PDF Security/Encryption with LuaLaTeX

Question

I was wondering whether it is possible to protect pdf files (e.g. control printing/copying/... access) with LuaLaTeX without using third-party software like QPDF or those listed in the pdfcrypt package.

Some older post says its possible with the \special command, but its example uses XeTeX and while testing with LuaLaTeX its not really working as expected. They indeed say probably there is no equivalent for LuaLaTeX:

\special{pdf:encrypt ownerpw (abc) userpw (xyz) length 128 perm 2052}
\documentclass{article}
\begin{document}
This is a test.
\end{document}

Is it maybe possible with LuaLaTeXs backend primitives like \pdfextension?

I'd be grateful if someone can show me a MWE if its possible with LuaLaTeX only.

Thanks,
Dave

Relevant discussion at https://tex.stackexchange.com/questions/19797/pdf-with-un-copyable-text-in-lualatex?rq=1 After you encrypt your pdf, send it to me and I will OCR it and decrypt it! — yannisl, Dec 23 '23 at 03:34
@YiannisLazarides "Encrypt" means "set a password for the PDF" in this context, not "prevent the text to be copied". — user202729, Dec 23 '23 at 06:04
@Dave I don't think it's possible, nevertheless, you may be happy with workarounds such as using an external tool but call it within the LaTeX run itself, such as seen in compiling - Can TeX post-process the PDF it produces? - TeX - LaTeX Stack Exchange — user202729, Dec 23 '23 at 06:33

score 3 · Answer 1 · answered Dec 24 '23 at 00:14

Thanks to @user202729 I was able to automate securing/encrypting pdf files via QPDF (Documentation) within LuaLaTeX.

During post-processing of the PDF in the wrapup_run phase, the output directory is read via Lua's pattern matching:

local outdir = nil
for i = 0, #arg, 1 do
    if string.find(arg[i], "%-+output%-d[irectory]*%=?%s?") then
        outdir = string.gsub(arg[i], "%-+output%-d[irectory]*%=?%s?", "")
    end
end

Note: I cannot say exactly whether all argument formats are covered by this, at least all formats that @user202729 has posted here are recognised.

QPDF is then executed with the desired arguments:

os.execute((outdir ~= nil and "cd " .. outdir .. " &&" or "") .. "qpdf " .. tex.jobname .. ".pdf --encrypt userpw ownerpw 256 --accessibility=y --assemble=y --extract=y --form=y --modify-other=y --print=full -- " .. tex.jobname .. "_secure.pdf")

Full MWE (Windows 10):

\documentclass{book}
\usepackage{luacode}
\begin{luacode}
    luatexbase.add_to_callback("wrapup_run", function()
        local outdir = nil
        for i = 0, #arg, 1 do
            if string.find(arg[i], "%-+output%-d[irectory]%=?%s?") then
                outdir = string.gsub(arg[i], "%-+output%-d[irectory]%=?%s?", "")
            end
        end
        os.execute((outdir ~= nil and "cd " .. outdir .. " &&" or "") .. "qpdf " .. tex.jobname .. ".pdf --encrypt userpw ownerpw 256 --accessibility=y --assemble=y --extract=y --form=y --modify-other=y --print=full -- " .. tex.jobname .. "_secure.pdf")
    end, "Callback to secure pdf")
\end{luacode}
\begin{document}
    Hello World!
\end{document}

PDF Security/Encryption with LuaLaTeX

1 Answers1