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6. Public Key Confirmation

Resource servers MUST be able to reliably identify whether an access token is DPoP-bound and ascertain sufficient information to verify the binding to the public key of the DPoP proof (see Section 7.1). Such a binding is accomplished by associating the public key with the token in a way that can be accessed by the protected resource, such as embedding the JWK hash in the issued access token directly, using the syntax described in Section 6.1, or through token introspection as described in Section 6.2. Other methods of associating a public key with an access token are possible per an agreement by the authorization server and the protected resource; however, they are beyond the scope of this specification.

Resource servers supporting DPoP MUST ensure that the public key from the DPoP proof matches the one bound to the access token.

6.1. JWK Thumbprint Confirmation Method​

When access tokens are represented as JWTs [RFC7519], the public key information is represented using the jkt confirmation method member defined herein. To convey the hash of a public key in a JWT, this specification introduces the following JWT Confirmation Method [RFC7800] member for use under the cnf claim.

jkt: JWK SHA-256 Thumbprint confirmation method. The value of the jkt member MUST be the base64url encoding (as defined in [RFC7515]) of the JWK SHA-256 Thumbprint (according to [RFC7638]) of the DPoP public key (in JWK format) to which the access token is bound.

The following example JWT in Figure 8 with a decoded JWT payload shown in Figure 9 contains a cnf claim with the jkt JWK Thumbprint confirmation method member. The jkt value in these examples is the hash of the public key from the DPoP proofs in the examples shown in Section 5. The example uses "" line wrapping per [RFC8792].

eyJhbGciOiJFUzI1NiIsImtpZCI6IkJlQUxrYiJ9.eyJzdWIiOiJzb21lb25lQGV4YW1\
wbGUuY29tIiwiaXNzIjoiaHR0cHM6Ly9zZXJ2ZXIuZXhhbXBsZS5jb20iLCJuYmYiOjE\
1NjIyNjI2MTEsImV4cCI6MTU2MjI2NjIxNiwiY25mIjp7ImprdCI6IjBaY09DT1JaTll\
5LURXcHFxMzBqWnlKR0hUTjBkMkhnbEJWM3VpZ3VBNEkifX0.3Tyo8VTcn6u_PboUmAO\
YUY1kfAavomW_YwYMkmRNizLJoQzWy2fCo79Zi5yObpIzjWb5xW4OGld7ESZrh0fsrA

    Figure 8: JWT Containing a JWK SHA-256 Thumbprint Confirmation
{
  "sub":"[email protected]",
  "iss":"https://server.example.com",
  "nbf":1562262611,
  "exp":1562266216,
  "cnf":
  {
    "jkt":"0ZcOCORZNYy-DWpqq30jZyJGHTN0d2HglBV3uiguA4I"
  }
}

 Figure 9: JWT Claims Set with a JWK SHA-256 Thumbprint Confirmation

6.2. JWK Thumbprint Confirmation Method in Token Introspection​

"OAuth 2.0 Token Introspection" [RFC7662] defines a method for a protected resource to query an authorization server about the active state of an access token. The protected resource also determines metainformation about the token.

For a DPoP-bound access token, the hash of the public key to which the token is bound is conveyed to the protected resource as metainformation in a token introspection response. The hash is conveyed using the same cnf content with jkt member structure as the JWK Thumbprint confirmation method, described in Section 6.1, as a top-level member of the introspection response JSON. Note that the resource server does not send a DPoP proof with the introspection request, and the authorization server does not validate an access token's DPoP binding at the introspection endpoint. Rather, the resource server uses the data of the introspection response to validate the access token binding itself locally.

If the token_type member is included in the introspection response, it MUST contain the value DPoP.

The example introspection request in Figure 10 and corresponding response in Figure 11 illustrate an introspection exchange for the example DPoP-bound access token that was issued in Figure 6.

POST /as/introspect.oauth2 HTTP/1.1
Host: server.example.com
Content-Type: application/x-www-form-urlencoded
Authorization: Basic cnM6cnM6TWt1LTZnX2xDektJZHo0ZnNON2tZY3lhK1Rp

token=Kz~8mXK1EalYznwH-LC-1fBAo.4Ljp~zsPE_NeO.gxU

               Figure 10: Example Introspection Request
HTTP/1.1 200 OK
Content-Type: application/json
Cache-Control: no-store

{
  "active": true,
  "sub": "[email protected]",
  "iss": "https://server.example.com",
  "nbf": 1562262611,
  "exp": 1562266216,
  "cnf":
  {
    "jkt": "0ZcOCORZNYy-DWpqq30jZyJGHTN0d2HglBV3uiguA4I"
  }
}

  Figure 11: Example Introspection Response for a DPoP-Bound Access
                                Token
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