Re: Postgres 11 release notes

On Wed, May 16, 2018 at 08:59:23PM +0900, Michael Paquier wrote:
> On Wed, May 16, 2018 at 01:09:07PM +0300, Heikki Linnakangas wrote:
> > I have to agree with Bruce, that it's pretty useless to implement channel
> > binding, if there is no way to require it in libpq. IMHO that must be
> > fixed.
>
> Wouldn't we want to also do something for the case where a client is
> willing to use SCRAM but that the server forces back MD5? In which
> case, one possibility is a connection parameter like the following,
> named say authmethod:
> - An empty value is equivalent to the current behavior, and is the
> default.
> - 'scram' means that client is willing to use SCRAM, which would cause a
> failure if server attempts to enforce MD5.
> - 'scram-plus' means that client enforces SCRAM and channel binding.
>
> Or we could just have a channel_binding_mode, which has a "require"
> value like sslmode, and "prefer" mode, which is the default and the
> current behavior... Still what to do with MD5 requests in this case?

Just to reiterate, MD5 and SCRAM-less-binding is designed to avoid
packet _replay_. It assumes no man-in-the-middle has adjusted what is
supported by the two endpoints.

SCRAM-with-binding is the first password method that attempts to avoid
man-in-the-middle attacks, and therefore is much less likely to be able
to trust what the endpoints supports. I think it is really the
channel_binding_mode that we want to control at the client. The lesser
modes are much more reasonable to use an automatic best-supported
negotiation, which is what we do now.

FYI, I think the server could also require channel binding for SCRAM. We
already have scram-sha-256 in pg_hba.conf, and I think
scram-sha-256-plus would be reasonable.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

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