Many may have been googled around with this very topic and have came across with many example; but most of them haven’t depicted the practical one.
So, here I tried to conduct practical one. Hope this will help 
We’ll started with how to generated the SecretKey without creating a new one every time you have run the software (no more random).I’ll illustrate this example with the “DES” algorithm. The “DES” algorithm required at least 8 bytes key. Here is SecretKey generator function.
1: public static SecretKey generateKey(String pass)
2: throws InvalidKeyException
3: {
4: SecretKey key = null;
5:
6: DESKeySpec ks = new DESKeySpec( pass.getBytes() ); // required 8 bytes at least
7: SecretKeyFactory factory;
8: try {
9: factory = SecretKeyFactory.getInstance( "DES" );
10: key = factory.generateSecret( ks );
11: } catch (NoSuchAlgorithmException e) {
12: e.printStackTrace();
13: } catch (InvalidKeySpecException e) {
14: e.printStackTrace();
15: }
16: return key;
17: }
After you have a function for generated the key; Let’s translate your class to array of byte and vice versa.
1: public static byte[] createByteArrayFromObject(Serializable o) throws IOException {
2: ByteArrayOutputStream baos = new ByteArrayOutputStream();
3: ObjectOutputStream oos;
4: oos = new ObjectOutputStream(baos);
5: oos.writeObject(o);
6: oos.close();
7: return baos.toByteArray();
8: }
9:
10: public static Object createObjectFromByteArray(byte[] byteObject) throws IOException, ClassNotFoundException {
11: ByteArrayInputStream bais;
12: ObjectInputStream ois;
13: bais = new ByteArrayInputStream(byteObject);
14: ois = new ObjectInputStream(bais);
15: return ois.readObject();
16:
17: }
This is how to encrypt the class; supply the class’s array of byte through bytes parameter.
1: public static String encryptBytes(SecretKey key, byte[] bytes) {
2: try {
3: Cipher ecipher = Cipher.getInstance("DES");
4: ecipher.init(Cipher.ENCRYPT_MODE, key);
5:
6: // re-adjust the bytes accordingly to the protocol
7: byte[] arranged = bytes;
8:
9: // Encrypt
10: byte[] enc = ecipher.doFinal(arranged);
11:
12: // Encode bytes to base64 to get a string
13: return new sun.misc.BASE64Encoder().encode(enc);
14: } catch (javax.crypto.BadPaddingException e) {
15: } catch (IllegalBlockSizeException e) {
16: } catch (javax.crypto.NoSuchPaddingException e) {
17: } catch (java.security.NoSuchAlgorithmException e) {
18: } catch (java.security.InvalidKeyException e) {
19: }
20: return null;
21: }
Now to decrypt:
1: public byte[] decryptBytes(String str) {
2: try {
3: Cipher dcipher;
4:
5: dcipher = Cipher.getInstance("DES");
6: dcipher.init(Cipher.DECRYPT_MODE, key);
7:
8: // Decode base64 to get bytes
9: byte[] dec = new sun.misc.BASE64Decoder().decodeBuffer(str);
10:
11: // Decrypt
12: byte[] unarranged = dcipher.doFinal(dec);
13: // re-adjust the bytes accordingly to the protocol
14: byte[] arranged = unarranged;
15:
16: return arranged;
17: } catch (javax.crypto.BadPaddingException e) {
18: } catch (IllegalBlockSizeException e) {
19: } catch (UnsupportedEncodingException e) {
20: } catch (java.io.IOException e) {
21: } catch (javax.crypto.NoSuchPaddingException e) {
22: } catch (java.security.NoSuchAlgorithmException e) {
23: } catch (java.security.InvalidKeyException e) {
24: }
25: return null;
26: }
Now encrypted this class’s array of byte with the key generated by the first function.
encrypt:
1: String password = "this is the password string used as key to encrypt";
2:
3: SecretKey key = generateKey(password);
4:
5: ObjectToEncrypt ote = new ObjectToEncrypt();
6: String encryptedString = encryptBytes(key, createByteArrayFromObject(ote));
decrypt:
1: String password = "the same password that used earlier";
2:
3: String encryptedString = "...."; // this is encrypted string
4:
5: SecretKey key = generateKey(password);
6:
7: ObjectToEncrypt ote = (ObjectToEncrypt) createObjectFromByteArray(decryptBytes(key, encryptedString));
Note that the object to encrypted/decrypted must implements Serializable.
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