diff --git a/src/mathquiz/backend_1/EmailService.java b/src/mathquiz/backend_1/EmailService.java
new file mode 100644
index 0000000..fe1f3c1
--- /dev/null
+++ b/src/mathquiz/backend_1/EmailService.java
@@ -0,0 +1,172 @@
+package mathquiz.backend;
+
+import java.io.FileInputStream;
+import java.io.IOException;
+import java.util.Properties;
+import java.util.Random;
+
+/**
+ * Email Service Class - Handles email verification code sending
+ */
+public class EmailService {
+    
+    // Email configuration - loaded from properties file
+    private String smtpHost;
+    private String smtpPort;
+    private String emailUsername;
+    private String emailPassword;
+    private String fromEmail;
+    
+    public EmailService() {
+        loadEmailConfig();
+    }
+    
+    /**
+     * Load email configuration from properties file
+     */
+    private void loadEmailConfig() {
+        Properties config = new Properties();
+        try {
+            config.load(new FileInputStream("email_config.properties"));
+            smtpHost = config.getProperty("smtp.host", "smtp.gmail.com");
+            smtpPort = config.getProperty("smtp.port", "587");
+            emailUsername = config.getProperty("email.username", "your-email@gmail.com");
+            emailPassword = config.getProperty("email.password", "your-app-password");
+            fromEmail = config.getProperty("email.from", "your-email@gmail.com");
+        } catch (IOException e) {
+            System.err.println("Warning: Could not load email configuration, using default settings");
+            // Use default settings
+            smtpHost = "smtp.gmail.com";
+            smtpPort = "587";
+            emailUsername = "your-email@gmail.com";
+            emailPassword = "your-app-password";
+            fromEmail = "your-email@gmail.com";
+        }
+    }
+    
+    /**
+     * Send verification code to email
+     */
+    public String sendVerificationCode(String email) throws Exception {
+        // Generate 6-digit numeric verification code
+        String verificationCode = generateVerificationCode();
+        
+        // Send real email
+        sendRealEmail(email, verificationCode);
+        
+        // Also save to file for backup verification
+        saveVerificationCodeToFile(email, verificationCode);
+        
+        return verificationCode;
+    }
+    
+    /**
+     * Generate verification code
+     */
+    private String generateVerificationCode() {
+        Random random = new Random();
+        StringBuilder code = new StringBuilder();
+        
+        for (int i = 0; i < 6; i++) {
+            code.append(random.nextInt(10));
+        }
+        
+        return code.toString();
+    }
+    
+    /**
+     * Send real email using SMTP protocol
+     */
+    private void sendRealEmail(String email, String verificationCode) throws Exception {
+        try {
+            // Create email content
+            String subject = "Math Quiz System - Registration Verification Code";
+            String content = createEmailContent(verificationCode);
+            
+            // Use SimpleEmailSender
+            SimpleEmailSender sender = new SimpleEmailSender(
+                smtpHost, 
+                Integer.parseInt(smtpPort), 
+                emailUsername, 
+                emailPassword, 
+                fromEmail
+            );
+            
+            sender.sendEmail(email, subject, content);
+            
+            System.out.println("Verification code sent successfully to: " + email);
+            System.out.println("Verification code: " + verificationCode);
+            
+        } catch (Exception e) {
+            System.err.println("Failed to send email: " + e.getMessage());
+            // Fall back to console output
+            System.out.println("=== EMAIL VERIFICATION CODE ===");
+            System.out.println("To: " + email);
+            System.out.println("Subject: Math Quiz System - Registration Verification Code");
+            System.out.println("Verification Code: " + verificationCode);
+            System.out.println("Please check your email or use the code above.");
+            System.out.println("================================");
+        }
+    }
+    
+    
+    /**
+     * Create email content with HTML formatting
+     */
+    private String createEmailContent(String verificationCode) {
+        return "<html>" +
+               "<head><meta charset='UTF-8'></head>" +
+               "<body style='font-family: Arial, sans-serif; line-height: 1.6; color: #333;'>" +
+               "<div style='max-width: 600px; margin: 0 auto; padding: 20px;'>" +
+               "<h2 style='color: #0066cc; text-align: center;'>Math Quiz System</h2>" +
+               "<h3 style='color: #333;'>Registration Verification</h3>" +
+               "<p>Hello,</p>" +
+               "<p>Thank you for registering with Math Quiz System. Please use the following verification code to complete your registration:</p>" +
