Alternative to using SecureRandom to generate AES key and IV

Question

I am trying to encrypt text using the AES encryption algorithm , save this encrypted text to a file and then reopen later and decrypt these. Following is my encryption and decryption logic

    Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
    cipher.init(Cipher.ENCRYPT_MODE, key, iv);
    byte[] stringBytes = clear_text.getBytes();
    byte[] raw = cipher.doFinal(stringBytes);
    return Base64.encodeBase64String(raw);

And this is the decryption logic

    Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
    cipher.init(Cipher.DECRYPT_MODE, key, iv);
    byte[] raw = Base64.decodeBase64(encText);
    byte[] stringBytes = cipher.doFinal(raw);
    String clear_text = new String(stringBytes, "UTF8");
    return clear_text;

I get a BadPaddingSize exception. My guess is that by using the SecureRandom class, both the methods use different keys while encrypting or decypting text. Is there a way I can use the same key in both the routines?

Answer 1

Yes, you can use the same key; it is even required to use the same key. You should however never use the same key / IV combination as that is not secure. So often the IV gets prefixed to the ciphertext instead.

Note that the following implementation shows you how to generate the random IV without SecureRandom, but that's a bit disingenious as the Cipher class will just use the default one internally to create the IV. For CBC the IV may be known to an attacker, but the attacker should not be able to distinguish it from random data.

In this example the key data is simply stored within a "constant". Storing the key within source code may not provide enough security. Instead it is often encrypted with a public key, password, stored on a USB key, stored within a smartcard or HSM etc. etc. Key management is however a vast subject so I won't discuss it further for this answer.

In Java you should however use SecretKey/SecretKeySpec to create keys from known data and IvParameterSpec for a known IV (or Nonce).

import java.nio.charset.StandardCharsets;
import java.security.AlgorithmParameters;
import java.security.InvalidAlgorithmParameterException;
import java.security.InvalidKeyException;
import java.security.NoSuchAlgorithmException;
import java.security.spec.InvalidParameterSpecException;

import javax.crypto.BadPaddingException;
import javax.crypto.Cipher;
import javax.crypto.IllegalBlockSizeException;
import javax.crypto.NoSuchPaddingException;
import javax.crypto.ShortBufferException;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;

public class AESWithStaticKeyAndRandomIV {

    private static byte[] KEY = new byte[] { (byte) 0x14, (byte) 0x0b,
            (byte) 0x41, (byte) 0xb2, (byte) 0x2a, (byte) 0x29, (byte) 0xbe,
            (byte) 0xb4, (byte) 0x06, (byte) 0x1b, (byte) 0xda, (byte) 0x66,
            (byte) 0xb6, (byte) 0x74, (byte) 0x7e, (byte) 0x14 };

    public static byte[] encrypt(byte[] plaintext) {
        try {
            Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
            SecretKey key = new SecretKeySpec(KEY, "AES");
            cipher.init(Cipher.ENCRYPT_MODE, key);

            AlgorithmParameters params = cipher.getParameters();
            byte[] iv = params.getParameterSpec(IvParameterSpec.class).getIV();

            byte[] ciphertext = new byte[iv.length
                    + cipher.getOutputSize(plaintext.length)];
            System.arraycopy(iv, 0, ciphertext, 0, iv.length);
            cipher.doFinal(plaintext, 0, plaintext.length, ciphertext,
                    iv.length);
            return ciphertext;
        } catch (InvalidKeyException | NoSuchAlgorithmException
                | NoSuchPaddingException | InvalidParameterSpecException
                | ShortBufferException | IllegalBlockSizeException
                | BadPaddingException e) {
            throw new IllegalStateException(
                    "CBC encryption with standard algorithm should never fail",
                    e);
        }
    }

    public static byte[] decrypt(byte[] ciphertext) throws IllegalBlockSizeException,
            BadPaddingException {
        try {
            Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
            SecretKeySpec key = new SecretKeySpec(KEY, "AES");

            if (ciphertext.length < cipher.getBlockSize()) {
                throw new IllegalArgumentException(
                        "Ciphertext too small to contain IV");
            }

            IvParameterSpec ivSpec = new IvParameterSpec(ciphertext, 0,
                    cipher.getBlockSize());
            cipher.init(Cipher.DECRYPT_MODE, key, ivSpec);

            byte[] plaintext = new byte[cipher.getOutputSize(ciphertext.length
                    - cipher.getBlockSize())];
            cipher.doFinal(ciphertext, cipher.getBlockSize(), ciphertext.length
                    - cipher.getBlockSize(), plaintext, 0);
            return plaintext;
        } catch (InvalidKeyException | NoSuchAlgorithmException
                | NoSuchPaddingException | ShortBufferException
                | InvalidAlgorithmParameterException e) {
            throw new IllegalStateException(
                    "CBC decryption with standard algorithm should be available",
                    e);
        }
    }

    public static void main(String[] args) throws Exception {
           byte[] plaintext = decrypt(encrypt("owlstead".getBytes(StandardCharsets.UTF_8)));
           System.out.println(new String(plaintext, StandardCharsets.UTF_8));
    }
}

