The Caesar cipher is named after
Julius Caesar, who, according to
Suetonius, used it with a
shift of three to protect messages of military significance:
If he had anything confidential to say, he wrote it in
cipher, that is, by so changing the order of the letters of the
alphabet, that not a word could be made out. If anyone wishes to
decipher these, and get at their meaning, he must substitute the
fourth letter of the alphabet, namely D, for A, and so with the
others. — Suetonius, Life of Julius Caesar 56
[1].