Sorry, I'm a bit late to the party. While I have attempted to follow
the discussion, here are some initial thoughts after a bit of pondering.
I might have missed if these were discussed elsewhere and I might very
well be wrong in my understanding of how this system might work.
DKIM replay.
I appears to me that most of what has been discussed with regards to
DKIM replay is an attempt to abuse systems that use DKIM for positive
reputation. However, such replay does require that the messages pass
DKIM signing.
Hypothetically, if I were evil[1], I would sign up for a target domain's
newsletter and mutate messages with this DKIM2, and resend them. While
forensic investigation would reveal the subterfuge, what gets displayed
via the user's MUA is verifiable via DKIM2 and presumably trusted. I
expect overuse of m=nomodify and this Could make the motivation for
DKIM2 somewhat moot.
An example;
1) I sign up for email from loudmouth@political-party.example.
2) When I receive new email message I mutate them hijacking the donation
links, maybe modify the message is subtle ways, DKIM2 sign the emails
appropriately, and resend them to my list of victims.
3) Receiving systems validate the DKIM2 and accept the messages.
[1] I do realize that some reading this might believe that the
hypothetical in that statement is the word "if"
Security gateways and ARC
It appears to me that the recipient of email messages has a relationship
with their message store provider, and most likely also the security
gateway, even if indirect. This is an interesting use case for ARC, as
a relationship exists, the trust issues of ARC signatures from either
the message store provider and/or the security gateway might not exist,
even down to a future MUA configured to trust signatures from those
organizations.
Bounce pathing
Current architecture of an overall mail systems may result in a
forwarder not being directly accessible for the general Internet. I.e.
there might not be a path to port 25, or any other port, to anyone
outside of the local site. This might create issues.
--
SgtChains
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