Getting values out of types at runtime

Key.hs

{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE ScopedTypeVariables #-}

module Key (Key, makeKey) where

import Data.ByteString (ByteString)
import qualified Data.ByteString as B
import Data.Proxy
import GHC.TypeLits

newtype Key (size :: Nat) = Key ByteString
    deriving (Show)

makeKey :: forall size. KnownNat size => ByteString -> Maybe (Key size)
makeKey s | B.length s == (keyLength `div` 8) = Just (Key s)
          | otherwise                         = Nothing
  where
    keyLength = fromInteger $ natVal (Proxy :: Proxy size)

--encrypt :: Key 96 -> Key 256 -> String -> String
--encrypt (Key nonce) (Key cipherKey) plaintext = undefined