Filling Out the Gaps: A Padding Algorithm for Transforming Out Timing Leaks

It has been shown that secret information can be leaked to external observers through covert timing channels. In this paper we are concerned with a kind of timing attack that wants to differentiate two processes, presented as probabilistic transition systems, by observing their timing behaviour. Our goal is to make the processes indistinguishable i.e. bisimilar, by adding virtual (dummy) states and transitions to the original processes (padding). Instead of padding the processes with whole virtual copies of their counterparts - as done by some padding algorithms - we present an algorithm that uses the bisimulation equivalence relation - computed as a lumping partition - as the main criterion to optimise the padding procedure.