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
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