Note and TODO regarding crypto packet padding.

1 files changed, 29 insertions, 1 deletions

diff --git a/src/Network/Tox/Crypto/Transport.hs b/src/Network/Tox/Crypto/Transport.hs
index b89bde46..3133ee9b 100644
--- a/src/Network/Tox/Crypto/Transport.hs
+++ b/src/Network/Tox/Crypto/Transport.hs
@@ -154,10 +154,38 @@ data CryptoData = CryptoData
       -- | [  uint32_t packet number if lossless
       --   , sendbuffer buffer_end  if lossy   , (big endian)]
     , bufferEnd :: Word32
-      -- | [data]
+      -- | [data] (TODO See Note [Padding])
     , bufferData :: CryptoMessage
     }
 
+{-
+Note [Padding]
+
+TODO: The 'bufferData' field of 'CryptoData' should probably be something like
+/Padded CryptoMessage/ because c-toxcore strips leading zeros on incoming and
+pads leading zeros on outgoing packets.
+
+After studying c-toxcore (at commit c49a6e7f5bc245a51a3c85cc2c8b7f881c412998),
+I've determined the following behavior.
+
+Incoming: All leading zero bytes are stripped until possibly the whole packet
+is consumed (in which case it is discarded).  This happens at
+toxcore/net_crypto.c:1366:handle_data_packet_core().
+
+Outgoing: The number of zeros added is:
+
+    padding_length len = (1373 - len) `mod` 8 where
+
+where /len/ is the size of the non-padded CryptoMessage.  This happens at
+toxcore/net_crypto.c:936:send_data_packet_helper()
+
+The number 1373 is written in C as MAX_CRYPTO_DATA_SIZE which is defined in
+terms of the max /NetCrypto/ packet size (1400) minus the minimum possible size
+of an id-byte (1) and a /CryptoPacket Encrypted/ ( 2 + 4 + 4 + 16 ).
+
+One effect of this is that short messages will be padded to at least 5 bytes.
+-}
+
 instance Serialize CryptoData where
     get = CryptoData <$> get <*> get <*> get
     put (CryptoData start end dta) = put start >> put end >> put dta