Optics

OPTimistic Interchain Communication

Abstract

We present Optics - a system for sending messages between consensus systems without paying header validation costs by creating the illusion of cross-chain communication. Similar to an atomic swap, Optics uses non-global protocol validation to simulate cross-chain communication. Optics can carry arbitrary messages (raw byte vectors), uses a single-producer multi-consumer model, and has protocol overhead sublinear in the number of messages being sent.

Key Points

System sketch:

1. A "home" chain commits messages in a merkle tree
2. A bonded "updater" attests to the commitment
3. The home chain ensures the attestation is accurate, and slashes if not
4. Attested updates are replayed on any number of "replica" chains, after a time delay

As a result, one of the following is always true:

1. All replicas have a valid commitment to messages from the home chain
2. Failure was published before processing, and the updater can be slashed on the home chain

This guarantee, although weaker than header-chain validation, is still likely acceptable for most applications.

Summary

Optics is a new strategy for simulating cross-chain communication without validating headers. The goal is to create a single short piece of state (a 32-byte hash) that can be updated regularly. This hash represents a merkle tree containing a set of cross-chain messages being sent by a single chain (the "home" chain for the Optics system). Contracts on the home chain can submit messages, which are put into a merkle tree (the "message tree"). The message tree's root may be transferred to any number of "replica" chains.

Rather than proving validity of the commitment, we put a delay on message receipt, and ensure that failures are publicly visible. This ensure that participants in the protocol have a chance to react to failures before the failure can harm them. Which is to say, rather than preventing the inclusion of bad messages, Optics guarantees that message recipients are aware of the inclusion, and have a chance to refuse to process them.

To produce this effect, the home chain designates a single "updater." The updater places a bond ensuring her good behavior. She is responsible for producing signed attestations of the new message tree root. The home chain accepts and validates these attestations. It ensures that they extend a previous attestation, and contain a valid new root of the message set. These attestations are then sent to each replica.

The replica accepts an update attestation signed by the updater, and puts it in a pending state. After a timeout, it accepts the update from that attestation and stores a new local root. Because this root contains a commitment of all messages sent by the home chain, these messages can be proven (using the replica's root) and then dispatched to contracts on the replica chain.

The timeout on new updates to the replica serves two purposes:

1. It ensures that any misbehavior by the updater is published in advance of message processing. This guarantees that data necessary for home chain slashing is available for all faults.
2. It gives message recipients a chance to opt-out of message processing for the update. If an incorrect update is published, recipients always have the information necessary to take defensive measures before any messages can be processed.