AES Encryption using Ciphers in JAVA

In this article, we will make a simple Java application that encrypts and decrypts strings using the AES algorithm.

Let’s get started by making a java class

public class AES{
}

The first step is to initialize some variables. For this step, we only need one (spoiler, there are at least two)
I will also need some final variables

private SecretKey key;

/**
 * Possible AES KEY_SIZE values are 128, 192 and 256
 * Possible tag length (T_LEN) values are 128, 120, 112, 104 and 96
 */
private final int KEY_SIZE = 128;
private final int T_LEN = 128;

Let’s get into my favorite method : init()

You can use a try-catch and print the stack trace if you want to learn about the possible errors there are, but I will just ignore all the errors for the sake of the simplicity

public void init() throws Exception{
    KeyGenerator generator = KeyGenerator.getInstance("AES");
    generator.init(KEY_SIZE);
    key = generator.generateKey();
}

Now that we have initialized our key, we can add the two encode/decode methods that use Base64 (same as the RSA code)

private String encode(byte[] data){
    return Base64.getEncoder().encodeToString(data);
}

private byte[] decode(String data){
    return Base64.getDecoder().decode(data);
}

Now, we are ready to roll with the encryption!

public String encrypt(String message) throws Exception{
    byte[] messageInBytes = message.getBytes();
    Cipher encryptionCipher = Cipher.getInstance("AES/GCM/NoPadding");
    encryptionCipher.init(Cipher.ENCRYPT_MODE,key);
    byte[] encryptedBytes = encryptionCipher.doFinal(messageInBytes);
    return encode(encryptedBytes);
}

Shall we move to the decrypt method? 🤔
Not really because we need something that only the encryption cipher has! HIS IV. So let’s modify our code

private Cipher encryptionCipher;

public String encrypt(String message) throws Exception{
    byte[] messageInBytes = message.getBytes();
    encryptionCipher = Cipher.getInstance("AES/GCM/NoPadding");
    encryptionCipher.init(Cipher.ENCRYPT_MODE,key);
    byte[] encryptedBytes = encryptionCipher.doFinal(messageInBytes);
    return encode(encryptedBytes);
}

NOTE: You don’t need to put the entire as a class variable, you can just save the IV by using the method .getIV(); on a Cipher object and save it in a byte array.

Now it’s time for the decryption

public String decrypt(String encryptedMessage) throws Exception{
    byte[] messageInBytes = decode(encryptedMessage);
    Cipher decryptionCipher = Cipher.getInstance("AES/GCM/NoPadding");
    GCMParameterSpec spec = new GCMParameterSpec(T_LEN,encryptionCipher.getIV());
    decryptionCipher.init(Cipher.DECRYPT_MODE,key,spec);
    byte[] decryptedBytes = decryptionCipher.doFinal(messageInBytes);
    return  new String(decryptedBytes);
}

We’re done! Time to squeeze in the main

public static void main(String[] args) {
    try{
        AES aes = new AES();
        aes.init();
        String encryptedMessage = aes.encrypt("TheXCoders");
        String decryptedMessage = aes.decrypt(encryptedMessage);

        System.err.println("Encrypted Message : "+encryptedMessage);
        System.err.println("Decrypted Message : "+decryptedMessage);
    }catch (Exception ignored){}
}

And here’s the result:

Encrypted Message : qMOVIjYfr8gwBGnIUHh2YuX48AFnk9TH+U0=
Decrypted Message : TheXCoders

And the final code can be found in my gist:

If you want to watch the video here it isss:

Thank you for taking the time to read! If you want to suggest what I would do next, feel free to comment on the video!