By the debut of the Digital Equipment Corporation PDP-10 in 1967, computers had found many viable niches and were spreading, but they were still major capital investments. The rise of time-sharing operating systems such as TOPS-10--with the ability to provide useful services to many users at once--sent communications links snaking across campuses and through office buildings. Mere physical control over the central hardware was no longer sufficient for system security.
User accounts, passwords, the distinction between ordinary `user mode' and privileged `supervisor mode' operation--all sorts of mechanisms for creating fences, enforcing separations, and permitting limited sharing--date from this period. Now for the first time, the internal design and operation of the computer begins to reflect the divisions and separations of the world outside it. Trust is no longer implicit and automatic; now it is explicit and conditional. Now for the first time, both in hardware and software the computer itself manifests a distinction between self and other, and the system administrator appears explicitly in the design, playing a third role, that of the `trusted other'.