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1. Overview

The symmetric-key block cipher plays an important role in data encryption. It means that the same key is used for both encryption and decryption. The Advanced Encryption Standard (AES) is a widely used symmetric-key encryption algorithm.

In this tutorial, we’ll see how to implement AES encryption and decryption using the Java Cryptography Architecture (JCA) within the JDK.

2. AES Algorithm

The AES algorithm is an iterative, symmetric-key block cipher that supports cryptographic keys (secret keys) of 128, 192, and 256 bits to encrypt and decrypt data in blocks of 128 bits. The below figure shows the high-level AES algorithm:

High Level AES Algorithm

If the data to be encrypted does not meet the block size of 128 bits requirement, it must be padded. Padding is a process of filling up the last block to 128 bits.

3. AES Variations

The AES algorithm has six modes of operation:

  1. ECB (Electronic Code Book)
  2. CBC (Cipher Block Chaining)
  3. CFB (Cipher FeedBack)
  4. OFB (Output FeedBack)
  5. CTR (Counter)
  6. GCM (Galois/Counter Mode)

The mode of operation may be applied in order to strengthen the effect of the encryption algorithm. Moreover, the mode of operation may convert the block cipher into a stream cipher. Each mode has its strength and weakness. Let’s have a quick review.

3.1. ECB

This mode of operation is the simplest of all. The plaintext is divided into blocks with a size of 128 bits. Then each block will be encrypted with the same key and algorithm. Therefore, it produces the same result for the same block. This is the main weakness of this mode and it is not recommended for encryption. It requires padding data.

3.2. CBC

In order to overcome the ECB weakness, CBC mode uses an Initialization Vector (IV) to augment the encryption. First, CBC uses the plaintext block xor with the IV. Then it encrypts the result to the ciphertext block. In the next block, it uses the encryption result to xor with the plaintext block until the last block.

In this mode, encryption can not be parallelized, but decryption can be parallelized. It also requires padding data.

3.3. CFB

This mode can be used as a stream cipher. First, it encrypts the IV, then it will xor with the plaintext block to get ciphertext. Then CFB encrypts the encryption result to xor the plaintext. It needs an IV.

In this mode, decryption can be parallelized but encryption can not be parallelized.

3.4. OFB

This mode can be used as a stream cipher. First, it encrypts the IV. Then it uses the encryption results to xor the plaintext to get ciphertext.

It doesn’t require padding data and will not be affected by the noisy block.

3.5. CTR

This mode uses the value of a counter as an IV. It's very similar to OFB, but it uses the counter to be encrypted every time instead of the IV.

This mode has two strengths, including encryption/decryption parallelization, and noise in one block does not affect other blocks.

3.6. GCM

This mode is an extension of the CTR mode. The GCM has received significant attention and is recommended by NIST. The GCM model outputs ciphertext and an authentication tag. The main advantage of this mode, compared to other operation modes of the algorithm, is its efficiency.

In this tutorial, we'll use the AES/CBC/PKCS5Padding algorithm because it is widely used in many projects.

3.7. Size of Data After Encryption

As mentioned earlier, the AES has a block size of 128 bits or 16 bytes. The AES does not change the size, and the ciphertext size is equal to the cleartext size. Also, in ECB and CBC modes, we should use a padding algorithm likes PKCS 5. So, the size of data after encryption is:

ciphertext_size (bytes) = cleartext_size + (16 - (cleartext_size % 16))

For storing IV with ciphertext, we need to add 16 more bytes.

4. AES Parameters

In the AES algorithm, we need three parameters: input data, secret key, and IV. IV is not used in ECB mode.

4.1. Input Data

The input data to the AES can be string, file, object, and password-based.

4.2. Secret Key

There are two ways for generating a secret key in the AES: generating from a random number or deriving from a given password.

In the first approach, the secret key should be generated from a Cryptographically Secure (Pseudo-)Random Number Generator like the SecureRandom class.

For generating a secret key, we can use the KeyGenerator class. Let’s define a method for generating the AES key with the size of n (128, 192, and 256) bits:

public static SecretKey generateKey(int n) throws NoSuchAlgorithmException {
    KeyGenerator keyGenerator = KeyGenerator.getInstance("AES");
    keyGenerator.init(n);
    SecretKey key = keyGenerator.generateKey();
    return key;
}

In the second approach, the AES secret key can be derived from a given password using a password-based key derivation function like PBKDF2. We also need a salt value for turning a password into a secret key. The salt is also a random value.

We can use the SecretKeyFactory class with the PBKDF2WithHmacSHA256 algorithm for generating a key from a given password.

