Since case does not matter for the cipher, we can use the convention that plaintext is represented in lowercase letters, and ciphertext in uppercase. Technically speaking, the Caesar cipher may be differentiated from other, more complex substitution ciphers by terming it either a shift cipher or a mono-alphabetic cipher both are correct. To modern readers, the Caesar cipher is perhaps better known through the Captain Midnight Code-O-Graph and secret decoder rings that even came inside Kix cereal boxes. Perhaps the simplest substitution cipher is the Caesar cipher, named after the man who used it. Substitution ciphers encrypt the plaintext by swapping each letter or symbol in the plaintext by a different symbol as directed by the key. Spring, in Introduction to Information Security, 2014 Substitution Ciphers