Caesar cipher
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An early and simple substitution cipher was the Caesar cipher, in which each letter in the plaintext was replaced by a letter some fixed number of positions further down the alphabet. It was named after Julius Caesar who used the cipher with a shift of 3 in order to communicate with his generals during his various military campaigns.
For example, with a shift of 3, "BAD DOG" would become "EDG GRJ". This type of encryption is trivial to cryptanalize using frequency analysis.
One Caesar cipher is still in use; rot 13 is used to hide "spoilers" such as the ending of a movie in Internet discussions. It is not used for any serious security.