Commit to a message without leaking information (?)

commitment.hs

{-# LANGUAGE OverloadedStrings #-}

module Commitment ( Message, Commitment, Opening, commit, reveal ) where

import Crypto.Cipher.AES (initAES, encryptCTR, decryptCTR)
import qualified Data.ByteString.Lazy as LB
import qualified Data.ByteString as SB

--
-- Type aliases to avoid mistakes
-- 

newtype Message = Message SB.ByteString
  deriving (Eq)
           
newtype Commitment = Commitment SB.ByteString
  deriving (Eq)
           
newtype Opening = Opening SB.ByteString
  deriving (Eq)

--
-- Commitment and revelation
--

commit :: Message -> IO (Commitment, Opening)
commit (Message msg) = do
  -- AES128 in CTR mode with random key & constant IV
  key <- (LB.toStrict . LB.take 16) `fmap` LB.readFile "/dev/urandom"
  return $! (Commitment $! encryptCTR (initAES key) kNONCE msg, Opening key)

reveal :: Commitment -> Opening -> Message
reveal (Commitment c) (Opening o) = Message $ decryptCTR (initAES o) kNONCE c

kNONCE :: SB.ByteString
kNONCE = SB.replicate 16 0