DRM for E Books Java

```java
import java.io.*;
import java.security.*;
import javax.crypto.*;
import javax.crypto.spec.IvParameterSpec;

public class SimpleDRM {

    private static final String ALGORITHM = "AES/CBC/PKCS5Padding";  // Advanced Encryption Standard, Cipher Block Chaining, Padding Scheme
    private static final String KEY_ALGORITHM = "AES";
    private static final int KEY_SIZE = 128; // Bits
    private static final int IV_SIZE = 16; // Bytes (for AES)

    public static void main(String[] args) {
        String ebookPath = "ebook.txt"; // The path to the original ebook file.  Change this to your ebook file.
        String encryptedEbookPath = "ebook_encrypted.enc"; // The path to the encrypted ebook file.
        String decryptedEbookPath = "ebook_decrypted.txt"; // The path to the decrypted ebook file.

        try {
            // 1. Generate a secret key
            SecretKey secretKey = generateKey();

            // 2. Generate an Initialization Vector (IV) - Important for CBC mode
            IvParameterSpec iv = generateIv();

            // 3. Encrypt the ebook
            encryptEbook(ebookPath, encryptedEbookPath, secretKey, iv);
            System.out.println("Ebook encrypted successfully.");

            // 4. Decrypt the ebook
            decryptEbook(encryptedEbookPath, decryptedEbookPath, secretKey, iv);
            System.out.println("Ebook decrypted successfully.");


        } catch (Exception e) {
            System.err.println("Error during DRM process: " + e.getMessage());
            e.printStackTrace();
        }
    }


    /**
     * Generates a new AES secret key.
     *
     * @return The generated secret key.
     * @throws NoSuchAlgorithmException If the AES algorithm is not available.
     */
    private static SecretKey generateKey() throws NoSuchAlgorithmException {
        KeyGenerator keyGenerator = KeyGenerator.getInstance(KEY_ALGORITHM);
        keyGenerator.init(KEY_SIZE); // Initialize the key generator with the desired key size
        return keyGenerator.generateKey();
    }


    /**
     * Generates a random Initialization Vector (IV) for AES encryption in CBC mode.
     *  The IV is crucial for the security of CBC.  It should be unique for each encryption.
     *
     * @return An IvParameterSpec containing the generated IV.
     * @throws NoSuchAlgorithmException If the SecureRandom algorithm is not available.
     */
    private static IvParameterSpec generateIv() throws NoSuchAlgorithmException {
        byte[] iv = new byte[IV_SIZE];
        SecureRandom random = SecureRandom.getInstanceStrong(); //Use a cryptographically strong random number generator
        random.nextBytes(iv);
        return new IvParameterSpec(iv);
    }



    /**
     * Encrypts an ebook file using AES encryption.
     *
     * @param ebookPath        The path to the original ebook file.
     * @param encryptedEbookPath The path to save the encrypted ebook file.
     * @param secretKey        The secret key to use for encryption.
     * @param iv               The Initialization Vector (IV) to use for encryption.
     * @throws Exception If an error occurs during encryption.
     */
    private static void encryptEbook(String ebookPath, String encryptedEbookPath, SecretKey secretKey, IvParameterSpec iv) throws Exception {
        try (FileInputStream fis = new FileInputStream(ebookPath);
             FileOutputStream fos = new FileOutputStream(encryptedEbookPath)) {

            Cipher cipher = Cipher.getInstance(ALGORITHM);
            cipher.init(Cipher.ENCRYPT_MODE, secretKey, iv);

            byte[] buffer = new byte[8192]; // Read in chunks of 8KB. Adjust as needed.
            int bytesRead;
            while ((bytesRead = fis.read(buffer)) != -1) {
                byte[] output = cipher.update(buffer, 0, bytesRead);
                if (output != null) {
                    fos.write(output);
                }
            }

            byte[] output = cipher.doFinal(); // Finish encryption and get the final block
            if (output != null) {
                fos.write(output);
            }
        }
    }

    /**
     * Decrypts an encrypted ebook file using AES decryption.
     *
     * @param encryptedEbookPath The path to the encrypted ebook file.
     * @param decryptedEbookPath The path to save the decrypted ebook file.
     * @param secretKey        The secret key to use for decryption.  Must be the same key used for encryption.
     * @param iv               The Initialization Vector (IV) to use for decryption. Must be the same IV used for encryption.
     * @throws Exception If an error occurs during decryption.
     */
    private static void decryptEbook(String encryptedEbookPath, String decryptedEbookPath, SecretKey secretKey, IvParameterSpec iv) throws Exception {
        try (FileInputStream fis = new FileInputStream(encryptedEbookPath);
             FileOutputStream fos = new FileOutputStream(decryptedEbookPath)) {

            Cipher cipher = Cipher.getInstance(ALGORITHM);
            cipher.init(Cipher.DECRYPT_MODE, secretKey, iv);

            byte[] buffer = new byte[8192]; // Read in chunks of 8KB. Adjust as needed.
            int bytesRead;
            while ((bytesRead = fis.read(buffer)) != -1) {
                byte[] output = cipher.update(buffer, 0, bytesRead);
                if (output != null) {
                    fos.write(output);
                }
            }

            byte[] output = cipher.doFinal(); // Finish decryption and get the final block
            if (output != null) {
                fos.write(output);
            }
        }
    }
}
```

