Hello Emanuel,
the PASS / MODIFIED_PASS / NULL_PASS taxonomy is useful, but the
equivalence between NULL_PASS and ARC pass requires a qualification.
ARC pass establishes that a chain of intermediaries asserts the message
was authenticated at the origin. It does not bind the SMTP envelope
cryptographically. MAIL FROM can change along the path without breaking
the chain. The message can be replayed to arbitrary recipients. There is
no way to verify that the envelope at any hop corresponds to what the
previous hop actually transmitted.
DKIM2 NULL_PASS with envelope binding is fundamentally stronger. Without
a body recipe, mf= binds MAIL FROM to d= at each hop and rt= binds RCPT
TO to the signing domain. The message cannot be replayed to different
recipients because rt= would not match. The envelope sender cannot be
changed without breaking the signature. You lose visibility into what
changed in the body (not useful for delivery decisions), but you retain
cryptographic proof of who handled the message, where it was supposed to
go and where it came from at every hop.
This is the difference between a chain of assertions (ARC) and a chain
of cryptographic bindings (DKIM2 with envelope binding). These
properties alone represent a qualitative advance over ARC that should
not be conflated with it.
Regards
Vittorio Moccia
Il 2026-04-30 20:14 Emanuel Schorsch ha scritto:
Thanks Todd and Wei for some interesting thoughts!
To start with some clarifications and terms.
RFC5322.From:
As far as I can see DKIM2 says nothing about the RFC5322.From which
seems like an omission if the intention is to replace DMARC. Todd,
where do you envision the guidance and policies around RFC5322.From
sitting in the long term future?
DKIM1 vs DKIM2 pass semantics:
DKIM1 pass has clear semantics for a pass, the message content is
unmodified from the original message. This is more ambiguous in the
DKIM2 world. It can mean:
1) Content is unmodified. Chain of custody is just added.
2) Content has reversible modifications that are recorded and can be
verified.
3) Null recipe. Equivalent to an ARC pass today where all you can
verify is the most recent hop and the rest is that hop's assertion.
I think it is important that these are clearly recorded as different
and that we have consistent terms to refer to them. The idea that 2
and 3 are equally a pass as 1 is makes me hesitant and concerned. I'm
bad at naming but for the sake of this email I'll call them PASS,
MODIFIED_PASS, NULL_PASS.
[...]
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