Julius Caesar (100–44 BC) systematically used a monoalphabetic shift substitution cipher for his military and political correspondence: each letter of the message was replaced by the letter three positions ahead in the Latin alphabet. Suetonius documents it in The Twelve Caesars (c.121 AD) with enough precision to reconstruct the method. Cicero mentions receiving ciphered letters from Caesar in his own letters (Ad Atticum, c.50 BC). It is the first systematic substitution cipher documented by primary sources close to the event. The Caesar cipher is trivially vulnerable to frequency analysis — as Al-Kindi would demonstrate 900 years later — but established the alphabetic substitution paradigm that dominated Western cryptography until the 20th century.