EIP4337 - Account Abstraction via Entry Point Contract specification

# Abstract

An account abstraction proposal which completely avoids the need for consensus-layer protocol changes. Instead of adding new protocol features and changing the bottom-layer transaction type, this proposal instead introduces a higher-layer pseudo-transaction object called a UserOperation. Users send UserOperation objects into a separate mempool. A special class of actor called bundlers (either miners, or users that can send transactions to miners through a bundle marketplace) package up a set of these objects into a transaction making a handleOps call to a special contract, and that transaction then gets included in a block.

# Motivation

See also "Implementing Account Abstraction as Part of Eth 1.x" (opens new window) and the links therein for historical work and motivation, and EIP-2938 for a consensus layer proposal for implementing the same goal.

This proposal takes a different approach, avoiding any adjustments to the consensus layer. It seeks to achieve the following goals:

Achieve the key goal of account abstraction: allow users to use smart contract wallets containing arbitrary verification logic instead of EOAs as their primary account. Completely remove any need at all for users to also have EOAs (as status quo SC wallets and EIP-3074 both require)
Decentralization
- Allow any bundler (think: miner) to participate in the process of including account-abstracted user operations
- Work with all activity happening over a public mempool; users do not need to know the direct communication addresses (eg. IP, onion) of any specific actors
- Avoid trust assumptions on bundlers
Do not require any Ethereum consensus changes: Ethereum consensus layer development is focusing on the merge and later on scalability-oriented features, and there may not be any opportunity for further protocol changes for a long time. Hence, to increase the chance of faster adoption, this proposal avoids Ethereum consensus changes.
Try to support other use cases
- Privacy-preserving applications
- Atomic multi-operations (similar goal to EIP-3074)
- Pay tx fees with ERC-20 tokens, allow developers to pay fees for their users, and EIP-3074-like sponsored transaction use cases more generally

# Specification

To avoid Ethereum consensus changes, we do not attempt to create new transaction types for account-abstracted transactions. Instead, users package up the action they want their wallet to take in an ABI-encoded struct called a UserOperation:

Field	Type	Description
`sender`	`Address`	The wallet making the operation
`nonce`	`uint256`	Anti-replay parameter; also used as the salt for first-time wallet creation
`initCode`	`bytes`	The initCode of the wallet (only needed if the wallet is not yet on-chain and needs to be created)
`callData`	`bytes`	The data to pass to the `sender` during the main execution call
`callGas`	`uint256`	The amount of gas to allocate the main execution call
`verificationGas`	`uint256`	The amount of gas to allocate for the verification step
`preVerificationGas`	`uint256`	The amount of gas to pay for to compensate the bundler for for pre-verification execution and calldata
`maxFeePerGas`	`uint256`	Maximum fee per gas (similar to EIP 1559 `max_fee_per_gas`)
`maxPriorityFeePerGas`	`uint256`	Maximum priority fee per gas (similar to EIP 1559 `max_priority_fee_per_gas`)
`paymaster`	`address`	Address sponsoring the transaction (or zero for regular self-sponsored transactions)
`paymasterData`	`bytes`	Extra data to send to the paymaster
`signature`	`bytes`	Data passed into the wallet along with the nonce during the verification step

Users send UserOperation objects to a dedicated user operation mempool. A specialized class of actors called bundlers (either miners running special-purpose code, or users that can relay transactions to miners eg. through a bundle marketplace such as Flashbots that can guarantee next-block-or-never inclusion) listen in on the user operation mempool, and create bundle transactions. A bundle transaction packages up multiple UserOperation objects into a single handleOps call to a pre-published global entry point contract.

The core interface of the entry point contract is as follows:

function handleOps
    (UserOperation[] calldata ops, address payable redeemer)
    external;

function simulateWalletValidation
    (UserOperation calldata userOp)
    external returns (uint gasUsedByPayForSelfOp);

function simulatePaymasterValidation
    (UserOperation calldata userOp, uint gasUsedByPayForSelfOp)
    external view returns (bytes memory context, uint gasUsedByPayForOp);

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The core interface required for a wallet to have is:

function validateUserOp
    (UserOperation calldata userOp, uint requiredPrefund)
    external;

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# Required entry point contract functionality

The entry point's handleOps function must perform the following steps (we first describe the simpler non-paymaster case). It must make two loops, the verification loop and the execution loop. In the verification loop, the handleOps call must perform the following steps for each UserOperation:

Create the wallet if it does not yet exist, using the initcode provided in the UserOperation (if the wallet does not exist and the initcode is empty, the call must fail)
Call validateUserOp on the wallet, passing in the UserOperation and the required fee. The wallet should verify the operation's signature, and pay the fee if the wallet considers the operation valid. If any validateUserOp call fails, handleOps must skip execution of at least that operation, and may revert entirely.

