[Openid-specs-mobile-profile] [Openid-specs-fapi] Initial review of MODRNA Client initiated Backchannel Authentication Flow 1.0

Axel.Nennker at telekom.de Axel.Nennker at telekom.de
Fri May 26 13:21:32 UTC 2017

Hi Dave,

thanks for the review!

The latest editor version in HTML of CIBA is always at this location:
The raw xml2rfc version is here: https://bitbucket.org/openid/mobile/raw/tip/draft-mobile-client-initiated-backchannel-authentication.xml

I created an issue regarding the Terminology Consumption Device:

Typo fixed by https://bitbucket.org/openid/mobile/commits/be7ab6b50447b82dd7d77f4fc2044c101be44069

Regarding Client Authentication:
I see CIBA as a generic spec introducing back-channel to MODRNA Authentication or OIDC.
So FAPI should reference CIBA if back-channel is used in FAPI AND profile CIBA to FAPI’s higher security needs
e.g. by restricting the Client Authentication Methods.
We currently RECOMMEND a signed request object for request authentication while prohibiting alg=none.
The rationale is this: OAuth2 was specified as a simple authorization framework with client_id and client_secret as a simple form of client authentication.
This was because developers and admins never managed to get the Liberty Alliance implementation right.
So CIBA allows client_id and client_secret but use cases for Banks and PSPs need better security and should profile CIBA for better Client Authentication.

Binding_message: Binding_message was introduced in MODRNA Authentication<https://xml2rfc.tools.ietf.org/cgi-bin/xml2rfc.cgi?Submit=Submit&format=ascii&mode=html&type=ascii&url=https://bitbucket.org/openid/mobile/raw/tip/openid-connect-modrna-authentication-1_0.xml?at=default#rfc.section.7> . Protects the user not the RP.
If you want it to be MANDATORY for payments then please profile MODRNA Authentication and CIBA and specify FAPI.
CIBA does not talk much about binding_message and binding_message potential use cases because CIBA is a quite generic protocol for back-channel authentication.
Payment and handling of binding_messages is out-of-scope for CIBA. If CIBA is used for payment transaction authorization then there probably should be a contract between the OP and the PSP specifying how the binding_message looks like. I like your petrol station use case that is exactly what CIBA is designed for. Also I am not sure what PSD2 actually mandates to be shown at the Authentication Device. The pump is showing the price and the binding_message “e.g. ‘Correct Horse Battery Staple’” and the binding_message is also displayed on the user’s AT to protect the user. Potential PIN entry is a separate issue. Reentering the binding_message at the pump does improve the pump’s assurance but might introduce an error prone UX. YMMW.

Regarding Token Request Validation:
I did not reference 3.1.3 because we have no redirect_url

Regarding the authentication of the OP at the Client Notification Endpoint:

Regarding signing results:

kind regards

From: Openid-specs-fapi [mailto:openid-specs-fapi-bounces at lists.openid.net] On Behalf Of Dave Tonge via Openid-specs-fapi
Sent: Freitag, 26. Mai 2017 12:59
To: Openid-specs Fapi <openid-specs-fapi at lists.openid.net>
Subject: [Openid-specs-fapi] Initial review of MODRNA Client initiated Backchannel Authentication Flow 1.0

Hi all,

We discussed on the last call that it would be good to review the MODRNA backchannel auth and user questioning specs from a FAPI perspective.

I've started to go through them both and here are my initial thoughts on the Backchannel Authentication spec<http://openid.net/specs/openid-connect-modrna-client-initiated-backchannel-authentication-1_0.html#rfc.section.4.1>:

2. Terminology

The definition for Consumption Device states that it is “most probably a browser”. It is envisaged that this will often not be the case, for payment APIs, e.g. the flow could be initiated via a POS device.

3. Overview

Spelling mistake - “inmediatly” -> “immediately”

4.1 Authentication Request

Client Authentication - this should be restricted to the methods listed in FAPI (i.e. private_key_jwt or MTLS)

Client_notification_token - this is a bearer token used by the OP when notifying the client via the `client_notification_endpoint`. In the FAPI spec we are moving away from bearer tokens to either sender contained tokens or OATB tokens. While this token is used for OP -> client rather than client -> OP should we still consider constraining the tokens?

A simpler approach may be to ensure that the OP signs any payload sent to the `client_notification_endpoint`. Currently, these payloads (error or success) are plain JSON payloads. The success payload contains an `id_token`, but this token contains claims about the identity of the user rather than acting as a detached signature for the access token (and refresh token)

Binding_message - as I understand it the binding message is shown to the user on both the consumption and the authentication device. The spec then relies on the user checking that both messages are the same. If the spec were to be used for payments then I would suggest that the binding message is shown on the consumption device and entered by the user on the authentication device. This would be more secure than relying on the user to cross-check both messages are the same.

For example, this spec could be used to support bank payments at a petrol station - the user could enter their phone number into the POS device, they would then receive a notification from their banking app asking them to authorise a payment. However what is to prevent someone entering the user’s phone number at the same time at the same petrol station when paying for fuel of the same amount. If the POS device displayed a 6 digit pin to the user, and the user entered that pin as part of the authorization flow in their banking app, then all parties would have a higher degree of confidence about the transaction.

The binding message should be required and not optional for payments.

4.2.1 Authentication Request Validation

The authentication request is a plain JSON payload, but because this is happening over a channel protected by client authentication this should be sufficient.

5. OpenID Provider Obtains End-user Consent/Authorization

The spec is focussed on user authentication rather than a user authorising a specific action, e.g. making a payment. The wording in this section could be confusing to those implementing it for a financial API.

In order to support fine-grained authorisation, the spec would need to support the OIDC claims parameter and guidance would need to be provided to implementers on example flows for payments and account information access.

6.1 Token Request Using Polling Mechanism

The spec should refer back to OIDC Core - 3.1.3 - token endpoint - as all the verification steps described there should be performed.

In addition, it should be made clear in the spec that the `auth_req_id` MUST be bound to the client to which it was issued. At the moment it would seem that any client could send any `auth_req_id`.

6.2 Successful Token Polling Response

To conform with FAPI, the tokens returned in this response should be bound to the client either using MTLS or OATB.

6.3.1 Succesful Token Notification

As per the notes on client_notification_token, I suggest that this payload is signed to ensure source authentication and integrity.

Other notes:

·         No mention of what happens if the client notification endpoint is down (e.g. retries)

  *   No mention of what acknowledgement the client should give to the OP when it receives a notification

I believe that this spec could be useful for Financial APIs, however, it is more coupled to OIDC than the current FAPI drafts. It could be that we have to draft a new part to the FAPI spec that references this spec, but is more in line with the rest of the FAPI draft and geared towards the use-case of financial APIs.

I have started a review of the user questioning API and will send that to the list shortly. However, on initial inspection, it doesn't result in access tokens being issued to the client and would, therefore, be unsuitable for "account information" access.

This was my first review of the MODRNA specs and I may well have misinterpreted some of the specs. I hope that members of this group or are also members of the MODRNA group will help to correct any mistakes I may have made.


Dave Tonge
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