Re: [MLS] Hiding content type

[Chelsea Komlo <[e-mail]>]

I realize this is off topic to the discussion of hiding content type/epoch
ids in MLS, but addressing the below for completeness. I'm happy to
continue this discussion in a separate thread if there is further interest.

On Tue, Jul 28, 2020 at 4:05 PM Brendan McMillion <[e-mail]>
wrote:

> While the network layer would need to protect against network-level
>> traffic analysis such as timing and frequency fingerprinting, this can be
>> done in a generalized way that is agnostic to the application (such as if a
>> mix network were used to route traffic between the client and the DS).
>
>
> In the specific case of a mix network, that's not true. The DS has at
> least these variables for traffic analysis:
>
> 1. Messages which are requested in quick succession (associating messages
> that are requested together with the same group)
>

The DS can observe access patterns, but the important point is whether it
can then use that information to deanonymize users.

In a mix network instantiated with protocols like Loopix [1], users
generate and send dummy packets (i.e, cover traffic that is dropped at
random points in the network) along with real packets, all which are
delayed by some random time period at each hop in the path. So performing
end-to-end correlation attacks in such systems is harder.

However, even when Tor is used, the question is at what point the DS *can*
learn enough to deanonymize users if it can only observe access patterns
and any exposed metadata (assuming it cannot perform end-to-end correlation
attacks). See next.


> 2. How many times a message is requested (associating a message with the
> number of participants in the group it was sent to)
>

Note this problem of access pattern observability is not unique to MLS and
without the use of PIR, access patterns will be observable by any content
provider even when traffic is routed through an anonymity network.


> 3. Messages requested over the same circuit (Tor and similar networks
> often reuse circuits for efficiency)
>

This is only an issue in circuit-switched networks like Tor (where users
maintain a long-lived connections), whereas mix networks like Loopix are
packet-switched, meaning every message is routed independently. Further,
applications using Tor can rotate through circuits more quickly, if desired
(Tor Browser does this).

[1]
https://www.usenix.org/conference/usenixsecurity17/technical-sessions/presentation/piotrowska