Apple ID Sign In: GeneratePrivateKey

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 PrivateKey generatePrivateKey() throws Exception {
  var file = new File("CERTIFICATE_PATH");

  var pemParser = new PEMParser(new FileReader(file));
  var converter = new JcaPEMKeyConverter();
  var object = (PrivateKeyInfo) pemParser.readObject();

  return converter.getPrivateKey(object);
}