Optimism Dai Bridge

Optimism Dai and upgradable token bridge

Contracts

• dai.sol - Improved DAI contract.
• L1DAITokenBridge.sol - L1 side of the bridge. Escrows L1 DAI in L1Escrow contract. Unlocks L1 DAI upon withdrawal message from L2DAITokenBridge.
• L2DAITokenBridge.sol - L2 side of the bridge. Mints new L2 DAI after receiving a message from L1DAITokenBridge. Burns L2 DAI tokens when withdrawals happen.
• L1Escrow - Hold funds on L1. Allows having many bridges coexist on L1 and share liquidity.
• L1GovernanceRelay & L2GovernanceRelay - allows to execute a governance spell on L2.

Upgrade guide

Deploying new token bridge

This bridge stores funds in an external escrow account rather than on the bridge address itself. To upgrade, deploy new bridge independently and connect to the same escrow. Thanks to this, no bridge will ever run out of funds.

Closing bridge

After deploying a new bridge you might consider closing the old one. The procedure is slightly complicated due to async messages like finalizeDeposit and finalizeWithdraw that can be in progress.

An owner calls L2DAITokenBridge.close() and L1DAITokenBridge.close() so no new async messages can be sent to the other part of the bridge. After all async messages are done processing (can take up to 1 week) bridge is effectively closed. Now, you can consider revoking approval to access funds from escrow on L1 and token minting rights on L2.

Emergency shutdown

If ES is triggered, ESM contract can be used to deny access from the PauseProxy (governance). In such scenario the bridge continues to work as usual and it's impossible to be closed.

Known Risks

Optimism's bug

In this section, we describe various risks caused by possible bugs in Optimism system.

L1 -> L2 message passing bug

Bug allowing to send arbitrary messages from L1 to L2 ie. making OVM_L2CrossDomainMessenger to send arbitrary messages, could result in minting of uncollateralized L2 DAI. This can be done via:

• sending finalizeDeposit messages directly to L2DAITokenBridge
• granting minting rights by executing malicious spell with L2GovernanceRelay

Immediately withdrawing L2 DAI to L1 DAI is not possible because of the dispute period (1 week). In case of such bug, governance can disconnect L1DAITokenBridge from L1Escrow, ensuring that no L1 DAI can be stolen. Even with 2 days delay on governance actions, there should be plenty of time to coordinate action. Later off-chain coordination is required to send DAI back to rightful owners or redeploy Optimism system.

L2 -> L1 message passing bug

Bug allowing to send arbitrary messages from L2 to L1 is potentially more harmful. This can happen two ways:

1. Bug in OVM_L1CrossDomainMessenger allows sending arbitrary messages on L1 bypassing the dispute period,
2. The fraud proof system stops working which allows submitting incorrect state root. Such state root can be used to proof an arbitrary message sent from L2 to L1. This will be a subject to a dispute period (1 week).

If (1) happens, an attacker can immediately drain L1 DAI from L1Escrow.

If (2) happens, governance can disconnect L1DAITokenBridge from L1Escrow and prevent from stealing L1 DAI.

Governance mistake during upgrade

Bridge upgrade is not a trivial procedure due to the async messages between L1 and L2. Whole process is described in Upgrade guide in this document.

If governance spell mistakenly revokes old bridge approval to access escrow funds async withdrawal messages will fail. Fortunately reverted messages can be replied at later date, so governance has to re-approve old L1DAITokenBridge to escrow funds and process again pending withdrawals.

Invariants

L1 DAI Locked and L2 DAI Minted

L1DAI.balanceOf(escrow) ≥ L2DAI.totalSupply()

All DAI available on L2 should be locked on L1. This should hold true with more bridges as well.

It's >= because:

a) when depositing on L1, locking is instant but minting is an async message

b) when withdrawing from L2, burning is instant but unlocking on L1 is an async message and is subject to a dispute period (1 week)

c) someone can send L1DAI directly to escrow

Scripts

Some of these scripts may require valid .env file. Copy .env.example as .env and fill it out.

• scripts/deployMainnet.ts - deploys a full solution to mainnet and optimism mainnet. Run with yarn deploy:mainnet
• scripts/deployKovan.ts - deploys a full solution to kovan and optimism testnet on kovan. Run with yarn deploy:kovan

Running

yarn
yarn build
yarn test  # runs unit tests

Running E2E tests

# clone optimism monorepo and run dockerized infrastructure
git clone https://github.com/ethereum-optimism/optimism.git
cd optimism
git checkout ae1ac05d7032422a71caf25d16f6e548df5b8d7f
cd ops
export COMPOSE_DOCKER_CLI_BUILD=1
export DOCKER_BUILDKIT=1
docker-compose build
docker-compose up

# in other terminal window run this project
yarn build
yarn test-e2e  # runs unit tests

Development

Run yarn test:fix to run linting in fix mode, auto-formatting and unit tests.

Running yarn test makes sure that contracts are compiled. Running yarn test-e2e doesn't.

Fuzzing

Install Echidna

• Precompiled Binaries (recommended)

Before starting, make sure Slither is installed:

$ pip3 install slither-analyzer

To quickly test Echidna in Linux or MacOS: release page

Local Dependencies

• Slither:

  $ pip3 install slither-analyzer
  

• solc-select:

  $ pip3 install solc-select
  

Run Echidna Tests

• Install solc version:

  $ solc-select install 0.7.6
  

• Select solc version:

  $ solc-select use 0.7.6
  

• Run Echidna Tests:

  $ echidna-test . --contract DaiEchidnaTest --config echidna.config.yml
  

Certora

Install Certora

• Install Java

  sudo apt install openjdk-14-jdk
  

• Install Certora Prover

  pip3 install certora-cli
  

• Set Certora Key

  export CERTORAKEY=<key>
  

Local Dependencies

• solc-select:

  pip3 install solc-select
  

Run Certora Specs

• Install solc version:

  solc-select install 0.7.6
  

• Run Certora Specs:

  yarn certora
  

Deployments:

Mainnet:

{
  "l1Dai": "0x6B175474E89094C44Da98b954EedeAC495271d0F",
  "l1Escrow": "0x467194771dAe2967Aef3ECbEDD3Bf9a310C76C65",
  "l1DAITokenBridge": "0x10E6593CDda8c58a1d0f14C5164B376352a55f2F",
  "l1GovernanceRelay": "0x09B354CDA89203BB7B3131CC728dFa06ab09Ae2F",
  "l2Dai": "0xDA10009cBd5D07dd0CeCc66161FC93D7c9000da1",
  "l2DAITokenBridge": "0x467194771dAe2967Aef3ECbEDD3Bf9a310C76C65",
  "l2GovernanceRelay": "0x10E6593CDda8c58a1d0f14C5164B376352a55f2F"
}

Kovan:

{
  "l1Dai": "0x4F96Fe3b7A6Cf9725f59d353F723c1bDb64CA6Aa",
  "l1Escrow": "0x8FdA2c4323850F974C7Abf4B16eD129D45f9E2e2",
  "l1DAITokenBridge": "0xb415e822C4983ecD6B1c1596e8a5f976cf6CD9e3",
  "l1GovernanceRelay": "0xAeFc25750d8C2bd331293076E2DC5d5ad414b4a2",
  "l2Dai": "0xDA10009cBd5D07dd0CeCc66161FC93D7c9000da1",
  "l2DAITokenBridge": "0x467194771dAe2967Aef3ECbEDD3Bf9a310C76C65",
  "l2GovernanceRelay": "0x10E6593CDda8c58a1d0f14C5164B376352a55f2F"
}