Re: [dmarc-ietf] ARC questions

Michael Thomas <> Wed, 02 December 2020 20:38 UTC

Return-Path: <>
Received: from localhost (localhost []) by (Postfix) with ESMTP id EDBFB3A15C2 for <>; Wed, 2 Dec 2020 12:38:37 -0800 (PST)
X-Virus-Scanned: amavisd-new at
X-Spam-Flag: NO
X-Spam-Score: -1.652
X-Spam-Status: No, score=-1.652 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, HEADER_FROM_DIFFERENT_DOMAINS=0.249, NICE_REPLY_A=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001] autolearn=no autolearn_force=no
Authentication-Results: (amavisd-new); dkim=pass (2048-bit key)
Received: from ([]) by localhost ( []) (amavisd-new, port 10024) with ESMTP id 93CQzIaiGr05 for <>; Wed, 2 Dec 2020 12:38:36 -0800 (PST)
Received: from ( [IPv6:2607:f8b0:4864:20::636]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by (Postfix) with ESMTPS id 4D0193A15A0 for <>; Wed, 2 Dec 2020 12:38:35 -0800 (PST)
Received: by with SMTP id l11so1800696plt.1 for <>; Wed, 02 Dec 2020 12:38:35 -0800 (PST)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;; s=20150623; h=subject:to:cc:references:from:message-id:date:user-agent :mime-version:in-reply-to:content-transfer-encoding:content-language; bh=J5SPn7dDi8uA8BRZxZEJ6OVzRE5qHHFvL/CjPS2urFg=; b=BhoT3U2f+WpyvIxVgja5ec4WJwynKIki05FPZ2nNIhupvJadgp2xz4jL9RKqr/afGJ HoCR5rsoGREI+thgZmjM/TEmeTN00ZIONqVkMu4uQfaFhckQTy3ZbqsrGwLCTZgpQgIR rQWCYjXxl1gmusJLZMcXc5CtR4PFFtkpiSaExK3aVm7PmWkfm6HocxjDfB5Lg4LxqyvQ btRhWuWZowN/dtsWiBSXlgjBxasV9UPAMaAXuhitnLaNOFuK3OVtUrJyLiCC6/ojl8s+ R7GL7L4IVjdGWJiUVKwQfLij19XK78v1wdSMH5uuS6Pub/BR98d9hNrOscKObrTSWu7P DS/A==
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-transfer-encoding :content-language; bh=J5SPn7dDi8uA8BRZxZEJ6OVzRE5qHHFvL/CjPS2urFg=; b=WJ3VLk30UpaImlTinlBmJ6A7ZB+6uLHw9Nu4dQb/LK8L5dDAYDJEilRtQNpejK05ij 7ZnKI0R1H0juZ2xs8NuYY/LTqd6/VIHCKiZIA2AYq5nis0GOv+5NS4gxS55u2ZZSmGOL 0Vd3X0VTSWfR8fxsOCeqou4RYlnE+owKIR5v5V2x0y1N8kob6llfokrxR0NEEoh8OJpc ylBlvQy4HYyGeDXGmfAkBEe7MRm32NvmDcyoY1iuLezPQEcAltufgXZ0D0TVnKEJsQ+1 6ePCYg06eaHxdM0DAKwBLSyPJ68uqU0IstPjk8Az0h/W/aFfBnCi6jDyL9T2naZMOJCv 8DOA==
X-Gm-Message-State: AOAM531+VLzVm6wikau22XkA62PxqpsJ7IQTqZiQh8lNVJDZeI01f1bP rMpHE8og53OGmGfXIvcA+JJaRVO4U0cg3A==
X-Google-Smtp-Source: ABdhPJyCPYKdqiYahxmrJi6YA57MFGmUxw34twLJzhE+HV277rzrh2ptsA5vPy3kFZPJ09cEFdWUFg==
X-Received: by 2002:a17:902:6acc:b029:d8:c6ee:6d6c with SMTP id i12-20020a1709026accb02900d8c6ee6d6cmr4202105plt.7.1606941514496; Wed, 02 Dec 2020 12:38:34 -0800 (PST)
Received: from mike-mac.lan ( []) by with ESMTPSA id x16sm430479pjh.39.2020. (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 02 Dec 2020 12:38:33 -0800 (PST)
To: John R Levine <>, Brandon Long <>
References: <20201124020453.AFDC027CE5C8@ary.qy> <> <> <> <> <> <> <> <> <> <>
From: Michael Thomas <>
Message-ID: <>
Date: Wed, 2 Dec 2020 12:38:31 -0800
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.5.0
MIME-Version: 1.0
In-Reply-To: <>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 8bit
Content-Language: en-US
Archived-At: <>
Subject: Re: [dmarc-ietf] ARC questions
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: "Domain-based Message Authentication, Reporting, and Compliance \(DMARC\)" <>
List-Unsubscribe: <>, <>
List-Archive: <>
List-Post: <>
List-Help: <>
List-Subscribe: <>, <>
X-List-Received-Date: Wed, 02 Dec 2020 20:38:45 -0000

On 12/2/20 12:35 PM, John R Levine wrote:
> On Wed, 2 Dec 2020, Michael Thomas wrote:
>>>> different in that respect. In fact as far as I can tell they are 
>>>> identical modulo the i= difference.
>>> Please reread the ARC spec.  The ARC-Authentication-Results at level 
>>> N tells you whether the ARC and DKIM signatures were good at level N-1.
>> That's why I said "modulo the i= difference"
> Well, yeah.  That i= is why we have ARC seals rather than just using a 
> DKIM signature.
> Remember that ARC is only useful if the last system sending the 
> message to you is reasonably trustworthy, not in the sense that it 
> never sends spam, but in the sense that its ARC tells the truth about 
> what it saw.  That's a low bar that any mailing list should be able to 
> meet.
Which could trivially be added as an extension to DKIM and Auth-Res 
negating the need for the Seal altogether since DKIM can directly sign 
the old (renamed) auth-res. I can understand for an experiment not 
wanting to touch dkim or auth-res, but for something standards track 
less is more.