Refactoring crypto box to secret box

Good morning guys, 

With the goal of represent properly the symmetric encryption, I would like to suggest a class refactoring. Let me try to explain, currently I'm working to provide support for asymmetric encryption using NaCl (http://nacl.cr.yp.to/).

This library has the concept of secret box (authenticated symmetric encryption - http://nacl.cr.yp.to/secretbox.html) and crypto box (public key encryption - http://nacl.cr.yp.to/box.html). For the former we've implemented it with common crypto and the later will make use of the C library, in other words we need separated classes to avoid compatibility issues between those libraries.

In Java was a different story (but we can change to keep the API symmetry if necessary), because we made use of a single API.