[Ace] AD review of draft-ietf-ace-oscore-profile-08

[Benjamin Kaduk <[e-mail]>]

Hi all,

Some high-level points before the section-by-section commentary:

I'm a little confused by the registry we are creating in Section 9.2.
While it's clear that we need something to specify the CBOR map key
labels to encode the structure for transit, it's not clear that we need
easy extensibility vs.  a fixed table.  A registry implies expectations
of future additions, but draft-ietf-core-object-security did not feel a
need to make a registry for future extensions; what has changed in the
past few months that makes extensibility needed now?  Why is this
document the right place to create the registry as opposed to that one?

We are in some sense defining a new substructure within "kid" values (a
CBOR-encoded array of one or two elements) for usage by things using
this profile.  That's probably not problematic, as the "kid" can be an
arbitrary bstr and its interpretation is up to the recipient -- any
recipient that implements this profile will know to handle the
substructure, but it does feel a little unusual and perhaps worth an
early note.  (IIUC, we also need to be careful about actually providing
the bstr framing in our examples.)

I'm concerned about the usage of "cnonce" to carry both N1 and N2,
neither of which correspond to the "cnonce" behavior in the core
framework; I think we should pick a different parameter name for the
differeng semantic usage.  Specifically, the core-framework "cnonce" is
a tool to give RSes some modicum of replay protection -- it verifies
that the client's authorization in the token is "fresh", since the token
includes the random nonce generated by the RS and conveyd to the AS via
the client.  This document uses a parameter named "cnonce" in two ways:
(1) to convey the client-generated nonce N1 to the RS in the authz-info
request; this is giving the client contributory behavior to the
ephemeral randomness input to the Security Context computation but does
not provide any sort of replay-protection in and of itself
(2) to convey the RS-generated nonce N2 back to the client in the
authz-info response; this does serve as replay protection in some sense,
but in that it serves to cause repeated posting of the same token (as
might occur in a replay attack) to generate a different OSCORE context
(due to the RSes contributory nature) so that the resulting key material
will be unique for a given association, as opposed to detecting and
blocking replay outright.  I think all three uses should have distinct
parameter names, based on my current understanding.

We mention several times about getting protection from AEAD nonce/key
reuse, but to some extent leave it to the reader to figure out how that
works.  Specifically, the only guidance we give on nonce selection is
for our N1 and N2 that go into the Master Salt; we don't say how varying
the Master Salt like that will cause the generated keys and (common) IVs
to differ across runs, thus by construction preventing the collision of
(key, nonce) pairs.  (Implementation carelessness that just reuses
key+nonce for a given Security Context is still possible, of course.)

In a related vein, we do not get into full detail about the
cryptographic role played by N1 and N2, so as to make the reader
confident that our 64-bit recommendation does provide full
Internet-grade security (as opposed to a weakened "IoT" variant).  I'm
not specifically concerned, but we may get some reviewers asking for
more detail.

I am, however, concerned about the exposure to an RFC 3552-style
attacker for unprotected authz-info responses.  There are two main
aspects I'm concerned about: identifier selection and nonce selection.
Letting the server override the client's idea of its own identity (even
when the AS has given the client an identifier!) seems to increase the
risk of a client being tricked into impersonating a different client,
and the stated justification (letting the RS preserve uniqueness)
doesn't seem to hold up in our one-AS-per-RS stated scope, where the AS
can equally well do so.  Based on what I know right now, it doesn't feel
like we're at the right point on the cost/benefit curve.  For the nonce
selection, this comes into play due to the lack of injectivity for the
mapping of initial AS-generated salt, N1, and N2 into the Master Salt.
Specifically, if a client chooses an N1 that's a prefix of some other N1
that was used, then the attacker can send an "N2" that contains the tail
end of that other N1 plus the actual N2 picked by the server in that
other exchange, causing a full nonce collision and having the
AS-generated portion be the only difference in Master Salt between
security contexts.  Using a length prefix for the fields being
concatenated provides injectivity and thwarts this type of attack.

There are a few places where I'm not entirely sure how the protocol flow
works for the case where the RS is (re)assigning the clientId; I've
tried to note them in the section-by-section comments.

On to the section-by-section notes:

Section 1

I'd suggest noting that this is a symmetric-crypto-based scheme in some
manner (e.g., that the AS is assigning a shared symmetric key used as the
OSCORE master secret), and/or that asymmetric PoP semantics are not possible
with this mechanism (or OSCORE in general).

I'd also suggest noting that proof of possession is not done by a dedicated
protocol element, but rather occurs implicitly based on knowledge of the
OSCORE Security Context.  (This is essentially analogous to Kerberos, but a
reference is not strictly needed.)  We do have a good treatment in
toplevel Section 4, but it might be worth saying earlier.

Section 2

   [I-D.ietf-ace-oauth-authz].  The access token request and response
   MUST be confidentiality-protected and ensure authenticity.  This
   profile RECOMMENDS the use of OSCORE between client and AS, but TLS
   or DTLS MAY be used additionally or instead.

Since the client/AS and client/RS profile are supposed to be completely
severable, I don't think we should use "MAY" here; perhaps "but other
protocols (such as TLS or DTLS) can be used as well".

   Once the client has retrieved the access token, it generates a nonce
   N1 and posts both the token and N1 to the RS using the authz-info
   endpoint and mechanisms specified in section 5.8 of
   [I-D.ietf-ace-oauth-authz] and Content-Format = application/ace+cbor.

