Client authentication using private_key_jwt method

Preface

private_key_jwt is one of client authentication methods defined in OpenID Connect Core 1.0, 9. Client Authentication .

On a token request, a client crafts a digitally signed JWT assertion and includes it to the request. Then an authorization server authenticates the client by verifying the signature and payload of the assertion.

Authlete supports private_key_jwt as a client authentication method so that authorization servers can enable it. This article describes overview of the method and setup instructions with Authlete.

Requirements for private_key_jwt

The following section describes details on both client and authorization server side.

Client

A client must include the following parameters in a token request when using the private_key_jwt method.

Parameter	Description
client_assertion_type	A type of client_assertion. Its value must be “urn:ietf:params:oauth:client-assertion-type:jwt-bearer ”.
client_assertion	A JWT that contains information for client authentication. It must be digitally signed using a private key . See below for details.

The value of client_assertion must satisfy the following requirements on its JWT payload and JWT signature. You can find an example JWT in the “Generating a JWT assertion " section.

Payload

A JWT assertion must contain the REQUIRED claims listed below.

Claim	Description
iss	[REQUIRED] Issuer. This must contain the client_id of the OAuth client.
sub	[REQUIRED] Subject. This must contain the client_id of the OAuth client.
aud	[REQUIRED] Audience. A value that identifies the authorization server as an intended audience. The authorization server must verify that it is an intended audience for the token. The audience should be the URL of the authorization server’s token endpoint.
jti	[REQUIRED] JWT ID. A unique identifier for the token, which can be used to prevent reuse of the token. These tokens must only be used once unless conditions for reuse were negotiated between the parties; any such negotiation is beyond the scope of this specification.
exp	[REQUIRED] Expiration time on or after which the JWT must not be accepted for processing.
iat	[OPTIONAL] Time at which the JWT was issued.

Signature

A JWT assertion must be digitally signed using a private key in asymmetric cryptography (e.g. RS256).
A client using the authentication method has to register its public key to an authorization server in advance so that the server can verify the assertion.

Authorization server

An authorization server must process a token request as per the specifications listed below. The details are omitted here because you can offload these operations from your authorization server to Authlete .

Configuration settings

This section explains settings for enabling the private_key_jwt method. You have to configure both Authlete service and its client to be authenticated with the method.

Authlete service

Configure the following setting in Service Owner Console.

Tab	Key	Value
Authorization	Supported Client Authentication Methods	Enable “PRIVATE_KEY_JWT ”

Client of the service

Configure the following setting in Client Developer Console.

Tab	Key	Value
Basic	Client Type	CONFIDENTIAL
Authorization	Client Authentication Method	PRIVATE_KEY_JWT
Authorization	Assertion Signature Algorithm	RS256 , RS384 , RS512 ,ES256 , ES384 , ES512 ,PS256 , PS384 , or PS512
JWK Set	JWK Set Content	A JWK set including a public key used for verifying the client's JWT assertion

client-auth-private-key-jwt_2 — Basic Tab

client-auth-private-key-jwt_3 — Authorization Tab

client-auth-private-key-jwt_4 — JWK Set Tab

Example

This example shows client authentication using private_key_jwt at the token endpoint of the authorization server.

Generating a JWT assertion

Let’s generate a JWT that will be used as a value of the client_assertion in a token request.

Preparing a JWT payload

First, create a JSON formatted payload and save it as “payload.json”.

{
    "jti": "myJWTId001",
    "sub": "38174623762",
    "iss": "38174623762",
    "aud": "http://localhost:4000/api/auth/token/direct/24523138205",
    "exp": 1536165540,
    "iat": 1536132708
}

Preparing a JWK set

Then, prepare a JWK set for signing and verification. Here we use mkjwk.org to generate a JWK set.

The examples below are ones generated using ES256 algorithm, and saved the public and private key pair as a file “key_pair.jwk”.

Make sure to register the public key (the latter one) to Authlete and configure Client Authentication Method (PRIVATE_KEY_JWT) and Client Assertion Algorithm (ES256). These settings can be done via Client Developer Console, as described in the previous section.

Public and private key pair

{
    "kty": "EC",
    "d": "ukQKQexNI8PtEv7SKpqUDnbZ-WkN6HaQqcVrVV8ZWRQ",
    "use": "sig",
    "crv": "P-256",
    "x": "9Yxd2TvwBbgmupZh3bpg3umKihM_FNAk2_uI_-Edv_Q",
    "y": "BOUFuyvWoBZ9-RVSeHJLF-L4I3ORv0xbaM1CKCFJr54",
    "alg": "ES256"
}

Public key (to be registered to Authlete via Client Developer Console)

{
    "kty": "EC",
    "use": "sig",
    "crv": "P-256",
    "x": "9Yxd2TvwBbgmupZh3bpg3umKihM_FNAk2_uI_-Edv_Q",
    "y": "BOUFuyvWoBZ9-RVSeHJLF-L4I3ORv0xbaM1CKCFJr54",
    "alg": "ES256"
}

