[Gen-art] Gen-ART Last Call review of draft-ietf-ace-dtls-authorize-12

[Paul Kyzivat <[e-mail]>]

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Document: draft-ietf-ace-dtls-authorize-12
Reviewer: Paul Kyzivat
Review Date: 2020-07-19
IETF LC End Date: 2020-07-20
IESG Telechat date: ?

Summary:

This draft is on the right track but has open issues, described in the 
review.

General:

TBD

Issues:

Major: 2
Minor: 1
Nits:  1

1) MAJOR: Management of token storage in RS

There seems to be an expectation that when the client uploads an access 
token that the RS will retain it until the client attempts to establish 
a DTLS association. This seems to require some sort of management of 
token lifetime in the RS. The only discussion I can find of this issue 
is the following in section 7:

    ... A similar issue exists with the
    unprotected authorization information endpoint where the resource
    server needs to keep valid access tokens until their expiry.
    Adversaries can fill up the constrained resource server's internal
    storage for a very long time with interjected or otherwise retrieved
    valid access tokens.

This seems to imply a normative requirement to keep tokens until their 
expiry. But I find no supporting normative requirements about this. And, 
this section only presents it as a DoS attack, rather than a potential 
problem with valid usage.

ISTM that there is an implied requirement that the RS have the capacity 
to store one access token for every PoP key of every authorized client. 
If so, that ought to be stated. If not, then some other way of bounding 
storage ought to be discussed.

2) MAJOR: Missing normative language

I found several places where the text seems to suggest required behavior 
but fails to do so using normative language:

* In section 3.3:

    ... Instead of
    providing the keying material in the access token, the authorization
    server includes the key identifier in the "kid" parameter, see
    Figure 7.  This key identifier enables the resource server to
    calculate the symmetric key used for the communication with the
    client using the key derivation key and a KDF to be defined by the
    application, for example HKDF-SHA-256.  The key identifier picked by
    the authorization server needs to be unique for each access token
    where a unique symmetric key is required.
    ...
    Use of a unique (per resource server) "kid" and the use of a key
    derivation IKM that is unique per authorization server/resource
    server pair as specified above will ensure that the derived key is
    not shared across multiple clients.

The uniqueness seems to be a requirement. Perhaps "needs to be unique" 
should be "MUST be unique". (And something similar for the IKM.)

* Also in section 3.3:

    All CBOR data types are encoded in CBOR using preferred serialization
    and deterministic encoding as specified in Section 4 of
    [I-D.ietf-cbor-7049bis].  This implies in particular that the "type"
    and "L" components use the minimum length encoding.  The content of
    the "access_token" field is treated as opaque data for the purpose of
    key derivation.

IIUC the type of serialization and encoding is a requirement. Will need 
some rewording to make it so.

* In section 3.3.1:

    ... To
    be consistent with the recommendations in [RFC7252] a client is
    expected to offer at least the ciphersuite TLS_PSK_WITH_AES_128_CCM_8
    [RFC6655] to the resource server.

I think "is expected" should be "MUST".

* Also in section 3.3.1:

    ... This
    specification assumes that the access token is a PoP token as
    described in [I-D.ietf-ace-oauth-authz] unless specifically stated
    otherwise.

I think "assumes ... unless" should be "MUST ... unless".

* Also in section 3.3.1:

    ... New access tokens issued by the
    authorization server are supposed to replace previously issued access
    tokens for the respective client.

Is this normative? Should "are supposed to" be "MUST"?

3) MINOR: Insufficient specification

(I'm waffling whether this is minor or major.)

There are a couple of places where what seem to be requirements are 
stated too vaguely to be implemented consistently:

* In the previously mentioned paragraph in 3.3.1:

    ... This
    specification assumes that the access token is a PoP token as
    described in [I-D.ietf-ace-oauth-authz] unless specifically stated
    otherwise.

The "unless specifically stated otherwise" is too vague to be normative. 
How would the alternative be indicated? Is this an escape hatch for 
future extensions? If so, it needs more work to make that clear and to 
open a path for that future work.

* Also in section 3.3.1:

    ... The resource server therefore must
    have a common understanding with the authorization server how access
    tokens are ordered.

The last statement ("must have a common understanding") is mysterious. 
IIUC section 4 is covering the same topic in a less mysterious way.

4) NIT: Subsection organization

Both 3.2 and 3.3 share a common structure:

* The section begins with discussion of the interaction between the 
client and the AS.

* it is followed by a subsection discussing the interaction between the 
client and the RS.

It is odd to have a section with a single subsection. And the structure 
isn't easily discerned from the TOC.

I suggest it would be clearer if each of these sections had *two* 
subsections, one covering the AS interactions and the other the RS 
interactions. IOW, put the material covering the AS interactions into a 
new subsection.