Phone-To-Server communication encryption in Java

This goes along with another thread, here: How to implement Java 256-bit AES encryption with CBC

Mainly, trying to get AES encryption on a phone using Java.

My question here is how to handle the encryption key. I don't know if I should store the key, hash the key and use that, or do a public-key-encyption scheme. I would rather have a way to not do an initial message from the server to the phone to communicate a key. I want the phone to be ready to encrypt and the server waiting for an encrypted message. To implement something like a key-sharing algorithm, I would have to modify our server application which is not very desireable. It's not impossible, but I'm going for code-reuse here =).

Are you trying to find a way to store the same key on every phone, yet prevent any attackers from getting the key and doing bad things with it? That's just not possible.

Estimate how much it could potentially cost if the key is disclosed. Then estimate the cost of understanding and implementing something like the following key agreement scheme. Choose the alternative that best serves your objectives.

Use ephemeral–static Diffie-Hellman key agreement. To intercept traffic, an attacker would have to hack a particular phone, and replace the server's public key, which is embedded in the application, with her own. That's pretty challenging, and each user has to be targeted individually, so the attack doesn't scale well.

RSA encryption would also work. It is secure and widely supported. But ephemeral Diffie-Hellman has one advantage. RSA would rely on a fixed key pair, so if an attacker eventually recovered the private key, she could decrypt any previously recorded messages.

Since the speed of the two algorithms should be comparable, I recommend Diffie-Hellman unless phones don't support it. Here's an example. (I'm sorry for the excessive length.)

import java.math.BigInteger;
import java.security.KeyFactory;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.spec.PKCS8EncodedKeySpec;
import java.security.spec.X509EncodedKeySpec;
import java.util.Arrays;

import javax.crypto.Cipher;
import javax.crypto.KeyAgreement;
import javax.crypto.interfaces.DHPublicKey;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;

public class DHExample
{

  public static void main(String... argv)
    throws Exception
  {
    if (argv.length == 0)
      argv = new String[]{"swright,password,transfer(10000USD,erickson)"};

    /* In reality, the private key should be stored in a KeyStore, protected by 
     * a strong password that is provided interactively every time the server 
     * is started.
     */
    PrivateKey pvt = Server.loadDemoKey();
    Server server = new Server(pvt);

    for (String message : argv) {
      Client client = new Client();
      byte[] packet = client.encrypt(message);
      /* The client (phone) would then send the packet to the server... */
      /* ... Now, at the server: */
      System.out.println(server.decrypt(packet));
    }

  }

}

class Client
{

  /* This public key corresponds to the private key used by the server. 
   * Generate your own Diffie-Hellman key pair, encode the public key, and 
   * embed it here.
   */
  private static final String server = "1GG80QCC4204DGC29AGP48DTOD041G2C42046050C103UNUKS13LQH4AAIRT59OBNCSJJVC4DNA8UEUH00OCF3V05MA4J6IHAT80H53UQP7M6LHULVONQRKC7MPEDLAR6NG4TO079KDVP6CO5NDECL19D4JUFUG13R20HC4JTRL7BVTDU63FS3MLV7OQKAC58F0JTO7TMJOKFC60HLAG9LK5KH6BR7BSTE5DGTEANFU8H066CTQ5403HO2G60G1TV1K22TD6PTRR5RPAQS6QS5FEBPIINRNUHQTA1FILQC1CUGF0J7A5CLF3LQQHCKVPJH0S88305K94B728V89GK1C4TNPS4J5368KRGJO5JQHDA7P398S2HQS7HBMEJ7B4BEKDVGNUH16LHF3UR2F80I8EUCKJORTA2HI24QH0UVS5DEB7O6IA5MCNK0FDAIAP019GTVTJQ9581040G00E0O8002G600TNIVRQ4KUFKVHTM685L9LRNURUUJ8B1EC45DN5TM70MI0T8RM17QCBSQ2F1FMQM3K63MVVQBMVD7N5MRVMGRVM09B1LGNU92SMS42RSROH3FMP4532CMDHO6E72ICI7USNGGDHPFLTSS7AAFRM9T8E6ELU4E3P5EBA6JPDRUN0PDNG08HBVSRJM5U7VVAQ1PD8FIJMSN9EM3";

