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 libra

Answer 1

Use 'base64Url.normalize()' function. That's what _decodeBase64() does from the answer above!

String getJsonFromJWT(String splittedToken){
  String normalizedSource = base64Url.normalize(encodedStr);
  return utf8.decode(base64Url.decode(normalizedSource));
}

Answer 2

As of this writing, the jaguar_jwt package is being actively maintained. Although it is not clearly documented, it does have a public method that will decode Base64Url encoding. It does basically the same thing as the accepted answer.

//import 'package:jaguar_jwt/jaguar_jwt.dart';

final String token = 'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJleHAiOjE1NTQ4MjAxNjIsImlhdCI6MTU1NDc3Njk2MiwiaXNzIjoiU3VyYWdjaCIsInN1YiI6IjQifQ.bg5B_k9WCmxiu2epuZo_Tpt_KZC4N9ve_2GEdrulcXM';
final parts = token.split('.');
final payload = parts[1];
final String decoded = B64urlEncRfc7515.decodeUtf8(payload);

This gives a JSON string, which for this particular example is:

{
  "exp":1554820162,
  "iat":1554776962,
  "iss":"Suragch",
  "sub":"4"
}

See also:

• JWT: The Complete Guide to JSON Web Tokens
• String based data encoding: Base64 vs Base64url

Answer 3

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));
}