etesync-android/app/src/main/java/at/bitfire/davdroid/webdav/TlsSniSocketFactory.java

/*******************************************************************************
 * Copyright (c) 2014 Ricki Hirner (bitfire web engineering).
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the GNU Public License v3.0
 * which accompanies this distribution, and is available at
 * http://www.gnu.org/licenses/gpl.html
 ******************************************************************************/
package at.bitfire.davdroid.webdav;

import java.io.IOException;
import java.net.InetSocketAddress;
import java.net.Socket;
import java.net.UnknownHostException;
import java.util.Arrays;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.List;
import javax.net.ssl.HostnameVerifier;
import javax.net.ssl.SSLPeerUnverifiedException;
import javax.net.ssl.SSLSession;
import javax.net.ssl.SSLSocket;
import org.apache.commons.lang.StringUtils;
import android.annotation.SuppressLint;
import android.annotation.TargetApi;
import android.net.SSLCertificateSocketFactory;
import android.os.Build;
import android.util.Log;
import org.apache.http.HttpHost;
import org.apache.http.conn.socket.LayeredConnectionSocketFactory;
import org.apache.http.conn.ssl.BrowserCompatHostnameVerifier;
import org.apache.http.protocol.HttpContext;

public class TlsSniSocketFactory implements LayeredConnectionSocketFactory {
	private static final String TAG = "davdroid.SNISocketFactory";
	
	final static TlsSniSocketFactory INSTANCE = new TlsSniSocketFactory();
	
	private final static SSLCertificateSocketFactory sslSocketFactory =
			(SSLCertificateSocketFactory)SSLCertificateSocketFactory.getDefault(0);
	private final static HostnameVerifier hostnameVerifier = new BrowserCompatHostnameVerifier();

	
	/*
	For SSL connections without HTTP(S) proxy:
	   1) createSocket() is called
	   2) connectSocket() is called which creates a new SSL connection
	   2a) SNI is set up, and then
	   2b) the connection is established, hands are shaken and certificate/host name are verified    	 
	
	Layered sockets are used with HTTP(S) proxies:
	   1) a new plain socket is created by the HTTP library
	   2) the plain socket is connected to http://proxy:8080
	   3) a CONNECT request is sent to the proxy and the response is parsed
	   4) now, createLayeredSocket() is called which wraps an SSL socket around the proxy connection,
	      doing all the set-up and verfication
	   4a) Because SSLSocket.createSocket(socket, ...) always does a handshake without allowing
	       to set up SNI before, *** SNI is not available for layered connections *** (unless
	       active by Android's defaults, which it isn't at the moment).
	*/


	@Override
	public Socket createSocket(HttpContext context) throws IOException {
		SSLSocket ssl = (SSLSocket)sslSocketFactory.createSocket();
		setReasonableEncryption(ssl);
		return ssl;
	}

	@Override
	public Socket connectSocket(int timeout, Socket plain, HttpHost host, InetSocketAddress remoteAddr, InetSocketAddress localAddr, HttpContext context) throws IOException {
		Log.d(TAG, "Preparing direct SSL connection (without proxy) to " + host);
		
		// we'll rather use an SSLSocket directly
		plain.close();
		
		// create a plain SSL socket, but don't do hostname/certificate verification yet
		SSLSocket ssl = (SSLSocket)sslSocketFactory.createSocket(remoteAddr.getAddress(), host.getPort());
		setReasonableEncryption(ssl);
		
		// connect, set SNI, shake hands, verify, print connection info
		connectWithSNI(ssl, host.getHostName());

		return ssl;
	}

	@Override
	public Socket createLayeredSocket(Socket plain, String host, int port, HttpContext context) throws IOException, UnknownHostException {
		Log.d(TAG, "Preparing layered SSL connection (over proxy) to " + host);
		
		// create a layered SSL socket, but don't do hostname/certificate verification yet
		SSLSocket ssl = (SSLSocket)sslSocketFactory.createSocket(plain, host, port, true);
		setReasonableEncryption(ssl);

