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Re: [TLS] Security review of TLS1.3 0-RTT

Nico Williams <nico@cryptonector.com> Tue, 02 May 2017 18:03 UTC

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Date: Tue, 02 May 2017 13:00:50 -0500
From: Nico Williams <nico@cryptonector.com>
To: Colm MacCárthaigh <colm@allcosts.net>
Cc: TLS WG <tls@ietf.org>
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Subject: Re: [TLS] Security review of TLS1.3 0-RTT
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On Tue, May 02, 2017 at 10:48:29AM -0700, Colm MacCárthaigh wrote:
> On Tue, May 2, 2017 at 10:39 AM, Nico Williams <nico@cryptonector.com>
> wrote:
> > With existing APIs, dealing with "pools of meaningfully distinct
> > tickets" seems meaningfully non-trivial.
> 
> I would actually prefer if the client could request N tickets, but was
> advised that this was too large a change to the protocol.
> 
> > > There's also an observation there that it should really be that
> > > > clients "MUST" use tickets only once. Any re-use likely discloses
> > > > the obfuscated ticket age, which is intended to be secret. Right now
> > > > it's a "SHOULD".
> >
> > Why should ticket age disclosure be a problem?  How does ticket one-time
> > use not do the same?
> >
> 
> The draft writes that it is to prevent connection correlation attacks.

I would think that the ticket itself is enough for that when using
0-rtt.  I.e., if you don't want connection correlation to be possible,
you can't use 0-rtt.  The age business (which I hadn't looked into
before) seems incidental.

(Also, one would think that the client would send a timestamp in an
authenticator...  You know, a lot like what Kerberos does.)

Nico
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