Flutter: How to decode JWT token using Dart?

JWT tokens are 3 Base64 strings separated by “.”.

• Part 1 is called Header. it contains an algorithm and token type.
• Part 2 is called Payload. it contains user data.
• Part 3 is called Signature. Which is calculated like following:

<Hashing Algo: e.g. HMACSHA256 etc>(
    base64UrlEncode(header) + "." + base64UrlEncode(payload), "secret string<Known to JWT creator>"
);

As a Application developer we are mostly interested in decoding the Payload to get User Details like mail-id, session time etc. So we start by splitting the JWT in 3 parts.

final parts = token.split('.');
if (parts.length != 3) {
  throw Exception('invalid token');
}

Now, Decode Base64 encoded string to get Payload JSON.

import 'dart:convert';
String decodeBase64(String str) {
  //'-', '+' 62nd char of encoding,  '_', '/' 63rd char of encoding
  String output = str.replaceAll('-', '+').replaceAll('_', '/');
  switch (output.length % 4) { // Pad with trailing '='
    case 0: // No pad chars in this case
      break;
    case 2: // Two pad chars
      output += '==';
      break;
    case 3: // One pad char
      output += '=';
      break;
    default:
      throw Exception('Illegal base64url string!"');
  }

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

Above function will return Json string with Payload.

//0: Header, 1: Payload, 2: Signature
final payload = _decodeBase64(parts[1]); 
final payloadMap = json.decode(payload); //Map dictionary 

Complete Code: