Features Required:

1. QR Code Generation: Ability to generate QR codes for two-factor authentication.

2. Token Generation: Generate time-based tokens for authentication.

3. Token Verification: Verify the correctness of the entered token.

4. Secret Key Storage: Securely store and retrieve secret keys associated with users.

5. User Registration: Register new users for two-factor authentication.

6. User Management: Manage users, including adding, updating, and removing them.

7. Algorithm Flexibility: Support for different hashing algorithms.

8. Time Synchronization: Handle time synchronization issues.

Design Patterns Involved or Used:

1. Singleton Pattern: For the GoogleAuthenticator instance, ensuring a single point of access to the authentication system.

2. Factory Method Pattern: For creating instances of authentication tokens, allowing flexibility for different token types.

3. Strategy Pattern: For supporting different hashing algorithms and making them interchangeable.

4. Facade Pattern: Simplifying the complexity of the authentication system by providing a higher-level interface.

5. Observer Pattern: For notifying users or systems about events like successful or failed authentication attempts.

6. Command Pattern: For encapsulating token generation and verification requests as objects.

7. Data Access Object (DAO) Pattern: For handling data access and storage, especially for user-related data.

Diagram

Code

import java.security.Key;
import java.util.Base64;
import java.util.HashMap;
import java.util.Map;

// Command Interface
interface TokenCommand {
    void execute();
}

// Token Generation Command
class GenerateTokenCommand implements TokenCommand {
    private TokenFactory tokenFactory;
    private String secretKey;
    private long currentTimeMillis;

    public GenerateTokenCommand(TokenFactory tokenFactory, String secretKey, long currentTimeMillis) {
        this.tokenFactory = tokenFactory;
        this.secretKey = secretKey;
        this.currentTimeMillis = currentTimeMillis;
    }

    public void execute() {
        // Execute token generation
        String generatedToken = tokenFactory.generateToken(secretKey, currentTimeMillis);
        System.out.println("Generated Token: " + generatedToken);
        // Additional logic, if needed
    }
}

// Token Verification Command
class VerifyTokenCommand implements TokenCommand {
    private TokenVerifier tokenVerifier;
    private String enteredCode;
    private String generatedCode;

    public VerifyTokenCommand(TokenVerifier tokenVerifier, String enteredCode, String generatedCode) {
        this.tokenVerifier = tokenVerifier;
        this.enteredCode = enteredCode;
        this.generatedCode = generatedCode;
    }

    public void execute() {
        // Execute token verification
        boolean isVerified = tokenVerifier.verifyToken(enteredCode, generatedCode);
        System.out.println("Token Verification Result: " + isVerified);
        // Additional logic, if needed
    }
}

// Token Operation Invoker
class TokenOperationInvoker {
    private TokenCommand tokenCommand;

    public void setCommand(TokenCommand tokenCommand) {
        this.tokenCommand = tokenCommand;
    }

    public void executeOperation() {
        // Execute the assigned command
        tokenCommand.execute();
    }
}

// Singleton Pattern
class GoogleAuthenticator {
    private static GoogleAuthenticator instance;
    private TokenFactory tokenFactory;
    private TokenVerifier tokenVerifier;
    private UserDao userDao;
    private TokenOperationInvoker tokenOperationInvoker;

    private GoogleAuthenticator() {
        this.tokenFactory = new DefaultTokenFactory();
        this.tokenVerifier = new DefaultTokenVerifier();
        this.userDao = new InMemoryUserDao();
        this.tokenOperationInvoker = new TokenOperationInvoker();
    }

    public static GoogleAuthenticator getInstance() {
        if (instance == null) {
            instance = new GoogleAuthenticator();
        }
        return instance;
    }

    // Other methods for QR code generation, user registration, etc.

    public void generateAndExecuteTokenGenerationCommand(String userId, long currentTimeMillis) {
        User user = userDao.getUser(userId);
        if (user != null) {
            String secretKey = user.getSecretKey();
            // Create the token generation command
            TokenCommand generateTokenCommand = new GenerateTokenCommand(tokenFactory, secretKey, currentTimeMillis);
            // Set the command
            tokenOperationInvoker.setCommand(generateTokenCommand);
            // Execute the command
            tokenOperationInvoker.executeOperation();
        }
    }

    public void generateAndExecuteTokenVerificationCommand(String userId, String enteredCode, String generatedCode) {
        User user = userDao.getUser(userId);
        if (user != null) {
            // Create the token verification command
            TokenCommand verifyTokenCommand = new VerifyTokenCommand(tokenVerifier, enteredCode, generatedCode);
            // Set the command
            tokenOperationInvoker.setCommand(verifyTokenCommand);
            // Execute the command
            tokenOperationInvoker.executeOperation();
        }
    }

    // Other methods...
}

// Factory Method Pattern
interface TokenFactory {
    String generateToken(String secretKey, long currentTimeMillis);
}

class DefaultTokenFactory implements TokenFactory {
    @Override
    public String generateToken(String secretKey, long currentTimeMillis) {
        // Implementation details...
        return "generated_token";
    }
}

// Strategy Pattern
interface TokenVerifier {
    boolean verifyToken(String enteredCode, String generatedCode);
}

class DefaultTokenVerifier implements TokenVerifier {
    @Override
    public boolean verifyToken(String enteredCode, String generatedCode) {
        // Implementation details...
        return enteredCode.equals(generatedCode);
    }
}

// Data Access Object (DAO) Pattern
interface UserDao {
    void addUser(User user);

    User getUser(String userId);

    void updateUser(User user);

    void removeUser(String userId);
}

class User {
    private String userId;
    private String name;
    private String secretKey;

    public User(String userId, String name, String secretKey) {
        this.userId = userId;
        this.name = name;
        this.secretKey = secretKey;
    }

    public String getUserId() {
        return userId;
    }

    public String getSecretKey() {
        return secretKey;
    }
}

class InMemoryUserDao implements UserDao {
    private Map<String, User> users;

    public InMemoryUserDao() {
        this.users = new HashMap<>();
    }

    @Override
    public void addUser(User user) {
        users.put(user.getUserId(), user);
    }

    @Override
    public User getUser(String userId) {
        return users.get(userId);
    }

    @Override
    public void updateUser(User user) {
        users.put(user.getUserId(), user);
    }

    @Override
    public void removeUser(String userId) {
        users.remove(userId);
    }
}


public class GoogleAuthenticatorApp {
    public static void main(String[] args) {
        GoogleAuthenticator authenticator = GoogleAuthenticator.getInstance();

        User user = new User("123", "John Doe", "secretKey123");
        authenticator.generateAndExecuteTokenGenerationCommand(user.getUserId(), System.currentTimeMillis());

        // Simulating user entering the code
        String enteredCode = "generated_token"; // Replace with the actual user input
        authenticator.generateAndExecuteTokenVerificationCommand(user.getUserId(), enteredCode, "generated_token");
    }
}

This is a simplified implementation showcasing some aspects of the design. The actual implementation may require additional considerations and refinements based on specific use cases and security requirements.