1 The paper considers several definitions of information flow security for intransitive policies from the point of view of the complexity of verifying whether a finite-state system is secure. The results are as follows. Checking (i) P-security (Goguen and Meseguer), (ii) IPsecurity (Haigh and Young), and (iii) TA-security (van der Meyden) are all in PTIME, while checking TO-security (van der Meyden) is undecidable. The most important ingredients in the proofs of the PTIME upper bounds are new

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... aracterizations of the respective security notions, which also enable the algorithms to return simple counterexamples demonstrating insecurity. Our results for IPsecurity improve a previous doubly exponential bound of Hadj-Alouane et al.