[random] add a seed capability with export/import from integer.

It decomposes the drgNew call that was gathering entropy then
initializing a ChaChaDRG, into 2 new calls seedNew and drgNewSeed.

drgNew remains unchanged.

The integer importing capability, should be used when wanting to bring
reproducibility to a debugging problem or for testing, otherwise it's
probably a bad idea to use.

Crypto/Random.hs

@@ -5,14 +5,21 @@
 -- Stability   : stable
 -- Portability : good
 --
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
 module Crypto.Random
     (
     -- * Deterministic instances
       ChaChaDRG
     , SystemDRG
+    , Seed
+    -- * Seed
+    , seedNew
+    , seedFromInteger
+    , seedToInteger
     -- * Deterministic Random class
     , getSystemDRG
     , drgNew
+    , drgNewSeed
     , drgNewTest
     , withDRG
     , withRandomBytes
@@ -25,14 +32,37 @@ module Crypto.Random
 import Crypto.Random.Types
 import Crypto.Random.ChaChaDRG
 import Crypto.Random.SystemDRG
-import Data.ByteArray (ByteArray, ScrubbedBytes)
+import Data.ByteArray (ByteArray, ByteArrayAccess, ScrubbedBytes)
 import Crypto.Internal.Imports
 
+import qualified Crypto.Number.Serialize as Serialize
+
+newtype Seed = Seed ScrubbedBytes
+    deriving (ByteArrayAccess)
+
+-- Length for ChaCha DRG seed
+seedLength :: Int
+seedLength = 40
+
+-- | Create a new Seed from system entropy
+seedNew :: MonadRandom randomly => randomly Seed
+seedNew = Seed `fmap` getRandomBytes seedLength
+
+-- | Convert a Seed to an integer
+seedToInteger :: Seed -> Integer
+seedToInteger (Seed b) = Serialize.os2ip b
+
+-- | Convert an integer to a Seed
+seedFromInteger :: Integer -> Seed
+seedFromInteger i = Seed $ Serialize.i2ospOf_ seedLength (i `mod` 2^(seedLength * 8))
+
 -- | Create a new DRG from system entropy
 drgNew :: MonadRandom randomly => randomly ChaChaDRG
-drgNew = do
-    b <- getRandomBytes 40
-    return $ initialize (b :: ScrubbedBytes)
+drgNew = drgNewSeed `fmap` seedNew
+
+-- | Create a new DRG from a seed
+drgNewSeed :: Seed -> ChaChaDRG
+drgNewSeed (Seed seed) = initialize seed
 
 -- | Create a new DRG from 5 Word64.
 --