AC once said that Layer 2 is actually a sidechain + bridge, is this really the case?

Author: Geek Web3

Regarding the concept of Layer2, it is explained as: Layer 2 is “an independent blockchain that scales Ethereum + inherits Ethereum security”, and AC, the spokesperson of Fantom, once bluntly said that “Layer 2 is actually a sidechain + bridge”.

! [AC once said that Layer 2 is actually a sidechain + bridge, is that really the case?] ](https://cdn-img.panewslab.com//panews/2022/11/6/images/0dbed26e82a55fea9f2717c529baf780.)

Admittedly, AC’s summary of Layer 2 as “sidechain + bridge” is indeed close to its essence, but it is actually equivalent to a big, empty nonsense. The cross-chain bridge itself is a vague concept, which corresponds to a variety of different models of inter-chain interaction, and different models are very different in terms of security.

For example, ZK Bridge, which relies on the verification results of ZK proofs by on-chain contracts to ensure cross-chain validity, and Axie’s third-party verification bridge, which relies solely on the multi-signature of multiple off-chain nodes to allow cross-chain transactions, are not the same thing at all. Some cross-chain bridge researchers have pointed out that ZK Bridge is basically the most secure type of cross-chain bridge, for example:

Suppose there is a ZK bridge between ETH and Polygon, and the bridge contract on the ETH chain will record Polygon’s block header, which is submitted by a third-party Relayer node, but Relayer will also submit the corresponding validity proof to prove that the block header is indeed valid.

! [AC once said that Layer 2 is actually a sidechain + bridge, is that really the case?] ](https://cdn-img.panewslab.com//panews/2022/11/6/images/3ecdc88f4687d05e929b3546439ef0de.)

If someone cross-chains from polygon to ETH, they can submit a merkle proof to prove that their cross-chain request is recorded in the polygon block (in other words, it can be matched head-to-head with a polygon block).

Obviously, this kind of ZK bridge relies solely on zero-knowledge proof technology to ensure security, rather than trusting a few off-chain notary nodes like many third-party multi-signature bridges.

Most of the cross-chain bridge models between ZK Rollup and Ethereum, similar to the ZK bridges mentioned above, although the two are different in details, the security model is highly similar: the target chain will record and verify that the block header /Merkle root of the source chain is valid, and when judging whether a cross-chain Txn is valid, it relies on the superiority of cryptography technologies such as zero-knowledge proofs/Merkle Proof, rather than entrusting a few off-chain nodes to decide everything like the Axie bridge.

Let’s go back to the subtitle “Layer 2 = Ethereum-dependent chain + more secure bridge”. In addition to the greater security of cross-chain bridges, Layer 2 is fundamentally different from sidechains like Polygon, which are basically independent of Ethereum: there is a correlation between the longest chain of Layer 2 and the longest chain of Ethereum.

For example, a Layer 2 full node needs to run an Ethereum geth client to synchronize ETH blocks in real time, while all transaction batches of Layer 2 must be published to ETH blocks with an anchor relationship, which greatly increases the difficulty of rolling back the longest chain of Layer 2.

Vitalik has pointed out that “if you want to roll back Layer 2, you must first roll back Layer 1” and “Layer 2 must be able to read the latest Ethereum block”. Suppose that the longest chain of Ethereum is rolled back due to the instability of the distributed network, resulting in multiple L2 Txn Batches that have already been published in the ETH block being rolled back, then the Layer 2 full node will also jointly roll back the Layer 2 longest chain recorded locally. Otherwise, there will be no rollback for normal Layer 2.

But Polygon doesn’t meet these characteristics. For example, Polygon had multiple rollbacks of more than 158 blocks last year, with a history of 5 minutes of rollbacks, and it is clear that there is no anchoring relationship between Polygon blocks and Ethereum blocks, in other words, there is no shared security between the two.

! [AC once said that Layer 2 is actually a sidechain + bridge, is that really the case?] ](https://cdn-img.panewslab.com//panews/2022/11/6/images/27062898b893c079d0288c0e1df0464f.)

Therefore, we can simply conclude here: Layer 2 is actually a sidechain that is anchored to Ethereum, and they will use Ethereum to improve their security, and the bridging model between L2 and L1 is the most secure category under the category of generalized cross-chain bridges, which is not the same as the multi-signature bridge composed of third-party notaries like Axie.

Attacking Layer 2 with a vague statement like “sidechain + bridge” that kills everyone with a stick is not rigorous and simply untenable.