Apple ID Sign In: GenerateJWT

AppleAuth.java

import io.jsonwebtoken.Claims;
import io.jsonwebtoken.JwsHeader;
import io.jsonwebtoken.Jwts;
import io.jsonwebtoken.SignatureAlgorithm;
import org.bouncycastle.asn1.pkcs.PrivateKeyInfo;
import org.bouncycastle.openssl.PEMParser;
import org.bouncycastle.openssl.jcajce.JcaPEMKeyConverter;

import java.io.File;
import java.io.FileReader;
import java.math.BigInteger;
import java.security.KeyFactory;
import java.security.NoSuchAlgorithmException;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.RSAPublicKeySpec;
import java.util.Base64;

private String generateJWT() throws Exception {
  var pKey = generatePrivateKey();

  return Jwts.builder()
    .setHeaderParam(JwsHeader.KEY_ID, "KEY_ID")
    .setIssuer("TEAM_ID")
    .setAudience("https://appleid.apple.com")
    .setSubject("CLIENT_ID")
    .setExpiration(new Date(System.currentTimeMillis() + (1000 * 60 * 5)))
    .setIssuedAt(new Date(System.currentTimeMillis()))
    .signWith(SignatureAlgorithm.ES256, pKey)
    .compact();
}