Re: [OAUTH-WG] New Version Notification for draft-fett-oauth-dpop-03.txt

[Neil Madden <[e-mail]>]

I can't attend Singapore either in person or remotely due to other commitments. I broadly support adoption of this draft, but I have some comments/suggestions about it.

Section 2 lists the main objective as being to harden against compromised/malicious AS or RS, which may attempt to replay captured tokens elsewhere. While this is a good idea, a casual reader might wonder why a simple audience claim in the access token/introspection response is not sufficient to prevent this. Because interactions between the client and RS are supposed to be over TLS, is the intended threat model one in which these protections have broken down? ("counterfeit" in the description suggests this). Or is the motivation that clients want to get a single broad-scoped access token (for usability/performance reasons) and use it to access multiple resource servers without giving each of them the ability to replay the token to the other servers? Or are we thinking of a phishing-type vulnerability were a general-purpose client might accidentally visit a malicious site which prompts for an access token that the client then blindly goes off and gets? (UMA?) It's not clear to me which of these scenarios is being considered, so it would be good to tighten up this section.

Another potential motivation is for mobile apps. Some customers of ours would like to tie access/refresh tokens to private key material generated on a secure element in the device, that can only be accessed after local biometric authentication (e.g. TouchID/FaceID on iOS). I have suggested using mTLS cert-bound tokens for this, but have heard some pushback due to the difficulty of configuring support for client certs across diverse infrastructure. A simple JWT-based solution like DPoP could fill this need.

My main concerns with the draft though are about efficiency and scalability of the proposed approach:

1. The requirement to use public key signatures, along with the anti-replay nonce, means that the RS is required to perform an expensive signature verification check on every request. That is not going to scale up well. While there are more efficient schemes like Ed25519 now, these are still typically an order of magnitude slower than HMAC and the latency and CPU overhead is likely to be a non-starter for many APIs (especially when you're billed by CPU usage). Public key signatures are also notoriously fragile (see e.g. the history of nonce reuse/leakage vulnerabilities in ECDSA or 

2. The advice for the RS to store a set of previously used nonces to prevent replay will also hamper scalability, especially in large deployments where such state would need to be replicated to all servers (or use sticky load balancing, which comes with its own problems). This violates the statelessness of HTTP, and it also potentially breaks idempotency of operations: Think of the case where the JWT validation and replay protection is done at an API gateway but then the call to the backend API server fails for a transient reason. The client (or a proxy/library) cannot simply replay the (idempotent) request in this case because it will be rejected by the gateway. It must instead recreate the DPoP JWT, incurring additional overheads.

3. Minor: The use of a custom header for communicating the DPoP proof will require additional CORS configuration on top of that already done for the Authorization header, and so adds a small amount of additional friction for adoption. Given that CORS configuration changes often require approval by a security team, this may make more of an impact than you'd expect.

It's also not clear to me exactly what threat the anti-replay nonce is protecting against. It does nothing against the replay scenario discussed in section 2, as I understand it - which really seems to be more of a MitM scenario. Given that the connection between the client and the RS is supposed to be over TLS, and TLS is already protected against replay attacks, I think this part needs to be better motivated given the obvious costs of implementing it.

I have a tentative suggestion for an alternative design which avoids these problems, but at a cost of potentially more complexity elsewhere. I'll summarise it here for consideration:

1. The client obtains an access token in the normal way. When calling the token endpoint it provides an EC/okp public key as the confirmation key to be associated with the access/refresh tokens.

2. The first time the client calls an RS it passes its access token in the Authorization: Bearer header as normal. (If the RS doesn't support DPoP then this would just succeed and no further action is required by the client - allowing clients to opportunistically ask for DPoP without needing a priori knowledge of RS capabilities).

3. The RS introspects the access token and learns the EC public key associated with the access token. As there is no DPoP proof with the access token, the RS will generate a challenge in the following way:
    o The RS generates an ephemeral EC key pair for the same curve as the confirmation key (e.g. P-256 or X25519).
    o The RS stores the ephemeral private key somewhere, associated with this access token (see below for a scalable implementation choice)
    o The RS encodes the ephemeral public key into a JWK (epk) and base64url-encodes it. It uses this as a challenge to the client by sending back a 401 response with WWW-Authenticate: DPoP <encoded-epk>

4. The client decodes the epk challenge and performs an ECDH key agreement between its private key and the challenge epk as per the method described for the existing JWA ECDH-ES encryption algorithm. Rather than deriving an AES key however, it derives a HMAC key for HS256. The "apu" value is set to the access token (string value as ASCII bytes) and the "apv" value is set to the hostname of the RS (e.g. "api.example.com <http://api.example.com/>"). This ensures that the derived key is cryptographically bound to the context in which it is used.

5. The client uses the HMAC key to create a DPoP proof JWT much like the one in the current draft, but signed using the HS256 key. If a "kid" field was present in the challenge JWK sent by the RS then the same value MUST be used in the "kid" header of this discharge JWT. It retries its original request sending Authorization: DPoP <hmac-jwt> at=<access_token>.

