{-# LANGUAGE FlexibleInstances, TypeSynonymInstances #-}
 Maintainer: Thomas.DuBuisson@gmail.com
 Stability: beta
 Portability: portable 

 This module is for instantiating cryptographically strong determinitic random bit generators (DRBGs, aka PRNGs)
 For the simple use case of using the system random number generator ('System.Crypto.Random') to seed the DRBG:

@   g <- newGenIO

 Users needing to provide their own entropy can call 'newGen' directly
@    entropy <- getEntropy nrBytes
    let generator = newGen entropy


module Crypto.Random
	( CryptoRandomGen(..)
	, GenError (..)
	, newGenIO
	) where

import System.Crypto.Random (getEntropy)
import Crypto.Types
import Control.Monad (liftM)
import qualified Data.ByteString as B
import Data.Tagged
import Data.Bits (xor, setBit, shiftR, shiftL, (.&.))
import Data.List (foldl')

-- |many generators have these error conditions in common
data GenError =
	  GenErrorOther String	-- ^ Misc
	| RequestedTooManyBytes	-- ^ Requested more bytes than a single pass can generate (The maximum request is generator dependent)
	| RangeInvalid		-- ^ When using @genInteger g (l,h)@ and @logBase 2 (h - l) > (maxBound :: Int)@.
	| NeedReseed		-- ^ Some generators cease operation after too high a count without a reseed (ex: NIST SP 800-90)
	| NotEnoughEntropy	-- ^ For instantiating new generators (or reseeding)
  deriving (Eq, Ord, Show)

-- |A class of random bit generators that allows for the possibility of failure,
-- reseeding, providing entropy at the same time as requesting bytes
-- Minimum complete definition: `newGen`, `genSeedLength`, `genBytes`, `reseed`.
class CryptoRandomGen g where
	-- |Instantiate a new random bit generator.  The provided bytestring should
	-- be of length >= genSeedLength.  If the bytestring is shorter
	-- then the call may fail (suggested error: `NotEnoughEntropy`).  If the
	-- bytestring is of sufficent length the call should always succeed.
	newGen :: B.ByteString -> Either GenError g

	-- |Length of input entropy necessary to instantiate or reseed a generator
	genSeedLength :: Tagged g ByteLength

	-- | @genBytes len g@ generates a random ByteString of length @len@ and new generator.
	-- The "MonadCryptoRandom" package has routines useful for converting the ByteString
	-- to commonly needed values (but "cereal" or other deserialization libraries would also work).
	-- This routine can fail if the generator has gone too long without a reseed (usually this
	-- is in the ball-park of 2^48 requests).  Suggested error in this cases is `NeedReseed`
	genBytes	:: ByteLength -> g -> Either GenError (B.ByteString, g)

	-- |@genBytesWithEntropy g i entropy@ generates @i@ random bytes and use the
	-- additional input @entropy@ in the generation of the requested data to
	-- increase the confidence our generated data is a secure random stream.
	-- Some generators use @entropy@ to perturb the state of the generator, meaning:
	-- @
	--     (_,g2') <- genBytesWithEntropy len g1 ent
	--     (_,g2 ) <- genBytes len g1
	--     g2 /= g2'
	-- @
	-- But this is not required.
	-- Default:
	-- @
	--     genBytesWithEntropy g bytes entropy = xor entropy (genBytes g bytes)
	-- @
	genBytesWithEntropy	:: ByteLength -> B.ByteString -> g -> Either GenError (B.ByteString, g)
	genBytesWithEntropy len entropy g =
		let res = genBytes len g
		in case res of
			Left err -> Left err
			Right (bs,g') ->
				let entropy' = B.append entropy (B.replicate (len - B.length entropy) 0)
				in Right (zwp' entropy' bs, g')

	-- |If the generator has produced too many random bytes on its existing seed
	-- it will throw `NeedReseed`.  In that case, reseed the generator using this function and
	-- a new high-entropy seed of length >= `genSeedLength`.  Using bytestrings that are too short
	-- can result in an error (`NotEnoughEntropy`).
	reseed		:: B.ByteString -> g -> Either GenError g

-- |Use "System.Crypto.Random" to obtain entropy for `newGen`.
newGenIO :: CryptoRandomGen g => IO g
newGenIO = go 0
  go 1000 = error "The generator instance requested by newGenIO never instantiates (1000 tries).  It must be broken."
  go i = do
	let p = Proxy
	    getTypedGen :: (CryptoRandomGen g) => Proxy g -> IO (Either GenError g)
	    getTypedGen pr = liftM newGen (getEntropy $ proxy genSeedLength pr)
	res <- getTypedGen p
	case res of
		Left _ -> go (i+1)
		Right g -> return (g `asProxyTypeOf` p)

-- |Obtain a tagged value for a particular instantiated type.
for :: Tagged a b -> a -> b
for t _ = unTagged t

bs2i :: B.ByteString -> Integer
bs2i bs = B.foldl' (\i b -> (i `shiftL` 8) + fromIntegral b) 0 bs
{-# INLINE bs2i #-}

-- |zipWith xor + Pack
-- As a result of rewrite rules, this should automatically be optimized (at compile time) 
-- to use the bytestring libraries 'zipWith'' function.
zwp' a = B.pack . B.zipWith xor a
{-# INLINE zwp' #-}