HomeBITCOINBitcoin Nodes on the Verge of Achieving Instant Sync

Bitcoin Nodes on the Verge of Achieving Instant Sync

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Bitcoin light clients can now sync to the tip of the blockchain almost instantly, thanks to ZeroSync, a bitcoin startup that specializes in zero-knowledge (ZK) proofs. ZeroSync’s ultimate goal is to enable full nodes to achieve the same functionality.

ZK proofs allow a prover to generate a short mathematical statement that demonstrates to a verifier the correctness of a particular computation, without revealing any specific details about that computation. Although constructing these proofs can be computationally expensive, the resulting statement is always compact. This allows for blazingly fast verification of data correctness using mathematics. In the bitcoin network, this technique can be incredibly useful for nodes and clients.

Bitcoin full nodes are traditionally required to download and verify every single piece of data that comprises the blockchain from its inception until the present day. However, due to the scalability challenges associated with this approach, Satoshi Nakamoto proposed an alternative client type in the bitcoin white paper that would be capable of verifying incoming payments without running a full node.

Bitcoin light clients leverage a simplified payment verification (SPV) mechanism. Upon receiving a payment, the client queries network nodes to obtain the headers of the longest chain. This allows the client to locate the block in which the incoming transaction was added, providing evidence that network nodes have accepted it as valid. As more blocks are added to the chain, the light client gains additional confirmations that the payment has been accepted by the network and included in a block.

Without requiring a consensus change to the bitcoin protocol, ZK proofs enhance this setup by compressing the headers into a single proof. Similar to how each bitcoin block compresses its transaction data into a Merkle tree and includes the root of that tree in its header, ZeroSync’s work compiles every bitcoin block header into another Merkle tree. Consequently, the chain of headers can be synthesized into a compact and lightweight piece of data known as the proof.

The header chain proof quickly determines whether a given block header is part of the chain. A block header can then be used to verify whether a specific transaction was included in that block. This process is similar to the SPV method described earlier, but more efficient. Instead of storing a full copy of every header in the blockchain, light clients only need to retain the small header chain proof to synchronize with the latest state of the chain within seconds.

In essence, the header chain proof serves as evidence that each block in the chain met the difficulty requirement at the time of mining. Verifying the header chain proof allows users or clients to confidently assert that each bitcoin block up to a certain height was correctly mined and satisfied the mining difficulty criteria at that time.

ZeroSync’s first major achievement was the release of the complete header chain ZK proof. However, the team aims to fulfill their broader vision by enabling full nodes to verify the entire historical blockchain without requiring users to download and process it. The accomplishment of this vision necessitates the fulfillment of two additional requirements. The second requirement involves enhancing the header chain proof to enable nodes to sync in a manner similar to Bitcoin Core’s Assume Valid function. The third and final requirement involves providing a complete bitcoin blockchain sync as envisioned.

Assume Valid is an option in Bitcoin Core that assumes all scripts up to a specific block height are valid. New full nodes syncing the blockchain using initial block download (IBD) can skip script verification from the Genesis block until the block height specified by the Bitcoin Core client. Although users have the option to disable Assume Valid and perform full script verification, enabling Assume Valid by default assumes that enough proof of work has been demonstrated up to that block height, thereby implying that the preceding scripts are valid.

ZeroSync’s middle ground offering will allow bitcoin users to synchronize their nodes similarly to Bitcoin Core’s default IBD. The node will download all data from the inception of bitcoin until the present day, but only verify witness data after the assumevalid height. The UTXO set is also a crucial component of this synchronization mechanism. To accomplish this, ZeroSync utilizes Utreexo, a project that strives to enhance syncing efficiency in bitcoin nodes. Utreexo provides the latest UTXO set at a given block, which ZeroSync incorporates into its ZK proofs-based setup. Consequently, the result is a shorter header chain proof and a more compact and efficient UTXO set, satisfying clients’ payment verification requirements.

The top tier offering from ZeroSync goes a step further, allowing nodes to synchronize with bitcoin’s latest state without making assumptions about script validity. By leveraging ZK proofs, full nodes can achieve a significantly faster initial sync, potentially even with greater security guarantees than Bitcoin Core’s default setting, which employs assumevalid.

It is important to note that even if Bitcoin Core users disable assumevalid and verify all scripts for similar security assumptions as ZeroSync’s top tier solution, the latter’s main value proposition lies in the substantial gains in efficiency and speed when verifying this information. While the current size of the bitcoin blockchain is 510GB, ZeroSync’s approach will enable a much quicker process by producing a short and lightweight proof of slightly over 1MB. This represents a performance improvement of several orders of magnitude compared to a standard IBD using Bitcoin Core, while still adhering to the same consensus rules.

Gains in efficiency will become increasingly crucial as the bitcoin blockchain continues to grow. Eventually, downloading and verifying the entire chain could become impractical in terms of bandwidth and storage, particularly in regions with limited or expensive access to high-speed internet and larger hard drives.

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