Question

Write a java program that takes the following phrases and encrypts them using

a substitution cipher. The program should ask for a key and a phrase and then proceed to encrypt

that phrase using the given shared key. Make sure that you have the key for the cipher and

the output of encrypted phrases.

Encrypt the phrase:

He who fights with monsters should look to it that he himself does not

become a monster. And if you gaze long into an abyss , the abyss also

gazes into you.

Answer 1

import java.util.Scanner;

public class EncryptPhrase {

    public static String encode(String code, int key) {
        String result = "";
        char ch;
        for (int i = 0; i < code.length(); ++i) {
            ch = code.charAt(i);
            if (Character.isLowerCase(ch)) {
                ch = (char) ('a' + ((ch - 'a' + key + 26) % 26));
            } else if (Character.isUpperCase(ch)) {
                ch = (char) ('A' + ((ch - 'A' + key + 26) % 26));
            }
            result += ch;
        }
        return result;
    }

    public static String decode(String code, int key)
    {
        String result = "";
        char ch;
        for(int i = 0; i < code.length(); ++i) {
            ch = code.charAt(i);
            if(Character.isLowerCase(ch)) {
                ch = (char) ('a' + ((ch - 'a' - key + 26) % 26));
            } else if(Character.isUpperCase(ch)) {
                ch = (char) ('A' + ((ch - 'A' - key + 26) % 26));
            }
            result += ch;
        }
        return result;
    }

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        System.out.print("Enter a key: ");
        int key = Integer.parseInt(in.nextLine());
        System.out.print("Enter a single line of phrase: ");
        String phrase = in.nextLine();
        String cipher = encode(phrase, key);
        System.out.println("Cipher text is\n" + cipher + "\n\n");
        System.out.println("Plain text is\n" + decode(cipher, key));
        in.close();
    }
}