# Byzantine Lattice Agreement in Synchronous Systems

In this paper, we study the Byzantine lattice agreement problem in synchronous systems. The lattice agreement problem in crash failure model has been studied both in synchronous and asynchronous systems, which leads to the current best upper bound of O(log f) rounds in both systems. However, very few algorithmic results are known for the lattice agreement problem in Byzantine failure model. The paper [Nowak et al., DISC, 2019] first gives an algorithm for a variant of the lattice agreement problem on cycle-free lattices that tolerates up to f < n/(h(X) + 1) Byzantine faults, where n is the number of processes and h(X) is the height of the input lattice X. The recent preprint by Di et al. studies this problem with a slightly modified validity condition in asynchronous systems. They present a O(f) rounds algorithm by using the reliable broadcast primitive as a first step and following the similar algorithmic framework as the algorithms in crash failure model. In this paper, we propose the first algorithm for the Byzantine lattice agreement problem in synchronous systems, which runs in min{3h(X) + 6,6√(f) + 6}) rounds and takes O(n^2 min{h(X), √(f)}) messages, where h(X) is the height of the input lattice X, n is the total number of processes in the system and f is the maximum number of Byzantine processes such that n ≥ 3f + 1. In our algorithm, each process keeps track of a safe lattice and use this lattice to filter out invalid values from Byzantine processes. To get our desired O(√(f)) round guarantee, we apply a slightly modified version of the Gradecast algorithm [Feldman et al. ACM, 1988] to detect Byzantine processes at each round and ignore their messages in future rounds. As far as we know, this is the first algorithm which applies the Gradecast primitive to solve the lattice agreement problem.

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