Step 3 - Verifying the login

Success callback

If the user logs in, they get redirected to the URL provided, with token query parameter appended and containing a RS256-signed JWT token, e.g.:


The token will decode to something like this:

The Header:

  "typ": "JWT",
  "alg": "RS256"

The Payload:

  "applicationVersion": "1.2.7",
  "sub": "1-FgW7/lhAajlonnWAr7g3yB7eByYVkiV8O3UzsFPhEkoOYdr+bbeGeATwKyZUM6YenuUQb85G+PIijAxmP85GRnTIX6bdOdeJuuw=",
  "application": "hatappstaging",
  "iss": "",
  "exp": 1543692303,
  "iat": 1541100303,
  "jti": "1c73175cd84624efb3ead7ea21fb055ae6f95a519722d314927f251dd13d19aac438861c441ef255a33dae1e1762ee914070251998cbc723a1e2f03f47784470cf70bb6aa003e582e0226d5d8a116af362d9e522a9b29b8281ab2014cc3ae4808e55a6e4752634b3f5b4c4a37e105f57695c6def44e77a2db8e5c9f36a768749"

The key parts of the Payload are:

  • The applicationVersion, the version of the app on HAT
  • The sub (subject), the subject of the token
  • The application, the application name that requested the token
  • The iss (issuer), which is the address of the HAT that has created the token and that you should be logging in
  • The exp (expiry) time of the token as a Unix timestamp, defining whether or not the token is still valid
  • The iat (issued at time) time that the token has been created as a Unix timestamp, defining whether or not the token is still valid. Token is expiring after 30 days of the issued at date no matter if a refreshed token has been received from HAT
  • The jti (JWT ID) the ID of the token

And the Signature, which is generated from the token and the private key of the HAT. The signature must be verified to verify that the token has not been tampered with. A HAT’s public key can be accessed at the /publickey endpoint of the HAT (e.g. The precise handling of tokens with asymmetric keys will depend on your library, however you need to make sure that your library supports RS256 keys. contains a very useful tool for token debugging while in development as well as listing all the major JWT libraries that you can use in your project.

When you have received a successful redirect from HAT you can check with HAT API Android and verity the user. To do so you simply have to call:

            applicationName: Auth.serviceName,
            url: url,
            success: success,
            failed: failed)
  • applicationName is the name of the application that sent the user to log in
  • url is the full url that returned from the HAT, like the example at the beginning of the step 3.
  • success is a callback to execute when the library have successfully authorized user. You can use this callback to dismiss WebView activity, save the values needed and navigate to the next Activity
  • failed is a callback to execute when the library couldn’t authorize user. You can use this callback to dismiss WebView activity and show an error message to the user.

Having received the success callback you have to save the token, ALWAYS use Encryption to save the token. DO NOT save it in any non encrypted database. A good idea would be to also save the full HAT Address as well, as you are gonna need it many times in order to communicate with the HAT. You can store the values in SharedPreferences in Private mode.

As soon as you save the token you are free to navigate to your next Activity.

Fail callback

In case the authentication failed, you will redirected to the fallback url. In that case you should dismiss WebView Activity and then update the UI or show an error message back to the user.

In order to dismiss the browser you do it from the viewController that presented originally the Activity, like this: