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

[Hubert Kario <[e-mail]>]

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

-- 
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