Cryptography Reference
In-Depth Information
Had the cryptologists been a bit more careful, they would have encrypted only
the text itself to at least prevent plaintext attacks. Even then, however, it would
still be easy to break the code: you will see how and by what program in
Section 3.6.
3.6 The Vigenere Method Under the Magnifying Glass
The Vigenere method is very simple and (unfortunately) still often used.
3
This
is why it is a good and interesting candidate for discussing a few problems
in cryptanalysis. Similarly to the previous section, you will learn a specific C
program, but one that's much more powerful and universal. Again, we will limit
ourselves to using the 'bitwise Vigenere method', also called simple XOR.
3.6.1 The Index of Coincidence Supplies the Period Length. The
Kasiski Method
We determined the method's period length experimentally both in the discus-
sion of the Vigenere method in Section 2.4.1 and in deciphering encrypted
WordPerfect files in the previous section. But things would be much faster and
simpler if we considered the
index of coincidence
, also called
kappa
. Though
this sounds very scientific again, it's very simple.
If we write two equally long texts,
T
1
and
T
2
, in two lines one on top of the
other, then the kappa of both texts is defined as follows:
number of coinciding characters
number of characters in the text
κ
=
More specifically, it looks like this:
this is the first text
and this is the second
...........*..........
The two texts coincide only in one character (a blank) and are 22 characters
long each. Consequently, the kappa of the two texts is 1/22, or 4.5 %. The
special thing about this quantity kappa is that it always has roughly the same
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For example, the
wincrypt
program uses a 512-byte Vigenere key.
