On Tue, Apr 15, 2014 at 02:03:43PM +0100, Ximin Luo wrote:
## The problem
The problem with the above structure, is that it is incompatible with the metaphor of connecting to a specific endpoint. This is what the PT spec is about, even though it does not explicitly mention this viewpoint. Instead, meek and flashproxy provide the metaphor of connecting to a global homogeneous service.
This has positive consequences, such as the user no longer having to bother to find Bridges, but also has several negative consequences:
- The Tor client can no longer authenticate the endpoint. Although
currently Tor makes this optional, it is strongly recommended, to prevent a MitM between the client and the server. Even if the midpoint does this, this is not end-to-end authentication that we would require for strong security.
I see this somewhat differently. You still choose and authenticate the second and third hops. I heard from Roger that it is a sort of accident that bridge-using circuits use three hops, anyway. It should be that there are four: the first hop is your untrusted bridge address you got from wherever, and the second is your guard. Would a design like that make most of these issues go away?
There's an old ticket here, "Let bridge users specify that they don't care if their bridge changes fingerprint." https://trac.torproject.org/projects/tor/ticket/3292 which also ties with this blog post "Different Ways to Use a Bridge." https://blog.torproject.org/blog/different-ways-use-bridge Completion of #3292 would be a beautiful thing, I think, for flash proxy, as it would allow us easily to round-robin multiple websocket bridges. (Currently you can't do that because the tor client freaks out; see https://trac.torproject.org/projects/tor/ticket/7153#comment:5.)
Some other relevant tickets about non-authentication of bridges:
"analyze security tradeoffs from using a socks proxy vs a bridge to reach the Tor network" https://trac.torproject.org/projects/tor/ticket/2764 For "socks proxy", substitute "indirect proxy", and it works the same. I think of indirect proxies like flash proxy as untrusted unauthenticated things that just get you to the Tor network, which you then authenticate, the same as a socks proxy. The quotes there that I agree with are "from a *security* perspective (for a broad definition of security), is there really any difference between a socks proxy and a bridge relay?" and "I don't see any huge roadblocks to having bridges that are just vanilla proxies. We should deploy them if we can make them usable, and maybe someday somebody will show us it was a bad idea."
"Tor build variant to support lightweight socks bridge" https://trac.torproject.org/projects/tor/ticket/3466
David Fifield