Okhttp3 - Accept all certificates and use a certificatePinner

Question

I'm trying to pin my server's self-signed certificate. My OkHttpClient takes two parameters, the first one is the ssl Socket Factory :

final TrustManager[] trustAllCerts = new TrustManager[] {
        new X509TrustManager() {
            @SuppressLint("TrustAllX509TrustManager")
            @Override
            public void checkClientTrusted(java.security.cert.X509Certificate[] chain, String authType) throws CertificateException {}

            @SuppressLint("TrustAllX509TrustManager")
            @Override
            public void checkServerTrusted(java.security.cert.X509Certificate[] chain, String authType) throws CertificateException {}

            @Override
            public java.security.cert.X509Certificate[] getAcceptedIssuers() {
                  return new X509Certificate[0];
            }
         }
     };

// Install the all-trusting trust manager
SSLContext sslContext;
try {
     sslContext = SSLContext.getInstance("SSL");
     sslContext.init(null, trustAllCerts, new java.security.SecureRandom());
 } catch (NoSuchAlgorithmException | KeyManagementException e) {
     e.printStackTrace();
     FirebaseCrash.report(e);
     return null;
}

// Create an ssl socket factory with our all-trusting manager
final SSLSocketFactory sslSocketFactory = sslContext.getSocketFactory();

Second is a certificate pinner :

new CertificatePinner.Builder()
    .add("bogus.com", "sha1/BOGUS")
    .build()

Note: if I don't add a certificatePinner, then everything works fine. The problem is that when the request gets executed, CertificatePinner.check() gets called:

if (pins.isEmpty()) return;

Obviously, if I do set a (non-empty) certificatePinner, the method won't stop there and will continue. It then goes on to check that "at least one of the certificates pinned for my hostname is a trusted certificate".

The problem is that I passed an empty array in getAcceptedIssuers for my TrustManager - meaning the self-signed cert will trigger an exception since it hasn't been explicitly trusted in "getAcceptedIssues". Seems like it's impossible to pin a certificate that isn't trusted explicitly in "getAcceptedIssuers".

Is there any way to get around that? Is it by design?

This is how I build my OkHttpClient :

OkHttpClient client = new OkHttpClient.Builder()
    .sslSocketFactory(sslSocketFactory, (X509TrustManager) trustAllCerts[0])
    .certificatePinner(certPinner)
    .readTimeout(10, TimeUnit.SECONDS)
    .connectTimeout(10, TimeUnit.SECONDS)
    .build();

Answer 1

The TrustManager, CertificatePinner and Hostname verification all do different but important things. If you want to use self-signed certificates but still have security, as opposed to self-signed certificates purely for ease of local development then you probably want to create a valid TrustManager.

e.g. https://github.com/yschimke/oksocial/blob/3757196cde420b9d0fe37cf385b66f4cdafb1ae1/src/main/java/com/baulsupp/oksocial/security/CertificateUtils.java#L19

  public static X509TrustManager load(List<File> serverCerts)
      throws NoSuchAlgorithmException, KeyStoreException, IOException, CertificateException {
    return trustManagerForKeyStore(keyStoreForCerts(serverCerts));
  }

  public static X509TrustManager trustManagerForKeyStore(KeyStore ks)
      throws NoSuchAlgorithmException, KeyStoreException {
    TrustManagerFactory tmf =
        TrustManagerFactory.getInstance(TrustManagerFactory.getDefaultAlgorithm());

    tmf.init(ks);

    return (X509TrustManager) tmf.getTrustManagers()[0];
  }

  public static KeyStore keyStoreForCerts(List<File> serverCerts)
      throws CertificateException, KeyStoreException, IOException, NoSuchAlgorithmException {
    CertificateFactory cf = CertificateFactory.getInstance("X.509");

    KeyStore ks = KeyStore.getInstance(KeyStore.getDefaultType());
    ks.load(null);

    for (int i = 0; i < serverCerts.size(); i++) {
      try (InputStream is = new FileInputStream(serverCerts.get(i))) {
        X509Certificate caCert = (X509Certificate) cf.generateCertificate(is);
        ks.setCertificateEntry("cacrt." + i, caCert);
      }
    }
    return ks;
  }

This will start off with the system certificates loaded, so your client can still be used to load externally hosted images etc.

Then on top of that you can use CertificatePinner to require that only your trusted self-signed certificates are used for your domain.

Answer 2

private static OkHttpClient getUnsafeOkHttpClient() {
  try {
    // Create a trust manager that does not validate certificate chains
    final TrustManager[] trustAllCerts = new TrustManager[] {
        new X509TrustManager() {
          @Override
          public void checkClientTrusted(java.security.cert.X509Certificate[] chain, String authType) throws CertificateException {
          }

          @Override
          public void checkServerTrusted(java.security.cert.X509Certificate[] chain, String authType) throws CertificateException {
          }

          @Override
          public java.security.cert.X509Certificate[] getAcceptedIssuers() {
            return new java.security.cert.X509Certificate[]{};
          }
        }
    };

    // Install the all-trusting trust manager
    final SSLContext sslContext = SSLContext.getInstance("SSL");
    sslContext.init(null, trustAllCerts, new java.security.SecureRandom());
    // Create an ssl socket factory with our all-trusting manager
    final SSLSocketFactory sslSocketFactory = sslContext.getSocketFactory();

    OkHttpClient.Builder builder = new OkHttpClient.Builder();
    builder.sslSocketFactory(sslSocketFactory, (X509TrustManager)trustAllCerts[0]);
    builder.hostnameVerifier(new HostnameVerifier() {
      @Override
      public boolean verify(String hostname, SSLSession session) {
        return true;
      }
    });

    OkHttpClient okHttpClient = builder
        .connectTimeout(15, TimeUnit.SECONDS)
        .writeTimeout(15, TimeUnit.SECONDS)
        .readTimeout(15, TimeUnit.SECONDS)
        .build();
    return okHttpClient;
  } catch (Exception e) {
    throw new RuntimeException(e);
  }
}