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#### Obfuscating PDFs typeset with TeX, or modifying the underlying text in a PDF

I'd like to create PDFs using (La)TeX which are readable by humans but
cannot be read by naive automated techniques. For example, it should
not be possible to copy plain text out of the document. One easy way
to do this, I realize, would be typesetting the document normally then
taking a screenshot of each page and creating a new document
containing only images. Unfortunately, should the (human) reader want
to zoom in, he would find that the resolution is limited and
everything would become blocky.

A while back I did a Google search to determine how to obfuscate PDFs
in this way. I found a resource that described a simple and beautiful
idea: apply a permutation to the font glyph data and the inverse
permutation to the text in the TeX source. For example, suppose that
the glyph data for characters "e" and "l" were interchanged. Then, the
TeX source

Hleeo, wored!
\bye

would be used to produce what appears to be "Hello, world!" to a human
but, barring the implementation of OCR, "Hleeo, wored!" to a computer.

This will not yield exactly the same results, of course. I suppose
certain "professional" features of TeX such as kerning, ligatures, and
intelligent line breaking would be broken. However, this I am willing
to sacrifice.

I am somehow no longer able to locate this resource! Either it has
been taken offline, or it is simply eluding my efforts to find it. In
the latter case, can someone else direct me to the resource? Or, can
someone suggest another way of accomplishing my goal?

I notice that a lot of older papers available online appear not to
have been typeset in TeX but rather scanned from journals and the
like. However, it is still possible to copy the text out of them,
suggesting that they have been run through OCR and the text obtained
added somehow. Does this mean there is an easy way of independently
setting the text which would be obtained by copying? If so, this could
be another solution to my problem. Does anybody know how can it be
done?

 0
ng3r9h7w (2)
9/12/2009 6:43:58 PM
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J. Random Hacker wrote:
> Or, can
> someone suggest another way of accomplishing my goal?

I am not sure I understand completely what you are trying to do. But
newer pdf-versions have /ActualText that you can use to set the text
being copied in copy/paste. This seems to work with Adobe Reader 9.1 on
my system:

\pdfliteral direct{/Span << /ActualText (Hleeo, wored!) >> BDC }%
Hello World!%
\pdfliteral direct{EMC }%
\bye

 0
mr_heller (370)
9/12/2009 11:04:46 PM
On Sep 12, 7:43=A0pm, "J. Random Hacker" <ng3r9...@gmail.com> wrote:
> I'd like to create PDFs using (La)TeX which are readable by humans but
> cannot be read by naive automated techniques.

Whatever you do, as long as it is possible to print the document (to a
file) and run OCR on it, all is lost (unless you typeset the document
in a "CAPTCHA" style). Newer versions of Acrobat come with OCR built-
in.

> someone suggest another way of accomplishing my goal?

Convert all fonts to paths or in other words all text to graphics. The
same basic outcome as rasterizing the pages but with vector graphics
as the end result.

Cheers,

Tomek

 0
t34www (104)
9/13/2009 12:25:12 PM
On 2009-09-13 04:13:58 +0930, "J. Random Hacker" <ng3r9h7w@gmail.com> said:

> Does anybody know how can it be
> done?

The randtext package was designed for this:
<http://www.ctan.org/tex-archive/help/Catalogue/entries/randtext.html>

I think Adobe Reader might be clever enough to bypass its obfuscation,
however. (But automated scanning tools would presumably still be
foiled.)

Hope this helps,
Will


 0
wspr81 (1209)
9/13/2009 1:24:21 PM
Thanks guys for your responses.

On Sep 13, 8:25=A0am, T3X <t34...@googlemail.com> wrote:
> On Sep 12, 7:43=A0pm, "J. Random Hacker" <ng3r9...@gmail.com> wrote:
>
> > I'd like to create PDFs using (La)TeX which are readable by humans but
> > cannot be read by naive automated techniques.
>
> Whatever you do, as long as it is possible to print the document (to a
> file) and run OCR on it, all is lost (unless you typeset the document
> in a "CAPTCHA" style). Newer versions of Acrobat come with OCR built-
> in.
>
> > someone suggest another way of accomplishing my goal?
>
> Convert all fonts to paths or in other words all text to graphics. The
> same basic outcome as rasterizing the pages but with vector graphics
> as the end result.
>
> Cheers,
>
> Tomek

Anyway, I am assuming that I am not up against OCR. I'm a student who
has to submit assignments to Turnitin and I resent being forced to
enable Turnitin to use my work for commercial purposes. Since Turnitin
allows submitting as PDF, I would like to obfuscate the files I submit
so that the actual content of my assignments cannot be kept for
Turnitin's use. (Incidentally, this would also defeat the plagirism
detection if it worked, but this is not my actual purpose - honest!) I
suspect that Turnitin only does a quick-and-dirty automated conversion
to text and I highly doubt that they would go to the trouble of
implementing OCR because it would be unnecessary in 99.9% of cases.
(This is for an English class; the teacher would have no idea what is
going on, especially since she can download the PDF directly from the
Turnitin web site and see that there's "nothing wrong with it")

The randtext package is pretty good - I tried an online PDF to text
converter and it was certainly fooled. It does however have two faults
I noticed that make it difficult to use: one, it only properly handles
plain text as its argument; for example, if something in \emph{} is in
its argument, the macro expansion fails to produce correct TeX, and
two, spaces within its argument must be escaped as "\ " otherwise they
seem to be ignored (and no space is produced between pairs of adjacent
words).

So I'm most interested in your suggestion - how would I do this?

 0
ng3r9h7w (2)
9/13/2009 2:49:51 PM
On 2009-09-12 at 20:04 ADT, Martin Heller <mr_heller@yahoo.dk> wrote:
> J. Random Hacker wrote:
>> Or, can
>> someone suggest another way of accomplishing my goal?
>
> I am not sure I understand completely what you are trying to do. But
> newer pdf-versions have /ActualText that you can use to set the text
> being copied in copy/paste. This seems to work with Adobe Reader 9.1 on
> my system:
>
> \pdfliteral direct{/Span << /ActualText (Hleeo, wored!) >> BDC }%
> Hello World!%
> \pdfliteral direct{EMC }%
> \bye

Unfortunately, xpdf (and possibly whatever turnitin uses) grabs the
"expected" text, even with your code above.

However, what you have above is interesting in its own right.

Cheers.

Jim

 0
9/13/2009 6:10:07 PM

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