1. A system for distinguishing between a human and non-human user, comprising: a private data store of manually categorized images; and a human interactive proof (HIP) component that displays one or more images from the private data store as part of a HIP challenge to a user, wherein an owner of the private data store makes the private data store available to an owner of the HIP component in exchange for displaying one or more ads on behalf of the owner of the private data store, the HIP component displays at least one ad in conjunction with the HIP challenge, the ad is displayed on behalf of an owner of the private data store, the HIP component determines if the user is a human or non-human based upon whether the user correctly classifies the one or more images.