(Perl) 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.

Chilkat Perl Downloads Perl Module for Windows, MacOS, Linux, Alpine Linux, Solaris

use chilkat();

# 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 = chilkat::CkJsonObject->new();
$success = $jsonToken->LoadFile("qa_data/tokens/google_sample_id_token.json");
if ($success == 0) {
    print "Failed to load the JSON file..." . "\r\n";
    exit;
}

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

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

$jsonHeader = chilkat::CkJsonObject->new();
$jsonHeader->Load($jose);
$jsonHeader->put_EmitCompact(0);
print $jsonHeader->emit() . "\r\n";

# The JOSE header looks like this:

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

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

$jsonClaims = chilkat::CkJsonObject->new();
$jsonClaims->Load($claims);
$jsonClaims->put_EmitCompact(0);
print $jsonClaims->emit() . "\r\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 = chilkat::CkCrypt2->new();
$bdHash = chilkat::CkBinData->new();

$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 . "\r\n";
print "token_hash_computed: " . $token_hash_computed . "\r\n";