Generating a JWT

Generate a JWT assertion including the payload and signed with the private key. The example below is an instruction using an authlete-jose library . Or you can use mkjose.org website to do that.

bin/jose-generator \
--payload-file payload.json \
--sign \
--signing-alg ES256 \
--jwk-signing-alg-file key_pair.jwk

The generated JWT will look like this (with line breaks for display purposes only):

eyJhbGciOiJFUzI1NiJ9.
ewogICJqdGkiOiJteUpXVElkMDAxIiwKICAic3ViIjoiMzgxNzQ2MjM3NjIiL
AogICJpc3MiOiIzODE3NDYyMzc2MiIsCiAgImF1ZCI6Imh0dHA6Ly9sb2NhbG
hvc3Q6NDAwMC9hcGkvYXV0aC90b2tlbi9kaXJlY3QvMjQ1MjMxMzgyMDUiLAo
gICJleHAiOjE1MzYxNjU1NDAsCiAgImlhdCI6MTUzNjEzMjcwOAp9Cg.
YB4gdhWUGRjWEsEbKDs7-G2WFH2oYz7bAEP5AtegHXInkY9ncA2V3IoA6O_HV
QuFxyCRIklrxsMk32MfNF_ABA

This JWT is a value of client_assertion, to be used by the client on making a token request.

Token request and response

Token request from the client to the authorization server

Assume the client that has the assertion makes a token request to the authorization server. (folded for readability)

POST /token HTTP/1.1
Host: as.example.com
Content-Type: application/x-www-form-urlencoded

grant_type=authorization_code&
code=Gw30fMKJBHkcOBSde5awLrMm4ahvgCNM2cFSTUOUflY&
redirect_uri=https://example.com/redirection&
client_assertion_type=
 urn%3Aietf%3Aparams%3Aoauth%3Aclient-assertion-type%3Ajwt-bearer&
client_assertion=
 eyJhbGciOiJFUzI1NiJ9.
 ewogICJqdGkiOiJteUpXVElkMDAxIiwKICAic3ViIjoiMzgxNzQ2MjM3NjIiL
 AogICJpc3MiOiIzODE3NDYyMzc2MiIsCiAgImF1ZCI6Imh0dHA6Ly9sb2NhbG
 hvc3Q6NDAwMC9hcGkvYXV0aC90b2tlbi9kaXJlY3QvMjQ1MjMxMzgyMDUiLAo
 gICJleHAiOjE1MzYxNjU1NDAsCiAgImlhdCI6MTUzNjEzMjcwOAp9Cg.
 YB4gdhWUGRjWEsEbKDs7-G2WFH2oYz7bAEP5AtegHXInkY9ncA2V3IoA6O_HV
 QuFxyCRIklrxsMk32MfNF_ABA

API request from the authorization server to Authlete

The authorization server forwards the request content to Authlete’s /auth/token API . (folded for readability)

$ curl -s -X POST https://api.authlete.com/api/auth/token \
-H 'Content-Type: application/json' \
-u '...:...' \
-d '{
  "parameters":"grant_type=authorization_code&
  code=Gw30fMKJBHkcOBSde5awLrMm4ahvgCNM2cFSTUOUflY&
  redirect_uri=https://example.com/redirection&
  client\_assertion\_type= urn:ietf:params:oauth:client-assertion-type:jwt-bearer
  &client\_assertion=eyJhbGciOiJFUzI1NiJ9.
  ewogICJqdGkiOiJteUpXVElkMDAxIiwKICAic3ViIjoiMzgxNzQ2MjM3NjIiL
  AogICJpc3MiOiIzODE3NDYyMzc2MiIsCiAgImF1ZCI6Imh0dHA6Ly9sb2NhbG
  hvc3Q6NDAwMC9hcGkvYXV0aC90b2tlbi9kaXJlY3QvMjQ1MjMxMzgyMDUiLAo
  gICJleHAiOjE1MzYxNjU1NDAsCiAgImlhdCI6MTUzNjEzMjcwOAp9Cg.
  YB4gdhWUGRjWEsEbKDs7-G2WFH2oYz7bAEP5AtegHXInkY9ncA2V3IoA6O\_HV
  QuFxyCRIklrxsMk32MfNF\_ABA"

}'

API response from Authlete to the authorization server

Authlete processes the request and sends back an API response to the authorization server as follows. (folded for readability)

{
   "type": "tokenResponse",
   "resultCode": "A050001",
   "resultMessage": "[A050001] The token request (grant_type=authorization_code) was processed successfully.",
   "accessToken": "ni6uDszfkeR5GH96k3cUjt3R7MHG9-xRbMDObaKGY2A",
   "responseContent": {
      "access_token": "ni6uDszfkeR5GH96k3cUjt3R7MHG9-xRbMDObaKGY2A",
      "refresh_token": "dyzc8D96hSdrCmaPaB75uFiqjWTIWHXq-_OjVN17gAk",
      "scope": null,
      "token_type": "Bearer",
      "expires_in": 3600
   },
   ...
}

Token response from the authorization server to the client

The authorization server extracts the value of “responseContent” and sends it back to the client as a token response (details omitted).