+               "<div style='background-color: #f0f8ff; border: 2px solid #0066cc; border-radius: 8px; padding: 20px; text-align: center; margin: 20px 0;'>" +
+               "<h1 style='color: #0066cc; font-size: 32px; margin: 0; letter-spacing: 5px;'>" + verificationCode + "</h1>" +
+               "</div>" +
+               "<p><strong>Important:</strong></p>" +
+               "<ul>" +
+               "<li>This verification code will expire in 5 minutes</li>" +
+               "<li>Please enter this code exactly as shown</li>" +
+               "<li>If you did not request this code, please ignore this email</li>" +
+               "</ul>" +
+               "<p>Best regards,<br>Math Quiz System Team</p>" +
+               "<hr style='border: none; border-top: 1px solid #eee; margin: 20px 0;'>" +
+               "<p style='font-size: 12px; color: #666; text-align: center;'>This is an automated message, please do not reply.</p>" +
+               "</div>" +
+               "</body>" +
+               "</html>";
+    }
+    
+    /**
+     * Save verification code to file (for demonstration only)
+     */
+    private void saveVerificationCodeToFile(String email, String verificationCode) {
+        try {
+            java.nio.file.Files.createDirectories(java.nio.file.Paths.get("data"));
+            java.nio.file.Files.write(
+                java.nio.file.Paths.get("data/verification_codes.txt"),
+                (email + ":" + verificationCode + "\n").getBytes(),
+                java.nio.file.StandardOpenOption.CREATE,
+                java.nio.file.StandardOpenOption.APPEND
+            );
+        } catch (Exception e) {
+            // Ignore file save errors
+        }
+    }
+    
+    /**
+     * Validate email format
+     */
+    public boolean isValidEmail(String email) {
+        if (email == null || email.trim().isEmpty()) {
+            return false;
+        }
+        
+        String trimmedEmail = email.trim();
+        return trimmedEmail.contains("@") && 
+               trimmedEmail.contains(".") && 
+               trimmedEmail.length() > 5;
+    }
+}
\ No newline at end of file
diff --git a/src/mathquiz/backend_1/Question.java b/src/mathquiz/backend_1/Question.java
new file mode 100644
index 0000000..7149e1b
--- /dev/null
+++ b/src/mathquiz/backend_1/Question.java
@@ -0,0 +1,58 @@
+package mathquiz.backend;
+
+/**
+ * Question Class - Represents a multiple choice question
+ */
+public class Question {
+    private final String question;
+    private final String optionA;
+    private final String optionB;
+    private final String optionC;
+    private final String optionD;
+    private final String correctAnswer;
+    
+    public Question(String question, String optionA, String optionB, String optionC, String optionD, String correctAnswer) {
+        this.question = question;
+        this.optionA = optionA;
+        this.optionB = optionB;
+        this.optionC = optionC;
+        this.optionD = optionD;
+        this.correctAnswer = correctAnswer;
+    }
+    
+    public String getQuestion() {
+        return question;
+    }
+    
+    public String getOptionA() {
+        return optionA;
+    }
+    
+    public String getOptionB() {
+        return optionB;
+    }
+    
+    public String getOptionC() {
+        return optionC;
+    }
+    
+    public String getOptionD() {
+        return optionD;
+    }
+    
+    public String getCorrectAnswer() {
+        return correctAnswer;
+    }
+    
+    @Override
+    public String toString() {
+        return "Question{" +
+                "question='" + question + '\'' +
+                ", optionA='" + optionA + '\'' +
+                ", optionB='" + optionB + '\'' +
+                ", optionC='" + optionC + '\'' +
+                ", optionD='" + optionD + '\'' +
+                ", correctAnswer='" + correctAnswer + '\'' +
+                '}';
+    }
+}
\ No newline at end of file
diff --git a/src/mathquiz/backend_1/QuizService.java b/src/mathquiz/backend_1/QuizService.java
new file mode 100644
index 0000000..d4dc996
--- /dev/null
+++ b/src/mathquiz/backend_1/QuizService.java
@@ -0,0 +1,298 @@
+package mathquiz.backend;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import java.util.Random;
+import mathquiz.AccountType;
+import mathquiz.Deduplicator;
+import mathquiz.GeneratorFactory;
+import mathquiz.ProblemGenerator;
+
+/**
+ * Quiz Service Class - Handles question generation and quiz logic
+ */
+public class QuizService {
+    
+    /**
+     * Generate multiple choice questions
+     */
+    public List<Question> generateQuestions(AccountType accountType, int count) {
+        try {
+            // Get question generator
+            ProblemGenerator generator = GeneratorFactory.getGenerator(accountType);
+            
+            // Generate original questions
+            List<String> rawQuestions = generator.generate(count);
+            
+            // Deduplication processing - use default username if current user is null
