adaptively secure broadcast, revisited
We consider the classical problem of synchronous broadcast with dishonest majority, when a public-key infrastructure and digital signatures are available. In a surprising result, Hirt and Zikas (Eurocrypt 2010) recently observed that all existing protocols for this task are insecure against an adaptive adversary who can choose which parties to corrupt as the protocol progresses. Moreover, they prove an impossibility result for adaptively secure broadcast in their setting. We argue that the communication model adopted by Hirt and Zikas is unrealistically pessimistic. We revisit the problem of adaptively secure broadcast in a more natural synchronous model (with rushing), and show that broadcast is possible in this setting for an arbitrary number of corruptions. Our positive result holds under a strong, simulation-based definition in the universal-composability framework. We also study the impact of adaptive attacks on protocols for secure multi-party computation where broadcast is used as a sub-routine.
adaptive security broadcast cryptographic protocols fault-tolerant distributed computing