invalid_client for sign in with apple

What I try to achieve:

• iOS client sends a JWT token to the backend.
• Backend (Java) calls https://appleid.apple.com/auth/token to verify the token.

what I have so far:

to make Apple verification call:

        restTemplate = new RestTemplate();

        HttpHeaders headers = new HttpHeaders();
        headers.setContentType(MediaType.APPLICATION_FORM_URLENCODED);
        MultiValueMap<String, String> map = new LinkedMultiValueMap<>();
        map.add("client_id", clientId); // app_id like com.app.id
        String token = generateJWT();   // generated jwt
        map.add("client_secret", token); 
        map.add("grant_type", "authorization_code");
        map.add("code", authorizationCode);  // JWT code we got from iOS
        HttpEntity<MultiValueMap<String, String>> request = new HttpEntity<>(map, headers);

        final String appleAuthURL = "https://appleid.apple.com/auth/token";
        String response = restTemplate.postForObject(appleAuthURL, request, String.class);

token generation:

        final PrivateKey privateKey = getPrivateKey();
        final int expiration = 1000 * 60 * 5;

        String token = Jwts.builder()
                .setHeaderParam(JwsHeader.KEY_ID, keyId) // key id I got from Apple 
                .setIssuer(teamId)  
                .setAudience("https://appleid.apple.com")
                .setSubject(clientId) // app id com.app.id
                .setExpiration(new Date(System.currentTimeMillis() + expiration))
                .setIssuedAt(new Date(System.currentTimeMillis()))
                .signWith(SignatureAlgorithm.ES256, privateKey) // ECDSA using P-256 and SHA-256
                .compact();

        return token;

to get my private key from the file:

        final Reader pemReader = new StringReader(getKeyData());
        final PEMParser pemParser = new PEMParser(pemReader);
        final JcaPEMKeyConverter converter = new JcaPEMKeyConverter();
        final PrivateKeyInfo object = (PrivateKeyInfo) pemParser.readObject();
        final PrivateKey pKey = converter.getPrivateKey(object);

I confirmed my JWT has all required fields:

{
  "kid": "SAME KEY AS MY KEY ID",
  "alg": "ES256"
}

{
  "iss": "Blahblah",
  "aud": "https://appleid.apple.com",
  "sub": "com.app.id",
  "exp": 1578513833,
  "iat": 1578513533
}

This line caught my attention:

map.add("code", authorizationCode);  // JWT code we got from iOS

The authorizationCode is not a jwt

JSON Web Tokens consist of 3 parts separated by dots

but the authorizationCode has 4 parts like this:

text1.text2.0.text3

You are probably using the identityToken from the iOS app instead of the authorizationCode

This is how you retrieve it:

let authorizationCode = String(data: appleIDCredential.authorizationCode!, encoding: .utf8)!
print("authorizationCode: \(authorizationCode)")

Also good to have the following in mind for those who might come here after getting the same invalid_client error:

1. kid is the id for the private key from developer.apple.com/account/resources/authkeys/list

2. keyFile is the file holding the private key downloaded from developer.apple.com

3. teamID can be found by logging in to developer.apple.com and clicking on account, the teamID can be seen in the upper right corner

4. the value in aud should be https://appleid.apple.com

5. app_id is the bundle identifier for the app

In case it might help, here is a working solution in python to create a client_secret:

# $ pip install pyjwt
import jwt
import time

kid = "myKeyId"  
keyFile = "/pathToFile/AuthKey.p8"
key = ""
with open(keyFile, 'r') as myFile:
    key = myFile.read()

print(key)

timeNow = int(round(time.time()))
time3Months = timeNow + 86400*90

claims = {
    'iss': teamID,
    'iat': timeNow,
    'exp': time3Months,
    'aud': 'https://appleid.apple.com',
    'sub': app_id,
}


secret = jwt.encode(claims, key, algorithm='ES256', headers={'kid': kid})
print("secret:")
print(secret)
client_secret = secret.decode("utf-8")
print(client_secret)