March 2013 - tor-commits - lists.torproject.org

[torspec/master] Describe replay prevention for INTRODUCE2 cells
by nickm＠torproject.org 15 Mar '13

15 Mar '13

commit 70d87e56d55a0a4f696a896d0ffcd7bae5cad0fc Author: Nick Mathewson <nickm(a)torproject.org> Date: Mon Mar 11 17:31:15 2013 -0400 Describe replay prevention for INTRODUCE2 cells Resolves ticket #4608 --- rend-spec.txt | 15 ++++++++++++++- 1 files changed, 14 insertions(+), 1 deletions(-) diff --git a/rend-spec.txt b/rend-spec.txt index 6c90c70..9d207d7 100644 --- a/rend-spec.txt +++ b/rend-spec.txt @@ -683,7 +683,20 @@ determined later; the current implementation sends a single '1' byte on failure.) - When Bob's OP receives the RELAY_COMMAND_INTRODUCE2 cell, it decrypts it + When Bob's OP receives the RELAY_COMMAND_INTRODUCE2 cell, it first checks + for a replay. Because of the (undesirable!) malleability of the hybrid + encryption, Bob's OP should only check whether the RSA-encrypted part is + replayed. It does this by keeping, for each introduction key, a list of + cryptographic digests of all the RSA-encrypted parts of the INTRODUCE2 + cells that it's seen, and dropping any INTRODUCE2 cell whose RSA-encrypted + part it has seen before. When Bob's OP stops using a given introduction + key, it drops the replay cache corresponding to that key. + + (Versions of Tor before 0.2.3.9-alpha used the timestamp in the INTRODUCE2 + cell to limit the lifetime of entries in the replay cache. This proved to + be fragile, due to insufficiently synchronized clients.) + + Assuming that the cell has not been replayed, Bob's server decrypts it with the private key for the corresponding hidden service, and extracts the rendezvous point's nickname, the rendezvous cookie, and the value of g^x chosen by Alice.

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[torspec/master] Merge branch 'bug4608_v2'
by nickm＠torproject.org 15 Mar '13

15 Mar '13

commit 41c93e4f7f492043aa231fefc9d952f2b5f017fb Merge: 2ac6513 70d87e5 Author: Nick Mathewson <nickm(a)torproject.org> Date: Fri Mar 15 11:50:44 2013 -0400 Merge branch 'bug4608_v2' rend-spec.txt | 15 ++++++++++++++- 1 files changed, 14 insertions(+), 1 deletions(-)

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[torspec/master] Move incentives.txt over from the tor repository into torspec
by nickm＠torproject.org 15 Mar '13

