Cryptography coursera class exercise #2.

Main.hs

{-# LANGUAGE UnicodeSyntax #-}

import Control.Applicative ((<$>))
import Control.Monad ((<=<))

import Crypto.Cipher.AES (Key, IV(..), decryptCBC, decryptCTR, initKey)
import Data.ByteString (ByteString)
import qualified Data.ByteString as B


main ∷ IO ()
main = do
  mapM_ (print <=< cbc "data/key1.dat") ["data/ciphertext1.dat", "data/ciphertext2.dat"]
  mapM_ (print <=< ctr "data/key2.dat") ["data/ciphertext3.dat", "data/ciphertext4.dat"]
 where
  cbc = decryptWith decryptCBC
  ctr = decryptWith decryptCTR


decryptWith ∷ (Key → IV → ByteString → ByteString) → FilePath → FilePath → IO ByteString
decryptWith f keyfp datafp =
  do key ← initKey <$> B.readFile keyfp
     (iv, cipher) ← parse <$> B.readFile datafp
     return $ f key iv cipher


parse ∷ ByteString → (IV, ByteString)
parse bs = (IV iv, cipher)
 where
  (iv, cipher) = B.splitAt 16 bs

% runhaskell Main.hs
"Basic CBC mode encryption needs padding.\b\b\b\b\b\b\b\b"
"Our implementation uses rand. IV\DLE\DLE\DLE\DLE\DLE\DLE\DLE\DLE\DLE\DLE\DLE\DLE\DLE\DLE\DLE\DLE"
"CTR mode lets you build a stream cipher from a block cipher."
"Always avoid the two time pad!"