2 - From Julius Caesar to simple substitution

Summary

Julius Caesar ciphers and their solution

In the Julius Caesar cipher each letter of the alphabet was moved along 3 places circularly, that is A was replaced by D, B by E … W by Z, X by A, Y by B and Z by C. Although Julius Caesar moved the letters 3 places he could have chosen to move them any number of places from 1 to 25. There are therefore 25 versions of the Julius Caesar cipher and this indicates how such a cipher can be solved: write down the cipher message and on 25 lines underneath it write the 25 versions obtained by moving each letter 1, 2, 3, …, 25 places. One of these 25 lines will be the original message.

Example 2.1

The text of a message enciphered by the Julius Caesar System is

VHFX TM HGVX

Decrypt the message.

Solution

We write out the cipher message and the 25 shifted versions, indicating the shift at the left of each line (see Table 2.1), and we see that the cipher used a shift of 19, for the cipher text is shifted 7 places to give the plain and this means that the plaintext has to be shifted (26–7) = 19 places to give the cipher. It looks very likely, on the assumption that no other shift would have produced an intelligible message, that we have correctly decrypted the message and so there is no point in writing out the remaining lines.