Update signing logic for correct custom token format

[kosukesaigusa]

Background

Create custom token here:

final adminApp = FirebaseAdminApp.initializeApp(
  'project-id',
  Credential.fromServiceAccountParams(
    clientId: 'client-id',
    privateKey: 'private-key',
    email: 'email',
  ),
);
final auth = Auth(adminApp);
final customToken = await auth.createCustomToken('some-user-id');
print('customToken: $customToken');

Then, try to use the custom token to sign in Firebase Auth with on Flutter client app:

final userCredential = await FirebaseAuth.instance.signInWithCustomToken(customToken);

The exception like the following is thrown:

FirebaseAuthException ([firebase_auth/invalid-custom-token] The custom token format is incorrect. Please check the documentation.)

If I use the custom token created by JS (TS) SDK by the same project service account on my Flutter client app, it successfully signs in.

admin.initializeApp({
    credential: admin.credential.cert(serviceAccount),
    databaseURL: `https://${serviceAccount.projectId}.firebaseio.com`
})

const main = async () => {
    const customToken = await admin.auth().createCustomToken(`some-user-id`)
    console.log(`customToken: ${customToken}`)
}

main()

I noticed the length of custom tokens are different from each other.

# Example of created custom token by this package:
eyJ*****************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************z0A

# Example of created custom token by JS (TS) SDK:
eyJ****************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************uFA

Then this PR fixes the RSA-SHA256 signing logic using pointycastle package instead of crypto package.

After the change,

• The length of created custom tokens is the same as the ones created by JS (TS) SDK.
• Successful signing with custom token is confirmed.

---

I am so excited by and curious about this project, enabling us to write Firebase Admin SDK server-side code by Dart!

Thank you so much for developing such a wonderful project!

[CLAassistant]

All committers have signed the CLA.

[rrousselGit]

Hello! Sorry for the delay.

We'd need tests for this. Could you add those? Otherwise LGTM

[rrousselGit]

Too many casts and ! in there. Could you remove those? Such as by using is and != null

We'd also need tests for when those values are invalid

[kosukesaigusa]

@rrousselGit

Thank you! I'd like you to give some advice about this point!

I'm thinking to improve the code like the following to reduce as casting and !:

crypto_signer.dart

RSAPrivateKey _parsePrivateKeyFromPem() {
  final privateKeyString = credential.privateKey
      .replaceAll('-----BEGIN PRIVATE KEY-----', '')
      .replaceAll('-----END PRIVATE KEY-----', '')
      .replaceAll('\n', '');
  final privateKeyDER = base64Decode(privateKeyString);

  final asn1Parser = ASN1Parser(Uint8List.fromList(privateKeyDER));
  final topLevelSequence = asn1Parser.nextObject();
  if (topLevelSequence is! ASN1Sequence) {
    throw const FormatException('Invalid ASN1 format for private key');
  }
  final privateKeyOctet = topLevelSequence.elements?[2];
  if (privateKeyOctet is! ASN1OctetString) {
    throw const FormatException('Invalid ASN1 format for private key');
  }

  final privateKeyParser = ASN1Parser(privateKeyOctet.valueBytes);
  final privateKeySequence = privateKeyParser.nextObject();
  if (privateKeySequence is! ASN1Sequence) {
    throw const FormatException('Invalid ASN1 format for private key');
  }

  final modulus = privateKeySequence.elements?[1];
  final exponent = privateKeySequence.elements?[3];
  final p = privateKeySequence.elements?[4];
  final q = privateKeySequence.elements?[5];
  if (modulus is! ASN1Integer ||
      exponent is! ASN1Integer ||
      p is! ASN1Integer ||
      q is! ASN1Integer) {
    throw const FormatException('Invalid ASN1 format for private key');
  }

  return RSAPrivateKey(
    modulus.integer!,
    exponent.integer!,
    p.integer,
    q.integer,
  );
}

and tried to write the unit test for success case like the following:

crypto_signer_test.dart

test('parsePrivateKeyFromPem should parse valid private key', () {
  final credential = Credential.fromServiceAccountParams(
    clientId: 'id',
    privateKey: _fakeRSAKey,
    email: 'email',
  );
  final app = FirebaseAdminApp.initializeApp('project-id', credential);
  final signer = cryptoSignerFromApp(app);
  expect(signer, isA<CryptoSigner>());
  expect(signer.algorithm, 'RS256');

  final buffer = Uint8List.fromList(List.generate(32, (i) => i));

  expect(() async => signer.sign(buffer), returnsNormally);
});

However, when I try to write test to confirm the code throwing the FormatException, I need to give invalid private key string to Credential.fromServiceAccountParams, and it throws Invalid DER encoding error before reaching _ServiceAccountSigner.sign (_ServiceAccountSigner._parsePrivateKeyFromPem) method.

https://github.com/google/googleapis.dart/blob/3e5b68f7a81353f8a18ac75ec46c4e1fff2a9b06/googleapis_auth/lib/src/crypto/pem.dart#L100-L102

I think _ServiceAccountSigner is private, so unit test has to be started with the public cryptoSignerFromApp function, which is expected to given FirebaseAdminApp instance.

I appreciate if you have any ideas about it!

[kosukesaigusa]

@rrousselGit

Thank you for your comment!

I will add tests and update the code based on your feedback!