Ola Bini obini@thoughtworks.com writes:
Hi,
Here are some more comments to the latest version of the proposal, as seen here: https://github.com/twstrike/torspec
Filename: 259-guard-selection.txt Title: New Guard Selection Behaviour Author: Isis Lovecruft, George Kadianakis, [Ola Bini] Created: 2015-10-28 Status: Draft
§1. Overview
Tor uses entry guards to prevent an attacker who controls some fraction of the network from observing a fraction of every user's traffic. If users chose their entries and exits uniformly at random from the list of servers every time they build a circuit, then an adversary who had (k/N) of the network would deanonymize F=(k/N)^2 of all circuits... and after a given user had built C circuits, the attacker would see them at least once with probability 1-(1-F)^C. With large C, the attacker would get a sample of every user's traffic with probability 1.
To prevent this from happening, Tor clients choose a small number of guard nodes (currently 3). These guard nodes are the only nodes that the client will connect to directly. If they are not compromised, the user's paths are not compromised.
But attacks remain. Consider an attacker who can run a firewall between a target user and the Tor network, and make many of the guards they don't control appear to be unreachable. Or consider an attacker who can identify a user's guards, and mount denial-of-service attacks on them until the user picks a guard that the attacker controls.
In the presence of these attacks, we can't continue to connect to the Tor network unconditionally. Doing so would eventually result in the user choosing a hostile node as their guard, and losing anonymity.
This proposal outlines a new entry guard selection algorithm, which addresses the following concerns:
- Heuristics and algorithms for determining how and which guard(s) is(/are) chosen should be kept as simple and easy to understand as possible. - Clients in censored regions or who are behind a fascist firewall who connect to the Tor network should not experience any significant disadvantage in terms of reachability or usability. - Tor should make a best attempt at discovering the most appropriate behaviour, with as little user input and configuration as possible.§2. Design
Alice, an OP attempting to connect to the Tor network, should undertake the following steps to determine information about the local network and to select (some) appropriate entry guards. In the following scenario, it is assumed that Alice has already obtained a recent, valid, and verifiable consensus document.
The algorithm is divided into four components such that the full algorithm is implemented by first invoking START, then repeatedly calling NEXT while adviced it SHOULD_CONTINUE and finally calling END. For an example usage see §A. Appendix.
Several components of NEXT can be invoked asynchronously. SHOULD_CONTINUE is used for the algorithm to be able to tell the caller whether we consider the work done or not - this can be used to retry primary guards when we finally are able to connect to a guard after a long network outage, for example.
This algorithm keeps track of the unreachability status for guards in state private to the algorithm - this is re-initialized every time START is called.
Hmm, didn't we decide to persist the unreachability status over runs, right? Or not?
The argument for doing so is "What happens if I'm a hidden service that needs to make 5 circuits per second, and my first 2 guards happen to be offline? Do I have to spend time probing them everytime I make a new circuit?".
The algorithm expects several arguments to guide its behavior. These will be defined in §2.1.
The goal of this algorithm is to strongly prefer connecting to the same guards we have connected to before, while also trying to detect conditions such as a network outage or a network environment that blocks most ports. The way it does this is by keeping track of how many guards we have exposed ourselves to, and if we have connected to too many we will fall back to only retrying the ones we have already tried. The algorithm also decides on sample set that should be persisted - in order to minimize the risk of an attacker forcing enumeration of the whole network by triggering rebuilding of circuits.
§2.1. The START algorithm
In order to start choosing an entry guard, use the START algorithm. This takes four arguments that can be used to fine tune the workings:
USED_GUARDS This is a list that contains all the guards that have been used before by this client. We will prioritize using guards from this list in order to minimize our exposure. The list is expected to be sorted based on priority, where the first entry will have the highest priority.
SAMPLED_UTOPIC_GUARDS This is a set that contains all guards that should be considered for connection under utopic conditions. This set should be persisted between runs. It will be filled in by the algorithm if it's empty, or if it contains less than SAMPLE_SET_THRESHOLD guards after winnowing out older guards. It should be filled by using NEXT_BY_BANDWIDTH with UTOPIC_GUARDS as an argument.
Should we use UTOPIC_GUARDS or REMAINING_UTOPIC_GUARDS as the argument?
SAMPLED_DYSTOPIC_GUARDS This is a set that contains all guards that should be considered for connection under dystopic conditions. This set should be persisted between runs. It will be filled in by the algorithm if it's empty, or if it contains less than SAMPLE_SET_THRESHOLD guards after winnowing out older guards. It should be filled by using NEXT_BY_BANDWIDTH with DYSTOPIC_GUARDS as an argument.
EXCLUDE_NODES A set of nodes that we should not consider using as a guard.
N_PRIMARY_GUARDS The number of guards we should consider our primary guards. These guards will be retried more frequently and will take precedence in most situations. By default the primary guards will be the first N_PRIMARY_GUARDS guards from USED_GUARDS.
DIR If this argument is set, we should only consider guards that can be directory guards. If not set, we will consider all guards.
The primary work of START is to initialize the state machine depicted in §2.2 The NEXT algorithm. The initial state of the machine is defined by:
GUARDS This is a set of all guards from the consensus, without EXCLUDE_NODES and potentially filtered if DIR is set.
UTOPIC_GUARDS This is a set of all guards to use under utopic conditions. It will primarily be used to fill in SAMPLED_UTOPIC_GUARDS. This set will be initialized to be the same as GUARDS.
DYSTOPIC_GUARDS This is a set of all guards to use under dystopic conditions (usually when we are subject to a firewall that restricts the ports we can connect to). It will primarily be used to fill in SAMPLED_UTOPIC_GUARDS. This set will be initialized to be the
I guess you mean SAMPLED_DYSTOPIC_GUARDS.
subset of GUARDS that listen to ports that are allowed by dystopic conditions.REMAINING_UTOPIC_GUARDS This is a running set of the utopic guards we have not yet tried to connect to. It should be initialized to be SAMPLED_UTOPIC_GUARDS without USED_GUARDS.
Maybe here we should also mention that we will reinsert guards that we have not tried in a long time (GUARDS_RETRY_TIME) as specified by 2.2.2?
REMAINING_DYSTOPIC_GUARDS This is a running set of the dystopic guards we have not yet tried to connect to. It should be initialized to be SAMPLED_DYSTOPIC_GUARDS without USED_GUARDS.
TRIED_GUARDS This set keeps track of all utopic guards we have tried connecting to. This should be initialized to the empty set.
TRIED_DYSTOPIC_GUARDS This set keeps track of all dystopic guards we have tried connecting to. This should be initialized to the empty set.
STATE A variable that keeps track of which state in the state machine we are currently in. It should be initialized to STATE_PRIMARY_GUARDS.
PRIMARY_GUARDS This list keeps track of our primary guards. These are guards that we will prioritize when trying to connect, and will also retry more often in case of failure with other guards. It should be initialized by calling algorithm NEXT_PRIMARY_GUARD repeatedly until PRIMARY_GUARDS contains N_PRIMARY_GUARDS elements.
§2.2. The NEXT algorithm
The NEXT algorithm is composed of several different possibly flows. The first one is a simple state machine that can transfer between four different states. Every time NEXT is invoked, it will resume at the state where it left off previously. In the course of selecting an entry guard, a new consensus can arrive. When that happens we need to update the data structures used, but nothing else should change.
Before jumping in to the state machine, we should first check if it was at least PRIMARY_GUARDS_RETRY_INTERVAL minutes since we tried any of the PRIMARY_GUARDS. If this is the case, and we are not in STATE_PRIMARY_GUARDS, we should save the previous state and set the state to STATE_PRIMARY_GUARDS.
§2.2.1. The STATE_PRIMARY_GUARDS state
Return each entry in PRIMARY_GUARDS in turn. For each entry, if it was not possible to connect to it, mark the entry as unreachable, and add it to TRIED_GUARDS. [XXX defining "was not possible to connect" as "entry is not live" according to current definition of "live entry guard" in tor source code, seems to improve success rate on the flaky network scenario. See: https://github.com/twstrike/tor_guardsim/issues/1#issuecomment-187374942]
Hmm, I'm not sure what this XXX means exactly. I believe we should actually try to _connect_ to those primary guards and not just check if we think they are live.
If all entries have been tried, restore the previous state and go there. If there is no previous state, transition to STATE_TRY_UTOPIC.
§2.2.2. The STATE_TRY_UTOPIC state
In order to give guards that have been marked as unreachable a chance to come back, add all entries in TRIED_GUARDS that were marked as unreachable more than GUARDS_RETRY_TIME minutes ago back to REMAINING_UTOPIC_GUARDS.
I'm a bit puzzled by this mechanism. Maybe it's benefits can be explained a bit more clearly?
When we add guards back to REMAINING_UTOPIC_GUARDS, do we also remove them from TRIED_GUARDS?
Also, the value here is currently 20 minutes. So this will trigger only when this algorithm takes over 20 minutes to terminate. I guess this should only happen when the network is down.
