Cryptography coursera class exercise #1.

Main.hs

{-# LANGUAGE UnicodeSyntax #-}
module Main where

import Control.Applicative ((<$>))
import Control.Monad (forM_, when)
import Control.Monad.State (execState, modify)
import Data.Bits (Bits, xor)
import Data.Char (chr, isAlpha)
import Data.List (maximumBy, group, sort, zip4)
import Data.List.Split (splitEvery)
import Data.IntMap (IntMap)
import Data.Ord (comparing)
import qualified Data.IntMap as M

-- | Read ciphers from "ciphers.txt", try to guess a key and decrypt secret message
main ∷ IO ()
main = do
  ciphers ← readCiphers "ciphers.txt"
  let key = M.elems $ chooseMostFrequent $ distribution $ combineWith xor ciphers
  mapM_ (putStrLn . map chr . zipWith xor key) ciphers

-- | Read routine
readCiphers ∷ Read α ⇒ FilePath → IO [[α]]
readCiphers fp = map readCipher . lines <$> readFile fp
  where readCipher = map (read . ("0x" ++)) . splitEvery 2

-- | Get all xored ciphers pairs
combineWith ∷ (α → α → β) → [[α]] → [([α], [α], [β])]
combineWith f cs = [ (a,b,zipWith f a b) | a ← cs, b ← cs ]

-- | Construct frequency IntMap
distribution ∷ Bits α ⇒ [([α], [α], [Int])] → IntMap [α]
distribution xs = flip execState (M.empty) $
  forM_ xs $ \(as,bs,cs) →
    forM_ (zip4 [0..] as bs cs) $ \(i,a,b,c) → do
      let hui = M.insertWith (++) i $ map (xor 32) [a, b]
      when (isAlpha $ chr c) $ modify hui

-- | NOTE: head is safe here since there is no empty lists
-- while construction distribution IntMap
chooseMostFrequent ∷ (Eq α, Ord α) ⇒ IntMap [α] → IntMap α
chooseMostFrequent = M.map (head . maximumBy (comparing length) . group . sort)

Does Prof. Dan Boneh allow students to post homework solutions?