The Encrypt API module provides a low-level system for two-way encryption. While it provides developers and users with a few basic encryption methods and key providers, developers may wish to extend either of these areas to provide more robust options. Both encryption methods and key providers are implemented as CTools plugins, so providing your own encryption methods and key providers is really easy and pretty well-documented. The default encryption methods and key providers supplied by the Encrypt API module are also implemented as plugins, so for good examples of how to implement your own, looking at the code would be a good place to start. You can find them in the Encrypt module's directory, in the plugins folder.