A system for, and method of, controlling access to a resource of a computer system. In one embodiment, the system includes: (1) a database of problems and corresponding precalculated solutions, (2) a problem retriever that responds to a request from a client for access to the resource by retrieving one of the problems from the database and transmitting the one of the problems to the client and (3) a solution evaluator that, upon receiving a putative solution from the client, employs the database to validate the putative solution and, if the putative solution is valid, grants the client access to the resource.