Cryptography Reference
In-Depth Information
Where
p
1
represents the plaintext character 'L';
p
5
represents 'a';
c
1
and
c
5
denote the pertaining ciphertext characters;
s
represents the key character for
this position; and
⊕
denotes the bitwise XOR according to the operation
ˆ
in
the C language. We XOR the left and right sides of both equations and obtain
⊕
⊕
p
1
p
5
=c
1
c
5
This is a good criterion for checking a position in the text, since we already
know that
p
1
⊕
p
5
=
'
L
'
⊕
'
a
'. We will naturally run this test on the other
character pairs, too. If no possible position at all results, we have to try it with
a different period length.
Once we've eventually found the correct position of the word, we use it to
reveal the correct key, since the Vigenere method is not resistant to plaintext
attacks: the plaintext length is greater than the period length so that we can
compute the key (plaintext XOR ciphertext) directly. Again, it is remarkable
that this approach can do totally without statistic analyses.
3.5 WordPerfect Encryption as a Modern Example
The WordPerfect word processor let's you encrypt your files just like many other
application programs. Though the method hadn't been disclosed, it appeared
on the Internet nevertheless. First I suspected somebody might have reverse-
engineered parts of the program, but then I realized that this effort wasn't nec-
essary. Finding out this method is so unbelievably simple that I want to briefly
demonstrate it here without qualifying you for a hacker (after all, I'm not one
either). All you need is the right intuition. I once used WordPerfect Version 5.1
under UNIX (it's equivalent to the same version for other operating systems).
3.5.1 The Encryption Method: How to Find It, and How
to Break It
Let's first create a simple text in WordPerfect 5.1. For example, we opt for
lines composed of 'A's only:
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
...