		// already connected, but verify host name again and print some connection info
		Log.w(TAG, "Setting SNI/TLSv1.2 will silently fail because the handshake is already done");
		connectWithSNI(ssl, host);

		return ssl;
	}
	
	
	@TargetApi(Build.VERSION_CODES.JELLY_BEAN_MR1)
	private void connectWithSNI(SSLSocket ssl, String host) throws SSLPeerUnverifiedException {
		// - set SNI host name
		if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR1) {
			Log.d(TAG, "Using documented SNI with host name " + host);
			sslSocketFactory.setHostname(ssl, host);
		} else {
			Log.d(TAG, "No documented SNI support on Android <4.2, trying with reflection");
			try {
				java.lang.reflect.Method setHostnameMethod = ssl.getClass().getMethod("setHostname", String.class);
				setHostnameMethod.invoke(ssl, host);
			} catch (Exception e) {
				Log.w(TAG, "SNI not useable", e);
			}
		}
		
		// verify hostname and certificate
		SSLSession session = ssl.getSession();
		if (!hostnameVerifier.verify(host, session))
			throw new SSLPeerUnverifiedException("Cannot verify hostname: " + host);

		Log.d(TAG, "Established " + session.getProtocol() + " connection with " + session.getPeerHost() +
				" using " + session.getCipherSuite());
	}
	
	
	@SuppressLint("DefaultLocale")
	private void setReasonableEncryption(SSLSocket ssl) {
		// set reasonable SSL/TLS settings before the handshake
		
		// Android 5.0+ (API level21) provides reasonable default settings
		// but it still allows SSLv3
		// https://developer.android.com/about/versions/android-5.0-changes.html#ssl

		// - enable all supported protocols (enables TLSv1.1 and TLSv1.2 on Android <5.0, if available)
		// - remove all SSL versions (especially SSLv3) because they're insecure now
		List<String> protocols = new LinkedList<String>();
		for (String protocol : ssl.getSupportedProtocols())
			if (!protocol.toUpperCase().contains("SSL"))
				protocols.add(protocol);
		Log.v(TAG, "Setting allowed TLS protocols: " + StringUtils.join(protocols, ", "));
		ssl.setEnabledProtocols(protocols.toArray(new String[0]));

		if (android.os.Build.VERSION.SDK_INT < 21) {	
			// choose secure cipher suites
			List<String> allowedCiphers = Arrays.asList(new String[] {
				// allowed secure ciphers according to NIST.SP.800-52r1.pdf Section 3.3.1 (see docs directory)
				// TLS 1.2
				"TLS_RSA_WITH_AES_256_GCM_SHA384",
				"TLS_RSA_WITH_AES_128_GCM_SHA256",
				"TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256",
				"TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256",
				"TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384",
				"TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256",
				"TLS_ECHDE_RSA_WITH_AES_128_GCM_SHA256",
				// maximum interoperability
				"TLS_RSA_WITH_3DES_EDE_CBC_SHA",
				"TLS_RSA_WITH_AES_128_CBC_SHA",
				// additionally
				"TLS_RSA_WITH_AES_256_CBC_SHA",
				"TLS_ECDHE_ECDSA_WITH_3DES_EDE_CBC_SHA",
				"TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA",
				"TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA",
				"TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA",
			});
			
			List<String> availableCiphers = Arrays.asList(ssl.getSupportedCipherSuites());
			
			// preferred ciphers = allowed Ciphers \ availableCiphers
			HashSet<String> preferredCiphers = new HashSet<String>(allowedCiphers);
			preferredCiphers.retainAll(availableCiphers);
			
			// add preferred ciphers to enabled ciphers
			// for maximum security, preferred ciphers should *replace* enabled ciphers,
			// but I guess for the security level of DAVdroid, disabling of insecure
			// ciphers should be a server-side task
			HashSet<String> enabledCiphers = preferredCiphers;
			enabledCiphers.addAll(new HashSet<String>(Arrays.asList(ssl.getEnabledCipherSuites())));
			
			Log.v(TAG, "Setting allowed TLS ciphers: " + StringUtils.join(enabledCiphers, ", "));
			ssl.setEnabledCipherSuites(enabledCiphers.toArray(new String[0]));
		}
	}
	
}