Cryptography coursera class exercise #4.

Main.hs

{-# LANGUAGE UnicodeSyntax #-}
module Main where

import Control.Applicative ((<$>))
import Control.Monad (foldM, join)
import qualified Data.Bits as Bits
import Data.List (genericReplicate, inits)
import Data.Monoid ((<>))
import Data.Word (Word8)
import Text.Printf (printf)

import Data.ByteString (ByteString)
import qualified Data.ByteString as B
import qualified Network.Curl as C


randomByteString ∷ ByteString
randomByteString = B.replicate 16 57


main ∷ IO ()
main =
  do cipher ← B.readFile "ciphertext.dat"
     plainText ← decrypt cipher
     print plainText


decrypt ∷ ByteString → IO ByteString
decrypt cipher = B.concat . map (B.pack . B.zipWith Bits.xor randomByteString) <$> mapM decryptitionRound (ciphers cipher)
 where
  ciphers = map B.concat . drop 2 . inits . chunk 16



decryptitionRound ∷ ByteString → IO ByteString
decryptitionRound cipher = foldM (bruteforceByte cipher) (B.replicate 16 0) paddings
 where
  paddings = map (B.pack . reverse . take 16 . (<> repeat 0) . join genericReplicate) [1..16]


bruteforceByte ∷ ByteString → ByteString → ByteString → IO ByteString
bruteforceByte cipher acc padding = go 0
 where
  paddedCipher = cipher `xor` padding `xor` acc `xor` randomByteString
  go n =
    do let guess = pretty $ paddedCipher `xor` fromWord8 (B.length $ B.dropWhile (== 0) acc) n
       r ← C.withCurlDo $ C.curlGetResponse_
         ("http://crypto-class.appspot.com/po?er=" <> guess)
         [] ∷ IO (C.CurlResponse_ [(String, String)] ByteString)
       case C.respStatus r of
         403 → go (n + 1)
         _ → return (addByte n acc)


addByte ∷ Word8 → ByteString → ByteString
addByte n xs = B.replicate (16 - t - 1) 0 <> B.cons n ys
 where
  ys = B.dropWhile (== 0) xs
  t = B.length ys


xor ∷ ByteString → ByteString → ByteString
xor x y = B.concat as <> B.pack (B.zipWith Bits.xor c y) <> b
 where
  (as,c,b) = split $ chunk 16 x


fromWord8 ∷ Int → Word8 → ByteString
fromWord8 t n = B.pack $ replicate (16 - t - 1) 0 ++ n : replicate t 0


pretty ∷ ByteString → String
pretty = concatMap (printf "%02x") . B.unpack


chunk ∷ Int → ByteString → [ByteString]
chunk n bs
  | B.length bs <= n = [bs]
  | otherwise = B.take n bs : chunk n (B.drop n bs)


split ∷ [α] → ([α], α, α)
split = go []
 where
  go as [x,y] = (reverse as, x, y)
  go as (x:xs) = go (x:as) xs
  go _ _ = error "Main.split: [_]/empty list"

I'm sure, as noted by my CS collegefellows here at Cambridge (I'm currently diversifying into management). I merely put in an implementation to get the work done, not really concerned with optimizing either, but the program took way too long for comfort. In the end, didn't really mind as it got the job done. Regarding this, it was clever for me, as I am just starting out with Haskell and Ruby; quite a way to go even before I reach this level of coding in hs, so to speak. Best Regards, Khurram

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