Fifth-generation computer systems will use large numbers of processors to achieve high performance. In this paper a capability-based operating system designed for this environment is discussed. Capability-based operating systems have traditionally required large, complex kernels to manage the use of capabilities. In our proposal, capability management is done entirely by user programs without giving up any of the protection aspects normally associated with capabilities. The basic idea is to use one-way functions and encryption to protect sensitive information. Various aspects of the proposed system are discussed.