Develop C++ API for PC and Mac from C# application functions

We are looking for an experienced developer who has designed, developed and documented SDK tools previously.

We have an existing C# GUI Windows application, its source code and full technical documentation. We need to remove the GUI element and rework the existing functionality of the application in a C++ environment to allow 3rd party developers to integrate our software into theirs.

The application gathers data on a users computer and sends it to a server for processing. The server responds with a return code that needs to be accessible and documented.

Key areas of expertise required are:

HTTP communications
Knowledge of dynamic XML parsing and generation (e.g. using TinyXML++)
Knowledge of C# to at least understand existing functionality

There is the potential for more work for the right developer. We are looking to integrate this completed SDK into a browser plugin (using Firebreath) so experience in this is a bonus (but not a must).