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Re: [TLS] Fwd: Clarification on interleaving app data and handshake records

Hubert Kario <hkario@redhat.com> Sun, 06 December 2015 15:53 UTC

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From: Hubert Kario <hkario@redhat.com>
To: Watson Ladd <watsonbladd@gmail.com>
Date: Sun, 06 Dec 2015 16:53:20 +0100
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Subject: Re: [TLS] Fwd: Clarification on interleaving app data and handshake records
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On Saturday 05 December 2015 19:20:11 Watson Ladd wrote:
> On Sat, Dec 5, 2015 at 6:54 PM, Peter Gutmann 
<pgut001@cs.auckland.ac.nz> wrote:
> > Hubert Kario <hkario@redhat.com> writes:
> >>miTLS does accept Application Data when it is send between Client
> >>Hello and Client Key Exchange and rejects it when it is sent
> >>between Change Cipher Spec and Finished.
> >>
> > Given that miTLS is a formally verified implementation, would this
> > imply that there's a problem with the verification?  "Beware of
> > bugs in the above code; I have only proved it correct, not tried
> > it"?
> 
> Are you saying there is a security flaw with the behavior described?
> Because I don't believe there is after one adopts Extended Master
> Secret. (Someone more familiar with the security should check this)

Extended Master Secret doesn't come into play here at all. The attack 
requires just a passive observation of a legitimate exchange for the 
attacker to have enough information to fake its identity, provided that 
TLS library returns data to application from a new handshake in 
renegotiation before the renegotiation finished.

-- 
Regards,
Hubert Kario
Senior Quality Engineer, QE BaseOS Security team
Web: www.cz.redhat.com
Red Hat Czech s.r.o., Purkyňova 99/71, 612 45, Brno, Czech Republic

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