15 Mar '13

commit 2ac6513f3769ccc28ce3cef1fa3b91331b4216c4 Author: Nick Mathewson <nickm(a)torproject.org> Date: Fri Mar 15 11:26:48 2013 -0400 Move incentives.txt over from the tor repository into torspec --- proposals/ideas/incentives.txt | 479 ++++++++++++++++++++++++++++++++++++++++ 1 files changed, 479 insertions(+), 0 deletions(-) diff --git a/proposals/ideas/incentives.txt b/proposals/ideas/incentives.txt new file mode 100644 index 0000000..850a0d0 --- /dev/null +++ b/proposals/ideas/incentives.txt @@ -0,0 +1,479 @@ + + Tor Incentives Design Brainstorms + +1. Goals: what do we want to achieve with an incentive scheme? + +1.1. Encourage users to provide good relay service (throughput, latency). +1.2. Encourage users to allow traffic to exit the Tor network from + their node. + +2. Approaches to learning who should get priority. + +2.1. "Hard" or quantitative reputation tracking. + + In this design, we track the number of bytes and throughput in and + out of nodes we interact with. When a node asks to send or receive + bytes, we provide service proportional to our current record of the + node's value. One approach is to let each circuit be either a normal + circuit or a premium circuit, and nodes can "spend" their value by + sending and receiving bytes on premium circuits: see section 4.1 for + details of this design. Another approach (section 4.2) would treat + all traffic from the node with the same priority class, and so nodes + that provide resources will get and provide better service on average. + + This approach could be complemented with an anonymous e-cash + implementation to let people spend reputations gained from one context + in another context. + +2.2. "Soft" or qualitative reputation tracking. + + Rather than accounting for every byte (if I owe you a byte, I don't + owe it anymore once you've spent it), instead I keep a general opinion + about each server: my opinion increases when they do good work for me, + and it decays with time, but it does not decrease as they send traffic. + Therefore we reward servers who provide value to the system without + nickle and diming them at each step. We also let them benefit from + relaying traffic for others without having to "reserve" some of the + payment for their own use. See section 4.3 for a possible design. + +2.3. Centralized opinions from the reputation servers. + + The above approaches are complex and we don't have all the answers + for them yet. A simpler approach is just to let some central set + of trusted servers (say, the Tor directory servers) measure whether + people are contributing to the network, and provide a signal about + which servers should be rewarded. They can even do the measurements + via Tor so servers can't easily perform only when they're being + tested. See section 4.4. + +2.4. Reputation servers that aggregate opinions. + + The option above has the directory servers doing all of the + measurements. This doesn't scale. We can set it up so we have "deputy + testers" -- trusted other nodes that do performance testing and report + their results. + + If we want to be really adventurous, we could even + accept claims from every Tor user and build a complex weighting / + reputation system to decide which claims are "probably" right. + One possible way to implement the latter is something similar to + EigenTrust [http://www.stanford.edu/~sdkamvar/papers/eigentrust.pdf] + where the opinion of nodes with high reputation more is weighted + higher. + +3. Related issues we need to keep in mind. + +3.1. Relay and exit configuration needs to be easy and usable. + + Implicit in all of the above designs is the need to make it easy to + run a Tor server out of the box. We need to make it stable on all + common platforms (including XP), it needs to detect its available + bandwidth and not overreach that, and it needs to help the operator + through opening up ports on his firewall. Then we need a slick GUI + that lets people click a button or two rather than editing text files. + + Once we've done all this, we'll hit our first big question: is + most of the barrier to growth caused by the unusability of the current + software? If so, are the rest of these incentive schemes superfluous? + +3.2. The network effect: how many nodes will you interact with? + + One of the concerns with pairwise reputation systems is that as the + network gets thousands of servers, the chance that you're going to + interact with a given server decreases. So if 90% of interactions + don't have any prior information, the "local" incentive schemes above + are going to degrade. This doesn't mean they're pointless -- it just + means we need to be aware that this is a limitation, and plan in the + background for what step to take next. (It seems that e-cash solutions + would scale better, though they have issues of their own.) + +3.3. Guard nodes + + As of Tor 0.1.1.11, Tor users pick from a small set of semi-permanent + "guard nodes" for their first hop of each circuit. This seems like it + would have a big impact on pairwise reputation systems since you + will only be cashing in on your reputation to a few people, and it is + unlikely that a given pair of nodes will use each other as guard nodes. + + What does this imply? For one, it means that we don't care at all + about the opinions of most of the servers out there -- we should + focus on keeping our guard nodes happy with us. + + One conclusion from that is that our design needs to judge performance + not just through direct interaction (beginning of the circuit) but + also through indirect interaction (middle of the circuit). That way + you can never be sure when your guards are measuring you. + + Both 3.2 and 3.3 may be solved by having a global notion of reputation, + as in 2.3 and 2.4. However, computing the global reputation from local + views could be expensive (O(n^2)) when the network is really large. + +3.4. Restricted topology: benefits and roadmap. + + As the Tor network continues to grow, we will need to make design + changes to the network topology so that each node does not need + to maintain connections to an unbounded number of other nodes. For + anonymity's sake, we may partition the network such that all + the nodes have the same belief about the divisions and each node is + in only one partition. (The alternative is that every user fetches + his own random subset of the overall node list -- this is bad because + of intersection attacks.) + + Therefore the "network horizon" for each user will stay bounded, + which helps against the above issues in 3.2 and 3.3. + + It could be that the core of long-lived servers will all get to know + each other, and so the critical point that decides whether you get + good service is whether the core likes you. Or perhaps it will turn + out to work some other way. + + A special case here is the social network, where the network isn't + partitioned randomly but instead based on some external properties. + Social network topologies can provide incentives in other ways, because + people may be more inclined to help out their friends, and more willing + to relay traffic if most of the traffic they are relaying comes + from their friends. It also opens the door for out-of-band incentive + schemes because of the out-of-band links in the graph. + +3.5. Profit-maximizing vs. Altruism. + + There are some interesting game theory questions here. + + First, in a volunteer culture, success is measured in public utility + or in public esteem. If we add a reward mechanism, there's a risk that + reward-maximizing behavior will surpass utility- or esteem-maximizing + behavior. + + Specifically, if most of our servers right now are relaying traffic + for the good of the community, we may actually *lose* those volunteers + if we turn the act of relaying traffic into a selfish act. + + I am not too worried about this issue for now, since we're aiming + for an incentive scheme so effective that it produces tens of + thousands of new servers. + +3.6. What part of the node's performance do you measure? + + We keep referring to having a node measure how well the other nodes + receive bytes. But don't leeching clients receive bytes just as well + as servers? + + Further, many transactions in Tor involve fetching lots of + bytes and not sending very many. So it seems that we want to turn + things around: we need to measure how quickly a node is _sending_ + us bytes, and then only send it bytes in proportion to that. + + However, a sneaky user could simply connect to a node and send some + traffic through it, and voila, he has performed for the network. This + is no good. The first fix is that we only count if you're receiving + bytes "backwards" in the circuit. Now the sneaky user needs to + construct a circuit such that his node appears later in the circuit, + and then send some bytes back quickly. + + Maybe that complexity is sufficient to deter most lazy users. Or + maybe it's an argument in favor of a more penny-counting reputation + approach. + + Addendum: I was more thinking of measuring based on who is the service + provider and service receiver for the circuit. Say Alice builds a + circuit to Bob. Then Bob is providing service to Alice, since he + otherwise wouldn't need to spend his bandwidth. So traffic in either + direction should be charged to Alice. Of course, the same attack would + work, namely, Bob could cheat by sending bytes back quickly. So someone + close to the origin needs to detect this and close the circuit, if + necessary. -JN + +3.7. What is the appropriate resource balance for servers vs. clients? + + If we build a good incentive system, we'll still need to tune it + to provide the right bandwidth allocation -- if we reserve too much + bandwidth for fast servers, then we're wasting some potential, but + if we reserve too little, then fewer people will opt to become servers. + In fact, finding an optimum balance is especially hard because it's + a moving target: the better our incentive mechanism (and the lower + the barrier to setup), the more servers there will be. How do we find + the right balance? + + One answer is that it doesn't have to be perfect: we can err on the + side of providing extra resources to servers. Then we will achieve our + desired goal -- when people complain about speed, we can tell them to + run a server, and they will in fact get better performance. + +3.8. Anonymity attack: fast connections probably come from good servers. + + If only fast servers can consistently get good performance in the + network, they will stand out. "Oh, that connection probably came from + one of the top ten servers in the network." Intersection attacks over + time can improve the certainty of the attack. + + I'm not too worried about this. First, in periods of low activity, + many different people might be getting good performance. This dirties + the intersection attack. Second, with many of these schemes, we will + still be uncertain whether the fast node originated the traffic, or + was the entry node for some other lucky user -- and we already accept + this level of attack in other cases such as the Murdoch-Danezis attack + [http://freehaven.net/anonbib/#torta05] + +3.9. How do we allocate bandwidth over the course of a second? + + This may be a simple matter of engineering, but it still needs to be + addressed. Our current token bucket design refills each bucket once a + second. If we have N tokens in our bucket, and we don't know ahead of + time how many connections are going to want to send out how many bytes, + how do we balance providing quick service to the traffic that is + already here compared to providing service to potential high-importance + future traffic? + + If we have only two classes of service, here is a simple design: + At each point, when we are 1/t through the second, the total number + of non-priority bytes we are willing to send out is N/t. Thus if N + priority bytes are waiting at the beginning of the second, we drain + our whole bucket