JWK-based proof-of-possession

With proof-of-possession, the OAuth 2.0 client uses a known cryptographic key to prove its identity to the resource server.

How it works

Proof-of-possession has the OAuth 2.0 client include a JSON Web Key (JWK) with the client’s public key in a request for an access token:

The client presents the access token to a resource server.
The resource server gets the client’s public key from the access token.
The resource server uses the public key from the JWK to issue and validate a challenge-response with the client.
The client uses the private key to respond to the challenge.

Successfully solving the challenge cryptographically confirms proof-of-possession of the access token.

For details, refer to RFC 7800, Proof-of-Possession Key Semantics for JSON Web Tokens (JWTs). AM does not support jwe and jku format keys; the public key must be represented in jwk format.

This sequence diagram displays the flow:

Figure 1. OAuth 2.0 JWK-based proof-of-possession flow

The client requests an access token using an OAuth 2.0 grant flow with the JWK in the request.

Before using the flow, the client gets a public-private key pair and prepares the JWK to hold the public key.
The authorization server returns the access token to the client:
- When AM uses server-side OAuth 2.0 token storage, it keeps the JWK with the access token in the CTS token store and provides the client with the access token ID.
- When AM uses client-side OAuth 2.0 token storage, the access token is a JWT with the JWK embedded.
The client requests access to the protected resources from the resource server.
The resource server recovers the JWK associated with the access token:
- When AM uses server-side OAuth 2.0 token storage, the resource server introspects the access token to get the JWK.
- When AM uses client-side OAuth 2.0 token storage, the access token is a JWT with the JWK embedded.
The resource server creates a challenge using the public key from the JWK.

For example, the challenge could be a message or a nonce encrypted with the public key.
The resource server sends the challenge to the client.
The client solves the challenge using its private key.
The client sends the response to the challenge to the resource server.

The resource server validates the response, confirming the client’s proof-of-possession, and grants access.

Demonstrate proof-of-possession

Demonstrate the process with an OAuth 2.0 client that uses the client credentials grant:

Create an OAuth 2.0 client
Get an access token
Use the access token

Create an OAuth 2.0 client

Create a confidential OAuth 2.0 client account.

In the AM admin UI, click Realm > Realm Name > Applications > OAuth 2.0 > Clients > + Add Client, and create a new confidential client with the following settings:

Client ID

myClient

Client secret

forgerock

Scopes

access
Under the Advanced tab, add Grant Type: Client Credentials and save your work.

Get an access token

Generate a public-private key pair for the OAuth 2.0 client.

AM supports both RSA and elliptic curve (EC) key types.

To demonstrate the process, use an online JWK generator, such as https://mkjwk.org/.

For production deployments, store the key pair with the private key in a secure location. The OAuth 2.0 client must never reveal the private key. It uses the private key to solve the challenge from the resource server.
Represent the public key as a JWK.

Adapt the output to use the JWK format fields.

AM proof-of-possession requires a JWK where the key fields depend on the key type and the body is a single public key object, not an array, as in the following example:
```
{
  "jwk": {
    "kty": "EC",
    "use": "enc",
    "crv": "P-256",
    "kid": "myPublicJsonWebKey",
    "x": "D5kNqoGZbLZa77xdh4HSlSZIJcHxNw4UP0pgd5wbXvU",
    "y": "tX3SnRZgUOy48FV0XTCtaQNLG_DxXGbcVk94KvpyXrk"
  }
}
```

Base64-encode your JWK, as in the following example:

ewogICJqd2siOiB7CiAgICAia3R5IjogIkVDIiwKICAgICJ1c2UiOiAiZW5jIiwKICAgICJjc
nYiOiAiUC0yNTYiLAogICAgImtpZCI6ICJteVB1YmxpY0pzb25XZWJLZXkiLAogICAgIngiOi
AiRDVrTnFvR1piTFphNzd4ZGg0SFNsU1pJSmNIeE53NFVQMHBnZDV3Ylh2VSIsCiAgICAieSI
6ICJ0WDNTblJaZ1VPeTQ4RlYwWFRDdGFRTkxHX0R4WEdiY1ZrOTRLdnB5WHJrIgogIH0KfQ==

Include the base64-encoded JWK as the value of the cnf_key parameter to request an access token:

$ curl \
--request POST \
--user 'myClient:forgerock' \
--data 'grant_type=client_credentials' \
--data 'scope=access' \
--data 'cnf_key=eyJqd2siOnsia3R5IjoiRUMiLCJ1c2UiOiJlbmMiLCJjcnYiOiJQLTI1NiIsImtpZCI6Im15UHVibGljSnNvbldlYktleSIsIngiOiJENWtOcW9HWmJMWmE3N3hkaDRIU2xTWklKY0h4Tnc0VVAwcGdkNXdiWHZVIiwieSI6InRYM1NuUlpnVU95NDhGVjBYVEN0YVFOTEdfRHhYR2JjVms5NEt2cHlYcmsifX0=' \
'https://openam.example.com:8443/openam/oauth2/realms/root/realms/alpha/access_token'
{"access_token":"nhFC8RBEA6Zm672ir9GlWOMdJYc","scope":"access","token_type":"Bearer","expires_in":3599}

In this example, AM uses server-side OAuth 2.0 tokens. The response includes the access token ID.

If AM used client-side OAuth 2.0 tokens, the access_token field would contain the embedded JWK.

Use the access token

The client uses the access token to request a protected resource on the resource server.

This step is not shown in the demonstration.

The resource server uses the access token to get the public key.

In this demonstration where AM uses server-side OAuth 2.0 tokens, use the OAuth 2.0 client credentials to introspect the token on behalf of the resource server.

The cnf field contains the JWK:

$ curl \
--request POST \
--user 'myClient:forgerock' \
--data 'token=nhFC8RBEA6Zm672ir9GlWOMdJYc' \
'https://openam.example.com:8443/openam/oauth2/realms/root/realms/alpha/introspect'
{
  "active": true,
  "scope": "access",
  "realm": "/alpha",
  "client_id": "myClient",
  "user_id": "myClient",
  "username": "myClient",
  "token_type": "Bearer",
  "exp": 1671126270,
  "sub": "myClient",
  "subname": "myClient",
  "iss": "https://openam.example.com:8443/openam/oauth2/realms/root/realms/alpha",
  "cnf": {
    "jwk": {
      "kty": "EC",
      "use": "enc",
      "crv": "P-256",
      "kid": "myPublicJsonWebKey",
      "x": "D5kNqoGZbLZa77xdh4HSlSZIJcHxNw4UP0pgd5wbXvU",
      "y": "tX3SnRZgUOy48FV0XTCtaQNLG_DxXGbcVk94KvpyXrk"
    }
  },
  "authGrantId": "CBmKDJ1Ei8V7HhyXcKAyHaNR42I",
  "auditTrackingId": "3c47dc23-a35f-4a34-9f73-daf709d42400-385803"
}

The resource server uses the public key to confirm proof-of-possession with a challenge-response interaction.

This step is not shown in the demonstration.

By successfully responding to the challenge, the client proves it has the private key for the public key it presented to get the access token.
The resource server grants access to the resource.

Access Management 7.3.2