Crypt – http://programmingpraxis.com/2011/10/28/crypt/

crypt.hs

import Data.Function (on)
import Data.Char (ord, chr)
import Data.Bits (xor)
import System (getArgs)

crypt :: FilePath -> String -> IO String
crypt filename key = do
    text <- readFile filename
    let code = map chr $ zipWith (xor `on` ord) text $ cycle key
    return code

main = do
    [filename,key] <- getArgs
    encrypted <- crypt filename key
    putStr encrypted