Paper 2025/988

Dynamic Security: A Realistic Approach to Adaptive Security With Applications to Strong FaF Security

Abstract

Secure multiparty computation allows multiple parties to jointly compute a function while maintaining security even in the presence of malicious adversaries. There are two types of adversaries in the literature: static adversaries, which choose the parties to corrupt before the protocol begins; and adaptive adversaries, which can corrupt parties during the execution of the protocol based on the messages exchanged by the parties. While adaptive security provides a more robust security guarantee, it may require too much in certain scenarios. Indeed, the adversary must allocate some of its resources to corrupt the parties; however, certain parties might be more susceptible to corruption, for instance, if they have not updated their operating system to the latest version. To address this, we introduce a new security notion called \emph{dynamic security}. Here, adversaries may corrupt new parties \emph{during and after} the protocol's execution, but \emph{cannot choose} targets based on the messages. A protocol is said to be $(t,h)$-dynamically secure if it is possible to simulate any adversary that can corrupt up to $t$ parties during the execution and $h$ thereafter. Dynamic security provides meaningful security for many real-world scenarios. Moreover, it circumvents known lower bounds on the communication complexity of adaptive security, allowing for more efficient protocols such as committee-based ones, which would be insecure against adaptive adversaries. We further explore dynamic security and establish the following results. 1. We show a surprising connection between dynamic security and the seemingly unrelated notion of security with friends and foes (FaF security), introduced by Alon et al. (CRYPTO 2020), which aims to protect honest parties not only from adversaries but also against other honest parties. The notion of $(t,h)$-\emph{strong FaF security} strengthens this by requiring the simulatability of the joint view of any $t$ malicious parties alongside any $h$ honest parties to be indistinguishable from their real-world view. We show that $(t,h)$-dynamic security and $(t,h)$-strong FaF security are equivalent. 2. We consider the feasibility of $(t,h)$-dynamic security and show that every $n$-party functionality can be computed with computational $(t,h)$-dynamic security (with guaranteed output delivery) if and only if $2t+h<n$. By our previous result, this also solves an open problem left by Alon et al. on the feasibility of strong FaF security.