Question

Caesar Cipher in Java Problem?

Objective

Practice the cumulative sum and char data type

Problem

You want to create an app to encrypt the text messages that you send to your friends. Once your friend gets the message, the message should be decrypted so that your friend understands it. To implement this app Caesar cipher algorithm should be used. Caesar Cipher text is formed by rotating each letter by a given amount. For example, if you rotate the letter ‘A’ by 3 you should get ‘D’. rotate ‘B’ by 3 you should get ‘E’. Toward the end of the alphabet you wrap around, if example rotate ‘X’ by 3 you should get ‘A’. rotate ‘Y’ by 3 you should get ’B’

Methods

Public static void main(String[] args)

• Create a scanner object
• Call the method run

public static void run(Scanner kb)

• Ask the user how many times they want to use the app
• Create a for loop
  • Ask the user to enter a message
  • Ask the user to enter a key
  • Call the method encrypt
  • Call the method decrypt
  • Display the encrypted and decrypted message

Public static String encrypt (String message, int key)

{

Convert the message to uppercase using the method toUpperCase from the String class

Declare a variable of type string called result to hold the encrypted message, initialize it to “”;

Create a loop to go through each letter of the message

{

Get each letter and store it in a variable of type char called c, use the charAt method: char c = message.charAt(i)

If the letter is between ‘A’ and ‘Z’

{

Add the key to the letter: c = c + key

//checking for wrap around

If c is greater than ‘Z’

{

Subtract 26 from c

                                           }

else

{

Add 26 to c

                                            }

}//end of if

Add the content of the variable c to the variable result (cumulative sum)

   }//end of the loop

Return result

}//end of the method

Public static String decrypt (String message, int key):

{

Declare a String called result and initialize it to “”

Create a for loop to go through each letter of the message

{

Get the character at each index   char c = message.charAt(i)

If the variable c is between ‘A’ and ‘Z’

{

Subtract the value of the variable key from the variable c

If the content of the variable c is less than ‘A’ //check for wrap around

{

Find the difference between the letter ‘A’ and the variable c : int diff = ‘A’ - c

c = (char)(‘Z’ – diff + 1)

                                            }

                                           else if c > ‘Z’    //

                                        {

                                                Int diff = ‘Z’ - c

                                               C = (char)((‘A’ + diff + 1)

                                       }

                          }

                          Concatenate the variable c to the variable result

              }//end for

             Return result

}//end of the method

Requirements

• Must provide all the methods
• Must generate the given output
• Must follow the naming rules, and conventions
• Must follow the indentation rules

Sample output:

How many times to you want to use the app: 4

Your message? I love java programming

Encoding key? 5

The encrypted message is:

N QTAJ OFAF UWTLWFRRNSL

The decrypted message is:

I LOVE JAVA PROGRAMMING

Answer 1

import java.util.*;
public class CaesarCipherProgram {
    public static void main(String args[]) {
        Scanner sc = new Scanner(System.in);
        System.out.println(" Input the plaintext message : ");
        String plaintext = sc.nextLine();
        System.out.println(" Enter the value by which 
        each character in the plaintext         
        message gets shifted : ");
        int shift = sc.nextInt();
        String ciphertext = "";
        char alphabet;
        for(int i=0; i < plaintext.length();i++) 
        {
             // Shift one character at a time
            alphabet = plaintext.charAt(i);
            
            // if alphabet lies between a and z 
            if(alphabet >= 'a' && alphabet <= 'z') 
            {
             // shift alphabet
             alphabet = (char) (alphabet + shift);
             // if shift alphabet greater than 'z'
             if(alphabet > 'z') {
                // reshift to starting position 
                alphabet = (char) (alphabet+'a'-'z'-1);
             }
             ciphertext = ciphertext + alphabet;
            }
            
            // if alphabet lies between 'A'and 'Z'
            else if(alphabet >= 'A' && alphabet <= 'Z') {
             // shift alphabet
             alphabet = (char) (alphabet + shift);    
                
             // if shift alphabet greater than 'Z'
             if(alphabet > 'Z') {
                 //reshift to starting position 
                 alphabet = (char) (alphabet+'A'-'Z'-1);
             }
             ciphertext = ciphertext + alphabet;
            }
            else {
             ciphertext = ciphertext + alphabet;   
            }
        
    }
    System.out.println(" ciphertext : " + ciphertext);
  }
}