playfair.hs

import Data.List.Split
import qualified Data.Map as Map
import Data.List.HT

buildCipher pw = Map.fromList $ zip [ (x,y) | x <- [0..4], y <- [0..4] ] (concat $ build pw) 
  where
    build = chunk 5 . (`union` delete 'J' ['A'..'Z']) . nub . process --calculate the list of characters in the right order

process = replace "J" "I" . map toUpper . filter isLetter

newDecode k = concatMap (\[x,y] -> code k (-1) x y) . chunk 2

newEncode _ []        = []
newEncode c [x]       = newEncode c (x : "X") --always at the end of the message
newEncode c (x:y:xs)  | (x==y)    = newEncode c [x] ++ newEncode c (y:xs) 
                      | otherwise = code c 1 x y ++ newEncode c xs

code c dir a b
  | y1 == y2  = get c (x1 + dir, y1) : [get c (x2 + dir, y2)] --same col
  | x1 == x2  = get c (x1, y1 + dir) : [get c (x2, y2 + dir)] --same row
  | otherwise = get c (x1, y2) : [get c (x2, y1)]
    where
    (x1, y1)    = findCharInCipher c a
    (x2, y2)    = findCharInCipher c b
    get m (x,y) = m Map.! (mod x 5, mod y 5)
    findCharInCipher k c = head . Map.keys $ Map.filter (==c) k

encode m k = (newEncode (buildCipher k) . process) m
decode m k = (newDecode (buildCipher k) . process) m

main = do print $ encode "PROGRAMMING PRAXIS" "PLAYFAIR" 
          print $ decode "LIVOBLKZEDOELIYWCN" "PLAYFAIR" 

compare this one to the readable version: https://gist.github.com/2794373