How to use SSL in ftp4j library

Question

I'm currently stuck as I find the documentation on ftp4j very light. According to this: Document

I can set up the library to connect using SSL certificates. The problem I'm having is figuring out how the library will know which SSL certificate to accept and which not(In case of an impostor). Any helps? Thanks!

Edit: I want to know how to import the SSL certificate and make the library only connect to an specific server with the specific SSL certificate.

Answer 1

You can use Sun's InstallCert.java to get the certificate you want to accept saved in a separate store. Then use this separate store when you setup your FTPClient.

I included a version of InstallCert.java below since it's harder and harder to find on the Internet after Oracle took over...

/*
 * Copyright 2006 Sun Microsystems, Inc.  All Rights Reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 *   - Redistributions of source code must retain the above copyright
 *     notice, this list of conditions and the following disclaimer.
 *
 *   - Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 *
 *   - Neither the name of Sun Microsystems nor the names of its
 *     contributors may be used to endorse or promote products derived
 *     from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
 * IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
 * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR
 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
/**
 * http://blogs.sun.com/andreas/resource/InstallCert.java
 * Use:
 * java InstallCert hostname
 * Example:
 *% java InstallCert ecc.fedora.redhat.com
 */

import javax.net.ssl.*;
import java.io.*;
import java.security.KeyStore;
import java.security.MessageDigest;
import java.security.cert.CertificateException;
import java.security.cert.X509Certificate;

/**
 * Class used to add the server's certificate to the KeyStore
 * with your trusted certificates.
 */
public class InstallCert {

    public static void main(String[] args) throws Exception {
    String host;
    int port;
    char[] passphrase;
    if ((args.length == 1) || (args.length == 2)) {
        String[] c = args[0].split(":");
        host = c[0];
        port = (c.length == 1) ? 443 : Integer.parseInt(c[1]);
        String p = (args.length == 1) ? "changeit" : args[1];
        passphrase = p.toCharArray();
    } else {
        System.out.println("Usage: java InstallCert <host>[:port] [passphrase]");
        return;
    }

    File file = new File("jssecacerts");
    if (file.isFile() == false) {
        char SEP = File.separatorChar;
        File dir = new File(System.getProperty("java.home") + SEP
                + "lib" + SEP + "security");
        file = new File(dir, "jssecacerts");
        if (file.isFile() == false) {
            file = new File(dir, "cacerts");
        }
    }
    System.out.println("Loading KeyStore " + file + "...");
    InputStream in = new FileInputStream(file);
    KeyStore ks = KeyStore.getInstance(KeyStore.getDefaultType());
    ks.load(in, passphrase);
    in.close();

    SSLContext context = SSLContext.getInstance("TLS");
    TrustManagerFactory tmf =
            TrustManagerFactory.getInstance(TrustManagerFactory.getDefaultAlgorithm());
    tmf.init(ks);
    X509TrustManager defaultTrustManager = (X509TrustManager) tmf.getTrustManagers()[0];
    SavingTrustManager tm = new SavingTrustManager(defaultTrustManager);
    context.init(null, new TrustManager[]{tm}, null);
    SSLSocketFactory factory = context.getSocketFactory();

    System.out.println("Opening connection to " + host + ":" + port + "...");
    SSLSocket socket = (SSLSocket) factory.createSocket(host, port);
    socket.setSoTimeout(10000);
    try {
        System.out.println("Starting SSL handshake...");
        socket.startHandshake();
        socket.close();
        System.out.println();
        System.out.println("No errors, certificate is already trusted");
    } catch (SSLException e) {
        System.out.println();
        e.printStackTrace(System.out);
    }

    X509Certificate[] chain = tm.chain;
    if (chain == null) {
        System.out.println("Could not obtain server certificate chain");
        return;
    }

    BufferedReader reader =
            new BufferedReader(new InputStreamReader(System.in));

    System.out.println();
    System.out.println("Server sent " + chain.length + " certificate(s):");
    System.out.println();
    MessageDigest sha1 = MessageDigest.getInstance("SHA1");
    MessageDigest md5 = MessageDigest.getInstance("MD5");
    for (int i = 0; i < chain.length; i++) {
        X509Certificate cert = chain[i];
        System.out.println
                (" " + (i + 1) + " Subject " + cert.getSubjectDN());
        System.out.println("   Issuer  " + cert.getIssuerDN());
        sha1.update(cert.getEncoded());
        System.out.println("   sha1    " + toHexString(sha1.digest()));
        md5.update(cert.getEncoded());
        System.out.println("   md5     " + toHexString(md5.digest()));
        System.out.println();
    }

    System.out.println("Enter certificate to add to trusted keystore or 'q' to quit: [1]");
    String line = reader.readLine().trim();
    int k;
    try {
        k = (line.length() == 0) ? 0 : Integer.parseInt(line) - 1;
    } catch (NumberFormatException e) {
        System.out.println("KeyStore not changed");
        return;
    }

    X509Certificate cert = chain[k];
    String alias = host + "-" + (k + 1);
    ks.setCertificateEntry(alias, cert);