+            String username = getCurrentUsername();
+            if (username == null || username.trim().isEmpty()) {
+                username = "default_user";
+            }
+            Deduplicator deduplicator = new Deduplicator(username);
+            List<String> uniqueQuestions = deduplicator.ensureUnique(rawQuestions, count, generator);
+            
+            // Convert to multiple choice questions
+            List<Question> questions = new ArrayList<>();
+            for (String questionText : uniqueQuestions) {
+                Question question = convertToMultipleChoice(questionText, accountType);
+                questions.add(question);
+            }
+            
+            return questions;
+        } catch (Exception e) {
+            System.err.println("Error in generateQuestions: " + e.getMessage());
+            e.printStackTrace();
+            return new ArrayList<>(); // Return empty list instead of null
+        }
+    }
+    
+    /**
+     * Convert expression questions to multiple choice questions
+     */
+    private Question convertToMultipleChoice(String expression, AccountType accountType) {
+        try {
+            // Calculate correct answer
+            double correctAnswer = calculateExpression(expression, accountType);
+            
+            // Handle invalid answers
+            if (Double.isNaN(correctAnswer) || Double.isInfinite(correctAnswer)) {
+                System.out.println("Invalid answer for expression: " + expression + ", using 0.0");
+                correctAnswer = 0.0;
+            }
+            
+            System.out.println("Expression: " + expression + ", Answer: " + correctAnswer);
+            
+            // Generate wrong options
+            List<Double> options = generateOptions(correctAnswer);
+            System.out.println("Generated " + options.size() + " wrong options: " + options);
+            
+            // Randomly arrange options
+            Random random = new Random();
+            String[] optionTexts = new String[4];
+            String correctAnswerText = "";
+            
+            // Create a list with correct answer and wrong options
+            List<Double> allOptions = new ArrayList<>();
+            allOptions.add(correctAnswer);
+            allOptions.addAll(options);
+            
+            System.out.println("All options before shuffle: " + allOptions);
+            
+            // Shuffle the options
+            Collections.shuffle(allOptions);
+            
+            System.out.println("All options after shuffle: " + allOptions);
+            
+            // Find the correct answer index after shuffling
+            int correctAnswerIndex = allOptions.indexOf(correctAnswer);
+            System.out.println("Correct answer index: " + correctAnswerIndex);
+            
+            for (int i = 0; i < 4; i++) {
+                optionTexts[i] = formatAnswer(allOptions.get(i));
+                if (i == correctAnswerIndex) {
+                    correctAnswerText = getOptionLetter(i);
+                }
+            }
+            
+            System.out.println("Final options: " + java.util.Arrays.toString(optionTexts));
+            System.out.println("Correct answer: " + correctAnswerText);
+            
+            return new Question(
+                "Calculate: " + expression + " = ?",
+                optionTexts[0], optionTexts[1], optionTexts[2], optionTexts[3],
+                correctAnswerText
+            );
+        } catch (Exception e) {
+            System.err.println("Error in convertToMultipleChoice: " + e.getMessage());
+            e.printStackTrace();
+            // Return a default question
+            return new Question(
+                "Calculate: 1 + 1 = ?",
+                "1", "2", "3", "4",
+                "B"
+            );
+        }
+    }
+    
+    /**
+     * Calculate expression result
+     */
+    private double calculateExpression(String expression, AccountType accountType) {
+        try {
+            // Handle special functions
+            String processedExpression = preprocessExpression(expression, accountType);
+            
+            // Simple expression calculation (in actual application should use more comprehensive expression parser)
+            return evaluateExpression(processedExpression);
+        } catch (Exception e) {
+            // If calculation fails, return a default value
+            return 0.0;
+        }
+    }
+    
+    /**
+     * Preprocess expression, handle special functions
+     */
+    private String preprocessExpression(String expression, AccountType accountType) {
+        String result = expression;
+        
+        // Handle square root
+        result = result.replaceAll("sqrt\\(([^)]+)\\)", "Math.sqrt($1)");
+        
+        // Handle square
+        result = result.replaceAll("\\(([^)]+)\\)\\^2", "($1) * ($1)");
+        
+        // Handle trigonometric functions
