Re: [TLS] Universal PSKs

Nikos Mavrogiannopoulos <nmav@redhat.com> Mon, 18 June 2018 10:03 UTC

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From: Nikos Mavrogiannopoulos <nmav@redhat.com>
To: David Benjamin <davidben@chromium.org>, Hubert Kario <hkario@redhat.com>
Cc: tls@ietf.org
Date: Mon, 18 Jun 2018 12:03:21 +0200
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Subject: Re: [TLS] Universal PSKs
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On Fri, 2018-06-15 at 09:11 -0400, David Benjamin wrote:
> On Fri, Jun 15, 2018 at 7:14 AM Hubert Kario <hkario@redhat.com>;
> wrote:
> > On Thursday, 14 June 2018 21:46:27 CEST David Benjamin wrote:
> > > Thoughts? If the WG likes this design, I would suggest:
> > > 
> > > - Most folks who want to use TLS 1.3 with external PSKs should
> > probably
> > > design their protocols to provision universal PSKs instead, after
> > it
> > > stabilizes.
> > > 
> > > - Folks who want to use TLS 1.3 with existing TLS 1.2 PSKs should
> > use the
> > > compatibility derivation in this draft, after it stabilizes.
> > > 
> > > - Folks who want to ship TLS 1.3 before then and have a TLS 1.2
> > PSK API
> > > should not use the 1.2 PSK as a 1.3 PSK. For now, just turn TLS
> > 1.3 off by
> > > default if that API is used and, in a future release, use the
> > compatibility
> > > derivation after it stabilizes.
> > 
> > that's not workable.
> > 
> > the reason why implementations chose to use old API to provision
> > TLS 1.3 PSKs 
> > was to make the upgrade process as smooth as possible, disabling
> > TLS 1.3 is 
> > quite antithetical to that
> 
> Indeed. That is why the TLS 1.2 compatibility section exists. :-) So
> that implementations in that position can reuse TLS 1.2 PSK APIs in
> TLS 1.3 while honoring the security proof.
> 
> But, unfortunately, there's a slight timing issue. There's no way
> some random draft published yesterday will be finalized before TLS
> 1.3. So implementations with TLS 1.2 PSK APIs would need to either
> violate the TLS 1.3 security proof or not ship TLS 1.3 until this
> draft finalizes.

Is key separation between TLS1.3 and TLS1.2 something that TLS1.3
provides or intended to provide? As I mentioned in my reply TLS1.3
design goals were very apparent that keys will be re-used from TLS1.2,
and this is what is happening today for any kind of keys from RSA to
PSKs. I'm not sure I see a new cross-protocol violation here that was
not discussed during the TLS1.3 process.

regards,
Nikos