Do we want to say explicitly that there is no cryptographic protection
applied for this POST to authz-info?

   which contains a nonce N2 in a CBOR map.  Moreover, the server
   concatenates N1 with N2 and appends the result to the Master Salt in
   the Security Context (see section 3 of
   [I-D.ietf-core-object-security]).  The RS then derives the complete
   Security Context associated with the received token from it plus the
   parameters received in the AS, following section 3.2 of
   [I-D.ietf-core-object-security].

This passage feels strange to me, probably because it seems to use "Master
Salt" to both the portion with and without N1+N2, and also because it does
not introduce "the Master Salt" as a term or some other object that deserves
the definite article.  Would it be equivalent to say "Moreover, the server
concatenates the input salt, N1, and N2 to obtain the Master Salt of the
OSCORE Security Context (see section 3 of [I-D.ietf-core-object-security]"
instead?  (Though the details may yet change based on later comments...)
Is "from it" the resulting Master Salt?
There seems to be a word or two missing in "the parameters received in
the AS".  (Presumably these parameters are contained in the token?)

   After receiving the nonce N2, the client concatenates it with N1 and
   appends the result to the Master Salt in its Security Context (see
   section 3 of [I-D.ietf-core-object-security]).  The client then
   derives the complete Security Context from the nonces plus the
   parameters received from the AS.

Similarly, IIUC we should be able to word this so that the "Master Salt" is
unique (and includes the nonces), as opposed to having it apparently refer to
both the pre- and post-nonce-includion strings.
Are these "parameters" received from the AS in the token response?

   Finally, the client sends a request protected with OSCORE to the RS.
   If the request verifies, then this Security Context is stored in the
   server, and used in the response, and in further communications with
   the client, until token expiration.  This Security Context is
   discarded if the same token is re-used to successfully derive a new
   Security Context.

The conditional on storing the Security Context only if the request
verifies reads oddly to me, as the RS seems to have needed to store some
amount of security context information for the period betweeen receiving
the token and receiving the authenticated request, as a prerequisite to
being able to verify the request.
Also, is this security context discarded if a new token from the same
client is successfully used?

   The use of random nonces during the exchange prevents the reuse of
   AEAD nonces and keys with different messages, in case of re-
   derivation of the Security Context both for Clients and Resource
   Servers from an old non-expired access token, e.g. in case of re-boot
   of either the client or RS.  In fact, by using random nonces as part

This sentence is pretty long and a little hard to parse.  I'd suggest
something more like:

%  The use of random nonces during the exchange prevents the reuse of
%  an AEAD nonces/key pair for two different messages.  This situation
%  might occur when client and RS derive a new Security Context from an
%  existing (non-expired) access token, as might occur when either party
%  has just rebooted.  [...]

I'd also consider s/In fact/Instead/ since we are contrasting the
previously described hypothetical (bad) scenario with what actually
happens.

   of the Master Salt, the request to the authz-info endpoint posting
   the same token results in a different Security Context, since Master
   Secret, Sender ID and Recipient ID are the same but Master Salt is
   different.  Therefore, the main requirement for the nonces is that

nit: [more usage of "Master Salt" for pre- and post-nonce forms]
I'd also suggest to s/since/since even though the/ (and the s/but/the/
needed to fix up the end of the sentence).

I also want to check my understanding: this "different Security Context" is
still going to use the same identifiers for the endpoints, and will just have
different keys/IVs/etc?  This would tie in to the (later) recommendation to
have such a new Security Context completely replace the old one, since
otherwise we'd have to resort to trial decryption to determine which
Security Context to use for processing a given message.

   they have a good amount of randomness.  If random nonces were not
   used, a node re-using a non-expired old token would be susceptible to
   on-path attackers provoking the creation of OSCORE messages using old
   AEAD keys and nonces.

I guess the focus on "good amount of randomness" is trying to emphasize
that we merely need to avoid a single node accidentally producing the
same value, as opposed to a stricter property of unguessability by an
on-path attacker?  Or is unguessability also needed when the attacker
can delay traffic?

Section 3

   Section 3.2 of [I-D.ietf-core-object-security] defines how to derive
   a Security Context based on a shared master secret and a set of other
   parameters, established between client and server, which the client
   receives from the AS in this exchange.  The proof-of-possession key

We say that the client gets the OSCORE parameters from the AS in the
token response; where do we say how the RS gets the OSCORE parameters
from the AS?

Section 3.1

   The client MUST send this POST request to the token endpoint over a
   secure channel that guarantees authentication, message integrity and
   confidentiality (see Section 5).

(The framework already requires this, so we may not need the "MUST"
keyword again, though I'm not proposing to remove all mention of the
need for these properties.)

   An example of such a request, in CBOR diagnostic notation without the
   tag and value abbreviations is reported in Figure 2

nit: only the payload is using CBOR diagnostic notation; the headers are
using CoAP notation.

   If the client wants to update its access rights without changing an
   existing OSCORE Security Context, it MUST include in its POST request
   to the token endpoint a req_cnf object.  The req_cnf MUST include a
   kid field carrying a CBOR array object containing the client's
   identifier (assigned in section Section 3.2) and optionally the
   context identifier (if assigned in section Section 3.2).  The CBOR

I'm not sure the best way to write a clear description that includes the
"bstr" framing for "kid".
Also, nit: the "assigned" language in the parentheticals feels weird to
me, here.
What's the workflow when the RS has overridden the AS's idea of the
clientId?  Does the client have to remember the original AS-assigned
clientId and use it here instead of the RS-assigned one?

