Dart

Validate the at_hash Claim of an ID Token

See more JSON Web Token (JWT) Examples

Demonstrates how to hash an access token to compare it with the at_hash claim of an ID token. Dart

import 'package:chilkat/chilkat.dart';

void main() {
  // This example requires the Chilkat API to have been previously unlocked.
  // See Global Unlock Sample for sample code.

  // This example uses a Google access_token + id_token that looks like this:

  //  {
  //   "access_token": "ya29.a0...0f",
  //   "expires_in": 3599,
  //   "scope": "openid https://www.googleapis.com/auth/userinfo.email",
  //   "token_type": "Bearer",
  //   "id_token": "eyJhb...o5nQ"
  // }

  final jsonToken = CkJsonObject();
  try {
    jsonToken.loadFile('qa_data/tokens/google_sample_id_token.json');
  } on ChilkatException {
    print('Failed to load the JSON file...');
    return;
  }

  // Use Chilkat's JWT API to examine the id_token..
  final jwt = CkJwt();
  final idToken = jsonToken.stringOf('id_token');

  // Extract the JOSE header..
  final jose = jwt.getHeader(idToken);

  final jsonHeader = CkJsonObject();
  jsonHeader.load(jose);
  jsonHeader.emitCompact = false;
  print(jsonHeader.emit());

  // The JOSE header looks like this:

  // {
  //   "alg": "RS256",
  //   "kid": "e8799db06287515556213c80acbcfd022fb302a9",
  //   "typ": "JWT"
  // }

  final claims = jwt.getPayload(idToken);

  final jsonClaims = CkJsonObject();
  jsonClaims.load(claims);
  jsonClaims.emitCompact = false;
  print(jsonClaims.emit());

  // The claims look like this:

  // {
  //   "iss": "https://accounts.google.com",
  //   "azp": "258999997753-5ni8lu5f15r7mno97d82f5lir9i9f6i1.apps.googleusercontent.com",
  //   "aud": "258999997753-5ni8lu5f15r7mno97d82f5lir9i9f6i1.apps.googleusercontent.com",
  //   "sub": "111787341816486547572",
  //   "email": "somebody@gmail.com",
  //   "email_verified": true,
  //   "at_hash": "HYJZImlW3mUK-UfjRfXjKw",
  //   "iat": 1615315968,
  //   "exp": 1615319568
  // }

  // The at_hash is the Access Token hash value. Its value is the base64url encoding of the
  // left-most half of the hash of the octets of the ASCII representation of the access_token value,
  // where the hash algorithm used is the hash algorithm used in the alg Header Parameter of the
  // ID Token's JOSE Header. For instance, if the alg is RS256, hash the access_token value with SHA-256,
  // then take the left-most 128 bits and base64url encode them. The at_hash value is a case sensitive string.

  final tokenToHash = jsonToken.stringOf('access_token');
  final tokenHashExpected = jsonClaims.stringOf('at_hash');

  // Step 1. hashes the access token using SHA-256 (Google uses `RS256` as the ID Token `alg`).
  final crypt = CkCrypt2();
  final bdHash = CkBinData();

  crypt.hashAlgorithm = 'sha256';
  // This encoding mode must match the encoding mode passed in the 2nd arg to AppendEncoded.
  // The encoding mode can be anything, as long as they are the same in both places.
  crypt.encodingMode = 'hex';

  bdHash.appendEncoded(crypt.hashStringENC(tokenToHash), 'hex');
  final sz = bdHash.numBytes;

  final tokenHashComputed = bdHash.getEncodedChunk(0, sz ~/ 2, 'base64url');

  // If the hashes are identical, then the access_token as issued for the given id_token.
  print('token_hash_expected: $tokenHashExpected');
  print('token_hash_computed: $tokenHashComputed');
}