Can you do that and it's still possible to validate that site2 signed it? Brandon
On Aug 18, 2017 5:53 PM, "Bron Gondwana" <br...@fastmailteam.com> wrote: > 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 > > > > _______________________________________________ > dmarc mailing list > dmarc@ietf.org > https://www.ietf.org/mailman/listinfo/dmarc > >
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