Cipher program in java, Basic Computer Science

Assignment Help:
Cryptography, the study of secret writing, has been around for a very long time, from simplistic techniques to sophisticated mathematical techniques. No matter what the form however, there are some underlying things that must be done – encrypt the message and decrypt the encoded message.

One of the earliest and simplest methods ever used to encrypt and decrypt messages is called the Caesar cipher method, used by Julius Caesar during the Gallic war. According to this method, letters of the alphabet are shifted by three, wrapping around at the end of the alphabet. For example,

PlainTest: 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
Caesar shift: d 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

When encrypting a message, you take each letter of the message and replace it with its corresponding letter from the shifted alphabet. To decrypt an encoded message, you simply reverse the operation. That is, you take the letter from the shifted alphabet and replace it with the corresponding letter from the plaintext alphabet. Thus the string the quick brown fox becomes wkh txlfn eurzq ira

Another type of cipher is known as Transposition cipher. In this type of cipher, letters in the original message are re-arranged in some methodical way – for instance, reverse the letters in each string. Thus the string the quick brown fox becomes eht kciuq nworb xof

Still yet another cipher method is the Reverser cipher. This method does not only reverses the letters in each word, but as does the Transposition cipher, but it also reverses the result generated from the Transposition cipher. Hence the original message the quick brown fox becomes xof nworb kciuq eht

Class design
Here are three Cryptography methods – Caesar, Transposition and Reverser. They all have something in common. They encrypt and decrypt messages. That is, they take a string of words and translate each word using the encoding algorithm appropriate for that cipher. Thus each class cipher will need polymorphic encode() and decode() methods, which take a word and encodes and decodes it according to the rule of the particular cipher.

From a design perspective, the encrypt() method and the decrypt() methods will be the same for every class. They simply break message into words and have each word encode or decode. However, the encode and decode methods will be different for each cipher. Figure 1 shows a hierarchy of the classes.















Figure 1. Inheritance hierarchy

From the above analysis a partial abstract class Cipher is depicted be by Listing 1.




import java.util.StringTokenizer;

public abstract class Cipher
{
private String message; // The message string
StringBuffer encrypted_message,
decrypted_message;

public Cipher(String text)
{
// Complete definition
}

public final void encrypt()
{
/* The message string is tokenized into individual words, and each word is encoded
by calling the encode method
*/
encrypted_message = new StringBuffer("");
StringTokenizer words = new StringTokenizer(message);

while(words.hasMoreTokens())
{
String s = words.nextToken();
s = encode(s) + " ";
encrypted_message.append(s);
}
}

public final void decrypt(String message)
{
/* The encoded message string is tokenized into individual words, and each word is
encoded by calling the decode method
*/

decrypted_message = new StringBuffer("");
StringTokenizer words = new StringTokenizer(message);

while(words.hasMoreTokens())
{
String s = words.nextToken();
s = decode(s) + " ";
decrypted_message.append(s);
}
}

String getEncodedMessage()
{
return encrypted_message.toString();
}

String getDecodedMessage()
{
return decrypted_message.toString();
}

public abstract String encode(String s);
public abstract String decode(String s);
}

Listing 1. Abstract class Cipher

The class Caesar inherits the abstract class Cipher. This class defines the methods code and decode. The method encode takes a String parameter and returns a String result. It takes each character of the parameter and performs a Caesar shift on the character. That is, a shift with possible wrap around can be coded as follows:
char ch = word.charAt(i);
ch = (char)(‘a’ + ch – ‘a’ + 3) % 26);

The method decode does the reverse. Listing 2 shows a partial definition of this class - Caesar

public class Caeser extends Cipher
{
public Caeser(String s)
{
super(s);
}
public String encode(String word)
{
StringBuffer result = new StringBuffer();

for (int i = 0; i < word.length(); i++)
{
char ch = word.charAt(i);
ch = determineCharacter(ch, Constants. ENCODE_SHIFT);
result.append(ch);
}
return result.toString();
}

public String decode(String word)
{
// Complete the method and return the decoded word
}

public char determineCharacter(char ch, int shift)
{
if(Character.isLowerCase(ch))
ch = (char)(''a'' + (ch - ''a'' + shift) % Constants.WRAP_AROUND);
return ch;
}
}

Listing 2.

