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Re: [TLS] Last Call: <draft-ietf-tls-downgrade-scsv-03.txt> (TLS Fallback Signaling Cipher Suite Value (SCSV) for Preventing Protocol Downgrade Attacks) to Proposed Standard

Adam Langley <agl@google.com> Fri, 16 January 2015 20:10 UTC

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From: Adam Langley <agl@google.com>
Date: Fri, 16 Jan 2015 12:10:18 -0800
Message-ID: <CAL9PXLyeE+Nh5hKf3zigDHe3UXpNrMr=Dn14WudsaKUwvzprTw@mail.gmail.com>
Subject: Re: [TLS] Last Call: <draft-ietf-tls-downgrade-scsv-03.txt> (TLS Fallback Signaling Cipher Suite Value (SCSV) for Preventing Protocol Downgrade Attacks) to Proposed Standard
To: Hanno Böck <hanno@hboeck.de>
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On Fri, Jan 16, 2015 at 12:03 PM, Hanno Böck <hanno@hboeck.de> wrote:
> Recently Mozilla has disabled the now so-called protocol dance, which
> makes adding another workaround (SCSV) pretty much obsolete:

Until they add TLS 1.3 support, when they'll need it again.

> (By the way: Has anyone thought what happens when people implement TLS
> hardware that is version intolerant to versions > 1.2 and at the same
> time send SCSV in the handshake? I'm pretty sure that at some point
> some hardware will appear that does exactly that. Will we need another
> SCSV standard for every TLS version then?)

The draft specifies that servers should compare the ClientHello
version to the version that they implement. If a client tries TLS 1.3,
fails because of intolerance then tries TLS 1.2 + FALLBACK_SCSV, the
server will accept the connection because the ClientHello version is
>= its maximum version.


Cheers

AGL

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