OpenID Connect Authorization Code Flow#

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BankID with biometrics supports the OpenID Connect Authorization Code Flow⁠ (opens in a new tab).

Steps

[TOC]

The Authorization Code Flow

sequenceDiagram
    actor u as User
    participant b as App Backend
    participant bid as BankID OIDC

    u ->>+ b: Request login
    b ->>+ bid: /auth/realms/.../.well-known/openid-configuration
    bid -->>- b: OIDC Configuration
    b -->>- u: Redirect to the authorization endpoint
    u ->>+ bid: Initiate the BankID with biometrics authentication flow
    break Authentication failed
        bid -->> u: Redirect to the redirect URI with the error
    end
    bid -->>- u: Redirect to the redirect URI with the authorization code
    u ->>+ b: Invoke callback endpoint
    b ->>+ bid: Token exchange
    bid -->>- b: Access token and ID token
    Note right of b: The App Backend verifies the ID token
    b -->>- u: Store user session and mark the user as logged in

1. Register a client with BankID OIDC#

If you don't have a client registered with BankID OIDC⁠ (opens in a new tab) yet, you'll need to register one. Follow the instructions here⁠ (opens in a new tab) to register your application and obtain the necessary client credentials⁠ (opens in a new tab) (client_id and client_secret).

2. Get the BankID OpenID Connect Configuration#

To ensure seamless integration with BankID OIDC and avoid hardcoding specific endpoints in you application, follow these steps to dynamically fetch the OpenID Connect configuration⁠ (opens in a new tab):

Determine the appropriate configuration URL based on the environment:

Environment	Url
Production	https://auth.bankid.no/auth/realms/prod/.well-known/openid-configuration⁠ (opens in a new tab)
Current (public test environment)	https://auth.current.bankid.no/auth/realms/current/.well-known/openid-configuration⁠ (opens in a new tab)

2
Send an HTTP GET request to the respective configuration URL using your preferred programming language or API tool.
3
Capture the response which will contain a JSON document with the configuration. By dynamically fetching the OIDC configuration, your application remains flexible, allowing for future changes or updates to the endpoints without requiring modifications to your code.
4
To enhance the reliability and performance of your integration, we strongly advise implementing response caching for the OIDC configuration.
Example response from Current (relevant fields only)
{ ... "token_endpoint": "https://auth.current.bankid.no/auth/realms/current/protocol/openid-connect/token", ... }
This is a sample response. The actual response may differ.

3. Implement the authorization code flow#

To successfully implement the OIDC Authorization Code Flow⁠ (opens in a new tab) in your application, follow the steps below:

The flow is specified RFC 6749 §4.1⁠ (opens in a new tab).

1
Retrieve the authorization endpoint URL dynamically from the OIDC configuration. Avoid hardcoding this URL in your application to allow for future changes as described in step 2.
2
Generate a random and non-guessable⁠ (opens in a new tab) string, such as a UUID, and store it in a variable called state. The value is used to protecting against cross-site request forgery (CSRF)⁠ (opens in a new tab) attacks. It will be sent back to your application in the callback.
3
Generate a cryptographically random and non-guessable⁠ (opens in a new tab) string and store it in a variable called nonce. This value ensures the integrity of the ID token and mitigates replay attacks.
4
Store the state and nonce value in your application using your preferred session mechanism, so they can be retrieved later. These values should be stored together for proper verification.
5
(Optional, but HIGHLY recommended) Generate a cryptographically random and non-guessable⁠ (opens in a new tab) string and store it in a variable called code_verifier. This value is used to mitigate against authorization code interception attacks through the use of Proof Key for Code Exchange (PKCE)⁠ (opens in a new tab).
6
Build the authorization endpoint URI by adding the following parameters:
- client_id: The client ID for your BankID OIDC client.
- scope: Specifies the requested information and resources. In this example we use openid and profile and get a regular ID token⁠ (opens in a new tab), and the nnin_altsub scope to get the Norwegian national identity number.
- response_type=code: Indicates the usage of the Authorization Code Flow.
- redirect_uri: The callback URI on your server where you want to receive the callback from BankID OIDC.
- acr_values=urn:bankid:bis: Specifies the authentication context class reference (ACR) value. This value indicates that the user should be authenticated using BankID Substantial/BankID with biometrics.
- login_hint=:<nnin>: Optionally supply the user's national identity number (nnin) to pre-fill the BankID login form.
- state: Use the previously generated state value.
- nonce: Use the previously generate nonce value.
- code_challenge: Base64-URL encoded value of the SHA-256 hash of your generated value for code_verifier. Use the formula code_challenge = BASE64-ENCODE(SHA256(code_verifier)). See RFC 7636 §4.2⁠ (opens in a new tab) for more details.
- code_challenge_method=S256: Specify the code challenge method as SHA-256.

