mitmproxy/netlib/websockets/protocol.py

"""
Colleciton of utility functions that implement small portions of the RFC6455
WebSockets Protocol Useful for building WebSocket clients and servers.

Emphassis is on readabilty, simplicity and modularity, not performance or
completeness

This is a work in progress and does not yet contain all the utilites need to
create fully complient client/servers #
Spec: https://tools.ietf.org/html/rfc6455

The magic sha that websocket servers must know to prove they understand
RFC6455
"""

from __future__ import absolute_import
import base64
import hashlib
import os

import six

from netlib import http, strutils

websockets_magic = b'258EAFA5-E914-47DA-95CA-C5AB0DC85B11'
VERSION = "13"


class Masker(object):

    """
        Data sent from the server must be masked to prevent malicious clients
        from sending data over the wire in predictable patterns

        Servers do not have to mask data they send to the client.
        https://tools.ietf.org/html/rfc6455#section-5.3
    """

    def __init__(self, key):
        self.key = key
        self.offset = 0

    def mask(self, offset, data):
        result = bytearray(data)
        if six.PY2:
            for i in range(len(data)):
                result[i] ^= ord(self.key[offset % 4])
                offset += 1
            result = str(result)
        else:

            for i in range(len(data)):
                result[i] ^= self.key[offset % 4]
                offset += 1
            result = bytes(result)
        return result

    def __call__(self, data):
        ret = self.mask(self.offset, data)
        self.offset += len(ret)
        return ret


class WebsocketsProtocol(object):

    def __init__(self):
        pass

    @classmethod
    def client_handshake_headers(self, key=None, version=VERSION):
        """
            Create the headers for a valid HTTP upgrade request. If Key is not
            specified, it is generated, and can be found in sec-websocket-key in
            the returned header set.

            Returns an instance of http.Headers
        """
        if not key:
            key = base64.b64encode(os.urandom(16)).decode('ascii')
        return http.Headers(
            sec_websocket_key=key,
            sec_websocket_version=version,
            connection="Upgrade",
            upgrade="websocket",
        )

    @classmethod
    def server_handshake_headers(self, key):
        """
          The server response is a valid HTTP 101 response.
        """
        return http.Headers(
            sec_websocket_accept=self.create_server_nonce(key),
            connection="Upgrade",
            upgrade="websocket"
        )

    @classmethod
    def check_client_handshake(self, headers):
        if headers.get("upgrade") != "websocket":
            return
        return headers.get("sec-websocket-key")

    @classmethod
    def check_server_handshake(self, headers):
        if headers.get("upgrade") != "websocket":
            return
        return headers.get("sec-websocket-accept")

    @classmethod
    def create_server_nonce(self, client_nonce):
        return base64.b64encode(hashlib.sha1(strutils.always_bytes(client_nonce) + websockets_magic).digest())