
//Loads an existing steam
PdfLoadedDocument loadedDocument = new PdfLoadedDocument(stream);
//Access the Document Security Store details
PdfDocumentSecureStore pdfDocumentSecureStore = loadedDocument.DocumentSecureStore;
//Store the DSS certificates on X509Certificate2 certificates.
X509Certificate2[] cert2 = pdfDocumentSecureStore.Certificates;
foreach(X509Certificate2 cert in cert2)
{
    PdfCertificate certificate = new PdfCertificate(cert);
}
// Close the document
loadedDocument.Close(true);

VB.NET [Windows-specific]

'Loads an existing signed PDF document
Dim document As PdfLoadedDocument = New PdfLoadedDocument("Input.pdf")
'Gets the signature field
Dim signatureField As PdfLoadedSignatureField = CType(document.Form.Fields(0),PdfLoadedSignatureField)
'Add public Certificates
Dim x509Certificate2s As List(Of X509Certificate2) = New List(Of X509Certificate2)
'Create long term validation of the signature.
signatureField.Signature.CreateLongTermValidity(x509Certificate2s, true)
Dim stream As MemoryStream = New MemoryStream
document.Save(stream)
'Close the document
document.Close(true)
Dim loadedDocument As PdfLoadedDocument = New PdfLoadedDocument(stream)
'Access the Document Security Store details
Dim pdfDocumentSecureStore As PdfDocumentSecureStore = loadedDocument.DocumentSecureStore
'Store the DSS certificates on X509Certificate2 certificates.
Dim cert2() As X509Certificate2 = pdfDocumentSecureStore.Certificates
For Each cert As X509Certificate2 In cert2
    Dim certificate As PdfCertificate = New PdfCertificate(cert)
Next
' Close the document
loadedDocument.Close(true)

Troubleshooting

Signature is not visible after signing the document.

Reason	The signature bounds is not properly set, due to this may appear invisible.
Solution	We recommend ensuring that the bounds value of the signature field is correctly set and drawing the image with the signature to ensure its visibility C# //Set the signature bounds. signature.Bounds= new RectangleF(new PointF(0, 0), new SizeF(100, 100));