Firebase authentication with custom token - ruby-on-rails

Question

I have a firebase project which Im trying to authenticate from my rails server creating a custom token with the library ruby-jwt as it says on the docs, but i keep getting the same error:
auth/invalid-custom-token, The custom token format is incorrect. Please check the documentation.
The credentials.json is from the service account I made in google console, uid is sent from the front end to the api.
def generate_auth_token(uid)
now_seconds = Time.now.to_i
credentials = JSON.parse(File.read("credentials.json"))
private_key = OpenSSL::PKey::RSA.new credentials["private_key"]
payload = {
:iss => credentials["client_email"],
:sub => credentials["client_email"],
:aud => 'https://identitytoolkit.googleapis.com/google.identity.identitytoolkit.v1.IdentityToolkit',
:iat => now_seconds,
:exp => now_seconds+(60*60), # Maximum expiration time is one hour
:uid => uid.to_s,
:claims => {:premium_account => true}
}
JWT.encode(payload, private_key, 'RS256')
end
it looks like this in jwt.io
{
"iss": "defered#defered.iam.gserviceaccount.com",
"sub": "defered#defered.iam.gserviceaccount.com",
"aud": "https://identitytoolkit.googleapis.com/google.identity.identitytoolkit.v1.IdentityToolkit",
"iat": 1486824545,
"exp": 1486828145,
"uid": "4",
"claims": {
"premium_account": true
}
}

Answer 1

It looks like the accepted answer found a way to link authentication from Firebase to Rails, but the original question seems to be asking how to link Rails authentication to Firebase (which is what I was trying to do).
To keep your authentication logic in Rails (ex: from Devise) and share it with Firebase, first get a Firebase server key as a .json file from your Service Accounts page in your project's settings.
You'll only need the private_key and client_id from this file, which I recommend storing as environment variables so they're not potentially leaked in source code.
Next, make a Plain ol' Ruby object (PORO) that will take in a User and spit out a JSON Web Token (JWT) that Firebase can understand:
class FirebaseToken
def self.create_from_user(user)
service_account_email = ENV["FIREBASE_CLIENT_EMAIL"]
private_key = OpenSSL::PKey::RSA.new ENV["FIREBASE_PRIVATE_KEY"]
claims = {
isCool: "oh yeah"
}
now_seconds = Time.now.to_i
payload = {
iss: service_account_email,
sub: service_account_email,
aud: "https://identitytoolkit.googleapis.com/google.identity.identitytoolkit.v1.IdentityToolkit",
iat: now_seconds,
exp: now_seconds + (60*60), # Maximum expiration time is one hour
uid: user.id,
# a hash to pass to the client as JSON
claims: claims
}
JWT.encode payload, private_key, "RS256"
end
end
Now send this JWT to authenticated users through javascript in your application layout:
window.firebaseJWT = "#{FirebaseToken.create_from_user(current_user)}";
In your frontend code, you can now use this token to authenticate users:
firebase
.auth()
.signInWithCustomToken(window.firebaseJWT)
.catch(error => {
console.error(error);
});
Remember to sign them out of firebase when they sign out of your application:
firebase
.auth()
.signOut()
.then(() => {
// Sign-out successful.
})
.catch(error => {
console.error(error);
});

Answer 2

I found a better way to authenticate, I'm just sending the token that firebase gives you and verifying it on rails with the information I need and that's it.

Answer 3

Check if your secret key is wrapped in double quotes and not single as they contain '\n' escape sequences. An auth/invalid-custom-token error is thrown if the secret key is not as specified in the documentation.