+        result = result.replaceAll("sin\\(([^)]+)\\)", "Math.sin($1)");
+        result = result.replaceAll("cos\\(([^)]+)\\)", "Math.cos($1)");
+        result = result.replaceAll("tan\\(([^)]+)\\)", "Math.tan($1)");
+        
+        // Handle operators
+        result = result.replace("×", "*");
+        result = result.replace("÷", "/");
+        
+        return result;
+    }
+    
+    /**
+     * Simple expression calculation
+     */
+    private double evaluateExpression(String expression) {
+        try {
+            // Simplified expression calculation, only handle basic arithmetic operations
+            return evaluateSimpleExpression(expression);
+        } catch (Exception e) {
+            // If calculation fails, return 0
+            return 0.0;
+        }
+    }
+    
+    /**
+     * Simplified expression calculation
+     */
+    private double evaluateSimpleExpression(String expression) {
+        // Implement a simple expression calculator here
+        // For simplification, only handle basic number operations
+        try {
+            // Remove all spaces
+            expression = expression.replaceAll("\\s+", "");
+            
+            // Handle parentheses
+            while (expression.contains("(")) {
+                int start = expression.lastIndexOf("(");
+                int end = expression.indexOf(")", start);
+                if (end == -1) break;
+                
+                String subExpr = expression.substring(start + 1, end);
+                double result = evaluateSimpleExpression(subExpr);
+                expression = expression.substring(0, start) + result + expression.substring(end + 1);
+            }
+            
+            // Handle multiplication and division
+            while (expression.contains("*") || expression.contains("/")) {
+                expression = processOperators(expression, new String[]{"*", "/"});
+            }
+            
+            // Handle addition and subtraction
+            while (expression.contains("+") || (expression.contains("-") && !expression.startsWith("-"))) {
+                expression = processOperators(expression, new String[]{"+", "-"});
+            }
+            
+            return Double.parseDouble(expression);
+        } catch (Exception e) {
+            return 0.0;
+        }
+    }
+    
+    /**
+     * Process operators
+     */
+    private String processOperators(String expression, String[] operators) {
+        for (String op : operators) {
+            int index = expression.indexOf(op);
+            if (index > 0) {
+                // Find left operand
+                int leftStart = index - 1;
+                while (leftStart >= 0 && (Character.isDigit(expression.charAt(leftStart)) || expression.charAt(leftStart) == '.')) {
+                    leftStart--;
+                }
+                leftStart++;
+                
+                // Find right operand
+                int rightEnd = index + 1;
+                while (rightEnd < expression.length() && (Character.isDigit(expression.charAt(rightEnd)) || expression.charAt(rightEnd) == '.')) {
+                    rightEnd++;
+                }
+                
+                double left = Double.parseDouble(expression.substring(leftStart, index));
+                double right = Double.parseDouble(expression.substring(index + 1, rightEnd));
+                double result = 0;
+                
+                switch (op) {
+                    case "+": result = left + right; break;
+                    case "-": result = left - right; break;
+                    case "*": result = left * right; break;
+                    case "/": result = right != 0 ? left / right : 0; break;
+                }
+                
+                expression = expression.substring(0, leftStart) + result + expression.substring(rightEnd);
+                break;
+            }
+        }
+        return expression;
+    }
+    
+    /**
+     * Generate wrong options
+     */
+    private List<Double> generateOptions(double correctAnswer) {
+        List<Double> options = new ArrayList<>();
+        Random random = new Random();
+        
+        // Generate 3 wrong options
+        for (int i = 0; i < 3; i++) {
+            double wrongAnswer;
+            do {
+                // Generate wrong answers near correct answer
+                double variation = (random.nextDouble() - 0.5) * Math.abs(correctAnswer) * 0.5;
+                wrongAnswer = correctAnswer + variation;
+                
+                // Ensure wrong answer is not 0 and different from correct answer
+                if (wrongAnswer == 0) {
+                    wrongAnswer = random.nextDouble() * 10 + 1;
+                }
+            } while (Math.abs(wrongAnswer - correctAnswer) < 0.01);
+            
+            options.add(wrongAnswer);
+        }
+        
+        return options;
+    }
+    
+    /**
+     * Format answer
+     */
+    private String formatAnswer(double answer) {
