[tor-dev] Alternative directory format for v3 client auth

[George Kadianakis]

Michael Rogers <michael at briarproject.org> writes:

> On 11/07/18 14:22, George Kadianakis wrote:
>> Michael Rogers <michael at briarproject.org> writes:
>> 
>>> On 10/07/18 19:58, George Kadianakis wrote:
>>>> here is a patch with an alternative directory format for v3 client auth
>>>> crypto key bookkeeping as discussed yesterday on IRC:
>>>>        https://github.com/torproject/torspec/pull/23
>>>>
>>>> Thanks for making me edit the spec because it made me think of various
>>>> details that had to be thought of.
>>>>
>>>> Let me know if you don't like it or if something is wrong.
>>>
>>> Minor clarification: line 2298 says the keypair is stored, it might be
>>> clearer to say the private key is stored.
>>>
>>> Nitpick: should the directory be called "client_authorized_privkeys" if
>>> it might contain private keys, public keys, or a mixture of the two?
>>>
>> 
>> Good points in both cases. Will fix soon (along with other feedback if received).
>> 
>> Other than that, what do you think about the whole concept? Too complex?
>> Logical? Too much?
>> 
>> Cheers for the feedback! :)
>
> Sorry for being late to the party - I just this morning finished reading
> the thread from 2016 where the client auth design was hashed out. :-/
>
> I think putting each client's keys in a separate file makes a lot of sense.
>
> At a higher level there are some things I'm not sure about. Sorry if
> this is threadjacking, but you said the magic words "whole concept". ;-)
>

Thanks for raising these issues and for taking the time to read the
previous thread. We really need feedback like this from people who have
used our systems like you :)

> First, Ed25519-based authentication ("intro auth"). Could this be punted
> to the application layer, or is there a reason it has to happen at the
> Tor layer?
>

Yes, it could be stuffed into the application layer. However that could be
an argument for everything (including end-to-end encryption of onions).

It might be the case that some application-layer protocols don't allow
any sort of pluggable authentication to happen on top of them, or that
users wouldn't want to enable them for some reason. Does this feel like
an artificial reason to you?

Another positive thing about intro auth is that it allows fine-grained
control over authentication, potentially allowing different tiers of
users etc.

Also see https://lists.torproject.org/pipermail/tor-dev/2018-May/013155.html

> Second, X25519-based authorization ("desc auth"). If I understand right,
> using asymmetric keypairs here rather than symmetric keys makes it
> possible for the client to generate a keypair and send the public key to
> the service over an authenticated but not confidential channel. But the
> client may not know how to do that, so we also need to support an
> alternative workflow where the service generates the keypair and sends
> the private key to the client over an authenticated and confidential
> channel.
>
> The upside of this design is the ability to use an authenticated but not
> confidential channel (as long as the client and service understand which
> workflow they need to use). The downside is extra complexity. I'm not
> really convinced this is a good tradeoff. But I'm guessing this argument
> has already been had, and my side lost. :-)
>

Yes, you have described it very well.
And I agree that the tradeoff is complicated.

> Third, what's the purpose of the fake auth-client lines for a service
> that doesn't use client auth? I understand that when a service does use
> client auth, it may not want clients (or anyone else who knows the onion
> address) to know the exact number of clients. But when a service doesn't
> use client auth, anyone who can decrypt the first layer of the
> descriptor can also decrypt the second layer, and therefore knows that
> the auth-client lines are fake. So are they just for padding in that
> case? But the first layer's padded before encryption anyway.
>

Yes, fake auth-client lines when client auth is disabled are not very
useful as you point out (also see #23641).

> Fourth, what goals does desc auth achieve in the v3 design? If I
> understand right, in v2 its major goal was to hide the intro points from
> everyone except authorised clients (including HSDirs). In v3 the intro
> points are already hidden from anyone who doesn't know the onion address
> (including HSDirs), so this goal can be achieved by not revealing the
> onion address to anyone except authorised clients.
>
> I'm probably missing something, but as far as I can see the only other
> goal achieved by desc auth is the ability to revoke a client's access
> without needing to distribute a new onion address to other clients. This
> seems useful. But again, I'd ask whether it could be punted to the
> application layer. The only advantage I can see from putting it at the
> Tor layer is that the list of intro points is hidden from revoked
> clients. Is there a real world use case where that's a big enough
> advantage to justify putting all this authorisation machinery at the Tor
> layer? Or maybe there are other things this design achieves that I
> haven't thought of.
>

Yes, you identified the point of desc auth correctly.

Another very important reason to have an authorization system inside
Tor, is because it allows only authorized clients to rendezvous (and in
general directly interact) with the onion service. That can mitigate all
sorts of guard discovery and correlation attacks that could be doable by
anyone, and restrict them only to authorized users.

Of course the above is achieved with either desc auth or intro
auth. Having both of them does not offer any benefits in this direction.

> Anyway, sorry for the bag of assorted questions. I've been meaning to
> catch up on all the discussions where they've probably been answered
> already, but it's becoming clear that's a losing battle, so I'd better
> just send them. Apologies if they're redundant or uninformed.
>

Thanks for raising these questions, they are very important. Please keep
them coming.

We might be trying to achieve too many things here, or trying to design
a too convoluted system, so we need feedback to understand the user
expectations and threat models we should be trying to satisfy.