Noninterference is a high-level security property that guarantees the absence of illicit information flow at runtime. Noninterference can be enforced statically using information flow type systems; however, these are criticized for being overly conservative and rejecting secure programs. More precision can be achieved by using program logics, but such an approach lacks its own verification tools. In this work we propose a novel, alternative approach: utilizing symbolic execution in combination with ideas from program logics in an attempt to increase the precision of analyses and automate noninterference testing. Dealing with policies incorporating declassification is also explored. The feasibility of the proposal is illustrated using a prototype tool based on the KLEE symbolic execution engine.

Abstract.A hyperproperty is a set of sets of finite or infinite traces over some fixed alphabet and can be seen as a very generic system specification. In this work, we define the notions of holistic and incremental hyperproperties. Systems specified holistically tend to be more intuitive but difficult to reason about, whereas incremental specifications have a straightforward verification approach. Since most interesting securityrelated hyperproperties are in the syntactic class of holistic hyperproperties, we introduce the process of incrementalization to convert holistic specifications into incremental ones. We then present three incrementalizable classes of holistic hyperproperties and a respective verification method.

Security enforcement mechanisms like execution monitors are used to make sure that some untrusted program complies with a policy. Different enforcement mechanisms have different strengths and weaknesses and hence it is important to understand the qualities of various enforcement mechanisms.This paper studies runtime enforcement mechanisms for reactive programs. We study the impact of two important constraints that many practical enforcement mechanisms satisfy: (1) the enforcement mechanism must handle each input/output event in finite time and on occurrence of the event (as opposed to for instance Ligatti's edit automata that have the power to buffer events for an arbitrary amount of time), and (2) the enforcement mechanism treats the untrusted program as a black box: it can monitor and/or edit the input/output events that the program exhibits on execution and it can explore alternative executions of the program by running additional copies of the program and providing these different inputs. It can not inspect the source or machine code of the untrusted program.Such enforcement mechanisms are important in practice: they include for instance many execution monitors, virtual machine monitors, and secure multi-execution or shadow executions.We establish upper and lower bounds for the class of policies that are enforceable by such black box mechanisms, and we propose a generic enforcement mechanism that works for a wide range of policies. We also show how our generic enforcement mechanism can be instantiated to enforce specific classes of policies, at the same time showing that many existing enforcement mechanisms are optimized instances of our construction.

Hyperproperties were proposed as an abstract formalization of security policies, but unfortunately they lack a generic verification methodology. In an attempt to remedy this, we introduced the notion of incremental hyperproperties (IHPs), motivated by the observation that they have a clearer and more feasible verification methodology. To show that verification is indeed feasible, a decidable IHP verification methodology via games is presented and evaluated. The main advantage of the approach is that the games in combination with winning strategy evidence give valuable intuition about the security of a system and are very helpful when analyzing systems w.r.t. policy specifications.