+        if (answer == (long) answer) {
+            return String.valueOf((long) answer);
+        } else {
+            return String.format("%.2f", answer);
+        }
+    }
+    
+    /**
+     * Get option letter
+     */
+    private String getOptionLetter(int index) {
+        return String.valueOf((char) ('A' + index));
+    }
+    
+    /**
+     * Get current username
+     */
+    private String getCurrentUsername() {
+        return mathquiz.gui.QuizSession.getInstance().getCurrentUser();
+    }
+}
\ No newline at end of file
diff --git a/src/mathquiz/backend_1/SimpleEmailSender.java b/src/mathquiz/backend_1/SimpleEmailSender.java
new file mode 100644
index 0000000..4c1c18a
--- /dev/null
+++ b/src/mathquiz/backend_1/SimpleEmailSender.java
@@ -0,0 +1,184 @@
+package mathquiz.backend;
+
+import java.io.*;
+import java.net.*;
+import javax.net.ssl.*;
+
+/**
+ * Simple Email Sender using SMTP protocol
+ * This class provides a basic SMTP client implementation
+ */
+public class SimpleEmailSender {
+    
+    private String smtpHost;
+    private int smtpPort;
+    private String username;
+    private String password;
+    private String fromEmail;
+    
+    public SimpleEmailSender(String smtpHost, int smtpPort, String username, String password, String fromEmail) {
+        this.smtpHost = smtpHost;
+        this.smtpPort = smtpPort;
+        this.username = username;
+        this.password = password;
+        this.fromEmail = fromEmail;
+    }
+    
+    /**
+     * Send email using SMTP protocol with SSL/TLS support
+     */
+    public void sendEmail(String toEmail, String subject, String content) throws Exception {
+        Socket socket = null;
+        BufferedReader reader = null;
+        PrintWriter writer = null;
+        
+        try {
+            System.out.println("Connecting to SMTP server: " + smtpHost + ":" + smtpPort);
+            
+            // Connect to SMTP server
+            socket = new Socket(smtpHost, smtpPort);
+            reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
+            writer = new PrintWriter(socket.getOutputStream(), true);
+            
+            // Read server greeting
+            String response = reader.readLine();
+            System.out.println("Server greeting: " + response);
+            if (!response.startsWith("220")) {
+                throw new Exception("SMTP server error: " + response);
+            }
+            
+            // Send EHLO command
+            String hostname = InetAddress.getLocalHost().getHostName();
+            if (hostname == null || hostname.isEmpty()) {
+                hostname = "localhost";
+            }
+            writer.println("EHLO " + hostname);
+            response = reader.readLine();
+            System.out.println("EHLO response: " + response);
+            if (!response.startsWith("250")) {
+                throw new Exception("EHLO failed: " + response);
+            }
+            
+            // Read additional EHLO responses
+            while (response.startsWith("250-")) {
+                response = reader.readLine();
+                System.out.println("EHLO continuation: " + response);
+            }
+            
+            // Start TLS if supported
+            writer.println("STARTTLS");
+            response = reader.readLine();
+            System.out.println("STARTTLS response: " + response);
+            if (response.startsWith("220")) {
+                // Upgrade to SSL socket
+                SSLSocketFactory factory = (SSLSocketFactory) SSLSocketFactory.getDefault();
+                SSLSocket sslSocket = (SSLSocket) factory.createSocket(socket, smtpHost, smtpPort, true);
+                sslSocket.startHandshake();
+                
+                // Update streams to use SSL socket
+                reader = new BufferedReader(new InputStreamReader(sslSocket.getInputStream()));
+                writer = new PrintWriter(sslSocket.getOutputStream(), true);
+                
+                // Send EHLO again after TLS
+                writer.println("EHLO " + hostname);
+                response = reader.readLine();
+                System.out.println("EHLO after TLS: " + response);
+                if (!response.startsWith("250")) {
+                    throw new Exception("EHLO after TLS failed: " + response);
+                }
+                
+                // Read additional EHLO responses
+                while (response.startsWith("250-")) {
+                    response = reader.readLine();
+                    System.out.println("EHLO continuation after TLS: " + response);
+                }
+            }
+            
+            // Authenticate using AUTH LOGIN
+            writer.println("AUTH LOGIN");
+            response = reader.readLine();
+            System.out.println("AUTH LOGIN response: " + response);
+            if (!response.startsWith("334")) {
+                throw new Exception("AUTH LOGIN failed: " + response);
+            }
+            
