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

Ilari Liusvaara <ilariliusvaara@welho.com> Tue, 02 May 2017 18:49 UTC

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Date: Tue, 02 May 2017 21:46:03 +0300
From: Ilari Liusvaara <ilariliusvaara@welho.com>
To: Nico Williams <nico@cryptonector.com>
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 12:39:06PM -0500, Nico Williams wrote:
> On Tue, May 02, 2017 at 01:33:37PM -0400, Viktor Dukhovni wrote:
> 
> > > 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?

Ticket re-use is the reason why the masking uses add modulo 2^32 instead
of xor (which is more common).

Using add for masking does not leak identity of the parent session (xor
would leak it) even on re-use.

And ticket-reuse leaks the fact that the child sessions are related
anyway, nothing that can be done about that.


-Ilari

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