Cryptography Reference
In-Depth Information
the search tree appear to be very short. Additional tests for greater search
depth are missing. This is a point where the method could be significantly
speeded up.
Conclusions
It is remarkable that the actual encryption method plays a role only in two
expressions — see the 'ˆ' (XOR) character in lines 99, 100, and 107 through
109. There could be an addition corresponding to the Vigenere method in the
classical sense instead. With true polyalphabetic methods, the set of substitu-
tions possible could be limited, at least with long files. However, the program
would then have to work more effectively, and it would be much more complex.
compress
is not the only compression program around.
pkzip
, which is popular
in the DOS world, also implements the Ziv - Lempel algorithm, among other
things. Of course, the file format differs from that of
compress
. Nevertheless,
the attack remains basically the same. The well-known Huffmann method,
implemented in
pack
for UNIX, writes character frequencies to a header and
subsequently appends a bit stream.
gzip
is a free and very effective program
(available for UNIX and DOS) and is also based on a Ziv - Lempel algorithm.
Each one of these methods requires a different approach.
You can see that, for the examples discussed in this section, we don't need any
information about the plaintext, except that it was compressed with
compress
.
We need to look at the text itself only if there are several possible passwords
and we can't guess the right one,
and
if several passwords let us decompress
the plaintext. Naturally, the code writer can select a different compression
method and add cryptological elements to it, for example, 'disturbing' bits. A
prerequisite is that the code writer knows exactly how a cryptanalyst would
proceed against his method. This means that he would basically jump out of
the frying pan into the fire.
Making a method appear more complicated by previously compressing things
can sometimes have the opposite effect. As a sideline, compression also makes
a brute-force search much easier. We will get back to this issue in Section 4.4.1.
After all, 'more complicated' doesn't always mean 'more secure'.
3.7
fcrypt
: How Differential Cryptanalysis Works
The discussion in this section takes us back to
fcrypt
, the encryption method I
mentioned at the end of Chapter 2, which is described in detail in [Wobfcrypt]
