Encrypt data, send it with UDP to a server, then decrypt it

What I am trying to do is encrypt a string into a byte[] with javax.crypto, send it through a DatagramSocket, then on the receiving side, decrypt it.

public static final String UNICODE = "UTF-8";

private SecretKey key;
private Cipher cipher;

public StringHandler() {
	try {
		key = generateKey("AES");
		cipher = Cipher.getInstance("AES");
	} catch (Exception e) {
		e.printStackTrace();
	}
}

private SecretKey generateKey(String type) throws Exception {
	KeyGenerator gen = KeyGenerator.getInstance(type);
	SecretKey key = gen.generateKey();
	return key;
}

public byte[] encrypt(String msg) {
	try {
		byte[] data = msg.getBytes(UNICODE);
		cipher.init(Cipher.ENCRYPT_MODE, key);
		return cipher.doFinal(data);
	} catch (Exception e) {
		e.printStackTrace();
		return null;
	}
}

public String decrypt(byte[] data) {
	try {
		cipher.init(Cipher.DECRYPT_MODE, key);
		return new String(cipher.doFinal(data), UNICODE);
	} catch (Exception e) {
		e.printStackTrace();
		return null;
	}
}

I read on a website how to encrypt and decrypt data, and I wrote this class. From what I can tell, the security key has to be the same on both sides for it to decrypt properly. Is there any way to convert it to a string or something, then get it from a string on the server side? Currently I have no idea how decrypt it on a different program.

You can turn a key represented as an array of bytes into a form that can be sent as "text". Base64 encoding is a common way to do that.

But that doesn't solve your real problem:

> Is there any way to convert it to a string or something, then get it from a string on the server side?

The real problem is how to send the string that represents to secret key to the server securely; i.e. without someone else being able to steal the key while it is in transit.

And the answer is that you can't ... without using another encryption mechanism:

• One possibility to encrypt the secret key with a different secret key that both the client and server already know.
• A second possibility is to use public key encryption.
• A third possibility is to use a secure communication system to transmit the secret key. For example a network based on quantum cryptography.

This is far too large a topic to cover here. If you want to understand the "key distribution problem" and its solutions, find a good textbook. Or start with these Wikipedia articles:

• https://en.wikipedia.org/wiki/Cryptography
• https://en.wikipedia.org/wiki/Public-key_cryptography
• https://en.wikipedia.org/wiki/Quantum_key_distribution