   These identifiers can be used by the AS to determine the shared
   secret to construct the proof-of-possession token and therefore MUST
   identify a symmetric key that was previously generated by the AS as a
   shared secret for the communication between the client and the RS.

nit: I suggest s/shared secret to construct/shared secret bound to/

Also, I think the AS will need to use some of the other token request
parameters (e.g., req_aud) in order to determine the client and RS for
which it needs to retrieve the shared PoP key.  We might want to mention
that as well.

   The AS MUST verify that the received value identifies a proof-of-
   possession key and token that have previously been issued to the
   requesting client.  If that is not the case, the Client-to-AS request

nit: It's not clear that we need it to identify a specific token, just a
key.  (And in fact for multiple successive access-rights updates, there
will be more than one previous token.)

The "kid" "req_cnf" member in Figure 4's example does not include the
bstr wrapper that draft-ietf-ace-cwt-proof-of-possession suggests is
required.

Section 3.2

   Moreover, the AS MUST provision the following data:

It's frequently the case in IETF documents that "provision" is a
euphimism for "out-of-band configuration", so if we mean "include in the
token response" it's probably better to say that directly.

   Additionally, the AS MAY provision the following data, in the same
   response.

When the AS does not do so, how are these parameters determined?

   The OSCORE_Security_Context is a CBOR map object, defined in
   Section 3.2.1.  The master secret MUST be communicated as the 'ms'
   field in the OSCORE_Security_Context in the 'cnf' parameter of the
   access token response as defined in Section 3.2 of
   [I-D.ietf-ace-oauth-params].  The AEAD algorithm MAY be included as

nit: s/in the OSCORE_Security_Context/in the OSCORE_Security_Context
field/, I think?

   the 'alg' parameter in the OSCORE_Security_Context; the HKDF
   algorithm MAY be included as the 'hkdf' parameter of the
   OSCORE_Security_Context, a salt MAY be included as the 'salt'
   parameter of the OSCORE_Security_Context, and the replay window type
   and size MAY be included as the 'rpl' of the OSCORE_Security_Context,
   as defined in Section 3.2.1.

I think we want to reword all of these, as I understand what we're doing
to be defining the parameters in which these values are conveyed, if
they are conveyed at all.  Just saying "<X> MAY be conveyed in <Y>" has
the connotation that <X> might alternatively be conveyed in some other
<Z> that is not mentioned.  The relevant MAY for our purposes is the one
above the list -- "the AS MAY provision the following data".

   The same parameters MUST be included as metadata of the access token.
   This profile RECOMMENDS the use of CBOR web token (CWT) as specified
   in [RFC8392].  If the token is a CWT, the same
   OSCORE_Security_Context structure defined above MUST be placed in the
   'cnf' claim of this token.

nit: "metadata of the access token" sounds like it's trying to be a term
of art, but it is not presently used as such.  s/metadata/attributes/
would, IIUC, be more in keeping with OAuth 2.0 tradition.
not-nit: the second sentence seems redundant with the second sentence of
the previous paragraph.

   The AS MUST also assign an identifier to the RS (serverId), MAY
   assign an identifier to the client (clientId), and MAY assign an
   identifier to the context (contextId).  These identifiers are then
   used as Sender ID, Recipient ID and ID Context in the OSCORE context
   as described in section 3.1 of [I-D.ietf-core-object-security].  The
   couple (client identifier, context identifier) MUST be unique in the
   set of all clients on a single RS.  Moreover, when assigned,
   serverId, clientId and contextId MUST be included in the
   OSCORE_Security_Context, as defined in Section 3.2.1.

[Any time we're assigning identities we have to say something about the
privacy properties thereof, or a bunch of reviewers will complain.]
Is the Recipient ID expected to be stable/persistent for a given RS as seen
by all clients?
nit: "all clients on a single RS" reads oddly to me; maybe s/on/of/?
nit: I'd suggest to replace "couple" with "tuple" or "pair".
nit: serverId is always assigned by the AS, so the "when assigned"
doesn't quite bind properly, grammar-wise.  While the clientId is
"always assigned" by the time of the Master Secret derivation, this is
just discussing the AS token response, so we can't necessarily assume
it's assigned here, at least with the current protocol spec.

OSCORE requires a Sender ID and Recipient ID, but we only have it as
optional for the AS to assign the clientId -- what does OSCORE use as
the Sender/Recipient ID when the AS does not assign a clientId?

When certain fields are optional, we typically have to ask whether it's
possible for the two parties to disagree about whether the field is
present.  IIUC, the OSCORE context derivation procedure includes enough
information to prevent that possibility, but it would be good if someone
would confirm that.

   We assume in this document that a resource is associated to one
   single AS, which makes it possible to assume unique identifiers for
   each client requesting a particular resource to a RS.  If this is not

I feel like this might be more clear if we can frame it in terms of the
AS enforcing the uniqueness of client identifiers (or, if you will, that
we delegate the enforcement thereof to the AS...)

   the case, collisions of identifiers may appear in the RS, in which
   case the RS needs to have a mechanism in place to disambiguate
   identifiers or mitigate their effect.

nit: s/appear in/occur at/
nit: s/mitigate their effect/mitigate the effect of the collisions/

   Note that in Section 4.3 C sets the Sender ID of its Security Context
   to the clientId value received and the Recipient ID to the serverId
   value, and RS does the opposite.