Let’s define a method for generating the AES key from a given password with 65,536 iterations and a key length of 256 bits:

public static SecretKey getKeyFromPassword(String password, String salt)
    throws NoSuchAlgorithmException, InvalidKeySpecException {
    
    SecretKeyFactory factory = SecretKeyFactory.getInstance("PBKDF2WithHmacSHA256");
    KeySpec spec = new PBEKeySpec(password.toCharArray(), salt.getBytes(), 65536, 256);
    SecretKey secret = new SecretKeySpec(factory.generateSecret(spec)
        .getEncoded(), "AES");
    return secret;
}

4.3. Initialization Vector (IV)

IV is a pseudo-random value and has the same size as the block that is encrypted. We can use the SecureRandom class to generate a random IV.

Let’s define a method for generating an IV:

public static IvParameterSpec generateIv() {
    byte[] iv = new byte[16];
    new SecureRandom().nextBytes(iv);
    return new IvParameterSpec(iv);
}

5. Encryption and Decryption

5.1. String

To implement input string encryption, we first need to generate the secret key and IV according to the previous section. As the next step, we create an instance from the Cipher class by using the getInstance() method.

Additionally, we configure a cipher instance using the init() method with a secret key, IV, and encryption mode. Finally, we encrypt the input string by invoking the doFinal() method. This method gets bytes of input and returns ciphertext in bytes:

public static String encrypt(String algorithm, String input, SecretKey key,
    IvParameterSpec iv) throws NoSuchPaddingException, NoSuchAlgorithmException,
    InvalidAlgorithmParameterException, InvalidKeyException,
    BadPaddingException, IllegalBlockSizeException {
    
    Cipher cipher = Cipher.getInstance(algorithm);
    cipher.init(Cipher.ENCRYPT_MODE, key, iv);
    byte[] cipherText = cipher.doFinal(input.getBytes());
    return Base64.getEncoder()
        .encodeToString(cipherText);
}

For decrypting an input string, we can initialize our cipher using the DECRYPT_MODE to decrypt the content:

public static String decrypt(String algorithm, String cipherText, SecretKey key,
    IvParameterSpec iv) throws NoSuchPaddingException, NoSuchAlgorithmException,
    InvalidAlgorithmParameterException, InvalidKeyException,
    BadPaddingException, IllegalBlockSizeException {
    
    Cipher cipher = Cipher.getInstance(algorithm);
    cipher.init(Cipher.DECRYPT_MODE, key, iv);
    byte[] plainText = cipher.doFinal(Base64.getDecoder()
        .decode(cipherText));
    return new String(plainText);
}

Let's write a test method for encrypting and decrypting a string input:

@Test
void givenString_whenEncrypt_thenSuccess()
    throws NoSuchAlgorithmException, IllegalBlockSizeException, InvalidKeyException,
    BadPaddingException, InvalidAlgorithmParameterException, NoSuchPaddingException { 
    
    String input = "baeldung";
    SecretKey key = AESUtil.generateKey(128);
    IvParameterSpec ivParameterSpec = AESUtil.generateIv();
    String algorithm = "AES/CBC/PKCS5Padding";
    String cipherText = AESUtil.encrypt(algorithm, input, key, ivParameterSpec);
    String plainText = AESUtil.decrypt(algorithm, cipherText, key, ivParameterSpec);
    Assertions.assertEquals(input, plainText);
}

5.2. File

Now let's encrypt a file using the AES algorithm. The steps are the same, but we need some IO classes to work with the files. Let's encrypt a text file:

public static void encryptFile(String algorithm, SecretKey key, IvParameterSpec iv,
    File inputFile, File outputFile) throws IOException, NoSuchPaddingException,
    NoSuchAlgorithmException, InvalidAlgorithmParameterException, InvalidKeyException,
    BadPaddingException, IllegalBlockSizeException {
    
    Cipher cipher = Cipher.getInstance(algorithm);
    cipher.init(Cipher.ENCRYPT_MODE, key, iv);
    FileInputStream inputStream = new FileInputStream(inputFile);
    FileOutputStream outputStream = new FileOutputStream(outputFile);
    byte[] buffer = new byte[64];
    int bytesRead;
    while ((bytesRead = inputStream.read(buffer)) != -1) {
        byte[] output = cipher.update(buffer, 0, bytesRead);
        if (output != null) {
            outputStream.write(output);
        }
    }
    byte[] outputBytes = cipher.doFinal();
    if (outputBytes != null) {
        outputStream.write(outputBytes);
    }
    inputStream.close();
    outputStream.close();
}

Please note that trying to read the entire file – particularly if it is large – into memory is not recommended. Instead, we encrypt a buffer at a time.

For decrypting a file, we use similar steps and initialize our cipher using DECRYPT_MODE as we saw before.

