[Ntp] Re: Wrong NTS key exporter context in use for AES-128-GCM-SIV

Miroslav Lichvar <mlichvar@redhat.com> Wed, 09 October 2024 06:42 UTC

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Date: Wed, 09 Oct 2024 08:40:26 +0200
From: Miroslav Lichvar <mlichvar@redhat.com>
To: Martin Mayer <martin.mayer=40m2-it-solutions.de@dmarc.ietf.org>
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Subject: [Ntp] Re: Wrong NTS key exporter context in use for AES-128-GCM-SIV
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It seems this email was stuck in the list for a few weeks. I'm
answering for completeness.

On Mon, Sep 16, 2024 at 11:42:40AM +0000, Martin Mayer wrote:
> Per RFC a KoD should be used if the data cannot be validated or authenticated, which will happen on NTP requests from an RFC-conformant client using AES-128-GCM-SIV and also from a patched chrony version.
> Chrony could remember that first (failed) stage to re-initiate one (and only one) second NTS-KE run in 'compatibility mode' using the non-conformant context.

Yes, this should work. The client can also keep both sets of keys and
switch between them without rerunning NTS-KE. It should do that only
if there was no authenticated response from the server, just a valid
NTS NAK. That's what the new chrony version is doing.

Thanks again for reporting the problem.

-- 
Miroslav Lichvar