Re: [Ohai] DoubleCheck and CONNECT-UDP

[Matthew Finkel <[e-mail]>]

On Tue, Jul 26, 2022 at 10:57:33PM -0400, Ben Schwartz wrote:
> During my presentation today, I suggested that the DoubleCheck procedure might not need CONNECT-UDP if it's acceptable for the Service Description Host (SDH), which is presumed to be affiliated with the gateway, to learn the pool of client IPs.  On second thought, I believe the CONNECT-UDP tunnel is indeed necessary.
>
> Without the CONNECT-UDP tunnel, clients would authenticate the KeyConfig by fetching it directly from the SDH.   In this arrangement, it is often possible for the gateway to identify the precise client IP that issued a particular OHTTP request, due to timing correlation.  If the rate of queries to this gateway (through this relay) is relatively low (<1 QPS), correlation is trivial.  The gateway can constantly rotate its KeyConfig, forcing clients to reauthenticate it before each OHTTP request.
>
> At higher query rates, other tricks are possible, such as rotating the KeyConfig and then slow-walking the authentication responses.  By sending authentication responses to each client "one at a time", the SDH and Gateway can observe the OHTTP request that follows each authentication response.
>
> In other words, OHTTP requests are linkable (via timing) to the corresponding KeyConfig authentication requests, so these authentication requests must not be linkable to anything else.  This means authentication requests from all clients on the relay must be indistinguishable, so they must all go through a single proxy.

I agree, thanks for thinking through this attack and writing a
concrete proposal for achieving key consistency and correctness. We
should probably update the referenced informational KCCS document with
some of these insights, as well. I also appreciate how this design
leverages multiple discovery methods.

As for feedback on the draft:
1) Is there a specific reason why this depends on the Access Service
Descriptions? I see why they are useful, but they seem more like a
convenience than a required dependency, is this true?

2) The concern described at the end of the introduction around not
using the same update mechanism for key rotation as the one used for
configuring the "Service Description" seems like a bit of a stretch.
First, key rotation should not be occurring at such a high frequency
that a slower (software release process) update mechanism is
inadequate. Second, as mentioned in the KCCS document, we recommend
specifying a "minimum validity period" for these keys such that a
service can't easily partition users based on which key they know/use.
While we didn't recommend any specific minimum period length, I would
certainly recommend a lower-bound on the order of days (with typical
rotation on the order of weeks, at a minimum). Providing more
flexibility and agility opens more avenues for abuse, but I am
interested in hearing any trade-offs you thought about for this, too.

3) Not directly feedback on this draft, but most obviously, the
consistency guarantees are only limited to users of the same relay (I
believe you mentioned this, but I can't find it now), so we may be
missing "global" consistency (if the service is used via
multiple/arbitrary relays). As discussed in other contexts, "global
consistency" really reduces to ensuring each client request originates
from a sufficiently large client anonymity set. Defining "sufficiently
large" is likely context and application-specific, but it should be
explicitly considered in deployments. This is related to "precondition
(1)", but it is not the same. Do you have thoughts you can share on
this?

4) A possible TODO: Regarding the response by a Service when the
Service Description changes but a request's If-Match header identifies
a previous version of the resource: Should the Service cache old
versions for a certain time? Maybe something like an additional
max-age length? The draft doesn't seem to specify when the Service
should stop returning a success response for old resources.

5) A possible TODO: What should be the client's behavior if the
response from the relay is rejected or if establishing the CONNECT-UDP
tunnel fails? Fail-open? Fail-closed? Attempt check via alternative
relay and/or choose different service? This could be
application-specific logic, but it could be worth describing
trade-offs of different behaviors in this situation. In particular,
the relay is in a position to maliciously perform a denial of service
during this check and it could force the client to choose a different
service (possibly one that will collude with the relay).

On a more nitty-note, I'm not sure how much you like the "DoubleCheck"
name, but as another option we've referred to similar behavior as
"multi-path verification".

Thanks,
Matt