Previous | Next | (P-PDF) Acrobat 6.0
Topic: Re: Effective security in Acrobat 6
Conf: (P-PDF) Acrobat 6.0, Msg: 110671
From: vkatalov
Date: 4/30/2004 06:42 PM
>The Elcomsoft approach is
>essentially to modify an
>already open document. In order
>to display a document, you will
>at one time have to open, and to
>expose its contents. This means that this
>point is the place where anything can be
>attacked (if that term is
>appropriate).
Sorry, this is not correct. The approach you described has been used only in our "Advanced eBook Processor" product (which is not available anymore -- after our DMCA case), and only for files encrypted using 3rd party plug-ins. I think this is a major fail in Adobe security model: if someone can open (view) PDF file, even very resticted, then it can create a completely unprotected copy. Simply because the whole PDF file is encrypted using single RC4 key (which should be known to viewer such as Adobe Reader) -- it is being returned by security plug-in.
Our current software (Advanced PDF Password Recovery) does not require the PDF file to be opened -- it obtains the key directly from PDF file. This is applicable to documents protected with "owner" password only, of course.
>If the document has an opening
>password, the Elcomsoft
>software does
>rely on the user to provide
>that password. Otherwise, it
>has to use the brute-force approach,
>which takes more than exponentially
>more time the longer the opening
>password is. And if you add some extra
>characters, even the dictionary search
>approach (which is a considerable
>speedup) will fail.
If 40-bit protection has been used, it is possible (also with our software, as well as a few other programs) to decrypt the file, so removing the "open" password, by trying all possible keys. It takes only a few days on good desktop PC.
>It gets even more extreme with
>a document which uses digital
>signatures/certificates for
>protection. Without the
>private key, whose
>public key has been used to
>encrypt the document, such
>documents can not be opened.
>Period.
Correction: digital signatures could not be used for protection. Period ;) One can add them only to ensure the "authority" of the documents, and also to detect changes in the file. Only certificates (public/private key pairs) really protect.
>However, once the document is
>opened for
>viewing, you do have access to
>it.
Absolutely correct.
>This also applies for any kind
>of third-party securing
>software.
Correct as well ;)
> I know
>of solutions which actually
>verify first if any known
>"protection
>removing" software is
>installed on the user's
>machine, and, if so, they
>simply refuse to work.
The only PDF protection that does that I know of is FileOpen plug-in. However, FileOpen protection (including one implemented in expensive and "very secure" WebPublisher) can be removed either instantly or in a matter of a few hours -- without Adobe Reader, FileOpen plug-in itself or whatever. I'd call it "snake-oil" (see Bruce Schneier's definition of the term) because of multible vulnerabilities. Their main protection is DMCA ;)
--
Sincerely yours,
Vladimir
Vladimir Katalov
Managing Director
ElcomSoft Co.Ltd.
Member of Association of Shareware Professionals (ASP)
Member of Russian Cryptology Association
mailto:vkatalov@elcomsoft.com
http://www.elcomsoft.com (Corporate site)
http://www.crackpassword.com (Password Recovery Software)
