I think you mean Caesar Cipher. It's a simple encryption scheme based on substitution (which means you replace one character with another, for example, replace all the letter 'A' in the plaintext message with 'Z', and 'B' with 'Y', and 'C' with 'X', and so on.). You need to make sure no two letters in your plaintext (unencrypted) alphabet encrypts into the same letter in the encrypted alphabet and vice versa.