International Association
for Cryptologic Research

Recent works have studied Byzantine agreement (BA) in the mixed-fault model with resilience threshold $n>2t+r+s$, where $n$ is the number of parties, $t$ is the number of Byzantine parties, $r$ is the number of receive-faulty parties, and $s$ is the number of send-faulty parties. In particular, Feng et al.~(ASIACRYPT~2025) proposed a BA protocol with optimal resilience, expected asymptotically quadratic communication complexity, and expected constant round complexity. However, existing BA protocols guarantee a meaningful output $v$ only when all parties hold the same input $v$. Such a guarantee is insufficient for efficient construction of atomic broadcast protocols, leaving efficient atomic broadcast in the mixed-fault setting as an open problem.

In this paper, we present the first atomic broadcast protocol (UABC) with optimal resilience in the mixed-fault model. In addition, our protocol achieves communication complexity $\mathcal{O}(n\ell+n^2\lambda^2)$ when $t=\Theta(n)$, where $\ell$ is the input size and $\lambda$ is the security parameter. This complexity is asymptotically optimal for moderately large $\ell$, namely when $\ell\ge n\lambda^2$. At the core of our design is an undead verifiable information dispersal (UVID) protocol, which allows an input to be efficiently dispersed and later reconstructed in the mixed-fault setting. Leveraging UVID together with the undead multi-valued consensus (ASIACRYPT~2025), we further construct a new primitive called weak undead multi-valued validated Byzantine agreement (wUMVBA) and its optimized variant (Opt-wUMVBA). Unlike standard multi-valued validated Byzantine agreement, wUMVBA and Opt-wUMVBA guarantee that the output is either a meaningful value satisfying the validity predicate or $\bot$. Moreover, the probability of outputting $\bot$ is at most $(t+s)/n$. Finally, by leveraging Opt-wUMVBA, we obtain our optimal atomic broadcast protocol for the mixed-fault setting.