PasswordHasher.java

import java.util.HashMap;

// Authentication layer for the inventory system
// Handles user registration and login with salted password hashing
public class PasswordHasher {
    private HashMap<String, PasswordEntry> users;
    private static final int HASH_ROUNDS = 5;

    // Store salt and hashed password together
    static class PasswordEntry {
        String salt;
        String hash;

        PasswordEntry(String salt, String hash) {
            this.salt = salt;
            this.hash = hash;
        }
    }

    public PasswordHasher() {
        users = new HashMap<>();
    }

    // Custom hash function using polynomial rolling hash + bit mixing
    // Takes input and salt, produces consistent hash output
    public String hash(String input, String salt) {
        String combined = input + salt;
        long hashValue = 5381;
        long secondary = 0;

        // First pass: polynomial rolling hash (DJB2 style)
        for (int i = 0; i < combined.length(); i++) {
            char c = combined.charAt(i);
            hashValue = ((hashValue << 5) + hashValue) ^ c;
            secondary = secondary * 37 + c;
        }

        // Second pass: bit mixing rounds to improve distribution
        for (int round = 0; round < HASH_ROUNDS; round++) {
            hashValue ^= (hashValue >>> 13);
            hashValue *= 0x9e3779b97f4a7c15L;
            hashValue ^= (hashValue >>> 16);
            secondary = secondary * 31 + (hashValue & 0xFF);
        }

        // Combine both values for final hash
        long finalHash = hashValue ^ secondary;
        return Long.toHexString(Math.abs(finalHash));
    }

    // Generate unique salt from system nanosecond timer
    public String generateSalt() {
        long timestamp = System.nanoTime();
        return Long.toHexString(timestamp);
    }

    // Verify password by rehashing with stored salt and comparing
    public boolean verify(String input, String username) {
        if (!users.containsKey(username)) {
            return false;
        }
        PasswordEntry entry = users.get(username);
        String computedHash = hash(input, entry.salt);
        return computedHash.equals(entry.hash);
    }

    // Register new user - check exists, hash password, store in HashMap
    public boolean registerUser(String username, String password) {
        if (users.containsKey(username)) {
            System.out.println("  [ERROR] User already exists!");
            return false;
        }
        String salt = generateSalt();
        String hashedPassword = hash(password, salt);
        users.put(username, new PasswordEntry(salt, hashedPassword));
        System.out.println("  [SUCCESS] Account created!");
        return true;
    }

    // Login by verifying password matches stored hash
    public boolean loginUser(String username, String password) {
        if (!verify(password, username)) {
            System.out.println("  [ERROR] Invalid credentials!");
            return false;
        }
        System.out.println("  [SUCCESS] Logged in as " + username);
        return true;
    }
}