Again, let's define a test method for encrypting and decrypting a text file. In this method, we read the baeldung.txt file from the test resource directory, encrypt it into a file called baeldung.encrypted, and then decrypt the file into a new file:

@Test
void givenFile_whenEncrypt_thenSuccess() 
    throws NoSuchAlgorithmException, IOException, IllegalBlockSizeException, 
    InvalidKeyException, BadPaddingException, InvalidAlgorithmParameterException, 
    NoSuchPaddingException {
    
    SecretKey key = AESUtil.generateKey(128);
    String algorithm = "AES/CBC/PKCS5Padding";
    IvParameterSpec ivParameterSpec = AESUtil.generateIv();
    Resource resource = new ClassPathResource("inputFile/baeldung.txt");
    File inputFile = resource.getFile();
    File encryptedFile = new File("classpath:baeldung.encrypted");
    File decryptedFile = new File("document.decrypted");
    AESUtil.encryptFile(algorithm, key, ivParameterSpec, inputFile, encryptedFile);
    AESUtil.decryptFile(
      algorithm, key, ivParameterSpec, encryptedFile, decryptedFile);
    assertThat(inputFile).hasSameTextualContentAs(decryptedFile);
}

5.3. Password-Based

We can do the AES encryption and decryption using the secret key that is derived from a given password.

For generating a secret key, we use the getKeyFromPassword() method. The encryption and decryption steps are the same as those shown in the string input section. We can then use the instantiated cipher and the provided secret key to perform the encryption.

Let's write a test method:

@Test
void givenPassword_whenEncrypt_thenSuccess() 
    throws InvalidKeySpecException, NoSuchAlgorithmException, 
    IllegalBlockSizeException, InvalidKeyException, BadPaddingException, 
    InvalidAlgorithmParameterException, NoSuchPaddingException {
    
    String plainText = "www.baeldung.com";
    String password = "baeldung";
    String salt = "12345678";
    IvParameterSpec ivParameterSpec = AESUtil.generateIv();
    SecretKey key = AESUtil.getKeyFromPassword(password,salt);
    String cipherText = AESUtil.encryptPasswordBased(plainText, key, ivParameterSpec);
    String decryptedCipherText = AESUtil.decryptPasswordBased(
      cipherText, key, ivParameterSpec);
    Assertions.assertEquals(plainText, decryptedCipherText);
}

5.4. Object

For encrypting a Java object, we need to use the SealedObject class. The object should be Serializable. Let's begin by defining a Student class:

public class Student implements Serializable {
    private String name;
    private int age;

    // standard setters and getters
}

Next, let's encrypt the Student object :

public static SealedObject encryptObject(String algorithm, Serializable object,
    SecretKey key, IvParameterSpec iv) throws NoSuchPaddingException,
    NoSuchAlgorithmException, InvalidAlgorithmParameterException, 
    InvalidKeyException, IOException, IllegalBlockSizeException {
    
    Cipher cipher = Cipher.getInstance(algorithm);
    cipher.init(Cipher.ENCRYPT_MODE, key, iv);
    SealedObject sealedObject = new SealedObject(object, cipher);
    return sealedObject;
}

The encrypted object can later be decrypted using the correct cipher:

public static Serializable decryptObject(String algorithm, SealedObject sealedObject,
    SecretKey key, IvParameterSpec iv) throws NoSuchPaddingException,
    NoSuchAlgorithmException, InvalidAlgorithmParameterException, InvalidKeyException,
    ClassNotFoundException, BadPaddingException, IllegalBlockSizeException,
    IOException {
    
    Cipher cipher = Cipher.getInstance(algorithm);
    cipher.init(Cipher.DECRYPT_MODE, key, iv);
    Serializable unsealObject = (Serializable) sealedObject.getObject(cipher);
    return unsealObject;
}

Let's write a test case:

@Test
void givenObject_whenEncrypt_thenSuccess() 
    throws NoSuchAlgorithmException, IllegalBlockSizeException, InvalidKeyException,
    InvalidAlgorithmParameterException, NoSuchPaddingException, IOException, 
    BadPaddingException, ClassNotFoundException {
    
    Student student = new Student("Baeldung", 20);
    SecretKey key = AESUtil.generateKey(128);
    IvParameterSpec ivParameterSpec = AESUtil.generateIv();
    String algorithm = "AES/CBC/PKCS5Padding";
    SealedObject sealedObject = AESUtil.encryptObject(
      algorithm, student, key, ivParameterSpec);
    Student object = (Student) AESUtil.decryptObject(
      algorithm, sealedObject, key, ivParameterSpec);
    assertThat(student).isEqualToComparingFieldByField(object);
}

6. Conclusion

In summary, we've learned how to encrypt and decrypt input data like strings, files, objects, and password-based data, using the AES algorithm in Java. Additionally, we've discussed the AES variations and the size of data after encryption.

As always, the full source code of the article is available over on GitHub.

Security bottom

I just announced the new Learn Spring Security course, including the full material focused on the new OAuth2 stack in Spring Security 5:

>> CHECK OUT THE COURSE
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