Apple ID Sign In: CreatePublicKeyApple

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.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 PublicKey createPublicKeyApple(String keyIdentifier) throws InvalidKeySpecException, NoSuchAlgorithmException {
  // Http Requests Client
  var applePublicKeysList = httpClient.getListPublicKeys()
    
  var applePublicKey = applePublicKeysList.getKeys().stream()
    .filter(publicKey -> keyIdentifier.equals(publicKey.getKid()))
    .findFirst();

  if (!applePublicKey.isPresent()) throw new AppleKeysException("Not matching key");

  var modulus = new BigInteger(1, Base64.getUrlDecoder().decode(applePublicKey.get().getN()));
  var exponent = new BigInteger(1, Base64.getUrlDecoder().decode(applePublicKey.get().getE()));

  return KeyFactory.getInstance(applePublicKey.get().getKty()).generatePublic(new RSAPublicKeySpec(modulus, exponent));
}