Re: [dmarc-ietf] ARC questions

Michael Thomas <mike@mtcc.com> Mon, 23 November 2020 18:49 UTC

Return-Path: <mike@fresheez.com>
X-Original-To: dmarc@ietfa.amsl.com
Delivered-To: dmarc@ietfa.amsl.com
Received: from localhost (localhost [127.0.0.1]) by ietfa.amsl.com (Postfix) with ESMTP id 219D93A0C02 for <dmarc@ietfa.amsl.com>; Mon, 23 Nov 2020 10:49:36 -0800 (PST)
X-Virus-Scanned: amavisd-new at amsl.com
X-Spam-Flag: NO
X-Spam-Score: 0.248
X-Spam-Level:
X-Spam-Status: No, score=0.248 tagged_above=-999 required=5 tests=[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: ietfa.amsl.com (amavisd-new); dkim=pass (2048-bit key) header.d=mtcc-com.20150623.gappssmtp.com
Received: from mail.ietf.org ([4.31.198.44]) by localhost (ietfa.amsl.com [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id aPSD4TJDxxx4 for <dmarc@ietfa.amsl.com>; Mon, 23 Nov 2020 10:49:35 -0800 (PST)
Received: from mail-pg1-x52d.google.com (mail-pg1-x52d.google.com [IPv6:2607:f8b0:4864:20::52d]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by ietfa.amsl.com (Postfix) with ESMTPS id E00E83A04BC for <dmarc@ietf.org>; Mon, 23 Nov 2020 10:49:34 -0800 (PST)
Received: by mail-pg1-x52d.google.com with SMTP id s63so5211570pgc.8 for <dmarc@ietf.org>; Mon, 23 Nov 2020 10:49:34 -0800 (PST)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=mtcc-com.20150623.gappssmtp.com; s=20150623; h=subject:to:cc:references:from:message-id:date:user-agent :mime-version:in-reply-to:content-transfer-encoding:content-language; bh=QFmxF3jqGKvLZ3lbRmFOg7dXkYek2g3zJvlWzBg8Dcs=; b=TQZWVN4tA8IPT5F0uO26BIBGT/TOSrbUaR+NX48teN4zKXlgT+eb/GV3z0yz0Vz6mM 5U9onnqLyd92NAL6st6RiRw0CgznSVAcVI/ObIR4b2/yvf1eGwcc4mRjMhp9ITajqSKP eXvU33fJUBGJ1/7UNVJMc3jC8FZPIdGkpB9wImOJ/FxkbvNUy066Giiw9sSImwG3yXME z5BCgERvrkFPIf8hLg+TEwYhOIzFk2KHw2K54etRnc4pzUz/uKwBi57zHhTxKC9w+zeM DbWry6qZDpg3JCyRUpCnnwJsQvoREk6w4Gz0LsuR+u8JcrnYsXMd/RvW/cFBlkHpTwvY WBvw==
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; 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=QFmxF3jqGKvLZ3lbRmFOg7dXkYek2g3zJvlWzBg8Dcs=; b=S4i72e+FuCns/L6SVTi9BUMM6XyIhRrfENlVhGDBDbXNNv8fS62CG9sVDHFg9TNpYD 6k/VjMIwLT5lvN+/gVWCJOUxqH7IOWadwIRuXjgRM4jooLOhDBW7lC3OvpLRuDkpm6Ke m6A1UQEVToQLLa56NWVxXEW1DFO4FaJnHTZvvnZhIgU2MLoNTaMoeVwG1t3XjhYp5Wem kyY64CPpLmaFpjQ6Va7M+Hibhzn2oxVBg+ZdVpsnzFwKiviojLSnijuCRSHBJUvlwh/L PM1LuTHednz4cuJyJcQTLCAoE3zjRzkwluWIiy/Wy3EzOYh/Rn5whRJwXdffNAIpv3vU 6pXA==
X-Gm-Message-State: AOAM5330r0wFfXju/Yv+0rAKaWDVZsr+bBhj2Sy5fg0MdgM/usABZXYY PVTqAFcPH95NO7268kYR7jvZTcXEb7OmiQ==
X-Google-Smtp-Source: ABdhPJzedOcE4+kBdVImVuOTfqi850qbpr48NITFkCVy3UBnyLbrXBTMBJqkB+mKligV+apGOyPSBw==
X-Received: by 2002:a17:90a:4482:: with SMTP id t2mr336413pjg.44.1606157373761; Mon, 23 Nov 2020 10:49:33 -0800 (PST)
Received: from mike-mac.lan (107-182-37-5.volcanocom.com. [107.182.37.5]) by smtp.gmail.com with ESMTPSA id v7sm12269989pfu.39.2020.11.23.10.49.32 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 23 Nov 2020 10:49:32 -0800 (PST)
To: John R Levine <johnl@taugh.com>
Cc: "dmarc@ietf.org" <dmarc@ietf.org>
References: <dcc265f9-a143-5093-eba0-94ee059c7cc7@mtcc.com> <20201122021417.B5E6E27B3E59@ary.qy> <CABuGu1pX=5ZC4RLsv19qrosRN9nCrPdeSk5Xg4O7ViEZit6dnA@mail.gmail.com> <453c4db4-fc62-dc76-5b15-707623d66f9f@mtcc.com> <64f18b-ae8-8c15-3d33-ff2d864c35bc@taugh.com> <884541e6-5076-7f8f-d1d2-d68ea9c5a2bc@mtcc.com> <8fa2d88c-55df-aa8e-932f-8f7bc97d741@taugh.com>
From: Michael Thomas <mike@mtcc.com>
Message-ID: <77854271-296a-b4f6-202e-c085036289d4@mtcc.com>
Date: Mon, 23 Nov 2020 10:49:31 -0800
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.4.3
MIME-Version: 1.0
In-Reply-To: <8fa2d88c-55df-aa8e-932f-8f7bc97d741@taugh.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 8bit
Content-Language: en-US
Archived-At: <https://mailarchive.ietf.org/arch/msg/dmarc/ZRqGm18CXzsySt6gv4CGTIpIMH8>
Subject: Re: [dmarc-ietf] ARC questions
X-BeenThere: dmarc@ietf.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: "Domain-based Message Authentication, Reporting, and Compliance \(DMARC\)" <dmarc.ietf.org>
List-Unsubscribe: <https://www.ietf.org/mailman/options/dmarc>, <mailto:dmarc-request@ietf.org?subject=unsubscribe>
List-Archive: <https://mailarchive.ietf.org/arch/browse/dmarc/>
List-Post: <mailto:dmarc@ietf.org>
List-Help: <mailto:dmarc-request@ietf.org?subject=help>
List-Subscribe: <https://www.ietf.org/mailman/listinfo/dmarc>, <mailto:dmarc-request@ietf.org?subject=subscribe>
X-List-Received-Date: Mon, 23 Nov 2020 18:49:36 -0000

