WebSocket frame decoding in Python
decode, mask, python, websocket
Solution
I've just looked at this frames one more time and I've realized that the first byte:
193[DEC] == 11000001[BIN]
which according to the specification means that `RSV1` is used (the second bit is `1`). This seems to mean that the extension is used and indeed in my handshake code I've found the following line:
ext = req.headers.get("Sec-WebSocket-Extensions", "").strip()
if ext:
res += "Sec-WebSocket-Extensions: %s\r\n" % ext
and in Chrome I can see:
Sec-WebSocket-Extensions:x-webkit-deflate-frame
when I do the handshake.
I'm not sure yet how these extensions work but removing these 3 lines from the handshake code fixes the issue.
Problem
I've tried to implement WebSocket frame unmasking algorithm (based on this: How can I send and receive WebSocket messages on the server side?) on the server side. Here's what I've got: ``` def decode(data): frame = bytearray(data) length = frame[1] & 127 indexFirstMask = 2 if length == 126: indexFirstMask = 4 elif length == 127: indexFirstMask = 10 indexFirstDataByte = indexFirstMask + 4 mask = frame[indexFirstMask:indexFirstDataByte] i = indexFirstDataByte j = 0 decoded = [] while i < len(frame): decoded.append(frame[i] ^ mask[j%4]) i += 1 j += 1 print decoded return "".join(chr(byte) for byte in decoded) ``` However I get very strange results. On JavaScript side: ``` w = new WebSocket("ws://localhost:2000"); w.send("test"); w.send("test"); w.send("test"); w.send("test"); ``` produces on the server side: ``` [42, 73, 45, 46, 1, 0] [42, 1, 98, 0, 0] [2, 97, 0, 0] [2, 97, 0, 0] ``` More calls to `w.send("test");` produces `[2, 97, 0, 0]`. Also first two arrays have length >4 (number of characters in word `test`). And none of these gets converted to word `test`. It seeems that I must be doing something wrong in my decoding code. What's causing that? Any help? EDIT Have look at raw frames: ``` [193, 134, 48, 166, 232, 11, 26, 239, 197, 37, 49, 166] [193, 133, 57, 161, 169, 218, 19, 160, 203, 218, 57] [193, 132, 150, 97, 124, 54, 148, 0, 124, 54] [193, 132, 163, 26, 102, 249, 161, 123, 102, 249] [193, 132, 238, 212, 210, 156, 236, 181, 210, 156] ``` Of course these numbers are a bit random (due to masking), but note that second byte (which is supposed to represent the length of payload) is `134`, then `133` and then always `132`. Also first two frames are longer then other frames. What's going on here?