On Tue, Nov 27, 2018 at 08:23:21AM -0500, Nick Mathewson wrote: > ### Traffic Fingerprinting of TCP-like systems > > Today, because Tor terminates TCP at the guard node, there is > limited ability for the exit node to fingerprint client TCP > behavior (aside from perhaps measuring some effects on traffic > volume, but those are not likely preserved across the Tor network). > > However, when using a TCP-like system for end-to-end congestion > control, flow control, and reliability, the exit relay will be able > to make inferences about client implementation and conditions based > on its behavior. > > Different implementations of TCP-like systems behave differently. > Either party on a stream can observe the packets as they arrive to > notice cells from an unusual implementation. They can probe the > other side of the stream, nmap-style, to see how it responds to > various inputs. > > If two TCP-like implementations differ in their retransmit or timeout > behavior, an attacker can use this to distinguish them by carefully > chosen patterns of dropped traffic. Such an attacker does not even > need to be a relay, if it can cause DTLS packets between relays to > be dropped or reordered. > > This class of attacks is solvable, especially if the exact same > TCP-like implementation is used by all clients, but it also requires > careful consideration and additional constraints to be placed on the > TCP stack(s) in use that are not usually considered by TCP > implementations -- particularly to ensure that they do not depend on > OS-specific features or try to learn things about their environment > over time, across different connections.
Thanks, this is nice and thoughtful analysis. Does the word "clients" in the last paragraph meant to exclude servers? Or should I understand something like "peers" that includes clients and servers? I'm trying to think of how fingerprinting a server could be useful to an attacker. An onion service doesn't count as a server--at the layer of the TCP-like protocol, it's a client, with the RP as server. Related to implementation differences is configuration. If there are knobs that let a user control, say, the reassembly buffer size, then some users will use them and make their protocol fingerprint differ. _______________________________________________ tor-dev mailing list tor-dev@lists.torproject.org https://lists.torproject.org/cgi-bin/mailman/listinfo/tor-dev
