Читать книгу Cryptography, Information Theory, and Error-Correction - Aiden A. Bruen - Страница 48

To demonstrate this method for finding , suppose we received the following cipher text:

EHMVL	VDWLP	WIWXW	PMMYD	PTKNF	RHWXS
LTWLP	OSKNF	WDGNF	DEWLP	SOXWP	HIWLL
EHMYD	LNGPT	EEUWE	QLLSX	TUP

Our task is to search for repetitions in the above text. For a small cipher text, the brute‐force method is not too difficult. We focus on trigrams, and highlight some as follows:

After having found these, we compute the distances between the trigrams.

EHM	‐	60
WLP	‐	25,15,40
XWP	‐	39
MYD	‐	45

We note that with the exception of 39, 5 divides all of the distances. In fact, if we proceed with frequency analysis, it turns out that we can decipher this message with a key length of 5. The codeword is “ladel,” and the plain text is “Thor and the little mouse thought that they should douse the house with a thousand liters of lithium” (Who said that secret messages have to have a clear meaning!) Frequency analysis is used in our next example below.

It is purely by chance that we had the repeated trigram WXP – this repeated trigram was not the result of the same three letters being enciphered by the same part of the keyword. This highlights the fact that the above method is probabilistic.