{-# LANGUAGE DataKinds #-}
{-# LANGUAGE DeriveAnyClass #-}
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE DuplicateRecordFields #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE NamedFieldPuns #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TupleSections #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE TypeFamilies #-}
{-# OPTIONS_GHC -fno-warn-redundant-constraints #-}

module Simplex.Messaging.Crypto.Ratchet where

import Control.Monad.Except
import Control.Monad.Trans.Except
import Crypto.Cipher.AES (AES256)
import Crypto.Hash (SHA512)
import qualified Crypto.KDF.HKDF as H
import Data.Aeson (FromJSON, ToJSON)
import qualified Data.Aeson as J
import Data.ByteString.Char8 (ByteString)
import qualified Data.ByteString.Char8 as B
import qualified Data.ByteString.Lazy as LB
import qualified Data.List.NonEmpty as L
import Data.Map.Strict (Map)
import qualified Data.Map.Strict as M
import Data.Maybe (fromMaybe)
import Data.Typeable (Typeable)
import Data.Word (Word32)
import Database.SQLite.Simple.FromField (FromField (..))
import Database.SQLite.Simple.ToField (ToField (..))
import GHC.Generics
import Simplex.Messaging.Agent.QueryString
import Simplex.Messaging.Crypto
import Simplex.Messaging.Encoding
import Simplex.Messaging.Encoding.String
import Simplex.Messaging.Parsers (blobFieldDecoder, parseE, parseE')
import Simplex.Messaging.Util (tryE)
import Simplex.Messaging.Version

e2eEncryptVersion :: Version
e2eEncryptVersion = 1

e2eEncryptVRange :: VersionRange
e2eEncryptVRange = mkVersionRange 1 e2eEncryptVersion

data E2ERatchetParams (a :: Algorithm)
  = E2ERatchetParams Version (PublicKey a) (PublicKey a)
  deriving (Eq, Show)

instance AlgorithmI a => Encoding (E2ERatchetParams a) where
  smpEncode (E2ERatchetParams v k1 k2) = smpEncode (v, k1, k2)
  smpP = E2ERatchetParams <$> smpP <*> smpP <*> smpP

instance VersionI (E2ERatchetParams a) where
  type VersionRangeT (E2ERatchetParams a) = E2ERatchetParamsUri a
  version (E2ERatchetParams v _ _) = v
  toVersionRangeT (E2ERatchetParams _ k1 k2) vr = E2ERatchetParamsUri vr k1 k2

instance VersionRangeI (E2ERatchetParamsUri a) where
  type VersionT (E2ERatchetParamsUri a) = (E2ERatchetParams a)
  versionRange (E2ERatchetParamsUri vr _ _) = vr
  toVersionT (E2ERatchetParamsUri _ k1 k2) v = E2ERatchetParams v k1 k2

data E2ERatchetParamsUri (a :: Algorithm)
  = E2ERatchetParamsUri VersionRange (PublicKey a) (PublicKey a)
  deriving (Eq, Show)

instance AlgorithmI a => StrEncoding (E2ERatchetParamsUri a) where
  strEncode (E2ERatchetParamsUri vs key1 key2) =
    strEncode $
      QSP QNoEscaping [("v", strEncode vs), ("x3dh", strEncodeList [key1, key2])]
  strP = do
    query <- strP
    vs <- queryParam "v" query
    keys <- L.toList <$> queryParam "x3dh" query
    case keys of
      [key1, key2] -> pure $ E2ERatchetParamsUri vs key1 key2
      _ -> fail "bad e2e params"

generateE2EParams :: (AlgorithmI a, DhAlgorithm a) => Version -> IO (PrivateKey a, PrivateKey a, E2ERatchetParams a)
generateE2EParams v = do
  (k1, pk1) <- generateKeyPair'
  (k2, pk2) <- generateKeyPair'
  pure (pk1, pk2, E2ERatchetParams v k1 k2)

data RatchetInitParams = RatchetInitParams
  { assocData :: Str,
    ratchetKey :: RatchetKey,
    sndHK :: HeaderKey,
    rcvNextHK :: HeaderKey
  }
  deriving (Eq, Show)