(The way this is written implicitly assumes that both clientId and
serverId are received from the AS.)

   Figure 5 shows an example of such an AS response, in CBOR diagnostic
   notation without the tag and value abbreviations.

I'd suggest s/such an AS response/an AS token response/

         "access_token" : h'a5037674656d7053656e73 ...'
          (remainder of access token omitted for brevity)',

In addition to the invalid-CBOR-diagnostic-notation-syntax comment, we
also have unbalanced quotes.

         "profile" : "coap_oscore",
         "expires_in" : "3600",
         "cnf" : {
           "OSCORE_Security_Context" : {
             "alg" : "AES-CCM-16-64-128",
             "clientId" : b64'qA',
             "serverId" : b64'Qg',

Any reason to prefer b64'qA' over h'a8' which is shorter?

   If the client has requested an update to its access rights using the
   same OSCORE Security Context, which is valid and authorized, the AS
   MUST omit the 'cnf' parameter in the response, and MUST carry the
   client identifier and optionally the context identifier in the 'kid'
   field in the 'cnf' parameter of the token, with the same structure

I think we should be careful about referring to this as "optionally the
context identifier" -- the choice for whether there is a context
identifier is made once (at initial issuance), and subsequent usage
depends on whether one has been allocated or not.  It's not like the AS
can decide at runtime "I gave you a context identifier for your original
security context, but I don't feel like giving it to you again" -- the
derived keys won't match up and it won't be reusable as the same
security context.
Also, I'm not sure this is fully defined for the case when the RS
overrides the AS's clientId assignment, as the kid array in the token
needs to be the value the RS will know, but the kid in req_cnf needs to
be the one the AS knows, and we don't really define a mechanism for the
AS to be able to do translation from one to the other.

   defined in Figure 3.  These identifiers need to be provisioned, in
   order for the RS to identify the previously generated Security
   Context.

nits: there's only one (not "these") identifier in the token, and
nothing in the token response; and a similar remark about "provisioned"
as was made above.

   Figure 9 shows an example CWT, containing the necessary OSCORE
   parameters in the 'cnf' claim for update of access rights, in CBOR
   diagnostic notation without tag and value abbreviations.

     {
       "aud" : "tempSensorInLivingRoom",
       "iat" : "1360189224",
       "exp" : "1360289224",
       "scope" :  "temperature_h",
       "cnf" : {
         "kid" : b64'qA'

I don't think this "kid" is a valid CBOR bstr wrapping the one- or
two-element array.

Section 3.2.1

   [I-D.ietf-core-object-security]).  The OSCORE_Security_Context object
   can either be encoded as JSON object or as CBOR map.  In both cases,

nits "a JSON object", "a CBOR map".

   defined below.  All parameters are optional.  Table 1 provides a
   summary of the OSCORE_Security_Context parameters defined in this
   section.

They're optional to appear in the OSCORE_Security_Context object that's
included in a "cnf" value, but not necessarily optional for OSCORE to be
usable.  What do we do for OSCORE when any given parameter is not
present in this structure?  (Also, doesn't the AS always have to provide
things like 'ms' and 'serverId'?)

   | hkdf      | 4     | bstr / int     | COSE         | OSCORE HKDF   |
   |           |       |                | Algorithm    | value         |
   |           |       |                | Values       |               |
   |           |       |                | (HMAC-based) |               |

Should be tstr, not bstr, right?

Also, why do we need the string form of either hkdf or alg?  More
possibilities makes the decoder more complicated and increases the
attack surface.

   | rpl       | 8     | bstr / int     |              | OSCORE Replay |
   |           |       |                |              | Window Type   |
   |           |       |                |              | and Size      |

Should just be bstr (not int), right?

   hkdf:  This parameter identifies the OSCORE HKDF Algorithm.  For more
      information about this field, see section 3.1 of
      [I-D.ietf-core-object-security].  The values used MUST be
      registered in the IANA "COSE Algorithms" registry and MUST be
      HMAC-based HKDF algorithms.  The value can either be the integer

It's a little unfortunate that we basically are relying on "I know it
when I see it" for determining which algorithms are HKDF algorithms.

Also, "HKDF" expands to "HMAC-based Extract-and-Expand Key Derivation
Function", i.e., is definitionally HMAC-based.  So this should be
reworded to reflect the actual intent.

   alg:  This parameter identifies the OSCORE AEAD Algorithm.  For more
      information about this field, see section 3.1 of
      [I-D.ietf-core-object-security] The values used MUST be registered
      in the IANA "COSE Algorithms" registry and MUST be AEAD
      algorithms.  The value can either be the integer or the text

Similarly, this seems to be "I know it when I see it".

   contextId:  This parameter identifies the security context as a byte
      string.  This identifier is used as OSCORE ID Context.  For more
      information about this field, see section 3.1 of
      [I-D.ietf-core-object-security].  In JSON, the "contextID" value
      is a Base64 encoded byte string.  In CBOR, the "contextID" type is
      bstr, and has label 7.

