Sign in with Apple = invalid_client

I'm facing a very bad issue because I read so many guides and tutorials and nothing works.

The result is always the same: {"error":"invalid_client"}

I get the code, identityToken and everything I need - except the call to https://appleid.apple.com/auth/token - because of invalid_client.

Here is my url for getting the code.

https://appleid.apple.com/auth/authorize?response_type=code&client_id=org.example.service&redirect_uri=https%3A%2F%2Fexample.org

So then I have the default workflow. And after accepting / loggin in I will be redirected to my page.

https://example.org/?code=a277243e2ec324fb09ba1c3333a8e6576.0.abcde.u4xiTDP2qHXoNEaxrcrIGx

(When I'm using the JavaScript API I'll get other informations like state, code and id_token. I already tried it with the "code" there, too.)

Back to the main function.

This is my request for Apple.

'client_id' => 'org.example.service',  
'client_secret' => JWT-Data encoded (OPENSSL_ALGO_SHA256) see below  
'grant_type' => 'authorization_code',  
'code' => 'a277243e2ec324fb09ba1c3333a8e6576.0.abcde.u4xiTDP2qHXoNEaxrcrIGx'

JWT Header:

{
  "alg": "ES256",
  "kid": "1ABC2345DE"
}

JWT Payload:

{
  "iss": "1A234BCD56",
  "iat": 1571269964,
  "exp": 1571273564,
  "aud": "https://appleid.apple.com",
  "sub": "org.example.service"
}

Response:

{  
  "error": "invalid_client"  
}

The useless error message of the world.

I dont know why the client should be invalid.

I have a key in https://developer.apple.com/account/resources/authkeys/list with downloaded file name AuthKey_1ABC2345DE.p8. (means 1ABC2345DE is my key id)

Then I have a native iOS app with identifier "org.example" and a service with identifier "org.example.service".

Its not working with both ids and mixed different things.

Nothing. invalid_client.

Can anyone help me please? I'm sitting here for hours and getting only invalid_client

My testing page:

<html>
<head>
</head>
<body>
<script type="text/javascript" src="https://appleid.cdn-apple.com/appleauth/static/jsapi/appleid/1/en_US/appleid.auth.js"></script>
<div id="appleid-signin" data-color="black" data-border="true" data-type="sign in" data-width="330px" data-height="100px"></div>
<script type="text/javascript">
    AppleID.auth.init({
        clientId : 'org.example.service',
        scope : 'email',
        redirectURI: 'https://example.org',
        state : 'EN'
    });
</script>
</body>
</html>

And PHP:

<?php
// index.php

// function by https://stackoverflow.com/q/56459075/1362858
function encode($data) {
    $encoded = strtr(base64_encode($data), '+/', '-_');
    return rtrim($encoded, '=');
}

// function by https://stackoverflow.com/q/56459075/1362858
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 = '';
    $success = openssl_sign($payload, $signature, $privKey, OPENSSL_ALGO_SHA256);
    if (!$success) return false;

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

$client_id = 'org.example.service';
$data = [
    'client_id' => $client_id,
    'client_secret' => generateJWT('1ABC2345DE', '1A234BCD56', $client_id, file_get_contents('AuthKey_1ABC2345DE.p8')),
    'code' => 'a277243e2ec324fb09ba1c3333a8e6576.0.abcde.u4xiTDP2qHXoNEaxrcrIGx',
    'grant_type' => 'authorization_code'
];
$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, $data);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);

$serverOutput = curl_exec($ch);

curl_close ($ch);

/**
 * {"error":"invalid_client"}
 */
var_dump($serverOutput);

Solution

The problem was this special encryption. In this blog they use PHP for everything except the client_secret generation. https://developer.okta.com/blog/2019/06/04/what-the-heck-is-sign-in-with-apple

And in the text the author explains this sentence:

Some JWT libraries don’t support elliptic curve methods, so make sure yours does before you start trying this out.

Now it's working fine with exactly the code in the top - only replaced the client_secret generation.