Are you looking for a way to setup a registration system for your shareware software?
There are several commercial products out there, but Aquatic has released a free (donations accepted) open-source framework: AquaticPrime. It supports both Cocoa and Carbon integration and also includes a php-based library for use on your online e-commerce shopping cart system.
AquaticPrime utilizes the strong one-way encryption algorithm, RSA to provide for a powerful way of locking down your product.
Aquatic also has simple Developer Documentation available to help you along your way when integrating the framework into your product.