then, and otherwise we provide some delayed service + to the non-priority bytes. + + Does this design expand to cover the case of three priority classes? + Ideally we'd give each remote server its own priority number. Or + hopefully there's an easy design in the literature to point to -- + this is clearly not my field. + + Is our current flow control mechanism (each circuit and each stream + start out with a certain window, and once they've exhausted it they + need to receive an ack before they can send more) going to have + problems with this new design now that we'll be queueing more bytes + for less preferred nodes? If it turns out we do, the first fix is + to have the windows start out at zero rather than start out full -- + it will slow down the startup phase but protect us better. + + While we have outgoing cells queued for a given server, we have the + option of reordering them based on the priority of the previous hop. + Is this going to turn out to be useful? If we're the exit node (that + is, there is no previous hop) what priority do those cells get? + + Should we do this prioritizing just for sending out bytes (as I've + described here) or would it help to do it also for receiving bytes? + See next section. + +3.10. Different-priority cells arriving on the same TCP connection. + + In some of the proposed designs, servers want to give specific circuits + priority rather than having all circuits from them get the same class + of service. + + Since Tor uses TCP's flow control for rate limiting, this constraints + our design choices -- it is easy to give different TCP connections + different priorities, but it is hard to give different cells on the + same connection priority, because you have to read them to know what + priority they're supposed to get. + + There are several possible solutions though. First is that we rely on + the sender to reorder them so the highest priority cells (circuits) are + more often first. Second is that if we open two TCP connections -- one + for the high-priority cells, and one for the low-priority cells. (But + this prevents us from changing the priority of a circuit because + we would need to migrate it from one connection to the other.) A + third approach is to remember which connections have recently sent + us high-priority cells, and preferentially read from those connections. + + Hopefully we can get away with not solving this section at all. But if + necessary, we can consult Ed Knightly, a Professor at Rice + [http://www.ece.rice.edu/~knightly/] for his extensive experience on + networking QoS. + +3.11. Global reputation system: Congestion on high reputation servers? + + If the notion of reputation is global (as in 2.3 or 2.4), circuits that + go through successive high reputation servers would be the fastest and + most reliable. This would incentivize everyone, regardless of their own + reputation, to choose only the highest reputation servers in its + circuits, causing an over-congestion on those servers. + + One could argue, though, that once those servers are over-congested, + their bandwidth per circuit drops, which would in turn lower their + reputation in the future. A question is whether this would overall + stabilize. + + Another possible way is to keep a cap on reputation. In this way, a + fraction of servers would have the same high reputation, thus balancing + such load. + +3.12. Another anonymity attack: learning from service levels. + + If reputation is local, it may be possible for an evil node to learn + the identity of the origin through provision of differential service. + For instance, the evil node provides crappy bandwidth to everyone, + until it finds a circuit that it wants to trace the origin, then it + provides good bandwidth. Now, as only those directly or indirectly + observing this circuit would like the evil node, it can test each node + by building a circuit via each node to another evil node. If the + bandwidth is high, it is (somewhat) likely that the node was a part of + the circuit. + + This problem does not exist if the reputation is global and nodes only + follow the global reputation, i.e., completely ignore their own view. + +3.13. DoS through high priority traffic. + + Assume there is an evil node with high reputation (or high value on + Alice) and this evil node wants to deny the service to Alice. What it + needs to do is to send a lot of traffic to Alice. To Alice, all traffic + from this evil node is of high priority. If the choice of circuits are + too based toward high priority circuits, Alice would spend most of her + available bandwidth on this circuit, thus providing poor bandwidth to + everyone else. Everyone else would start to dislike Alice, making it + even harder for her to forward other nodes' traffic. This could cause + Alice to have a low reputation, and the only high bandwidth circuit + Alice could use would be via the evil node. + +3.14. If you run a fast server, can you run your client elsewhere? + + A lot of people want to run a fast server at a colocation facility, + and then reap the rewards using their cablemodem or DSL Tor client. + + If we use anonymous micropayments, where reputation can literally + be transferred, this is trivial. + + If we pick a design where servers accrue reputation and can only + use it themselves, though, the clients can configure the servers as + their entry nodes and "inherit" their reputation. In this approach + we would let servers configure a set of IP addresses or keys that get + "like local" service. + +4. Sample designs. + +4.1. Two classes of service for circuits. + + Whenever a circuit is built, it is specified by the origin which class, + either "premium" or "normal", this circuit belongs. A premium circuit + gets preferred treatment at each node. A node "spends" its value, which + it earned a priori by providing service, to the next node by sending + and receiving bytes. Once a node has overspent its values, the circuit + cannot stay as premium. It either breaks or converts into a normal + circuit. Each node also reserves a small portion of bandwidth for + normal circuits to prevent starvation. + + Pro: Even if a node has no value to spend, it can still use normal + circuits. This allow casual user to use Tor without forcing them to run + a server. + + Pro: Nodes have incentive to forward traffic as quick and as much as + possible to accumulate value. + + Con: There is no proactive method for a node to rebalance its debt. It + has to wait until there happens to be a circuit in the opposite + direction. + + Con: A node needs to build circuits in such a way that each node in the + circuit has to have good values to the next node. This requires + non-local knowledge and makes circuits less reliable as the values are + used up in the circuit. + + Con: May discourage nodes to forward traffic in some circuits, as they + worry about spending more useful values to get less useful values in + return. + +4.2. Treat all the traffic from the node with the same service; + hard reputation system. + + This design is similar to 4.1, except that instead of having two + classes of circuits, there is only one. All the circuits are + prioritized based on the value of the interacting node. + + Pro: It is simpler to design and give priority based on connections, + not circuits. + + Con: A node only needs to keep a few guard nodes happy to forward their + traffic. + + Con: Same as in 4.1, may discourage nodes to forward traffic in some + circuits, as they worry about spending more useful values to get less + useful values in return. + +4.3. Treat all the traffic from the node with the same service; + soft reputation system. + + Rather than a guaranteed system with accounting (as 4.1 and 4.2), + we instead try for a best-effort system. All bytes are in the same + class of service. You keep track of other Tors by key, and give them + service proportional to the service they have given you. That is, in + the past when you have tried to push bytes through them, you track the + number of bytes and the average bandwidth, and use that to weight the + priority of their connections if they try to push bytes through you. + + Now you're going to get minimum service if you don't ever push bytes + for other people, and you get increasingly improved service the more + active you are. We should have memories fade over time (we'll have + to tune that, which could be quite hard). + + Pro: Sybil attacks are pointless because new identities get lowest + priority. + + Pro: Smoothly handles periods of both low and high network load. Rather + than keeping track of the ratio/difference between what he's done for + you and what you've done for him, simply keep track of what he's done + for you, and give him priority based on that. + + Based on 3.3 above, it seems we should reward all the nodes in our + path, not just the first one -- otherwise the node can provide good + service only to its guards. On the other hand, there might be a + second-order effect where you want nodes to like you so that *when* + your guards choose you for a circuit, they'll be able to get good + performance. This tradeoff needs more simulation/analysis. + + This approach focuses on incenting people to relay traffic, but it + doesn't do much for incenting them to allow exits. It may help in + one way through: if there are few exits, then they will attract a + lot of use, so lots of people will like them, so when they try to + use the network they will find their first hop to be particularly + pleasant. After that they're like the rest of the world though. (An + alternative would be to reward exit nodes with higher values. At the + extreme, we could even ask the directory servers to suggest the extra + values, based on the current availability of exit nodes.) + + Pro: this is a pretty easy design to add; and it can be phased in + incrementally simply by having new nodes behave differently. + +4.4. Centralized opinions from the reputation servers. + + Have a set of official measurers who spot-check servers from the + directory to see if they really do offer roughly the bandwidth + they advertise. Include these observations in the directory. (For + simplicity, the directory servers could be the measurers.) Then Tor + servers give priority to other servers. We'd like to weight the + priority by advertised bandwidth to encourage people to donate more, + but it seems hard to distinguish between a slow server and a busy + server. + + The spot-checking can be done anonymously to prevent selectively + performing only for the measurers, because hey, we have an anonymity + network. + + We could also reward exit nodes by giving them better priority, but + like above this only will affect their first hop. Another problem + is that it's darn hard to spot-check whether a server allows exits + to all the pieces of the Internet that it claims to. If necessary, + perhaps this can be solved by a distributed reporting mechanism, + where clients that can reach a site from one exit but not another + anonymously submit that site to the measurers, who verify. + + A last problem is that since directory servers will be doing their + tests directly (easy to detect) or indirectly (through other Tor + servers), then we know that we can get away with poor performance for + people that aren't listed in the directory. Maybe we can turn this + around and call it a feature though -- another reason to get listed + in the directory. + +5. Recommendations and next steps. + +5.1. Simulation. + + For simulation trace, we can use two: one is what we obtained from Tor + and one from existing web traces. + + We want to simulate all the four cases in 4.1-4. For 4.4, we may want + to look at two variations: (1) the directory servers check the + bandwidth themselves through Tor; (2) each node reports their perceived + values on other nodes, while the directory servers use EigenTrust to + compute global reputation and broadcast those. + +5.2. Deploying into existing Tor network. +

