PROJECT    OOP in Java - Data Encryption and The Caesar Cipher     Solution


Objective To use the Caesar cipher to encrypt login passwords.

PROJECT DESCRIPTION
Using the Caesar cipher, you will encrypt a set of passwords.
Information about this Project

A type of substitution cipher is the Caesar cipher. This cipher is mono - alphabetic since only one alphabet was used. In a substitution cipher, every letter of the plain text is substituted by the some other letter or symbol. The substitution in the Caesar cipher is based on a shift operation performed on the letters of the plaintext. For example if the shift 4 , then the standard alphabet



A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z



becomes



E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D



and a plaintext message such as " PURCHASE NOW " is encrypted as " TYVGLEWI RSA " .

Notice that the letter P was shifted as T , the letter U as Y , and so on.




Steps to Complete this Project



STEP 1 Open Eclipse, JCreator or Similar Java IDE



Open Eclipse, JCreator or similar Java editor on your computer and create a new Java Project called Caesar. Next add a class to your project called Encryptor. Within your new class file, type the Java code from in Figure 1 below.



STEP 2 Test the File1



Run your program. Your program should display an input box similar to the

screen snapshot segment given below.











Into the text field, enter a password to encrypt, such as the word subterfuge and then in the next field key, enter a Caesar key shift such as the number 6 .







Observe the output that results.



PROJECT OOP in Java - Data Encryption and The Caesar Cipher











STEP 3 Dissect the Initial Program Code



Examine in detail the code given in Figure 1 . The bulk of the program code lies within the Encryptor() constructor that has a String return type.



STEP 4 Modify the File



Return to your Java IDE and modify the file such that the user can enter a cipher text ( encrypted ) message and a key and your program will decrypt the message.



One way to modify the program is to first declare a class level variable.



public String cipherText;



Then supplement the main() method with the instantiation of a new object that will be used to perform the decryption.



Notice that in the main method the cipher text, i.e. the decrypted text, is assigned to a global variable in order for it to be used in the Decrypt() method.










public static void main(String [] args) {



// encryption block

Encryptor d = new Encryptor();

String strCipherText = d.Encrypt();

System.out.println(strCipherText);



// decrypt block

Encryptor d = new Encryptor();



// cipher text becomes the input text to the Decrypt method

d.cipherText = strCipherText;



String strPlainText = d.Decrypt();

System.out.println(strPlainText);



System.exit(0);

}









PROJECT OOP in Java - Data Encryption and The Caesar Cipher



Within the class definition add a Decrypt() method. Copy the code from the Encrypt() method and place it into the Decrypt() method.



Now alter the copied code in the Decrypt() method such that the following occur.







plainText = ((String)JOptionPane.showInputDialog("enter words " +

"to decrypt")).toLowerCase().trim();


Change the above line of code to:



plainText = cipherText;



Finally change this copied block of code in the Decrypt() method such that instead of performing a forward shift you will now perform a backward shift.



offset += shift;

if(offset 25)

{

newOffset = offset % 26;

sb.append(alphabet.charAt(newOffset));

}

else

{

sb.append(alphabet.charAt(offset) );

}





A sample trial run of your modified program could appear as follows.







STEP 5 Submit the File



Submit your original and modified source code file for credit. Include screen snapshots showing the operation of your program with the above snapshotted encrypted and decrypted text results.











PROJECT OOP in Java - Data Encryption and the Caesar Cipher



Figure 1 Caesar Cipher Program Code








import javax.swing.*;

public class Encryptor {

private String plainText;

private int shift;



public Encryptor() {

plainText = null;

shift = 0;

}

public static void main(String [] args) {

Encryptor e = new Encryptor();

String strCipherText = e.Encrypt();

System.out.println(strCipherText);

System.exit(0);

}

public String Encrypt()

{

plainText =

((String)JOptionPane.showInputDialog("enter words " +

"to encrypt")).toLowerCase().trim();



shift =

Integer.parseInt(JOptionPane.showInputDialog("enter offset"));



int offset = 0;

int newOffset = 0;

String alphabet = "abcdefghijklmnopqrstuvwxyz";

StringBuffer sb = new StringBuffer();

int index = plainText.length();

for(int i = 0; i < index; i++)

{

String temp = "" + plainText.charAt(i);

offset = alphabet.indexOf(temp);

offset += shift;

if(offset 25)

{

newOffset = offset % 26;

sb.append(alphabet.charAt(newOffset));

}

else

{

sb.append(alphabet.charAt(offset));

}

}

return sb.toString();

}

}
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