Re: [TLS] TLS 1.3 - method to request uncached shared secrets

Viktor Dukhovni <ietf-dane@dukhovni.org> Sun, 19 July 2015 21:42 UTC

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Date: Sun, 19 Jul 2015 21:42:57 +0000
From: Viktor Dukhovni <ietf-dane@dukhovni.org>
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Subject: Re: [TLS] TLS 1.3 - method to request uncached shared secrets
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On Sun, Jul 19, 2015 at 05:28:27PM -0400, Brian Smith wrote:

> Thus, because of that
> possibility, it is valuable to have the client be able to say "don't cache
> the session" and/or limit the session's lifetime, so the client can help
> direct the level of forward secrecy for the session. Right now, only the
> server has a say in how long a session will be forward-secret.

I think this is too much complexity, for too little gain.  Servers
will limit the lifetime of local session storage rather severely
to avoid running out of space.

> Note also that the NewSessionTicket extension precedes any application
> data, so without a way to prevent an unwanted NewSessionTicket message from
> being sent, the client has to waste effort and time to consume the
> NewSessionTicket before it can do anything useful.

This is typically much smaller than the server certificate chain,
so the cost saving is marginal.

> Anyway, I don't understand why you keep directing your question to server
> vendors. The people that would be interested in such a feature are client
> software vendors, for client software that wants to control the level of
> forward secrecy for a session.

I don't think the client would gain much if any control by signalling
a lifetime hint.  With actual session tickets, the key rotation
lifecycle determines the effective forward-secrecy exposure (to
server state compromise before a key is retired) of the ticket,
and no client hint can influence that in any way.

So I agree with Eric that the issue is fundamentally a server-side
issue, modulo the fact that the CPU and bandwidth cost of processing
the unwanted ticket is borne on both sides.

-- 
	Viktor.