How to add invisible signature to existing PDF

[Kamil Zmeskal]

Hi ,
I cannot figure out how to add an invisible signature to an existing PDF.

My test code:

var pdfIn = "C:\\Prg\\Test\\TestTelerikPDF\\pdf\\demo.pdf";
var pdfSigned = $"{pdfIn}.sign.pdf";
var pfx = "C:\\Prg\\Test\\TestTelerikPDF\\pdf\\Cert.pfx";

using (var input = new FileStream(pdfIn, FileMode.Open, FileAccess.Read))
{
    var document = new PdfFormatProvider().Import(input);

    var certificate = new X509Certificate2(pfx, "passs");
    var signatureField = new SignatureField("Blabla");
    signatureField.Signature = new Signature(certificate);

    document.AcroForm.FormFields.Add(signatureField);

    var settings = new PdfExportSettings();
    settings.ComplianceLevel = PdfComplianceLevel.PdfA3B;

    var formatProvider = new PdfFormatProvider();
    formatProvider.ExportSettings = settings;

    using (var ms = new FileStream(pdfSigned, FileMode.Create))
        formatProvider.Export(document, ms);
}

This code create an output PDF but the signature is missing :-(

Can you help me please?

Best

Kamil 

[Yoan]

Hello Kamil,

By default, a document with an invisible signature must have its signature flags set. With this functionality, you can specify that there is a signature, even if the signature itself does not have a visual representation. You can read more about it in the New Features in the Telerik PdfProcessing Library in R2 2022 blog or in the Signature Flags section in the documentation.

If you are interested in a visual representation of the signature and a full, detailed example, you might find the Signing a document with a digital signature topic helpful.

If you happen to have any further questions, feel free to contact us.

Regards, Yoan Progress Telerik