Ceasar Cipher

Ceasar Cipher: Being one of the first documented encryption algorithms, works based on substitution. By means of a key of addition or subtraction of indexes, the encryption is obtained and with the reverse operation the deciphering. 

Example: 

Encryption key: letter + 1 
HELLO -> IFMMP 
When deciphering, just use: letter - 1 
IFMMP -> HELLO