How to verify code from "Sign In with Apple"?

Question

I'm trying to verify the code I got from the "Sign In with Apple" service on my Redirect Uri. I used the information from the documentation to create the post data and generate the "client_secret".

The response I'm getting is: {"error":"invalid_client"}.

My functions to generate the "client_secret" can be found below:

function encode($data) {
    $encoded = strtr(base64_encode($data), '+/', '-_');
    return rtrim($encoded, '=');
}

function generateJWT($kid, $iss, $sub, $key) {
    $header = [
        'alg' => 'ES256',
        'kid' => $kid
    ];
    $body = [
        'iss' => $iss,
        'iat' => time(),
        'exp' => time() + 3600,
        'aud' => 'https://appleid.apple.com',
        'sub' => $sub
    ];

    $privKey = openssl_pkey_get_private($key);
    if (!$privKey) return false;

    $payload = encode(json_encode($header)).'.'.encode(json_encode($body));
    $signature = '';
    $success = openssl_sign($payloads, $signature, $privKey, OPENSSL_ALGO_SHA256);
    if (!$success) return false;

    return $payload.'.'.encode($signature);
}

My variables in this example:

$kid is my identifier for my private key. In this example it is JYJ5GS7N9K. I got the identifier from here https://developer.apple.com/account/resources/authkeys/list

$iss is my team identifier from my developer account. In this example it is WGL33ABCD6.

$sub is the same value as "client_id". My "client_id" in this example is "dev.hanashi.sign-in-with-apple". I got the client id from the app identifiers here: https://developer.apple.com/account/resources/identifiers/list

$key is my generated private key by developer account. The key has format like this:

-----BEGIN PRIVATE KEY-----
myrandomgeneratedkeybyappledeveloperaccount
-----END PRIVATE KEY-----

This is the php code to make the request:

$key = <<<EOD
-----BEGIN PRIVATE KEY-----
myrandomgeneratedkeybyappledeveloperaccount
-----END PRIVATE KEY-----
EOD; // replaced with correct key

$kid = 'JYJ5GS7N9K'; // identifier for private key
$iss = 'WGL33ABCD6'; // team identifier
$sub = 'dev.hanashi.sign-in-with-apple'; // my app id

$jwt = generateJWT($kid, $iss, $sub, $key);

$data = [
    'client_id' => $sub,
    'client_secret' => $jwt,
    'code' => $_POST['code'],
    'grant_type' => 'authorization_code',
    'request_uri' => 'https://myurl.tld/redirect.php'
];
$ch = curl_init();

curl_setopt($ch, CURLOPT_URL, 'https://appleid.apple.com/auth/token');
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($data));
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_USERAGENT, 'Mozilla/5.0 (Windows NT 6.2) AppleWebKit/536.6 (KHTML, like Gecko) Chrome/20.0.1090.0 Safari/536.6');

$serverOutput = curl_exec($ch);

curl_close ($ch);
echo $serverOutput;

I get now the response {"error":"invalid_client"} from the apple server. What am I doing wrong? Could it be that I'm generating the JWT token wrong?

Answer 1

The problem for me was that I forgot to verify my domain under the Service Id section of the Apple dev portal.

You need to download the key they give you, and upload it to: https://example.com/.well-known/apple-developer-domain-association.txt

The website doesn't verify automatically, you have to click the verify button and get a green tick next to the domain to be sure. After this, I had no more invalid_client issues.

Update 2020.07

As the flow was changed, you just have to add the Domain and the Communication Email to:

Certificates, Identifiers & Profiles > More > Configure