[number] remove the ByteString from generation of high bits

Crypto/Number/Generate.hs

@@ -16,9 +16,9 @@ import           Crypto.Internal.Imports
 import           Crypto.Number.Basic
 import           Crypto.Number.Serialize
 import           Crypto.Random.Types
-import qualified Data.ByteString as B
-import           Crypto.Internal.ByteArray (Bytes)
 import           Data.Bits ((.|.), (.&.), shiftR)
+import           Crypto.Internal.ByteArray (Bytes, ScrubbedBytes)
+import qualified Crypto.Internal.ByteArray as B
 
 
 -- | generate a positive integer x, s.t. 0 <= x < m
@@ -47,9 +47,12 @@ generateBetween low high = (low +) <$> generateMax (high - low + 1)
 -- the number of bits need to be multiple of 8. It will always returns
 -- an integer that is close to 2^(1+bits/8) by setting the 2 highest bits to 1.
 generateOfSize :: MonadRandom m => Int -> m Integer
-generateOfSize bits = unmarshall <$> getRandomBytes (bits `div` 8)
+generateOfSize bits = os2ip . setHighest <$> getRandomBytes (bits `div` 8)
   where
-    unmarshall bs = os2ip $ snd $ B.mapAccumL (\acc w -> (0, w .|. acc)) 0xc0 bs
+    setHighest :: ScrubbedBytes -> ScrubbedBytes
+    setHighest ran = case B.unpack ran of
+                        []     -> B.empty
+                        (w:ws) -> B.pack ((w .|. 0xc0) : ws)
 
 -- | Generate a number with the specified number of bits
 generateBits :: MonadRandom m => Int -> m Integer