Do we need to forbid present-but-empty contextIds in order to avoid
cryptographic collision with absent-contextId?

   rpl:  This parameter is used to carry the OSCORE value, encoded as a
      bstr.  This parameter identifies the OSCORE Replay Window Size and
      Type value, which is a byte string.  For more information about
      this field, see section 3.1 of [I-D.ietf-core-object-security].
      In JSON, the "rpl" value is a Base64 encoded byte string.  In
      CBOR, the "rpl" type is bstr, and has label 8.

I tried to follow the reference, but I couldn't really find a
description of the encoding for this as a byte string (especially
noteworthy since we claim that it represents both size and type).
(Relatedly, I'm not sure why it's allowed to be either bstr or int.)

Section 4

Can we get some CDDL or similar for both C-to-RS and RS-to-C (in the
respective subsections)?

I'd naively expect a section titled "Client-RS Communication" to include
some discussion of normal protected OSCORE request/response pairs once
the security context is fully established, but this initial section
basically only talks about the authz-info interaction and disclaims that
the subsections will cover anything else.

   The following subsections describe the details of the POST request
   and response to the authz-info endpoint between client and RS.  The
   client generates a nonce N1 and posts it together with the token that
   includes the materials provisioned by the AS to the RS.  The RS then
   derives a nonce N2 and use Section 3.2 of

nits: "provisioned" again, and the RS is going to be generating a nonce
from scratch, not deriving one from ... anything else, really.

   [I-D.ietf-core-object-security] to derive a security context based on
   a shared master secret and the two nonces, established between client
   and server.

(The shared master secret is part of what the AS provisions^Wgives to
the RS, right?)

   Note that the proof-of-possession required to bind the access token
   to the client is implicitly performed by generating the shared OSCORE
   Security Context using the pop-key as master secret, for both client
   and RS.  An attacker using a stolen token will not be able to
   generate a valid OSCORE context and thus not be able to prove
   possession of the pop-key.

It may be worth saying something roughly equating "proof of possesion
performed by the RS" and "the RS authenticating itself to the client",
since there is not an external action for server authentication.
Also, ISTR getting some pushback on a different document that talked
about an attacker with a "stolen token", as something that can steal a
token from the client's local storage might also steal the associated
key.  I forget if we ended up changing to some other phrasing or not,
though :(

Section 4.1

   The client MUST generate a nonce N1 very unlikely to have been
   previously used with the same input keying material.  This profile
   RECOMMENDS to use a 64-bit long random number as nonce.  The client

Why 64 instead of 128?  I know that 128 is enough without having to
think about it; for 64 we have to be clear about what role the nonce is
serving.

   Note that the use of the payload and the Content-Format is different
   from what described in section 5.8.1 of [I-D.ietf-ace-oauth-authz],
   which only transports the token without any CBOR wrapping.  In this
   profile, the client MUST wrap the token and N1 in a CBOR map.  The
   client MUST use the Content-Format "application/ace+cbor" defined in
   section 8.14 of [I-D.ietf-ace-oauth-authz].  The client MUST include
   the access token using the correct CBOR label (e.g., "cwt" for CWT,
   "jwt" for JWT) and N1 using the 'cnonce' parameter defined in section
   5.1.2 of [I-D.ietf-ace-oauth-authz].

That's not what the 'cnonce' parameter is for -- 'cnonce' is protecting
the RS against replay, but N1 is protecting the client.  I think we need
to use a different name (and maybe define some integer map label
values?) for this authz-info POST body.

   The access token MUST be encrypted, since it is transferred from the
   client to the RS over an unprotected channel.

This is probably worth stating in Section 3.2 as well, where we discuss
the AS-to-C token response in more detail.

   Note that a client may be required to re-POST the access token, since
   an RS may delete a stored access token, due to lack of memory.

I'd suggest using the phrase "at any time" and noting that the client
will detect this by receiving an AS Request Creation Hints message since
the RS treated the request as an Unauthorized Resource Request, perhaps:

%  Note that a client may be required to re-POST the access token in
%  order to complete a request, since an RS may delete a stored access
%  token at any time, for example due to all storage space being
%  consumed.  This situation is detected by the client when it receives
%  an AS Request Creation Hints response.

         Payload:
           {
             "access_token": h'a5037674656d7053656e73 ...'
          (remainder of access token omitted for brevity)',
             "cnonce": h'018a278f7faab55a'
           }

This does not appear to be using either a "cwt" label or "jwt" label, as
the second paragraph of the section would have us using.

Section 4.2

   token.  If the token is valid, the RS MUST respond to the POST
   request with 2.01 (Created).  If the token is valid but is associated

We probably don't need to repeat the normative requirements from
the core framework with normative keywords again.

   to claims that the RS cannot process (e.g., an unknown scope), or if
   any of the expected parameters in the OSCORE_Security_Context is

Hmm, "associated to claims that the RS cannot process" sounds like we're
telling the RS to abort if the token includes any unrecognized claims,
which IIUC is at odds with the OAuth 2.0 philosophy of ignoring
unrecognized parameters.  (Hopefully someone will correct me if I'm
misremembering that.)

   missing (e.g. any of the mandatory parameters from the AS), or if any

nit: comma after "e.g.".

