Digital signature

[pamapa]

part of #237

I am not so sure if the taken approach is the right way to:

• PdfItem.WriteObject vs PdfItem.Write seems error prone
• is the position tracker approach really needed, can't that be done more elegant?

We current do not need to sign PDFs for our project, therefore i am not investing time for this feature. In order somebody else wants to work on this, i am pushing my changes on a draft branch. This branch is not intended to be merged like it is now...

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[mmmilan]

Are you sure this works? For me, Adobe Reader validates the signature OK, but in the place of the visible signature I got an empty rectangle.

[pamapa]

I'm not 100% sure, its some time ago i did this. At the end we did not need that feature so i stopped and archived this MR as a starting point for somebody else. In the issue i have referenced the places where i did take the code parts from. In general i am not 100% sure if this approach is the right way to do it anyway, as it is very invasive (see description above). So feel free to hack on this and improve it :-)

Have you seen SignatureTestConsole, this might be a good starting point.

[mmmilan]

Thanks for your reply! Yes, I'm fiddling with SignatureTestConsole, I've just added my own certificate. If this had worked for you, I must have messed up something...

[ascott18]

@mmmilan @pamapa Its not visible because XForm.Finish() is not implemented in PdfSharpCore. https://github.com/ststeiger/PdfSharpCore/blob/8b9076c465123bc195c0d844dcec81eafae4fa3f/PdfSharpCore/Drawing/XForm.cs#L178 versus https://github.com/empira/PDFsharp/blob/3205bd933b464d150c0f42e8bcdff3314b6c6164/src/PdfSharp/Drawing/XForm.cs#L231

It also doesn't work in SignatureTestConsole by default, because that project's first target framework is netcoreapp3.1, and in the version of PdfSharpCore that this PR is based on, the particular XGraphics ctor that this work requires just silently doesn't create the renderer unless the TFM is net5.0 or higher: https://github.com/pamapa/PdfSharpCore/blob/b3e115417785be2f84150b14b7210854b0767997/PdfSharpCore/Drawing/XGraphics.cs#L211-L217 (this was fixed recently upstream in https://github.com/ststeiger/PdfSharpCore/pull/295)

If you change SignatureTestConsole to target .NET 6 specifically, and also go add the following implementation of XForm.Finish(), then it works:

        internal virtual void Finish()
        {
            if (_formState == FormState.NotATemplate || _formState == FormState.Finished)
                return;

            Debug.Assert(_formState == FormState.Created || _formState == FormState.UnderConstruction);
            _formState = FormState.Finished;
            Gfx.Dispose();
            Gfx = null;

            if (PdfRenderer != null)
            {
                //pdfForm.CreateStream(PdfEncoders.RawEncoding.GetBytes(PdfRenderer.GetContent()));
                PdfRenderer.Close();
                Debug.Assert(PdfRenderer == null);

                if (_document.Options.CompressContentStreams)
                {
                    _pdfForm.Stream.Value = Filtering.FlateDecode.Encode(_pdfForm.Stream.Value, _document.Options.FlateEncodeMode);
                    _pdfForm.Elements["/Filter"] = new PdfName("/FlateDecode");
                }
                int length = _pdfForm.Stream.Length;
                _pdfForm.Elements.SetInteger("/Length", length);
            }
        }

[mmmilan]

It works! Thank you!

[pamapa]

@ascott18 nice work