---

With a key store (you have to use JCEKS for now):

import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.security.AlgorithmParameters;
import java.security.InvalidAlgorithmParameterException;
import java.security.InvalidKeyException;
import java.security.KeyStore;
import java.security.KeyStore.ProtectionParameter;
import java.security.KeyStore.SecretKeyEntry;
import java.security.KeyStoreException;
import java.security.NoSuchAlgorithmException;
import java.security.UnrecoverableEntryException;
import java.security.cert.CertificateException;
import java.security.spec.InvalidParameterSpecException;

import javax.crypto.BadPaddingException;
import javax.crypto.Cipher;
import javax.crypto.IllegalBlockSizeException;
import javax.crypto.NoSuchPaddingException;
import javax.crypto.SecretKey;
import javax.crypto.ShortBufferException;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;

public class AESWithStaticKeyAndRandomIV {

    private static final String KEY_ALIAS = "secret";

    private static byte[] KEY = new byte[] { (byte) 0x14, (byte) 0x0b,
            (byte) 0x41, (byte) 0xb2, (byte) 0x2a, (byte) 0x29, (byte) 0xbe,
            (byte) 0xb4, (byte) 0x06, (byte) 0x1b, (byte) 0xda, (byte) 0x66,
            (byte) 0xb6, (byte) 0x74, (byte) 0x7e, (byte) 0x14 };

    private static ProtectionParameter PASSWORD = new KeyStore.PasswordProtection(
            new char[] {'p', 'a', 's', 's', 'w', 'o', 'r', 'd'});

    private final KeyStore store;

    private AESWithStaticKeyAndRandomIV(KeyStore store) {
        this.store = store;
    }

    public byte[] encrypt(byte[] plaintext) {
        try {
            Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
            SecretKey key;
            try {
                key = ((SecretKeyEntry) store.getEntry(KEY_ALIAS, PASSWORD))
                        .getSecretKey();
            } catch (UnrecoverableEntryException | KeyStoreException e) {
                throw new IllegalStateException("What key?", e);
            }
            cipher.init(Cipher.ENCRYPT_MODE, key);

            AlgorithmParameters params = cipher.getParameters();
            byte[] iv = params.getParameterSpec(IvParameterSpec.class).getIV();

            byte[] ciphertext = new byte[iv.length
                    + cipher.getOutputSize(plaintext.length)];
            System.arraycopy(iv, 0, ciphertext, 0, iv.length);
            cipher.doFinal(plaintext, 0, plaintext.length, ciphertext,
                    iv.length);
            return ciphertext;
        } catch (InvalidKeyException | NoSuchAlgorithmException
                | NoSuchPaddingException | InvalidParameterSpecException
                | ShortBufferException | IllegalBlockSizeException
                | BadPaddingException e) {
            throw new IllegalStateException(
                    "CBC encryption with standard algorithm should never fail",
                    e);
        }
    }

    public byte[] decrypt(byte[] ciphertext) throws IllegalBlockSizeException,
            BadPaddingException {
        try {
            Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
            SecretKey key;
            try {
                key = ((SecretKeyEntry) store.getEntry(KEY_ALIAS, PASSWORD))
                        .getSecretKey();
            } catch (UnrecoverableEntryException | KeyStoreException e) {
                throw new IllegalStateException("What key?", e);
            }

            if (ciphertext.length < cipher.getBlockSize()) {
                throw new IllegalArgumentException(
                        "Ciphertext too small to contain IV");
            }

            IvParameterSpec ivSpec = new IvParameterSpec(ciphertext, 0,
                    cipher.getBlockSize());
            cipher.init(Cipher.DECRYPT_MODE, key, ivSpec);

            byte[] plaintext = new byte[cipher.getOutputSize(ciphertext.length
                    - cipher.getBlockSize())];
            cipher.doFinal(ciphertext, cipher.getBlockSize(), ciphertext.length
                    - cipher.getBlockSize(), plaintext, 0);
            return plaintext;
        } catch (InvalidKeyException | NoSuchAlgorithmException
                | NoSuchPaddingException | ShortBufferException
                | InvalidAlgorithmParameterException e) {
            throw new IllegalStateException(
                    "CBC decryption with standard algorithm should be available",
                    e);
        }
    }

    public static KeyStore createStoreWithSecretKey() {

        try {
            KeyStore keyStore = KeyStore.getInstance("JCEKS");
            keyStore.load(null);
            SecretKey key = new SecretKeySpec(KEY, "AES");
            keyStore.setEntry(KEY_ALIAS, new KeyStore.SecretKeyEntry(key), PASSWORD);
            return keyStore;
        } catch (KeyStoreException | NoSuchAlgorithmException
                | CertificateException | IOException e) {
            throw new IllegalStateException("Unable to create key store", e);
        }
    }

    public static void main(String[] args) throws Exception {
        AESWithStaticKeyAndRandomIV crypt = new AESWithStaticKeyAndRandomIV(
                createStoreWithSecretKey());

        byte[] plaintext = crypt.decrypt(crypt.encrypt("owlstead"
                .getBytes(StandardCharsets.UTF_8)));
        System.out.println(new String(plaintext, StandardCharsets.UTF_8));
    }
}