Title: Vanguards lite
Author: George Kadianakis, Mike Perry
Created: 2021-05-20
Status: Closed

0. Introduction & Motivation

This proposal specifies a simplified version of Proposal 292 "Mesh-based vanguards" for the purposes of implementing it directly into the C Tor codebase.

For more details on guard discovery attacks and how vanguards defend against it, we refer to Proposal 292 PROP292_REF.

1. Overview

We propose an identical system to the Mesh-based Vanguards from proposal 292, but with the following differences:

  • No third layer of guards is used.
  • The Layer2 lifetime uses the max(x,x) distribution with a minimum of one day and maximum of 12 days. This makes the average lifetime approximately a week.
  • We let NUM_LAYER2_GUARDS=4. We also introduce a consensus parameter guard-hs-l2-number that controls the number of layer2 guards (with a maximum of 19 layer2 guards).
  • We don't write guards on disk. This means that the guard topology resets when tor restarts.

By avoiding a third-layer of guards we avoid most of the linkability issues of Proposal 292. This means that we don't add an extra hop on top of most of our onion service paths, which increases performance. However, we do add an extra middle hop at the end of service-side introduction circuits to avoid linkability of L2s by the intro points.

This is how onion service circuits look like with this proposal:

 Client rend:   C -> G -> L2 -> Rend
 Client intro:  C -> G -> L2 -> M -> Intro
 Client hsdir:  C -> G -> L2 -> M -> HSDir
 Service rend:  C -> G -> L2 -> M -> Rend
 Service intro: C -> G -> L2 -> M -> Intro
 Service hsdir: C -> G -> L2 -> M -> HSDir

2. Rotation Period Analysis

From the table in Section 3.1 of Proposal 292, with NUM_LAYER2_GUARDS=4 it can be seen that this means that the Sybil attack on Layer2 will complete with 50% chance in 187 days (126 days) for the 1% adversary, 47 days (one month) for the 5% adversary, and 2*7 days (two weeks) for the 10% adversary.

3. Tradeoffs from Proposal 292

This proposal has several advantages over Proposal 292:

By avoiding a third-layer of guards we reduce the linkability issues of Proposal 292, which means that we don't have to add an extra hop on top of our paths. This simplifies engineering and makes paths shorter by default: this means less latency and quicker page load times.

This proposal also comes with disadvantages:

The lack of third-layer guards makes it easier to launch guard discovery attacks against clients and onion services. Long-lived services are not well protected, and this proposal might provide those services with a false sense of security. Such services should still use the vanguards addon VANGUARDS_REF.

4. Implementation nuances

Tor replaces an L2 vanguard whenever it is no longer listed in the most recent consensus, with the goal that we will always have the right number of vanguards ready to be used.

For implementation reasons, we also replace a vanguard if it loses the Fast or Stable flag, because the path selection logic wants middle nodes to have those flags when it's building preemptive vanguard-using circuits.

The design doesn't have to be this way: we might instead have chosen to keep vanguards in our list as long as possible, and continue to use them even if they have lost some flags. This tradeoff is similar to the one in about whether to continue using Entry Guards if they lose the Guard flag -- and Tor's current choice is "no, rotate" for that case too.

5. References