Probably the first substitution cipher to be described is that used by Julius Caesar in parts of his private correspondence to friends and colleagues in Rome while he was campaigning in Gaul. The code Caesar used to plan a political intrigue upon his return to Rome was described by Suetonius in his Twelve Caesars 150 years later. It involves a simple “shift” of the alphabetical order (known as the algorithm); if the letter A is to be encrypted, and the shift used is four letters, then A appears encrypted as E, M as Q, and so on. The Caesar Shift cipher therefore has 25 potential ciphers, and in turn 25 separate keys depending on which shift is chosen. However, it is not very secure: cryptanalysts, if they suspect a simple Caesar Shift has been used, only have to check 25 potential keys. — Excerpted from The Book of Codes, page 103.
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