Subject: [Ietf-dkim] Re: My concerns
From: "Bron Gondwana" <[email-masked]>
To: "Dave Crocker" <[email-masked]>, "Larry M. Smith" <[email-masked]>, [email-masked]
Date: Thu, 24 Apr 2025 03:14:46 -0400
List-Id: IETF DKIM List <ietf-dkim.ietf.org>

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
  [email-masked]