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

user_2921408 0
Viewed 21154

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?

jwt dart flutter
4 Answers

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));
}
user_24619570

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));
}
user_99870590

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:

user_36818800

you can decode JWT base64 by seperate first part of it that contains "."

      String decodeUserData(String code) {
      String normalizedSource = base64Url.normalize(code.split(".")[1]);
      return utf8.decode(base64Url.decode(normalizedSource));
      }
user_106771570
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