   Additionally, the RS MUST generate a nonce N2 very unlikely to have
   been previously used with the same input keying material, and send it
   within the 2.01 (Created) response.  The payload of the 2.01
   (Created) response MUST be a CBOR map containing the 'cnonce'
   parameter defined in section 5.1.2 of [I-D.ietf-ace-oauth-authz], set
   to N2.  This profile RECOMMENDS to use a 64-bit long random number as
   nonce.  Moreover, if the OSCORE_Security_Context in the token did not

As above, we need to use a different name than 'cnonce', as it's
performing a different role (this time, contributing entropy to the
master secret).  (Also, if we change our recommendation about nonce
length above we should do so here as well, of course.)

   contain a 'clientId' parameter, the RS MUST generate an identifier,
   unique in the set of all its existing client identifiers, and send it
   in a 'clientId' parameter in the CBOR map as a CBOR bstr.  The RS MAY
   generate and send a 'ClientId' identifier even though the
   OSCORE_Security_Context contained such a parameter, in order to
   guarantee the uniqueness of the client identifier.  The RS MUST use

I'm not sure I understand the case where the AS wouldn't be able to
guarantee uniqueness of ClientId, and wonder if we could simplify by
always having the AS make ID assignment.  Is there some more
background/usage scenarios here that I'm missing?

   When receiving an updated access token with updated authorization
   information from the client (see section Section 3.1), it is
   RECOMMENDED that the RS overwrites the previous token, that is only
   the latest authorization information in the token received by the RS
   is valid.  This simplifies for the RS to keep track of authorization
   information for a given client.

[this same recommendation is already in the core framework]

Section 4.3

   the 'clientId' in the CBOR map in the payload of the response.  Then,
   the client MUST set the Master Salt of the Security Context created
   to communicate with the RS to the concatenation of salt, N1, and N2,
   in this order: Master Salt = salt | N1 | N2, where | denotes byte
   string concatenation, and where salt was received from the AS in
   Section 3.2.  The client MUST set the Master Secret and Recipient ID

I think it's general best practice to use an injective mapping of the inputs
to the Master Salt value (i.e., length-prefixes for the various fields, since
there are no reserved octet values).

   from the parameters received from the AS in Section 3.2.  The client
   MUST set the AEAD Algorithm, ID Context, HKDF, and Replay Window from
   the parameters received from the AS in Section 3.2, if present.  In
   case these parameters are omitted, the default values are used as
   described in section 3.2 of [I-D.ietf-core-object-security].  The

Ah, okay; I'd asked about default values earlier.  That said, if stock OSCORE
ever changes the defaults we'd have to come up with a compatibility story for
communication between old/new nodes.

   client MUST set the Sender ID from the 'clientId in the 2.01
   (Created) response, if present; otherwise, the client MUST set the
   Sender ID from the parameters received from the AS in Section 3.2.

[As written, this has the clientId from the RS take precedence over the one
from the AS when both are present.  I understand that the claimed
justification is for the RS to be able to enforce clientId uniqueness, but it
still feels risky, since the value from the RS is unauthenticated.]

   If any of the expected parameters is missing (e.g. any of the
   mandatory parameters from the AS, or the 'clientId', either received
   from the AS or in the 2.01 (Created) response from the RS), the
   client MUST stop the exchange, and MUST NOT derive the Security
   Context.  The client MAY restart the exchange, to get the correct
   security material.

I think this means "restart at the AS" but it's probably worth being very
explicit about it.
Also, is there anything to be said about rate-limiting/backoff for persistent
missing parameters?

   After sending the 2.01 (Created) response, the RS MUST set the Master
   Salt of the Security Context created to communicate with the client
   to the concatenation of salt, N1, and N2, in this order: Master Salt
   = salt | N1 | N2, where | denotes byte string concatenation, and

[It might be worth breaking this computation off into a standalone piece of
text that applies to both C and RS, including the edits for injectivity.]

   where salt was received from the AS in Section 4.2.  The RS MUST set
   the Master Secret, Sender ID and Recipient ID from the parameters,
   received from the client in the access token in Section 4.1 after

nit: I'd place the emphasis more on "in the access token" or even "from the
AS" rather than "from the client" -- the RS/AS preestablished relationship is
what authenticates these values.

   validation of the token as specified in Section 4.2.  The RS MUST set

Does Section 4.2 really say much about validation other than "RS MUST verify
the validity of the token"?

   [I-D.ietf-core-object-security].  After that, the RS MUST derive the
   complete Security Context following section 3.2.1 of
   [I-D.ietf-core-object-security], and MUST associate this Security
   Context with the authorization information from the access token.

side note: it's sometimes useful as a rhetorical device to have a discussion
of the abstract data model the RS uses -- in this case, that it maintains a
collection of Security Contexts with associated authorization information,
for all the clients that it's currently communicating with, and that the
authorization information is policy that's used as input to processing
requests from those clients.  Otherwise, this text about "MUST associate" can
be a bit vague to the reader.

   The RS then uses this Security Context to verify the request and send
   responses to C using OSCORE.  If OSCORE verification fails, error

(nit?) what is "the request"?  I don't think we've talked about a specific
OSCORE request yet, so this would probably be better if talking about generic
incoming requests from a given client.

   responses are used, as specified in section 8 of
   [I-D.ietf-core-object-security].  Additionally, if OSCORE
   verification succeeds, the verification of access rights is performed
   as described in section Section 4.4.  The RS MUST NOT use the
   Security Context after the related token has expired, and MUST
   respond with a unprotected 4.01 (Unauthorized) error message.

nit: I suggest "MUST respond with an unprotected [...] error message to
requests received that correspond to a security context with an expired
token" to tie it back to which requests are affected.

