fapi-ciba July 2024
Tonge Standards Track [Page]
D. Tonge
Moneyhub Financial Technology

FAPI Client Initiated Backchannel Authentication Profile - draft


The OpenID Foundation (OIDF) promotes, protects and nurtures the OpenID community and technologies. As a non-profit international standardizing body, it is comprised by over 160 participating entities (workgroup participant). The work of preparing implementer drafts and final international standards is carried out through OIDF workgroups in accordance with the OpenID Process. Participants interested in a subject for which a workgroup has been established have the right to be represented in that workgroup. International organizations, governmental and non-governmental, in liaison with OIDF, also take part in the work. OIDF collaborates closely with other standardizing bodies in the related fields.

Final drafts adopted by the Workgroup through consensus are circulated publicly for the public review for 60 days and for the OIDF members for voting. Publication as an OIDF Standard requires approval by at least 50% of the members casting a vote. There is a possibility that some of the elements of this document may be subject to patent rights. OIDF shall not be held responsible for identifying any or all such patent rights.


The FAPI Working Group produces security profiles based on the OAuth 2.0 Authorization Framework and related specifications suitable for protecting APIs in high-value scenarios.

This document is a profile of the OpenID Connect Client Initiated Backchannel Authentication Flow [CIBA]. The CIBA spec allows a client that gains knowledge of an identifier for the user to obtain tokens from the authorization server. The user consent is given at the user's authentication device mediated by the authorization server. This document profiles the CIBA specification to bring it in line with FAPI Security Profiles and provides security recommendations for its use with APIs that require high levels of security.

This profile may be used with either:

  1. FAPI API Security Profile 1.0 - Parts 1 and 2 or
  2. FAPI 2.0 Security Profile


This document is not an OIDF International Standard. It is distributed for review and comment. It is subject to change without notice and may not be referred to as an International Standard.

Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they are aware and to provide supporting documentation.

Notational conventions

The keywords "shall", "shall not", "should", "should not", "may", and "can" in this document are to be interpreted as described in ISO Directive Part 2 [ISODIR2]. These keywords are not used as dictionary terms such that any occurrence of them shall be interpreted as keywords and are not to be interpreted with their natural language meanings.

Table of Contents

1. Scope

This document specifies the method for an application to:

2. Terms and definitions

For the purpose of this standard, the terms defined in RFC6749, RFC6750, RFC7636, OpenID Connect Core and OpenID Connect Client Initiated Backchannel Authentication Core apply.

3. Symbols and abbreviated terms

API – Application Programming Interface

CCTV – Closed Circuit Television

CIBA – Client Initiated Backchannel Authentication

HTTP – Hyper Text Transfer Protocol

HTTPS – Hypertext Transfer Protocol Secure

JSON – JavaScript Object Notation

JWE – JSON Web Encryption

JWS – JSON Web Signature

JWT – JSON Web Token

OIDF – OpenID Foundation

QR – Quick Response

POS – Point-of-Sale

REST – Representational State Transfer

TLS – Transport Layer Security

4. Client initiated backchannel authentication security profile

The standard OAuth method for the client to send the resource owner to the authorization server is to use an HTTP redirect. FAPI specification support this interaction model and are suitable for use cases where the resource owner is interacting with the client on a device they control that has a web browser. There are however many use-cases for initiating payments where the resource owner is not interacting with the client in such a manner. For example, the resource owner may want to authorize a payment at a "point of sale" terminal at a shop or fuel station.

The Client Initiated Backchannel Authentication Flow [CIBA] specifies an alternate method of users granting access to their resources whereby the flow is started from a consumption device, but authorized on an authentication device.

The following sections specify a profile of CIBA that is suited for high-value transactions and sensitive (personal and other) data.

4.1. Client initiated backchannel authentication security provisions

When this profile is used with the FAPI 1.0 specifications then it should be read in conjunction with OpenID Connect Client Initiated Backchannel Authentication Core [CIBA] and with parts 1 [FAPI1.1] and 2 [FAPI1.2] of the FAPI 1.0 Specifications.

When this profile is used with the FAPI 2.0 specifications then it should be read in conjunction with OpenID Connect Client Initiated Backchannel Authentication Core [CIBA] and the FAPI 2.0 Security Profile [FAPI2].

4.1.1. Authorization server

When this profile is used with the FAPI 1.0 specifications, the authorization server shall support the provisions specified in clause 5.2.2 of [FAPI1.1] and [FAPI1.2].

When this profile is used with the FAPI 2.0 specifications, the authorization server shall support the general requirements for authorization servers listed in clause of [FAPI2].

