On Sat, 12 Aug 2017, at 10:16, Kurt Andersen (b) wrote: > On Fri, Aug 11, 2017 at 4:54 PM, Bron Gondwana > <br...@fastmailteam.com> wrote:>> __ >> . . . it's a bad idea to sign if you're not modifying, because then >> everybody has to trust you or their chain breaks, even though you >> didn't do anything which required signing.> > <elided>
I would like to address this point, but maybe we should have a separate thread for it? I would strongly argue that sites not changing the message SHOULD NOT add ARC headers. I spelled out the reasons in my initial posting on this thread. >> In state #1, you don't need a chain of ARC Seal. You just need each >> site to sign their own AAR and each AAR to include "arc=pass" for the >> previous AMS. You trust the sites, so you trust them to verify the >> ARC status on ingress.> > In the current layout, "signing [the] AAR" is done via the AS. We > wanted to keep the AAR as close to the A-R header as we could to > maximize leverage of previous definitions rather than inventing an > entirely new one. Initially, we had intended the AMS to sign over the > AAR, but people objected to signing the AAR within both the AMS and > AS scopes. I can understand that. I would fix it by not having AS scopes rather than removing AAR from AMS. > <elided> >> And this is the crux of our disagreement. Seth thinks it's necessary >> to do more than signing a statement that you believed the message was >> authenticated when you got it, in a way that the next hop can verify >> your signature over your own Authentication Results plus the content >> of the message. I disagree.> > One could replace the AMS with an "egress DKIM" signature, but then > you would still have to decide what to do about alignment on this new > DKIM signature. That's why we built the AMS - to avoid the question > of alignment and have the ARCset as a self-contained "package". Yes - calling it something different from DKIM-Signature is good, so that nobody tries to check alignment with the "From:" domain. But I don't see any reason to replace AMS - it does what's needed (apart from not signing the AAR). It's AS that bothers me. > I see the point that you are driving at regarding the claim of > "forgery", but I don't consider that any more or less of a forgery > than what the IETF mailman will do to this message en route to the > recipients. Mailman changes the headers (Subject) and body. Seems like > that's about what you've done in the sample message...but at least you > took responsibility for doing so with ARCset[7] (or someone with the > private key for brong.net ;-) ). It's true, anybody at FastMail could have done that. At least anybody with production access to our DKIM keys database :) The point with forgery is that "a chain of unbroken ARC-Seals" is meaningless, because they're not protecting anything. Bron. -- Bron Gondwana, CEO, FastMail Pty Ltd br...@fastmailteam.com
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