The method is named after Julius Caesar, who used it in his private correspondence. According to Suetonius, he used it with a shift of three to protect messages of military significance. A Caesar cipher is one of the simplest and most widely known encryption techniques. It is a type of substitution cipher in which each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet. For example:
Plain: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Cipher: X Y Z A B C D E F G H I J K L M N O P Q R S T U V W
Plaintext: attack the eastern camp at sunrise
Ciphertext: xqqxzh qeb bxpqbok zxjm xq prkofpb
The replacement remains the same throughout the message, so the cipher is classed as a type of monoalphabetic substitution, as opposed to polyalphabetic substitution.