A guard discovery attack allows attackers to determine the guard relay of a Tor client. The hidden service protocol provides an attack vector for a guard discovery attack since anyone can force an HS to construct a 3-hop circuit to a relay, and repeat this process until one of the adversary's middle relays eventually ends up chosen in a circuit. These attacks are also possible to perform against clients, by causing an application to make repeated connections to multiple unique onion services.
The adversary must use a protocol side channel to confirm that their relay was chosen in this position (see Proposal #344), and then learns the guard relay of a client, or service.
When a successful guard discovery attack is followed with compromise or coercion of the guard relay, the onion service or onion client can be deanonymized. Alternatively, Netflow analytics data purchase can be (and has been) used for this deanonymization, without interacting with the Guard relay directly (see again Proposal #344).
This specification assumes that Tor protocol side channels have 100% accuracy and are undetectable, for simplicity in reasoning about expected attack times. Presently, such 100% accurate side channels exist in silent form, in the Tor Protocol itself.
As work on addressing Tor's protocol side channels progresses, these attacks will require application-layer activity that can be monitored and managed by service operators, as opposed to silent and unobservable side channel activity via the Tor Protocol itself. Application-layer side channels are also expected to have less accuracy than native Tor protocol side channels, due to the possibility of false positives caused by similar application activity elsewhere on the Tor network. Despite this, we will preserve the original assumption of 100% accuracy, for simplicity of explanation.
In this specification, we specify two forms of a multi-layered Guard system: one for long-lived services, called Full Vanguards, and one for onion clients and short-lived services, called Vanguards-Lite.
Both approaches use a mesh topology, where circuits can be created from any relay in a preceding layer to any relay in a subsequent layer.
The core difference between these two mechanisms is that Full Vanguards has two additional layers of fixed vanguard relays, which results in longer path lengths and higher latency. In contrast, Vanguards-Lite has only one additional layer of fixed vanguard relays, and preserves the original path lengths in use by onion services. Thus, Full Vanguards comes with a performance cost, where as Vanguards-Lite does not. The rotation periods of the two approaches also differ.
Vanguards-Lite MUST be the default for all onion service and onion client activity; Full Vanguards SHOULD be available as an optional configuration option for services.
Neither system applies to Exit activity.
Tor's original guards are called First Layer Guards.
The first layer of vanguards is at the second hop, and is called the Second Layer Guards.
The second layer of vanguards is at the third hop, and is called the Third Layer Guards.
Full Vanguards pins these two middle positions into a mesh topology, where any relay in a layer can be used in that position in a circuit, as follows:
-> vanguard_2A -> vanguard_3A -> guard_1A -> vanguard_2B -> vanguard_3B HS -> vanguard_3C -> guard_1B -> vanguard_2C -> vanguard_3D -> vanguard_3E -> vanguard_2D -> vanguard_3F
Additionally, to avoid trivial discovery of the third layer, and to minimize linkability, we insert an extra middle relay after the third layer guard for client side intro and hsdir circuits, and service-side rendezvous circuits. This means that the set of paths for Client (C) and Service (S) side look like this:
Client hsdir: C - G - L2 - L3 - M - HSDIR Client intro: C - G - L2 - L3 - M - I Client rend: C - G - L2 - L3 - R Service hsdir: S - G - L2 - L3 - HSDIR Service intro: S - G - L2 - L3 - I Service rend: S - G - L2 - L3 - M - R
Vanguards-Lite uses only one layer of vanguards:
-> vanguard_2A -> guard_1A -> vanguard_2B HS -> guard_1B -> vanguard_2C -> vanguard_2D
This yields shorter path lengths, of the following form:
Client hsdir: C -> G -> L2 -> M -> HSDir Client intro: C -> G -> L2 -> M -> Intro Client rend: C -> G -> L2 -> Rend Service hsdir: C -> G -> L2 -> M -> HSDir Service intro: C -> G -> L2 -> M -> Intro Service rend: C -> G -> L2 -> M -> Rend
An alternative to vanguards for client activity is to restrict the number of onion services that a Tor client is allowed to connect to, in a certain period of time. This defense was explored in Onion Not Found.
We have opted not to deploy this defense, for three reasons:
- It does not generalize to the service-side of onion services
- Setting appropriate rate limits on the number of onion service content elements on a page for Tor Browser is difficult. Sites like Facebook use multiple onion domains for various content elements on a single page.
- It is even more difficult to limit the number of service connections for arbitrary applications, such as cryptocurrency wallets, mining, and other distributed apps deployed on top of onion services that connect to multiple services (such as Ricochet).