    OutputStream out = new FileOutputStream("jssecacerts");
    ks.store(out, passphrase);
    out.close();

    System.out.println();
    System.out.println(cert);
    System.out.println();
        System.out.println
                ("Added certificate to keystore 'jssecacerts' using alias '"
                        + alias + "'");
    }

    private static final char[] HEXDIGITS = "0123456789abcdef".toCharArray();

    private static String toHexString(byte[] bytes) {
        StringBuilder sb = new StringBuilder(bytes.length * 3);
        for (int b : bytes) {
            b &= 0xff;
            sb.append(HEXDIGITS[b >> 4]);
            sb.append(HEXDIGITS[b & 15]);
            sb.append(' ');
        }
        return sb.toString();
    }

private static class SavingTrustManager implements X509TrustManager {

        private final X509TrustManager tm;
        private X509Certificate[] chain;

        SavingTrustManager(X509TrustManager tm) {
            this.tm = tm;
        }

        public X509Certificate[] getAcceptedIssuers() {
            throw new UnsupportedOperationException();
        }

        public void checkClientTrusted(X509Certificate[] chain, String authType)
                throws CertificateException {
            throw new UnsupportedOperationException();
        }

        public void checkServerTrusted(X509Certificate[] chain, String authType)
                throws CertificateException {
            this.chain = chain;
            tm.checkServerTrusted(chain, authType);
        }
    }
}

That code makes a copy of the certificates supplied by the JRE before adding the new certificate you supply. If you don't want that, but instead only store your certificate, you will need to remove this part:

if (file.isFile() == false) {
        char SEP = File.separatorChar;
        File dir = new File(System.getProperty("java.home") + SEP
                + "lib" + SEP + "security");
        file = new File(dir, "jssecacerts");
        if (file.isFile() == false) {
            file = new File(dir, "cacerts");
        }
    }

Now when you have a file containing your certificate you can use this to create a SSLSocketFactory that you supply to the FTPClient.

You can create some kind of certificate manager like this:

import java.io.File;
import java.io.FileInputStream;
import java.io.InputStream;
import java.security.KeyStore;
import java.security.cert.CertificateException;
import java.security.cert.X509Certificate;

import javax.net.ssl.SSLContext;
import javax.net.ssl.SSLSocketFactory;
import javax.net.ssl.TrustManager;
import javax.net.ssl.TrustManagerFactory;
import javax.net.ssl.X509TrustManager;

public class CertManager {

private static final char[] passphrase = "changeit".toCharArray();
private String rootPath;
private SSLSocketFactory factory;
private SavingTrustManager trustManager;
private KeyStore keyStore;


/**
 * Creates a CertManager.
 * @param rootPath Path to directory where the file 'jssecacerts' is located.
 */
public CertManager(String rootPath){
    this.rootPath = rootPath;       
}

/**
 * Gets a SSLSocketFactory with the trusted certs.
 * @return
 * @throws Exception
 */
public SSLSocketFactory getSSLSocketFactory() throws Exception {
    //Load trusted certs
    File file = new File(rootPath+"jssecacerts");

    System.out.println("Loading KeyStore " + file + "...");
    InputStream in = new FileInputStream(file);
    keyStore = KeyStore.getInstance(KeyStore.getDefaultType());
    keyStore.load(in, passphrase);
    in.close();

    //Use these certs 
    SSLContext context = SSLContext.getInstance("TLS");
    TrustManagerFactory tmf =
        TrustManagerFactory.getInstance(TrustManagerFactory.getDefaultAlgorithm());
    tmf.init(keyStore);
    X509TrustManager defaultTrustManager = (X509TrustManager)tmf.getTrustManagers()[0];
    trustManager = new SavingTrustManager(defaultTrustManager);
    context.init(null, new TrustManager[] {trustManager}, null);
    factory = context.getSocketFactory();
    return factory;
}

private static class SavingTrustManager implements X509TrustManager {

    private final X509TrustManager tm;
    private X509Certificate[] chain;

    SavingTrustManager(X509TrustManager tm) {
        this.tm = tm;
    }

    public X509Certificate[] getAcceptedIssuers() {         
        return tm.getAcceptedIssuers();
    }

    public void checkClientTrusted(X509Certificate[] chain, String authType)
    throws CertificateException {
        this.chain = chain;
        tm.checkClientTrusted(chain, authType);
    }

    public void checkServerTrusted(X509Certificate[] chain, String authType)
    throws CertificateException {
        this.chain = chain;
        tm.checkServerTrusted(chain, authType);
    }
}
}

Use your certificate manager to get the SSLSocketFactory that you supply to the FTPClient:

...
FTPClient client = new FTPClient();
CertManager certMan = new CertManager("");
SSLSocketFactory sslSocketFactory = null;
try{
    sslSocketFactory = certMan.getSSLSocketFactory();
} catch (Exception e) {
    e.printStackTrace();
}
client.setSSLSocketFactory(sslSocketFactory);
client.setSecurity(FTPClient.SECURITY_FTPS); // or client.setSecurity(FTPClient.SECURITY_FTPES);
...