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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)

Andrei Popov <Andrei.Popov@microsoft.com> Mon, 20 October 2014 20:52 UTC

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From: Andrei Popov <Andrei.Popov@microsoft.com>
To: Hubert Kario <hkario@redhat.com>
Thread-Topic: [TLS] The TLS_FALLBACK_SCSV time bomb (was: Re: Working Group Last Call for draft-ietf-tls-downgrade-scsv-00)
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Date: Mon, 20 Oct 2014 20:52:15 +0000
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Cc: "tls@ietf.org" <tls@ietf.org>
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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>> On the server side, SCSV (not extension) is only needed if a) SSL3 is 
>> enabled, and b) the server tries to protect clients that offer TLS1.0 
>> (not
>> 1.1 or 1.2) at the first connection attempt. Am I missing something?

>1.3 to 1.2. Over 10% of Internet web servers are intolerant to 1.3 ClientHello

1.3 to 1.2 could be protected by an extension; I think SCSV is only needed to protect the downgrade from TLS1.0 to SSL3.

Cheers,

Andrei

-----Original Message-----
From: Hubert Kario [mailto:hkario@redhat.com] 
Sent: Monday, October 20, 2014 3:14 AM
To: Andrei Popov
Cc: Martin Thomson; tls@ietf.org
Subject: Re: [TLS] The TLS_FALLBACK_SCSV time bomb (was: Re: Working Group Last Call for draft-ietf-tls-downgrade-scsv-00)

----- Original Message -----
> From: "Andrei Popov" <Andrei.Popov@microsoft.com>
> To: "Martin Thomson" <martin.thomson@gmail.com>
> Cc: tls@ietf.org
> Sent: Saturday, 18 October, 2014 3:08:20 AM
> Subject: Re: [TLS] The TLS_FALLBACK_SCSV time bomb (was: Re: Working 
> Group Last Call for
> draft-ietf-tls-downgrade-scsv-00)
> 
> Certainly a lot of implementations will keep SSL3 code around for a 
> while, just disabled by default.
> 
> On the client side, SCSV (not extension) is only needed if fallback to 
> SSL3 is allowed, correct? This is the very fallback that browser 
> vendors are disabling right now. So the only browsers that fall back 
> to SSL3 will be the ones without the Downgrade-SCSV support.

We don't know yet if it will be disabled in all browsers.

> On the server side, SCSV (not extension) is only needed if a) SSL3 is 
> enabled, and b) the server tries to protect clients that offer TLS1.0 
> (not
> 1.1 or 1.2) at the first connection attempt. Am I missing something?

1.3 to 1.2. Over 10% of Internet web servers are intolerant to 1.3 ClientHello

> However, a browser that implements a shiny new Downgrade-something RFC 
> is unlikely to offer TLS1.0 on the first connection attempt.

Browsers maybe not (time will tell), but it got implemented in OpenSSL 0.9.8 which supports TLS 1.0 max.

--
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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