Bridges provide a way to send data across chains. For example, you may want to send data from Ethereum mainnet to Optimism to vote on an onchain proposal.

They form a critical component of blockchain infrastructure and take 3 out of the top 5 spots on rekt leaderboard!

Protocols enabling this are generally called cross-chain messaging protocols. And since any data can be sent across chains using these protocols, the most popular use case they enable is bridging ERC20 tokens.

We'll focus on token transfer bridges in this post.

Consider a bridge that transfers ETH from Ethereum mainnet to Optimism.

A bridge has a smart contract deployed on each chain (B1 and B2). Addresses on mainnet that want to bridge ETH to Optimism call B1 with the ETH they want to bridge. B1 stores this ETH.

A trusted actor observes this transfer and tells B2 to mint the same amount of OP ETH to the related receiver address. OP ETH is the native token on Optimism.

To bridge this ETH back to mainnet, the same process is followed but in reverse. User address on Optimism sends OP ETH to B2. B2 burns it, and a trusted off-chain actor tells B1 to send the same amount of ETH to the receiver address on mainnet.

For Optimism's native bridge, this trusted actor is replaced by their infrastructure (for bridging to Optimism) and the 7 day challenge period (for bridging to mainnet). This native bridge can mint and burn Optimism's native token (OP ETH).

You can read more about Optimism's bridge in their official docs.

Continuing with the Optimism example, bridging ETH back to mainnet is a slow process since it needs to go through the 7 day challenge period. To circumvent that, non-native bridges like Hop protocol enable fast transfer. These bridges enable rely on off-chain actors observing the native bridge transfer and instantly sending ETH to the receiver. The off-chain actor then receives ETH bridged through the native bridge.

These off-chain actors can be incentivized through a fee.

Now there can be various bridges between 2 chains with different features and trade off. For example, one may take a lower fee, the other may be faster.

For users, it can be a headache to keep track of multiple bridges and pick one. Bridge aggregators aim to solve them.

Example of an aggregator is li.fi connect to various bridges and try to offer the best route to bridge tokens. Now, users just have to interact with one protocol.

Although now the problem becomes which aggregator to choose from!

There can be bridges between non-compatible blockchains, like Bitcoin and Ethereum, or Solana and Ethereum. WBTC, for example, represents the bridged BTC from Bitcoin to Ethereum.

That’s it for this post! Thanks for reading, and I hope you found this post useful.