Most General Winning Secure Equilibria Synthesis in Graph Games

This paper considers the problem of co-synthesis in $k$-player games over a finite graph where each player has an individual $\omega$-regular specification $\phi_i$. In this context, a secure equilibrium (SE) is a Nash equilibrium w.r.t. the lexicographically ordered objectives of each player to first satisfy their own specification, and second, to falsify other players' specifications. A winning secure equilibrium (WSE) is an SE strategy profile $(\pi_i)_{i\in[1;k]}$ that ensures the specification $\phi:=\bigwedge_{i\in[1;k]}\phi_i$ if no player deviates from their strategy $\pi_i$. Distributed implementations generated from a WSE make components act rationally by ensuring that a deviation from the WSE strategy profile is immediately punished by a retaliating strategy that makes the involved players lose. In this paper, we move from deviation punishment in WSE-based implementations to a distributed, assume-guarantee based realization of WSE. This shift is obtained by generalizing WSE from strategy profiles to specification profiles $(\varphi_i)_{i\in[1;k]}$ with $\bigwedge_{i\in[1;k]}\varphi_i = \phi$, which we call most general winning secure equilibria (GWSE). Such GWSE have the property that each player can individually pick a strategy $\pi_i$ winning for $\varphi_i$ (against all other players) and all resulting strategy profiles $(\pi_i)_{i\in[1;k]}$ are guaranteed to be a WSE. The obtained flexibility in players' strategy choices can be utilized for robustness and adaptability of local implementations. Concretely, our contribution is three-fold: (1) we formalize GWSE for $k$-player games over finite graphs, where each player has an $\omega$-regular specification $\phi_i$; (2) we devise an iterative semi-algorithm for GWSE synthesis in such games, and (3) obtain an exponential-time algorithm for GWSE synthesis with parity specifications $\phi_i$.