Now that we have persistent SAMPLED_UTOPIC_GUARDS is this still useful? Won't we have fully populated our SAMPLED_*_GUARDS structures by the point this rule triggers?
Sorry for the confusion :)
Return each entry in USED_GUARDS that is not in PRIMARY_GUARDS in turn. For each entry, if it was not possible to connect to it, mark the entry as unreachable and add it to TRIED_GUARDS.
Return each entry from REMAINING_UTOPIC_GUARDS using NEXT_BY_BANDWIDTH. For each entry, if it was not possible to connect to it, remove the entry from REMAINING_UTOPIC_GUARDS, mark it as unreachable and add it to TRIED_GUARDS.
If no entries remain in REMAINING_UTOPIC_GUARDS, transition to STATE_TRY_DYSTOPIC.
§2.2.3. The STATE_TRY_DYSTOPIC state
In order to give guards that have been marked as unreachable a chance to come back, add all entries in TRIED_DYSTOPIC_GUARDS that were marked as unreachable more than GUARDS_RETRY_TIME minutes ago back to REMAINING_DYSTOPIC_GUARDS.
Return each dystopic entry in USED_GUARDS that is not in PRIMARY_GUARDS in turn. For each entry, if it was not possible to connect to it, mark the entry as unreachable and add it to TRIED_DYSTOPIC_GUARDS.
Return each entry from REMAINING_DYSTOPIC_GUARDS using NEXT_BY_BANDWIDTH. For each entry, if it was not possible to connect to it, remove the entry from REMAINING_DYSTOPIC_GUARDS, mark it as unreachable and add it to TRIED_DYSTOPIC_GUARDS.
If no entries remain in REMAINING_DYSTOPIC_GUARDS, transition to STATE_PRIMARY_GUARDS.
§2.2.5. ON_NEW_CONSENSUS
First, ensure that all guard profiles are updated with information about whether they were in the newest consensus or not. If not, the guard is considered bad.
Maybe instead of "If not" we could say "If a guard is not included in the newest consensus" to make it a bit clearer.
If any PRIMARY_GUARDS have become bad, remove the guard from PRIMARY_GUARDS. Then ensure that PRIMARY_GUARDS contain N_PRIMARY_GUARDS entries by repeatedly calling NEXT_PRIMARY_GUARD.
If any guards in USED_GUARDS have switched from being bad to being non-bad, add it back in the place it should have been in PRIMARY_GUARDS if it had been non-bad when populating PRIMARY_GUARDS. If this results in PRIMARY_GUARDS being larger than N_PRIMARY_GUARDS, truncate the list to be N_PRIMARY_GUARDS entries long. [XXX Does "add it back in the place it should have been in PRIMARY_GUARDS if it had been non-bad" implies keeping original order?]
If I understand correctly, I think the answer to this XXX is "Ideally, yes.".
I'm curious to see how this mechanism will be implemented because it's important and it would be nice if it's done cleanly.
Also, we should be careful about when we count 'bad' guards. After a few weeks of operation, the USED_GUARDS list can accumulate multiple bad guards, and we should make sure we don't count them when we do our threshold checks.
§2.3. The SHOULD_CONTINUE algorithm
This algorithm takes as an argument a boolean indicating whether the circuit was successfully built or not.
After the caller have tried to build a circuit with a returned guard, they should invoke SHOULD_CONTINUE to understand if the algorithm is finished or not. SHOULD_CONTINUE will always return true if the circuit failed. If the circuit succeeded, SHOULD_CONTINUE will always return false, unless the guard that succeeded was the first guard to succeed after INTERNET_LIKELY_DOWN_INTERVAL minutes - in that case it will set the state to STATE_PRIMARY_GUARDS and return true.
ACK.
Just a reminder that we also discussed adding the "Retry primary guards if we have looped over the whole guardlist" heuristic somewhere here. Because in many cases the network can go down and then back up in less than a minute.
§2.4. The END algorithm
The goal of this algorithm is simply to make sure that we keep track of successful connections made. This algorithm should be invoked with the guard that was used to correctly set up a circuit.
Once invoked, this algorithm will mark the guard as used, and make sure it is in USED_GUARDS. [XXX if the guard is not in USED_GUARDS should be added *first*? Important because it will affect on the building of PRIMARY_GUARDS.]
IIUC, if the guard is not in USED_GUARDS it should be added *last* (that is, with lowest priority).
§2.5. Helper algorithms
These algorithms are used in the above algorithms, but have been separated out here in order to make the flow clearer.
NEXT_PRIMARY_GUARD - Return the first entry from USED_GUARDS that is not in PRIMARY_GUARDS and that is in the most recent consensus. - If USED_GUARDS is empty, use NEXT_BY_BANDWIDTH with REMAINING_UTOPIC_GUARDS as the argument.
