Zk-Rollup
Zk-Rollup is a Layer 2 scaling solution that addresses the challenges of Layer 2 protocols such as Plasma and Channels. While these protocols can be used for specific use cases, generalizing them is difficult. The Summary of Ethereum Scaling Technologies provides an overview of these challenges.
The concept of a "Shadow Chain" was introduced in a blog post by Vitalik Buterin in 2014, where blocks are provisionally accepted and finalized after a certain period of time. During this period, challenges can be raised against the provisionally accepted blocks. If a block is found to be invalid, the chain is reverted and penalties are imposed. This concept is similar to the current Optimistic Rollup approach. It's interesting to note that this concept was proposed in 2014, showing a glimpse into the future.
In recent proposals like ZK Rollup 1, the validity of blocks is verified using zk-SNARK, allowing for similar functionality without the challenge period. This approach is referred to as "Layer 2 with zero-knowledge proof". The SNARK mentioned here seems to be a proof of the validity of the ZK Rollup package structure, which compresses hundreds of "internal transactions" that affect the state of the ZK Rollup system to about 10 bytes per internal transaction. The package containing the SNARK proof is compressed to about 100-300 bytes.
Zk-Rollup aims to reduce the computational load by compressing transaction data using Zero Knowledge Proof for signature verification instead of computation. Each transaction is compressed to just 10 bytes, allowing for 500 transactions per second, which is 30 times the performance of Layer 1 Ethereum chain. The signatures are verified through zero-knowledge proofs, eliminating the need to include them in the transaction data.
Zero-Knowledge Proofs for Scaling offer two approaches. The first approach involves writing off-chain transaction bundles to the main chain in a verifiable form, compressing the data size. An example of this approach is the idea of Zk-Rollup, which uses zk-SNARK to verify the validity of blocks. Matter Labs has developed the zkSync scaling protocol based on ZK Rollup, which provides a high UX.
The second approach involves using Recursive snarks to reduce the size of the blockchain. This approach leverages the compatibility of cryptography and zero-knowledge proofs.
When considering scalability, multiple Zk-Rollup solutions are often seen as the most effective, taking into account factors such as chain rewards and fees. Shingen_crypto has discussed the potential of Zk-Rollup as the best solution in terms of scalability and sustainability. October 6, 2021
According to , one advantage of Zk-Rollup over Optimistic Rollup is the immediate coordination with Layer 1 through validity proofs, allowing nodes to verify the state. However, Zk-Rollup has higher costs and implementation difficulties with zk-SNARKs compared to Optimistic Rollup and EVM.