Question

Using the website Repl.it

Write a Java program that can perform the Caesar cipher for English messages that include both upper and lowercase alphabetic characters. The Caesar cipher replaces each plaintext letter with a different one, by a fixed number of places down the alphabet. The program should be able to shift a specific number of places based on the user's input number.

For example

Plaintext: THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG.

Ciphertext: QEBNRFZH YOLTK CLU GRJMP LSBO QEB IXWV ALD

Also enable ROT13. Algorithm ROT13 is a common example of a cipher that was used quite frequently on numerous sites. With ROT13, you are shifting each character 13 places in the alphabet, the same shift will return the text to its original position.

And if you have time...write a program to decipher both. For example, change QEB NRFZH YOLTK CLU GRJMP LSBO QEB IXWV ALD back to THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG.

Answer 1

CODE:

import java.util.Scanner;

public class CaesarCipher {
// Encrypts text using a shift od s
public static StringBuffer caesarCipher(String plaintext, int shift) {
StringBuffer result= new StringBuffer(); //the string that will be returned
char ch;
for (int i=0; i<plaintext.length(); i++) {
if (Character.isUpperCase(plaintext.charAt(i))) { // if the character is an uppercase character
if(((((int)plaintext.charAt(i) - shift - 65) % 26) + 65) < 65) {// if the new character is lower than A
ch = (char)((((int)plaintext.charAt(i) - shift - 65) % 26) + 65 + 26) ;// we add 26 more to it
}
else {
ch = (char)((((int)plaintext.charAt(i) - shift - 65) % 26) + 65);//else we add the base 65 to teh character
}
//System.out.println(" Character = " + plaintext.charAt(i) + "=" + ((int)plaintext.charAt(i) - shift - 65) % 26);
result.append(ch);
}
else if(Character.isLowerCase(plaintext.charAt(i))) {// if the character is an uppercase character
if(((((int)plaintext.charAt(i) - shift - 65) % 26) + 97) < 97) { // if the new character gooes lower than a
ch = (char)((((int)plaintext.charAt(i) - shift - 97) % 26) + 97 + 26);// we add 26 to it
}
else {
ch = (char)((((int)plaintext.charAt(i) - shift - 97) % 26) + 97);
}
result.append(ch);
}
else { // if the character is an special character, number or spaces
  
result.append(plaintext.charAt(i));
}
}
return result;
}
  
// Driver code
public static void main(String[] args) {

Scanner scan = new Scanner(System.in);

String plaintext = "THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG";// plain text
System.out.println("Please enter the cipher shift:");// prompt to get the shift input
int shift = scan.nextInt(); // getting the shift
System.out.println("Text : " + plaintext); // writing the plain text
System.out.println("Shift : " + shift); // writing the shift
System.out.println("Cipher: " + caesarCipher(plaintext, shift)); // writing the ciphered text by calling caesarCipher function
}
}

OUTPUT:

HERE IS A CODE ALONG WITH DECRYPTION:

import java.util.Scanner;

public class CaesarCipher {
    // Encrypts text using a shift od s
    public static StringBuffer caesarCipher(String plaintext, int shift) {
        StringBuffer result= new StringBuffer(); //the string that will be returned
        char ch;
        for (int i=0; i<plaintext.length(); i++) {
            if (Character.isUpperCase(plaintext.charAt(i))) { // if the character is an uppercase character
                if(((((int)plaintext.charAt(i) - shift - 65) % 26) + 65) < 65) {// if the new character is lower than A
                    ch = (char)((((int)plaintext.charAt(i) - shift - 65) % 26) + 65 + 26) ;// we add 26 more to it
                }
                else {
                    ch = (char)((((int)plaintext.charAt(i) - shift - 65) % 26) + 65);//else we add the base 65 to teh character
                }
                //System.out.println(" Character = " + plaintext.charAt(i) + "=" + ((int)plaintext.charAt(i) - shift - 65) % 26);
                result.append(ch);
            }
            else if(Character.isLowerCase(plaintext.charAt(i))) {// if the character is an uppercase character
             if(((((int)plaintext.charAt(i) - shift - 65) % 26) + 97) < 97) { // if the new character gooes lower than a
                ch = (char)((((int)plaintext.charAt(i) - shift - 97) % 26) + 97 + 26);// we add 26 to it
             }
             else {
                ch = (char)((((int)plaintext.charAt(i) - shift - 97) % 26) + 97);
             }
                result.append(ch);
            }
            else { // if the character is an special character, number or spaces
          
                result.append(plaintext.charAt(i));
            }
        }
        return result;
    }
    // decryption function
    public static StringBuffer decryptCaesar(String ciphertext, int shift) {
        StringBuffer result= new StringBuffer(); //the string that will be returned
        char ch;
        for (int i=0; i<ciphertext.length(); i++) {
            if (Character.isUpperCase(ciphertext.charAt(i))) { // if the character is an uppercase character
                if(((((int)ciphertext.charAt(i) + shift - 65) % 26) + 65) > 90) {// if the new character is greater than A
                    ch = (char)((((int)ciphertext.charAt(i) + shift - 65) % 26) + 65 - 26) ;// we subtract 26 from it
                }
                else {
                    ch = (char)((((int)ciphertext.charAt(i) + shift - 65) % 26) + 65);//else we add the base 65 to the character
                }
             
                result.append(ch);
            }
            else if(Character.isLowerCase(ciphertext.charAt(i))) {// if the character is an uppercase character
             if(((((int)ciphertext.charAt(i) + shift - 65) % 26) + 97) > 122) { // if the new character gooes higher than a
                ch = (char)((((int)ciphertext.charAt(i) + shift - 97) % 26) + 97 - 26);// we subtract 26 from it
             }
             else {
                ch = (char)((((int)ciphertext.charAt(i) + shift - 97) % 26) + 97);// else we only add 97 to it
             }
                result.append(ch);
            }
            else { // if the character is an special character, number or spaces
          
                result.append(ciphertext.charAt(i));
            }
        }
        return result;
    }

    // Driver code
    public static void main(String[] args) {

        Scanner scan = new Scanner(System.in);

        String plaintext = "THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG";// plain text
        System.out.println("Please enter the cipher shift:");// prompt to get the shift input
        int shift = scan.nextInt(); // getting the shift
        System.out.println("Text : " + plaintext); // writing the plain text
        System.out.println("Shift : " + shift); // writing the shift
        String encrypted = caesarCipher(plaintext, shift).toString();// getting the ciphertext in a variable
        System.out.println("Cipher: " + encrypted); // writing the ciphered text by calling caesarCipher function

        System.out.println("Decrypyted Cipher:" + decryptCaesar(encrypted,shift));
    }
}

OUTPUT:

As the question asks, you can simply edit the value of shift = 13 for implementing the same logic for Rot13. The shift size woulf be fixed,the rest of the things will be the same.