Here I explore providing with capabilities, access control reminiscent of the access control lists of some conventional operating systems.
The access control list can be changed to add or delete access by a principal P to a particular file F. With capabilities this may be partially modeled by creating an indirection object N that will hold a capability to F while providing access is desired and holding nothing when it is not. Two capabilities to N arise as it is created:
(In Keykos F would be a segment and N would be a node. Ns is the node key and Nu is a segment key to N.)
When it is desired to allow access by P to F, F’s owner puts the capability to F in N by invoking Ns. By placing a null capability in N access by P to F is temporarily or permanently revoked. (Better service to P might be provided by putting a capability to a static but stale version of F in N while embargoed information is developed in F.)
Subtle differences remain between this form of access control and classic ACLs. I will argue that while not equivalent, they generally favor the form provided by capabilities. Access by a principal to a file depends on how access was acquired. If P1 conveys rescindable access to P2 and then P2 passes that to P3, capability discipline would appear weaker than access lists for the owner had not intended to provide access to P3. With access lists, however, P2 could still pass the file’s content to P3 and would arguable seem likely to do so under similar circumstances. The extra power of access lists seems empty. Note that either of these cases requires P2’s ability to deliver to P3. Capability discipline can deny this ability.
The ostensible reason to rescind P2’s access, is to hide information subsequently put into the file. Again the information protection is the same, for P3 also looses access to this new data. If P2 is a contractor whose job is defined by the content of the file is should not concern P1 that P2 has a sub-contractor. If P1 wants to guard the disemination of the file’s content then confinement can provide this with capabilities but not, as far as I can see, with access lists. With confinement P1 need not even trust P2 with the file or its content.