JWT Bearer Grant

The JSON Web Token(JWT) is simply a JSON string containing claim values. The JWT Bearer grant handler will evaluate and validate the claims in the JWT token, and issue an access token at the Authorization Server end.

WSO2 API Manager, as an OAuth 2.0 Authorization Server with its Key Manager features, can accept JWT Assertions from OAuth 2.0 clients as means of resource owner authentication and authorization. Additionally, it can exchange the JWT token with OAuth 2.0 access tokens in order to access protected resources on behalf of the resource owner.

The Flow

A client can exchange a JWT token to an OAuth 2.0 access token using this grant type.  Once an application is created on the API Store, keys for the application must be generated. When the keys are generated, there will be a Service Provider created on the WSO2 API Manager.  The service provider entity  is used by the WSO2 APIM server to obtain information of the application created on the API Store.  There needs to be an Identity Provider configuration corresponding to the IDP created on the WSO2 APIM Server as well. This IDP is who creates and signs the JWT assertion.  This is required so that the server can identify the issuer of the JWT and obtain the public certificate of the IDP, in order to validate the JWT. 

When a request is made to the token endpoint with the grant type, the JWT assertion, the client key and client  secret, the WSO2 APIM Server will read the grant type and trigger the JWT Bearer Grant Handler. This handler will check for the issuer of the JWT token and retrieve the IDP configuration. It will then obtain the public certificate of the IDP  stored in the IDP configuration, and validate the JWT. Once the JWT is validated, it will create an OAuth2.0 access token for the application holding the provided client key and client secret.  

Configuring the  Identity Provider and the Service Provider

1. Sign in to the WSO2 API Store.
2. Create a new application if an application is not available already.
3. Generate keys for the application.
4. Sign in to the WSO2 API Manager. Enter your username and password to log on to the Management Console (https://localhost:9443/carbon). 
5. Navigate to the Identity Providers section under the Main tab of the management console and click Add.
6. Provide the following values to configure the IDP:
   • Identity Provider Name: Enter the JWT issuer name as the identity provider name. This is used to generate the JWT assertion.

   • Identity Provider Public Certificate: The certificate used to sign the JWT assertion. 

     Identity provider Public Certificate

     The Identity Provider Public Certificate is the public certificate belonging to the identity provider. Uploading this is necessary to authenticate the response from the identity provider.

     This can be any certificate. If the identity provider is another API Manager or Identity Server, this can be a wso2.crt file.

     To create the identity provider certificate from the wso2carbon.jks file, follow the steps below.

     1. Open your Command Line interface, go to the <APIM_HOME>/repository/resources/security/directory. Run the following command.

     keytool -export -alias wso2carbon -file wso2.crt -keystore wso2carbon.jks -storepass wso2carbon

     2. Once you run this command, the wso2.crt file is generated and can be found in the <APIM_HOME>/repository/resources/security/ directory. Click Choose File and navigate to this location in order to select and upload this file.

     See Using Asymmetric Encryption in the WSO2 Product Administration Guide for more information on how public keys work and how to sign these keys by a certification authority.
   • Alias: Give the name of the alias if the Identity Provider identifies this token endpoint by an alias (e.g., https://localhost:9443/oauth2/token). For more information, see Adding a new identity provider.
     
     
7. Navigate to the Main menu to access the Identity menu. Click List under Service Providers.
8. Check if there is a Service provider listed for the application used to generate the keys in step 3.  The Service Provider name will have the  format <application owner>_<application name>_<generated key type>
   
   
   
   

Using the JWT grant

The cURL commands below can be used to retrieve the access token and refresh the token using a JWT.

curl -i -X POST -u <clientid>:<clientsecret> -k -d 'grant_type=urn:ietf:params:oauth:grant-type:jwt-bearer&assertion=<JWT>' -H 'Content-Type: application/x-www-form-urlencoded' https://localhost:9443/oauth2/token

• The -u flag should specify the “<Client Id>:<Client Secret>” value.
•  The assertion parameter value is the signed base64 encoded JWT. The value of the assertion parameter MUST contain a single JWT. You can refer JWT Bearer Grant for more information about assertion.
  
