Bitcoin light clients can now sync to the tip of the blockchain almost instantly, thanks to a new development by bitcoin startup ZeroSync. This breakthrough is made possible by their work in zero-knowledge (ZK) proofs. ZeroSync’s ultimate goal is to enable full nodes to achieve the same level of synchronization speed.
ZK proofs allow a prover to generate a concise mathematical statement that proves the correctness of a computation to a verifier, without revealing any specific details about the computation itself. While constructing these proofs can be computationally demanding, the resulting statement is always compact, regardless of the size of the input data. This enables rapid verification of data correctness with mathematical certainty, making ZK proofs highly valuable for bitcoin nodes and clients.
Bitcoin full nodes are traditionally required to download and verify every piece of data that comprises the blockchain, from its inception in 2009 to the present day. However, this comprehensive setup poses scalability challenges. In the bitcoin white paper, Satoshi Nakamoto proposed an alternative client type that could verify incoming payments without running a full node.
Bitcoin light clients utilize a simplified payment verification (SPV) mechanism. When they receive a payment, these clients query network nodes to obtain the headers of the longest chain. This allows them to locate the block in which the transaction was included, indicating that network nodes have accepted it as valid. As more blocks are added to the chain, the light client gains more confirmations that the payment was indeed valid and accepted by the network.
ZeroSync’s innovation improves this setup by compressing the headers into a single proof, without requiring any consensus changes to bitcoin. Similar to how each bitcoin block compresses its transactions’ data into a Merkle tree and includes the root of that tree in its header, ZeroSync’s work batches every bitcoin block header into another Merkle tree. This process synthesizes the chain of headers into a lightweight and concise piece of data, known as the proof.
The header chain proof allows for quick verification of whether a specific block header is part of the chain. Leveraging a block header, it is possible to confirm whether a particular transaction was included in that block. This process is similar to SPV, but more efficient. Instead of storing a complete copy of every header in the blockchain, light clients only need to store the small header chain proof, allowing them to sync to the latest state of the chain within seconds.
The header chain proof verifies that each block in the chain met the required difficulty at the time of mining. In other words, it ensures that every bitcoin block up to a given height was correctly mined and met the mining difficulty criteria at that time.
ZeroSync has achieved its first milestone by releasing the first complete header chain ZK proof. To fulfill their broader vision of providing full verification of the historical blockchain to full nodes without requiring users to download and process it, the team must accomplish two more goals. The second goal involves enhancing the header chain proof to enable node synchronization similar to Bitcoin Core’s Assume Valid function. The final goal is to achieve complete synchronization with the entire bitcoin blockchain.
Bitcoin Core’s Assume Valid is an option that assumes all scripts up to a specific block height are valid. This allows new full nodes to skip script verification from the Genesis block until the block height established by the Bitcoin Core client at a particular release. While users can choose to disable Assume Valid and perform full script verification, enabling it by default is based on the assumption that sufficient proof of work has been demonstrated up to that block height, making it reasonable to believe that the preceding scripts are valid.
ZeroSync’s intermediate solution will enable bitcoin users to sync their nodes similar to Bitcoin Core’s default initial block download (IBD). Nodes will download all data from bitcoin’s inception to the present day but will only verify witness data after the assumevalid height. The UTXO set is also a crucial component of this setup. ZeroSync utilizes Utreexo, another project focused on increasing efficiency in syncing bitcoin nodes, to provide the latest UTXO set at a given block. By incorporating this into their ZK proofs-based setup, ZeroSync achieves a shorter header chain proof and a more compact and efficient UTXO set, meeting the needs of clients for payment verification.
ZeroSync’s top-tier offering takes synchronization a step further by allowing nodes to sync with bitcoin’s latest state without assuming the validity of any script. Leveraging ZK proofs, full nodes can achieve a faster initial sync with potentially greater security assurances than Bitcoin Core’s default setting.
It is crucial to note that even if Bitcoin Core users disable Assume Valid and achieve similar security assumptions to ZeroSync’s top-tier offering, the latter still provides significant efficiency and speed gains in verifying all this information. While the bitcoin blockchain currently occupies 510GB of data, ZeroSync’s approach will enable a much quicker process by producing a compact proof of slightly over 1MB. This represents a performance improvement of several orders of magnitude compared to a standard IBD using Bitcoin Core, while ensuring adherence to the same consensus rules.
As the bitcoin blockchain continues to grow, efficiency gains become increasingly important. Downloading and verifying the entire chain could become impractical in terms of bandwidth and storage, especially in regions with limited access to high-speed internet and expensive storage options.
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About the author - J-S Tremblay
I've been involved in the cryptocurrency world since 2016 and trading since 2019. I started Moon and Lambo in 2021. I'm passionate about crypto and love to share my knowledge. I hate bankers and I hope that cryptocurrency will change the financial world for the better. View full profile...
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