NEXT_BY_BANDWIDTH - Takes G as an argument, which should be a set of guards to choose from. - Return a randomly select element from G, weighted by bandwidth.
§3. Consensus Parameters, & Configurable Variables
This proposal introduces several new parameters that ideally should be set in the consensus but that should also be possible to set or override in the client configuration file. Some of these have proposed values, but for others more simulation and trial needs to happen.
PRIMARY_GUARDS_RETRY_INTERVAL In order to make it more likely we connect to a primary guard, we would like to retry the primary guards more often than other types of guards. This parameter controls how many minutes should pass before we consider retrying primary guards again. The proposed value is 3.
GUARDS_RETRY_TIME In the normal course of selecting guards, we want to continue retrying unreachable guards in case they have become reachable. This parameter controls the minimum amount of minutes before we should start to consider guards for connecting again. Proposed value is 20.
SAMPLE_SET_THRESHOLD In order to allow us to recognize dystopic situations or a completely unreachable network, we would like to avoid connecting to too many guards before switching modes. We also want to avoid exposing ourselves to too many nodes in a potentially hostile situation. This parameter, expressed as a fraction, determines the number of guards we should keep as the sampled set of the only guards we will consider connecting to. It will be used as a fraction for both the utopic and the dystopic sampled set. If we assume there are 1900 utopic guards and of them there are 500 dystopic guards, a setting of 0.02 means we will have a sample set of 38 utopic guards and 10 dystopic guards. This limits our total exposure. Proposed value is 0.02.
We should decide if we want to actually use a dynamic percentage here, or just set the threshold to a constant value.
A dynamic percentage might give us better security and reachability as the network evolves, but might also cause unpredictable behaviors if we suddently get too many guards or too many of them disappear.
I don't have a strong opinion here.
INTERNET_LIKELY_DOWN_INTERVAL The number of minutes since we started trying to find an entry guard before we should consider the network down and consider retrying primary guards before using a functioning guard found. Proposed value 20.
It seems to me that the value 20 here could get reduced to something like 5 or even less. Of course 5 is also an arbitrary value and to actually find out the "best" number here we should test the algorithm ourselves in various network types.
Let's say we are restricting ourselves to SAMPLE_SET_THRESHOLD guards. If the network is down, we will start cycling through guards and we will have walked through all of them in a matter of minutes. After we have exhausted our guard list once and we did not manage to connect, our network is effectively down. This might give extra reasons to do the "Retry guards if we have exhausted our guard list once" heuristic.
§4. Security properties and behavior under various conditions
Under normal conditions, this algorithm will allow us to quickly connect and use guards we have used before with high likelihood of working. Assuming the first primary guard is reachable and in the consensus, this algorithm will deterministically always return that guard.
Under dystopic conditions (when a firewall is in place that blocks all ports except for potentially port 80 and 443), this algorithm will try to connect to 2% of all guards before switching modes to try dystopic guards. Currently, that means trying to connect to circa 40 guards before getting a successful connection. If we assume a connection try will take maximum 10 seconds, that means it will take up to 6 minutes to get a working connection.
When the network is completely down, we will try to connect to 2% of all guards plus 2% of all dystopic guards before realizing we are down. This means circa 50 guards tried assuming there are 1900 guards in the network.
In terms of exposure, we will connect to a maximum of 2% of all guards plus 2% of all dystopic guards, or 3% of all guards, whichever is lower. If N is the number of guards, and k is the number of guards an attacker controls, that means an attacker would have a probability of 1-(1-(k/N)^2)^(N * 0.03) to have one of their guards selected before we fall back. In real terms, this means an attacker would need to control over 10% of all guards in order to have a larger than 50% chance of controlling a guard for any given client.
In addition, since the sampled set changes slowly (the suggestion here is that guards in it expire every month) it is not possible for an attacker to force a connection to an entry guard that isn't already in the users sampled set.
§A. Appendix: An example usage
In order to clarify how this algorithm is supposed to be used, this pseudo code illustrates the building of a circuit:
OPEN_CIRCUIT: context = ALGO_CHOOSE_ENTRY_GUARD_START(used_guards, [], 3, false) while True: entryGuard = ALGO_CHOOSE_ENTRY_GUARD_NEXT(context) circuit = composeCircuitAndConnect(entryGuard) if not SHOULD_CONTINUE(isSuccessful(circuit)): ALGO_CHOOSE_ENTRY_GUARD_END(context, entryGuard) return circuit-*- coding: utf-8 -*-