Thread: Basic Symmetric Encryption

Hello!

The code is technically ok but I don't understand my output. A "hello world" message encryption is displayed correctly on the Decryption output. A longer message seems to cause the problems. For example, a string any longer would cause characters from the beginning to be cut off. Eventually only the last character would display.

I am wondering if this is just a basic configuration for one word and a different approach is required for longer messages. Or is this my own OS configuration that is giving the issue.

 
import java.security.Key;
import javax.crypto.Cipher;
import javax.crypto.KeyGenerator;
 
public class SymmetricAES {
    public static void main(String[] args) throws Exception {
        SymmetricAES ctrl = new SymmetricAES();
        byte[] msg = "Hello, World!".getBytes();
 
        // Generate AES key
        ctrl.keyGenerator = KeyGenerator.getInstance("AES");
        ctrl.keyGenerator.init(256);
        ctrl.key = ctrl.keyGenerator.generateKey();
 
        // Configure cipher
        ctrl.cipher = Cipher.getInstance("AES");
        ctrl.cipher.init(Cipher.ENCRYPT_MODE, ctrl.key);
        ctrl.cipher.update(msg);
 
        // Generate & output ciphertext
        byte[] cipherTxt = ctrl.cipher.doFinal();
        String encrypted = new String(cipherTxt);
        System.out.println("Encryption: " + encrypted);
 
        // Decrypt Cipher
        ctrl.cipher.init(Cipher.DECRYPT_MODE, ctrl.key);
        ctrl.cipher.update(cipherTxt);
        msg = ctrl.cipher.doFinal();
        encrypted = new String(msg, "UTF8");
 
        System.out.println("Decrypted: " + encrypted);
        for (byte b : ctrl.key.getEncoded()) {
            System.out.print(b);
        }
 
    }
 
    private KeyGenerator keyGenerator;
    private Key key;
    private Cipher cipher;
}

It seems like the issue lies in how you're updating the cipher with the message. When you call `cipher.update(msg)`, it's only processing a single block of data. For short messages like "Hello, World!", this might work fine, but for longer messages, it's not sufficient. The `update()` method of `Cipher` is typically used for streaming encryption, where you encrypt or decrypt data in chunks.

To fix this issue, you need to handle longer messages correctly. You can either read the message in chunks and call `update()` for each chunk, or you can directly pass the entire message to `doFinal()`. Since you're encrypting the entire message at once, you can use `doFinal()` directly.

Here's the modified part of your code:

```java
// Configure cipher
ctrl.cipher = Cipher.getInstance("AES");
ctrl.cipher.init(Cipher.ENCRYPT_MODE, ctrl.key);

// Generate ciphertext
byte[] cipherTxt = ctrl.cipher.doFinal(msg);
String encrypted = new String(cipherTxt);
System.out.println("Encryption: " + encrypted);

// Decrypt Cipher
ctrl.cipher.init(Cipher.DECRYPT_MODE, ctrl.key);
byte[] decryptedMsg = ctrl.cipher.doFinal(cipherTxt);
String decrypted = new String(decryptedMsg, "UTF8");

System.out.println("Decrypted: " + decrypted);
```

This change ensures that the entire message is encrypted and decrypted correctly, regardless of its length.

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