On 11/22/20 11:56 AM, John R Levine wrote:
> On Sun, 22 Nov 2020, Michael Thomas wrote:
>>> The ARC signature has a sequence number so you can track the chain 
>>> of custody.  You are right that it is similar to the DKIM signature 
>>> but the extra ovehead doesn't seem excessive.
>>>
>> Did the wg consider just grafting that onto the DKIM signature itself 
>> instead of having essentially a duplicate signature? Receivers are 
>> already supposed to ignore any tags they don't understand so it 
>> shouldn't hurt backward compatibility.
>
> ARC is an experiment that came from the people who designed DMARC.  
> It's not a WG product.
>
> Having adapted the perl DKIM module to handle ARC signing and 
> verification, I can say that the extra signature is not a big deal.  
> If you look at mail coming from large mail systems, they're full of 
> other junk headers and the extra overhead of AMS along with DKIM is 
> not important.
>
 From what I can tell, the main thing that ARC is doing is binding an 
auth-res to a dkim signature-like thing. But as I recall -- it's been a 
long time -- there were ordering requirements ala received headers for 
where new dkim-signatures and auth-res go in the header. Assuming my 
memory is correct, that means you can reconstruct "what this looked like 
before i messed with it" already by signing the incoming auth-res as 
part of the new DKIM signature.

Is there something more going on here?

Mike