Ethereum Smart Contracts
There are five smart contracts that make up an Ethereum bridge deployment.
The Proxy contract serves as a dumb storage for holding the addresses of other contracts, specifically the Governance contract, the Vault contract and the current Bridge contract. Once deployed, it is modifiable only by the Governance contract, to update the address for which contract is the current Bridge contract.
The Proxy contract is fixed forever once the bridge has been deployed.
The Bridge contract is the only contract that unprivileged users of the bridge may interact with. It provides methods for transferring ERC20s to Namada (holding them in escrow in the Vault), as well as releasing escrowed ERC20s from the Vault for transfers made from Namada to Ethereum. It holds a whitelist of ERC20s that may cross the bridge, and this whitelist may be updated by the Governance contract.
The Governance contract may "upgrade" the bridge by updating the Proxy contract to point to a new Bridge contract and/or a new Governance contract. It may also withdraw all funds from the Vault to any specified Ethereum address, if a quorum of validators choose to do so.
The wNAM contract is a simple ERC20 token with a fixed supply, which is all minted when the bridge is first deployed. After initial deployment, the entire supply of wNAM belongs to the Vault contract. As NAM is transferred from Namada to Ethereum, wNAM may be released from the Vault by the Bridge.
The wNAM contract is fixed forever once the bridge has been deployed.
The Vault contract holds in escrow any ERC20 tokens that have been sent over the bridge to Namada, as well as a supply of wNAM ERC20s to represent NAM that has been sent from Namada to Ethereum. Funds held by the Vault may only be spendable by the current Bridge contract. When ERC20 tokens are transferred from Ethereum to Namada, they must be deposited to the Vault via the Bridge contract.
The Vault contract is fixed forever once the bridge has been deployed.
When an account on Namada becomes a validator, they must provide two Ethereum secp256k1 keys:
- the bridge key - a hot key for normal operations
- the governance key - a cold key for exceptional operations, like emergency withdrawal from the bridge
These keys are used to control the bridge smart contracts, via signing of messages. Validators should be challenged periodically to prove they still retain knowledge of their governance key, which is not regularly used.
The contracts should be deployable by anyone to any EVM chain using an automated script. The following configuration should be agreed up front by Namada governance before deployment:
- details of the initial active validator set that will control the bridge -
specifically, for each validator:
- their hot Ethereum address
- their cold Ethereum address
- their voting power on Namada for the epoch when the bridge will launch
- the total supply of the wNAM ERC20 token, which will represent Namada-native NAM on the EVM chain
- an initial whitelist of ERC20 tokens that may cross the bridge from Ethereum
to Namada - specifically, for each whitelisted ERC20:
- the Ethereum address of the ERC20 contract
- a cap on the total amount that may cross the bridge, in units of ERC20
After a deployment has finished successfully, the deployer must not have any privileged control of any of the contracts deployed. Any privileged actions must only be possible via a message signed by a validator set that the smart contracts are storing details of.
From Ethereum to Namada
A Namada chain's validators are configured to listen to events emitted by the
smart contracts pointed to by the Proxy contract. The address of the Proxy
contract is set in a governance parameter in Namada storage. Namada validators
treat emitted events as authoritative and take action on them. Namada also knows
the address of the wNAM ERC20 contract via a governance parameter, and treats
transfers of this ERC20 to Namada as an indication to release native NAM from
#EthBridgeEscrow account on Namada, rather than to mint a wrapped ERC20 as
is the case with all other ERC20s.
From Namada to Ethereum
At any time, the Governance and Bridge contracts must store:
- a hash of the current Namada epoch's active validator set
- a hash of another epoch's active validator set. When the bridge is first deployed, this will also be the current Namada epoch's active validator set, but after the first validator set update is submitted to the Governance smart contract, this hash will always be an adjacent Namada epoch's active validator set i.e. either the previous epoch's, or the next epoch's
In the case of the Governance contract, these are hashes of a map of validator's cold key addresses to their voting powers, while for the Bridge contract it is hashes of a map of validator's hot key addresses to their voting powers. Namada validators may post signatures as on chain of relevant messages to be relayed to the Ethereum bridge smart contracts (e.g. validator set updates, pending transfers, etc.). Methods of the Ethereum bridge smart contracts should generally accept:
- some message
- full details of some active validator set (i.e. relevant Ethereum addresses + voting powers)
- signatures over the message by validators from the this active validator set
Given this data, anyone should be able to make the relevant Ethereum smart contract method call, if they are willing to pay the Ethereum gas. A call is then authorized to happen if:
- The active validator set specified in the call hashes to either of the validator set hashes stored in the smart contract
- A quorum (i.e. more than 2/3 by voting power) of the signatures over the message are valid
Validator set updates
Initial deployment aside, at the beginning of each epoch, the smart contracts will contain details of the current epoch's validator set and the previous epoch's validator set. Namada validators must endeavor to sign details of the next epoch's validator set and post them on Namada chain in a protocol transaction. Details of the next epoch's validator set and a quorum of signatures over it by validators from the current epoch's validator set must then be relayed to the Governance contract before the end of the epoch, which will update both the Governance and Bridge smart contracts to have the hash of the next epoch's validator set rather than the previous epoch's validator set. This should happen before the current Namada epoch ends. If this does not happen, then the Namada chain must either halt or not progress to the next epoch, to avoid losing control of the bridge.
When a validator set update is submitted, the hashes for the oldest validator set are effectively "evicted" from the Governance and Bridge smart contracts. At that point, messages signed by that evicted validator set will no longer be accepted by the bridge.
- Namada epoch
10begins. Currently, the Governance contract knows the hashes of the validator sets for epochs
10, as does the Bridge contract.
- Validators for epoch
10post signatures over the hash of details of the validator set for epoch
11to Namada as protocol transactions
- A point is reached during epoch
10at which a quorum of such signatures is present on the Namada chain
- A relayer submits a validator set update for epoch
11to Governance, using a quorum of signatures from the Namada chain
- The Governance and Bridge contracts now know the hashes of the validator
sets for epochs
11, and will accept messages signed by either of them. It will no longer accept messages signed by the validator set for epoch
- Namada progresses to epoch
11, and the flow repeats
NB: the flow for when the bridge has just launched is similar, except the
contracts know the details of only one epoch's validator set - the launch
epoch's. E.g. if the bridge launches at epoch
10, then initially the contracts
know the hash only for epoch
10 and not epochs
11, until the first
validator set update has been submitted