+            // Send username (base64 encoded)
+            String encodedUsername = java.util.Base64.getEncoder().encodeToString(username.getBytes());
+            writer.println(encodedUsername);
+            response = reader.readLine();
+            System.out.println("Username auth response: " + response);
+            if (!response.startsWith("334")) {
+                throw new Exception("Username authentication failed: " + response);
+            }
+            
+            // Send password (base64 encoded)
+            String encodedPassword = java.util.Base64.getEncoder().encodeToString(password.getBytes());
+            writer.println(encodedPassword);
+            response = reader.readLine();
+            System.out.println("Password auth response: " + response);
+            if (!response.startsWith("235")) {
+                throw new Exception("Password authentication failed: " + response);
+            }
+            
+            // Send MAIL FROM
+            writer.println("MAIL FROM:<" + fromEmail + ">");
+            response = reader.readLine();
+            System.out.println("MAIL FROM response: " + response);
+            if (!response.startsWith("250")) {
+                throw new Exception("MAIL FROM failed: " + response);
+            }
+            
+            // Send RCPT TO
+            writer.println("RCPT TO:<" + toEmail + ">");
+            response = reader.readLine();
+            System.out.println("RCPT TO response: " + response);
+            if (!response.startsWith("250")) {
+                throw new Exception("RCPT TO failed: " + response);
+            }
+            
+            // Send DATA
+            writer.println("DATA");
+            response = reader.readLine();
+            System.out.println("DATA response: " + response);
+            if (!response.startsWith("354")) {
+                throw new Exception("DATA failed: " + response);
+            }
+            
+            // Send email headers and content
+            writer.println("From: " + fromEmail);
+            writer.println("To: " + toEmail);
+            writer.println("Subject: " + subject);
+            writer.println("MIME-Version: 1.0");
+            writer.println("Content-Type: text/html; charset=UTF-8");
+            writer.println();
+            writer.println(content);
+            writer.println(".");
+            
+            response = reader.readLine();
+            System.out.println("Email send response: " + response);
+            if (!response.startsWith("250")) {
+                throw new Exception("Email sending failed: " + response);
+            }
+            
+            // Quit
+            writer.println("QUIT");
+            response = reader.readLine();
+            System.out.println("QUIT response: " + response);
+            
+            System.out.println("Email sent successfully to: " + toEmail);
+            
+        } catch (Exception e) {
+            System.err.println("Failed to send email: " + e.getMessage());
+            e.printStackTrace();
+            throw e;
+        } finally {
+            try {
+                if (writer != null) writer.close();
+                if (reader != null) reader.close();
+                if (socket != null) socket.close();
+            } catch (IOException e) {
+                // Ignore cleanup errors
+            }
+        }
+    }
+}
diff --git a/src/mathquiz/backend_1/UserService.java b/src/mathquiz/backend_1/UserService.java
new file mode 100644
index 0000000..c518f36
--- /dev/null
+++ b/src/mathquiz/backend_1/UserService.java
@@ -0,0 +1,209 @@
+package mathquiz.backend;
+
+import java.io.*;
+import java.nio.file.*;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.Objects;
+
+/**
+ * User Service Class - Handles user registration, login, password management
+ */
+public class UserService {
+    private static final String USERS_FILE = "data/users.txt";
+    private static final String PASSWORDS_FILE = "data/passwords.txt";
+    
+    private Map<String, String> userPasswords;
+    private Map<String, String> userEmails;
+    
+    public UserService() {
+        loadUserData();
+    }
+    
+    /**
+     * User registration
+     */
+    public void registerUser(String email, String username) throws IOException {
+        if (userEmails.containsValue(email)) {
+            throw new IllegalArgumentException("This email has already been registered");
+        }
+        
+        userEmails.put(username, email);
+        
+        // Save user data
+        saveUserData();
+    }
+    
+    /**
+     * Set password
+     */
+    public void setPassword(String email, String password) throws IOException {
+        if (!isValidPassword(password)) {
+            throw new IllegalArgumentException("Password does not meet requirements");
+        }
+        
+        String username = getUsernameByEmail(email);
+        if (username == null) {
+            throw new IllegalArgumentException("User does not exist");