In addition the authorization server

  1. shall only support confidential clients for client initiated backchannel authentication flows;
  2. shall ensure unique authorization context exists in the authentication request or require a binding_message in the authentication request;
  3. shall not support CIBA push mode;
  4. shall support CIBA poll mode;
  5. may support CIBA ping mode;
  6. shall, if it supports the acr claim and the client has requested acr, return an 'acr' claim in the resulting ID token;
  7. should not use the login_hint or login_hint_token to convey "intent ids" or any other authorization metadata; and
  8. may require clients to provide a request_context claim as defined in Section 4.3 of this profile.

When this profile is used with the FAPI 1.0 specifications, the authorization server

  1. shall support unsigned and signed backchannel authentication endpoint requests as described in [CIBA] 7.1.1; and
  2. shall require the signed authentication request to contain nbf and exp claims that limit the lifetime of the request to no more than 60 minutes.

NOTE: As per [CIBA], login_hint, login_hint_token and id_token_hint are used only to determine who the user is. In scenarios where complex authorization parameters need to be conveyed from the client to the authorization server, implementers should consider using OAuth 2.0 Rich Authorization Requests [RAR]. The use of parameterized scope values or the use of an additional request parameter are both supported by this specification.

NOTE: The binding message is required to protect the user by binding the session on the consumption device with the session on the authentication device. An example use case is when a user is paying at POS terminal. The user will enter their user identifier to start the [CIBA] flow, the terminal will then display a code, the user will receive a notification on their phone (the authentication device) to ask them to authenticate and authorize the transaction, as part of the authorization process the user will be shown a code and will be asked to check that it is the same as the one shown on the terminal.

NOTE: The FAPI CIBA profile only supports CIBA ping and poll modes, therefore it is only possible to retrieve access tokens and optionally refresh tokens from the token endpoint.

NOTE: While the format of the login_hint and login_hint_token parameters are not defined by [CIBA] or this profile, implementers may wish to consider https://tools.ietf.org/html/draft-ietf-secevent-subject-identifiers for a standards based method of communicating user identifiers.

4.1.2. Confidential client

When this profile is used with the FAPI 1.0 specifications, a confidential client shall support the provisions specified in clause 5.2.4 of [FAPI1.1] and [FAPI1.2].

When this profile is used with the FAPI 2.0 specifications, a confidential client shall support the general requirements for clients listed in clause of [FAPI2].

In addition, the confidential client

  1. shall ensure sufficient authorization context exists in authorization request or shall include a binding_message in the authentication request; and

When this profile is used with the FAPI 1.0 specifications, the confidential client

  1. should only send signed authentication requests as defined in [CIBA] 7.1.1 to the backchannel authentication endpoint.

##NOTE:## When used with FAPI 2.0, both signed and unsigned requests are supported.

4.1.3. Extensions to CIBA authentication request

This profile defines the following extensions to the authentication request defined in [CIBA] Section 7.1.

  1. request_context: OPTIONAL. a JSON object (the contents of which are not defined by this specification) containing information to inform fraud and threat decisions. For example, an ecosystem may require relying parties to provide geolocation for the consumption device.

4.2. Accessing protected resources

The benefit of the CIBA specification is that once tokens are issued they can be used in the same manner as tokens issued via authorization code flows.

When this profile is used with the FAPI 1.0 specifications, the provisions for accessing protected resources detailed in [FAPI1.1] and [FAPI1.2] apply fully.

When this profile is used with the FAPI 2.0 specifications, the provisions for accessing protected resources detailed in [FAPI2] apply fully.

4.2.1. Client provisions

In situations where the client does not control the consumption device, the client

  1. shall not send x-fapi-customer-ip-address or x-fapi-auth-date headers; and
  2. should send metadata about the consumption device, for example geolocation and device type.

4.3. Registration and discovery metadata

This specification adds additional metadata parameters for OAuth authorization server metadata as defined in [RFC8414] and dynamic client registration metadata as defined in [RFC7591].

OAuth authorization server metadata:

  1. backchannel_endpoint_login_hint_token_types_supported: OPTIONAL. JSON array of strings that the authorization server can use to advertise which types of login_hint_token it supports. The values in this parameter are likely to be ecosystem specific.

Dynamic client registration metadata:

  1. backchannel_endpoint_login_hint_token_types: OPTIONAL. JSON array of strings that the client can use to register the type of login_hint_token that it will use.

5. Security considerations

5.1. Introduction

The [CIBA] specification introduces some new attack vectors not present in OAuth 2 redirect based flows. This profile aims to help implementers of [CIBA] for higher security needs to reduce or eliminate these attack vectors. There are however further security considerations that should be taken into account when implementing this specification.

