Subject: Re: [Ace] CoAP-EAP draft

From: Christian Amsüss <[email-masked]>
Date: Mon, 16 Aug 2021 16:40:03 +0200

Hello CoAP-EAP authors and involved groups,
(CC'ing core@ as this is a review on CoAP usage),

I've read the -03 draft and accumulated a few comments; largely in
sequence of occurrence.

Over all, the protocol has improved a lot since I've last had my eyes on
it. Several comments below are about how prescriptive the message types
are. I believe that this should be resolved towards generality, or else
the usability of this protocol with generic CoAP components will be
limited (or, worse, still implemented and then surprisingly
incompatible).

* Figure 1: For readers new to the topic of EAP, I think that it might
  be useful to extend this to cover also the EAP server or AAA
  infrastructure, if that can be covered without too much complication.

  Suggestion (without illusions of correctness):

           IoT device            Controller
         +------------+        +------------+       
         | EAP peer/  |        | EAP auth./ |+-----+[ AAA infra. ]
         | CoAP server|+------+| CoAP client|+-----+[ EAP server?]
         +------------+  CoAP  +------------+  EAP?
        
         \_____ Scope of this document _____/

                      Figure 1: CoAP-EAP Architecture

* `/.well-known/a`: [note: May be irrelevant, see next two items]

  If the designated experts don't go along with a
  very-short option (I'd kind of doubt you'd get anything shorter than
  `/.well-known/eap`) and if that puts you up against practical limits,
  using a short-hand option might be viable.

  So far there's no document for it and I've only pitched the idea
  briefly at an interim[1] (slides at [2]), but if push comes to shove
  and you need the compactness, let me know and that work can be
  expedited.

  [1]: https://datatracker.ietf.org/meeting/interim-2021-core-05/session/core
  [2]: https://datatracker.ietf.org/meeting/interim-2021-core-05/materials/slides-interim-2021-core-05-sessa-core-option-for-well-known-resources-00

* Discovering the Controller is described rather generically, but with
  CoAP discovery as an example.

  As long as CoAP discovery (as per RFC6690/7252) is used, that already
  produces a URI, which can contain any path the server picked. It has
  thus no need for a well-known path.

  Are there other discovery options envisioned that'd only result in a
  network address? Only for these, a well-known path would make sense.
  (And then it's up to the envisioned client complexity if one is
  warranted).

  For comparison, RD[3] explores some of the options. A path may be
  discovered using CoAP discovery as `?rt=core.rd*` right away from
  multicast. Or an address may be discovered using an IPv6 RA option,
  with CoAP discovery acting on that address. Only for cases of very
  simple endpoints, it also defines a `/.well-known/rd` name that can be
  used without CoAP discovery (and thus link parsing) happening
  beforehand. The same rationales may apply for EAP (the devices using
  the resource are mostly servers, otherwise, and send a very simple
  request to start things), but again that's only if the address was
  discovered through something that's not CoAP discovery already.

  [3]: https://www.ietf.org/archive/id/draft-ietf-core-resource-directory-28.html#name-rd-discovery-and-other-inte

* For message 1, why does this need to go to a fixed resource? There has
  been previous communication in message 0 in which the resource could
  have been transported.

  Granted, it's not as easy as in messages 2-to-3 etc where the
  Location-* options are around, but the original message 0 POST could
  just as well contain the path in the payload.

  There are options as to how to do that precisely (just the URI
  reference in text form, or a RFC6690 link, or a CBOR list of path
  segments, or a CRI reference[4] -- if the latter were in WGLC already
  I'd recommend it wholeheartedly), but either of them would stay more
  true to the style of the other messages in that the earlier message
  informs the path choice of the next ones.

  An upside of this would be that it allows better behavior in presence
  of proxies (see later), even though it may be practical to not spec
  that out in full here. (But the path would be open for further specs,
  and they'd just need some setting down of paving stones).

  [4]: https://datatracker.ietf.org/doc/draft-ietf-core-href/

* (Bycatch of suggesting URIs): It may be worth mentioning that the
  NON's source address can easily be spoofed. Thus, any device on the
  network can trigger what the authenticator may think of as a
  device-triggered reauthentication, and the device may think of as an
  authenticator-triggered reauthentication (provided it works that way,
  see below when reauthentication is mentioned again).

  Even sending full URIs in message 0 would be no worse than the current
  source spoofing.

  Sending URI paths in message 0 would make this minimally better
  because the attacker would need to guess (or observe from the network)
  the CoAP server's path.

* In 3.1 General flow, the message types are described in high detail.
  CoAP can generally be used with different transports (some of which
  don't even do NON/CON). Also, while I think it's reasonable to expect
  that a CoAP implementation can deal with requests coming in as either
  CON or NON, I'd expect that some don't offer all possible choices to
  applications. (A very constrained device may only send NON requests,
  or an implementation may decide autonomously whether to send
  piggy-backed or not).