+        }
+        
+        userPasswords.put(username, password);
+        savePasswordData();
+    }
+    
+    /**
+     * User login
+     */
+    public boolean login(String email, String password) {
+        String username = getUsernameByEmail(email);
+        if (username == null) {
+            return false;
+        }
+        
+        String storedPassword = userPasswords.get(username);
+        if (storedPassword == null) {
+            return false;
+        }
+        
+        if (Objects.equals(storedPassword, password)) {
+            // Set current user (use email for session)
+            mathquiz.gui.QuizSession.getInstance().setCurrentUser(email);
+            return true;
+        }
+        
+        return false;
+    }
+    
+    /**
+     * Change password
+     */
+    public boolean changePassword(String username, String oldPassword, String newPassword) throws IOException {
+        if (!isValidPassword(newPassword)) {
+            throw new IllegalArgumentException("New password does not meet requirements");
+        }
+        
+        String storedPassword = userPasswords.get(username);
+        if (!Objects.equals(storedPassword, oldPassword)) {
+            return false;
+        }
+        
+        userPasswords.put(username, newPassword);
+        savePasswordData();
+        return true;
+    }
+    
+    /**
+     * Validate password format
+     */
+    private boolean isValidPassword(String password) {
+        if (password.length() < 6 || password.length() > 10) {
+            return false;
+        }
+        
+        boolean hasUpperCase = false;
+        boolean hasLowerCase = false;
+        boolean hasDigit = false;
+        
+        for (char c : password.toCharArray()) {
+            if (Character.isUpperCase(c)) {
+                hasUpperCase = true;
+            } else if (Character.isLowerCase(c)) {
+                hasLowerCase = true;
+            } else if (Character.isDigit(c)) {
+                hasDigit = true;
+            }
+        }
+        
+        return hasUpperCase && hasLowerCase && hasDigit;
+    }
+    
+    /**
+     * Get username by email
+     */
+    private String getUsernameByEmail(String email) {
+        for (Map.Entry<String, String> entry : userEmails.entrySet()) {
+            if (entry.getValue().equals(email)) {
+                return entry.getKey();
+            }
+        }
+        return null;
+    }
+    
+    /**
+     * Generate username
+     */
+    private String generateUsername(String email) {
+        String baseName = email.split("@")[0];
+        String username = baseName;
+        int counter = 1;
+        
+        while (userEmails.containsKey(username)) {
+            username = baseName + counter;
+            counter++;
+        }
+        
+        return username;
+    }
+    
+    /**
+     * Load user data
+     */
+    private void loadUserData() {
+        userPasswords = new HashMap<>();
+        userEmails = new HashMap<>();
+        
+        try {
+            // Create data directory
+            Files.createDirectories(Paths.get("data"));
+            
+            // Load user email mapping
+            if (Files.exists(Paths.get(USERS_FILE))) {
+                for (String line : Files.readAllLines(Paths.get(USERS_FILE))) {
+                    String[] parts = line.split(",");
+                    if (parts.length == 2) {
+                        userEmails.put(parts[0], parts[1]);
+                    }
+                }
+            }
+            
+            // Load user passwords
+            if (Files.exists(Paths.get(PASSWORDS_FILE))) {
+                for (String line : Files.readAllLines(Paths.get(PASSWORDS_FILE))) {
+                    String[] parts = line.split(",");
+                    if (parts.length == 2) {
+                        userPasswords.put(parts[0], parts[1]);
+                    }
+                }
+            }
+            
+        } catch (IOException e) {
+            // If loading fails, use empty data
+            userPasswords = new HashMap<>();
+            userEmails = new HashMap<>();
+        }
+    }
+    
+    /**
+     * Save user data
+     */
+    private void saveUserData() throws IOException {
+        try (BufferedWriter writer = Files.newBufferedWriter(Paths.get(USERS_FILE))) {
+            for (Map.Entry<String, String> entry : userEmails.entrySet()) {
+                writer.write(entry.getKey() + "," + entry.getValue());
+                writer.newLine();
+            }
+        }
+    }
+    
+    /**
+     * Save password data
+     */
+    private void savePasswordData() throws IOException {
+        try (BufferedWriter writer = Files.newBufferedWriter(Paths.get(PASSWORDS_FILE))) {
+            for (Map.Entry<String, String> entry : userPasswords.entrySet()) {
+                writer.write(entry.getKey() + "," + entry.getValue());
+                writer.newLine();
+            }
+        }
+    }
+}
\ No newline at end of file