Vitalik Buterin Skeptical of Cross-Chain Bridges

Ethereum co-founder and developer Vitalik Buterin tweeted a link to a Reddit post today in which he discussed his belief in a multi-chain future, but expressed doubt concerning cross-chain ecosystems.

Cross-chain Vulnerabilities

In his argument, Buterin cited the “fundamental security limits of bridges” as the key reason for his disapproval of a cross-chain environment.

In his explanation, Buterin mentioned that he disagrees with the mentality that all security mechanisms fail if and when a blockchain suffers a 51% attack. The objective of a 51% attack is to manipulate the integrity of the transactions being registered in a blockchain by controlling more than 50% of the network’s mining hash rate or computing power.

Buterin alleged that in the case of a 51% attack, the attacker/s cannot propose a block that takes away someone’s ETH because such a block would violate the consensus rules and would therefore be rejected by the network. In other words, he contended, even if 99% of the hashpower were aligned toward illegally taking away another wallet’s ETH, the nodes would simply follow the chain of the remaining 1% because it is  the only set of blocks following the protocol rules. Thus, Buterin claimed, the “honest” blocks would preserve the consistency of the state.

The problem, Buterin argued, emerges when the user bridges assets from their native blockchains to a non-native blockchains. If the native blockchain suffers a 51% attack that reverts the bridge transaction, then as soon as that same transaction gets confirmed in the non-native (destination) blockchain, those assets could be left “orphaned” or “siloed,” therefore leaving the user with a contract that is no longer fully backed in the native blockchain.

Furthermore, Buterin went on to explain that the same principle applies to any Layer 2 that is built on the Ethereum main chain. In this regard, he wrote: 

“If Ethereum gets 51% attacked and reverts, Arbitrum and Optimism revert too, and so “cross-rollup” applications that hold state on Arbitrum and Optimism are guaranteed to remain consistent even if Ethereum gets 51% attacked. And if Ethereum does not get 51% attacked, there’s no way to 51% attack Arbitrum and Optimism separately.”

The author goes on to say that using dApps that are interrelated between different chains is where we can witness a “contagion effect” in which a 51% attack can compromise an entire ecosystem. To further justify this idea, Vitalik clarifies that he is in favor of zones of sovereignty in which several native Layer 1 applications interface closely with each other instead of interacting with other blockchain environments.

Buterin concluded by saying that he did not expect these problems to arise immediately, but as the volume of cryptocurrency held in bridges grows, so too will the incentive to attack them. 

Disclosure: At the time of writing this article, the author owned ETH and several other cryptocurrencies.