crypto example

import javax.crypto.BadPaddingException;
import javax.crypto.Cipher;
import javax.crypto.IllegalBlockSizeException;
import javax.crypto.NoSuchPaddingException;
import javax.crypto.SecretKeyFactory;
import javax.crypto.spec.GCMParameterSpec;
import javax.crypto.spec.PBEKeySpec;
import javax.crypto.spec.SecretKeySpec;
import java.security.InvalidAlgorithmParameterException;
import java.security.InvalidKeyException;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;
import java.security.spec.InvalidKeySpecException;

public class CryptoHelper {

    private static final byte[] SALT = new byte[16];
    private static final String PBKDF_2_WITH_HMAC_SHA_256 = "PBKDF2WithHmacSHA256";
    private static final String ALGORITHM = "AES/GCM/NoPadding";
    private static final int GCM_IV_LENGTH = 12;
    private static final int GCM_TAG_LENGTH = 128;
    private static final SecureRandom SECURE_RANDOM = new SecureRandom();
    private static final int ITERATION_COUNT = 10000;
    private static final int KEY_LENGTH = 256;
    private final SecretKeySpec secretKeySpec;

    public CryptoHelper(final String password) {
        this.secretKeySpec = createSecretKey(password);
    }

    public byte[] encrypt(byte[] data) {
        try {
            Cipher cipher = Cipher.getInstance(ALGORITHM);
            byte[] iv = new byte[GCM_IV_LENGTH];
            SECURE_RANDOM.nextBytes(iv);
            GCMParameterSpec gcmParameterSpec = new GCMParameterSpec(GCM_TAG_LENGTH, iv);
            cipher.init(Cipher.ENCRYPT_MODE, secretKeySpec, gcmParameterSpec);
            byte[] encrypted = cipher.doFinal(data);
            byte[] bytes = new byte[iv.length + encrypted.length];
            System.arraycopy(iv, 0, bytes, 0, iv.length);
            System.arraycopy(encrypted, 0, bytes, iv.length, encrypted.length);
            return bytes;
        } catch (NoSuchAlgorithmException | NoSuchPaddingException | InvalidKeyException |
                 InvalidAlgorithmParameterException | IllegalBlockSizeException | BadPaddingException e) {
            throw new CryptoException(e);
        }
    }

    public byte[] decrypt(byte[] bytes) {
        try {
            Cipher cipher = Cipher.getInstance(ALGORITHM);
            byte[] iv = new byte[GCM_IV_LENGTH];
            byte[] encrypted = new byte[bytes.length - GCM_IV_LENGTH];
            System.arraycopy(bytes, 0, iv, 0, iv.length);
            System.arraycopy(bytes, iv.length, encrypted, 0, encrypted.length);
            GCMParameterSpec gcmParameterSpec = new GCMParameterSpec(GCM_TAG_LENGTH, iv);
            cipher.init(Cipher.DECRYPT_MODE, secretKeySpec, gcmParameterSpec);
            return cipher.doFinal(encrypted);
        } catch (NoSuchAlgorithmException | NoSuchPaddingException | InvalidKeyException |
                 InvalidAlgorithmParameterException | IllegalBlockSizeException | BadPaddingException e) {
            throw new CryptoException(e);
        }
    }

    private static SecretKeySpec createSecretKey(String password) {
        try {
            char[] passwordChars = password.toCharArray();
            PBEKeySpec spec = new PBEKeySpec(passwordChars, CryptoHelper.SALT, CryptoHelper.ITERATION_COUNT, CryptoHelper.KEY_LENGTH);
            SecretKeyFactory keyFactory = SecretKeyFactory.getInstance(PBKDF_2_WITH_HMAC_SHA_256);
            byte[] keyBytes = keyFactory.generateSecret(spec).getEncoded();
            return new SecretKeySpec(keyBytes, "AES");
        } catch (NoSuchAlgorithmException | InvalidKeySpecException e) {
            throw new CryptoException(e);
        }
    }
}