CREATE2 and counterfactual addresses (2019)

CREATE derives a new contract’s address from the deployer and its nonce, so the address depends on everything the deployer did before. EIP-1014, by Vitalik Buterin, activated in the Constantinople upgrade of February 2019, added CREATE2:

\[\mathrm{address} = H(\mathtt{0xff} \,\|\, \mathrm{deployer} \,\|\, \mathrm{salt} \,\|\, H(\mathrm{init\ code}))_{[12:]} .\]

The address depends only on who deploys, a chosen salt and the code, so it is known before anything is deployed: it is counterfactual. A user can receive funds at a smart-contract wallet that does not exist yet, and the wallet is deployed only when it is first needed, by whoever pays for it. State channels use the same trick for contracts that are deployed only in a dispute.

import matplotlib.pyplot as plt

import blockchainkit as bk

Pay first, deploy later

world = bk.contracts.World()
factory = world.deploy("dev", bk.contracts.Factory, name="factory")
address = world.view(factory, "predict", 7, "alice")
world.fund("bob", 100)
world.transact("bob", address, value=60)
print("before deployment: code", world.code(address), "balance", world.balance(address))
assert world.code(address) is None and world.balance(address) == 60

deployed = world.transact("relayer", factory, "deploy", 7, "alice").result
assert (
    deployed == address == bk.contracts.create2_address(factory, 7, bk.contracts.Forwarder, "alice")
)
assert world.transact("anyone", address, "sweep").result == 60 and world.balance("alice") == 60
assert "already in use" in world.transact("relayer", factory, "deploy", 7, "alice").error

before deployment: code None balance 60

What the address depends on

creates = [bk.contracts.create_address(factory, nonce) for nonce in range(3)]
salts = [world.view(factory, "predict", salt, "alice") for salt in range(3)]
owners = [world.view(factory, "predict", 7, owner) for owner in ("alice", "bob", "carol")]
assert len(set(creates)) == len(set(salts)) == len(set(owners)) == 3
assert world.view(factory, "predict", 7, "alice") == address  # Reproducible.

fig, ax = plt.subplots(figsize=(8, 3.5))
rows = [
    ("CREATE, nonce 0..2", creates),
    ("CREATE2, salt 0..2", salts),
    ("CREATE2, owner varies", owners),
]
for row, (_, addresses) in enumerate(rows):
    for column, value in enumerate(addresses):
        ax.text(
            column,
            -row,
            value[:12] + "...",
            ha="center",
            va="center",
            family="monospace",
            bbox={"facecolor": "#e0e7ff" if value != address else "#bbf7d0", "pad": 4},
        )
ax.set_yticks([-r for r in range(len(rows))], [label for label, _ in rows])
ax.set(
    xlim=(-0.6, 2.6),
    ylim=(-2.6, 0.6),
    xticks=[],
    title="Every input of the formula moves the address",
)
fig.tight_layout()

plt.show()

Exercise

The deployed wallet sends everything to its owner. Bob funded Alice’s address before it existed, trusting that only a Forwarder for Alice can ever be deployed there. Which inputs of create2_address guarantee that, and why must the constructor’s argument be part of the code hash?