commit d88aa2b009af679172881ecc8891da32a56b9ea9 Author: George Kadianakis desnacked@riseup.net Date: Wed Apr 16 04:43:20 2014 +0300

Add xxx-single-guard-node.txt . --- proposals/xxx-single-guard-node.txt | 274 +++++++++++++++++++++++++++++++++++ 1 file changed, 274 insertions(+)

diff --git a/proposals/xxx-single-guard-node.txt b/proposals/xxx-single-guard-node.txt new file mode 100644 index 0000000..94cb736 --- /dev/null +++ b/proposals/xxx-single-guard-node.txt @@ -0,0 +1,274 @@ +Filename: xxx-single-guard-node.txt +Title: The move to a single guard node +Author: George Kadianakis +Created: 2014-03-22 +Status: Draft and potentially a bad idea + +0. Introduction + + It has been suggested that reducing the number of guard nodes of + each user and increasing the guard node rotation period will make + Tor more resistant against certain attacks [0]. + + For example, an attacker who sets up guard nodes and hopes for a + client to eventually choose them as their guard will have much less + probability of succeeding in the long term. + + Currently, every client picks 3 guard nodes and keeps them for 2 to + 3 months (since 0.2.4.12-alpha) before rotating them. In this + document, we propose the move to a single guard per client and an + increase of the rotation period to 9 to 10 months. + +1. Proposed changes + +1.1. Switch to one guard per client + + When this proposal becomes effective, clients will switch to using + a single guard node. + + That is, in its first startup, Tor picks one guard and stores its + identity persistently to disk. Tor uses that guard node as the + first hop of its circuits from thereafter. + + If that Guard node ever becomes unusable, rather than replacing it, + Tor picks a new guard and adds it to the end of the list. When + choosing the first hop of a circuit, Tor tries all guard nodes from + the top of the list sequentially till it finds a usable guard node. + + A Guard node is considered unusable according to section "5. Guard + nodes" in path-spec.txt. The rest of the rules from that section + apply here too. XXX which rules specifically? + + XXX Do we need to specify how already existing clients migrate? + +1.1.1. Alternative behavior to section 1.1 + + Here is an alternative behavior than the one specified in the + previous section. It's unclear which one is better. + + Instead of picking a new guard when the old guard becomes unusable, + we pick a number of guards in the beginning but only use the top + usable guard each time. When our guard becomes unusable, we move to + the guard below it in the list. + + This behavior _might_ make some attacks harder; for example, an + attacker who shoots down your guard in the hope that you will pick + his guard next, is now forced to have evil guards in the network at + the time you first picked your guards. + + However, this behavior might also influence performance, since a + guard that was fast enough 7 months ago, might not be this fast + today. Should we reevaluate our opinion based on the last + consensus, when we have to pick a new guard? Also, a guard that was + up 7 months ago might be down today, so we might end up sampling + from the current network anyway. + +1.2. Increase guard rotation period + + When this proposal becomes effective, Tor clients will set the + lifetime of each guard to a random time between 9 to 10 months. + + If Tor tries to use a guard whose age is over its lifetime value, + the guard gets discarded (also from persistent storage) and a new + one is picked in its place. + + XXX We didn't do any analysis on extending the rotation period. + For example, we don't even know the average age of guards, and + whether all guards stay around for less than 9 months anyway. + Maybe we should do some analysis before proceeding? + + XXX The guard lifetime should be controlled using the + (undocumented?) GuardLifetime consensus option, right? + +1.2.1. Alternative behavior to section 1.2 + + Here is an alternative behavior than the one specified in the + previous section. It's unclear which one is better. + + Similar to section 1.2, but instead of rotating to completely new + guard nodes after 9 months, we pick a few extra guard nodes in the + beginning, and after 9 months we delete the already used guard + nodes and use the one after them. + + This has approximately the same tradeoffs as section 1.1.1. + + Also, should we check the age of all of our guards periodically, or + only check them when we try to use them? + +1.3. Age of guard as a factor on guard probabilities + + By increasing the guard rotation period we also increase the lack + of clients for young guards since clients will rotate guards even + more infrequently now (see 'Phase three' of [1]). + + We can try to mitigate this phenomenon by giving higher priority to + young guards to be picked as guards: + + To do so, everytime an authority needs to vote for a guard, it + reads a set of consensus documents spanning the past NNN months, and + calculates the age of the guard; that is, in how many consensuses + its public key has been included in the past. + + The authorities include the age of each guard by appending + '[SP "Age=" INT]' in the guard's "w" line. + + When a client picks a guard, it applies the age of each guard as a + weight on its guard probability. XXX unspecified how + + XXX How much should the age of a guard influence its probability? + Should we say that a guard that just appeared should have 10% + more chance of being selected as a guard node than the oldest + guard in town? + + XXX Should the authorities include the age itself, or just the + weight that clients should apply to the probability? + + XXX Is this risky? Maybe we shouldn't give too much priority to new + guards, otherwise an adversary can start up a few new relays + every month, enjoy maximum priority when they get the guard + flag, leave them