For more information about the parameters, read here⁠ (opens in a new tab)

Here is an example of a valid authorization request URI:

GET https://auth.current.bankid.no/auth/realms/current/precheck/auth
    ?client_id=bioexample-bankid-current
    &scope=openid profile nnin_altsub
    &response_type=code
    &redirect_uri=https://localhost:3000/api/auth/callback/bid
    &acr_values=urn:bankid:bis
    &state=4eJYruvMul0SOs5GGuJTjERgtVIovp7PFZt3Qw4EC94
    &nonce=6ZlR7ik_7YI740Ayt8DmMV75IoNfI2jLflVDfYexMQc
    &code_challenge=rMU4NcLC3_O_tsTZ3gQE1ONoXD6OMZph_2zRuobajhQ
    &code_challenge_method=S256

Make sure to set acr_values to urn:bankid:bis. BIS is an identity provider⁠ (opens in a new tab) and this parameter specifies that the user should be authenticated using BankID with biometrics authentication.

By initializing the authorization code flow with the constructed URI, you will redirect the user to the authorization endpoint. There, the user will authenticate with BankID with biometrics and consent to the requested scopes. For more information on available scopes, read here⁠ (opens in a new tab).

4. Handle callback and exchange the auth code for tokens#

After the user has authenticated, BankID OIDC will redirect the user back to the callback URI specified in the redirect_uri parameter. See RFC 6749 §4.1.2⁠ (opens in a new tab) for more information about the authorization response.

graph TD
    classDef error stroke: #ef4444
    
    A[Callback Received] --> B[Verify State]
    B -->|Mismatch| C[Authentication failed]:::error
    B -->|Match| D[Check for error]
    D -->|Present| C
    D -->|Not present| E[Exchange code for token]
    E --> F[Verify the token]
    F -->|Fail| C
    F -->|OK| G[Authentication succeeded]

The application's server is responsible for performing the token exchange:

1
Retrieve the code parameter from the callback request.
2
Build the token request by including the following parameters:
- grant_type=authorization_code: Indicates the usage of the Authorization Code Flow.
- client_id: The client ID for your BankID OIDC client.
- client_secret: The client secret for your BankID OIDC client.
- redirect_uri: The same callback URI used in the authorization request.
- code: The authorization code received in the callback.
- code_verifier: If PKCE was used in the authorization request, include the code_verifier used.
Obtain the token_endpoint from the BankID OIDC provider configuration.
3
Send a POST request to the token endpoint with the constructed parameters. The parameters should be encoded as application/x-www-form-urlencoded. Remember to include the client_id and client_secret as part of the basic authentication header in the request. See RFC 6749 §4.1.3⁠ (opens in a new tab) and RFC 6749 §2.3.1⁠ (opens in a new tab) for more details on sending the request.

Example token request:

POST https://auth.current.bankid.no/auth/realms/current/protocol/openid-connect/token
    ?grant_type=authorization_code
    &redirect_uri=https://localhost:3000/api/auth/callback/bid
    &code=AUTHORIZATION_CODE
    &code_verifier=code_verifier

Authorization: Basic YmlvZXhhbXBsZS1iYW5raWQtY3VycmVudDpzZWNyZXQ=

The example is formatted for readability.

For more detailed info see the BankID OIDC documentation⁠ (opens in a new tab).

5. Validate the ID token#

To ensure the authenticity and integrity of the received ID token, you must perform the following verification:

Verify that the nonce claim in the received ID token corresponds to the nonce value belonging to the same session (i.e. the nonce associated with the state).
Verify that the acr claim include LOA=3 (Level of Assurance 3). This indicating the desired level of trust in the authentication process.
Verify that the algorithm used to sign the ID token is found in the BankID with biometrics OIDC configuration under id_token_signing_alg_values_supported.
Verify that the token is signed by a key from the JWKS (JSON Web Key Set)⁠ (opens in a new tab) document. The URI for the JWKS document can be found under the jwks_uri key in the BankID OIDC configuration you retrieved in step 2. We recommend caching the JWKS for 30 minutes, and also fetching a new JWKS if the kid in the ID token header does not match any key in the cached JWKS.
We recommend verifying these additional properties:
- iss (issuer) - The issuer of the token must match the issuer in the BankID with biometrics OIDC configuration.
- aud (audience) - The audience of the token must match your client ID.
- exp (expiration time) - The token must not be expired.
- iat (issued at) - The token must not be issued in the future.

Further information on how to validate the ID token can be found in the OpenID Connect Core specification⁠ (opens in a new tab).