[Poppler-bugs] [Bug 99416] Sign PDF with digital signature

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=99416

--- Comment #17 from Adrian Johnson <ajohnson at redneon.com> ---
(In reply to Hans-Ulrich Jüttner from comment #16)
> > FormWidgetSignature::prepareSignature is writing the entire PDF to memory.
> > It would be better to write it to a temp file then search it on disk. I
> > don't like the way you search the PDF file for matching strings. It could
> > end up matching some random data in another stream. It would be better to
> > get the offset of the dict object you want to search and start the search
> > for there for up to some reasonable maximum.
> > 
> > eg
> > 
> > XRefEntry *entry = doc->getXRef()->getEntry(ref);
> > Goffset objOffset = entry->offset;
> > 
> > Also, don't search for %%EOF to get the file size. Use
> > doc->getBaseStream()->getLength() or Gfseek(f, 0, SEEK_END), Gftell().
> > 
> 
> Ok, but is it guaranteed that this offset and the length of
> doc->getBaseStream()
> are always updated when some objects in the PDF have been changed?

After saving the PDF, open the saved PDF in a new PDFDoc object.

> > I don't fully understand what signDocument() and prepareSignature() are
> > doing. Which means some comments would be helpful. It appears you are saving
> > the PDF, computing the signature, updating the signature object, then saving
> > the PDF again. This assumes that the second save will produce an identical
> > file except for the updated signature. I'm not sure if we can assume this
> > will always be true.
> > 
> 
> As the user later wants to save the signed document for example with the
> PDFConverter from the Qt5 interface, saving the document again has to produce
> an identical byte stream. Otherwise the signature would no longer be valid.

I don't really understand how this works. Do you mean save a copy just after we
signed the document? Or save any PDF that already has a signature? If we want
to save an identical copy with the signature preserved we should just copy the
file. It will be a lot faster. It may work now but what happens if we decide to
always update the ModDate each time the PDF is saved?

> > My naive understanding is that we could just create a signature dict with a
> > dummy signature and offsets (large enough for any size, XRef allows up to 10
> > digits), save the PDF, compute the signature, and overwrite the dummy
> > signature and offsets in the disk file with the real signature. What am I
> > missing?
> 
> I was striving not to waste to much space for the signature. That's why I
> calculated a signature with the user's certificate and a hash over an empty
> string first to see how much space it will take. I had the problem that the
> byte range object itself changed it's size in the byte stream when some
> number
> in it gets an additional digit. Therefore I recalculated the byte range and
> after it the signature once again. May bee this can be avoided if some extra
> bytes are added to the space reserved for the signature.

A byte offset can not exceed 10 digits. Typically at least 6 digits will be
used. I don't think it is worth trying to save a few bytes.

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