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Frank's Java Code Stack #1 Working with Cipher Streams
Frank's Java Code Stack #1 Working with Cipher Streams

(October 11, 2002) - Often it is necessary to encrypt confidential messages and send them across a stream for better Data veracity. With JCE, you can encrypt streams of data and decrypt them using public/private key pair or using a single signature key file. The technique involves associating a Cipher object to Input or Output Stream. Please note that JCE is different from JSA and you need Java Cryptography Extension APIs to work with this example. JDK 1.4 comes with JCE APIs.

Let's create a simple program, which initializes a KeyGenerator class, instantiates a Cipher Object using Cipher feedback Mode (CFB), which is a mode, provided by SunJCE Provider. The ciphstream code, which we have developed, encrypts a simple text message and writes it to a stream and creates a key file, which is necessary to decrypt this stream later.

1.    import javax.crypto.*;
2.    import javax.crypto.spec.*;
3.    import java.io.*;
4.    import java.security.*;
5.    public class ciphstream{
6.    /* A simple String encrypting method */
7.     public void encryptMessage(String message){
8.      try{
9.        /* Algorithm of our choice is DES..
10.        You can try SHA */
11.        KeyGenerator
12.        kg=KeyGenerator.getInstance("DES");
13.        /* SecureRandom provides a
14.        cryptographically strong
15.        pseudo-random number generator
16.              for implementation independent
17.        algorithms
18.        */
19.        kg.init(new SecureRandom());
20.
21.        SecretKey key=kg.generateKey();
22.        SecretKeyFactory
23.        skf=SecretKeyFactory.getInstance("DES");
24.
25.        Class al=Class.forName
26.        ("javax.crypto.spec.DESKeySpec");
27.        DESKeySpec ks=
28.        (DESKeySpec)skf.getKeySpec(key,al);
29.
30.        ObjectOutputStream oos=new
31.        ObjectOutputStream(new
32.        FileOutputStream("keyfile"));
33.        oos.writeObject(ks.getKey());
34.
35.        /* Transformation Format: Algo/Mode/Padding
36.        And CFB8 Mode does not need any Padding */
37.        Cipher c=
38.        Cipher.getInstance("DES/CFB8/NoPadding");
39.        c.init(Cipher.ENCRYPT_MODE,key);
40.
41.        /* Write the Cipher stream to a file */
42.        CipherOutputStream cos=new
43.        CipherOutputStream(new
44.        FileOutputStream("ciphertext"),c);
45.        PrintWriter pw=new PrintWriter(new
46.        OutputStreamWriter(cos));
47.        pw.println(message);
48.        pw.close();
49.
50.        oos.writeObject(c.getIV());
51.        oos.close();
52.    }catch(Exception e){ System.out.println(""+e);}
53.   }
54.    public void decryptMessage(){
55.    try{
56.        ObjectInputStream ois=new
57.        ObjectInputStream(new
58.        FileInputStream("keyfile"));
59.
60.        /* Read the Secret Key from the Key File */
61.        DESKeySpec ks=new DESKeySpec((byte[])
62.        ois.readObject());
63.
64.        SecretKeyFactory
65.        skf=SecretKeyFactory.getInstance("DES");
66.        SecretKey key=skf.generateSecret(ks);
67.
68.        /* Read the Cipher text */
69.        Cipher c=
70.        Cipher.getInstance("DES/CFB8/NoPadding");
71.        c.init(Cipher.DECRYPT_MODE,key,new
72.        IvParameterSpec((byte[]) ois.readObject()));
73.        CipherInputStream cis=new
74.        CipherInputStream(new
75.        FileInputStream("ciphertext"),c);
76.
77.        cis.read(new byte[0]); BufferedReader br=new
78.        BufferedReader(new InputStreamReader(cis));
79.        System.out.println("Retrieved Message..");
80.        System.out.println(br.readLine());
81.    }catch(Exception e){System.out.println(""+e);}
82.   }
83.
84.    public static void main(String ar[]){
85.        ciphstream myciph=new ciphstream();
86.        myciph.encryptMessage("Karl Mark is dead.
87.        Patriots report at 7. Keller Square");
88.        myciph.decryptMessage();
89.     }
90.  }

Before Creating a Cipher object, you need a valid Secret Key to operate on the message. We take the aid from SecretKeyFactory and the DESKeySpec class to generate a random secret key, which we can use to encrypt our message. And also we save the secret key in a file for decrypting the message at a later instance. Note that anyone having this key file can decrypt the message. After generating the key, we create a Cipher object and initialize it with the key. By default the CFB mode works on 8-byte block (CFB8). You can, however, change this. But make sure that the value is a multiple of 8. Decryption is straightforward and we use Initialization Vector for decrypting because of the inherent requirement of CFB mode.

Assignment:
Create a client server application, which can exchange crypted messages over the stream. Don't save the key as a file. Directly pipe it to a Socket Output Stream.

About Frank Jennings
Frank Jennings works in the Communication Designs Group of Pramati Technologies

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Reader Feedback: Page 1 of 1

For generating keypair:

KeyPairGenerator kpg=KeyPairGenerator.getInstance("DSA");
//Strength Value
kpg.initialize(512);
KeyPair kp=kpg.generateKeyPair();

Do check out these two interface DSAPrivateKey and DSAPublicKey

This deals with the basic symmetric key encryption. How do you encrypt with a private/public key pair. I don't want to generated a signature. Cleartext should be encrypted by the public key and decrypted directly with the private key.

I found the article interesting, showing how to use high-level cryptography API's in an almost simple way (well, you still have to know how the machinery works to use it correctly).

What surprises me, on the other side, is the low-level way used to access the various services. Nearly all parameters are sent in string form. One even needs the reflection mechanism to build a class (again with its name in string form!), followed by a cast to regain the proper object.

The conclusion I draw from this is that enumeration types are cruelly missing in Java. Tricks exist to simulate them, and I can only be surprised to see how little they are generally used in the various API's I can see. This one is no exception.




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