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[tor/maint-0.2.4] Remove incentives.txt from tor git; putting it into torspec.
by nickm＠torproject.org 15 Mar '13

15 Mar '13

commit 18da1e0cf268bb56adc1a45fa4877e6a1bd2b470 Author: Nick Mathewson <nickm(a)torproject.org> Date: Fri Mar 15 11:25:45 2013 -0400 Remove incentives.txt from tor git; putting it into torspec. --- doc/contrib/incentives.txt | 479 -------------------------------------------- 1 files changed, 0 insertions(+), 479 deletions(-) diff --git a/doc/contrib/incentives.txt b/doc/contrib/incentives.txt deleted file mode 100644 index 850a0d0..0000000 --- a/doc/contrib/incentives.txt +++ /dev/null @@ -1,479 +0,0 @@ - - Tor Incentives Design Brainstorms - -1. Goals: what do we want to achieve with an incentive scheme? - -1.1. Encourage users to provide good relay service (throughput, latency). -1.2. Encourage users to allow traffic to exit the Tor network from - their node. - -2. Approaches to learning who should get priority. - -2.1. "Hard" or quantitative reputation tracking. - - In this design, we track the number of bytes and throughput in and - out of nodes we interact with. When a node asks to send or receive - bytes, we provide service proportional to our current record of the - node's value. One approach is to let each circuit be either a normal - circuit or a premium circuit, and nodes can "spend" their value by - sending and receiving bytes on premium circuits: see section 4.1 for - details of this design. Another approach (section 4.2) would treat - all traffic from the node with the same priority class, and so nodes - that provide resources will get and provide better service on average. - - This approach could be complemented with an anonymous e-cash - implementation to let people spend reputations gained from one context - in another context. - -2.2. "Soft" or qualitative reputation tracking. - - Rather than accounting for every byte (if I owe you a byte, I don't - owe it anymore once you've spent it), instead I keep a general opinion - about each server: my opinion increases when they do good work for me, - and it decays with time, but it does not decrease as they send traffic. - Therefore we reward servers who provide value to the system without - nickle and diming them at each step. We also let them benefit from - relaying traffic for others without having to "reserve" some of the - payment for their own use. See section 4.3 for a possible design. - -2.3. Centralized opinions from the reputation servers. - - The above approaches are complex and we don't have all the answers - for them yet. A simpler approach is just to let some central set - of trusted servers (say, the Tor directory servers) measure whether - people are contributing to the network, and provide a signal about - which servers should be rewarded. They can even do the measurements - via Tor so servers can't easily perform only when they're being - tested. See section 4.4. - -2.4. Reputation servers that aggregate opinions. - - The option above has the directory servers doing all of the - measurements. This doesn't scale. We can set it up so we have "deputy - testers" -- trusted other nodes that do performance testing and report - their results. - - If we want to be really adventurous, we could even - accept claims from every Tor user and build a complex weighting / - reputation system to decide which claims are "probably" right. - One possible way to implement the latter is something similar to - EigenTrust [http://www.stanford.edu/~sdkamvar/papers/eigentrust.pdf] - where the opinion of nodes with high reputation more is weighted - higher. - -3. Related issues we need to keep in mind. - -3.1. Relay and exit configuration needs to be easy and usable. - - Implicit in all of the above designs is the need to make it easy to - run a Tor server out of the box. We need to make it stable on all - common platforms (including XP), it needs to detect its available - bandwidth and not overreach that, and it needs to help the operator - through opening up ports on his firewall. Then we need a slick GUI - that lets people click a button or two rather than editing text files. - - Once we've done all this, we'll hit our first big question: is - most of the barrier to growth caused by the unusability of the current - software? If so, are the rest of these incentive schemes superfluous? - -3.2. The network effect: how many nodes will you interact with? - - One of the concerns with pairwise reputation systems is that as the - network gets thousands of servers, the chance that you're going to - interact with a given server decreases. So if 90% of interactions - don't have any prior information, the "local" incentive schemes above - are going to degrade. This doesn't mean they're pointless -- it just - means we need to be aware that this is a limitation, and plan in the - background for what step to take next. (It seems that e-cash solutions - would scale better, though they have issues of their own.) - -3.3. Guard nodes - - As of Tor 0.1.1.11, Tor users pick from a small set of semi-permanent - "guard nodes" for their first hop of each circuit. This seems like it - would have a big impact on pairwise reputation systems since you - will only be cashing in on your reputation to a few people, and it is - unlikely that a given pair of nodes will use each other as guard nodes. - - What does this imply? For one, it means that we don't care at all - about the opinions of most of the servers out there -- we should - focus on keeping our guard nodes happy with us. - - One conclusion from that is that our design needs to judge performance - not just through direct interaction (beginning of the circuit) but - also through indirect interaction (middle of the circuit). That way - you can never be sure when your guards are measuring you. - - Both 3.2 and 3.3 may be solved by having a global notion of reputation, - as in 2.3 and 2.4. However, computing the global reputation from local - views could be expensive (O(n^2)) when the network is really large. - -3.4. Restricted topology: benefits and roadmap. - - As the Tor network continues to grow, we will need to make design - changes to the network topology so that each node does not need - to maintain connections to an unbounded number of other nodes. For - anonymity's sake, we may partition the network such that all - the nodes have the same belief about the divisions and each node is - in only one partition. (The alternative is that every user fetches - his own random subset of the overall node list -- this is bad because - of intersection attacks.) - - Therefore the "network horizon" for each user will stay bounded, - which helps against the above issues in 3.2 and 3.3. - - It could be that the core of long-lived servers will all get to know - each other, and so the critical point that decides whether you get - good service is whether the core likes you. Or perhaps it will turn - out to work some other way. - - A special case here is the social network, where the network isn't - partitioned randomly but instead based on some external properties. - Social network topologies can provide incentives in other ways, because - people may be more inclined to help out their friends, and more willing - to relay traffic if most of the traffic they are relaying comes - from their friends. It also opens the door for out-of-band incentive - schemes because of the out-of-band links in the graph. - -3.5. Profit-maximizing vs. Altruism. - - There are some interesting game theory questions here. - - First, in a volunteer culture, success is measured in public utility - or in public esteem. If we add a reward mechanism, there's a risk that - reward-maximizing behavior will surpass utility- or esteem-maximizing - behavior. - - Specifically, if most of our servers right now are relaying traffic - for the good of the community, we may actually *lose* those volunteers - if we turn the act of relaying traffic into a selfish act. - - I am not too worried about this issue for now, since we're