Table of Contents

Client authentication using client_secret_jwt method

Preface

client_secret_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 JWT assertion that contains a message authentication code (MAC) in its signature part, and includes it to the request. Then an authorization server authenticates the client by verifying the signature and payload of the assertion.

Authlete supports client_secret_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 client_secret_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 client_secret_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 contain a MAC using a shared 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

The signature of the JWT must be calculated using HMAC-SHA algorithm e.g. HS256.
Client secret must be used as a shared key on calculating the signature.

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 client_secret_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 “CLIENT_SECRET_JWT ”

Client of the service

Configure the following setting in Client Developer Console.

Tab	Key	Value
Basic	Client Type	CONFIDENTIAL
Authorization	Client Authentication Method	CLIENT_SECRET_JWT
Authorization	Assertion Signature Algorithm	HS256 , HS384 or HS256

Example

This example shows client authentication using client_secret_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
}

Generating a JWT

Generate a JWT assertion including the payload, and a MAC using the client’s shared key (client secret). 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 HS256 \
  --signing-alg-key TzPTZDtcw9ek41H1VmofRoXQddP5cWCXPWidZHSA2spU6gZN9eIFUiXaHD7OfxtBhTxJsg_I1tdFI_CkKl8t8Q

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

eyJhbGciOiJIUzI1NiJ9.
ewogICJqdGkiOiJteUpXVElkMDAxIiwKICAic3ViIjoiMzgxNzQ2MjM3NjIiLAogIC
Jpc3MiOiIzODE3NDYyMzc2MiIsCiAgImF1ZCI6Imh0dHA6Ly9sb2NhbGhvc3Q6NDAw
MC9hcGkvYXV0aC90b2tlbi9kaXJlY3QvMjQ1MjMxMzgyMDUiLAogICJleHAiOjE1Mz
YxNjU1NDAsCiAgImlhdCI6MTUzNjEzMjcwOAp9Cg.
Vin3IxRPMLQ0SKNJ8Ba_59dYHBGLb4Ft-JLbJVKFd3E

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=
 eyJhbGciOiJIUzI1NiJ9.
 ewogICJqdGkiOiJteUpXVElkMDAxIiwKICAic3ViIjoiMzgxNzQ2MjM3NjIiLAogIC
 Jpc3MiOiIzODE3NDYyMzc2MiIsCiAgImF1ZCI6Imh0dHA6Ly9sb2NhbGhvc3Q6NDAw
 MC9hcGkvYXV0aC90b2tlbi9kaXJlY3QvMjQ1MjMxMzgyMDUiLAogICJleHAiOjE1Mz
 YxNjU1NDAsCiAgImlhdCI6MTUzNjEzMjcwOAp9Cg.
 Vin3IxRPMLQ0SKNJ8Ba_59dYHBGLb4Ft-JLbJVKFd3E

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%3Aietf%3Aparams%3Aoauth%3Aclient-assertion-type%3Ajwt-bearer&
    client_assertion=
     eyJhbGciOiJIUzI1NiJ9.
     ewogICJqdGkiOiJteUpXVElkMDAxIiwKICAic3ViIjoiMzgxNzQ2MjM3NjIiLAogIC
     Jpc3MiOiIzODE3NDYyMzc2MiIsCiAgImF1ZCI6Imh0dHA6Ly9sb2NhbGhvc3Q6NDAw
     MC9hcGkvYXV0aC90b2tlbi9kaXJlY3QvMjQ1MjMxMzgyMDUiLAogICJleHAiOjE1Mz
     YxNjU1NDAsCiAgImlhdCI6MTUzNjEzMjcwOAp9Cg.
     Vin3IxRPMLQ0SKNJ8Ba_59dYHBGLb4Ft-JLbJVKFd3E
}'

API response from Authlete to the authorization server

Authlete processes the request and send 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": "kwXY57oN4nBOqxk57vW2fo-WzgezrwSl2h1N_xW8aKI",
    "responseContent": {
        "access_token": "kwXY57oN4nBOqxk57vW2fo-WzgezrwSl2h1N_xW8aKI",
        "refresh_token": "5zBNsdrlMojcMH3wCrfaXpmAY6vKqOqeV3q1ebRJzGM",
        "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 send it back to the client as a token response (details omitted).