Computer Laboratory

Technical reports

Improving security and performance of capability systems

Paul Ashley Karger

October 1988, 273 pages

This technical report is based on a dissertation submitted March 1988 by the author for the degree of Doctor of Philosophy to the University of Cambridge, Wolfson College.


This dissertation examines two major limitations of capability systems: an inability to support security policies that enforce confinement and a reputation for relatively poor performance when compared with non-capability systems.

The dissertation examines why conventional capability systems cannot enforce confinement and proposes a new secure capability architecture, called SCAP, in which confinement can be enforced. SCAP is based on the earlier Cambridge Capability System, CAP. The dissertation shows how a non-discretionary security policy can be implemented on the new architecture, and how the new architecture can also be used to improve traceability of access and revocation of access.

The dissertation also examines how capability systems are vulnerable to discretionary Trojan horse attacks and proposes a defence based on rules built into the command-language interpreter. System-wide garbage collection, commonly used in most capability systems, is examined in the light of the non-discretionary security policies and found to be fundamentally insecure. The dissertation proposes alternative approaches to storage management to provide at least some of the benefits of system-wide garbage collection, but without the accompanying security problems.

Performance of capability systems is improved by two major techniques. First, the doctrine of programming generality is addressed as one major cause of poor performance. Protection domains should be allocated only for genuine security reasons, rather than at every subroutine boundary. Compilers can better enforce modularity and good programming style without adding the expense of security enforcement to every subroutine call. Second, the ideas of reduced instruction set computers (RISC) can be applied to capability systems to simplify the operations required. The dissertation identifies a minimum set of hardware functions needed to obtain good performance for a capability system. This set is much smaller than previous research had indicated necessary.

A prototype implementation of some of the capability features is described. The prototype was implemented on a re-microprogrammed VAX-11/730 computer. The dissertation examines the performance and software compatibility implications of the new capability architecture, both in the context of conventional computers, such as the VAX, and in the context of RISC processors.

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BibTeX record

  author =	 {Karger, Paul Ashley},
  title = 	 {{Improving security and performance of capability systems}},
  year = 	 1988,
  month = 	 oct,
  url = 	 {},
  institution =  {University of Cambridge, Computer Laboratory},
  number = 	 {UCAM-CL-TR-149}