Traditional storage systems have considered security as a problem to be solved at the perimeter: once a user is authenticated, each device internal to the system trusts the decision made elsewhere. However, as storage systems become ever more distributed, shared, and dynamic, it becomes necessary to enforce security at the boundaries of each storage device, rather than around the system as a whole. This form of decentralized security presents several new challenges in the design and implementation of distributed storage systems. We explore challenges in distributed file systems, third party transfer, active storage, and group management in the context of a 200-node cooperative storage system deployed at the University of Notre Dame. These explorations result in three recommendations for future system designs.