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Refs: #750 Here we present the smart contracts necessary to enable the L2 direct bridging (a.k.a native bridging) feature. This mechanism allows getting canonical TBTC on the given supported L2 chain, without the need to touch the L1 Ethereum chain the tBTC protocol is deployed on. Changes made as part of this pull request introduce a generic mechanism that can be deployed on all supported L2 EVM-based chains and deploy the mechanism on Ethereum Sepolia and Base Sepolia chains for testing purposes. ### Motivation Right now, a user of the supported L2 chain willing to obtain canonical L2 TBTC has to go the following path: 1. Generate a Bitcoin deposit address (using tBTC dApp or SDK) 2. Make a deposit transaction on Bitcoin 3. Reveal the Bitcoin transaction to the tBTC Bridge to start the minting process (Ethereum transaction) 4. Once TBTC is minted on Ethereum, go to the [Wormhole Token Portal](https://portalbridge.com) and bridge minted TBTC to the given L2 (another Ethereum transaction) 5. Canonical L2 TBTC lands on the user account automatically This flow is unwieldy and has major drawbacks: - It's complicated and requires multiple transactions - It requires L2 users to interact with the L1 Ethereum chain - It requires interacting with the Wormhole Token Portal The idea behind direct bridging is simplifying the above flow to something like: 1. Generate a Bitcoin deposit address (using dApp or SDK) 2. Make a deposit transaction on Bitcoin 3. Reveal the Bitcoin transaction to the tBTC Bridge **using a single transaction on the L2 chain** 4. Canonical L2 TBTC lands on the user account automatically Although this flow still relies on Wormhole underneath, the advantages are: - Simpler flow with fewer transactions - L2 users interact only with the given L2 chain - No need to touch the Wormhole Token Portal - As a next iteration, we can even get rid of the reveal deposit transaction on L2 and use Bitcoin deposit transactions as a trigger. This will improve UX even more. See the **Next iteration: Gasless bridging** section for details. ### High-level architecture The high-level architecture of the direct briding mechanism is presented on the chart below: <img width="2144" alt="bob-i1" src="https://github.com/keep-network/tbtc-v2/assets/11180469/6a32050d-6bc4-44cb-a299-1bc3e8009364"> - The **green** contracts are existing tBTC contracts that form the current bridging flow based on the Wormhole Token Portal (see [RFC 8](https://github.com/keep-network/tbtc-v2/blob/main/docs/rfc/rfc-8.adoc#37-smart-contracts)). The `TBTC Bridge` component is the `Bridge` contract deployed on L1 Ethereum responsible for minting the L1 TBTC token. The `L2WormholeGateway` contract has the authority to mint canonical L2 TBTC on the given L2, based on received Wormhole L2 TBTC tokens. The `L2TBTC` component is the canonical L2 TBTC token contract. - The **blue** contracts are the new contracts that enable the new direct bridging flow. The `AbstractTBTCDepositor` contract (introduced by #778) provides some useful tooling facilitating integration with the tBTC `Bridge` and its new **deposit with extra data** function (developed in #760) which is the foundation of the L2 direct bridging mechanism. The `L1BitcoinDepositor` and `L2BitcoinDepositor` components are smart contracts handling the actual direct bridging actions on the L1 and L2 chains respectively. Those two contracts are introduced by this pull request. - The **red** contracts belong to the Wormhole protocol that handles cross-chain operations. In the context of the direct bridging mechanism, that means the transfer of minted L1 TBTC to the L2 chain. The `TokenBridge` contract handles the bookkeeping part of the transfer. The `Relayer` contract handles the actual execution of it. - The **yellow** off-chain relayer bot is the planned component (implemented as an immediate next step) that will "turn the crank" of the direct bridging mechanism. It will initialize deposits on the L1 chain (based on L2 events in the first iteration) and finalize them once L1 TBTC is minted by the tBTC `Bridge` contract. The above components interact with each other in the following way: 1. The user makes the BTC deposit funding transaction and calls the `L2BitcoinDepositor.initializeDeposit` function to initiate the deposit process on L2 (the call is made through a dApp and tBTC SDK). 2. The off-chain relayer bot observes the `DepositInitialized` event emitted by the `L2BitcoinDepositor` contract. 3. After assessing the deposit validity, the off-chain relayer bot calls the `L1BitcoinDepositor.initializeDeposit` function on L1. 4. The `L1BitcoinDepositor` contract calls the `Bridge.revealDepositWithExtra` function under the hood (through the `AbstractTBTCDepositor` abstract contract). 5. After some time (several hours), the `Bridge` contract mints L1 TBTC to the `L1BitcoinDepositor` contract. 6. The off-chain bot observes the mint event and calls `L1BitcoinDepositor.finalizeDeposit` to start the finalization process and move L1 TBTC to the L2 chain. 7. The `L1BitcoinDepositor` calls Wormhole's `TokenBridge.transferTokensWithPayload` function to initiate the cross-chain transfer of L1 TBTC. This call pulls out the L1 TBTC from the `L1BitcoinDepositor` contract and locks it in the `TokenBridge`. 8. The `L1BitcoinDepositor` calls Wormhole's `Relay.sendVaasToEvm` to send a cross-chain message to `L2BitcoinDepositor` and notify it about a pending cross-chain transfer. 9. The Wormhole protocol calls the `L2BitcoinDepositor.receiveWormholeMessages` function to deliver the cross-chain message. 10. The `L2BitcoinDepositor` contract calls the `L2WormholeGateway.receiveTbtc` function under the hood. It passes the VAA representing the cross-chain transfer as an argument of the call. 11. The `L2WormholeGateway` uses the obtained VAA to finalize the cross-chain transfer by calling the Wormhole's `TokenBridge.completeTransferWithPayload` function. This call redeems Wormhole-wrapped L2 TBTC from the `TokenBridge`. 12. The `L2WormholeGateway` uses obtained Wormhole-wrapped L2 TBTC to call `L2TBTC.mint` and mint canonical L2 TBTC. 13. Minted canonical L2 TBTC is transferred to the L2 user. ### Immediate next steps Changes presented in this pull request introduce the on-chain components of the direct bridging mechanism. To make the mechanism complete, the following steps need to take place: - Expose the L2 direct bridging feature in the tBTC Typescript SDK. This feature will be incrementally exposed for specific L2 chains the mechanism will be deployed for. The first L2 chain will be Base. - Implement the off-chain relayer bot ### Next iteration: Gasless bridging The plans for the future include some improvements to the first iteration of the direct bridging mechanism described above. Namely, the next iteration will bring gasless direct bridging that will not require any L2 transaction to initiate the process and will rely on a single Bitcoin funding transaction issued by the L2 user. On the technical level, the first two steps of the flow will be replaced by a direct call to the off-chain relayer's REST endpoint: <img width="2144" alt="bob-i2" src="https://github.com/keep-network/tbtc-v2/assets/11180469/f6a01138-0354-4b41-b3f1-4f4a38be7f91">
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