WHAT IS THE PURPOSE OF THIS CODE?

This code is used to encrypt or can also decrypt plaintext using Java as the main language.

⚠️This code only runs on terminal/shell and does not display GUI!!

INSTALLING GUIDE IN TERMUX

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How to running the Code in Termux?

First clone this project

Open Termux and run the following commands to install OpenJDK v21 or latest:

termux-setup-storage
pkg upgrade
pkg update 
pkg install openjdk-21

Compile Java Code Run the following command to compile the Java file:

javac crypto.java

If there are no errors, this will create a file named crypto.class

Run the Program:

java crypto.java

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By following these steps, you can run this code in Termux, read text from user input, and produce an encrypted or decrypted result based on the chosen mode.

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CODE IDENTIFICATION

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I will explain in detail the meaning of the code I wrote to make it easier for you to read this code.

-- First, import the scanner library

Scanner Library

import java.util.Scanner;

Imports the Scanner class from java.util

Main Class Declaration

public class crypto {

This declares a class named crypto. Inside this class, we define all methods needed for encryption, decryption, and creating the shifted alphabet.

createShiftedAlphabet Function

public static String createShiftedAlphabet(String key) {

String alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

This function takes key (as a string) from the user and creates a shifted version of the alphabet based on that key.

String alphabet contains the basic A-Z alphabet as a reference.

int keyLength = key.length();
    int startIndex = alphabet.indexOf(key.charAt(0));
    String shiftedAlphabet = key;

keyLength stores the length of key, and startIndex determines the position of the first letter of key in alphabet. shiftedAlphabet is initialized with the key.

for (int i = startIndex + keyLength; i < alphabet.length(); i++) {
        shiftedAlphabet += alphabet.charAt(i);
    }

This first loop appends the letters after the key until the end of the alphabet.

for (int i = 0; i < startIndex; i++) {
        shiftedAlphabet += alphabet.charAt(i);
    }

And the second loop appends the letters that come before the key (letters not already in shiftedAlphabet).

return shiftedAlphabet;
}

This returns shiftedAlphabet, which contains the alphabet shifted according to the key.

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Encrypt Function

public static String encrypt(String text, String shiftedAlphabet) {

The encrypt function takes two parameters: text (the text to be encrypted) and shiftedAlphabet (the shifted alphabet created using key).

String alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    text = text.toUpperCase();
    StringBuilder encryptedText = new StringBuilder();

This initializes alphabet, converts text to uppercase, and creates a StringBuilder to store the encrypted result.

for (char c : text.toCharArray()) {

The for loop iterates over each character in text.

if (Character.isLetter(c)) {
            int index = alphabet.indexOf(c);
            encryptedText.append(shiftedAlphabet.charAt(index));
        } else {
            encryptedText.append(c);
        }
    }

If the character is a letter, the program finds its index in alphabet and appends the corresponding character in shiftedAlphabet to encryptedText. If it’s not a letter, the character is appended directly.

return encryptedText.toString();
}

This returns encryptedText as the final encrypted string.

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Decrypt Function

public static String decrypt(String text, String shiftedAlphabet) {

The decrypt function takes text and shiftedAlphabet and reverses the encryption to restore the original text.

String alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    text = text.toUpperCase();
    StringBuilder decryptedText = new StringBuilder();

This initializes alphabet, converts text to uppercase, and creates a StringBuilder to store the decrypted text.

for (char c : text.toCharArray()) {
        if (Character.isLetter(c)) {
            int index = shiftedAlphabet.indexOf(c);
            decryptedText.append(alphabet.charAt(index));
        } else {
            decryptedText.append(c);
        }
    }

If the character is a letter, the program finds its index in shiftedAlphabet and retrieves the corresponding letter in alphabet to restore the original character. If it’s not a letter, it’s directly appended to decryptedText.

return decryptedText.toString();
}

This returns decryptedText as the final decrypted string.

## main Function - User Interface

public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);

This creates a Scanner object to read user input from the console.

System.out.println("Choose a mode:");
    System.out.println("1. Encrypt");
    System.out.println("2. Decrypt");
    System.out.print("Enter your choice (1 or 2): ");
    int choice = scanner.nextInt();
    scanner.nextLine();

The program prompts the user to select a mode: 1 for encryption or 2 for decryption. scanner.nextLine() clears the buffer after reading the integer.

System.out.print("Enter text: ");
    String text = scanner.nextLine();

    System.out.print("Enter KEY (e.g., RSTU): ");
    String key = scanner.nextLine().toUpperCase();

The program then prompts the user to enter the text to be encrypted or decrypted, as well as the key.

String shiftedAlphabet = createShiftedAlphabet(key);
    System.out.println("Shifted alphabet: " + shiftedAlphabet);

createShiftedAlphabet is called with key to create the shiftedAlphabet, which is then printed.

if (choice == 1) {
        String encryptedText = encrypt(text, shiftedAlphabet);
        System.out.println("Encrypted result: " + encryptedText);
    } else if (choice == 2) {
        String decryptedText = decrypt(text, shiftedAlphabet);
        System.out.println("Decrypted result: " + decryptedText);
    } else {
        System.out.println("Invalid choice. Program exits.");
    }
    scanner.close();
}

If choice is 1, the text is encrypted and the result is printed; if it is 2, the text is decrypted. If the choice is invalid, the program displays a message and exits.

That's all for those of you who read my explanation and thank you.

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MIT License

Copyright (c) 2024 DimazRizky