Bankapp/src/main/java/me/zacharias/bank/Utils.java

package me.zacharias.bank;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

import javax.crypto.Cipher;
import javax.crypto.SecretKey;
import javax.crypto.SecretKeyFactory;
import javax.crypto.spec.GCMParameterSpec;
import javax.crypto.spec.PBEKeySpec;
import javax.crypto.spec.SecretKeySpec;
import java.io.*;
import java.nio.charset.StandardCharsets;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;
import java.util.Base64;

public class Utils {
    
    public static final Gson gson;
    
    static {
        GsonBuilder gsonBuilder = new GsonBuilder();
        //gsonBuilder.registerTypeAdapter(Account.class, new AccountAdapter());
        
        gson = gsonBuilder.create();
        
        File users = new File("./users");
        
        if(!users.exists()) {
            users.mkdir();
        }
    }
    
    public static String SHA256(String input) {
        MessageDigest sha256 = null;
        try {
            sha256 = MessageDigest.getInstance("SHA-256");
        } catch (NoSuchAlgorithmException e) {
            throw new RuntimeException(e);
        }
        byte[] hash = sha256.digest(input.getBytes(StandardCharsets.UTF_8));

        StringBuilder hexString = new StringBuilder();
        for (byte b : hash) {
            String hex = Integer.toHexString(0xff & b);
            if (hex.length() == 1) hexString.append('0');
            hexString.append(hex);
        }

        return hexString.toString();
    }

    private static final int KEY_SIZE = 256;
    private static final int ITERATIONS = 65536;
    private static final int SALT_LENGTH = 16;
    private static final int IV_LENGTH = 12;

    public static String Encrypt(String data, String username) throws Exception {
        SecureRandom random = new SecureRandom();

        byte[] salt = new byte[SALT_LENGTH];
        random.nextBytes(salt);

        byte[] iv = new byte[IV_LENGTH];
        random.nextBytes(iv);

        SecretKey key = deriveKey(username, salt);

        Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");
        GCMParameterSpec gcmSpec = new GCMParameterSpec(128, iv);
        cipher.init(Cipher.ENCRYPT_MODE, key, gcmSpec);
        byte[] encryptedData = cipher.doFinal(data.getBytes());

        byte[] combined = new byte[salt.length + iv.length + encryptedData.length];
        System.arraycopy(salt, 0, combined, 0, salt.length);
        System.arraycopy(iv, 0, combined, salt.length, iv.length);
        System.arraycopy(encryptedData, 0, combined, salt.length + iv.length, encryptedData.length);

        return Base64.getEncoder().encodeToString(combined);
    }

    public static String Decrypt(String data, String username) throws Exception {
        byte[] combined = Base64.getDecoder().decode(data);

        // Extract salt, IV, and encrypted data
        byte[] salt = new byte[SALT_LENGTH];
        byte[] iv = new byte[IV_LENGTH];
        byte[] cipherText = new byte[combined.length - SALT_LENGTH - IV_LENGTH];

        System.arraycopy(combined, 0, salt, 0, salt.length);
        System.arraycopy(combined, salt.length, iv, 0, iv.length);
        System.arraycopy(combined, salt.length + iv.length, cipherText, 0, cipherText.length);

        SecretKey key = deriveKey(username, salt);

        Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");
        GCMParameterSpec gcmSpec = new GCMParameterSpec(128, iv);
        cipher.init(Cipher.DECRYPT_MODE, key, gcmSpec);
        byte[] decrptedData = cipher.doFinal(cipherText);

        return new String(decrptedData);
    }

    private static SecretKey deriveKey(String password, byte[] salt) throws Exception {
        SecretKeyFactory factory = SecretKeyFactory.getInstance("PBKDF2WithHmacSHA256");
        PBEKeySpec spec = new PBEKeySpec(password.toCharArray(), salt, ITERATIONS, KEY_SIZE);
        return new SecretKeySpec(factory.generateSecret(spec).getEncoded(), "AES");
    }

    public static String ReadFile(File file)
    {
        if(!file.exists())
        {
            throw new RuntimeException(new FileNotFoundException(file.getPath()+" not found"));
        }

        try {
            BufferedReader br = new BufferedReader(new FileReader(file));
            StringBuilder sb = new StringBuilder();
            String line;
            while((line = br.readLine()) != null)
            {
                sb.append(line);
            }
            return sb.toString();
        }
        catch (IOException e)
        {
            throw new RuntimeException(e);
        }
    }

    public static void WriteFile(File file, String data) {
        if(file.exists())
        {
            file.delete();
        }

        try {
            file.createNewFile();

            BufferedWriter bw = new BufferedWriter(new FileWriter(file));
            bw.write(data);
            bw.close();
        }catch (IOException e)
        {
            throw new RuntimeException(e);
        }
    }
}