aiming - for an incentive scheme so effective that it produces tens of - thousands of new servers. - -3.6. What part of the node's performance do you measure? - - We keep referring to having a node measure how well the other nodes - receive bytes. But don't leeching clients receive bytes just as well - as servers? - - Further, many transactions in Tor involve fetching lots of - bytes and not sending very many. So it seems that we want to turn - things around: we need to measure how quickly a node is _sending_ - us bytes, and then only send it bytes in proportion to that. - - However, a sneaky user could simply connect to a node and send some - traffic through it, and voila, he has performed for the network. This - is no good. The first fix is that we only count if you're receiving - bytes "backwards" in the circuit. Now the sneaky user needs to - construct a circuit such that his node appears later in the circuit, - and then send some bytes back quickly. - - Maybe that complexity is sufficient to deter most lazy users. Or - maybe it's an argument in favor of a more penny-counting reputation - approach. - - Addendum: I was more thinking of measuring based on who is the service - provider and service receiver for the circuit. Say Alice builds a - circuit to Bob. Then Bob is providing service to Alice, since he - otherwise wouldn't need to spend his bandwidth. So traffic in either - direction should be charged to Alice. Of course, the same attack would - work, namely, Bob could cheat by sending bytes back quickly. So someone - close to the origin needs to detect this and close the circuit, if - necessary. -JN - -3.7. What is the appropriate resource balance for servers vs. clients? - - If we build a good incentive system, we'll still need to tune it - to provide the right bandwidth allocation -- if we reserve too much - bandwidth for fast servers, then we're wasting some potential, but - if we reserve too little, then fewer people will opt to become servers. - In fact, finding an optimum balance is especially hard because it's - a moving target: the better our incentive mechanism (and the lower - the barrier to setup), the more servers there will be. How do we find - the right balance? - - One answer is that it doesn't have to be perfect: we can err on the - side of providing extra resources to servers. Then we will achieve our - desired goal -- when people complain about speed, we can tell them to - run a server, and they will in fact get better performance. - -3.8. Anonymity attack: fast connections probably come from good servers. - - If only fast servers can consistently get good performance in the - network, they will stand out. "Oh, that connection probably came from - one of the top ten servers in the network." Intersection attacks over - time can improve the certainty of the attack. - - I'm not too worried about this. First, in periods of low activity, - many different people might be getting good performance. This dirties - the intersection attack. Second, with many of these schemes, we will - still be uncertain whether the fast node originated the traffic, or - was the entry node for some other lucky user -- and we already accept - this level of attack in other cases such as the Murdoch-Danezis attack - [http://freehaven.net/anonbib/#torta05] - -3.9. How do we allocate bandwidth over the course of a second? - - This may be a simple matter of engineering, but it still needs to be - addressed. Our current token bucket design refills each bucket once a - second. If we have N tokens in our bucket, and we don't know ahead of - time how many connections are going to want to send out how many bytes, - how do we balance providing quick service to the traffic that is - already here compared to providing service to potential high-importance - future traffic? - - If we have only two classes of service, here is a simple design: - At each point, when we are 1/t through the second, the total number - of non-priority bytes we are willing to send out is N/t. Thus if N - priority bytes are waiting at the beginning of the second, we drain - our whole bucket then, and otherwise we provide some delayed service - to the non-priority bytes. - - Does this design expand to cover the case of three priority classes? - Ideally we'd give each remote server its own priority number. Or - hopefully there's an easy design in the literature to point to -- - this is clearly not my field. - - Is our current flow control mechanism (each circuit and each stream - start out with a certain window, and once they've exhausted it they - need to receive an ack before they can send more) going to have - problems with this new design now that we'll be queueing more bytes - for less preferred nodes? If it turns out we do, the first fix is - to have the windows start out at zero rather than start out full -- - it will slow down the startup phase but protect us better. - - While we have outgoing cells queued for a given server, we have the - option of reordering them based on the priority of the previous hop. - Is this going to turn out to be useful? If we're the exit node (that - is, there is no previous hop) what priority do those cells get? - - Should we do this prioritizing just for sending out bytes (as I've - described here) or would it help to do it also for receiving bytes? - See next section. - -3.10. Different-priority cells arriving on the same TCP connection. - - In some of the proposed designs, servers want to give specific circuits - priority rather than having all circuits from them get the same class - of service. - - Since Tor uses TCP's flow control for rate limiting, this constraints - our design choices -- it is easy to give different TCP connections - different priorities, but it is hard to give different cells on the - same connection priority, because you have to read them to know what - priority they're supposed to get. - - There are several possible solutions though. First is that we rely on - the sender to reorder them so the highest priority cells (circuits) are - more often first. Second is that if we open two TCP connections -- one - for the high-priority cells, and one for the low-priority cells. (But - this prevents us from changing the priority of a circuit because - we would need to migrate it from one connection to the other.) A - third approach is to remember which connections have recently sent - us high-priority cells, and preferentially read from those connections. - - Hopefully we can get away with not solving this section at all. But if - necessary, we can consult Ed Knightly, a Professor at Rice - [http://www.ece.rice.edu/~knightly/] for his extensive experience on - networking QoS. - -3.11. Global reputation system: Congestion on high reputation servers? - - If the notion of reputation is global (as in 2.3 or 2.4), circuits that - go through successive high reputation servers would be the fastest and - most reliable. This would incentivize everyone, regardless of their own - reputation, to choose only the highest reputation servers in its - circuits, causing an over-congestion on those servers. - - One could argue, though, that once those servers are over-congested, - their bandwidth per circuit drops, which would in turn lower their - reputation in the future. A question is whether this would overall - stabilize. - - Another possible way is to keep a cap on reputation. In this way, a - fraction of servers would have the same high reputation, thus balancing - such load. - -3.12. Another anonymity attack: learning from service levels. - - If reputation is local, it may be possible for an evil node to learn - the identity of the origin through provision of differential service. - For instance, the evil node provides crappy bandwidth to everyone, - until it finds a circuit that it wants to trace the origin, then it - provides good bandwidth. Now, as only those directly or indirectly - observing this circuit would like the evil node, it can test each node - by building a circuit via each node to another evil node. If the - bandwidth