[OAUTH-WG] Comments on ietf-oauth-dpop

[Rohan Mahy <[e-mail]>]

Hi,
Here are some comments on draft-ietf-oauth-dpop-06:

1) With such a significant attack possible as DPoP proof pre-generation,
why isn't using the server nonce a SHOULD? Preventing a significant attack
and making lifetime handling sane are two excellent reasons to use a server
nonce. If an implementation has a good reason to not use a server nonce, we
can give guidance about what additional steps the implementation needs to
take.

2) The handling of lifetimes of DPoP proofs is vague: "acceptable
timeframe" (Section 4.3), "relatively brief period" (Section 11.1). Is that
1 day,15 minutes, or 30 seconds?
The normative text in the two sections seem contradictory.
I think you need a lifetime parameter if a server nonce isn't included, or
just pick a number (5 minutes?).

3) I had a similar thought to Nicolas Mora about including other
assertions/tokens. There should be a way to chain, include, or reference
other OAuth assertions and bind them somehow with the DPoP. This will be a
common and important model.

4. Right now you describe the access token hash before describing the
access token itself. I think it would be very useful to show the a worked
example of an access token and then its hash used subsequently. Also
Section 4.3 step 11 feels like a circular description. Please rewrite more
verbosely to be clearer:
Currently:
"when presented to a protected resource in conjunction with an access
token, ensure that the value of the ath claim equals the hash of that
access token and confirm that the public key to which the access token is
bound matches the public key from the DPoP proof."

5. Re: IANA registration of the MIME type. TL;DR: Just register
application/dpop+jwt.
Long version: The semantics of the thing you want to register is
application/dpop. The first syntax you are defining is jwt. For example,
iCalendar has three formats: text/calendar (iCal),
application/calendar+json (jCal), and application/calendar+xml (xCal).

NITS:
- Spell out first use of acronyms: JWT, JWK, JWS, TLS, JOSE, PKCE,
- Add reference to TLS, XSS, Crime/Heartbleed/BREACH/etc., HTTP, JOSE, on
first use
- First sentence of Section 2 (Objectives): add a comma (access tokens_,_
by binding) to make it clear that "binding a token" is doing the preventing
instead of the stealing in the sentence.
- Section 2 para 5: s/XXS/XSS/
- Maybe mention why you are using ASCII (7-bit) when the charset in the
examples is UTF-8.

I hope these comments are useful.
Many thanks,
-rohan


*Rohan Mahy  *l  Vice President Engineering, Architecture

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