Auth.java

import java.io.*;
import java.security.*;
import java.security.spec.*;
import javax.crypto.*;
import javax.crypto.spec.*;
import java.util.*;

class Auth {
    static String usersFile = "users.txt";

    static void registerUser(String username,String displayName, String password) {
        try {
            if (!isUsernameUnique(username)) {
                System.out.println("Username already exists. Please choose another username.");
                return;
            }

            KeyPair keyPair = RSA.generateKeyPair();
            PublicKey publicKey = keyPair.getPublic();
            PrivateKey privateKey = keyPair.getPrivate();

            byte[] encryptedPassword = RSA.encrypt(password, publicKey);

            BufferedWriter writer = new BufferedWriter(new FileWriter(usersFile, true));
            writer.write(username + ":" + displayName + ":" + Base64.getEncoder().encodeToString(encryptedPassword) +
                    ":" + Base64.getEncoder().encodeToString(publicKey.getEncoded()) +
                    ":" + Base64.getEncoder().encodeToString(privateKey.getEncoded()) + "\n");
            writer.close();

            System.out.println("Registration successful!");
        } catch (Exception e) {
            System.err.println("Error: " + e.getMessage());
        }
    }

    static boolean loginUser(String username, String password) {
        try {
            BufferedReader reader = new BufferedReader(new FileReader(usersFile));
            String line;
            while ((line = reader.readLine()) != null) {
                String[] parts = line.split(":");
                if (parts.length >= 2 && parts[0].equals(username)) {
                    byte[] encryptedPassword = Base64.getDecoder().decode(parts[2]);

                    byte[] privateKeyBytes = Base64.getDecoder().decode(parts[4]);
                    PrivateKey privateKey = KeyFactory.getInstance("RSA").generatePrivate(new PKCS8EncodedKeySpec(privateKeyBytes));

                    String decryptedPassword = RSA.decrypt(encryptedPassword, privateKey);
                    if (password.equals(decryptedPassword)) {
                        return true;
                    } else {
                        System.out.println("Invalid password. Please try again.");
                        return false;
                    }
                }
            }
            System.out.println("User not found. Please register.");
            return false;
        } catch (Exception e) {
            System.err.println("Error: " + e.getMessage());
            return false;
        }
    }


    static boolean isUsernameUnique(String username) throws IOException {
        BufferedReader reader = new BufferedReader(new FileReader(usersFile));
        String line;
        while ((line = reader.readLine()) != null) {
            String[] parts = line.split(":");
            if (parts[0].equals(username)) {
                return false;
            }
        }
        return true;
    }

    static boolean checkPasswordMatching(String password1, String password2) {
        return password1.equals(password2);
    }
}