vominhtrius/aes-example

Learn how to build AES encryption which interoperates between Java and JavaScript

Interoperable AES encryption with Java and JavaScript

AES implementations are available in many languages, including Java and JavaScript. In Java, the javax.crypto.* packages are part of the standard, and in JavaScript, the excellent CryptoJS provides an implementation for many cryptographic algorithms. However, due to different default settings and various implementation details, it is not trivial to use the APIs in a way, that the result is the same on all platforms.

This example demonstrates implementations of the algorithm in Java and JavaScript that produces identical results using passphrase based encryption. For AES encryption, you cannot - or shouldn't - simply use a password in order to encrypt data. Instead, many parameters need to be defined, such as:

• iteration count used for the salting process
• padding mode
• key derivation function
• key length

Then, additional initialization parameters need to be defined, such as the salt and the initialization vector (IV). With all parameters defined, the encryption process is the same for both, Java and JavaScript:

1. Generate salt and IV (this is typically done using a secure psuedo-random number generator; in my example tests both are fixed in order to produce predictable results).
2. Generate the key (using the PBKDF2 function) from the given passphrase, salt, key size and number of iterations (for the salting process.
3. Encrypt the plaintext using key and IV.

The decryption process is even simpler, because IV and salt have already been generated. These have to be reused to successfully reproduce the plaintext. Therefore, for successful encryption, you have to store IV, salt and iteration count (as long as it is not fixed for your application) along with the cipher text. Since these parameters don't need to get generated the decryption process only has 2 steps:

1. Generate key (same as step 2. above).
2. Decrypt cipher text using key and IV.

In this example, I have created a utility class for each language: AesUtil.java and AesUtil.js. In the test, all data (salt, passpharse, IV, plaintext, ciphertext) are represented as String. The ciphertext is encoded using base64, in order to get a proper and compact representation of the bytes (AES produces a byte array, not a String). The other parameters, salt and IV are encoded in hex. This is useful to effectively count and read the number of bytes used (and see if the length of both parameters is correct).

JavaScript implementation AesUtil.js

1. Generate key:

      var key = CryptoJS.PBKDF2(
          passPhrase, 
          CryptoJS.enc.Hex.parse(salt),
          { keySize: this.keySize, iterations: this.iterationCount });
   

   Note, that this.keySize is the size of the key in 4-byte blocks. So, if you want to use a 128-bit key, you have to divide the number of bits by 32 to get the key size used for CryptoJS.

2. Encrypt plaintext:

   The object returned by the encrypt method is not a String, but a object that contains the parameters of the algorithm and the ciphertext.

      var encrypted = CryptoJS.AES.encrypt(
          plainText,
          key,
          { iv: CryptoJS.enc.Hex.parse(iv) });
   

   To convert the encryption result into base64 format, you have to use the toString() function:

      var ciphertext = encrypted.ciphertext.toString(CryptoJS.enc.Base64);
   

3. Decrypt ciphertext:

   To decrypt, a parameter object is created first, that contains the ciphertext (note base64 encoding is used here):

      var cipherParams = CryptoJS.lib.CipherParams.create({
        ciphertext: CryptoJS.enc.Base64.parse(ciphertext)
      });
      var decrypted = CryptoJS.AES.decrypt(
          cipherParams,
          key,
          { iv: CryptoJS.enc.Hex.parse(iv) });
   

   Again, to get the result in text form, you use the toString() function:

      var plaintext = decrypted.toString(CryptoJS.enc.Utf8);
   

Java implementation AesUtil.java

The Java implementation looks a bit different, but the structure is the same:

1. Create a cipher instance:

        Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
   

2. Generate key:

        SecretKeyFactory factory = SecretKeyFactory.getInstance("PBKDF2WithHmacSHA1");
        KeySpec spec = new PBEKeySpec(passphrase.toCharArray(), hex(salt), iterationCount, keySize);
        SecretKey key = new SecretKeySpec(factory.generateSecret(spec).getEncoded(), "AES");
   

3. Encrypt:

        cipher.init(Cipher.ENCRYPT_MODE, key, new IvParameterSpec(hex(iv)));
        byte[] encrypted = cipher.doFinal(bytes);
   

4. Decrypt:

        cipher.init(Cipher.DECRYPT_MODE, key, new IvParameterSpec(hex(iv)));
        byte[] decrypted = cipher.doFinal(bytes);
   

Running the tests

The project uses Maven as build environment. After cloning the repository, you just need to type:

#> mvn test

That executes both, the Java unit tests and the JavaScript Jasmine specs.

Browser example

To run a simple example to encrypt a text in the browser and send it to a servlet, you just need to run:

#> mvn jetty:run

Then open http://localhost:8080. The example encrypts a text using a password, which is then sent to the server. The request contains everything required to encrypt the password, such as salt, IV, iteration count, and the passphrase. In the real world, you need to pass all these to the server, except passphrase, of course.