Re: [TLS] (strict) decoding of legacy_record_version?

Brian Smith <> Tue, 08 November 2016 03:01 UTC

Return-Path: <>
Received: from localhost (localhost []) by (Postfix) with ESMTP id 8C74212948F for <>; Mon, 7 Nov 2016 19:01:22 -0800 (PST)
X-Virus-Scanned: amavisd-new at
X-Spam-Flag: NO
X-Spam-Score: -2.6
X-Spam-Status: No, score=-2.6 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, HTML_MESSAGE=0.001, RCVD_IN_DNSWL_LOW=-0.7, SPF_PASS=-0.001] autolearn=ham autolearn_force=no
Authentication-Results: (amavisd-new); dkim=pass (2048-bit key)
Received: from ([]) by localhost ( []) (amavisd-new, port 10024) with ESMTP id XDvcxWzusxBc for <>; Mon, 7 Nov 2016 19:01:21 -0800 (PST)
Received: from ( [IPv6:2607:f8b0:4001:c0b::229]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by (Postfix) with ESMTPS id D610A129411 for <>; Mon, 7 Nov 2016 19:01:20 -0800 (PST)
Received: by with SMTP id e187so130654200itc.0 for <>; Mon, 07 Nov 2016 19:01:20 -0800 (PST)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;; s=20150623; h=mime-version:in-reply-to:references:from:date:message-id:subject:to :cc; bh=o6uVGoDbjOsUZByA3SpQzxAts9zKqsQu3dpJbWevnd8=; b=GFDWdSroEcZIsGC8AEbxuuoSANGprzOCKiQePxJNHlVT3NSZrJFjjgVTiwp7gvC9ZL AEGZVc6GCveWBNL+J/4rRRaWs40SGXgeNfqm2pAUQ0A4sLZf/IIDlGc1Ha4VieHnwxig ZLy1xhGswFafBNYzygEEpJgD/xkDGROCvwbpv41g18sZC4TdDAt+GwkddcOcUsxqNyx7 ZpW4GXD2KdEKT0Huf0B4QkHaAxdv+e/aVGCpdYa+6/eR8pFKKGt5yLLrvIoBlzxiqIOQ bbxLszdsurctG6T6amU0kYZxB3Bwg1cExKs9SWtfW4Fwy+s3TMo4F6ZNpNKLHR+BB0HZ tpuw==
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; bh=o6uVGoDbjOsUZByA3SpQzxAts9zKqsQu3dpJbWevnd8=; b=JBvd8hbFju8fWYTVTPGLziW1q0gdtjiAyklwY8WbHyoaCd/Z+roMorCfORW/4CMdGo +Ue93W3K5Xn57IeRz2n9YxQHgEMFMi8tUIOeFBfXOqoP87oEr0keitwwepO3tgIPFEkY UTQ2aNzjFKRykz+fivtDvAove+JvGhenoL0RFtab2beFCNvJF++28YMSRLaKfByQdfW3 5ryBG/bCP9gDMUOkNimfOsFZnb1vQGnhj5aWIpjiUOA6JtZyZzStT76SaMzHQujZnZkz hnatUQlWiniXRi6LYfKunJXyOck73jqE3b72IC0015ohqnWBat1q/dYFAmwSxU3R0GbC TlVg==
X-Gm-Message-State: ABUngveHCAvkbGEUVo8Ogpx+zYeDEzmCIRWi1McuCe7S6buiD+kE8cBurDxbXZ0hnNS1hp42eLY3rZgEGdYQkw==
X-Received: by with SMTP id m196mr8534621itb.58.1478574079103; Mon, 07 Nov 2016 19:01:19 -0800 (PST)
MIME-Version: 1.0
Received: by with HTTP; Mon, 7 Nov 2016 19:01:18 -0800 (PST)
In-Reply-To: <>
References: <> <> <> <> <> <> <>
From: Brian Smith <>
Date: Mon, 7 Nov 2016 17:01:18 -1000
Message-ID: <>
To: David Benjamin <>
Content-Type: multipart/alternative; boundary=001a1140467ec282dc0540c15a6a
Archived-At: <>
Cc: Matt Caswell <>, "" <>
Subject: Re: [TLS] (strict) decoding of legacy_record_version?
X-Mailman-Version: 2.1.17
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: Tue, 08 Nov 2016 03:01:22 -0000

David Benjamin <> wrote:

> Once you've gotten as far as to switch to TLSCiphertext, I don't see a
> reason not to enforce. Keying on versions is problematic (which is why we
> avoided a transition to enforcement), but keying on whether the record is
> encrypted seems fine. I think it just didn't occur to us to base it on
> that. :-)

Since this field isn't included in the additional_data of the AEAD in TLS
1.3 any more, it isn't authenticated. That means an active MitM can use
this to transport up to 2 bytes of information hop-to-hop if the receiver
doesn't check it. That seems like a good reason to check it, and also to
check TLSCiphertext.opaque_type is application_data. Assuming this is the
reason, the reasoning should be explicitly called out because it is

If that isn't a reason to do the check, then I don't think there's any
reason to mandate that implementations do it.