[Teep] Reliably getting TEE properties

[Dave Thaler <[e-mail]>]

I just filed issue https://github.com/ietf-teep/teep-protocol/issues/189
based on thinking about the discussion Brendan and I were having at the
tail end of the SUIT WG meeting today.

Currently evidence in the QueryResponse is an optional field, and the
QueryRequest can specify the attestation bit to request it.  Since the evidence
is not mandatory, the QueryResponse today has no reliable way to learn
the device identifying information and TEE identifying information (to use
the terms in the architecture draft), which seem critical to the task of being
able to determine which Trusted Components to install in the TEE.

Options:
1. Make evidence mandatory in the QueryResponse and remove the attestation bit from data-items-requested
2. Make attestation mandatory but allow it to be either passed inside the TEEP protocol or in a transport protocol (e.g., HTTPS terminating inside the TEE as in model C of figure 5 in the architecture draft). The attestation bit shouldn't be needed in data-items-requested here either since an implementation could know whether the transport included it inside the TEE or not.
3. Make use of SUIT Reports in the QueryResponse mandatory and put the system information there. This means that a TAM would be able to access them without relying on a Verifier to put relevant claims in Attestation Results so could ease interop with more Verifiers.   The only downside I see is that today it is not a
requirement that SUIT Reports be generated at "boot" time (meaning start time
for whatever components are relevant) on a TEEP device, and this option would
make that a requirement.
4. Some hybrid that allows more than one of the above in implementations but
only uses one in any particular message exchange.

I have to admit that I do see advantages in option 3 as noted above, and so I
filed https://github.com/ietf-teep/teep-protocol/pull/187 with what that might
look like if we go that route though I temporarily made suit reports optional
so it's technically option 4, but if we go with option 3, I will update to make
them mandatory.

Looking for feedback from the WG and especially from other implementers.

Dave