   If the exchange was an update of access rights, i.e. a new Security

nit: comma after "i.e.".

   Context was derived from a client that already had a Security Context
   in place, the is RECOMMENDED to delete the old Security Context after
   OSCORE verification and verification of access rights succeed.  The
   RS MUST delete the Security Context if it deletes the access token
   associated to it.

I'm not sure I understand what the workflow is, here -- I assume that the
deletion of the old security context occurs after a successful update via
the authz-info endpoint (as opposed to some particular OSCORE-protected
request), but I don't know what "verification of access rights" could be
performed at that time.  Aren't access rights mostly tied to a specific
operation, or is this just supposed to be a more generic audience/scope
check?

Section 4.4

   The RS MUST follow the procedures defined in section 5.8.2 of
   [I-D.ietf-ace-oauth-authz]: if an RS receives an OSCORE-protected
   request from a client, then the RS processes it according to
   [I-D.ietf-core-object-security].  If OSCORE verification succeeds,
   and the target resource requires authorization, the RS retrieves the

nit(?): is it appropriate to say "requires additional authorization" (over
the implicit authorization given to any authenticated client)?  I'm not sure
if that would be confusing or not.

   authorization information from the access token associated to the
   Security Context.  The RS then MUST verify that the authorization
   information covers the resource and the action requested.

In the vein of my previous note about an abstract data model, we may not want
to be overly prescriptive about "retrieves [...] from the access token".
Per-request introspection is allowed, as is caching the authorization
information in a non-token local datastore.
I'd also consider rephrasing to "MUST verify that the resource and action
requested are authorized", since "covers" is perhaps a little vague.

   The response code MUST be 4.01 (Unauthorized) in case the client has
   not used the Security Context associated with the access token, or if
   RS has no valid access token for the client.  If RS has an access

The causality here seems a bit weird; how would the RS know what token was
intended just from an OSCORE request that's using a random or different
security context?

   token for the client but not for the resource that was requested, RS
   MUST reject the request with a 4.03 (Forbidden).  If RS has an access

I'd suggest rephrasing this to be in terms of authorization rather than
"token for X", perhaps as "if the RS has an access token for the client but
no actions are authorized on the target resource"

   token for the client but it does not cover the action that was
   requested on the resource, RS MUST reject the request with a 4.05
   (Method Not Allowed).

Similarly, perhaps "but the requested action is not authorized".

Also, don't both of these descriptions have high overlap with Section 5.8.2
of the core framework?  Is it necessary to reiterate those behavior
specifications?

Section 5

Thank you for clearly stating that the OSCORE security contexts between RS/AS
and C/AS must be established independently of any of the mechanisms defined
in this document, and that the protocols used for the C/RS, C/AS, and RS/AS
steps can be selected independently.

   Furthermore the requesting entity and the AS communicate using OSCORE
   ([I-D.ietf-core-object-security]) through the introspection endpoint
   as specified in section 5.7 of [I-D.ietf-ace-oauth-authz] and through
   the token endpoint as specified in section 5.6 of
   [I-D.ietf-ace-oauth-authz].

I'm not entirely sure what this sentence is trying to say -- it comes across
a little bit as saying that when this document is in use, the indicated
interactions also use OSCORE, but that of course can't be what's actually
intended.  Perhaps just saying that the behavior of the introspection
endpoing is specified in section 5.7 of [framework] and that of the token
endpoint is specified in section 5.6 of [framework] would suffice?

Section 6

   o  the client receives a number of 4.01 Unauthorized responses to
      OSCORE requests using the same security context.  The exact number
      needs to be specified by the application.

Is this a global counter or does it get reset by a successful OSCORE
response?

   o  the client receives a new nonce in the 2.01 (Created) response
      (see Section 4.2) to a POST request to the authz-info endpoint,
      when re-posting a non-expired token associated to the existing
      context.

Why is "non-expired" specifically relevant?

   The RS MUST discard the current security context associated with a
   client when:

   o  Sequence Number space ends.

   o  Access token associated with the context expires.

nit: we should make the grammar of the list elements parallel to the client's
list, so "the Sequence Number space ends." and "the access token associated
with the context expires".

Section 7

We talk about both nonces as needing to be "very unlikely to have been
previously used", and it might be worth trying to quantify that (e.g.,
"chance of inadvertent collision less than 2^-32").

As alluded to in my high-level point, we should have more discussion of
the consequences of the authz-info exchange being completely
unprotected.

   a secure binding between the request and the response(s).  Thus the
   basic OSCORE protocol is not intended for use in point-to-multipoint
   communication (e.g. multicast, publish-subscribe).  Implementers of

nit: comma after "e.g.".

   provoke re-use.  If that is not guaranteed, nodes are still
   susceptible to re-using AEAD nonces and keys, in case the Security
   Context is lost, and on-path attacker replay messages.

This text is talking about AEAD nonces, but the nonces (and 64-bit
recommendation) previously discussed are the nonces used in the Master Salt
(and thus Master Secret) calculation; we don't really talk about AEAD nonces
specifically anywhere in the document, other than to point out the risk of
(nonce, key) reuse.  IIUC, vulnerable nodes would need to reuse both the
N1 or N2 values defined in this spec as well as the OSCORE partial IV in
order to collide the AEAD (nonce, key) pair (noting that both key and common
IV are influenced by the Master Salt).  That said, if the N1 generation is
flawed it seems quite plausible that PIV generation is similarly flawed, so
we are right to document the concern.
Also, the grammar here is a little bit weird, so perhaps a rewording to:

%  provoke re-use.  If that is not guaranteed, nodes are
%  susceptible to re-use of AEAD (nonces, keys) pairs, especially since an on-path
%  attacker can cause the client to use an arbitrary nonce for Security
%  Context establishment.  Even partial loss of Security Context information
%  can allow an on-path attacker to replay messages.

though I'm not entirely sure I understand which replay scenarios you had in
mind.

