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Re: [TLS] [PSA] 0-RTT tolerance is mandatory

Eric Rescorla <ekr@rtfm.com> Wed, 04 July 2018 16:50 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Wed, 04 Jul 2018 09:50:14 -0700
Message-ID: <CABcZeBOybKcqWBqXxJ5fyX12vhJrq3n=dupVL5NcKR5jCfnYRA@mail.gmail.com>
To: Hubert Kario <hkario@redhat.com>
Cc: Ilari Liusvaara <ilariliusvaara@welho.com>, "<tls@ietf.org>" <tls@ietf.org>
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Subject: Re: [TLS] [PSA] 0-RTT tolerance is mandatory
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On Wed, Jul 4, 2018 at 6:27 AM, Hubert Kario <hkario@redhat.com> wrote:

> On Wednesday, 4 July 2018 15:06:27 CEST Ilari Liusvaara wrote:
> > On Wed, Jul 04, 2018 at 02:42:51PM +0200, Hubert Kario wrote:
> > > All the implementations I deal with in my day-to-day work fail to
> handle
> > > the 0-RTT client hello correctly when the 0-RTT support is not enabled
> on
> > > the server.
> > >
> > > I.e. they ignore the MUST clause from
> > > https://tools.ietf.org/html/draft-ietf-tls-tls13-28#page-58 stating
> that
> > > the server can handle an early_data extension (and following encrypted
> > > data) in only one of three ways, neither which allows for unconditional
> > > connection abort.
> > >
> > > This also runs afoul the recommendation from
> https://tools.ietf.org/html/
> > > draft-ietf-tls-tls13-28#section-D.3 on 0-RTT backwards compatibility.
> >
> > OTOH, such servers probably do not send out tickets allowing 0-RTT, so
> > any client attempting 0-RTT with such server is very broken.
>
> please read Section D.3, it is spelled out in detail there why that's not
> the
> case
>
>
> the short of it: the client has no way of knowing if it is connecting to
> the
> same server/instance/process it received the ticket from
>

That's true if you have some kind of mixed rollout, but if you never
supported
0-RTT, then there is something wrong with the client.

-Ekr


> --
> Regards,
> Hubert Kario
> Senior Quality Engineer, QE BaseOS Security team
> Web: www.cz.redhat.com
> Red Hat Czech s.r.o., Purkyňova 115, 612 00  Brno, Czech Republic
>
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