How to externally sign a PDF document using ASP.NET Core

 
@{Html.BeginForm("CreateDocument", "Home", FormMethod.Get);
    {
        <div>
            <input type="submit" value="Create Document" style="width:150px;height:27px" />
        </div>
    }
    Html.EndForm();
}
 

 
using Syncfusion.Drawing;
using Syncfusion.Pdf;
using Syncfusion.Pdf.Security;
using System.Security.Cryptography;
using System.Security.Cryptography.X509Certificates;
using Syncfusion.Pdf.Parsing;
 

string filePath = Path.Combine(_hostingEnvironment.ContentRootPath, "Data/PDF.pfx");
 
//Creates a new PDF document.
 
PdfDocument document = new PdfDocument();
 
//Creating the stream object.
 
MemoryStream stream = new MemoryStream();
 
//Save a document as a stream.
 
document.Save(stream);
 
//Close a document.
 
document.Close(true);
 
PdfLoadedDocument loadedDocument = new PdfLoadedDocument(stream);
 
//Creates a digital signature.
 
PdfSignature signature = new PdfSignature(loadedDocument, loadedDocument.Pages[0], null, "Signed");
 
//Creates a certificate instance from the PFX file with a private key.
 
X509Certificate2 digitalId = new X509Certificate2(filePath, "password123");
 
X509Chain chain = new X509Chain();
 
chain.Build(digitalId);
 
List<X509Certificate2> certificates = new List<X509Certificate2>();
 
for (int i = 0; i < chain.ChainElements.Count; i++)
{
certificates.Add(chain.ChainElements[i].Certificate);
}
 
//Sets the signature information.
 
signature.EnableLtv = true;
 
signature.AddExternalSigner(new ExternalSigner(digitalId), certificates, null);
 
signature.TimeStampServer = new TimeStampServer(new Uri("http://timestamp.entrust.net/TSS/RFC3161sha2TS"));
 
signature.EnableLtv = true;
 
signature.Settings.CryptographicStandard = CryptographicStandard.CADES;
 
signature.Settings.DigestAlgorithm = DigestAlgorithm.SHA256;
 
signature.Bounds = new RectangleF(0, 0, 200, 100);
 
MemoryStream memorystream = new MemoryStream();
 
loadedDocument.Save(memorystream);
 
//Set the position as '0.'
memorystream.Position = 0;
 
//Download a PDF document in a browser.
FileStreamResult fileStreamResult = new FileStreamResult(memorystream, "application/pdf");
 
fileStreamResult.FileDownloadName = "Sample.pdf";
 
return fileStreamResult;

internal class ExternalSigner : IPdfExternalSigner
{
 
public string HashAlgorithm => "SHA256";
 
X509Certificate2 digitalID;
 
public ExternalSigner(X509Certificate2 certificate)
{
digitalID = certificate;
}
 
public byte[] Sign(byte[] message, out byte[] timeStampResponse)
{
byte[] signedBytes = null;
 
//This condition is based on the provided certificate private key.    
 
if (digitalID.PrivateKey is RSACng)
{
Console.WriteLine("RSACng");
 
RSACng rsa = (RSACng)digitalID.PrivateKey;
 
signedBytes = rsa.SignData(message, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);
}
else if (digitalID.PrivateKey is RSACryptoServiceProvider)
{
Console.WriteLine("RSACryptoServiceProvider");
 
RSACryptoServiceProvider rsa = (RSACryptoServiceProvider)digitalID.PrivateKey;
 
signedBytes = rsa.SignData(message, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);
}
else if (digitalID.PrivateKey is RSAOpenSsl
{
 
Console.WriteLine("RSAOpenSsl");
 
RSAOpenSsl rsa = (RSAOpenSsl)digitalID.PrivateKey;
 
signedBytes = rsa.SignData(message, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);
}
 
timeStampResponse = null;
 
return signedBytes;
}
}