Add PRG typeclass for use in pure code

Crypto/Random/ChaChaDRG.hs

@@ -23,6 +23,10 @@ import qualified Crypto.Cipher.ChaCha as C
 instance DRG ChaChaDRG where
     randomBytesGenerate = generate
 
+instance PRG ChaChaDRG where
+    newPrgFromSeed seed = ChaChaDRG $ C.initializeSimple seed
+    prgSeedLength = 40
+
 -- | ChaCha Deterministic Random Generator
 newtype ChaChaDRG = ChaChaDRG C.StateSimple
     deriving (NFData)
@@ -32,7 +36,7 @@ newtype ChaChaDRG = ChaChaDRG C.StateSimple
 initialize :: ByteArrayAccess seed
            => seed        -- ^ 40 bytes of seed
            -> ChaChaDRG   -- ^ the initial ChaCha state
-initialize seed = ChaChaDRG $ C.initializeSimple seed
+initialize = newPrgFromSeed
 
 -- | Initialize a new ChaCha context from 5-tuple of words64.
 -- This interface is useful when creating a RNG out of tests generators (e.g. QuickCheck).

Crypto/Random/Types.hs

@@ -5,11 +5,17 @@
 -- Stability   : experimental
 -- Portability : Good
 --
+{-# LANGUAGE AllowAmbiguousTypes #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TypeApplications #-}
 module Crypto.Random.Types
     (
       MonadRandom(..)
     , MonadPseudoRandom
     , DRG(..)
+    , PRG(..)
+    , newPrgFromEntropy
+    , newPrg
     , withDRG
     ) where
 
@@ -25,6 +31,26 @@ class DRG gen where
     -- | Generate N bytes of randomness from a DRG
     randomBytesGenerate :: ByteArray byteArray => Int -> gen -> (byteArray, gen)
 
+-- | A Psuedo Random Generator (PRG) class
+--
+-- Like 'DRG' but also supports initialisation from some fixed seed.
+class DRG gen => PRG gen where
+    -- | Initialize the DRG from some fixed seed.
+    newPrgFromSeed :: ByteArrayAccess seed => seed -> gen
+    -- | Length of seed in bytes
+    prgSeedLength :: Int
+
+-- | Initialize the PRG from some entropy supplier.
+newPrgFromEntropy :: forall gen f. (PRG gen, Functor f)
+                  => (Int -> f ScrubbedBytes)
+                  -> f gen
+newPrgFromEntropy myGetEntropy =
+    newPrgFromSeed <$> myGetEntropy (prgSeedLength @gen)
+
+-- | Initialize the PRG from a 'MonadRandom'.
+newPrg :: (PRG gen, MonadRandom f) => f gen
+newPrg = newPrgFromEntropy getRandomBytes
+
 instance MonadRandom IO where
     getRandomBytes = getEntropy