x3dhSnd :: DhAlgorithm a => PrivateKey a -> PrivateKey a -> E2ERatchetParams a -> RatchetInitParams
x3dhSnd spk1 spk2 (E2ERatchetParams _ rk1 rk2) =
  x3dh (publicKey spk1, rk1) (dh' rk1 spk2) (dh' rk2 spk1) (dh' rk2 spk2)

x3dhRcv :: DhAlgorithm a => PrivateKey a -> PrivateKey a -> E2ERatchetParams a -> RatchetInitParams
x3dhRcv rpk1 rpk2 (E2ERatchetParams _ sk1 sk2) =
  x3dh (sk1, publicKey rpk1) (dh' sk2 rpk1) (dh' sk1 rpk2) (dh' sk2 rpk2)

x3dh :: DhAlgorithm a => (PublicKey a, PublicKey a) -> DhSecret a -> DhSecret a -> DhSecret a -> RatchetInitParams
x3dh (sk1, rk1) dh1 dh2 dh3 =
  RatchetInitParams {assocData, ratchetKey = RatchetKey sk, sndHK = Key hk, rcvNextHK = Key nhk}
  where
    assocData = Str $ pubKeyBytes sk1 <> pubKeyBytes rk1
    (hk, rest) = B.splitAt 32 $ dhBytes' dh1 <> dhBytes' dh2 <> dhBytes' dh3
    (nhk, sk) = B.splitAt 32 rest

type RatchetX448 = Ratchet 'X448

data Ratchet a = Ratchet
  { -- ratchet version range sent in messages (current .. max supported ratchet version)
    rcVersion :: VersionRange,
    -- associated data - must be the same in both parties ratchets
    rcAD :: Str,
    rcDHRs :: PrivateKey a,
    rcRK :: RatchetKey,
    rcSnd :: Maybe (SndRatchet a),
    rcRcv :: Maybe RcvRatchet,
    rcNs :: Word32,
    rcNr :: Word32,
    rcPN :: Word32,
    rcNHKs :: HeaderKey,
    rcNHKr :: HeaderKey
  }
  deriving (Eq, Show, Generic, FromJSON)

instance AlgorithmI a => ToJSON (Ratchet a) where
  toEncoding = J.genericToEncoding J.defaultOptions

data SndRatchet a = SndRatchet
  { rcDHRr :: PublicKey a,
    rcCKs :: RatchetKey,
    rcHKs :: HeaderKey
  }
  deriving (Eq, Show, Generic, FromJSON)

instance AlgorithmI a => ToJSON (SndRatchet a) where
  toEncoding = J.genericToEncoding J.defaultOptions

data RcvRatchet = RcvRatchet
  { rcCKr :: RatchetKey,
    rcHKr :: HeaderKey
  }
  deriving (Eq, Show, Generic, FromJSON)

instance ToJSON RcvRatchet where
  toEncoding = J.genericToEncoding J.defaultOptions

type SkippedMsgKeys = Map HeaderKey SkippedHdrMsgKeys

type SkippedHdrMsgKeys = Map Word32 MessageKey

data SkippedMsgDiff
  = SMDNoChange
  | SMDRemove HeaderKey Word32
  | SMDAdd SkippedMsgKeys

-- | this function is only used in tests to apply changes in skipped messages,
-- in the agent the diff is persisted, and the whole state is loaded for the next message.
applySMDiff :: SkippedMsgKeys -> SkippedMsgDiff -> SkippedMsgKeys
applySMDiff smks = \case
  SMDNoChange -> smks
  SMDRemove hk msgN -> fromMaybe smks $ do
    mks <- M.lookup hk smks
    _ <- M.lookup msgN mks
    let mks' = M.delete msgN mks
    pure $
      if M.null mks'
        then M.delete hk smks
        else M.insert hk mks' smks
  SMDAdd smks' ->
    let merge hk mks = M.alter (Just . maybe mks (M.union mks)) hk
     in M.foldrWithKey merge smks smks'

type HeaderKey = Key

data MessageKey = MessageKey Key IV

instance Encoding MessageKey where
  smpEncode (MessageKey (Key key) (IV iv)) = smpEncode (key, iv)
  smpP = MessageKey <$> (Key <$> smpP) <*> (IV <$> smpP)

