CPS3498 Computer Security Homework Name: Chapter 2 1. Write two computer program

Question

CPS3498 Computer Security Homework Name: Chapter 2 1. Write two computer programs to simulate a stream cipher that consists of both encryption and decryption. The encryption program accepts inputs from an existing text file, called "letter.txt. The encryption program produces an output ciphertext file, called "secret The decryption program takes "secret as input and decrypts it into original plaintext. (Hint: the random "seed must be known, but be kept secure, by the pseudorandom number generators in both encryption and decryption programs.)

Explanation / Answer

Encryption program:

import java.security.*;
import java.security.spec.*;

import java.io.*;
import javax.crypto.*;
import javax.crypto.spec.*;

public class Encrypt {

private final String ALGO = "DES";
private final String MODE = "ECB";
private final String PADDING = "PKCS5Padding";
private static int mode = 0;

public static void main(String args[]) {
Encrypt me = new Encrypt();
me.encrypt();
}

public void encrypt() {
try {
System.out.println("Start encryption ...");

/* Get Input Data */
String input = getInputData();
System.out.println("Input data : "+input);

/* Create Secret Key */
KeyGenerator keyGen = KeyGenerator.getInstance(ALGO);
SecureRandom random = SecureRandom.getInstance("SHA1PRNG", "SUN");
keyGen.init(56,random);
Key sharedKey = keyGen.generateKey();

/* Create the Cipher and init it with the secret key */
Cipher c = Cipher.getInstance(ALGO+"/"+MODE+"/"+PADDING);
c.init(Cipher.ENCRYPT_MODE,sharedKey);
byte[] ciphertext = c.doFinal(input.getBytes());
System.out.println("Input Encrypted : "+new String(ciphertext,"UTF8"));

/* Save key to a file */
save(sharedKey.getEncoded(),"shared.key");

/* Save encrypted data to a file */
save(ciphertext,"encrypted.txt");
  
} catch (Exception e) {
e.printStackTrace();
}
}

private String getInputData() {
// Replace this code with code to read data from file if you want
String id = "owner.id=...";
String name = "owner.name=...";
String contact = "owner.contact=...";
String tel = "owner.tel=...";
final String rc = System.getProperty("line.separator");
StringBuffer buf = new StringBuffer();
buf.append(id);
buf.append(rc);
buf.append(name);
buf.append(rc);
buf.append(contact);
buf.append(rc);
buf.append(tel);
return buf.toString();
}

private void save(byte[] buf, String file) throws IOException {
FileOutputStream fos = new FileOutputStream(file);
fos.write(buf);
fos.close();
}

}

Decryption program:

import java.security.*;
import java.security.spec.*;

import java.io.*;
import javax.crypto.*;
import javax.crypto.spec.*;

public class Decrypt {

private final String ALGO = "DES";
private final String MODE = "ECB";
private final String PADDING = "PKCS5Padding";
private static int mode = 0;

public static void main(String args[]) {
Decrypt me = new Decrypt();
me.decrypt();
}

public void decrypt() {
try {
System.out.println("Start decryption ...");

/* Get encoded shared key from file*/
byte[] encoded = load("shared.key");
SecretKeyFactory kf = SecretKeyFactory.getInstance(ALGO);
KeySpec ks = new DESKeySpec(encoded);
SecretKey ky = kf.generateSecret(ks);

/* Get encoded data */
byte[] ciphertext = load("encrypted.txt");
System.out.println("Encoded data = " + new String(ciphertext,"UTF8"));

/* Create a Cipher object and initialize it with the secret key */
Cipher c = Cipher.getInstance(ALGO+"/"+MODE+"/"+PADDING);
c.init(Cipher.DECRYPT_MODE,ky);

/* Update and decrypt */
byte[] plainText = c.doFinal(ciphertext);
System.out.println("Plain Text : "+new String(plainText,"UTF8"));
} catch (Exception e) {
e.printStackTrace();
}
}

private byte[] load(String file) throws FileNotFoundException, IOException {
FileInputStream fis = new FileInputStream(file);
byte[] buf = new byte[fis.available()];
fis.read(buf);
fis.close();
return buf;
}
}

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