Re: Password identifiers, protocol aging and SCRAM protocol

On Thu, Dec 8, 2016 at 5:55 PM, Heikki Linnakangas <hlinnaka@iki.fi> wrote:
> On 12/08/2016 10:18 AM, Michael Paquier wrote:
>> Hmmm. How do we handle the case where the user name does not match
>> then? The spec gives an error message e= specifically for this case.
>
> Hmm, interesting. I wonder how/when they imagine that error message to be
> used. I suppose you could send a dummy server-first message, with a made-up
> salt and iteration count, if the user is not found, so that you can report
> that in the server-final message. But that seems unnecessarily complicated,
> compared to just sending the error immediately. I could imagine using a
> dummy server-first messaage to hide whether the user exists, but that
> argument doesn't hold water if you're going to report an "unknown-user"
> error, anyway.

Using directly an error message would map with MD5 and plain, but
that's definitely a new protocol piece so I'd rather think that using
e= once the client has sent its first message in the exchange should
be answered with an appropriate SASL error...

> Actually, we don't give away that information currently. If you try to log
> in with password or MD5 authentication, and the user doesn't exist, you get
> the same error as with an incorrect password. So, I think we do need to give
> the client a made-up salt and iteration count in that case, to hide the fact
> that the user doesn't exist. Furthermore, you can't just generate random
> salt and iteration count, because then you could simply try connecting
> twice, and see if you get the same salt and iteration count. We need to
> deterministically derive the salt from the username, so that you get the
> same salt/iteration count every time you try connecting with that username.
> But it needs indistinguishable from a random salt, to the client. Perhaps a
> SHA hash of the username and some per-cluster secret value, created by
> initdb. There must be research papers out there on how to do this..

A simple idea would be to use the system ID when generating this fake
salt? That's generated by initdb, once per cluster. I am wondering if
it would be risky to use it for the salt. For the number of iterations
the default number could be used.

> To be really pedantic about that, we should also ward off timing attacks, by
> making sure that the dummy authentication is no faster/slower than a real
> one..

There is one catalog lookup when extracting the verifier from
pg_authid, I'd guess that if we generate a fake verifier things should
get pretty close.

>> If this is taken into account we need to perform sanity checks at
>> initialization phase I am afraid as the number of iterations and the
>> salt are part of the verifier. So you mean that just sending out a
>> normal ERROR message is fine at an earlier step (with *logdetails
>> filled for the backend)? I just want to be sure I understand what you
>> mean here.
>
> That's right, we can send a normal ERROR message. (But not for the
> "user-not-found" case, as discussed above.)

I'd think that the cases where the password is empty and the password
has passed valid duration should be returned with e=other-error. If
the caller sends a SCRAM request that would be impolite (?) to just
throw up an error once the exchange has begun.

> Although, currently, the whole pg_hba.conf file in that example is a valid
> file that someone might have on a real server. With the above addition, it
> would not be. You would never have the two lines with the same
> host/database/user combination in pg_hba.conf.

Okay.
-- 
Michael