Imagine you are an outsider (perhaps an adversary) who would like to know the content of the encrypted messages. You know that the messages are encrypted using a Caesar cipher, but do not know the key (shift value). Write a Java program to decrypt the text encrypted with a Caesar cipher (such as the quote below).

One of the oldest (and least complicated) ciphers is known as the Caesar cipher (after Julius Cae- sar). To use the Caesar cipher, the two people must agree upon a shift value. Each letter in the

original message will be shifted by this value. So, if they agree upon 7, the letter a would be- come h.

Here is an example of a quote encrypted with a Caesar cipher: Y jxyda secfkjuh isyudsu, ro qdt bqhwu, yi ijybb ijksa yd jxu Cetuhd qwu.

The final program can include an encrypter and a decrypter or just a decrypter if possible.

The decrypter should be able to find out what the shift value is without the user knowing; so it should work regardless of shift value.

Below is the code I have so far, it works, but I have to manually change what the shift value is in the code for it to be able to decrypt the cipher.

import java.util.Scanner;

public class CaesarCipher { public static final String ALPHABET = "abcdefghijklmnopqrstuvwxyz";

public static String encrypt(String plainText, int shiftKey) { plainText = plainText.toLowerCase(); String cipherText = ""; for (int i = 0; i < plainText.length(); i++) { int charPosition = ALPHABET.indexOf(plainText.charAt(i)); int keyVal = (shiftKey + charPosition) % 26; char replaceVal = ALPHABET.charAt(keyVal); cipherText += replaceVal; } return cipherText; }

public static String decrypt(String cipherText, int shiftKey) { cipherText = cipherText.toLowerCase(); String plainText = ""; for (int i = 0; i < cipherText.length(); i++) { int charPosition = ALPHABET.indexOf(cipherText.charAt(i)); int keyVal = (charPosition - shiftKey) % 26; if (keyVal < 0) { keyVal = ALPHABET.length() + keyVal; } char replaceVal = ALPHABET.charAt(keyVal); plainText += replaceVal; } return plainText; }

public static void main(String[] args) { Scanner sc = new Scanner(System.in); System.out.println("Enter the String for Encryption: "); String message = new String(); message = sc.next(); System.out.println(encrypt(message, 3)); System.out.println(decrypt(encrypt(message, 3), 3)); sc.close(); } }