Cryptography Reference
In-Depth Information
You can see that the method can be recovered within one to two hours at most
when proceeding cleverly. This is not an exceptional case. Vigenere methods
or modifications are thought to still be very popular in MS-DOS and Mac
software. The manufacturers say that their method 'is almost as secure as DES,
only much faster'. Though the second part of this statement may be true, let's
show compassion for the first part.
How do you break the WordPerfect encryption? The most obvious method is to
simply try all password lengths. There aren't many since the program limits the
maximum admissible length to 23. We use the statistical method (as described
in Section 2.4.1) to try and find the key for each length. This is not a problem,
for there is plenty of material to analyze — almost all WordPerfect files are
many Kbytes long — and there are sufficient peculiarities in how the characters
are distributed in these files.
But it can be even
simpler
than that. WordPerfect 4.0 is thought to have
encrypted only the text itself. In contrast, Version 5.1 'protects' headers, too.
The motivation for this expansion may have been to also encrypt the business
addresses included in the styles (which are included in the headers). But the
header contains plenty of known bytes that can be exploited in a plaintext
attack. We will see exactly how this is done in the next section.
3.5.2 The
newwpcrack
Program
WordPerfect encryption is known not only on the Internet; there are lots of
programs you can get for free that can break it. One of these programs was
written by Ron Dippold in 1991 and is included on the Web site to this topic.
As usual, nobody will guarantee the software's reliability, but that's not the
point. Even a success rate of 50 % would show that the method works.
Unaware of this program, I had developed and published one of my own
[Wobsymm]. I'll introduce an improvement in this section. The program is
still short (only 70 lines; see listing in Figure 3.10), but it shows how several
methods can be combined. Since cryptanalysis is very specific, you should
take your time and look at the details, even if you don't think you'll ever use
WordPerfect.
How does
newwpcrack
work?
•
We first have to identify the bytes that are constant in WordPerfect files.
To find these bytes, I used a program called
wph.c
, a program found on
our Web site. It compares bytes 17 through 56 of several WordPerfect