-- | Input key material for double ratchet HKDF functions
newtype RatchetKey = RatchetKey ByteString
  deriving (Eq, Show)

instance ToJSON RatchetKey where
  toJSON (RatchetKey k) = strToJSON k
  toEncoding (RatchetKey k) = strToJEncoding k

instance FromJSON RatchetKey where
  parseJSON = fmap RatchetKey . strParseJSON "Key"

instance AlgorithmI a => ToField (Ratchet a) where toField = toField . LB.toStrict . J.encode

instance (AlgorithmI a, Typeable a) => FromField (Ratchet a) where fromField = blobFieldDecoder J.eitherDecodeStrict'

instance ToField MessageKey where toField = toField . smpEncode

instance FromField MessageKey where fromField = blobFieldDecoder smpDecode

-- | Sending ratchet initialization, equivalent to RatchetInitAliceHE in double ratchet spec
--
-- Please note that sPKey is not stored, and its public part together with random salt
-- is sent to the recipient.
initSndRatchet ::
  forall a. (AlgorithmI a, DhAlgorithm a) => PublicKey a -> PrivateKey a -> RatchetInitParams -> Ratchet a
initSndRatchet rcDHRr rcDHRs RatchetInitParams {assocData, ratchetKey, sndHK, rcvNextHK} = do
  -- state.RK, state.CKs, state.NHKs = KDF_RK_HE(SK, DH(state.DHRs, state.DHRr))
  let (rcRK, rcCKs, rcNHKs) = rootKdf ratchetKey rcDHRr rcDHRs
   in Ratchet
        { rcVersion = e2eEncryptVRange,
          rcAD = assocData,
          rcDHRs,
          rcRK,
          rcSnd = Just SndRatchet {rcDHRr, rcCKs, rcHKs = sndHK},
          rcRcv = Nothing,
          rcPN = 0,
          rcNs = 0,
          rcNr = 0,
          rcNHKs,
          rcNHKr = rcvNextHK
        }

-- | Receiving ratchet initialization, equivalent to RatchetInitBobHE in double ratchet spec
--
-- Please note that the public part of rcDHRs was sent to the sender
-- as part of the connection request and random salt was received from the sender.
initRcvRatchet ::
  forall a. (AlgorithmI a, DhAlgorithm a) => PrivateKey a -> RatchetInitParams -> Ratchet a
initRcvRatchet rcDHRs RatchetInitParams {assocData, ratchetKey, sndHK, rcvNextHK} =
  Ratchet
    { rcVersion = e2eEncryptVRange,
      rcAD = assocData,
      rcDHRs,
      rcRK = ratchetKey,
      rcSnd = Nothing,
      rcRcv = Nothing,
      rcPN = 0,
      rcNs = 0,
      rcNr = 0,
      rcNHKs = rcvNextHK,
      rcNHKr = sndHK
    }

data MsgHeader a = MsgHeader
  { -- | max supported ratchet version
    msgMaxVersion :: Version,
    msgDHRs :: PublicKey a,
    msgPN :: Word32,
    msgNs :: Word32
  }
  deriving (Eq, Show)

data AMsgHeader
  = forall a.
    (AlgorithmI a, DhAlgorithm a) =>
    AMsgHeader (SAlgorithm a) (MsgHeader a)

-- to allow extension without increasing the size, the actual header length is:
-- 69 = 2 (original size) + 2 + 1+56 (Curve448) + 4 + 4
paddedHeaderLen :: Int
paddedHeaderLen = 88

-- only used in tests to validate correct padding
-- (2 bytes - version size, 1 byte - header size, not to have it fixed or version-dependent)
fullHeaderLen :: Int
fullHeaderLen = 2 + 1 + paddedHeaderLen + authTagSize + ivSize @AES256

instance AlgorithmI a => Encoding (MsgHeader a) where
  smpEncode MsgHeader {msgMaxVersion, msgDHRs, msgPN, msgNs} =
    smpEncode (msgMaxVersion, msgDHRs, msgPN, msgNs)
  smpP = do
    msgMaxVersion <- smpP
    msgDHRs <- smpP
    msgPN <- smpP
    msgNs <- smpP
    pure MsgHeader {msgMaxVersion, msgDHRs, msgPN, msgNs}

data EncMessageHeader = EncMessageHeader
  { ehVersion :: Version,
    ehIV :: IV,
    ehAuthTag :: AuthTag,
    ehBody :: ByteString
  }

