[zeromq-dev] Identities as cookies

Oliver Smith oliver at kfs.org
Fri May 13 10:11:46 CEST 2011

I'm not sure I've 100% understood identities correctly, or maybe I 
have... I'd like to use identities as a sort of validated login cookie 
to make associating incoming data with a connection a wee bit easier, 
something like:

	import zmq ; zc = zmq.Context(1)

	# Perform authentication.
	sockAuth = zc.socket(zmq.REQ)
	reply = sockAuth.recv()
	sockAuth = None

	if not reply or reply == "Go away":
		# If this was real code, the message would be far sillier.
		raise Exception("Login denied, or something. Possibly cake.")

	# Obviously, sending the sessionId in plain text like this
	# would be a bad idea, but I only want to write so much
	# pseduo code. Writing comments, that I could do all day...
	sessionId = reply.sessionId

	# Now log in to the service.
	sockSvc = zc.socket(zmq.XREQ)
	sockSvc.setsockopt(zmq.IDENTITY, sessionId)
	sockSvc.send("Hello! You may recall me from such credentials as " + fancySchmancyEncryptedCredentials + random.random())
	reply = sockSvc.recv()

	if reply[:8] != "Biscuits":
		raise Exception("Should have gone with the cake :(")

	# I wish I hadn't opened with a mention of cookies...
	return sockSvc, reply[8:]

If cookies work the way I think, the only issue with this would be the 
vulnerability to MITM injection?

If that's the case, my thinking is that the session ID + initial 
[encrypted, not clear] exchange would double as the initial 
application-level component for crypto seeding.

That would [effectively] prevent a different application instance or 
imposter or bug causing a different connection to correctly communicate 
as that identity, but would it also prevent the genuine application from 
re-establishing the same tcp connection in the case of intermediate 
network failure? e.g.

	sock, seed = aboveCode()
	system "ifconfig eth0 down"
	system "ifconfig eth0 up"
	response = sock.recv()

If this is all sounding feasible...
     - How would I poke ZeroMQ to give me a specific delivery tolerance?
             sock.setsockopt(zmq.TIMEOUT, 30 * 1000)
     - Is there a way to tell the XREP socket on the server to flush any 
backlog/close the underlying tcp socket associated with a particular 

	serverSideSock = zc.socket(zmq.XREP)
	def noMoreCakeFor(identity):
		serverSideSock.setsockopt(zmq.DISCONNECT_IDENTITY, identity)

     - Most importantly, where and how much overhead does the identity 
impose across a single ZMQ-hop?
                 client <----wan-but-no-devices---> server
        Is it only in the handshaking? Is it repeated periodically? Or 
is it a per-message overhead?

- Oliver

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