MtlsKeyStoreUtils.java

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package com.google.auth.mtls;

import com.google.api.client.util.PemReader;
import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.nio.charset.StandardCharsets;
import java.security.GeneralSecurityException;
import java.security.KeyFactory;
import java.security.KeyStore;
import java.security.PrivateKey;
import java.security.cert.Certificate;
import java.security.cert.CertificateException;
import java.security.cert.CertificateFactory;
import java.security.cert.X509Certificate;
import java.security.spec.PKCS8EncodedKeySpec;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.logging.Level;
import java.util.logging.Logger;
import org.jspecify.annotations.NullMarked;

/**
 * Builds the mTLS {@link KeyStore} from PEM-encoded certificates and a private key.
 *
 * <p>This replaces {@code com.google.api.client.util.SecurityUtils#createMtlsKeyStore}, which keeps
 * only the first certificate. Here the key entry holds every distinct certificate in the input, so
 * the intermediate certificates are presented too. Some peers (for example, another workload that
 * trusts only a root CA) need them to verify the client certificate. Because every certificate in
 * the input is sent in the TLS handshake, the input should hold only the leaf certificate and its
 * intermediates.
 *
 * <p>Everything else matches the upstream method: key store type (JKS), alias, empty password,
 * accepted key format (PKCS#8 {@code PRIVATE KEY}), which private key is chosen, and error
 * messages. Any valid certificate-and-key pair that the upstream method accepted is still accepted,
 * even though more of the input is now used: once a certificate and a private key have been found
 * (the point where the upstream method stopped reading), a malformed later PEM section ends the
 * input instead of failing, and an extra certificate that cannot be parsed is left out of the
 * chain. Unlike the upstream method, the input is decoded as UTF-8 rather than the platform default
 * charset, it is read to the end, and the private key is verified against the leaf certificate's
 * public key.
 */
@NullMarked
final class MtlsKeyStoreUtils {

  private static final Logger LOGGER = Logger.getLogger(MtlsKeyStoreUtils.class.getName());

  private MtlsKeyStoreUtils() {}

  /**
   * Creates a key store holding one private key entry.
   *
   * @param certAndKey PEM input containing one or more {@code CERTIFICATE} sections and a PKCS#8
   *     {@code PRIVATE KEY} section, in any order. The first certificate must be the one that
   *     matches the private key (the leaf); the remaining distinct certificates are kept in input
   *     order as the rest of the chain. If there are several private keys, the one used is the same
   *     as upstream: the last one before the first certificate, or else the first one after it. The
   *     stream is read to the end and is not closed.
   * @throws IllegalArgumentException if the input has no certificate or no private key, or if a PEM
   *     section before both a certificate and a private key have been read is malformed
   * @throws GeneralSecurityException if the first certificate or the private key cannot be parsed,
   *     if the private key does not match the leaf certificate's public key, or if the key store
   *     cannot be created
   * @throws IOException if the input cannot be read
   */
  static KeyStore createMtlsKeyStore(InputStream certAndKey)
      throws GeneralSecurityException, IOException {
    List<byte[]> certificates = new ArrayList<>();
    byte[] privateKey = null;
    PemReader reader = new PemReader(new InputStreamReader(certAndKey, StandardCharsets.UTF_8));
    while (true) {
      PemReader.Section section;
      try {
        section = reader.readNextSection();
      } catch (IllegalArgumentException e) {
        if (!certificates.isEmpty() && privateKey != null) {
          // Malformed trailing content after a usable certificate and key: stop here, as the
          // upstream method (which stopped reading at this point) would have.
          break;
        }
        throw e;
      }
      if (section == null) {
        break;
      }
      if ("CERTIFICATE".equals(section.getTitle())) {
        byte[] certBytes = section.getBase64DecodedBytes();
        if (!containsCertificate(certificates, certBytes)) {
          certificates.add(certBytes);
        }
      } else if ("PRIVATE KEY".equals(section.getTitle())
          && (privateKey == null || certificates.isEmpty())) {
        // Same choice as upstream: a later key replaces an earlier one until the first certificate
        // has been seen.
        privateKey = section.getBase64DecodedBytes();
      }
    }
    if (certificates.isEmpty()) {
      throw new IllegalArgumentException("certificate is missing from certAndKey string");
    }
    if (privateKey == null) {
      throw new IllegalArgumentException("private key is missing from certAndKey string");
    }

    CertificateFactory certFactory = CertificateFactory.getInstance("X.509");
    List<X509Certificate> chain = new ArrayList<>(certificates.size());
    chain.add(parseCertificate(certFactory, certificates.get(0)));
    for (int i = 1; i < certificates.size(); i++) {
      try {
        chain.add(parseCertificate(certFactory, certificates.get(i)));
      } catch (CertificateException | RuntimeException e) {
        // The upstream method never parsed extra certificates; leave this one out rather than
        // fail.
        LOGGER.log(Level.FINE, "Skipping unparsable extra certificate in mTLS chain", e);
      }
    }
    X509Certificate leaf = chain.get(0);
    String keyAlgorithm = leaf.getPublicKey().getAlgorithm();
    PrivateKey key =
        KeyFactory.getInstance("ECDSA".equals(keyAlgorithm) ? "EC" : keyAlgorithm)
            .generatePrivate(new PKCS8EncodedKeySpec(privateKey));
    MtlsUtils.verifyKeyPair(leaf, key);

    KeyStore keyStore = KeyStore.getInstance("JKS");
    keyStore.load(null);
    keyStore.setKeyEntry("alias", key, new char[] {}, chain.toArray(new X509Certificate[0]));
    return keyStore;
  }

  private static boolean containsCertificate(List<byte[]> certificates, byte[] candidate) {
    for (byte[] existing : certificates) {
      if (Arrays.equals(existing, candidate)) {
        return true;
      }
    }
    return false;
  }

  private static X509Certificate parseCertificate(CertificateFactory certFactory, byte[] der)
      throws CertificateException {
    Certificate cert = certFactory.generateCertificate(new ByteArrayInputStream(der));
    if (cert == null) {
      throw new CertificateException("Empty certificate");
    }
    return (X509Certificate) cert;
  }
}