  

Access Token Endpoint: https://localhost:9443/oauth2/token?tenantDomain=wso2.com

Sample

Request

curl -i -X POST -H 'Content-Type: application/x-www-form-urlencoded' -u bBhEoE2wIpU1zB8HA3GfvZz8xxAa:RKgXUC3pTRQg9xPpNwyuTPGtnSQa -k -d 'grant_type=urn:ietf:params:oauth:grant-type:jwt-bearer&assertion=eyJhbGciOiJSUzI1NiJ9.eyJleHAiOjE0NTgxNjY5ODUsInN1YiI6ImFkbWluIiwibmJmIjoxNDU4MTA2OTg1LCJhdWQiOlsiaHR0cHM6XC9cL2xvY2FsaG9zdDo5NDQzXC9vYXV0aDJcL3Rva2VuIiwid3NvMi1JUyJdLCJpc3MiOiJqd3RJRFAiLCJqdGkiOiJUb2tlbjU2NzU2IiwiaWF0IjoxNDU4MTA2OTg1fQ.ZcxdoTVEsWoil80ne42QzmsfelMWyjRZJEjUK1c2vMZJjjtrZnsWExyCA5tN6iXYFAXC_7rkFuuNSgOlBi51MNLPZw3WcgGI52j6apGEW92V2tib9zRRWOeLQLAdo8ae8KzLp7kuKZ2XunfQ2WYU9TvvLDm_vp5ruuYz3ZZrJOc' https://localhost:9443/oauth2/token

Response

You would have now received the response from the token endpoint. The response would contain the access token, refresh token, expiry time and token type. 

{"token_type":"Bearer","expires_in":3600,"refresh_token":"b1b4b78e2b0ef4956acb90f2e38a8833","access_token":"615ebcc943be052cf6dc27c6ec578816"} 

The JWT Assertion

JWT contains three parts that are separated by dots ".": header, payload, and a signature. The header identifies the algorithm used to generate the signature.

For example, see the following code block.

{
	"alg":"RS256"
}

The payload contains the claims mentioned below:

• iss (issuer) - The JWT must contain an iss (issuer) claim that contains a unique identifier that identifies the identity provider that issued the JWT.
• sub (subject) - The JWT must contain a sub (subject) claim that identifies the entity that the identity provider or the entity that issued the JWT vouches for.
• aud (audience) - The JWT must contain an aud (audience) claim which containing a value that identifies the authorization server as an intended audience. This value should be registered as token endpoint alias in the Identity Provider.
• exp (expiration time) - The JWT must contain an exp (expiration) claim that limits the time window during which the JWT can be used.
• nbf (not before) - The JWT may contain a nbf (not before time) claim that forces a JWT to be used only after a specified time.
• iat (issued at) - The JWT may contain an iat (issued at) claim that identifies the time at which the JWT was issued.
• jti (json web token ID) - The JWT may contain jti (JWT ID) claim that provides a unique identifier for the token.
• Other custom claims - JWT may contain claims other than the above mentioned ones. This is the extension point of the JWT specification.

For example, see the following code block.

{  
   "sub":"admin",
   "aud":[  
      "https://localhost:9443/oauth2/token"
   ],
   "nbf":1507546100,
   "iss":"jwtIDP",
   "exp":1507606100,
   "iat":1507546100,
   "jti":"Token56756"
}

The signature is calculated by base64 URL encoding the header and payload and concatenating them with a period as a separator and signing it:

Signature = sign(encodeBase64(header) + '.' + encodeBase64(payload))

The signature must then be base64 URL encoded. JWT assertion can be generated by concatenating these three encoded values with a separator dot ".".

assertion =  encodeBase64(header) + '.' + encodeBase64(payload) + '.' + encodeBase64(signature)

The result is as follows:

eyJhbGciOiJSUzI1NiJ9.eyJzdWIiOiJhZG1pbiIsImF1ZCI6WyJodHRwczpcL1wvbG9jYWxob3N0Ojk0NDNcL29hdXRoMlwvdG9rZW4iXSwibmJmIjoxNTA3NTQ2MTAwLCJpc3MiOiJqd3RJRFAiLCJleHAiOjE1MDc2MDYxMDAsImlhdCI6MTUwNzU0NjEwMCwianRpIjoiVG9rZW41Njc1NiJ9.iGMhjibB0W2QFQlM27gnHp6z47Eybv8cAHk2o2i-xqo2S4uJ_1VppFI4CCJXTj4qzV9vmkJ5HKNAayiTa6wOMXGL4XnwYwpOAoKXvboznlEDNRpw3htW34nLvyUu6PjHbdvAPVjh8kPRwf7esRr2p-luecGvC21mjWdhyGzM4hE