One of the oldest research threads regarding Tor is trying to figure out how close you could get in real life to the
global passive adversary that’s known to be able to deanonymize all communications. This is a new entry in that line of research, from HotNets 2014.
At the largest scale, the global Internet is administratively divided into
autonomous systems (ASes) that exchange traffic, using BGP for configuration. Any given AS can only communicate with a small number of direct
peers, so a stream of packets will normally pass through many different ASes on the way to its destination. It’s well-known that AS-operated backbone routers are in an excellent position to mount traffic-correlation attacks on Tor, particularly if they collude  . The key observation in this paper is that, by manipulating BGP, a malicious AS can observe traffic that wouldn’t naturally flow through it.
BGP is an old protocol, originally specified in 1989; like most of our older protocols, it assumes that all participants are cooperative and honest. Any backbone router can
announce that it is now capable of forwarding packets to a
prefix (set of IP addresses) and the rest of the network will believe it. Incidents where traffic is temporarily redirected to an AS that either can’t get it to the destination at all, or can only do so suboptimally, are commonplace, and people argue about how malicious these are.    Suppose an adversary can observe one end of a Tor circuit—perhaps they control the ISP for a Tor client. They also have some reason to suspect a particular destination for the traffic. They use BGP to hijack traffic to the suspected destination, passing it on so that the client doesn’t notice anything. They can now observe both ends of the circuit and confirm their suspicions. They might not get to see traffic in both directions, but the authors also demonstrate that a traffic-correlation attack works in principle even if you can only see the packet flow in one direction, thanks to TCP acknowledgments.
Making this worse, high-bandwidth, long-lived Tor relays (which are disproportionately likely to be used for either end of a circuit) are clustered in a small number of ASes worldwide. This means an adversary can do
dragnet surveillance by hijacking all traffic to some of those ASes; depending on its own position in the network, this might not even appear abnormal. The adversary might even be one of those ASes, or an agency in a position to lean on its operators.
The countermeasures proposed in this paper are pretty weak; they would only operate on a timescale of hours to days, whereas a BGP hijack can happen, and stop happening, in a matter of minutes. I don’t see a good fix happening anywhere but in the routing protocol itself. Unfortunately, routing protocols that do not assume honest participants are still a topic of basic research. (I may get to some of those papers eventually.) There are proposals for adding a notion of
this AS is authorized to announce this prefix to BGP  but those have all the usual problems with substituting
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