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That seemed like a long journey for no purpose. However, we can fix this problem we can modify the backing field itself to be read-only, as shown in Example 3-19.

private readonly string _identifier;

public class AESEncrypter { public static final int IV_SIZE = 16; // 128 bits public static final int KEY_SIZE = 16; // 128 bits public static final int BUFFER_SIZE = 1024; // 1KB Cipher cipher; SecretKey secretKey; AlgorithmParameterSpec ivSpec; byte[] buf = new byte[BUFFER_SIZE]; byte[] ivBytes = new byte[IV_SIZE]; public AESEncrypter(SecretKey key) throws Exception { cipher = Cipher.getInstance("AES/CBC/PKCS5Padding"); secretKey = key; } public void encrypt(InputStream in, OutputStream out) throws Exception { // create IV and write to output ivBytes = createRandBytes(IV_SIZE); out.write(ivBytes); ivSpec = new IvParameterSpec(ivBytes); cipher.init(Cipher.ENCRYPT_MODE, secretKey, ivSpec); // Bytes written to cipherOut will be encrypted CipherOutputStream cipherOut = new CipherOutputStream(out, cipher);

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That will foil the developer who wrote Example 3-15 and Example 3-18. But doesn t it also break our constructor again In fact, it doesn t: read-only fields behave differently from read-only properties. A read-only property can never be modified. A read-only field can be modified, but only by a constructor. Since read-only fields only become truly read-only after construction completes, it makes them perfect for properties that need to be able to be different from one instance to another, but which need to be fixed for the lifetime of an instance. Before we move on from const and readonly fields, there s another property our Plane needs for which const seems like it could be relevant, albeit in a slightly different way. In addition to monitoring the speed of an aircraft, we also need to know whether it is approaching or heading away from the airport. We could represent that with a bool property called something like IsApproaching (where true would mean that it was approaching, and false would, by implication, indicate that it was heading away). That s a bit clumsy, though. You can often end up having to negate Boolean properties you might need to write this sort of thing:

if (!plane.IsApproaching) { ... }

That reads as if not plane is approaching which sounds a bit awkward. We could go with:

if (somePlane.IsApproaching == false) { ... }

That s if is approaching is false which isn t much better. We could offer a second, calculated property called IsNotApproaching, but our code is likely to be simpler and easier to read (and therefore likely to contain fewer bugs) if, instead of using bool, we have a Direction property whose value could somehow be either Approaching or Leaving. We ve just seen a technique we could use for that. We could create two constant fields of any type we like (int, for example), and a property of type int called Direction (see Example 3-20).

// Read in the plaintext bytes and write to cipherOut to encrypt int numRead = 0; while ((numRead = in.read(buf)) >= 0) cipherOut.write(buf, 0, numRead); cipherOut.close(); } public void decrypt(InputStream in, OutputStream out) throws Exception { // read IV first in.read(ivBytes); ivSpec = new IvParameterSpec(ivBytes); cipher.init(Cipher.DECRYPT_MODE, secretKey, ivSpec); // Bytes read from in will be decrypted CipherInputStream cipherIn = new CipherInputStream(in, cipher); // Read in the decrypted bytes and write the plaintext to out int numRead = 0; while ((numRead = cipherIn.read(buf)) >= 0) out.write(buf, 0, numRead); out.close(); } public static byte [] createRandBytes(int numBytes) throws NoSuchAlgorithmException { byte [] bytesBuffer = new byte [numBytes]; SecureRandom sr = SecureRandom.getInstance("SHA1PRNG"); sr.nextBytes(bytesBuffer); return bytesBuffer; } public static void main(String argv[]) throws Exception { if (argv.length != 2) usage(); String operation = argv[0]; String keyFile = argv[1]; if (operation.equals("createkey")) { FileOutputStream fos = new FileOutputStream(keyFile); KeyGenerator kg = KeyGenerator.getInstance("AES"); kg.init(KEY_SIZE*8); SecretKey skey = kg.generateKey(); /* write key */ fos.write(skey.getEncoded()); fos.close(); } else { /* read key */

class Plane { public const int Approaching = 0; public const int Leaving = 1; // ... } public int Direction { get; set; }

This lets us write code that reads a bit more naturally than it would if we had used just true and false:

someBoeing777.Direction = Plane.Approaching; if (someAirbusA380.Direction == Plane.Leaving) { /* Do something */ }

Summary

But there s one problem: if our Direction property s type is int, there s nothing to stop us from saying something like:

someBoeing777.Direction = 72;

byte keyBytes [] = new byte [KEY_SIZE]; FileInputStream fis = new FileInputStream(keyFile); fis.read(keyBytes); SecretKeySpec keySpec = new SecretKeySpec(keyBytes, "AES"); /* initialize encrypter */ AESEncrypter aes = new AESEncrypter(keySpec); if (operation.equals("encrypt")) { aes.encrypt(System.in, System.out); } else if (operation.equals("decrypt")) { aes.decrypt(System.in, System.out); } else { usage(); } } } public static void usage () { System.err.println("java com.learnsecurity.AESEncrypter " + "createkey|encrypt|decrypt <keyfile>"); System.exit(-1); } } We now walk through bite-sized pieces of the code, one chunk at a time. We start with the imports and data members of the AESEncrypter class: package com.learnsecurity; import import import import import java.security.*; java.security.spec.*; javax.crypto.*; javax.crypto.spec.*; java.io.*;

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