Add TlsHelper (designed to be used with sks-keyservers.net)

We can't use the AndroidPinning library for this, because it requires the certificate to be signed using a system CA, sks-keyservers.net uses there own CA

OpenKeychain/src/main/java/org/sufficientlysecure/keychain/helper/TlsHelper.java

@@ -0,0 +1,83 @@
+/*
+ * Copyright (C) 2013-2014 Dominik Schürmann <dominik@dominikschuermann.de>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package org.sufficientlysecure.keychain.helper;
+
+import javax.net.ssl.HttpsURLConnection;
+import javax.net.ssl.SSLContext;
+import javax.net.ssl.TrustManagerFactory;
+import java.io.ByteArrayInputStream;
+import java.io.IOException;
+import java.net.URL;
+import java.security.KeyManagementException;
+import java.security.KeyStore;
+import java.security.KeyStoreException;
+import java.security.NoSuchAlgorithmException;
+import java.security.cert.*;
+
+public class TlsHelper {
+    public static class TlsHelperException extends Exception {
+        public TlsHelperException(Exception e) {
+            super(e);
+        }
+    }
+
+    /**
+     * Opens a Connection that will only accept certificates signed with a specific CA and skips common name check.
+     * This is required for some distributed Keyserver networks like sks-keyservers.net
+     *
+     * @param certificate The X.509 certificate used to sign the servers certificate
+     * @param url         Connection target
+     */
+    public static HttpsURLConnection openCAConnection(byte[] certificate, URL url)
+            throws TlsHelperException, IOException {
+        try {
+            // Load CA
+            CertificateFactory cf = CertificateFactory.getInstance("X.509");
+            Certificate ca = cf.generateCertificate(new ByteArrayInputStream(certificate));
+
+            // Create a KeyStore containing our trusted CAs
+            String keyStoreType = KeyStore.getDefaultType();
+            KeyStore keyStore = KeyStore.getInstance(keyStoreType);
+            keyStore.load(null, null);
+            keyStore.setCertificateEntry("ca", ca);
+
+            // Create a TrustManager that trusts the CAs in our KeyStore
+            String tmfAlgorithm = TrustManagerFactory.getDefaultAlgorithm();
+            TrustManagerFactory tmf = TrustManagerFactory.getInstance(tmfAlgorithm);
+            tmf.init(keyStore);
+
+            // Create an SSLContext that uses our TrustManager
+            SSLContext context = SSLContext.getInstance("TLS");
+            context.init(null, tmf.getTrustManagers(), null);
+
+            // Tell the URLConnection to use a SocketFactory from our SSLContext
+            HttpsURLConnection urlConnection = (HttpsURLConnection) url.openConnection();
+            urlConnection.setSSLSocketFactory(context.getSocketFactory());
+
+            return urlConnection;
+        } catch (CertificateException e) {
+            throw new TlsHelperException(e);
+        } catch (NoSuchAlgorithmException e) {
+            throw new TlsHelperException(e);
+        } catch (KeyStoreException e) {
+            throw new TlsHelperException(e);
+        } catch (KeyManagementException e) {
+            throw new TlsHelperException(e);
+        }
+    }
+}