Re: [dmarc-ietf] ARC-Seal is meaningless security theatre

[Seth Blank <[e-mail]>]

On Sun, Aug 6, 2017 at 10:21 PM, Bron Gondwana <[e-mail]>
wrote:
>
> *AS adds nothing over just having AMS signing its own AAR, and then you
> only have to verify ONE signature, the most recent.*
>
> <snip>
>
> You either trust the most recent signer and trust that THEY validated the
> previous signer/SPF (and so on) or the chain is broken anyway, so why add a
> parallel, easily falsified, chain of signatures?
>

There's a critical reason the ARC Seal exists that you're missing:

ARC is about maintaining a chain of custody so that a final receiver can be
certain of which domains modified a message in transit. Like DMARC, DKIM,
and SPF, we're trying to ascertain if the message was handled by the domain
it said it was handled by - we're not passing judgement on the contents of
the messages itself.

When validating an ARC signed message, one verifies the latest AMS (which
must validate), and *the entire chain* of ARC Seals, not only the latest.
This guarantees you a list of all message signatories - the chain of
custody we're talking about.

When evaluating the chain for final receipt, there are two states to worry
about as a matter of local policy:
1) you trust all the signatories on the chain
2) there is an untrusted signatory on the chain

In state #1, you're done - if you trust the signatories then you trust
they're not playing games with the AMS and AAR contents or manipulating the
message in malicious ways. Now you can make a delivery decision as local
policy dictates.

In state #2, you're also done - if you don't trust all the signatories,
then there are a multitude of routes for the message to be garbage,
including but not limited to everything you've outlined above.

The critical thing about the ARC Seal is that it guarantees this behavior
in state #1 - if you trust all the d= values in the ARC Seals, they all
validate, and you have cv=pass, then you know for certain everyone who has
manipulated the message (and maybe some who handled but did not modify).

Without the ARC Seal this determination is not possible and there is no way
to evaluate the ARC chain for delivery as a final receiver.

Seth