Cryptography Reference
In-Depth Information
In the simplest systems of the Vigenère type, the key is
a word or phrase that is repeated as many times as required
to encipher a message. If the key is DECEPTIVE and the
message is WE ARE DISCOVERED SAVE YOURSELF,
then the resulting cipher will be
In 1861 Friedrich W. Kasiski, formerly a German
army officer and cryptanalyst, published a solution of
repeated-key Vigenère ciphers based on the fact that
identical pairings of message and key symbols generate
the same cipher symbols. Cryptanalysts look for pre-
cisely such repetitions. In the example given above, the
group VTW appears twice, separated by six letters, sug-
gesting that the key (i.e., word) length is either three or
nine. Consequently, the cryptanalyst would partition the
cipher symbols into three and nine monoalphabets and
attempt to solve each of these as a simple substitution
cipher. With sufficient ciphertext, it would be easy to
solve for the unknown key word.
The periodicity of a repeating key exploited by Kasiski
can be eliminated by means of a running-key Vigenère
cipher. Such a cipher is produced when a nonrepeating
text is used for the key. Vigenère actually proposed con-
catenating the plaintext itself to follow a secret key word
in order to provide a running key in what is known as an
autokey.
Even though running-key or autokey ciphers elimi-
nate periodicity, two methods exist to cryptanalyze them.
In one, the cryptanalyst proceeds under the assumption
that both the ciphertext and the key share the same fre-
quency distribution of symbols and applies statistical
analysis. For example, E occurs in English plaintext with
 
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