(PHP Extension) Validate the at_hash Claim of an ID Token

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

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<?php

// The version number (9_5_0) should match version of the Chilkat extension used, omitting the micro-version number.
// For example, if using Chilkat v9.5.0.48, then include as shown here:
include("chilkat_9_5_0.php");

// 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"
// }

$jsonToken = new CkJsonObject();
$success = $jsonToken->LoadFile('qa_data/tokens/google_sample_id_token.json');
if ($success == false) {
    print 'Failed to load the JSON file...' . "\n";
    exit;
}

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

// Extract the JOSE header..
$jose = $jwt->getHeader($idToken);

$jsonHeader = new CkJsonObject();
$jsonHeader->Load($jose);
$jsonHeader->put_EmitCompact(false);
print $jsonHeader->emit() . "\n";

// The JOSE header looks like this:

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

$claims = $jwt->getPayload($idToken);

$jsonClaims = new CkJsonObject();
$jsonClaims->Load($claims);
$jsonClaims->put_EmitCompact(false);
print $jsonClaims->emit() . "\n";

// 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.

$token_to_hash = $jsonToken->stringOf('access_token');
$token_hash_expected = $jsonClaims->stringOf('at_hash');

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

$crypt->put_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->put_EncodingMode('hex');

$success = $bdHash->AppendEncoded($crypt->hashStringENC($token_to_hash),'hex');
$sz = $bdHash->get_NumBytes();

$token_hash_computed = $bdHash->getEncodedChunk(0,$sz / 2,'base64url');

// If the hashes are identical, then the access_token as issued for the given id_token.
print 'token_hash_expected: ' . $token_hash_expected . "\n";
print 'token_hash_computed: ' . $token_hash_computed . "\n";

?>