Unity3D build assembly encryption

I'd like to know whether it's possible to encrypt the compiled code in a Unity3D build in a way that de-compiling dlls won't give access to the legible source code to a potential hacker.

I've read elsewhere that it's possible to encrypt the assembly dlls and modify the mono loader (libmono.so).

My question is, has anybody done this? If so, how? And can it be done on Windows, Mac and Linux builds?

If this is not possible, is there another way?

The requirement is to protect the source code.