6. The RS uses its stored ephemeral private key to derive the same HMAC key and verify the DPoP discharge JWT. If it validates and all fields are correct then the request is allowed.

Efficient implementation trick:
Because the client is required to copy and "kid" value from the challenge JWK, the RS can preemptively carry out the ECDH key agreement immediately and generate the derived HMAC key. The RS can then encrypt this derived key using a local authenticated encryption key (e.g. AES-GCM) and use that encrypted value as the "kid" value in the challenge (perhaps along with some context or an expiry time). That way the RS only needs to decrypt this kid value rather than performing the ECDH key agreement on every request. This also avoids the need for the RS to store any per-client state locally.

The challenge-response nature of the scheme prevents traditional replay attacks in the case where a DPoP discharge JWT is accidentally leaked through server logs or some other flaw, without needing to store nonces on the server. Using the RS's hostname in the key derivation process prevents mitm attacks in a similar way to how FIDO/WebAuthn prevents this. Most importantly, once a HS256 key has been derived between a client and RS they can reuse that key for multiple requests, reducing the overhead of the ECDH key agreement step. Either side can decide as a matter of policy how long to let this occur and when to trigger a fresh challenge-response.

Because this fits within the standard HTTP authentication framework, it also requires no additional CORS configuration and is relatively easy to plug in to existing HTTP client libraries.

The main downside of this approach to me is the fact that you can't simply reuse an existing JWT library to implement it, and so it will take time for client libs to develop. (Although I think this might be achievable now with existing *COSE* libraries). This would increase the risk of people hand-rolling solutions, rather than using well-tested libraries. On the other hand, it uses fairly widely supported primitives so e.g. an implementation using WebCrypto is probably only a few dozen lines of code.

-- Neil


> On 31 Oct 2019, at 19:20, Brian Campbell <bcampbell=[e-mail]> wrote:
> 
> Hello WG, 
> 
> Just a quick note to let folks know that -03 of the DPoP draft was published earlier today. The usual various document links are in the forwarded message below and the relevant snippet from the doc history with a summary of the changes is included here for convenience. 
> 
> Hopefully folks will have time to read the (relativity) short document before the meeting(s) in Singapore where (spoiler alert) I plan to ask that the WG consider adoption of the draft. 
> 
> Thanks,
> 
>  -03
>    o  rework the text around uniqueness requirements on the jti claim in
>       the DPoP proof JWT
>    o  make tokens a bit smaller by using "htm", "htu", and "jkt" rather
>       than "http_method", "http_uri", and "jkt#S256" respectively
>    o  more explicit recommendation to use mTLS if that is available
>    o  added David Waite as co-author
>    o  editorial updates
> 
> ---------- Forwarded message ---------
> From: <[e-mail] <mailto:[e-mail]>>
> Date: Thu, Oct 31, 2019 at 11:53 AM
> Subject: New Version Notification for draft-fett-oauth-dpop-03.txt
> To: Torsten Lodderstedt <[e-mail] <mailto:[e-mail]>>, Michael Jones <[e-mail] <mailto:[e-mail]>>, John Bradley <[e-mail] <mailto:ve7jtb@ve7jtb..com>>, Brian Campbell <[e-mail] <mailto:[e-mail]>>, David Waite <[e-mail] <mailto:[e-mail]>>, Daniel Fett <[e-mail] <mailto:[e-mail]>>
> 
> 
> 
> A new version of I-D, draft-fett-oauth-dpop-03.txt
> has been successfully submitted by Brian Campbell and posted to the
> IETF repository.
> 
> Name:           draft-fett-oauth-dpop
> Revision:       03
> Title:          OAuth 2.0 Demonstration of Proof-of-Possession at the Application Layer (DPoP)
> Document date:  2019-10-30
> Group:          Individual Submission
> Pages:          15
> URL:            https://www.ietf.org/internet-drafts/draft-fett-oauth-dpop-03.txt <https://www.ietf.org/internet-drafts/draft-fett-oauth-dpop-03.txt>
> Status:         https://datatracker.ietf.org/doc/draft-fett-oauth-dpop/ <https://datatracker.ietf.org/doc/draft-fett-oauth-dpop/>
> Htmlized:       https://tools.ietf.org/html/draft-fett-oauth-dpop-03 <https://tools.ietf.org/html/draft-fett-oauth-dpop-03>
> Htmlized:       https://datatracker.ietf.org/doc/html/draft-fett-oauth-dpop <https://datatracker.ietf.org/doc/html/draft-fett-oauth-dpop>
> Diff:           https://www.ietf.org/rfcdiff?url2=draft-fett-oauth-dpop-03 <https://www.ietf.org/rfcdiff?url2=draft-fett-oauth-dpop-03>
> 
> Abstract:
>    This document describes a mechanism for sender-constraining OAuth 2.0
>    tokens via a proof-of-possession mechanism on the application level.
>    This mechanism allows for the detection of replay attacks with access
>    and refresh tokens.
> 
> 
> 
> 
> Please note that it may take a couple of minutes from the time of submission
> until the htmlized version and diff are available at tools.ietf.org <http://tools.ietf.org/>.
> 
> The IETF Secretariat
> 
> 
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