foo.hs

module SubstitutionCipher where
import Data.List
import Data.Maybe

chunkify :: Int -> [a] -> [[a]]
chunkify _ [] = []
chunkify n xs = hd : chunkify n tl
  where
    (hd, tl) = splitAt n xs

-- | Substitution cipher
--   Example: "abcdefghij" x "haskell" -> "gbskfll"
cipher :: String -> String -> String
cipher key text = map recode text
  where
    recode c = altChar c $ find (elem c) keyChunks
    altChar c (Just pair) = fromMaybe c $ find (/= c) pair
    altChar c _           = c
    keyChunks = chunkify 2 key