Re: [TLS] 0-RTT encrypted data limits

Hubert Kario <hkario@redhat.com> Thu, 01 September 2016 10:31 UTC

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From: Hubert Kario <hkario@redhat.com>
To: Eric Rescorla <ekr@rtfm.com>
Date: Thu, 01 Sep 2016 12:31:49 +0200
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Subject: Re: [TLS] 0-RTT encrypted data limits
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On Wednesday, 31 August 2016 11:23:11 CEST Eric Rescorla wrote:
> On Wed, Aug 31, 2016 at 11:14 AM, Hubert Kario <hkario@redhat.com> wrote:
> > Current draft has the following text in it:
> >     If any of these checks fail, the server MUST NOT respond
> >     with the extension and must discard all the remaining first
> >     flight data (thus falling back to 1-RTT). If the client attempts
> >     a 0-RTT handshake but the server rejects it, it will generally
> >     not have the 0-RTT record protection keys and must instead
> >     trial decrypt each record with the 1-RTT handshake keys
> >     until it finds one that decrypts properly, and then pick up
> >     the handshake from that point.
> > 
> > My understanding of that, in case client does 0-RTT but server rejects it
> > (because the PSK is too old or its time is different enough) is that the
> > server needs to keep on reading arbitrarily large amounts of data it has
> > no
> > idea what to do with
> > 
> > Why is there no limit on the amount of data that can be encrypted using
> > PSK keys (0-RTT)?
> 
> I don't think this would usefully improve things.

there's also second angle:

I'm afraid that requiring the server to keep the connection open for 
essentially arbitrary amount of time while it consumes garbage data is not 
unlike the Apache slowloris attack.
-- 
Regards,
Hubert Kario
Senior Quality Engineer, QE BaseOS Security team
Web: www.cz.redhat.com
Red Hat Czech s.r.o., Purky┼łova 99/71, 612 45, Brno, Czech Republic