Bug OAuth2 - Public flow - grant is always invalid

[stromb0li]

I am trying to follow OAuth2 public flow and I seem to always receive back provided_code_verifier_does_not_match_code_challenge when trying to obtain an access token.

Here's my code in generating the verify and challenge (it's in dart):
Code:

  // Generate a random code verifier
  String _generateCodeVerifier() {
    final random = Random.secure();
    final values = List<int>.generate(32, (i) => random.nextInt(256));
    return base64Url.encode(values).replaceAll('=', '');
  }

  //...

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