[ILUG] opinions on DKIM

[Jeroen Massar]

paul at clubi.ie wrote:
[..]
> So all the "authenticate mail by originating MTA/domain" efforts so far
> seem to be exercises in missing the point and/or reinventing wheels
> (poorly).

The 'case against' OpenPGP according to DKIM folks and a lot of other
such groups:
 - No established PGP web of trust (the key distribution)
 - hard to deploy for 100.000 clients
   (DKIM you simply install on the mailservers, other mail
    will get bounced as not being signed properly, users will need
    to use the upstream servers)
 - OpenPGP doesn't sign the headers, thus one can easily change
   the subject or the MIME boundaries etc. But of course parts are
   then not signed and one can simply choose to not accept the message
   or ignore those parts.
 - Some clients can't handle the MIME OpenPGP format properly.

[..]
> What *high-level* problem are we trying to solve here?

DKIM is for ISP deployment. One place, one admin, one huge amount of users.

OpenPGP only solves it for 1 user.

[..]
> *Why* are we trying to align email authentication along the DNS naming
> hierarchy, rather than the only existing, widely used PKI naming
> hierarchy (X.509 certificates) or the only existing PK Web-of-Trust
> (PGP)? The DNS naming hierarchy doesn't even have secure delegation.

I sign my mails, and due to PKA* one can actually figure out where my
signature is and how to retrieve it. This will though require that every
client gets upgraded. Please tell your 100k users to start: generating
PGP keys, find family to trust them etc...

*= http://lists.gnupg.org/pipermail/gnupg-users/2005-August/026388.html

I once proposed a solution to the mass-deployment way though: make a
'domain pgp key' which can be used for signing all mail from a domain,
eg for "example.com" the key would be "example.com". When a mail passes
through the MX of the domain, it checks if there is a (valid) signature
on it, if there is none and the message was authenticated to come from
valid a user, the mail gets signed by the domain key and relayed,
otherwise reject it.

This would solve the 100k users case partially, but there is no real
accepted standard here to find the 'domain key'.

[..]
>   - With DKIM, in order to tell whether I can trust that mail
>     from my bank, I'll need to know:
> 
>     - what security procedures my bank use (just as I would for end-end)
>     - what security procedures my email provider uses (ie do they
>       check DKIM? if they do, what do they do with the result? Should
>       my MUA check DKIM too???)

Of course MUA's can check it next to the MTA. The place where it gets
checked is the place where the message can be rejected.

> Anyway, feel free to pass on the crack pipe. Not sure you can convince
> me to smoke from it though..

I have been smoking the OpenPGP crack pipe for a long time already now.
Still, as no large entity uses it it doesn't help much, except that I
can easily claim that messages which are not signed are most likely not
written by me. (Note that when it is unsigned I can't prove that it is
not from me, as I can simply not sign it...)

Greets,
 Jeroen