How is the cross-chain communication is achieved?

The communication between Exocore and L1 chains is very specific, involving only a “reading” and a “writing” process from Exocore to client chains. The reading is for reflecting the restaked assets state and other related data from client chain to Exocore system. The writing is for Exocore to issue control actions to the client chain such as locking, unlocking and slashing.

The reading process is achieved using a ZK light-client-based bridge. Specifically, Exocore validators will host a light client for each integrated client chain. Events about staking and withdrawal from the Exocore contracts in client chains will be received and verified by the Exocore validators and corresponding state will be updated in the Exocore accounting system. No transaction is needed to be sent from client chain to Exocore since the message processing logic is built into the Exocore validator logic.

The writing process is achieved by Exocore validators collectively signing and sending transactions to the client chains. These transactions are signed using a Threshold Signature Scheme (TSS) for its cost-effectiveness. We chose not to use multi-sig based signing due to following tradeoff considerations.

Bridge solutions	Multisig-based	TSS-based
Cost	High (it cost 3000 gas units to verify a ECDSA signature. With our Exocore validator set to be at least 100 for decentralization, it will be extremely costly to issue a single action from Exocore)	Low (Only one signature is verified on the client chain and it costs the same as any normal transaction, the cost are saved because the complexity of the algorithm happens off-chain in MPC on Exocore side)
Complexity	Relatively easy to implement and most logic exist on smart contract level.	Complicated to implement (could be mitigated by adopting well-known open-source TSS implementations), but it’s easy to implement on the client side smart contract since only one signature needs to be verified.
Security	As secure as Exocore	As secure as Exocore
Existing Adopters	Wormhole, BSC bridge, Ronin Bridge	Thorchain, Multichain, Axelar (based on their whitepaper)
Implementation Risk	Medium	High