Map-to-integer for ciphers?

[박신환]

For use of ciphers (SHA-256, RSA-2048, etc.), a type must be able to be injectively mapped to integers. It seems `Enum` is currently the closest thing that does this. 
But `Enum` is supposed to be for arithmetic sequences, so it seems better to define a new typeclass. (Here named `Cipherable`) 
There are some types that aren't members of `Enum`. For example, `Maybe`, `[]`, etc. They seem okay to be Cipherable. Hence:
 
 
{-# LANGUAGE ScopedTypeVariables #-}

instance Cipherable a => Cipherable (Maybe a) where
toEnum 0 = Nothing
toEnum n = Just (toEnum n)
fromEnum Nothing = 0
fromEnum (Just x) = 1 + fromEnum x

instance forall a. (Cipherable a, Bounded a) => Cipherable [a] where
toEnum 0 = []
toEnum n = let
(q,r) = (n-1) `quotRem` (1 + fromEnum (maxBound :: a))
in toEnum r : toEnum q
fromEnum [] = 0
fromEnum (x:xs) = 1 + fromEnum x + (1 + fromEnum (maxBound :: a)) * fromEnum xs

instance Cipherable Void where
toEnum = errorWithoutStackTrace "Cipher.Cipherable.Void.toEnum"
fromEnum = absurd


(Besides, it is possible to re-write that of `[]` without ScopedTypeVariables? I see no way...)

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