Dynamic protection structures

A very general problem which pervades the entire field of operating system design is the construction of protection mechanisms. These come in many different forms, ranging from hardware which prevents the execution of input/output instructions by user programs, to password schemes for identifying customers when they log onto a time-sharing system. This paper deals with one aspect of the subject, which might be called the meta-theory of protection systems: how can the information which specifies protection and authorizes access, itself be protected and manipulated. Thus, for example, a memory protection system decides whether a program P is allowed to store into location T. We are concerned with how P obtains this permission and how he passes it on to other programs.