Caesar cipher

Caesar cipher is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

Implement a Caesar cipher, both encryption and decryption. The key is an integer from 1 to 25. This cipher rotates the letters of the alphabet (A to Z). The encryption replaces each letter with the 1st to 25th next letter in the alphabet (wrapping Z to A). So key 2 encrypts "HI" to "JK", but key 20 encrypts "HI" to "BC". This simple "monoalphabetic substitution cipher" provides almost no security, because an attacker who has the encrypted message can either use frequency analysis to guess the key, or just try all 25 keys.

Caesar cipher is identical to Vigenère cipher with key of length 1. Also, Rot-13 is identical to Caesar cipher with key 13.