Re: [TLS] WGLC for draft-ietf-tls-ticketrequests

Viktor Dukhovni <ietf-dane@dukhovni.org> Tue, 21 January 2020 09:16 UTC

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Date: Tue, 21 Jan 2020 04:16:18 -0500
From: Viktor Dukhovni <ietf-dane@dukhovni.org>
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Subject: Re: [TLS] WGLC for draft-ietf-tls-ticketrequests
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On Tue, Jan 21, 2020 at 07:57:27PM +1100, Martin Thomson wrote:

> On Tue, Jan 21, 2020, at 16:54, Viktor Dukhovni wrote:
> > There's no need to exclude valid use-cases.  The refined proposal
> > is rather non-invasive, and handles this case cost-effectively
> > on clients that re-use tickets (and don't use early-data, ...).
> 
> I don't find your arguments persuasive.  This adds complexity
> specifically to address a case that has - in the general case -
> suboptimal characteristics, both in terms of forward secrecy and
> linkability.  Whether or not there are specific cases that might
> tolerate these suboptimalities, the complexity and risks are borne by
> everyone.

    * Switching to a new ticket is of no help if the server is still
      using the same STEK.  Any forward-secrecy concerns are a function
      of the frequency of STEK rotation on the server.

    * We work in noticeably different application spaces.  SMTP client
      MTAs send 220 greetings and EHLO commands identifying themselves
      in the clear prior to the STARTTLS handshake.  MTAs don't roam
      from network to network, and linkability of traffic is not a
      concern.

    * Postfix, for example, rotates STEKs by default once an hour,
      making the forward-secrecy exposure quite limited. 

    * Clients don't use the same ticket indefinitely, they too discard
      stale tickets, if the server does not beat them to it.

The requested change is rather minimal, it merely asks the server to
intepret 0 as "optional ticket" rather than "no ticket" reserving the
last code point (not uncommon to have make it special) as a no-go
sentinel.

Even if the proposal is not compelling for some, it allows additional
legitimate use-cases at negligible cost.  So barring some actual
problem, it is hard to see on what basis it can be reasonably excluded. 

That's all from me.  Thanks.

-- 
    Viktor.