instance Encoding EncMessageHeader where
  smpEncode EncMessageHeader {ehVersion, ehIV, ehAuthTag, ehBody} =
    smpEncode (ehVersion, ehIV, ehAuthTag, ehBody)
  smpP = do
    (ehVersion, ehIV, ehAuthTag, ehBody) <- smpP
    pure EncMessageHeader {ehVersion, ehIV, ehAuthTag, ehBody}

data EncRatchetMessage = EncRatchetMessage
  { emHeader :: ByteString,
    emAuthTag :: AuthTag,
    emBody :: ByteString
  }

instance Encoding EncRatchetMessage where
  smpEncode EncRatchetMessage {emHeader, emBody, emAuthTag} =
    smpEncode (emHeader, emAuthTag, Tail emBody)
  smpP = do
    (emHeader, emAuthTag, Tail emBody) <- smpP
    pure EncRatchetMessage {emHeader, emBody, emAuthTag}

rcEncrypt :: AlgorithmI a => Ratchet a -> Int -> ByteString -> ExceptT CryptoError IO (ByteString, Ratchet a)
rcEncrypt Ratchet {rcSnd = Nothing} _ _ = throwE CERatchetState
rcEncrypt rc@Ratchet {rcSnd = Just sr@SndRatchet {rcCKs, rcHKs}, rcDHRs, rcNs, rcPN, rcAD = Str rcAD, rcVersion} paddedMsgLen msg = do
  -- state.CKs, mk = KDF_CK(state.CKs)
  let (ck', mk, iv, ehIV) = chainKdf rcCKs
  -- enc_header = HENCRYPT(state.HKs, header)
  (ehAuthTag, ehBody) <- encryptAEAD rcHKs ehIV paddedHeaderLen rcAD msgHeader
  -- return enc_header, ENCRYPT(mk, plaintext, CONCAT(AD, enc_header))
  let emHeader = smpEncode EncMessageHeader {ehVersion = minVersion rcVersion, ehBody, ehAuthTag, ehIV}
  (emAuthTag, emBody) <- encryptAEAD mk iv paddedMsgLen (rcAD <> emHeader) msg
  let msg' = smpEncode EncRatchetMessage {emHeader, emBody, emAuthTag}
      -- state.Ns += 1
      rc' = rc {rcSnd = Just sr {rcCKs = ck'}, rcNs = rcNs + 1}
  pure (msg', rc')
  where
    -- header = HEADER(state.DHRs, state.PN, state.Ns)
    msgHeader =
      smpEncode
        MsgHeader
          { msgMaxVersion = maxVersion rcVersion,
            msgDHRs = publicKey rcDHRs,
            msgPN = rcPN,
            msgNs = rcNs
          }

data SkippedMessage a
  = SMMessage (DecryptResult a)
  | SMHeader (Maybe RatchetStep) (MsgHeader a)
  | SMNone

data RatchetStep = AdvanceRatchet | SameRatchet
  deriving (Eq)

type DecryptResult a = (Either CryptoError ByteString, Ratchet a, SkippedMsgDiff)

