

Task

Implement a Caesar cipher, both encoding and decoding. 

The key is an integer from 1 to 25.

This cipher rotates (either towards left or right) the letters of the alphabet (A to Z).

The encoding 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 "mono-alphabetic substitution cipher" provides almost no security, because an attacker who has the encoded message can either use frequency analysis to guess the key, or just try all 25 keys.

Caesar cipher is identical to Vigenère cipher with a key of length 1. 

Also, Rot-13 is identical to Caesar cipher with key 13.



Related tasks

 Rot-13
 Substitution Cipher
 Vigenère Cipher/Cryptanalysis


open number char monad io string
 
chars = "ABCDEFGHIJKLMOPQRSTUVWXYZ"
 
caesar _ _ [] = ""
caesar op key (x::xs) = check shifted ++ caesar op key xs
  where orig = indexOf (string.upper $ toString x) chars
        shifted = orig `op` key
        check val | orig == -1 = x
                  | val > 24 = trans $ val - 25
                  | val < 0 = trans $ 25 + val
                  | else = trans shifted
        trans idx = chars:idx
 
cypher = caesar (+)
decypher = caesar (-)
 
key = 2
 
do
  putStrLn "A string to encode:"
  str <- readStr
  putStr "Encoded string: "
  cstr <- return <| cypher key str
  put cstr
  putStrLn ""
  putStr "Decoded string: "
  put $ decypher key cstr