Rust library for Java to decrypt classes on definition, utilising a hook in the JVM.

JVM ClassLoader hook

How it works

This library enhances the JVM by creating a hook when loaded in the JVM_DefineClassWithSource export in the jvm library.
This feature gives us the ability to read and modify the bytes of a class before it's constructed, providing greater control and customization options.

Note

Please note that this is a basic implementation of class decryption in Java.
While this method may provide a first line of defense against immediate decompiling, it's important to understand that more advanced tools and agents may still be able to read and modify the loaded classes.
Keep this in mind as you use this feature in your projects.

Compatibility

✅ Java

The export JVM_DefineClassWithSource was only added in JDK1.5, therefore all Java versions starting from Java 6 are supported (there is no JNI version for Java 5).
Thus far only Java 8, Java 20 and Java 21 were explicitly tested.

✅ Operating system

The build targets all major architectures

• Linux
• Windows
• MacOS

How to build

1. Install rust
2. Install nightly by running rustup install nightly
3. Enable nightly by running rustup override set nightly
4. Run cargo build --release

How to use

Integration

Load the library from the defined path like:

System.loadLibrary("classloader");

Encryption

Define your custom magic value.

The value is used by the classloader hook to determine if the given payload uses the encryption method defined in the ClassLoader::is_custom_payload method.
Note: Changes to this value also require corresponding changes in the constant in class_loader.rs.

private static final byte[] CUSTOM_MAGIC_VALUE = BigInteger.valueOf(0xDEADC0DE).toByteArray();

Encrypt the bytes of your class

For simplicity, every class is XOR encrypted using the same static key "my static key". Add ENCRYPTION_KEY as environment variable (or secret in GitHub) to take control over this value.
Changing the entire encryption part is highly recommended if you want to use a custom build. Modify ClassLoader::decrypt_payload to reverse your encryption method.

final byte[] keyBytes = "my static key".getBytes();
for (int i = 0; i < classBytes.length; i++) {
    classBytes[i] ^= keyBytes[i % keyBytes.length];
}

Override javas magic value with your custom one

This allows the library to detect a custom class, to forward it to the decryption procedure.

System.arraycopy(CUSTOM_MAGIC_VALUE, 0, classBytes, 0, CUSTOM_MAGIC_VALUE.length);