The ZeroSync proposal is released by the ZeroSync Association on the bitcoin dev mailing list

On May 12, 2023, ZeroSync was proposed to the bitcoin-dev mailing list by Robin Linus. ZeroSync aimed to address one of the most pressing challenges in the Bitcoin ecosystem: scalability. The proposal introduced a novel proof system that leveraged Succinct Non-Interactive Arguments of Knowledge (SNARKs) to compress the entire Bitcoin blockchain into a compact proof of validity. This innovation promised to enable instant verification of the blockchain, which could dramatically reduce the time and resources required to operate full nodes and participate in the network.
 

The core of ZeroSync was its ability to create chain state proofs using the Cairo language, Utreexo, and recursive STARKs. These proofs would allow for quick bootstrapping of full nodes and the creation of trustless light clients, enhancing the privacy of the Lightning Network and enabling secure cross-chain bridges. Importantly, the implementation of ZeroSync required no consensus changes to the Bitcoin protocol, which was crucial given the difficulty of implementing and achieving consensus for forks in Bitcoin.
 

Despite the existing bottleneck in prover performance, the ZeroSync team presented various optimization strategies and demonstrated the practicality of generating a complete chain state proof. Additionally, the proposal introduced zkCoins, a client-side validation protocol combined with zero-knowledge SNARKs, which could significantly improve the privacy and throughput of token transactions.