is high, it is (somewhat) likely that the node was a part of - the circuit. - - This problem does not exist if the reputation is global and nodes only - follow the global reputation, i.e., completely ignore their own view. - -3.13. DoS through high priority traffic. - - Assume there is an evil node with high reputation (or high value on - Alice) and this evil node wants to deny the service to Alice. What it - needs to do is to send a lot of traffic to Alice. To Alice, all traffic - from this evil node is of high priority. If the choice of circuits are - too based toward high priority circuits, Alice would spend most of her - available bandwidth on this circuit, thus providing poor bandwidth to - everyone else. Everyone else would start to dislike Alice, making it - even harder for her to forward other nodes' traffic. This could cause - Alice to have a low reputation, and the only high bandwidth circuit - Alice could use would be via the evil node. - -3.14. If you run a fast server, can you run your client elsewhere? - - A lot of people want to run a fast server at a colocation facility, - and then reap the rewards using their cablemodem or DSL Tor client. - - If we use anonymous micropayments, where reputation can literally - be transferred, this is trivial. - - If we pick a design where servers accrue reputation and can only - use it themselves, though, the clients can configure the servers as - their entry nodes and "inherit" their reputation. In this approach - we would let servers configure a set of IP addresses or keys that get - "like local" service. - -4. Sample designs. - -4.1. Two classes of service for circuits. - - Whenever a circuit is built, it is specified by the origin which class, - either "premium" or "normal", this circuit belongs. A premium circuit - gets preferred treatment at each node. A node "spends" its value, which - it earned a priori by providing service, to the next node by sending - and receiving bytes. Once a node has overspent its values, the circuit - cannot stay as premium. It either breaks or converts into a normal - circuit. Each node also reserves a small portion of bandwidth for - normal circuits to prevent starvation. - - Pro: Even if a node has no value to spend, it can still use normal - circuits. This allow casual user to use Tor without forcing them to run - a server. - - Pro: Nodes have incentive to forward traffic as quick and as much as - possible to accumulate value. - - Con: There is no proactive method for a node to rebalance its debt. It - has to wait until there happens to be a circuit in the opposite - direction. - - Con: A node needs to build circuits in such a way that each node in the - circuit has to have good values to the next node. This requires - non-local knowledge and makes circuits less reliable as the values are - used up in the circuit. - - Con: May discourage nodes to forward traffic in some circuits, as they - worry about spending more useful values to get less useful values in - return. - -4.2. Treat all the traffic from the node with the same service; - hard reputation system. - - This design is similar to 4.1, except that instead of having two - classes of circuits, there is only one. All the circuits are - prioritized based on the value of the interacting node. - - Pro: It is simpler to design and give priority based on connections, - not circuits. - - Con: A node only needs to keep a few guard nodes happy to forward their - traffic. - - Con: Same as in 4.1, may discourage nodes to forward traffic in some - circuits, as they worry about spending more useful values to get less - useful values in return. - -4.3. Treat all the traffic from the node with the same service; - soft reputation system. - - Rather than a guaranteed system with accounting (as 4.1 and 4.2), - we instead try for a best-effort system. All bytes are in the same - class of service. You keep track of other Tors by key, and give them - service proportional to the service they have given you. That is, in - the past when you have tried to push bytes through them, you track the - number of bytes and the average bandwidth, and use that to weight the - priority of their connections if they try to push bytes through you. - - Now you're going to get minimum service if you don't ever push bytes - for other people, and you get increasingly improved service the more - active you are. We should have memories fade over time (we'll have - to tune that, which could be quite hard). - - Pro: Sybil attacks are pointless because new identities get lowest - priority. - - Pro: Smoothly handles periods of both low and high network load. Rather - than keeping track of the ratio/difference between what he's done for - you and what you've done for him, simply keep track of what he's done - for you, and give him priority based on that. - - Based on 3.3 above, it seems we should reward all the nodes in our - path, not just the first one -- otherwise the node can provide good - service only to its guards. On the other hand, there might be a - second-order effect where you want nodes to like you so that *when* - your guards choose you for a circuit, they'll be able to get good - performance. This tradeoff needs more simulation/analysis. - - This approach focuses on incenting people to relay traffic, but it - doesn't do much for incenting them to allow exits. It may help in - one way through: if there are few exits, then they will attract a - lot of use, so lots of people will like them, so when they try to - use the network they will find their first hop to be particularly - pleasant. After that they're like the rest of the world though. (An - alternative would be to reward exit nodes with higher values. At the - extreme, we could even ask the directory servers to suggest the extra - values, based on the current availability of exit nodes.) - - Pro: this is a pretty easy design to add; and it can be phased in - incrementally simply by having new nodes behave differently. - -4.4. Centralized opinions from the reputation servers. - - Have a set of official measurers who spot-check servers from the - directory to see if they really do offer roughly the bandwidth - they advertise. Include these observations in the directory. (For - simplicity, the directory servers could be the measurers.) Then Tor - servers give priority to other servers. We'd like to weight the - priority by advertised bandwidth to encourage people to donate more, - but it seems hard to distinguish between a slow server and a busy - server. - - The spot-checking can be done anonymously to prevent selectively - performing only for the measurers, because hey, we have an anonymity - network. - - We could also reward exit nodes by giving them better priority, but - like above this only will affect their first hop. Another problem - is that it's darn hard to spot-check whether a server allows exits - to all the pieces of the Internet that it claims to. If necessary, - perhaps this can be solved by a distributed reporting mechanism, - where clients that can reach a site from one exit but not another - anonymously submit that site to the measurers, who verify. - - A last problem is that since directory servers will be doing their - tests directly (easy to detect) or indirectly (through other Tor - servers), then we know that we can get away with poor performance for - people that aren't listed in the directory. Maybe we can turn this - around and call it a feature though -- another reason to get listed - in the directory. - -5. Recommendations and next steps. - -5.1. Simulation. - - For simulation trace, we can use two: one is what we obtained from Tor - and one from existing web traces. - - We want to simulate all the four cases in 4.1-4. For 4.4, we may want - to look at two variations: (1) the directory servers check the - bandwidth themselves through Tor; (2) each node reports their perceived - values on other nodes, while the directory servers use EigenTrust to - compute global reputation and broadcast those. - -5.2. Deploying into existing Tor network. -

