Re: [Privacy-pass] Detecting malicious registries/servers

[Steven Valdez <[e-mail]>]

I don't think the Privacy Pass drafts can really solve the unlinkable
communication problem, and shouldn't focus on it besides listing it as a
requirement. Depending on the use case/caller, communicating with the
Issuer/Redeemer in an unlinkable way could involve making completely
disconnected TLS/HTTP connections, dropping all state (some sort of
anonymous mode), changing request patterns so they can't be time
correlated, etc.

PrivacyPass can make the protocol robust against unlinkability based on the
contents of protocol messages, but solving unlinkability based on the
transport seems outside of the PrivacyPass scope, though listing the
requirements that would be maintained could be useful for use cases built
on top of Privacy Pass.

On Mon, Aug 3, 2020 at 6:14 PM Chelsea Komlo <[e-mail]> wrote:

> It seems there was a slight miscommunication with the point I was trying
> to make during the meeting, so I'll clarify.
>
>
> I am strongly in favor of making the protocol as robust as possible, while
> being transparent about what risks cannot be mitigated in certain settings
> (such as servers building a lookup table).
>
>
> Placing the burden on implementations to figure out how to ensure the
> primary use case of the protocol- unlinkability- is counterintuitive to me.
>
>
>
>
> ------------------------------
> *From:* Privacy-pass <[e-mail]> on behalf of Ben
> Schwartz <bemasc=[e-mail]>
> *Sent:* Monday, August 3, 2020 9:59:59 AM
> *To:* Chelsea Komlo
> *Cc:* [e-mail]
> *Subject:* Re: [Privacy-pass] Detecting malicious registries/servers
>
> I found your and Richard's comment during the meeting particularly
> insightful on this point: we should clarify the assumptions Privacy Pass
> has to make about the intrinsic identifiability of client requests.  For
> example, if Privacy Pass necessarily assumes that the Client's connections
> to Verifiers are unlinkable, then perhaps we can simply declare that the
> Client's connections to Registries must be similarly unlinkable.
>
> The mechanism for ensuring that unlinkability might be out of scope for
> us, but we would need to describe the requirements for it.
>
> On Mon, Aug 3, 2020 at 5:16 PM Chelsea Komlo <[e-mail]>
> wrote:
>
>> Hi all,
>>
>> One question that has been raised is how clients can detect registry or
>> server misbehavior. One particular concern is the possibility that
>> registries or servers can perform targeted attacks by publishing different
>> keys to specific users without detection (resulting in a small anonymity
>> set).
>>
>> While client-side gossip protocols or use of anonymity networks give the
>> client an option to detect such targeted attacks, these options are not
>> necessarily generalizable.
>>
>> However, I think if we introduce the assumption that *at least one*
>> registry is acting honestly, then this problem may be easier to solve.
>>
>> Registries just need to add an additional capability to query other
>> registries on behalf of a client (if the client specifies this in their
>> request). In doing so, they can act as a one-hop proxy for the client. If
>> clients perform these requests periodically through a series of
>> randomly-chosen registries, they should eventually be able to detect
>> misbehavior, assuming one of the registries is acting honestly.
>>
>> Chris Wood pointed out to me that the above approach may have false
>> positives when a server rotates their key, so adding some kind of
>> "suspected" and "failed state" with retries might help get around such edge
>> cases.
>>
>> Thanks,
>> Chelsea
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>>
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-- 

Steven Valdez |  Chrome Privacy Sandbox |  [e-mail] |
 210-692-4742