Cryptography, Information Theory, and Error-Correction. Aiden A. Bruen

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      where is the cipher key, with . In the case of the Caesar cipher, . If is negative, then, as explained above, .

      To break such a cipher, one can decrypt the message by trying all 26 keys (this is referred to as an exhaustive search). For long messages, the likelihood of a cipher text decrypting to two intelligible messages is small.

      We mention here briefly affine ciphers. They are similar to Caesar ciphers in that they are simple substitution ciphers, but they differ in that enciphering involves not only addition, but also multiplication as well.

2.3 The Scytale Cipher

      The Scytale cipher was introduced around 500 BCE by the Spartans, who used this rather simple but effective method to send crucial planning data between generals and bureaucrats. Both the sender and receiver were in possession of a cylindrical tube of the same diameter. To encode the message, the sender would wrap a thin strip of paper around the tube, with the paper spiraling its way down the length of the tube. The message was then written on the strip, with letters being written one beneath the other until the end of the tube was reached. (The message was then continued by starting a new column of letters, and this process was repeated until the message was finished.) To encode, the sender would simply unwrap the paper, leaving a thin strip of unintelligible letters. To decode, the receiver only had to wrap the paper around their similar tube and read the message off in columns.

      It is often much simpler to duplicate the Scytale process with pencil and paper. Using a preselected number of rows (this number is the cipher key), write the message in columns. Then, “unwrap” the message by writing a string of characters consisting of the concatenated rows. For example, the message “THE ENEMY WILL ATTACK AT DAWN” is encrypted as follows: