Like word documents, .PDF
files created by Adobe's Acrobat software can be protected by two different
levels of passwords. A 'user' password prevents anyone from opening the
file unless they provide the correct identification, while an 'owner' password
allows reading but prevents various forms of modification unless the correct
password is entered.
Both of these forms of protection are
susceptible to dictionary and brute-force attacks, as are most other programs
that offer password protection. PDF files protected by older versions of Acrobat are
also vulnerable to key searches, in which each possible numerical key
(generated from the password to encrypt and decrypt the document) is tried.
Newer versions include support for encryption up to
128-bits, making this kind of password recovery impractical due to the amount of
time it would take. Many products can instantly remove 'owner' passwords
as long as the PDF document is unprotected by a 'user' password.
There do not appear to be any
freeware or open source .PDF password recovery programs easily available as of
now. Several companies offer professional password defeating and recovery
software products targeted at PDF files.
Here are a few examples which
have trial versions available:
Elcomsoft's Advanced PDF password Recovery Pro
AdultPDF.com's PDF password
GuaPDF program, like GuaWord, strips password protection from PDF documents by
using a key search. Useful if the password is too complex to be revealed with a
brute-force attack program. It
also strips 'owner' password automatically.
word about securing documents
If you are concerned about the safety of data that
you have password protected, the best step you can take is to
enforce complex passwords. While a password like 'sesame' can be broken a few minutes
or less by most modern password recovery programs, something more
complex like 'SeSam3E$eed' will take a prohibitive amount of time for any program to
Note the inherent weakness of
the security features of older versions of Microsoft Office and Adobe Acrobat to 'key-search'
attacks. If you are using these
programs and want to make sure your data is safe, strong passwords will not
It is also
possible to use some of the third party encryption programs we
list a little later in this article. Microsoft Office XP contains
enhanced security features which make cracking the password by key-searching
(matching the numerical key used to create the encryption) practically
impossible. Please keep in mind that there is a balance here. The
harder you make your passwords, the harder it will be to recover from a
forgotten password. Professional services are available, but there is a
limit to what they can do with a well-designed password backed up by solid
If you want to make sure that
your documents are protected from other methods of password cracking, like key-searches,
there are a couple of ways to do this.
If you are using Microsoft's Office XP, Word and Excel documents can take
advantage of XP's increased security features.
These programs use 128-bit
encryption, making the documents effectively impossible to crack by means of a key
search except with a few super-computers.