Account abstraction with ERC-4337 (2021)

An Ethereum key-pair account can start transactions but follows fixed rules: one ECDSA key, gas paid in ether. A contract account can follow any rule but cannot start a transaction. ERC-4337, proposed in 2021 by Vitalik Buterin, Yoav Weiss and others and deployed in 2023, lets contract accounts act on their own without a protocol change. Owners sign user operations; a bundler submits them to an EntryPoint contract, which asks each account to validate its operation and pay its fee, then runs it. The owner signs

\[h = \mathrm{SHA256}(\text{domain} \,\|\, \text{EntryPoint} \,\|\, \text{chain} \,\|\, \text{operation}),\]

so an operation cannot be replayed elsewhere. Because validation is code, the owner can rotate the key, batch payments, or plug in social recovery.

import matplotlib.pyplot as plt

import blockchainkit as bk

A smart account, a bundler, and a key rotation

world = bk.contracts.World()
entry = world.deploy("deployer", bk.wallets.EntryPoint, name="EntryPoint")
account = world.deploy("deployer", bk.wallets.SmartAccount, entry, bk.crypto.public_key(7))
world.fund("alice", 100)
world.transact("alice", account, value=100)


def operation(nonce, function, *args, key=7, fee=2):
    op = bk.wallets.UserOperation(account, nonce, function, args, fee)
    return bk.wallets.sign_user_operation(op, key, entry)


ops = [
    operation(0, "execute", "bob", 10),
    operation(1, "rotate_owner", bk.crypto.public_key(11)),
]
receipt = world.transact("bundler", entry, "handle_ops", ops, "bundler")
print("bundle:", receipt.success, receipt.result, "bundler earned", world.balance("bundler"))

stale = operation(2, "execute", "mallory", 50)  # Signed with the old key.
fresh = operation(2, "execute_batch", [("bob", 5), ("carol", 5)], key=11)
kept = bk.wallets.bundle(world, entry, [stale, fresh])
print("bundler keeps", len(kept), "of 2 operations")
assert kept == (fresh,)
receipt = world.transact("bundler", entry, "handle_ops", kept, "bundler")
assert receipt.result == (True,)

# An operation that would only revert in validation never reaches the chain:
# included anyway, it would revert the bundle at the bundler's expense.
assert not world.transact("bundler", entry, "handle_ops", [stale], "bundler").success

bundle: True (True, True) bundler earned 4
bundler keeps 1 of 2 operations

Where the ether went

holders = ["bob", "carol", "mallory", "bundler", account]
balances = [world.balance(h) for h in holders]
print(dict(zip(["bob", "carol", "mallory", "bundler", "account"], balances, strict=True)))
assert world.balance("mallory") == 0 and world.balance("bundler") == 6

fig, ax = plt.subplots(figsize=(6, 4))
ax.bar(
    ["bob", "carol", "mallory", "bundler", "account"],
    balances,
    color=["#2563eb", "#2563eb", "#dc2626", "#64748b", "#16a34a"],
)
ax.set(ylabel="ether")
ax.set_title("The old key's operation was dropped; fees paid the bundler")
fig.tight_layout()
plt.show()

{'bob': 15, 'carol': 5, 'mallory': 0, 'bundler': 6, 'account': 74}

Exercise

Sign an operation for another chain ID and submit it. Which check stops it, and why must the EntryPoint’s address be in the hash too? A worked solution is in Exercises: wallets.