[ietf-dkim] requirements

[Scott Kitterman]

On Wednesday 26 July 2006 16:36, Michael Thomas wrote:
> Scott Kitterman wrote:
> >On Wednesday 26 July 2006 14:14, Michael Thomas wrote:
> >>This is a really good instance of what the base level requirements are.
> >>On the one hand we can say that the requirement is that an ISP signing
> >>on behalf of a customer actually sign on behalf of the customer. That
> >>is, the d=customer.com rather than d=isp.com.
> >>
> >>What I see here is the desire to actually have d=isp.com with the policy
> >>saying that that is ok. One downside of this is that you'd require a
> >> policy lookup because the From: address would still be customer.com, not
> >> isp.com (ie, it looks like a third party). On the other hand, it doesn't
> >> seem like it's a very big burden on the signing software to know what
> >> domains it signs for, but I'm not as convinced about that from an
> >> operational standpoint.
> >
> >Agreed.
> >
> >I see it as a complexity tradeoff between managing multiple keys for
> > multiple domains versus a single key for the ISP's domain and doing
> > additional policy lookups for ISPs that sign 3rd party for their hosted
> > domains.
>
> I think the issue of multiple keys is orthogonal as you could use one
> key in either scenario. My code, for example, currently supports either
> key scenario: one key for multiple domains, different keys for different
> domains, etc. I think the issue is whether the signer needs to keep track
> of all the domains it signs for or not. It seems to me that it would
> though, and if that's the case I'm not sure if it's worthwhile having
> indirection at the policy layer instead of just doing at the signing layer.
>
It would also have to determine if, in fact, a message it was signing for a 
domain was actually from an authorized sender for that domain (a requirement 
no matter how this beast gets killed).

I don't know how detailed the requirements document you are working on is 
intended to be....

At a high level, I think it is a requirement that the DKIM with a sender 
policy protocol support the use case of multiple administratively separate 
domains sending signed mail via a shared (e.g. ISP) server.

This has several lower level requirements that I can think of offhand for the 
signer:

 - Ensuring that messages that are signed for a domain are from an authorized 
source for that domain.

 - Signing the message in a way that allows receivers to know that the 
signature is an authorized signature for the sending domain (and this could 
be first party using one or more keys or 3rd party with a policy record that 
indicates that the signer is authorized)

 - Public or Public/Private (depending on how the previous requirement is 
satisfied) key exchange and maintenance.

The domain owner will have to publish the key and possibly  a policy record in 
their DNS.

Scott K