Our Reply to Vitalik’s Concerns on Cross-Chain Bridges.
recently made a Reddit post about the limits of blockchain interoperability, and why cross-chain bridges won’t work. In this article, we attempt to assuage some of those concerns. Let’s dive deeper.
Vitalik essentially makes a simple argument that standalone blockchains can survive short-term 51% attacks, while bridges would go through catastrophic failure under these conditions. Actually, we will show that bridges can survive such attacks, in the same manner a standalone blockchain survives it.
Consider Vitalik’s first argument:-
“suppose that you have 100 ETH on Ethereum, and Ethereum gets 51% attacked, so some transactions get censored and/or reverted. No matter what happens, you still have your 100 ETH.”
This concept is often referred to as “finality”. It means that as a transaction (that gave you 100 ETH) gets deeper into the blockchain (as more blocks are added on top of it), the difficulty of reverting this transaction rises exponentially. This happens because, in order to revert a transaction, the 51% attacker would need to build a new chain starting from the block containing this transaction. If many blocks have been added on top of this transaction, the attacker would need to recreate all those blocks. This would be hard to do for old transactions. But for new transactions, only a few blocks would need to be spoofed, and hence a 51% attacker might be able to revert them. This is the reason Metamask users are advised to wait for several blocks of confirmations before considering their transaction complete.
So far, so good.
Now consider Vitalik’s second argument:-
“if you move 100 ETH onto a bridge on Solana to get 100 Solana-WETH, and then Ethereum gets 51% attacked. The attacker deposited a bunch of their own ETH into Solana-WETH and then reverted that transaction on the Ethereum side as soon as the Solana side confirmed it. The Solana-WETH contract is now no longer fully backed, and perhaps your 100 Solana-WETH is now only worth 60 ETH.it.”
In this case, Vitalik is assuming that the moment the user moves their assets into the bridge, the bridge will instantly make the cross-chain action on Solana. However, the bridge could easily wait for the transaction to be “finalized” on Ethereum (ie wait for the tx to become several blocks deep), before taking the cross-chain action. This is similar to how Metamask users wait for several blocks of confirmations. This way, the bridge can be sure that even if Ethereum is 51% attacked after it takes the cross-chain action, the transactions processed by the bridge will not be reversed, since they are already several blocks deep.
The rest of Vitalik’s arguments are built on top of this argument only, hence we will not discuss them separately.
Going further with BFT consensus
BFT consensus systems like tendermint and NEAR provide instant finality as a feature, and hence transactions do not have to wait for any block confirmations at all. Hence, for BFT chains, cross-chain transactions are indistinguishable from same-chain transactions.
