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{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE NamedFieldPuns #-}
{-# LANGUAGE Rank2Types #-}
module Data.Digest.Internal where
import Control.Exception (assert)
import Data.Bits (Bits((.&.)), shiftR)
import Data.ByteString (ByteString)
import qualified Data.ByteString as ByteString
import qualified Data.ByteString.Unsafe as ByteString
import Data.Char (intToDigit)
import Data.Word (Word8)
import Foreign (Ptr, Storable, alloca, allocaArray, throwIf_)
import Foreign.C.Types
import Foreign.Marshal.Unsafe (unsafeLocalState)
import Unsafe.Coerce (unsafeCoerce)
-- | A hash algorithm which follows the standard initialize-update-finalize
-- pattern.
data Algo = forall ctx. Storable ctx => Algo
{ mdLen :: CSize -- ^ The length of the digest.
, mdInit :: Ptr ctx -> IO CInt -- ^ Initializes the context. Must return 1.
-- | Adds the buffer to the context. Must not modify the buffer. Must return
-- 1.
, mdUpdate :: forall a. Ptr ctx -> Ptr a -> CSize -> IO CInt
-- | Adds final padding to the context and writes the digest to the buffer.
, mdFinal :: Ptr CUChar -> Ptr ctx -> IO CInt
}
-- The type signatures in 'Algo' are suggestive of the functions exposed by the
-- BoringSSL API. Those functions fall into two broad categories--those which
-- always return 1 and those which return 1 only on success.
alwaysSucceeds :: IO CInt -> IO ()
alwaysSucceeds f = do
r <- f
assert (r == 1) (return ())
requireSuccess :: IO CInt -> IO ()
requireSuccess f = throwIf_ (/= 1) (const "BoringSSL failure") f
-- | The result of a hash operation.
newtype Digest =
Digest ByteString
deriving (Eq, Ord)
instance Show Digest where
show (Digest d) = ByteString.foldr showHexPadded [] d
where
showHexPadded b xs =
hexit (b `shiftR` 4 .&. 0x0f) : hexit (b .&. 0x0f) : xs
hexit = intToDigit . fromIntegral :: Word8 -> Char
-- | Hashes according to the given 'Algo'.
hash :: Algo -> ByteString -> Digest
hash (Algo {mdLen, mdInit, mdUpdate, mdFinal}) bytes =
let mdLen' = fromIntegral mdLen :: Int
in unsafeLocalState $
alloca $ \ctx -> do
alwaysSucceeds $ mdInit ctx
-- 'mdUpdate' treats its @buf@ argument as @const@, so the sharing
-- inherent in 'ByteString.unsafeUseAsCStringLen' is fine.
ByteString.unsafeUseAsCStringLen bytes $ \(buf, len) ->
alwaysSucceeds $ mdUpdate ctx buf (fromIntegral len)
d <-
allocaArray mdLen' $ \mdOut -> do
requireSuccess $ mdFinal mdOut ctx
-- 'mdOut' is a 'Ptr CUChar'. However, to make life more interesting,
-- 'CString' is a 'Ptr CChar', and 'CChar' is signed. This is
-- especially unfortunate given that all we really want to do is
-- convert to a 'ByteString', which is unsigned. To work around it,
-- we're going to cheat and let Haskell reinterpret-cast 'mdOut' to
-- 'Ptr CChar' before it does its 'ByteString' ingestion.
ByteString.packCStringLen (unsafeCoerce mdOut, mdLen')
return (Digest d)
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