maxSkip :: Word32
maxSkip = 512

rcDecrypt ::
  forall a.
  (AlgorithmI a, DhAlgorithm a) =>
  Ratchet a ->
  SkippedMsgKeys ->
  ByteString ->
  ExceptT CryptoError IO (DecryptResult a)
rcDecrypt rc@Ratchet {rcRcv, rcAD = Str rcAD} rcMKSkipped msg' = do
  encMsg@EncRatchetMessage {emHeader} <- parseE CryptoHeaderError smpP msg'
  encHdr <- parseE CryptoHeaderError smpP emHeader
  -- plaintext = TrySkippedMessageKeysHE(state, enc_header, cipher-text, AD)
  decryptSkipped encHdr encMsg >>= \case
    SMNone -> do
      (rcStep, hdr) <- decryptRcHeader rcRcv encHdr
      decryptRcMessage rcStep hdr encMsg
    SMHeader rcStep_ hdr ->
      case rcStep_ of
        Just rcStep -> decryptRcMessage rcStep hdr encMsg
        Nothing -> throwE CERatchetHeader
    SMMessage r -> pure r
  where
    decryptRcMessage :: RatchetStep -> MsgHeader a -> EncRatchetMessage -> ExceptT CryptoError IO (DecryptResult a)
    decryptRcMessage rcStep MsgHeader {msgDHRs, msgPN, msgNs} encMsg = do
      -- if dh_ratchet:
      (rc', smks1) <- ratchetStep rcStep
      case skipMessageKeys msgNs rc' of
        Left e -> pure (Left e, rc', smkDiff smks1)
        Right (rc''@Ratchet {rcRcv = Just rr@RcvRatchet {rcCKr}, rcNr}, smks2) -> do
          -- state.CKr, mk = KDF_CK(state.CKr)
          let (rcCKr', mk, iv, _) = chainKdf rcCKr
          -- return DECRYPT (mk, cipher-text, CONCAT (AD, enc_header))
          msg <- decryptMessage (MessageKey mk iv) encMsg
          -- state . Nr += 1
          pure (msg, rc'' {rcRcv = Just rr {rcCKr = rcCKr'}, rcNr = rcNr + 1}, smkDiff $ smks1 <> smks2)
        Right (rc'', smks2) -> do
          pure (Left CERatchetState, rc'', smkDiff $ smks1 <> smks2)
      where
        smkDiff :: SkippedMsgKeys -> SkippedMsgDiff
        smkDiff smks = if M.null smks then SMDNoChange else SMDAdd smks
        ratchetStep :: RatchetStep -> ExceptT CryptoError IO (Ratchet a, SkippedMsgKeys)
        ratchetStep SameRatchet = pure (rc, M.empty)
        ratchetStep AdvanceRatchet =
          -- SkipMessageKeysHE(state, header.pn)
          case skipMessageKeys msgPN rc of
            Left e -> throwE e
            Right (rc'@Ratchet {rcDHRs, rcRK, rcNHKs, rcNHKr}, hmks) -> do
              -- DHRatchetHE(state, header)
              (_, rcDHRs') <- liftIO $ generateKeyPair' @a
              -- state.RK, state.CKr, state.NHKr = KDF_RK_HE(state.RK, DH(state.DHRs, state.DHRr))
              let (rcRK', rcCKr', rcNHKr') = rootKdf rcRK msgDHRs rcDHRs
                  -- state.RK, state.CKs, state.NHKs = KDF_RK_HE(state.RK, DH(state.DHRs, state.DHRr))
                  (rcRK'', rcCKs', rcNHKs') = rootKdf rcRK' msgDHRs rcDHRs'
                  rc'' =
                    rc'
                      { rcDHRs = rcDHRs',
                        rcRK = rcRK'',
                        rcSnd = Just SndRatchet {rcDHRr = msgDHRs, rcCKs = rcCKs', rcHKs = rcNHKs},
                        rcRcv = Just RcvRatchet {rcCKr = rcCKr', rcHKr = rcNHKr},
                        rcPN = rcNs rc,
                        rcNs = 0,
                        rcNr = 0,
                        rcNHKs = rcNHKs',
                        rcNHKr = rcNHKr'
                      }
              pure (rc'', hmks)
    skipMessageKeys :: Word32 -> Ratchet a -> Either CryptoError (Ratchet a, SkippedMsgKeys)
    skipMessageKeys _ r@Ratchet {rcRcv = Nothing} = Right (r, M.empty)
    skipMessageKeys untilN r@Ratchet {rcRcv = Just rr@RcvRatchet {rcCKr, rcHKr}, rcNr}
      | rcNr > untilN = Left CERatchetDuplicateMessage
      | rcNr + maxSkip < untilN = Left CERatchetTooManySkipped
