{-# 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
	( -- * Basic Interface
	, GenError (..)
	  -- * Helper functions and expanded interface
	, splitGen
	  -- * Instances
	, SystemRandom
	) where

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

-- |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)
	| NeedsInfiniteSeed	-- ^ This generator can not be instantiated or reseeded with a finite seed (ex: 'SystemRandom')
  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

	-- |By default this uses "System.Crypto.Random" to obtain entropy for `newGen`.
	newGenIO :: 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)

-- |get a random number generator based on the standard system entropy source
getSystemGen :: IO SystemRandom
getSystemGen = do
        ch <- openHandle
        let getBS = unsafeInterleaveIO $ do
                bs <- hGetEntropy ch ((2^15) - 16)
                more <- getBS
                return (bs:more)
        liftM (SysRandom . L.fromChunks) getBS

-- |Not that it is technically correct as an instance of 'CryptoRandomGen', but simply because
-- it's a reasonable engineering choice here is a CryptoRandomGen which streams the system randoms. Take note:
--  * It uses the default definition of 'genByteWithEntropy'
--  * 'newGen' will always fail!
--  * 'reseed' will always fail!
--  * the handle to the system random is never closed
data SystemRandom = SysRandom L.ByteString

instance CryptoRandomGen SystemRandom where
        newGen _ = Left NeedsInfiniteSeed
        genSeedLength = Tagged 0
        genBytes req (SysRandom bs) =
                let reqI = fromIntegral req
                    rnd = L.take reqI bs
                    rest = L.drop reqI bs
                in if L.length rnd == reqI
                        then Right (B.concat $ L.toChunks rnd, SysRandom rest)
                        else Left $ GenErrorOther "Error obtaining enough bytes from system random for given request"
        reseed _ _ = Left NeedsInfiniteSeed
	newGenIO = getSystemGen

-- | While the safety and wisdom of a splitting function depends on the properties of the generator being split,
-- several arguments from informed people indicate such a function is safe for NIST SP 800-90 generators.
-- (see libraries@haskell.org discussion ~ Sept, Oct 2010)
splitGen :: CryptoRandomGen g => g -> Either GenError (g,g)
splitGen g = do
	let e = genBytes (genSeedLength `for` g) g
	case e of
		Left e -> Left e
		Right (ent,g') -> 
			case newGen ent of
				Right new -> Right (g',new)
				Left e -> Left e

-- |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' #-}