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[tor/maint-0.2.4] Fix brackets in tor-fw-helper help output.
by nickm＠torproject.org 15 Mar '13

15 Mar '13

commit a0b92cd8008982a71b99e2938df19c8bc40c5436 Author: David Fifield <david(a)bamsoftware.com> Date: Thu Dec 20 12:40:50 2012 -0800 Fix brackets in tor-fw-helper help output. Change nesting from [(]) to [()]. Formerly it made it look to me at first glance that "internal port" was optional. [Trivial change; fixes #7767 --nickm] --- src/tools/tor-fw-helper/tor-fw-helper.c | 2 +- 1 files changed, 1 insertions(+), 1 deletions(-) diff --git a/src/tools/tor-fw-helper/tor-fw-helper.c b/src/tools/tor-fw-helper/tor-fw-helper.c index d92445e..adeb63b 100644 --- a/src/tools/tor-fw-helper/tor-fw-helper.c +++ b/src/tools/tor-fw-helper/tor-fw-helper.c @@ -100,7 +100,7 @@ usage(void) " [-T|--Test]\n" " [-v|--verbose]\n" " [-g|--fetch-public-ip]\n" - " [-p|--forward-port ([<external port>]:<internal port>])\n"); + " [-p|--forward-port ([<external port>]:<internal port>)]\n"); } /** Log commandline options to a hardcoded file <b>tor-fw-helper.log</b> in the