  public byte[] encrypt(String message)
    throws Exception
  {
    /* Convert the embedded server public key into a DHPublickKey object. */
    KeyFactory kf = KeyFactory.getInstance("DiffieHellman");
    byte[] decoded = new BigInteger(Client.server, 32).toByteArray();
    DHPublicKey server = (DHPublicKey) kf.generatePublic(new X509EncodedKeySpec(decoded));
    /* Generate an ephemeral key pair with the same parameters as server's. */
    KeyPairGenerator gen = KeyPairGenerator.getInstance("DiffieHellman");
    gen.initialize(server.getParams());
    KeyPair ephemeral = gen.generateKeyPair();
    /* Encode the client public key to be sent to server with ciphertext. */
    byte[] pub = ephemeral.getPublic().getEncoded();
    /* Generate a secret key using Diffie-Hellman between client and server. */
    KeyAgreement ka = KeyAgreement.getInstance("DiffieHellman");
    ka.init(ephemeral.getPrivate());
    ka.doPhase(server, true);
    byte[] raw = ka.generateSecret();
    SecretKeySpec secret;
    try {
      secret = new SecretKeySpec(raw, 0, 16, "AES");
    }
    finally {
      Arrays.fill(raw, (byte) 0);
    }
    /* Setup cipher for encryption with secret key. */
    Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
    cipher.init(Cipher.ENCRYPT_MODE, secret);
    /* Get the IV to be sent to server with ciphertext. */
    byte[] iv = cipher.getParameters().getParameterSpec(IvParameterSpec.class).getIV();
    /* Encrypt the message with the secret key. */
    byte[] plaintext = message.getBytes("UTF-8");
    byte[] ciphertext;
    try {
      ciphertext = cipher.doFinal(plaintext);
    }
    finally {
      Arrays.fill(plaintext, (byte) 0);
    }
    /* Package up ephemeral public key, iv, and cipher text to transmit to server. */
    byte[] packet = new byte[4 + pub.length + iv.length + ciphertext.length];
    for (int idx = 0; idx < 4; ++idx)
      packet[idx] = (byte) (pub.length >>> 8 * idx);
    System.arraycopy(pub, 0, packet, 4, pub.length);
    System.arraycopy(iv, 0, packet, 4 + pub.length, iv.length);
    System.arraycopy(ciphertext, 0, packet, 4 + pub.length + iv.length, ciphertext.length);
    return packet;
  }


}

class Server
{

  /* This key isn't private anymore, since it was posted on stackoverflow.com.
   * Generate your own Diffie-Hellman key pair, encode the private key, and 
   * embed it here. In real code, you don't embed private keys like this. This 
   * is just to enable the demonstration.
   */
  private static final String demo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

  private final PrivateKey pvt;

  static PrivateKey loadDemoKey()
    throws Exception
  {
    /* Convert the embedded server public key into a PrivateKey object. */
    KeyFactory kf = KeyFactory.getInstance("DiffieHellman");
    byte[] decoded = new BigInteger(Server.demo, 32).toByteArray();
    return kf.generatePrivate(new PKCS8EncodedKeySpec(decoded));
  }

  Server(PrivateKey pvt)
  {
    this.pvt = pvt;
  }

  public String decrypt(byte[] packet)
    throws Exception
  {
    /* Deconstruct packet. */
    int len = 0;
    for (int idx = 0; idx < 4; ++idx)
      len |= ((packet[idx] & 0xFF) << 8 * idx);
    byte[] pub = new byte[len];
    System.arraycopy(packet, 4, pub, 0, len);
    KeyFactory kf = KeyFactory.getInstance("DiffieHellman");
    PublicKey client = kf.generatePublic(new X509EncodedKeySpec(pub));
    IvParameterSpec iv = new IvParameterSpec(packet, len + 4, 16);
    /* Perform key agreement to determine secret key. */
    KeyAgreement ka = KeyAgreement.getInstance("DiffieHellman");
    ka.init(pvt);
    ka.doPhase(client, true);
    byte[] raw = ka.generateSecret();
    SecretKeySpec secret;
    try {
      secret = new SecretKeySpec(raw, 0, 16, "AES");
    }
    finally {
      Arrays.fill(raw, (byte) 0);
    }
    /* Setup cipher for decryption with secret key. */
    Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
    cipher.init(Cipher.DECRYPT_MODE, secret, iv);
    byte[] plaintext = cipher.doFinal(packet, len + 20, packet.length - (len + 20));
    try {
      return new String(plaintext, "UTF-8");
    }
    finally {
      Arrays.fill(plaintext, (byte) 0);
    }
  }

}