Hi!
On 6/29/26 14:25, Bron Gondwana wrote:
On Mon, Jun 29, 2026, at 07:51, Hannah Stern wrote:
Hi!
On 6/24/26 21:08, Richard Clayton wrote:
> In message <178232634716.1360785.12363329888242618970@dt-datatracker-
> f9b87776f-xzl65>, [email protected] writes
>
> > Internet-Draft draft-ietf-dkim-dkim2-spec-03.txt is now available.
> It is a
> > work item of the Domain Keys Identified Mail (DKIM) WG of the IETF.
>
> > Title: DomainKeys Identified Mail Signatures v2 (DKIM2)
> > Authors: Richard Clayton
> > Wei Chuang
> > Bron Gondwana
> > Name: draft-ietf-dkim-dkim2-spec-03.txt
> > Pages: 42
> > Dates: 2026-06-24
>
> the changes are (and these affect implementors, so if you own code you
> will need to look at the changes)
>
> #1 introducing nd= as an alternative to mf= rt= for an imaginary hop.
> This is essemtially the "forward" signing scheme that Tobias suggested
>
> viz instead of:
>
> i=1; d=example.com <http://example.com>; [email protected]
[email protected]; f=..flags..;
> i=2; d=esp.tld; [email protected]; [email protected];f=..flags..;
>
> hide the imaginary hop (because the ESP is creating both signatures)
>
> i=1; d=example.com <http://example.com>; nd=esp.tld; f=..flags..;
> i=2; d=esp.tld; [email protected]; [email protected];f=..flags..;
>
> similarly for a mailbox provider that forwards for vanity domains
>
> i=1; d=example.com <http://example.com>; [email protected];
[email protected];
> i=2; d=vanity.com <http://vanity.com>; nd=mailbox1.tld;
> i=3; d=mailbox1.tld; [email protected]; [email protected];
>
Questions:
- On verification, is it allowed that the latest signature (highest i)
has nd?
If you're receiving that message over the wire, no... unless you have
a contractural relationship with the previous hop. The point is to
say "we're managing the handover between domains directly". If you
get something with nd= over the wire, it's breaking the replay
protections.
Makes sense.
- Is it allowed to have more than one nd in a row? (I.e. i=4 has
nd=<domain of i=5>, i=5 has nd=<domain of i=6>)?
Yes, no reason why not to. Again, so long as all these domains have
contractural agreements.
As verifier I don't know about contractual agreements of earlier hops
though.
- How is the mf/rt chain checked in the presence of nd? Just skip the nd
hop? I.e. if i=4 has mf/rt, i=5 has nd, i=6 has mf=rt, do I check i=6
against i=4 using the usual rules?
No, you never skip over a hop. You check:
i=4 rt=foo@X
i=5 d=X; nd=Y
i=6 d=Y; mf=bar@Y
(Reformatted, your formatting gave me white on white in my message
composition editor.)
Those are the invariants which must pass.
They look much stricter than the usual checks.
If i=5 where a "normal" intermediate signature,
- i=6 the mf domain would be allowed to be a subdomain of the d domain
- The i=4 rt domain would be only very loosely related to the i=5 d
domain (indirectly via the i=5 mf domain) Basically i=5 mf could be any
subdomain of i=5 d, and i=4 rt domain could be any "super"domain
(suffix) of that. That is, i=4 rt can be either the same as i=5 d, or
one a subdomain of the other (this way or the other way)!
And once, we'd adjust for that, I'm not sure what we gain from nd over
just having some synthesized/fictitious mf/rt on i=5 instead of nd.
I.e. i=5 d=X mf=fictitious@X rt=fictitious@Y
[...]
By the way, I see this change:
All relevant DKIM2-Signature header fields (and Message-Instance
-header fields if the message body is supplied) MUST verify. The
-DSN itself MUST have appropriate Message-Instance and DKIM2-Signature
-fields, noting that the MAIL FROM to be used will be null ("<>").
+header fields if the message body is supplied) MUST verify.
Shouldn't the Message-Instance of the original mail be verifiable too,
even if no body is provided, at least for headers?
Hannah.
--
Hannah Stern
Software Developer
Mail Transfer Development
1&1 Mail & Media Development & Technology GmbH | | |
Phone: +49 721 91374-4519
E-Mail:[email protected] | Web:www.mail-and-media.com www.gmx.net www.web.de www.mail.com www.united-internet-media.de
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Geschäftsführer: Alexander Charles, Dr. Michael Hagenau, Thomas Ludwig, Dr.
Verena Patzelt
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