Re: [TLS] PR#625: Change alert requirements

Ilari Liusvaara <> Sat, 10 September 2016 14:53 UTC

Return-Path: <>
Received: from localhost (localhost []) by (Postfix) with ESMTP id 2240312B0E8 for <>; Sat, 10 Sep 2016 07:53:27 -0700 (PDT)
X-Virus-Scanned: amavisd-new at
X-Spam-Flag: NO
X-Spam-Score: -3.408
X-Spam-Status: No, score=-3.408 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, RCVD_IN_DNSWL_NONE=-0.0001, RP_MATCHES_RCVD=-1.508] autolearn=ham autolearn_force=no
Received: from ([]) by localhost ( []) (amavisd-new, port 10024) with ESMTP id F2VUEYPfR_hR for <>; Sat, 10 Sep 2016 07:53:25 -0700 (PDT)
Received: from ( []) by (Postfix) with ESMTP id 0B9BF128E18 for <>; Sat, 10 Sep 2016 07:53:24 -0700 (PDT)
Received: from localhost (localhost []) by (Postfix) with ESMTP id 4C066E086; Sat, 10 Sep 2016 17:53:23 +0300 (EEST)
X-Virus-Scanned: Debian amavisd-new at
Received: from ([IPv6:::ffff:]) by localhost ( [::ffff:]) (amavisd-new, port 10024) with ESMTP id dS5xXjmDphfz; Sat, 10 Sep 2016 17:53:23 +0300 (EEST)
Received: from LK-Perkele-V2 ( []) (using TLSv1 with cipher ECDHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by (Postfix) with ESMTPSA id 1246D21C; Sat, 10 Sep 2016 17:53:23 +0300 (EEST)
Date: Sat, 10 Sep 2016 17:53:21 +0300
From: Ilari Liusvaara <>
To: Martin Thomson <>
Message-ID: <>
References: <> <> <>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <>
User-Agent: NeoMutt/ (1.7.0)
Archived-At: <>
Cc: "" <>
Subject: Re: [TLS] PR#625: Change alert requirements
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: Sat, 10 Sep 2016 14:53:27 -0000

On Sat, Sep 10, 2016 at 08:43:33PM +1000, Martin Thomson wrote:
> I wouldn't say that this is a great argument, but it's one that could
> be made.  Generally, I've given up on TLS error codes being useful, or
> even making them useful; we've been stung in the past by being overly
> specific about what went wrong.

Well, regarding usefulness of TLS error codes, the error codes don't
even come close to useful-for-debugging. This isn't because of
information leaking (which AFAICT can only happen in very few places),
but because error codes just are not expressive enough.

Also, if you have a place where you worry about alerts leaking 
information to attacker, watch out, you might very well also have a
timing attack.

The places of TLS 1.3 that I can quickly name that are sensitive,
and so one should take care to only send one kind of alert for

- Deprotect failures (bad_record_mac)
- MAC (finished) failures (decrypt_error)
- key agreement failure (???, my code uses illegal_parameter[1])
- signature failures (internal_error)

Also, note that with some bad cryptographic algorithms, one can't
even let the other side know operation failed. So even just
resetting the connection leaks too much information.

[1] Logic being: It is likely provoked by peer sending invalid
share -> invalid field value -> illegal_parameter.