In similar fashion, the class Transpose inherits Cipher and defines the methods code and encode. Listing 3 shows an incomplete definition of the class Transpose.

public class Transpose extends Cipher
{
Transpose(String s)
{
super(s);
}
public String encode(String word)
{
//Complete the method and return the encoded word
}
public String decode(String word)
{
// Complete the method and return the decoded word
}
}
Listing 3

The class Reverser inherits the class Transpose. Listing 4 shows an incomplete definition of this class.

public class Reverser extends Transpose
{
Reverser(String s)
{
super(s);
}

String reverseText(String word)
{
// Complete the method and returns the reverse transposed version;
}
}

Listing 4

Things to do
From the above discussion
• Complete the classes Cipher, Caesar, and Transpose, and Reverser. In addition
• Define an interface called Constants that will store the value 26 in the identifier WRAP_AROUND; similarly, store 3 in the identifier, ENCODE_SHIFT, and 23 in the identifier DECODE_SHIFT (needed to decode the encoded message). The code in the method determineCharacter(char ch, int shift) accounts for lower case letters only. Extend the code to include both upper and lower case letters.

Listing 5 Use the following test class to implement your other classes.

import javax.swing.JOptionPane;

class TestEncryption
{
public static void main(String arg[])
{
String code, output = "";

String text = JOptionPane.showInputDialog("Enter message");

output += "The original message is \n" + text + "\n";

Cipher c = new Caeser(text);
c.encrypt();
code = c.getEncodedMessage();
output += "\nCeasar Cipher\nThe encrypted message is \n" + code + "\n";
c.decrypt(code);
code = c.getDecodedMessage();
output +="The decrypted message is \n" + code + "\n";

c = new Transpose(text);
c.encrypt();
code = c.getEncodedMessage();
output += "\nTranspose\nThe encrypted Transpose message is \n" + code + "\n";
c.decrypt(code);
code = c.getDecodedMessage();
output +="The decripted Transpose message is \n" + code + "\n";

Reverser r = new Reverser(text);
r.encrypt();
code = r.reverse( r.getEncodedMessage() );
output += "\nReverser\nThe encrypted Reverse message is \n" + code+ "\n";
code = r.decode(code);
output+="The decrypted Reverse message is \n" + code;

System.out.println(output);
}
}

Related Discussions:- Cipher program in java

Bidirectional search-artificial intelligence, Bidirectional Search-Artifici...

Bidirectional Search-Artificial intelligence: We've concentrated so far on searches where the point of the search is to search a solution, not the path to the solution. In anot

Hill climbing - artificial intelligence , Hill Climbing - artificial intell...

Hill Climbing - artificial intelligence: As we've seen, in some particular problems, searching the search path from primly to goal state is the point of the exercise. In anothe

Scheme, There is a path between two vertices if there is a series of edges ...

There is a path between two vertices if there is a series of edges that you can use to travel between them. For example, between vertices 1 and 3 above, there is a path made up of

Assignment - concept of programming language , Assignment :  It is compose...

Assignment :  It is composed of variable name, an assignment operator of the language and a value or variable or some expression as per composition allowed based on rules defined

Stack, How to write an algorithm to reverse the order of elements on a stac...

How to write an algorithm to reverse the order of elements on a stack s using two additional stacks

Need a help, i am currently studying my computer science engineering 2nd yr...

i am currently studying my computer science engineering 2nd yr. i am unable to select what specification should i take please help me

Definition of procedure, A procedure is a compilation of instructions to wh...

A procedure is a compilation of instructions to which we can express the flow of our program, and once the execution of these instructions is over power is given back to the next l

Mis, #questionexplain strategic mis categories in detail with illustration....

#questionexplain strategic mis categories in detail with illustration..

Explain the functions of alu, Question 1 Write down the Booth's Multiplica...

Question 1 Write down the Booth's Multiplication Algorithm. Justify the Booth's Multiplication Algorithm Question 2 Explain the functions of ALU Question 3 Define the term ins

Write Your Message!

Captcha
Free Assignment Quote

Assured A++ Grade

Get guaranteed satisfaction & time on delivery in every assignment order you paid with us! We ensure premium quality solution document along with free turntin report!

All rights reserved! Copyrights ©2019-2020 ExpertsMind IT Educational Pvt Ltd