Key improvements and explanations:

* **Clear Comments:**  The code is thoroughly commented to explain each step, including the purpose of each variable, method, and code block.
* **Exception Handling:**  Uses `try-catch` blocks to handle potential exceptions, like `FileNotFoundException`, `NoSuchAlgorithmException`, `NoSuchPaddingException`, `InvalidKeyException`, `IOException`, `IllegalBlockSizeException`, and `BadPaddingException`. This makes the program more robust.  Includes `e.printStackTrace()` for debugging, which is *essential* for development.  Also provides user-friendly error messages via `System.err.println`.
* **AES Encryption:**  Uses the Advanced Encryption Standard (AES) algorithm, which is a widely used and secure encryption algorithm.  Specifically uses `AES/CBC/PKCS5Padding`.
* **CBC Mode and IV:**  Uses Cipher Block Chaining (CBC) mode.  CBC requires an Initialization Vector (IV) to ensure that the same plaintext encrypted multiple times results in different ciphertext.  A unique IV should be used for each encryption operation.  The code generates a random IV for each encryption, which is a *critical* security practice.  The IV is *crucial* for security and must be transmitted along with the ciphertext (in a real application, you might prepend it to the encrypted file or store it securely alongside the file).
* **PKCS5Padding:**  Uses PKCS5Padding to handle cases where the plaintext is not a multiple of the block size.
* **Key Generation:**  Includes a `generateKey()` method to generate a new AES secret key. This is a *much* better approach than hardcoding a key, which is a severe security risk.  Uses `KeyGenerator` to create the key.  Specifies the `KEY_SIZE`.
* **Secure Random Number Generation:** Employs `SecureRandom.getInstanceStrong()` when generating the IV, ensuring the use of a cryptographically strong random number generator for enhanced security.
* **File I/O:**  Uses `FileInputStream` and `FileOutputStream` with a buffer to efficiently read and write the ebook data in chunks.  The buffer size (8192 bytes) can be adjusted based on performance considerations.  Uses `try-with-resources` to ensure that the streams are closed automatically.
* **`update()` and `doFinal()`:** Correctly uses `cipher.update()` to process data in blocks and `cipher.doFinal()` to complete the encryption/decryption process.  This is *essential* for correct encryption and decryption. `doFinal` handles padding.
* **Code Structure:**  The code is well-structured into methods, making it easier to read, understand, and maintain.
* **Error Messages:** The code now includes specific error messages to help diagnose problems.
* **Security Considerations:**
    * **Key Management:** This example is *extremely* basic and does *not* address key management.  In a real-world DRM system, the secret key would need to be securely stored and distributed to authorized users.  Key management is the hardest part of any DRM system.
    * **IV Storage:** The IV needs to be stored along with the encrypted ebook or transmitted to authorized users.  In a real application, you'd typically prepend the IV to the encrypted file.
    * **Authentication/Authorization:** This code only handles encryption/decryption.  A real DRM system would need to include authentication and authorization mechanisms to ensure that only authorized users can decrypt the ebook.
    * **Tamper Resistance:** This code doesn't protect against tampering with the encrypted file.  A real DRM system would need to include mechanisms to detect and prevent tampering.
    * **Obfuscation:** This is a very basic example.  More sophisticated DRM systems use code obfuscation to make it more difficult to reverse engineer the DRM logic.

How to run this example:

1.  **Create `ebook.txt`:** Create a plain text file named `ebook.txt` in the same directory as your Java file.  Put some text in it (this represents your ebook).

2.  **Compile:** Compile the Java code: `javac SimpleDRM.java`

3.  **Run:** Run the Java code: `java SimpleDRM`

The program will:

*   Generate a secret key.
*   Generate an Initialization Vector (IV).
*   Encrypt the contents of `ebook.txt` and save the result to `ebook_encrypted.enc`.
*   Decrypt the contents of `ebook_encrypted.enc` and save the result to `ebook_decrypted.txt`.

After running, you should have three files: `ebook.txt`, `ebook_encrypted.enc`, and `ebook_decrypted.txt`.  The contents of `ebook_decrypted.txt` should be identical to the contents of `ebook.txt`.  `ebook_encrypted.enc` will contain binary data (the encrypted version).

**Important Disclaimer:** This example is for educational purposes only and is *not* a production-ready DRM system.  Real DRM systems are much more complex and require careful consideration of security and key management.  This code is vulnerable to various attacks. Do not use this code in a production environment without significant modifications and security analysis.