Timed automata (TAs) are an extension of finite automata that can measure and react to the passage of time, providing the ability to handle real-time constraints using clocks. In 2009, Franck Cassez showed that the timed opacity problem, where an attacker can observe some actions with their timestamps and attempts to deduce information, is undecidable for TAs. Moreover, he showed that the undecidability holds even for subclasses such as event-recording automata. In this article, we consider the same definition of opacity, by restricting either the system or the attacker. Our first contribution is to prove the inter-reducibility of two variants of opacity: full opacity (for which the observations should be the same regardless of the visit of a private location) and weak opacity (for which it suffices that the attacker cannot deduce whether the private location was visited, but for which it is harmless to deduce that it was not visited); we also prove further results including a connection with timed language inclusion. Our second contribution is to study opacity for several subclasses of TAs: with restrictions on the number of clocks, the number of actions, the nature of time, or a new subclass called observable event-recording automata. We show that opacity is mostly decidable in these cases, except for one-action TAs and for one-clock TAs with $ε$-transitions, for which undecidability remains. Our third (and arguably main) contribution is to propose a new definition of opacity in which the number of observations made by the attacker is limited to the first $N$ observations, or to a set of $N$ timestamps after which the attacker observes the first action that follows immediately. This set can be defined either a priori or at runtime; all three versions yield decidability for the whole TA class.

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