Re: [TLS] Suspicious behaviour of TLS server implementations

Hubert Kario <hkario@redhat.com> Thu, 22 September 2016 11:41 UTC

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From: Hubert Kario <hkario@redhat.com>
To: tls@ietf.org
Date: Thu, 22 Sep 2016 13:41:28 +0200
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Subject: Re: [TLS] Suspicious behaviour of TLS server implementations
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On Wednesday, 21 September 2016 15:53:33 CEST Peter Gutmann wrote:
> Andreas Walz <andreas.walz@hs-offenburg.de>; writes:
> >Actually, I wasn't aware of the fact that the TLS 1.3 draft now explicitly
> >addresses this in the Presentation Language section:
> >
> >  "Peers which receive a message which cannot be parsed according to the
> >  syntax (e.g., have a length extending beyond the message boundary or
> >  contain an out-of-range length) MUST terminate the connection with a
> >  "decoding_error" alert."
> 
> And how many implementations are going to do this?  Consider the
> error-message litmus test I proposed earlier, reasons for failing to
> connect to (say) amazon.com:
> 
>   Error: Couldn't connect to Amazon because its certificate isn't valid.
>  
>   Error: Couldn't connect to Amazon because no suitable encryption was
>          available.
> 
>   Error: Couldn't connect to Amazon because <explanation for
>          decoding_error alert>

"received data was malformed."

> If you're writing a strict validating protocol parser than disconnecting in
> this case is a valid response, but if it's software that will be used by
> actual humans then failing a connect based on something like this makes no
> sense.

We are talking about a cryptographic protocol. Being very strict about what 
you receive is the raison d'être of the whole thing.

POODLE happened because SSL 3 in general and some TLS implementations in 
particular weren't strict.

Researchers look at protocol itself and only a handful of popular 
implementations, they won't be analysing if the TLS-as-implemented-by-ACME-
widget-7000 is secure even if it isn't strict.
-- 
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