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[tor/master] Improve asciidoc-helper's suggestion of what to install
by nickm＠torproject.org 15 Mar '13

15 Mar '13

commit aa051b7573d4b7ab9867560b190808d5e67a34c7 Author: Nick Mathewson <nickm(a)torproject.org> Date: Fri Mar 15 11:17:08 2013 -0400 Improve asciidoc-helper's suggestion of what to install Fixes bug #7766. Patch by David Fifield, revised by Peter Palfrader. --- doc/asciidoc-helper.sh | 4 ++-- 1 files changed, 2 insertions(+), 2 deletions(-) diff --git a/doc/asciidoc-helper.sh b/doc/asciidoc-helper.sh index dd420f7..c06b570 100755 --- a/doc/asciidoc-helper.sh +++ b/doc/asciidoc-helper.sh @@ -52,8 +52,8 @@ elif [ "$1" = "man" ]; then You need a working asciidoc installed to be able to build the manpage. a2x is installed, but for some reason it isn't working. Sometimes -This happens because required docbook support files are missing. -Please install docbook-xsl, docbook-xml, and libxml2-utils (Debian) or +this happens because required docbook support files are missing. +Please install docbook-xsl, docbook-xml, and xmlto (Debian) or similar. If you use homebrew on Mac OS X, install the docbook formula and add "export XML_CATALOG_FILES=/usr/local/etc/xml/catalog" to your .bashrc

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[tor/master] Merge remote-tracking branch 'origin/maint-0.2.4'
by nickm＠torproject.org 15 Mar '13

15 Mar '13

commit ed600832d9fcd9ac6b2aacd36114f65ff227daf9 Merge: 2810753 aa051b7 Author: Nick Mathewson <nickm(a)torproject.org> Date: Fri Mar 15 11:18:58 2013 -0400 Merge remote-tracking branch 'origin/maint-0.2.4' doc/asciidoc-helper.sh | 4 ++-- 1 files changed, 2 insertions(+), 2 deletions(-)

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[tor/maint-0.2.4] Improve asciidoc-helper's suggestion of what to install
by nickm＠torproject.org 15 Mar '13

15 Mar '13

commit aa051b7573d4b7ab9867560b190808d5e67a34c7 Author: Nick Mathewson <nickm(a)torproject.org> Date: Fri Mar 15 11:17:08 2013 -0400 Improve asciidoc-helper's suggestion of what to install Fixes bug #7766. Patch by David Fifield, revised by Peter Palfrader. --- doc/asciidoc-helper.sh | 4 ++-- 1 files changed, 2 insertions(+), 2 deletions(-) diff --git a/doc/asciidoc-helper.sh b/doc/asciidoc-helper.sh index dd420f7..c06b570 100755 --- a/doc/asciidoc-helper.sh +++ b/doc/asciidoc-helper.sh @@ -52,8 +52,8 @@ elif [ "$1" = "man" ]; then You need a working asciidoc installed to be able to build the manpage. a2x is installed, but for some reason it isn't working. Sometimes -This happens because required docbook support files are missing. -Please install docbook-xsl, docbook-xml, and libxml2-utils (Debian) or +this happens because required docbook support files are missing. +Please install docbook-xsl, docbook-xml, and xmlto (Debian) or similar. If you use homebrew on Mac OS X, install the docbook formula and add "export XML_CATALOG_FILES=/usr/local/etc/xml/catalog" to your .bashrc

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[tor/master] Be explicit that we want not only nacl but nacl-with-a-fast-curve25519
by nickm＠torproject.org 15 Mar '13

15 Mar '13

commit 10fb3398087133add9aff93740aff8f818edf242 Author: Nick Mathewson <nickm(a)torproject.org> Date: Fri Mar 15 10:48:33 2013 -0400 Be explicit that we want not only nacl but nacl-with-a-fast-curve25519 Resolves the user experience part of #8014. --- changes/bug8014 | 5 +++++ configure.ac | 4 ++-- 2 files changed, 7 insertions(+), 2 deletions(-) diff --git a/changes/bug8014 b/changes/bug8014 new file mode 100644 index 0000000..c09a860 --- /dev/null +++ b/changes/bug8014 @@ -0,0 +1,5 @@ + o Minor usability improvements (build): + - Clarify that when autconf is checking for nacl, it is checking + specifically for nacl with a fast curve25519 implementation. + Fixes bug 8014. + diff --git a/configure.ac b/configure.ac index 4bc0db3..d588984 100644 --- a/configure.ac +++ b/configure.ac @@ -693,7 +693,7 @@ if test x$enable_curve25519 != xno; then AC_CHECK_HEADERS([crypto_scalarmult_curve25519.h \ nacl/crypto_scalarmult_curve25519.h]) - AC_CACHE_CHECK([whether we can use curve25519 from nacl], + AC_CACHE_CHECK([for nacl compiled with a fast curve25519 implementation], tor_cv_can_use_curve25519_nacl, [tor_saved_LIBS="$LIBS" LIBS="$LIBS -lnacl" @@ -705,7 +705,7 @@ if test x$enable_curve25519 != xno; then #include <nacl/crypto_scalarmult_curve25519.h> #endif #ifdef crypto_scalarmult_curve25519_ref_BYTES - #error Hey, this is the reference implementation! + #error Hey, this is the reference implementation! That's not fast. #endif ], [ unsigned char *a, *b, *c; crypto_scalarmult_curve25519(a,b,c);

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[tor/master] Merge remote-tracking branch 'origin/maint-0.2.4'
by nickm＠torproject.org 15 Mar '13

15 Mar '13

commit 2810753928692ca86820f4df3b4a76caf2701d33 Merge: 9be3389 10fb339 Author: Nick Mathewson <nickm(a)torproject.org> Date: Fri Mar 15 11:01:06 2013 -0400 Merge remote-tracking branch 'origin/maint-0.2.4' changes/bug8014 | 5 +++++ configure.ac | 4 ++-- 2 files changed, 7 insertions(+), 2 deletions(-)

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