Why is it said that Layer2 is not just as simple as sidechain + bridge?

Why is Layer2 considered more complex than just a combination of sidechain and bridge?

Source: Geek web3 Source: X (former Twitter) @eternal1997L

Redefining Layer2 = Ethereum-based chain + higher security bridge.

About the concept of Layer2, http://ethereum.org explains it as “an independent blockchain that enables Ethereum scalability and inherits Ethereum security”. However, AC, a spokesman for Fantom, once bluntly said, “Layer2 is actually a sidechain + bridge”.

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Indeed, AC’s summary of Layer2 as “sidechain + bridge” is indeed close to its essence, but in actuality, it is just a big, empty talk. Cross-chain bridges themselves are a fuzzy concept, and under this concept, there are various different models of inter-chain interaction, which differ greatly in terms of security.

For example, ZK Bridge, which relies on on-chain contract verification of ZK proofs to ensure cross-chain validity, and third-party verification bridges like Axie that rely solely on multiple off-chain nodes for releasing cross-chain transactions, are fundamentally different. Some researchers have pointed out that ZK Bridge is basically the safest type of cross-chain bridge, for example:

Let’s assume there is a ZK bridge between ETH and Polygon. The bridge contract on the ETH chain will record Polygon’s block header, which is submitted by third-party Relayer nodes. However, the Relayer will also submit the corresponding validity proof, proving that this block header is indeed valid.

If someone wants to cross-chain from Polygon to ETH, they just need to submit a merkle proof, proving that their cross-chain request is recorded in a Polygon block (in other words, it matches a specific block header in Polygon).

Clearly, this type of ZK bridge relies solely on zero-knowledge proof technology to ensure security, rather than relying on a few off-chain notary nodes like many third-party multisig bridges.

Most ZK Rollup models for cross-chain bridges between ZK Rollup and Ethereum are similar to the ZK bridge mentioned above. Although there are differences in details, the security models are very similar: the target chain will record and verify the validity of the source chain’s block header/merkle root, and when determining the validity of cross-chain transactions, they rely on the advantages of cryptographic technologies such as zero-knowledge proofs and merkle proofs, rather than relying on a few off-chain nodes to make decisions like the Axie bridge.

Returning to the subtitle mentioned earlier, “Layer2 = Ethereum-based chain + higher security bridge”. Besides having higher security in cross-chain bridges, Layer2 also fundamentally differs from sidechains like Polygon, which are basically independent of Ethereum: the longest chain in Layer2 is connected to the longest chain in Ethereum.

For example, Layer2 full nodes need to run the geth client of Ethereum first, synchronizing with ETH blocks in real time. All transaction batches of Layer2 must be published on the ETH block with an anchoring relationship, which significantly increases the difficulty of rolling-back the longest chain of Layer2.

Vitalik once pointed out, “To roll back Layer2, you must first roll back Layer1,” “Layer2 needs to be able to read the latest Ethereum blocks.” Suppose the longest chain of Ethereum rolls back due to the instability of the distributed network, causing multiple L2 Txn Batches already published on the ETH block to be rolled back. At this time, Layer2 full nodes will also roll back the locally recorded longest chain of Layer2 in linkage. In normal circumstances, Layer2 will not roll back.

However, Polygon does not meet the above criteria. For example, Polygon has experienced multiple rollbacks last year, with more than 158 blocks rolled back, and the rollback history lasted for 5 minutes. Obviously, there is no anchoring relationship between Polygon blocks and Ethereum blocks. In other words, they do not share security.

Therefore, we can conclude simply: Layer2 is actually a sidechain anchored to Ethereum, which enhances its security with the help of Ethereum. The bridging model between L2 and L1 is the most secure category under the generalized cross-chain bridge, unlike Axie’s third-party notary multi-sig bridge.

It is not rigorous and unfounded to criticize Layer2 with the vague statement of “sidechain + bridge” to dismiss everyone.

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