The typed access matrix model

The typed access matrix (TAM) model is defined by introducing the notion of strong typing into the Harrison, Ruzzo, and Ullman model (HRU) (M. H. Harrison et al., 1978). It is shown that monotonic TAM (MTAM) has decidable, but NP-hard, safety for its acyclic creation cases. It is further shown that ternary MTAM has polynomial time safety analysis for its acyclic cases, even though it is, in general, equivalent to MTAM. Ternary MTAM thus has strong safety properties. The expressive power of ternary MTAM has been shown to be equivalent to MTAM in general. The results establish that strong typing is crucial to achieving a useful demarcation between decidable and undecidable safety, and ternary monotonic commands are critical for tractable safety analysis