John Davidson

Message:

I have a PHP application and I am trying to generate a token to authenticate users so that they can access Firebase from the browser.

I have generated a private key from service account in Firebase console and I use the firebase/php-jwt library. When trying to access with the signInWithCustomToken method from javascript I always get the error>

The custom token format is incorrect. Please see the documentation

The code with which I generate the token is:

use Firebase\JWT\JWT;



require_once("php-jwt-master/src/JWT.php");



$service_account_email = "[email protected]"; // Dots are app name



$key = "-----BEGIN PRIVATE KEY-----\n..........\n-----END PRIVATE KEY-----\n"; // Dots are private key from the downloaded file



$time = time();



$token = array( 'iat' => $time,

                'exp' => $time + 3000,

                'uid' => '1',

                'aud' => 'https://identitytoolkit.googleapis.com/google.identity.identitytoolkit.v1.IdentityToolkit',

                'iss' => $service_account_email,

                'sub' => $service_account_email

               );

               



$jwt = JWT::encode($token, $key, 'HS256');

The javascript code is:

function login(){



  var token = document.getElementById("token").value;



  firebase.auth().signInWithCustomToken(token)      

    .then((user) => {           

      console.log("Autenticado");         

    })      

    .catch((error) => {         

      var errorCode = error.code;

      var errorMessage = error.message;

      

      console.log(errorCode + " - " + errorMessage);          

    });

}

I have checked the jwt with jwt.io tool and it seems to be correct.

I am not sure in the key format, I have tried with the header and the "\ n" in a single line as in the file and also without the "\ n" and using the returns in a multiline variable with "<<<".

I have also tried using the encode function with or without the parameter 'HS256'. But I always get the same error.

Thanks in advance for any response.

---

My Answer:
It looks like you are using the wrong algorithm to encode the JWT token. Firebase requires the token to be signed with RS256 algorithm, not HS256.

You can update your PHP code to use the RS256 algorithm like this:

php
$jwt = JWT::encode($token, $key, 'RS256');


Make sure that the private key you are using is in the correct format and does not contain any extra characters. You can try using the following format for the private key:


-----BEGIN PRIVATE KEY-----
YOUR_PRIVATE_KEY_HERE
-----END PRIVATE KEY-----


Also, make sure that the service account email you are using is correct and has the necessary permissions to generate custom tokens.

After making these changes, try generating a new token and see if you are able to authenticate users successfully using the signInWithCustomToken method in JavaScript.

If you continue to face issues, you can refer to the Firebase documentation for more information on generating custom tokens: https://firebase.google.com/docs/auth/admin/create-custom-tokens

I hope this helps! Let me know if you have any other questions.