commit c9a9acb28f8b3ec536e2ff38f87fd2892d813f64 Author: Isis Lovecruft isis@torproject.org Date: Thu Oct 29 20:32:10 2015 +0000

Add draft proposal of new guard selection algorithm. --- proposals/xxx-guard-selection.txt | 303 +++++++++++++++++++++++++++++++++++++ 1 file changed, 303 insertions(+)

diff --git a/proposals/xxx-guard-selection.txt b/proposals/xxx-guard-selection.txt new file mode 100644 index 0000000..c294467 --- /dev/null +++ b/proposals/xxx-guard-selection.txt @@ -0,0 +1,303 @@ +Filename: xxx-guard-selection.txt +Title: New Guard Selection Behaviour +Author: Isis Lovecruft, George Kadianakis +Created: 2015-10-28 +Status: Draft +Extends: 241-suspicious-guard-turnover.txt + + +§1. Overview + + In addition to the concerns regarding path bias attacks, namely that the + space from which guards are selected by some specific client should not + consist of the entirely of nodes with the Guard flag (cf. §1 of proposal + #247), several additional concerns with respect to guard selection behaviour + remain. This proposal outlines a new entry guard selection algorithm, which + additionally 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. + + Before attempting the guard selection procedure, Alice initialises the guard + data structures and prepopulates the guardlist structures, including the + UTOPIC_GUARDLIST and DYSTOPIC_GUARDLIST (cf. §XXX). Additionally, the + structures have been designed to make updates efficient both in terms of + memory and time, in order that these and other portions of the code which + require an up-to-date guard structure are capable of obtaining such. + + 0. Determine if the local network is potentially accessible. + + Alice should attempt to discover if the local network is up or down, + based upon information such as the availability of network interfaces + and configured routing tables. See #16120. [0] + + [XXX: This section needs to be fleshed out more. I'm ignoring it for + now, but since others have expressed interest in doing this, I've added + this preliminary step. —isis] + + 1. Check that we have not already attempted to add too many guards + (cf. proposal #241). + + 2. Then, if the PRIMARY_GUARDS on our list are marked offline, the + algorithm attempts to retry them, to ensure that they were not flagged + offline erroneously when the network was down. This retry attempt + happens only once every 20 mins to avoid infinite loops. + + [Should we do an exponential decay on the retry as s7r suggested? —isis] + + 3. Take the list of all available and fitting entry guards and return the + top one in the list. + + 4. If there were no available entry guards, the algorithm adds a new entry + guard and returns it. [XXX detail what "adding" means] + + 5. Go through the steps 1-4 above algorithm, using the UTOPIC_GUARDLIST. + + 5.a. When the GUARDLIST_FAILOVER_THRESHOLD of the UTOPIC_GUARDLIST has + been tried (without success), Alice should begin trying steps 1-4 + with entry guards from the DYSTOPIC_GUARDLIST as well. Further, + if no nodes from UTOPIC_GUARDLIST work, and it appears that the + DYSTOPIC_GUARDLIST nodes are accessible, Alice should make a note + to herself that she is possibly behind a fascist firewall. + + 5.b. If no nodes from either the UTOPIC_GUARDLIST or the + DYSTOPIC_GUARDLIST are working, Alice should make a note to + herself that the network has potentially gone down. Alice should + then schedule, at exponentially decaying times, to rerun steps 0-5. + + [XXX Should we do step 0? Or just 1-4? Should we retain any + previous assumptions about FascistFirewall? —isis] + + 6. [XXX Insert potential other fallback mechanisms, e.g. switching to + using bridges? —isis] + + +§3. New Data Structures, Consensus Parameters, & Configurable Variables + +§3.1. Consensus Parameters & Configurable Variables + + Variables marked with an asterisk (*) SHOULD be consensus parameters. + + DYSTOPIC_GUARDS ¹ + All nodes listed in the most recent consensus which are marked with + the Guard flag and which advertise their ORPort(s) on 80, 443, or any + other addresses and/or ports controllable via the FirewallPorts and + ReachableAddresses configuration options. + + UTOPIC_GUARDS + All nodes listed in the most recent consensus which are marked with + the Guard flag and which do NOT advertise their ORPort(s) on 80, 443, + or any other addresses and/or ports controllable via the FirewallPorts + and ReachableAddresses configuration options. + + PRIMARY_GUARDS * + The number of first, active, PRIMARY_GUARDS on either the + UTOPIC_GUARDLIST or DYSTOPIC_GUARDLIST as "primary". We will go to + extra lengths to ensure that we connect to one of our primary guards, + before we fall back to a lower priority guard. By "active" we mean that + we only consider guards that are present in the latest consensus as + primary. + + UTOPIC_GUARDS_ATTEMPTED_THRESHOLD * + DYSTOPIC_GUARDS_ATTEMPTED_THRESHOLD * + These thresholds limit the amount of guards from the UTOPIC_GUARDS and + DYSTOPIC_GUARDS which should be partitioned into a single + UTOPIC_GUARDLIST or DYSTOPIC_GUARDLIST respectively. Thus, this + represents the maximum percentage of each of UTOPIC_GUARDS and + DYSTOPIC_GUARDS respectively which we will attempt to connect to. If + this threshold is hit we assume that we are offline, filtered, or under + a path bias attack by a LAN adversary. + + There are currently 1600 guards in the network. We allow the user to + attempt 80 of them before failing (5% of the guards). With regards to + filternet reachability, there are 450 guards on ports 80 or 443, so the + probability of picking such a guard guard here should be high. + + This logic is not based on bandwidth, but rather on the number of + relays which possess the Guard flag. This is for three reasons: First, + because each possible *_GUARDLIST is roughly equivalent to others of + the same category in terms of bandwidth, it should be unlikely [XXX How + unlikely? —isis] for an OP to select a guardset which contains less + nodes of high bandwidth (or vice versa). Second, the path-bias attacks + detailed in proposal #241 are best mitigated through limiting the + number of possible entry guards which an OP might attempt to use, and + varying the level of security an OP can expect based solely upon the + fact that the OP picked a higher number of low-bandwidth entry guards + rather than a lower number of high-bandwidth entry guards seems like a + rather cruel and unusual punishment in addition to the misfortune of + already having slower entry guards. Third, we favour simplicity in the + redesign of the guard selection algorithm, and introducing bandwidth + weight fraction computations seems like an excellent way to + overcomplicate the design and implementation. + + +§3.2. Data Structures + + UTOPIC_GUARDLIST + DYSTOPIC_GUARDLIST + These lists consist of a subset of UTOPIC_GUARDS and DYSTOPIC_GUARDS + respectively. The guards in these guardlists are the only guards to + which we will attempt connecting. + + When an OP is attempting to connect to the network, she will construct + hashring structure containing all potential guard nodes from both + UTOPIC_GUARDS and DYSTOPIC_GUARDS. The nodes SHOULD BE inserted into + the structure some number of times proportional to their consensus + bandwidth weight. From this, the client will hash some information + about themselves [XXX what info should we use? —isis] and, from that, + choose #P number of points on the ring, where #P is + {UTOPIC,DYSTOPIC}_GUARDLIST_ATTEMPTED_THRESHOLD proportion of the + total number of unique relays inserted (if a duplicate is selected, it + is discarded). These selected nodes comprise the + {UTOPIC,DYSTOPIC}_GUARDLIST for (first) entry guards. (We say "first" + in order to distinguish between entry guards and the vanguards + proposed for hidden services in proposal #247.) + + [Perhaps we want some better terminology for this. Suggestions + welcome. —isis] + + Each GUARDLIST SHOULD have the property that the total sum of + bandwidth weights for the nodes contained within it is roughly equal + to each other guardlist of the same type (i.e. one UTOPIC_GUARDLIST is + roughly equivalent in terms of bandwidth to another UTOPIC_GUARDLIST, + but necessarily equivalent