Implement SSL file-based session caching

Caching is beneficial because it can eliminate redundant cryptographic
computations and network traffic when re-establishing a connection to
the same server, thus saving time and conserving power.

src/com/fsck/k9/net/ssl/SslSessionCacheHelper.java

@@ -0,0 +1,89 @@
+package com.fsck.k9.net.ssl;
+
+import java.io.File;
+import java.lang.reflect.Method;
+
+import javax.net.ssl.SSLContext;
+import javax.net.ssl.SSLSessionContext;
+
+import com.fsck.k9.K9;
+
+import android.content.Context;
+import android.os.Build;
+import android.util.Log;
+
+/**
+ * A class to help with associating an {@code SSLContext} with a persistent
+ * file-based cache of SSL sessions.
+ * <p>
+ * This uses reflection to achieve its task.
+ * <p>
+ * The alternative to this would be to use {@link SSLCertificateSocketFactory}
+ * which also provides session caching. The problem with using that occurs when
+ * using STARTTLS in combination with
+ * {@code TrustedSocketFactory.hardenSocket(SSLSocket)}. The result is that
+ * {@code hardenSocket()} fails to change anything because by the time it is
+ * applied to the socket, the SSL handshake has already been completed. (This is
+ * because of another feature of {@link SSLCertificateSocketFactory} whereby it
+ * performs host name verification which necessitates initiating the SSL
+ * handshake immediately on socket creation.)
+ * <p>
+ * If eventually the use of hardenSocket() should become unnecessary, then
+ * switching to using {@link SSLCertificateSocketFactory} would be a better
+ * solution.
+ */
+public class SslSessionCacheHelper {
+    private static Object sSessionCache;
+    private static Method sSetPersistentCacheMethod;
+    private static boolean sIsDisabled = false;
+
+    static {
+        final String packageName;
+        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.KITKAT) {
+            packageName = "org.apache.harmony.xnet.provider.jsse";
+        } else {
+            packageName = "com.android.org.conscrypt";
+        }
+        final File cacheDirectory = K9.app.getDir("sslcache", Context.MODE_PRIVATE);
+        try {
+            Class<?> fileClientSessionCacheClass = Class.forName(packageName +
+                    ".FileClientSessionCache");
+            Method usingDirectoryMethod = fileClientSessionCacheClass
+                    .getMethod("usingDirectory", File.class);
+            sSessionCache = usingDirectoryMethod.invoke(null, cacheDirectory);
+
+            Class<?> sslClientSessionCacheClass = Class.forName(packageName +
+                    ".SSLClientSessionCache");
+            Class<?> clientSessionContextClass = Class.forName(packageName +
+                    ".ClientSessionContext");
+            sSetPersistentCacheMethod = clientSessionContextClass.getMethod(
+                    "setPersistentCache", sslClientSessionCacheClass);
+        } catch (Exception e) {
+            // Something went wrong. Proceed without a session cache.
+            Log.e(K9.LOG_TAG, "Failed to initialize SslSessionCacheHelper: " + e);
+            sIsDisabled = true;
+        }
+    }
+
+    /**
+     * Associate an {@code SSLContext} with a persistent file-based cache of SSL
+     * sessions which can be used when re-establishing a connection to the same
+     * server.
+     * <p>
+     * This is beneficial because it can eliminate redundant cryptographic
+     * computations and network traffic, thus saving time and conserving power.
+     */
+    public static void setPersistentCache(SSLContext sslContext) {
+        if (sIsDisabled) {
+            return;
+        }
+        try {
+            SSLSessionContext sessionContext = sslContext.getClientSessionContext();
+            sSetPersistentCacheMethod.invoke(sessionContext, sSessionCache);
+        } catch (Exception e) {
+            // Something went wrong. Proceed without a session cache.
+            Log.e(K9.LOG_TAG, "Failed to initialize persistent SSL cache: " + e);
+            sIsDisabled = true;
+        }
+    }
+}

src/com/fsck/k9/net/ssl/TrustedSocketFactory.java

@@ -138,6 +138,7 @@ public class TrustedSocketFactory {
 
         SSLContext context = SSLContext.getInstance("TLS");
         context.init(keyManagers, trustManagers, null);
+        SslSessionCacheHelper.setPersistentCache(context);
         SSLSocketFactory socketFactory = context.getSocketFactory();
         Socket trustedSocket;
         if (socket == null) {