It is strongly recommended to use OAuth 2 redirect based flows secured by [FAPI1.1], [FAPI1.2] or [FAPI2] for the same device use cases because decoupled flows suffer from session binding limitations that are not present in redirect flows. This specification is designed to cater for cross device and user not present scenarios.

5.3. Reliance on user to confirm binding messages

Depending on the hint used to identify the user and the client's user authentication processes, it may be possible for a fraudster to start a malicious [CIBA] flow at the same time as a genuine flow, with both flows using the genuine user’s identifier. If the scope of access requested is similar then the only way to ensure that a user is authorizing the correct transaction is for the user to compare the binding messages on the authentication and consumption devices.

If this risk is deemed unacceptable then implementers should either consider alternative mechanisms of verifying the binding message (e.g. conveying it to the authentication device via a QR code), or use ephemeral user identifiers generated on the authentication device.

5.4. Loss of fraud markers to authorization server

In a redirect-based flow, the authorization server can collect useful fraud markers from the user-agent. In a [CIBA] flow the separation of consumption and authentication devices reduces the data that can be collected. This could reduce the effectiveness of any fraud detection system.

5.5. Incomplete or incorrect implementations of the specifications

To achieve the full security benefits, it is important the implementation of this specification, and the underlying OpenID Connect and OAuth 2.0 specifications, are both complete and correct.

The OpenID Foundation provides tools that can be used to confirm that an implementation is correct:


The OpenID Foundation maintains a list of certified implementations:


Deployments that use this specification should use a certified implementation.

5.6. JWS/JWE Algorithm considerations

When this profile is used with the FAPI 1.0 specifications, authorization servers and clients shall follow the guidance around JWT signing and encryption algorithms in [FAPI1.2] 8.6 and 8.6.1.

When this profile is used with the FAPI 2.0 specifications, authorization servers and clients shall follow the guidance around cryptography and secrets in [FAPI2] 5.4.

5.7. Authentication device security

This profile and the underlying specifications do not specify how the authorization server should initiate and perform user authentication and authorization of consent on the authentication device.

Implementors must use appropriately strong methods to communicate with the authentication device and to authenticate the end user.

5.8. CIBA token delivery modes

[CIBA] defines 3 ways that tokens can be delivered to the client.

The push mode is not permitted by this specification as it delivers tokens to the client by calling an endpoint owned by the client. This substantially differs from the established pattern of retrieving tokens by presenting client authentication to the token endpoint, and it may have security concerns that are currently unknown.

The poll and ping modes both follow the established convention of retrieving tokens from the token endpoint and hence do not have this concern.

The ping mode delivers a notification to an endpoint owned by the client. The information contained in this notification is limited to the auth_req_id for the request, as described in [CIBA] 10.2. The bearer token used by the authorization server to access this resource is not sender constrained. If the backchannel_client_notification_endpoint, the auth_req_id and the client_notification_token are known to an attacker, they may be able to force the client to call the token endpoint repeatedly or before the authentication has completed. For most deployments this is not a significant issue.

5.9. TLS considerations

As confidential information is being exchanged, all interactions shall be encrypted with TLS (HTTPS).

The recommendations for Secure Use of Transport Layer Security in [BCP195] shall be followed, with the following additional requirements:

  1. TLS version 1.2 or later shall be used for all communications.
  2. A TLS server certificate check shall be performed, as per [RFC6125].
  3. For TLS versions below 1.3, only the following 4 cipher suites shall be permitted:

  4. When using the TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 or TLS_DHE_RSA_WITH_AES_256_GCM_SHA384 cipher suites, key lengths of at least 2048 bits are required.

5.10. Algorithm considerations

For JWS, both clients and authorization servers:

  1. shall use PS256 or ES256 algorithms;
  2. should not use algorithms that use RSASSA-PKCS1-v1_5 (e.g. RS256);
  3. shall not use none;

5.11. Encryption algorithm considerations

For JWE, both clients and authorization servers

  1. shall not use the RSA1_5 algorithm.

6. Privacy considerations

There are no additional privacy considerations beyond those in [CIBA] 15.

7. Acknowledgement

The following people contributed heavily towards this document:

8. IANA Considerations

8.1. OAuth authorization server metadata registration

This specification adds the following values to the IANA "OAuth Authorization Server Metadata" registry established by [RFC8414]:

  • Server Metadata Name: backchannel_endpoint_login_hint_token_types_supported
  • Server Metadata Description: Supported CIBA login hint token types.
  • Change Controller: OpenID Foundation Financial-Grade API Working Group - openid-specs-fapi@lists.openid.net
  • Specification Document(s): Section 7 of [[this specification]]

8.2. OAuth dynamic client registration metadata registration

This specification requests registration of the following client metadata definitions in the IANA "OAuth Dynamic Client Registration Metadata" registry established by [RFC7591]:

  • Client Metadata Name: backchannel_endpoint_login_hint_token_types
  • Client Metadata Description: The supported CIBA login hint token types that the client will use to initiate CIBA requests.
  • Change Controller: OpenID Foundation Financial-Grade API Working Group - openid-specs-fapi@lists.openid.net
  • Specification Document(s): Section 7 of [[this specification]]

9. Appendix A - Examples

The following are non-normative examples of the FAPI-CIBA requests and responses.