to a DYSTOPIC_GUARDLIST). + + For space and time efficiency reasons, implementations of the + GUARDLISTs SHOULD support prepopulation(), update(), insert(), and + remove() functions. A second data structure design consideration is + that the amount of "shifting" — that is, the differential between + constructed hashrings as nodes are inserted or removed (read: ORs + falling in and out of the network consensus) — SHOULD be minimised in + order to reduce the resources required for hashring update upon + receiving a newer consensus. + + The implementation we propose is to use a Consistent Hashring, + modified to dynamically allocate replications in proportion to + fraction of total bandwidth weight. As with a normal Consistent + Hashring, replications determine the number times the relay is + inserted into the hashring. The algorithm goes like this: + + router ← ⊥ + key ← 0 + replications ← 0 + bw_weight_total ← 0 + while router ∈ GUARDLIST: + | bw_weight_total ← bw_weight_total + BW(router) + while router ∈ GUARDLIST: + | replications ← FLOOR(CONSENSUS_WEIGHT_FRACTION(BW(router), bw_total) * T) + | factor ← (S / replications) + | while replications != 0: + | | key ← (TOINT(HMAC(ID)[:X] * replications * factor) mod S + | | INSERT(key, router) + | | replications <- replications - 1 + + where: + + - BW is a function for extracting the value of an OR's `w bandwith=` + weight line from the consensus, + - GUARDLIST is either UTOPIC_GUARDLIST or DYSTOPIC_GUARDLIST, + - CONSENSUS_WEIGHT_FRACTION is a function for computing a router's + consensus weight in relation to the summation of consensus weights + (bw_total), + - T is some arbitrary number for translating a router's consensus + weight fraction into the number of replications, + - H is some collision-resistant hash digest, + - S is the total possible hash space of H (e.g. for SHA-1, with + digest sizes of 160 bits, this would be 2^160), + - HMAC is a keyed message authentication code which utilises H, + - ID is an hexadecimal string containing the hash of the router's + public identity key, + - X is some (arbitrary) number of bytes to (optionally) truncate the + output of the HMAC to, + - S[:X] signifies truncation of S, some array of bytes, to a + sub-array containing X bytes, starting from the first byte and + continuing up to and including the Xth byte, such that the + returned sub-array is X bytes in length. + - INSERT is an algorithm for inserting items into the hashring, + - TOINT convert hexadecimal to decimal integers, + + For routers A and B, where B has a little bit more bandwidth than A, + this gets you a hashring which looks like this: + + B-´¯¯`-BA + A,` `. + / \ + B| |B + \ / + `. ,´A + AB--__--´B + + When B disappears, A remains in the same positions: + + _-´¯¯`-_A + A,` `. + / \ + | | + \ / + `. ,´A + A`--__--´ + + And similarly if B disappears: + + B-´¯¯`-B + ,` `. + / \ + B| |B + \ / + `. ,´ + B--__--´B + + Thus, no "shifting" problems, and recalculation of the hashring when a + new consensus arrives via the update() function is much more time + efficient. + + Alternatively, for a faster and simpler algorithm, but non-uniform + distribution of the keys, one could remove the "factor" and replace + the derivation of "key" in the algorithm above with: + + key ← HMAC(ID || replications)[:X] + + A reference implementation in Python is available². [1] + + +§4. Footnotes + +¹ "Dystopic" was chosen because those are the guards you should choose from if + you're behind a FascistFirewall. + +² One tiny caveat being that the ConsistentHashring class doesn't dynamically + assign replication count by bandwidth weight; it gets initialised with the + number of replications. However, nothing in the current implementation + prevents you from doing: + >>> h = ConsistentHashring('SuperSecureKey', replications=6) + >>> h.insert(A) + >>> h.replications = 23 + >>> h.insert(B) + >>> h.replications = 42 + >>> h.insert(C) + + +§5. References + + [0]: https://trac.torproject.org/projects/tor/ticket/16120 + [1]: https://gitweb.torproject.org/user/isis/bridgedb.git/tree/bridgedb/hashring.... + + +-*- coding: utf-8 -*-