Re: [Ntp] Post NTS, Is shared key authentication interesting?

Daniel Franke <dfoxfranke@gmail.com> Tue, 26 May 2020 15:34 UTC

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From: Daniel Franke <dfoxfranke@gmail.com>
Date: Tue, 26 May 2020 11:34:31 -0400
Message-ID: <CAJm83bAqnfu1zVQk30AHTHCVLNWDd5+FcLsn3mak3pqA69QNnw@mail.gmail.com>
To: Miroslav Lichvar <mlichvar@redhat.com>
Cc: Watson Ladd <watsonbladd@gmail.com>, NTP WG <ntp@ietf.org>
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Subject: Re: [Ntp] Post NTS, Is shared key authentication interesting?
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I could go either way on this. What moves me a bit toward Miroslav's point
of view is that the PSK handshakes in TLS 1.3 unfortunately did not get the
same care and attention from the TLS WG as the rest of the protocol did,
and as a result we got things like https://eprint.iacr.org/2019/347.pdf.
This particular attack doesn't harm us since we distinguish between client
and server packets at the application level, but it suggests that if we're
considering relying on TLS-PSK we can't simply assume that the TLS WG has
given us an adequate foundation to build on; we need to check their work
and make that assessment ourselves.

On Tue, May 26, 2020 at 11:23 AM Miroslav Lichvar <mlichvar@redhat.com>
wrote:

> On Tue, May 26, 2020 at 10:55:41AM -0400, Watson Ladd wrote:
> > On Mon, May 25, 2020 at 9:29 AM Daniel Franke <dfoxfranke@gmail.com>
> wrote:
> > > I would like NTPv5's shared key authentication to be a little more
> closely integrated with NTS. Either accomplish it by doing a PSK TLS
> handshake for NTS-KE, or skip NTS-KE and have pre-shared S2C/C2S keys and a
> shorter cookie giving just a key ID.
> >
> > I think PSK TLS is a better idea: all the complexity gets dumped on
> > the TLS stack, while the NTP specific parts don't change.
>
> A different point of view would be to avoid exposing the NTS+TLS stack
> to attackers if not necessary. The complexity of the NTP MAC is
> minimal when compared to that.
>
> --
> Miroslav Lichvar
>
>