Understanding SPKI/SDSI using first-order logic

SPKI/SDSI is a language for expressing distributed access control policy, derived from SPKI and SDSI. We provide a first-order logic (FOL) semantics for SDSI, and show that it has several advantages over previous semantics. For example, the FOL semantics is easily extended to additional policy concepts and gives meaning to a larger class of access control and other policy analysis queries. We prove that the FOL semantics is equivalent to the string rewriting semantics used by SDSI designers, for all queries associated with the rewriting semantics. We also provide a FOL semantics for SPKI/SDSI. This reveals some problems. For example, the standard proof procedure in RFC 2693 is semantically incomplete. In addition, as noted before by other authors, authorization tags in SPKI/SDSI are algorithmically problematic, making a complete proof procedure unlikely. We compare SPKI/SDSI with RT₁^C, which is a language in the RT role-based trust-management framework that can be viewed as an extension of SDSI. The constraint feature of ₁^C, based on constraint datalog, provides an alternative mechanism that is expressively similar to SPKI/SDSI tags, semantically natural, and algorithmically tractable.