Paper 2023/1773

Scalable and Adaptively Secure Any-Trust Distributed Key Generation and All-hands Checkpointing

Abstract

The classical distributed key generation protocols (DKG) are resurging due to their widespread applications in blockchain. While efforts have been made to improve DKG communication, practical large scale deployments are still yet to come, due to various challenges including broadcast channel scalability and worst-case complaint phase. In this paper, we propose a practical DKG for DL-based cryptosystems, with only (quasi-)linear computation/communication cost per participant, with the help of a public ledger, and beacon; Notably, our DKG only incurs constant-size blockchain storage cost for broadcast, even in the face of worst-case complaints. Moreover, our protocol satisfies adaptive security. The key to our improvements lies in delegating the most costly operations to an Any-Trust group. This group is randomly sampled and consists of a small number of individuals. The population only trusts that at least one member in the group is honest, without knowing which one. Additionally, we introduce an extended broadcast channel based on a blockchain and data dispersal network (such as IPFS), enabling reliable broadcasting of arbitrary-size messages at the cost of constant-size blockchain storage, which may be of independent interest. Our DKG leads to a fully practical instantiation of Filecoin's checkpointing mechanism, in which all validators of a Proof-of-Stake (PoS) blockcahin periodically run DKG and threshold signing to create checkpoints on Bitcoin, thereby enhancing the security of the PoS chain. In comparison with another checkpointing approach of Babylon (Oakland, 2023), ours enjoys a significally smaller monetary cost of Bitcoin transaction fees. For a PoS chain with $2^{12}$ validators, our cost is merely 0.6% of that incurred by Babylon's approach.