Trustless price oracle for ETH/stETH Curve pool

A trustless oracle for the ETH/stETH Curve pool using Merkle Patricia proofs of Ethereum state.

The oracle assumes that the pool's fee and A (amplification coefficient) values don't change between the time of proof generation and submission.

Audits

Commits 1033b3e and ae093b3 (the currently deployed version) were audited by MixBytes. Contracts in both commits were assumed as secure to use according to the auditors' security criteria. See the full report for details.

Mechanics

The oracle works by generating and verifying Merkle Patricia proofs of the following Ethereum state:

• Curve stETH/ETH pool contract account and the following slots from its storage trie:

  • admin_balances[0]
  • admin_balances[1]

• stETH contract account and the following slots from its storage trie:

  • shares[0xDC24316b9AE028F1497c275EB9192a3Ea0f67022]
  • keccak256("lido.StETH.totalShares")
  • keccak256("lido.Lido.beaconBalance")
  • keccak256("lido.Lido.bufferedEther")
  • keccak256("lido.Lido.depositedValidators")
  • keccak256("lido.Lido.beaconValidators")

Contracts

The repo contains two main contracts:

• StableSwapStateOracle.sol is the main oracle contract. It receives and verifies the report from the offchain code, and persists the verified state along with its timestamp.

• StableSwapPriceHelper.vy is a helper contract used by StableSwapStateOracle.sol and written in Vyper. It contains the code for calculating exchange price based on the values of pool's storage slots. The code is copied from the actual pool contract with minimal modifications.

Deploying and using the contracts

First, deploy StableSwapPriceHelper. Then, deploy StableSwapStateOracle, pointing it to StableSwapPriceHelper using the constructor param:

# assuming eth-brownie console

helper = StableSwapPriceHelper.deploy({ 'from': deployer })

price_update_threshold = 300 # 3%
price_update_threshold_admin = deployer

oracle = StableSwapStateOracle.deploy(
  helper,
  price_update_threshold_admin,
  price_update_threshold,
  { 'from': deployer }
)

To send proofs to the state oracle, call the submitState function:

header_rlp_bytes = '0x...'
proofs_rlp_bytes = '0x...'

tx = oracle.submitState(header_rlp_bytes, proofs_rlp_bytes, { 'from': reporter })

The function is permissionless and, upon successful verification, will generate two events, SlotValuesUpdated and PriceUpdated, and update the oracle with the verified pool balances and stETH price. You can access them by calling getState and getPrice:

(timestamp, etherBalance, stethBalance, stethPrice) = oracle.getState()
stethPrice = oracle.getPrice()
print("stETH/ETH price:", stethPrice / 10**18)

Sending oracle transaction

Use the following script to generate and submit a proof to the oracle contract:

python offchain/generate_steth_price_proof.py \
  --rpc <HTTP RPC endpoint of a geth full node> \
  --keyfile <path to a JSON file containing an encrypted private key> \
  --gas-price <tx gas price in wei> \
  --contract <state oracle contract address> \
  --block <block number>

Some flags are optional:

• Skip the --keyfile flag to print the proof without sending a tx.
• Skip the --gas-price flag to use gas price determined by the node.
• Skip the --block flag to generate a proof correnspoding to the block latest - 15.