All examples use private_key_jwt client authentication with the following key:

  "kty": "EC",
  "d": "gM__X2faDsb4s6QLer9h-y4KzLIgwt5Jz2dJi5r64Pc",
  "use": "sig",
  "kid": "thrwqnuer",
  "crv": "P-256",
  "x": "YPczq3aBrd8PjtFsXX_HPZNwnzp89vAGjgQXm4cOgdQ",
  "y": "eqE4OZu0V07qXi9ojhQAeqKndWp0QwUfB3aNp4dYYPQ",
  "alg": "ES256"

9.1. Signed authentication request with private_key_jwt client authentication

This example includes various optional fields, some of which may not be applicable to some deployments. Line wraps within values are for display purposes only.

POST /backchannel-authorization-endpoint HTTP/1.1
Host: server.example.com
Content-Type: application/x-www-form-urlencoded


which contains the JWT payload:

  "iss": "301183373814979",
  "aud": "https://server.example.com/",
  "iat": 1564902738,
  "nbf": 1564902738,
  "exp": 1564903038,
  "jti": "AJxZRpNqlg62UTdy37gu",
  "scope": "openid payments",
  "acr_values": "urn:mace:incommon:iap:silver urn:mace:incommon:iap:bronze",
  "client_notification_token": "_MiUOY07EOCvWR5BVuOL=",
  "login_hint": "john@example.com",
  "binding_message": "S24R",
  "user_code": "6365",
  "requested_expiry": "120",
  "request_context": {
    "location": {
      "lat": 51.17397,
      "lng": -1.82238
  "payment_intent": {
    "amount": "165.88",
    "currency": "GBP",
    "creditor_account": {
      "scheme_name": "UK.OBIE.SortCodeAccountNumber",
      "identification": "08080021325698",
      "name": "ACME Inc"

10. Normative References

Fett, D., "OpenID Connect Client Initiated Backchannel Authentication Core", , <http://openid.net/specs/openid-client-initiated-backchannel-authentication-core-1_0.html>.
Fett, D., "FAPI Part 1: Baseline Security Profile", , <https://openid.net/specs/openid-financial-api-part-1.html>.
Fett, D., "FAPI Part 2: Advanced Security Profile", , <https://openid.net/specs/openid-financial-api-part-2.html>.
Fett, D., "FAPI 2.0 Security Profile", , <https://openid.net/specs/fapi-2_0-security-profile-ID2.html>.
Fett, D., "OAuth 2.0 Rich Authorization Requests", , <https://datatracker.ietf.org/doc/html/draft-ietf-oauth-rar>.

11. Informative References

ISO/IEC, "ISO/IEC Directives, Part 2 - Principles and rules for the structure and drafting of ISO and IEC documents", <https://www.iso.org/sites/directives/current/part2/index.xhtml>.

Appendix A. Notices

Copyright (c) 2024 The OpenID Foundation.

The OpenID Foundation (OIDF) grants to any Contributor, developer, implementer, or other interested party a non-exclusive, royalty free, worldwide copyright license to reproduce, prepare derivative works from, distribute, perform and display, this Implementers Draft or Final Specification solely for the purposes of (i) developing specifications, and (ii) implementing Implementers Drafts and Final Specifications based on such documents, provided that attribution be made to the OIDF as the source of the material, but that such attribution does not indicate an endorsement by the OIDF.

The technology described in this specification was made available from contributions from various sources, including members of the OpenID Foundation and others. Although the OpenID Foundation has taken steps to help ensure that the technology is available for distribution, it takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this specification or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any independent effort to identify any such rights. The OpenID Foundation and the contributors to this specification make no (and hereby expressly disclaim any) warranties (express, implied, or otherwise), including implied warranties of merchantability, non-infringement, fitness for a particular purpose, or title, related to this specification, and the entire risk as to implementing this specification is assumed by the implementer. The OpenID Intellectual Property Rights policy requires contributors to offer a patent promise not to assert certain patent claims against other contributors and against implementers. The OpenID Foundation invites any interested party to bring to its attention any copyrights, patents, patent applications, or other proprietary rights that may cover technology that may be required to practice this specification.

Author's Address

Dave Tonge
Moneyhub Financial Technology