[Acme] Use cases / trust model for device certs

[Richard Barnes <[e-mail]>]

Hey Rifaat,

Owen and I were chatting about ACME and device certs this morning, and it
seemed like it might be useful to rekindle discussion on the topic here on
the ACME list.

I'd like to push a little more on the trust model here.  Just to establish
some terminology:

- Device: Uses certificates to authenticate identifiers
- Vendor: Makes the device that will get the end certificate
- Customer: Buys the device from the vendor and operates it
- CA: Validates identifiers and issues certificates
- Relying Party: Uses certificates to verify authentication for identifiers
- Device Identity: MAC address or similar

In the flows Owen and I have been discussing (more based on ANIMA/BRSKI),
the model is basically broken in two, with the customer in the middle:

1. The customer validates devices' device identity as part of the ANIMA
flow, based on the customer trusting the vendor, and assigns the device a
domain name
2. The customer uses ACME to issue domain name certificates (the CA is
unaware of the device identity)

That all pretty much just works with BRSKI and ACME as they are today.  But
it presumes that the RP is authenticating the device by domain name, as is
prevalent in most uses of TLS today.

In contrast, it seems like your draft presumes that the RP needs to know
the device identity; it's not satisfied by a domain name alone.  Can you
elaborate a bit more on what scenarios you have in mind for this?  If all
you care about is the customer tracking things, then the model above is
sufficient; the customer can simply assign domain names that encode the
device identity however it likes.

Thanks,
--Richard