Firebase IdToken generated using FirebaseAuth.currentUser.getIdToken() on flutter gives error "failed to verify token signature" on Go backend - firebase

Question

I am working on a flutter app where after sign-in, I have to validate a user's idToken on a custom backend (written in Go) using the Firebase admin SDK: firebase.google.com/go.
I am using the following snippet to sign-in the user via GoogleSignIn and retrieve the Firebase idToken on the client side:
final FirebaseAuth _auth = FirebaseAuth.instance;
final GoogleSignIn googleSignIn = GoogleSignIn();
Future<String> signInWithGoogle() async {
await Firebase.initializeApp();
final GoogleSignInAccount googleSignInAccount = await googleSignIn.signIn();
final GoogleSignInAuthentication googleSignInAuthentication =
await googleSignInAccount.authentication;
final AuthCredential credential = GoogleAuthProvider.credential(
accessToken: googleSignInAuthentication.accessToken,
idToken: googleSignInAuthentication.idToken,
);
final UserCredential authResult = await _auth.signInWithCredential(credential);
final User user = authResult.user;
String FirebaseIdToken = await _auth.currentUser.getIdToken();
print("FirebaseIdToken: " + FirebaseIdToken);
if (user != null) {
/* code to validate user and return it */
} return null;
}
I copy the token corresponding to the FirebaseIdToken variable and send it to the backend using Postman with a Authentication: Bearer <token> request header.
At the backend, there is the following:
/* am.cli here is basically the auth.Client in firebase admin SDK and clientToken is the token received from flutter app. */
idToken, err := am.cli.VerifyIDToken(context.Background(), clientToken)
log.Println("ERROR:", err)
I get the following error printed:
ERROR: failed to verify token signature
Based on the documentation for both client & backend, I believe that I'm using the correct methods to retrieve and to verify the token.
I have tried to retrieve the idToken with the following code as well:
IdTokenResult idTokRes = await _auth.currentUser.getIdTokenResult(true);
print("idTokRes: " + idTokRes.token);
But this fails the same way. (And idTokRes.token and the FirebaseIdToken from the previous method are not same.)
I have also tried to verify the token manually on https://jwt.io/ with the public certificate and the private key which fails as well.
Any help would be appreciated!

Answer 1

Thanks to a member of the Flutter community, I was able to solve the problem.
Turns out, for some reason, the FirebaseIdToken printed by
print("FirebaseIdToken: " + FirebaseIdToken);
is not the complete token. Because of being large, the output gets truncated.
(Still not sure why though. Does Dart's print() statement truncate large strings?)
Edit: Apparently, its the terminal window that truncates/wraps a large output by embedding linebreaks to it.
But, by using the following snippet
String firebaseIdToken = await user.getIdToken();
while (firebaseIdToken.length > 0) {
int startTokenLength =
(firebaseIdToken.length >= 500 ? 500 : firebaseIdToken.length);
print("TokenPart: " + firebaseIdToken.substring(0, startTokenLength));
int lastTokenLength = firebaseIdToken.length;
firebaseIdToken =
firebaseIdToken.substring(startTokenLength, lastTokenLength);
}
I was able to print the complete token in 3 broken parts, which I then concatenated, and sent to the backend via Postman and got no errors this time.
Thanks Rexford!