  Can you clarify as to what of this is meant to be normative and what
  exemplary?

  My recommendation is to state that what is prescribed is the flow of
  requests and responses (which is what CoAP provides to the next
  layer), while notes on reliable transmission are recommendations for
  CoAP-over-UDP/DTLS. A similar statement, which I like a lot, is
  already in 3.2 on error handling.

  (I can serve examples of how subtle incompatibilities can develop but
  go unnoticed, but I'd only go through that if this is all really
  intended to be prescriptive).

* The reuse of the empty token only works if the peers actually respond
  with piggy-backed responses, so that's where enforcing the above rules
  would give some benefit -- but at the cost of losing existing CoAP
  implementations that make no guarantees as to how the response will be
  sent as long as it's reliable.

* Proxying: As it is right now, this protocol just barely works across
  proxies, and only if they support CoAP-EAP explicitly. (And while it
  may sound odd to even consider that, bear in mind that they are used
  in a very similar way in RFC9031).

  While it's a bit open whether all CoAP-based protocols should
  reasonably be expected to work across proxies or not, a remark (maybe
  before 3.1?) that "If CoAP proxies are used between EAP peer and EAP
  authenticator, they must be configured with knowledge of this
  specification, or the operations will fail after step 0."

* 3.2.2: The use of RST is rather unusual here, for the same reasons as
  the prescriptive message types.

  A response of 5.03 (Service Unavailable) has roughly the same size,
  is available independent of transport, and on most libraries *way*
  easier to use, if they support sending an RST to a well-formed message
  at all.

  (Furthermore, the sender of the 5.03 can encode an estimate of the
  remaining unavailable time in the Max-Age option; not sure if that is
  of any help here).

* 3.3.1: "received with the ACK", "sends piggybacked response" are,
  again, overly specific. "received in the last response" and "sends a
  response" could work as replacements even if message types are
  presecriptive.

* 3.3.1: "after the maximum retransmission attempts, the CoAP client
  will remove the state from its side".

  So the device that's being kicked from the network can delay its own
  eviction for about a minute as long as it doesn't answer?

* 3.3.2: Is reauthentication always triggered by the EAP peer, or can it
  also be triggered by the authenticator? If the latter, will the
  authenticator use /.well-known/a again, or POST something to the
  resource from where it'd DELETE in 3.3.1?

* cryptosuites: What's the upgrade model of that hardcoded list? As it
  is now, it looks pretty static, so updates would be through updates of
  this document. The obvious alternative is an IANA registry with
  ranges, policies and the usual pros and cons.

  Then again, this is not the first nor last time AEAD algorithms with
  their parametrization and hash functions are assigned aggregate
  numbers (I-D.ietf-lake-edhoc comes to mind which has asymmetric algs
  in the mix too; probably others as well); can we deduplicate this with
  anything? (Possibly by bringing this up with COSE or OSCORE people).

* OSCORE derivation: Is it cryptographically necessary to derive *both*
  a master secret and a master salt through KDF? (Sounds like a
  needless step to me, as both only go into KDF once more when the
  actual OSCORE parameters are derived). I *guess* there's a good reason
  why the MSK is not used as the OSCORE IKM right away and the CSO as
  OSCORE master salt, but it'd help to have at list a comment here on
  why that's needed.

  (It may be useful to compare this step to the HKDF steps in OSCORE;
  their info element is always a 5-element array with a 4th "type"
  element of "Key" or "IV"; other extractions might just hook in there
  with different type values, maybe, and save everyone an extra handling
  step).

* OSCORE ID derivation:

  * Randomly assigned full-length ideas look like an odd choice. They
    are excessively long (nonce length - 6 is 7 for the MTI
    AES-CCM-16-64-128 and shorter for other current ones, but I doubt
    that keeping the IV *short* is necessarily a design criterion for
    future algorithms).

    What commonly happens here (eg. in the ACE-OSCORE profile, or in
    EDHOC) is that each party picks a recipient ID out of its pool of
    currently unused IDs. This makes for shorter keys, and allows the
    client to be sure that no two peers use the same context.

    Any chance something like that can still make it in?

  * If the parties happen to be assigned the same sender ID, bad things
    happen (identical key derivation, nonce reuse, nuclear meltdown).

    If the current pattern of KDF'ing IDs stands, this needs to be
    prevented explicitly.

  * The derivations of "OSCORE RECIPIENT ID" and "OSCORE SENDER ID" are
    confusing as they each need to happen on both sides, and the terms
    will match on one and need to be opposite on the other.

    (I couldn't even easily find which is intended to be which).

    My suggestion is to derive "OSCORE EAP PEER SENDER ID" and "OSCORE
    AUTHENTICATOR SENDER ID" instead. (Or preferably shorter strings).

* Exmaples: Do you envision particular EAP protocols to be used in the
  given examples?

Best regards
Christian