Re: [TLS] open issues for draft-ietf-tls-chacha20-poly1305-00

David Benjamin <> Tue, 04 August 2015 17:22 UTC

Return-Path: <>
Received: from localhost ( []) by (Postfix) with ESMTP id 4726C1A88ED for <>; Tue, 4 Aug 2015 10:22:14 -0700 (PDT)
X-Virus-Scanned: amavisd-new at
X-Spam-Flag: NO
X-Spam-Score: -1.388
X-Spam-Status: No, score=-1.388 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, HTML_MESSAGE=0.001, SPF_PASS=-0.001, T_RP_MATCHES_RCVD=-0.01] autolearn=no
Received: from ([]) by localhost ( []) (amavisd-new, port 10024) with ESMTP id LtWnxYuMG2vD for <>; Tue, 4 Aug 2015 10:22:12 -0700 (PDT)
Received: from ( [IPv6:2607:f8b0:4001:c05::229]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by (Postfix) with ESMTPS id 1F7E51A887D for <>; Tue, 4 Aug 2015 10:22:12 -0700 (PDT)
Received: by iggf3 with SMTP id f3so15900316igg.1 for <>; Tue, 04 Aug 2015 10:22:11 -0700 (PDT)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;; s=google; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc:content-type; bh=OFeuy3Bfx9wASLEO/ETJ+r/OFg89/Bt/P+OTGRuT648=; b=b9qcg+Gt50R31vPyp0khc3isalJIwsqmPUJwQApdFdk52oGscFoOLoQCsl7phqE7Jg UkUtkaBhH7s0bGvQRLxitnFBdnLYZiQLEBBnUqzoM3IUkf7AYkxIl+idYnl+RDZN2A2F yJNXEleJsOuR2qjlNj97NPItfXAvnKRKHu4Qo=
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;; s=20130820; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc:content-type; bh=OFeuy3Bfx9wASLEO/ETJ+r/OFg89/Bt/P+OTGRuT648=; b=J835TtE4nunCLYKJI/FXXAIOR9xDxhZZfOaw4xd2XI6s2dQnYVZoY+jMIagvo4xMh4 F0cCp37B0omotxZtjh6ulF4n0T3ouB96KZ1XYSLA4SGWMCJEmK3hTqmCIl5gcABzAxQH 8A6wSFNG50SLEkRj3mwlho3ImmMulqhnisO769ZrKJeF9zwDIkm46bz2dSaE2+hzh6Nx X8zvC4UduCCgBIztNxpCEFe8xiU5/jnxpQt53VT8Fb6jIMDR2ZYrfXiS2IaJioqp3Y0F juo7bq4Hq7QocbehGoGTyAzutUgvUODvpHjJUfYz88lFyMPwoWp++f06LdK31gCdiKsI Ov/g==
X-Gm-Message-State: ALoCoQnjykTtvWUORdrPjsvomd+ni7CtxbXUvv8+mxuRgD97mn6UIeV4BBAnotQUt4zpzJTXgpff
X-Received: by with SMTP id ir10mr5133301igb.69.1438708931401; Tue, 04 Aug 2015 10:22:11 -0700 (PDT)
MIME-Version: 1.0
References: <> <> <>
In-Reply-To: <>
From: David Benjamin <>
Date: Tue, 04 Aug 2015 17:22:01 +0000
Message-ID: <>
To: "Salz, Rich" <>, Martin Thomson <>, Nikos Mavrogiannopoulos <>
Content-Type: multipart/alternative; boundary="089e013c6a94cac9cc051c7f8696"
Archived-At: <>
Cc: "" <>
Subject: Re: [TLS] open issues for draft-ietf-tls-chacha20-poly1305-00
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: Tue, 04 Aug 2015 17:22:14 -0000

On Tue, Aug 4, 2015 at 12:20 PM Salz, Rich <> wrote:

> > Personally, I would rather see the nonce construction follow the form
> > defined in the respective TLS version. [DB: Adding back in for context:
> "That means including redundant bytes in TLS 1.2 and only getting the full
> advantage when we move to TLS 1.3."]
> Yes, consistency.  +1

I think this already came up:

This is a waste of bandwidth. Consistency is generally good, but a weak
argument here. The complexity is minimal. If you don't care about
decrypting in-place, none of this matters. If you do care, you already have
to deal with:

- RC4 and TLS 1.0 CBC use no prefix on the ciphertext
- AES-GCM, TLS 1.1+ 3DES use an 8-byte prefix
- TLS 1.1+ AES CBC use a 16-byte prefix

Adding a 0-byte prefix cipher doesn't make anything worse. It already must

As for following what's defined in TLS 1.2, RFC 5246 just says that
GenericAEADCipher has a nonce_explicit field and:

   Each AEAD cipher suite MUST specify how the nonce supplied to the
   AEAD operation is constructed, and what is the length of the
   GenericAEADCipher.nonce_explicit part.

It then describes the 1.2 AES-GCM scheme, but prefaced with "In many cases,
it is appropriate to [...]". Saying nonce_explicit is empty and the nonce
is built from the sequence number is perfectly consistent here.