Re: [Teep] TEEP Architecture Last Call Review

[Mingliang Pei <[e-mail]>]

Hi Ben,

Thank you very much for your review and suggestions as tracked here
<https://datatracker.ietf.org/doc/review-ietf-teep-architecture-16-secdir-lc-schwartz-2022-04-04/>.
Somehow we didn't notice it until recently and we have tracked it as Issue
#251 <https://github.com/ietf-teep/architecture/issues/251> in Github TEEP
project.

We have added inputs for each of your suggestions or questions, and adopted
suggestions into the doc revision. There are a few comments that we would
like to follow up with you. Please see full replies below. I am copying the
comments from #251 issue here. It will be great if you can reply as
comments in the github issue above. It is fine too we exchange here, and we
will incorporate our discussions to the issue tracker for further revision
references.

-----

> Section 1:
The use of the term "applications" carries an implication of a client-side
device with installable software, but TEEP seems to extend also to server
software sharing a kernel, hypervisors sharing a mainboard, etc. A term like
"software" would be more neutral.

This would be a major change in the doc. We authors will review for a
consensus.

> "An application component ... is referred to as a Trusted Application":
This is
confusing. A component, explicitly not an entire "application", is referred
to
as an "application". "Trusted Component" would be more consistent.

The doc has defined a TA to mean possibly an "application component" in the
Terminology Section as follows:

"- Trusted Application (TA): An application (or, in some implementations,
an application component) that runs in a TEE"

The change would be a significant change across the doc, including APIs to
be consistent when "Trusted Application" is replaced with "Trusted
Component" everywhere. Can we continue to use the above definition to cover
both cases?

> Also, "trusted" seems to be the wrong adjective here, as it is the
environment, and
not the software, that carries an elevated level of trust. "Isolated" might
be
a better descriptor.

> If this is common terminology for the field, a citation would be good.

Not sure that a name "Isolated Component" is more natural than "Trusted
Component".

> I would appreciate some discussion of whether the Device Owner needs to
trust
the Trusted Application, i.e. interaction between enclaves and sandboxes.

Sounds good. A Device Owner may not always have control over TAs that go to
its device regardless of the owner's trust. A Device Administrator may
often decide when the Administrator is different from the owner. Secondly,
the trust to a TA is delegated to the TAMs that may manage the TAs for
installation to devices.

@dthaler <https://github.com/dthaler> @hannestschofenig
<https://github.com/hannestschofenig> how do you think?

> "verify the ... rights of TA developers": "rights" is a loaded term.
Rather
than get into constitutional law, consider "permissions".

Agreed. Fixed.

> "so that the Untrusted Application can complete" -> "so that installation
of
the Untrusted Application can complete".

Agreed. Fixed.

> "is considered confidential" -- By whom? From whom? Consider "A developer
who
wants to provide a TA without revealing its code to the device owner..."

Agreed. Just note that the developer may not even want to share the code
with a TAM that distributes the binary. So it isn't only protecting from a
device owner. Proposed fix:

A Trusted Component might also be encrypted,
if the code is considered confidential, for example, when a developer wants to
provide a TA without revealing its code to others.

> "A TEE ... wants to determine" ... excessive personification. I suggest
"needs".

Agreed. Fixed.

> Section 2:
"it is more common for the enterprise to own the device, and any device user
has no or limited administration rights": Grammar issue. Perhaps "and for
any
device user to have ...".

Agreed. Adopted in fix.

> Section 3.1
"trusted user interface" ... can you cite an example of a mobile device
with a
trusted peripheral that is not accessible to the REE OS? This seems
theoretical.

Need help from co-authors here too.

> Section 3.3
Similarly, are there any examples of IoT devices that prevent the REE OS
from
operating certain actuators?

We will follow up on this.

