Nitro / Orbit

Arbitrum integration with Espresso

Espresso has developed an integration with the Arbitrum Nitro tech stack that allows Arbitrum Orbit chains to integrate with Espresso. The integration gives each Orbit chain decentralized Proof of Stake settlement and finality from the Espresso Network: the chain continues to run its own Nitro sequencer and execution stack, and Espresso finalizes the blocks it produces. Roadmap items include enhanced cross-chain interoperability and support for Espresso DA.

The first production integration launched with Rari on January 29, 2025 at block 1486407. Orbit chains can integrate by running Espresso's Docker image directly or through a RaaS provider.

If you just want to see the steps for getting up and running, you can skip to the guide.

Integration overview

This integration makes minimal changes to the Arbitrum Nitro stack, and ensures that each batch processed by the rollup is consistent with HotShot-finalized blocks within its namespace.

To ensure that the batch has been finalized by HotShot, the following checks are performed:

1. Namespace validation: Ensure that the set of transactions in a rollup's batch corresponds to the correct namespace. Namespacing allows multiple chains to use Espresso’s fast confirmation layer simultaneously by associating each chain’s transactions with a unique namespace within HotShot blocks.

2. Espresso block Merkle proof check: Confirm that the rollup's batch maps to a valid HotShot block. Specifically, verify that the HotShot block associated with a rollup batch is a valid leaf in the Merkle tree maintained by the light client contract, which stores the state of HotShot on L1.

3. The sequencer calls WriteSequencerMsg on the transaction streamer.

4. The batcher fetches the message from the transaction streamer and submits the transaction to HotShot via the transaction streamer.

5. The batcher then calls the query API to check if the transaction has been finalized by HotShot.

6. Once the transaction is finalized, the batcher performs batch consistency checks.

7. The batcher signs the transaction calldata that would be sent to the Sequencer Inbox contract.

8. The Sequencer Inbox contract is modified to verify the batcher’s signature.

This approach involves running a Nitro node with only the batcher enabled, operating in a TEE environment (such as Intel SGX). The batcher will sign the transaction calldata. In case the TEE is broken, the batch poster can't impact the safety of the Orbit chain. It could, however, temporarily halt the chain's progress, thereby breaking liveness. Bridges relying on Espresso confirmations for faster settlement need to trust the TEE as well in this integration. In a future update, the dependency on TEEs will be removed entirely.