      | rcNr == untilN = Right (r, M.empty)
      | otherwise =
        let (rcCKr', rcNr', mks) = advanceRcvRatchet (untilN - rcNr) rcCKr rcNr M.empty
            r' = r {rcRcv = Just rr {rcCKr = rcCKr'}, rcNr = rcNr'}
         in Right (r', M.singleton rcHKr mks)
    advanceRcvRatchet :: Word32 -> RatchetKey -> Word32 -> SkippedHdrMsgKeys -> (RatchetKey, Word32, SkippedHdrMsgKeys)
    advanceRcvRatchet 0 ck msgNs mks = (ck, msgNs, mks)
    advanceRcvRatchet n ck msgNs mks =
      let (ck', mk, iv, _) = chainKdf ck
          mks' = M.insert msgNs (MessageKey mk iv) mks
       in advanceRcvRatchet (n - 1) ck' (msgNs + 1) mks'
    decryptSkipped :: EncMessageHeader -> EncRatchetMessage -> ExceptT CryptoError IO (SkippedMessage a)
    decryptSkipped encHdr encMsg = tryDecryptSkipped SMNone $ M.assocs rcMKSkipped
      where
        tryDecryptSkipped :: SkippedMessage a -> [(HeaderKey, SkippedHdrMsgKeys)] -> ExceptT CryptoError IO (SkippedMessage a)
        tryDecryptSkipped SMNone ((hk, mks) : hks) = do
          tryE (decryptHeader hk encHdr) >>= \case
            Left CERatchetHeader -> tryDecryptSkipped SMNone hks
            Left e -> throwE e
            Right hdr@MsgHeader {msgNs} ->
              case M.lookup msgNs mks of
                Nothing ->
                  let nextRc
                        | maybe False ((== hk) . rcHKr) rcRcv = Just SameRatchet
                        | hk == rcNHKr rc = Just AdvanceRatchet
                        | otherwise = Nothing
                   in pure $ SMHeader nextRc hdr
                Just mk -> do
                  msg <- decryptMessage mk encMsg
                  pure $ SMMessage (msg, rc, SMDRemove hk msgNs)
        tryDecryptSkipped r _ = pure r
    decryptRcHeader :: Maybe RcvRatchet -> EncMessageHeader -> ExceptT CryptoError IO (RatchetStep, MsgHeader a)
    decryptRcHeader Nothing hdr = decryptNextHeader hdr
    decryptRcHeader (Just RcvRatchet {rcHKr}) hdr =
      -- header = HDECRYPT(state.HKr, enc_header)
      ((SameRatchet,) <$> decryptHeader rcHKr hdr) `catchE` \case
        CERatchetHeader -> decryptNextHeader hdr
        e -> throwE e
    -- header = HDECRYPT(state.NHKr, enc_header)
    decryptNextHeader hdr = (AdvanceRatchet,) <$> decryptHeader (rcNHKr rc) hdr
    decryptHeader k EncMessageHeader {ehBody, ehAuthTag, ehIV} = do
      header <- decryptAEAD k ehIV rcAD ehBody ehAuthTag `catchE` \_ -> throwE CERatchetHeader
      parseE' CryptoHeaderError smpP header
    decryptMessage :: MessageKey -> EncRatchetMessage -> ExceptT CryptoError IO (Either CryptoError ByteString)
    decryptMessage (MessageKey mk iv) EncRatchetMessage {emHeader, emBody, emAuthTag} =
      -- DECRYPT(mk, cipher-text, CONCAT(AD, enc_header))
      -- TODO add associated data
      tryE $ decryptAEAD mk iv (rcAD <> emHeader) emBody emAuthTag

rootKdf :: (AlgorithmI a, DhAlgorithm a) => RatchetKey -> PublicKey a -> PrivateKey a -> (RatchetKey, RatchetKey, Key)
rootKdf (RatchetKey rk) k pk =
  let dhOut = dhBytes' $ dh' k pk
      (rk', ck, nhk) = hkdf3 rk dhOut "SimpleXRootRatchet"
   in (RatchetKey rk', RatchetKey ck, Key nhk)

chainKdf :: RatchetKey -> (RatchetKey, Key, IV, IV)
chainKdf (RatchetKey ck) =
  let (ck', mk, ivs) = hkdf3 "" ck "SimpleXChainRatchet"
      (iv1, iv2) = B.splitAt 16 ivs
   in (RatchetKey ck', Key mk, IV iv1, IV iv2)

hkdf3 :: ByteString -> ByteString -> ByteString -> (ByteString, ByteString, ByteString)
hkdf3 salt ikm info = (s1, s2, s3)
  where
    prk = H.extract salt ikm :: H.PRK SHA512
    out = H.expand prk info 96
    (s1, rest) = B.splitAt 32 out
    (s2, s3) = B.splitAt 32 rest