> Section 4.1
the TAM cannot directly contact a TEEP
Agent, but must wait for the TEEP Broker to contact the TAM
requesting a particular service. This architecture is intentional
in order to accommodate network and application firewalls

> This is true in many use cases, but for Confidential Cloud the reverse
logic
applies. In fact, the TAM could be operating on-site inside an enterprise,
requiring a firewall exception to be reachable from the TEEP Broker. This
architecture is also unnatural: it converts an event-driven "update command"
into a polling loop that adds delay and wastes resources. Why is this part
of
the TEEP architecture? Surely it could be handled by a reversal-of-control
pattern one layer below TEEP (e.g. Server-sent events)?

> I think the real motivation here is (1) installation is presumed to be
triggered locally, by the Untrusted Application, so the TAM must be
reachable
as a "server", and the other side naturally should keep the client role; (2)
the TAM is intended to have O(1) state while serving N devices.

Right, that was the main use case and motivation for the architecture.

> For a TAM to be successful,
it must have its public key or certificate installed in a device's
Trust Anchor Store.
This needs discussion of threat model. What damage can a hostile TAM do?
What
does the device administrator need to know for adding a trust anchor to be
safe?

This has been addressed in "Section 9.5 Compromised TAM" in a version after
the review was posted.

> Section 4.4
Implementations must support encryption of
such Personalization Data to preserve the confidentiality of
potentially sensitive data contained within it,

> Implementations of what?

Changed to "Implementations of TEEP protocol"

> and must support
integrity protection of the Personalization Data.

Lower-case "must" without explanation. Why, and is this a normative
requirement?

We chose to not use MUST in the doc to differentiate "normative" from
"informal" requirements. To discuss this. @dthaler
<https://github.com/dthaler> @hannestschofenig
<https://github.com/hannestschofenig>

> Section 4.4.2
"e.g., OP-TEE" -> What is this?

Changed to "e.g., OP-TEE, an open source TEE"

> Section 5.4

> When a PKI is used, many intermediate CA certificates can chain to a
root certificate, each of which can issue many certificates.

> Intermediate CAs have a troubled history (e.g. [1]), and techniques that
make
them safer (e.g. x.509 name constraints) can't be deployed as a retrofit.
Does
TEEP need some rules about supported x.509 extensions?

We leave this to the device provider for the constraints on X509 extensions
it supports and uses in Trust Anchor validation. They may select to trust
only a selected intermediate CA instead of the root as the Trust Anchor.

*Is this fine?*

> Section 6.2.1

> If an Untrusted Application is summarily deleted, how do you avoid
leaking the
TA?

Similar to the removal of a buggy or malicious TA, this is up to a device
to have some scheme to contact TAM or be contacted to initiative a removal
of TAs that are not needed anymore.

> Section 7

> TEEP is format-agnostic for attestations, but what about
message-sequence-agnostic? Can it tunnel arbitrary challenge-response
sequences?

There are some limitations in TEEP about supporting arbitrary
challenge-response sequences. It generally complies with RATS recommended
sequence. A single pass attestation and verifier flow is assumed. A
challenge may be supported by sending it in a request from a TAM to the
device where the device will combine the challenge in its attestation
evidence generation.

> Section 9.3

> We have
already seen examples of attacks on the public Internet with billions
of compromised devices being used to mount DDoS attacks.

> Citation please. Also, are you sure it has reached into billions?

Changed to "a large number".

> Section 9:

> Nothing here seems to discuss attacks on the TEE's properties, and the
post-compromise implications of those attacks. For example, if all instances
of a TA share a secret key, used for decrypting the Personalization Data,
then
a single successful attack on a TEE is sufficient to decrypt all
Personalization Data (previous and future). Given the prevalence of such
attacks (especially via hardware fault injection), it seems likely to be
worth
mentioning.

In general, we expect that a different secret key is used for different
devices for Personalization Data encryption. Such a secret key is delivered
to a TA using the public key of the TEE in the device. Such a shared secret
key for all TA instances isn't a common case.

To mitigate this happening, I think we can add a statement to require that
Personalization Data must not be encrypted with the same key for all
devices.

How do you think? I will also get other co-authors's inputs on this.

Again, thank you very much for your detailed review and suggestions.

Best,

Ming

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