Calling C# code from Java?

Does anyone have a good solution for integrating some C# code into a java application?

The code is small, so I could re-write in java, but I would rather reuse the code if possible. Don't repeat yourself, etc.

Also, I know I can expose the C# as a web service or whatever, but it has some security/encryption stuff in there, so I would rather keep it tightly integrated if possible.

Edit: It's going to be on a server-based app, so "downloading" another runtime is irrelevant.