    -- |
    -- Module      : Crypto.Cipher.XSalsa
    -- License     : BSD-style
    -- Maintainer  : Brandon Hamilton <brandon.hamilton@gmail.com>
    -- Stability   : stable
    -- Portability : good
    --
    -- Implementation of XSalsa20 algorithm
    -- <https://cr.yp.to/snuffle/xsalsa-20081128.pdf>
    -- Based on the Salsa20 algorithm with 256 bit key extended with 192 bit nonce

    {-# LANGUAGE ForeignFunctionInterface #-}
    module Crypto.Cipher.XSalsa
        ( initialize
        , combine
        , generate
        , State
        ) where

    import           Crypto.Internal.ByteArray (ByteArrayAccess, ByteArray, ScrubbedBytes)
    import qualified Crypto.Internal.ByteArray as B
    import           Crypto.Internal.Compat
    import           Crypto.Internal.Imports
    import           Foreign.Ptr
    import           Foreign.Storable
    import           Foreign.C.Types
    import           Crypto.Cipher.Salsa hiding (initialize)

    -- | Initialize a new XSalsa context with the number of rounds,
    -- the key and the nonce associated.
    initialize :: (ByteArrayAccess key, ByteArrayAccess nonce)
               => Int    -- ^ number of rounds (8,12,20)
               -> key    -- ^ the key (256 bits)
               -> nonce  -- ^ the nonce (192 bits)
               -> State  -- ^ the initial XSalsa state
    initialize nbRounds key nonce
        | kLen /= 32                      = error "XSalsa: key length should be 256 bits"
        | nonceLen /= 24                  = error "XSalsa: nonce length should be 192 bits"
        | not (nbRounds `elem` [8,12,20]) = error "XSalsa: rounds should be 8, 12 or 20"
        | otherwise = unsafeDoIO $ do
            stPtr <- B.alloc 132 $ \stPtr ->
                B.withByteArray nonce $ \noncePtr  ->
                B.withByteArray key   $ \keyPtr ->
                    ccryptonite_xsalsa_init stPtr (fromIntegral nbRounds) kLen keyPtr nonceLen noncePtr
            return $ State stPtr
      where kLen     = B.length key
            nonceLen = B.length nonce

    foreign import ccall "cryptonite_xsalsa_init"
        ccryptonite_xsalsa_init :: Ptr State -> Int -> Int -> Ptr Word8 -> Int -> Ptr Word8 -> IO ()