running for a bit till the next batch gets the + guard flag and then trash them. + +1.4. Raise the bandwidth threshold for being a guard + + From dir-spec.txt: + "Guard" -- A router is a possible 'Guard' if its Weighted Fractional + Uptime is at least the median for "familiar" active routers, and if + its bandwidth is at least median or at least 250KB/s. + + When this proposal becomes effective, authorities should change the + bandwidth threshold for being a guard node to 2000KB/s instead of + 250KB/s. + + Implications of raising the bandwidth threshold are discussed in + section 2.3. + + XXX Is this insane? It's an 8-fold increase. + +2. Discussion + +2.1. Guard node set fingerprinting + + With the old behavior of three guard nodes per user, it was + extremely unlikely for two users to have the same guard node + set. Hence the set of guard nodes acted as a fingerprint to each + user. + + When this proposal becomes effective, each user will have one guard + node. We believe that this slightly reduces the effectiveness of + this fingerprint since users who pick a popular guard node will now + blend in with thousands of other users. However, clients who pick a + slow guard will still have a small anonymity set [2]. + + All in all, this proposal slightly improves the situation of guard + node fingerprinting, but does not solve it. See the next section + for a suggested scheme that would further fix the guard node set + fingerprinting problem + +2.1.1. Potential fingerprinting solution: Guard buckets + + One of the suggested alternatives that moves us closer to solving + the guard node fingerprinting problem, would be to split the list + of N guard nodes into buckets of K guards, and have each client + pick a bucket [3]. + + This reduces the fingerprint from N-choose-k to N/k guard set + choices; it also allows users to have multiple guard nodes which + provides reliability and performance. + + Unfortunately, the implementation of this idea is not as easy and + its anonymity effects are not well understood so we had to reject + this alternative for now. + +2.2. What about 'multipath' schemes like Conflux? + + By switching to one guard, we rule out the deployment of + 'multipath' systems like Conflux [4] which build multiple circuits + through the Tor network and attempt to detect and use the most + efficient circuits. + + On the other hand, the 'Guard buckets' idea outlined in section + 2.1.1 works well with Conflux-type schemes so it's still worth + considering. + +2.3. Implications of raising the bandwidth threshold for guards + + By raising the bandwidth threshold for being a guard we directly + affect the performance and anonymity of Tor clients. We performed a + brief analysis of the implications of switching to one guard and + the results imply that the changes are not tragic [2]. + + Specifically, it seems that the performance of about half of the + clients will degrade slightly, but the performance of the other + half will remain the same or even improve. + + Also, it seems that the powerful guard nodes of the Tor network + have enough total bandwidth capacity to handle client traffic even + if some slow guard nodes get discarded. + + On the anonymity side, by increasing the bandwidth threshold to + 2MB/s we half our guard nodes; we discard 1000 out of 2000 + guards. Even if this seems like a substantial diversity loss, it + seems that the 1000 discarded guard nodes had a very small chance + of being selected in the first place (7% chance of any of the being + selected). + + However, it's worth noting that the performed analysis was quite + brief and the implications of this proposal are complex, so we + should be prepared for surprises. + +2.4. Should we stop building circuits after a number of guard failures? + + Inspired by academic papers like the Sniper attack [5], a powerful + attacker can choose to shut down guard nodes till a client is + forced to pick an attacker controlled guard node. Similarly, a + local network attacker can kill all connections towards all guards + except the ones she controls. + + This is a very powerful attack that is hard to defend against. A + naive way of defending against it would be for Tor to refuse to + build any more circuits after a number of guard node failures have + been experienced. + + Unfortunately, we believe that this is not a sufficiently strong + countermeasure since puzzled users will not comprehend the + confusing warning message about guard node failures and they will + instead just uninstall and reinstall TBB to fix the issue. + +2.5. What this proposal does not propose + + Finally, this proposal does not aim to solve all the problems with + guard nodes. This proposal only tries to solve some of the problems + whose solution is analyzed sufficiently and seems harmless enough + to us. + + For example, this proposal does not try to solve: + - Guard enumeration attacks. We need guard layers or virtual + circuits for this [6]. + - The guard node set fingerprinting problem [7] + - The fact that each isolation profile or virtual identity should + have its own guards. + +XXX It would also be nice to have some way to easily revert back to 3 + guards if we later decide that a single guard was a very stupid + idea. + +References: + +[0]: https://blog.torproject.org/blog/improving-tors-anonymity-changing-guard-par... + http://freehaven.net/anonbib/#wpes12-cogs + +[1]: https://blog.torproject.org/blog/lifecycle-of-a-new-relay + +[2]: https://lists.torproject.org/pipermail/tor-dev/2014-March/006458.html + +[3]: https://trac.torproject.org/projects/tor/ticket/9273#comment:4 + +[4]: http://freehaven.net/anonbib/#pets13-splitting + +[5]: https://blog.torproject.org/blog/new-tor-denial-service-attacks-and-defenses + +[6]: https://trac.torproject.org/projects/tor/ticket/9001 + +[7]: https://trac.torproject.org/projects/tor/ticket/10969 +