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 <> Fri, 16 January 2015 20:10 UTC

Return-Path: <>
Received: from localhost ( []) by (Postfix) with ESMTP id 078871B2B2B for <>; Fri, 16 Jan 2015 12:10:40 -0800 (PST)
X-Virus-Scanned: amavisd-new at
X-Spam-Flag: NO
X-Spam-Score: 1.211
X-Spam-Level: *
X-Spam-Status: No, score=1.211 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, FM_FORGED_GMAIL=0.622, MANGLED_BACK=2.3, MIME_8BIT_HEADER=0.3, SPF_PASS=-0.001, T_RP_MATCHES_RCVD=-0.01] autolearn=no
Received: from ([]) by localhost ( []) (amavisd-new, port 10024) with ESMTP id tUJ4rANqYKIM for <>; Fri, 16 Jan 2015 12:10:39 -0800 (PST)
Received: from ( [IPv6:2607:f8b0:400d:c00::22f]) (using TLSv1 with cipher ECDHE-RSA-RC4-SHA (128/128 bits)) (No client certificate requested) by (Postfix) with ESMTPS id 1684E1B2B1D for <>; Fri, 16 Jan 2015 12:10:38 -0800 (PST)
Received: by with SMTP id n8so17029693qaq.6 for <>; Fri, 16 Jan 2015 12:10:38 -0800 (PST)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;; s=20120113; h=mime-version:in-reply-to:references:from:date:message-id:subject:to :cc:content-type:content-transfer-encoding; bh=dh7jIr/LvbcLCkdm7jzFVoVAYVeFBuNLbowrRNR1iyc=; b=oJ3oBCsNnZzOH5HHM7ODzE6eir/TWs9Z1a7NsRR9a0SuYhsa7kLifbZWHydcM+X1vc I5NhgZUWoWvuo+zacr4CNmqqktExC47QGLWW4ZCqpC1Gt/8aOr4DwxXcpzC1P69n7KTk mWbrC60J3adVmyV9p1qKBXKuuFWRwzNkRsZDLV+43XFCvkIc4Ifcyq41IqP1+hzqceTA 9rC3Mgy7xCZuDA1YBr8+mRv4XHM4Eaej7RBOSU0zDAm76Hr3rtz3ORrhSYAlpTsaZqh3 VovEsCE+Gc3sYE5SuNsGDQF4W+8JMxACm61FGGikNg0u8MP/ST0pTTnLVgb55+iZ3DGU zZxA==
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;; s=20130820; h=x-gm-message-state:mime-version:in-reply-to:references:from:date :message-id:subject:to:cc:content-type:content-transfer-encoding; bh=dh7jIr/LvbcLCkdm7jzFVoVAYVeFBuNLbowrRNR1iyc=; b=MNS6bUfsjcrrHSHgrZIbx2INvkPFEi34oDRpaxv9JEavfNe5K+3w0r3iwJwO52fe4O lrKsQe7co6Gh5Xjuy04N4OTOG6LCCYRBASQROEJ3tpqrh+TO2fbfqSbmiKe7UMaSbdZW /zGq+qHQjtujmXp/M1rm1JrvK8g+gJPs1QzZ2gGlH5kKyds+meEbKsS4BVJwTuQCnRBE rzRRb1Vub9gzh21zHkCY57F72dzSkKwvhPL2mOq1v5/eyriNOsi4RRK7fb3lFzaeRhzC SzlfMI6PtY6el5OgBgUlqA4pDXe20Q5QUElRKI+hncBy3lx3u5I9S8/QEln1mTPat6GM HW6Q==
X-Gm-Message-State: ALoCoQlA0PmRcr/tXUwGftgWXblCAenHCjMJAwOvwh9J7s+kWqLHNzHT8YsRPQE/8Ezkb+gyKU2M
X-Received: by with SMTP id w6mr28364540qas.78.1421439038177; Fri, 16 Jan 2015 12:10:38 -0800 (PST)
MIME-Version: 1.0
Received: by with HTTP; Fri, 16 Jan 2015 12:10:18 -0800 (PST)
In-Reply-To: <20150116210327.61046788@pc>
References: <> <20150116210327.61046788@pc>
From: Adam Langley <>
Date: Fri, 16 Jan 2015 12:10:18 -0800
Message-ID: <>
To: Hanno Böck <>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Archived-At: <>
Cc: "" <>,
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
X-Mailman-Version: 2.1.15
Precedence: list
List-Id: "This is the mailing list for the Transport Layer Security working group of the IETF." <>
List-Unsubscribe: <>, <>
List-Archive: <>
List-Post: <>
List-Help: <>
List-Subscribe: <>, <>
X-List-Received-Date: Fri, 16 Jan 2015 20:10:40 -0000

On Fri, Jan 16, 2015 at 12:03 PM, Hanno Böck <> 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.