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

I had this error several times. Here are the causes I could find:

• Improper Domain Verification and invalid redirect_uri
• Client ID is not correct: your client ID could be incorrect.
• JWT encoder is not working properly: maybe it doesn't support the ES256 algorithm?
• Request type: for /auth/authorize, you need to use x-www-form-urlencode, oherwise you get invalid_client errors.

When I solved these problems, I started to get invalid_grant error. Here were the steps I had been doing:

• I created client_secret via JWT
• I navigated to https://appleid.apple.com/auth/authorize?response_type=code&state=abcdefg&client_id=com.company.apple-sign-in-abcd&scope=openid&redirect_uri=https://app.com/redirect_uri manually on web browser,
• authorized
• it redirected back to backend url (which gave 404 because it was not deployed yet).
• I copied the code argument in the url bar when
• With the copied code, I POSTeed the https://appleid.apple.com/auth/token endpoint with x-www-form-urlencoded arguments:
  • code
  • client_id
  • client_secret
  • redirect_uri
  • grant_type="authorization_code"

If you lose a few seconds, code gets invalidated and you'll get invalid_grant error. If you copy and paste immediately within second, you'll get your response:

{
    "access_token": "abcdefg",
    "token_type": "Bearer",
    "expires_in": 3600,
    "refresh_token": "abcdefg",
    "id_token": "abcdefghijklmnopqrstu"
}

The next step would be decoding id_token with Apple's public key.

The {"error":"invalid_client"} message could be related to an invalid signature generated by the openssl_sign function. The ES256 algorithm must be used in order to sign the JWT and the generated signature should be the concatenation of two unsigned integers, denoted as R and S. It turns out that openssl_sign function generates a DER-encoded ASN.1 signature which is not correct for Apple (see here).

So the solution is to convert the DER-encoded ASN.1 signature generated by openSSL into a simple concatenation of the R and S values.

This can be done by using the following function:

/**
 * @param string $der
 * @param int    $partLength
 *
 * @return string
 */
public static function fromDER(string $der, int $partLength)
{
    $hex = unpack('H*', $der)[1];
    if ('30' !== mb_substr($hex, 0, 2, '8bit')) { // SEQUENCE
        throw new \RuntimeException();
    }
    if ('81' === mb_substr($hex, 2, 2, '8bit')) { // LENGTH > 128
        $hex = mb_substr($hex, 6, null, '8bit');
    } else {
        $hex = mb_substr($hex, 4, null, '8bit');
    }
    if ('02' !== mb_substr($hex, 0, 2, '8bit')) { // INTEGER
        throw new \RuntimeException();
    }
    $Rl = hexdec(mb_substr($hex, 2, 2, '8bit'));
    $R = self::retrievePositiveInteger(mb_substr($hex, 4, $Rl * 2, '8bit'));
    $R = str_pad($R, $partLength, '0', STR_PAD_LEFT);
    $hex = mb_substr($hex, 4 + $Rl * 2, null, '8bit');
    if ('02' !== mb_substr($hex, 0, 2, '8bit')) { // INTEGER
        throw new \RuntimeException();
    }
    $Sl = hexdec(mb_substr($hex, 2, 2, '8bit'));
    $S = self::retrievePositiveInteger(mb_substr($hex, 4, $Sl * 2, '8bit'));
    $S = str_pad($S, $partLength, '0', STR_PAD_LEFT);
    return pack('H*', $R.$S);
}
/**
 * @param string $data
 *
 * @return string
 */
private static function preparePositiveInteger(string $data)
{
    if (mb_substr($data, 0, 2, '8bit') > '7f') {
        return '00'.$data;
    }
    while ('00' === mb_substr($data, 0, 2, '8bit') && mb_substr($data, 2, 2, '8bit') <= '7f') {
        $data = mb_substr($data, 2, null, '8bit');
    }
    return $data;
}
/**
 * @param string $data
 *
 * @return string
 */
private static function retrievePositiveInteger(string $data)
{
    while ('00' === mb_substr($data, 0, 2, '8bit') && mb_substr($data, 2, 2, '8bit') > '7f') {
        $data = mb_substr($data, 2, null, '8bit');
    }
    return $data;
}

which can be found in this library. More details here Apple Sign In, sign JWT for authentication using PHP and openSSL

I made a little package to generate apple client secret in php, based on jwt-framework: https://github.com/kissdigital-com/apple-sign-in-client-secret-generator

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