dart - How to get the claims from a JWT in my Flutter Application

Question

I am writing a Flutter/Dart application and am getting a JWT back from an auth server that has some claims I need to use. I have looked at various (4 so far) Dart JWT libraries -- but all are either too old and no longer work with Dart 2, etc. or they need the secret to decode the JWT which makes no sense and isn't correct (or possible since I have no access ).

So -- how can one get a JWT and get the claims from it within a "modern" Dart/Flutter application?

Answer 1

JWT tokens are just base64 encoded JSON strings (3 of them, separated by dots):

import 'dart:convert';

Map<String, dynamic> parseJwt(String token) {
  final parts = token.split('.');
  if (parts.length != 3) {
    throw Exception('invalid token');
  }

  final payload = _decodeBase64(parts[1]);
  final payloadMap = json.decode(payload);
  if (payloadMap is! Map<String, dynamic>) {
    throw Exception('invalid payload');
  }

  return payloadMap;
}

String _decodeBase64(String str) {
  String output = str.replaceAll('-', '+').replaceAll('_', '/');

  switch (output.length % 4) {
    case 0:
      break;
    case 2:
      output += '==';
      break;
    case 3:
      output += '=';
      break;
    default:
      throw Exception('Illegal base64url string!"');
  }

  return utf8.decode(base64Url.decode(output));
}