So this is an interesting case that I'd like to spin into a separate thread. At the moment, ARC headers are purely additive. You receive a message with some ARC headers on it, you add some more on top and send it on. AR: arc=pass, ... // at receiver AS: i=3; cv=pass, d=site4.com AMS: i=3; d=site4.com AAR: i=3; arc=pass AS: i=2; cv=pass, d=site3.com AMS: i=2; d=site3.com AAR: i=2; arc=pass AS: i=1; cv=none, d=site2.com AMS: i=1; d=site2.com AAR: i=1; arc=none; dkim=pass DKIM-Signature: d=site1.com
site1 => site2 => site3 => site4 => receiver Somebody who obtains a copy of that message could then trim the message back: AS: i=2; cv=pass, d=site3.com AMS: i=2; d=site3.com AAR: i=2; arc=pass AS: i=1; cv=none, d=site2.com AMS: i=1; d=site2.com AAR: i=1; arc=none; dkim=pass DKIM-Signature: d=site1.com And pretend that the message was sent from site3 down a different path: AR: arc=pass, ... // at receiver AS: i=3; cv=pass, d=badsite.com AMS: i=3; d=badsite.com AAR: i=3; arc=pass AS: i=2; cv=pass, d=site3.com AMS: i=2; d=site3.com AAR: i=2; arc=pass AS: i=1; cv=none, d=site2.com AMS: i=1; d=site2.com AAR: i=1; arc=none; dkim=pass DKIM-Signature: d=site1.com And the message still arrives at receiver with a valid ARC chain, just via badsite.com instead of site3.com. It is possible to do things with crypto, mixing in hashes, such that you not only add new headers, but you rewrite past headers such that the original versions of them can't be reconstructed any more. Which would mean that if you could intercept a copy at the receiver, you couldn't trim back to i=2 and restart the chain on that message. It would mean header replacement rather than just header addition though. Is this something that would have enough interest to be worth pursuing? It's bound to be more complex than ARC-as-defined, but it also makes faking mail flows a lot harder, because you would have to intercept the message between site3 and site4 if you wanted to fake the mail flow from site3 - you couldn't just pick it up later. Bron. -- Bron Gondwana, CEO, FastMail Pty Ltd br...@fastmailteam.com
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