Re: Encrypted PDF isn't

[David Wright <deblis@lionunicorn.co.uk>]

On Wed 24 Feb 2016 at 23:10:42 (+0000), Lisi Reisz wrote:
> On Wednesday 24 February 2016 21:16:46 David Wright wrote:
> > encrypted.pdf
> 
> No, sadly it is not!
> 
> Sorry, David. ;-)

Well it turns out that the problem has affected other people, eg
http://superuser.com/questions/983368/both-pdftk-and-qpdf-fail-at-protecting-my-documents
(still unanswered), but
https://en.wikipedia.org/wiki/Portable_Document_Format#Security_and_signatures
knows the answer. You have to encrypt with
$ pdftk secret.pdf output /tmp/encrypted.pdf user_pw foo

The man page for pdftk is therefore quite wrong when it says:

    Encrypt a PDF using 128-bit strength (the default), withhold all permissions (the default)
       pdftk 1.pdf output 1.128.pdf owner_pw foopass

The owner_pw seems to be quite farcical when writing a pdf.
However, if you set both passwords (with different keys), giving
either key will decrypt it:

$ pdftk secret.pdf output /tmp/encrypted.pdf user_pw foo owner_pw bar
$ pdftk /tmp/encrypted.pdf input_pw foo cat output /tmp/u-used.pdf
WARNING: The creator of the input PDF:
   /tmp/encrypted.pdf
   has set an owner password (which is not required to handle this PDF).
   You did not supply this password. Please respect any copyright.
$ pdftk /tmp/encrypted.pdf input_pw bar cat output /tmp/o-used.pdf
$ 

(Both /tmp/u-used.pdf and /tmp/o-used.pdf are decrypted.)

One other wrinkle: xpdf seems to have problems distinguishing its
options from its arguments:

$ xpdf /tmp/encrypted.pdf 
Command Line Error: Incorrect password

$ xpdf -opw bar /tmp/encrypted.pdf 
error: "bar" file not found

$ xpdf -opw foo /tmp/encrypted.pdf 
error: "foo" file not found

$ xpdf /tmp/encrypted.pdf -opw foo
Command Line Error: Incorrect password

$ xpdf /tmp/encrypted.pdf -opw bar          (works)

$ xpdf -upw bar /tmp/encrypted.pdf 
Command Line Error: Incorrect password

$ xpdf -upw foo /tmp/encrypted.pdf          (works)

$ xpdf /tmp/encrypted.pdf -upw bar
Command Line Error: Incorrect password

$ xpdf /tmp/encrypted.pdf -upw foo          (works)

$ xpdf -upw /tmp/encrypted.pdf              (maps a grey window)

$ xpdf -opw /tmp/encrypted.pdf              (maps a grey window)

$ 

Mind you, though wheezy's xpdf behaves like this too,
the jessie one is far more broken: most of the interactive
commands and many of the options don't work anymore. Does
the stretch version work any better?

Cheers,
David.