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[Ietf-dkim] Re: My concerns

Bron Gondwana <brong@fastmailteam.com> Thu, 24 April 2025 07:15 UTC

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Date: Thu, 24 Apr 2025 03:14:46 -0400
From: Bron Gondwana <brong@fastmailteam.com>
To: Dave Crocker <dcrocker@bbiw.net>, "Larry M. Smith" <ietf.org@fahq2.com>, ietf-dkim@ietf.org
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Subject: [Ietf-dkim] Re: My concerns
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On Sun, Apr 20, 2025, at 02:50, Dave Crocker wrote:
> I believe there is something approximating rough consensus that simply 
> including the original recipient address in the DKIM signature, and 
> validating it by the receiver, should be enough to suppress DKIM Replay.

Certainly not consensus on that point as stated here.  Not unless you discount alumni forwarders and mailing lists; or expect them to all add a DKIM signature as well.

Adding the original recipient address certainly helps a lot, but it still doesn't allow you to get two messages and know if they're a replay or not.

The design I co-authored (once all involved parties support it) gives 100% certainty which signing domain split the message into multiple copies, if you receive two messages with the same i=1 first signature.  So you know where the replay occurred.

Bron.

--
  Bron Gondwana, CEO, Fastmail Pty Ltd
  brong@fastmailteam.com

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