PA4 on Haskell

{-# LANGUAGE ViewPatterns #-}

import Data.Bits (xor)
import Data.Char
import Data.Hex (hex, unhex)
import Text.Regex
import Data.Maybe (mapMaybe)
import Data.List

unHex :: String -> String
unHex = head . unhex

listXor k m = zipWith xor (cycle k) m
stringXor (map ord -> k) (map ord -> m) = map chr $ listXor k m

validPad = matchRegex (mkRegex "GET /([0-9a-f]+) HTTP/1.1\" 404")

process valids = concat $ zipWith stringXor guesses ciphers where
  guesses = tail $ map (stringXor "\16" . take 16) lastValids
  ciphers = init $ map (drop 16) lastValids
  lastValids = map (valids!!) [16-1,32-1..length valids - 1]

main = do
  f <- readFile "proj4-log.txt"
  putStrLn $ process $ map unHex $ concat  $ mapMaybe validPad $ lines f