[Teep] Implementation feedback on draft-ietf-teep-opentrustprotocol-01

[Dave Thaler <[e-mail]>]

During the Hackathon yesterday, I started trying to write code for OTrP
to see what issues I would run into.   Below are some issues I ran into, in
addition to what I already gave as feedback on the arch doc.


1)     Section 6.5 explains that the TAM needs to receive a list of one or more

TA's that are requested to be installed (S6.5.1 point 1A).  However, no

message is defined for doing so, which prevents interoperability.  I
think this message needs to be generated by the TEE (not the rich app),
for reasons I will explain in #3 below.

2)     Section 6.5 explains that the TAM needs to keep track of TAs installed
on all devices, even though its list might be wrong.   This has a scalability
issue.  Instead, I think there should be no such requirement.

3)     Putting my issue #1 and issue #2 together means there's an extra round

trip that is unnecessary.  6.5 says the TAM receive a list of TAs needed,

and then the TAM just goes back and asks what is installed, just to get
a list of what needs to be installed.  This is unnecessary, the TEE can just

send a list of one or more TAs that need to be installed and aren't already.

Hopefully this explains why I said in issue #1 above why I think the message
needs to be generated by the TEE.

4)     Section 9.1.1 requires a list of OCSP stapling data, but as far as I can see,

the document provides no information or citation about the correct format
for such data.

5)     The "did" field in 9.2.1.1 seems to be either (a) redundant and should be

removed, or (b) missing from other messages like Install TA.  The text explains
the field is to check that the message was received by the right device.  My
opinion is that since the TEE has to trust the TAM anyway, it's the TAM's
responsibility to send messages to the right device over an authenticated
channel (whether encrypted or not).  So I think it should be removed.

6)     Some fields, e.g. "signerreq", have boolean values that are encoded as
strings ("true", "false"). I think these should be boolean types, not strings,
which would also have the advantage of better compression if we can use
CBOR encoding.

7)     Section 6.5.1 point 9.A implies that to install a TA, one must have an extra
round trip first to create an SD if one isn't already there.  I would expect one
common case to be where there is one TA per SD, so that all TAs are isolated
from each other.  As such, requiring the extra delay is inefficient in time,
bandwidth, and processing.   All the fields in CreateSD are already present
in an InstallTA message (except the "did" field mentioned above in issue #5),
so it could be done automatically by the first InstallTA message itself.

8)     The scope of uniqueness of the "rid" and "tid" fields is underspecified.

They just say "unique".  I think "rid" is just supposed to be unique within

a given {session,"tid"} but I can't tell for sure.  And I think "tid" is just supposed
to be unique within a given session (not globally across all sessions, all TAMs,

all devices), but I can't tell.   They're also formatted as strings, but I'm not sure
why they can't be integers which I think would be much more efficient.

9)     I found it confusing that the names of the messages don't match the name values
in the messages themselves ("GetDeviceStateResponse" vs

"GetDeviceTEEStateTBSResponse", etc.)   Having these not match is bug-prone.

10) It's unclear whether a rich app can depend on two TA's from different TAMs,

and whether a TA can depend on a TA from a different TAM.  In the use case

where the device admin runs the TAM and controls all TAs on their devices the

answer would be no.  But in other use cases I'm not sure.   If so, then the question
arises about how dependencies are expressed and whether a dependency needs
to express which TAM is used.  This then begs the questions of whether a TA might
be via more than one TAM, or might change TAMs over time.   The answers here
probably belong in the arch doc.

Dave