Create Apple Client Secret

CreateSecret.java

import com.nimbusds.jose.*;
import com.nimbusds.jose.crypto.*;
import com.nimbusds.jose.jwk.*;
import com.nimbusds.jwt.*;
import java.io.*;
import java.security.*;
import java.security.spec.*;
import java.time.*;
import java.util.*;

public class JWTExample {

    public static void main(String[] args) throws Exception {

        // Your private key from Apple
        File keyFile = new File("key.txt");
        // Your 10-character Team ID
        String teamId = "";
        // Your Services ID, e.g. idp.applesignintest.com
        String clientId = "";
        // 10-char Key ID value from the Keys section
        String keyId = "";

        // Read the key file and create an EC key
        byte[] keyBytes = Files.readAllBytes(keyFile.toPath());
        KeyFactory kf = KeyFactory.getInstance("EC");
        PKCS8EncodedKeySpec spec = new PKCS8EncodedKeySpec(keyBytes);
        PrivateKey privateKey = kf.generatePrivate(spec);

        // Create the JWT header with the key ID
        JWSHeader header = new JWSHeader.Builder(JWSAlgorithm.ES256)
                .keyID(keyId)
                .build();

        // Set the claims for the JWT
        Date now = new Date();
        Date exp = Date.from(Instant.ofEpochSecond(now.getTime() / 1000 + 86400 * 180));
        JWTClaimsSet claims = new JWTClaimsSet.Builder()
                .issuer(teamId)
                .issueTime(now)
                .expirationTime(exp)
                .audience("https://appleid.apple.com")
                .subject(clientId)
                .build();

        // Sign the JWT with the private key
        JWSSigner signer = new ECDSASigner(privateKey);
        SignedJWT signedJWT = new SignedJWT(header, claims);
        signedJWT.sign(signer);

        // Print the resulting JWT
        System.out.println(signedJWT.serialize());
    }
}