   This profiles recommends that the RS maintains a single access token

nit: "This profile" singular.

   Also, tokens may contradict each other which may lead the server to
   enforce wrong permissions.  If one of the access tokens expires

Could you give me an example of the contradiction scenarios?  I see the
"common" case as being roughly "Token T1 gives access to the set X of
resources and token T2 gives access to a partially disjoint set Y of
resources", but I don't know that such a scenario would be considered a
"conflict".  To get a conflict I'd expect more like "token T1 indicates that
access to X is to be denied, but token T2 indicates that access to X is to be
allowed" with some sort of explicit negative ACL, but I don't think OAuth
really has a concept of negative ACL.

Section 8

There might be a few more privacy considerations worth mentioning:

- the token is sent in the clear to the authz-info endpoint, so if a single
  token is used with multiple audiences or RSes, a client that uses the same
  token from multiple locations can be tracked/correlated by the access
  token's value.
- the nonces exchanged in the authz-info exchange are also sent in the clear,
  so using random nonces is best for privacy (as opposed to, e.g., a counter,
  that might leak some information).
- the identifiers clientId/serverId that are created/assigned for this
  protocol have some privacy considerations; they overlap a lot with the core
  OSCORE considerations in terms of the potential privacy consequences but
  there might be a little more to say about guidelines for how the
  identifiers are generated.  If I'm reading OSCORE correctly, for the uses
  specified in this document only the clientId will actually go on the wire?
- similarly, there may be considerations for the ID context values.

Section 9.1

   o  Profile Description: Profile for using OSCORE to secure
      communication between constrained nodes using the Authentication
      and Authorization for Constrained Environments framework.

This is the registry for ACE profiles; "using the Authentication and
Authorization for Constrained Envirornments framework" feels redundant.
There's also perhaps something to say about defining "coap_oscore" for all
three interaction flows even though the procedures for two of the three are
essentially just "out of the scope of this specification", though having
*some* identifier for "use OSCORE" is probably still useful.

Section 9.2

   name  The JSON name requested (e.g., "ms").  Because a core goal of
      this specification is for the resulting representations to be
      compact, it is RECOMMENDED that the name be short.  This name is
      case sensitive.  Names may not match other registered names in a
      case-insensitive manner unless the Designated Experts state that
      there is a compelling reason to allow an exception.  The name is

nit: I'd suggest s/state/determine/

   CBOR label  The value to be used to identify this algorithm.  Key map
      labels MUST be unique.  The label can be a positive integer, a

nit: s/Key map/Map key/

      negative integer or a string.  Integer values between 0 and 255
      and strings of length 1 are designated as Standards Track Document
      required.  Integer values from 256 to 65535 and strings of length
      2 are designated as Specification Required.  Integer values of
      greater than 65535 and strings of length greater than 2 are
      designated as expert review.  Integer values less than -65536 are
      marked as private use.

So use of negative integers from -65535 to -1 and positive integers greater
than or equal to 65536 is not defined?


I'd also consider listing [I-D.ietf-core-object-security] as an
additional reference for the initial entries, since that specifies the
actual semantics of the fields in question.

Section 9.3

"OSCORE_Security_Context" feels like a somewhat long name in light of the
previous commentary about shorter names being preferred. :)

   o  Confirmation Method Description: OSCORE_Security_Context carrying
      the OSCORE Security Context parameters

I'd suggest rewording the description to "parameters for using OSCORE
per-message security with implicit key confirmation"

Section 9.4

[same suggested rewording]

Section 9.5

Thank you for indicating in which direction the various considerations should
sway the experts' decisions, as opposed to just what the experts should
consider. :)

   The IANA registry established in this document is defined as expert
   review.  This section gives some general guidelines for what the

nit: I suggest "defined to use the Expert Review registration policy".

      The zones tagged as private use are intended for testing purposes
      and closed environments, code points in other ranges should not be
      assigned for testing.

nit: comma splice

   o  Specifications are required for the standards track range of point
      assignment.  Specifications should exist for specification
      required ranges, but early assignment before a specification is
      available is considered to be permissible.  Specifications are

side note: this is a little awkward, as RFC 7120 formally does not allow
"early allocations" for the "Expert Review" policy, so there is not a formal
procedure in place to review and expire early allocations that end up not
being needed.  It might be worth something to chat with the IANA table about
in Vancouver.

Section 10.2

We might get some sticklers asking for the terminology references to be made
normative, but I'm not concerned about it one way or the other for now.

Appendix A

Thank you for assembling this in list form for me.
Do we expect to keep this content in the final RFC?  If we do, we should
resynchronize to the current list in Appendix C of the framework :)

   o  how/if the authz-info endpoint is protected: Security protocol
      above

I thought authz-info was fully unprotected, per Section 4.1's "The authz-info
endpoint is not protected, nor are the responses from this resource."

Thanks,

Ben