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Re: [TLS] The TLS_FALLBACK_SCSV time bomb (was: Re: Working Group Last Call for draft-ietf-tls-downgrade-scsv-00)

Hubert Kario <hkario@redhat.com> Fri, 17 October 2014 13:40 UTC

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Date: Fri, 17 Oct 2014 09:40:21 -0400
From: Hubert Kario <hkario@redhat.com>
To: Bodo Moeller <bmoeller@acm.org>
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Thread-Topic: The TLS_FALLBACK_SCSV time bomb (was: Re: Working Group Last Call for draft-ietf-tls-downgrade-scsv-00)
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Subject: Re: [TLS] The TLS_FALLBACK_SCSV time bomb (was: Re: Working Group Last Call for draft-ietf-tls-downgrade-scsv-00)
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----- Original Message -----

> From: "Bodo Moeller" <bmoeller@acm.org>
> To: "Hubert Kario" <hkario@redhat.com>
> Cc: tls@ietf.org
> Sent: Friday, 17 October, 2014 3:20:14 PM
> Subject: Re: [TLS] The TLS_FALLBACK_SCSV time bomb (was: Re: Working Group
> Last Call for draft-ietf-tls-downgrade-scsv-00)

> Hubert Kario < hkario@redhat.com > :

> > [...] I think the only thing we can do to prevent
> 
> > that is to be explicit in the document:
> 

> > "The client MUST NOT advertise TLS_FALLBACK_SCSV if it initiates connection
> 
> > with the highest protocol version it supports."
> 
> Right, I think stating this explicitly in the spec is probably the right
> thing to do (of course this has been the intended behavior all along).
> Thanks.

> (Arguably the OpenSSL API is too fragile, and SSL_MODE_SEND_FALLBACK_SCSV
> should be ignored in this case. So clarifying that in the spec is not the
> *only* thing we can do to prevent this.

NSS API does allow this too. 

Thing is, we want to have a way to test if the servers are tolerant to future clients 
that claim they had to downgrade. So having the ability to set TL1.2 with this 
SCSV is useful, just like is having an easy to use, widely distributed CLI tool 
(openssl command) that can do this should help with testing and interoperability, 
IMHO. 

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

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