In the execution loop, the handleOps call must perform the following steps for each UserOperation:

Call the wallet with the UserOperation's calldata. It's up to the wallet to choose how to parse the calldata; an expected worlflow is for the wallet to have an execute function that parses the remaining calldata as a series of one or more calls that the wallet should make.
Refund unused gas fees to the wallet

Before accepting a UserOperation, bundlers must use an RPC method to locally simulate calling the simulateWalletValidation function of the entry point, to verify that the signature is correct and the operation actually pays fees; see the Simulation section below for details.

# Extension: paymasters

We extend the entry point logic to support paymasters that can sponsor transactions for other users. This feature can be used to allow application developers to subsidize fees for their users, allow users to pay fees with ERC-20 tokens and many other use cases. When the paymaster is not equal to the zero address, the entry point implements a different flow:

During the verification loop, in addition to calling validateUserOp, the handleOps execution also must check that the paymaster has enough ETH staked with the entry point to pay for the operation, and then call validatePaymasterUserOp on the paymaster to verify that the paymaster is willing to pay for the operation. Additionally, the validateUserOp must be called with a requiredPrefund of 0 to reflect that it's the paymaster, and not the wallet, that's paying the fees.

During the execution loop, the handleOps execution must call postOp on the paymaster after making the main execution call. It must guarantee the execution of postOp, by making the main execution inside an inner call context, and if the inner call context reverts attempting to call postOp again in an outer call context.

Maliciously crafted paymasters can DoS the system. To prevent this, we use a paymaster reputation system; see the reputation, throttling and banning section for details.

The paymaster interface is as follows:

function validatePaymasterUserOp
    (UserOperation calldata userOp, uint maxcost)
    external view returns (bytes memory context);

function postOp
    (PostOpMode mode, bytes calldata context, uint actualGasCost)
    external;

enum PostOpMode {
    opSucceeded, // user op succeeded
    opReverted, // user op reverted. still has to pay for gas.
    postOpReverted // user op succeeded, but caused postOp to revert
}

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To prevent attacks involving malicious UserOperation objects listing other users' wallets as their paymasters, the entry point contract must require a paymaster to call the entry point to lock their stake and thereby consent to being a paymaster. Unlocking stake must have a delay. The extended interface for the entry point, adding functions for paymasters to add and withdraw stake, is:

function addStake() external payable
function lockStake() external
function unlockStake(address paymaster) external
function withdrawStake(address payable withdrawAddress) external

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# Client behavior upon receiving a UserOperation

When a client receives a UserOperation, it must first run some basic sanity checks, namely that:

Either the sender is an existing contract, or the initCode is not empty (but not both)
The verificationGas is sufficiently low (<= MAX_VERIFICATION_GAS) and the preVerificationGas is sufficiently high (enough to pay for the calldata gas cost of serializing the UserOperation plus PRE_VERIFICATION_OVERHEAD_GAS)
The paymaster is either the zero address or is a contract which (i) currently has nonempty code on chain, (ii) has registered and has sufficient stake, and (iii) has not been banned
The callgas is at least the cost of a CALL with non-zero value.
The maxFeePerGas and maxPriorityFeePerGas are above a configurable minimum value that the client is willing to accept
The UserOperation object is not already present in the pool (or it replaces an existing entry with the same sender, nonce with a higher priority)

If the UserOperation object passes these sanity checks, the client must next simulate the op, and if the simulation succeeds, the client must add the op to the pool. Simulation must also happen during bundling to make sure that the storage accessed is the same as the accessList that was saved during the initial simulation.

# Simulation

To simulate a UserOperation op, the client makes an eth_call with the following params:

{
    "from": 0x0000000000000000000000000000000000000000,
    "to": [entry point address],
    "input": [simulateWalletValidation header] + serialize(op),
    "gas": op.verificationGas,
}

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If the call returns an error, the client rejects the op. If it succeeds, the output is interpreted as an integer gasUsedByPayForSelfOp. If op.paymaster != ZERO_ADDRESS, the client must then make an eth_call with the following params:

{
    "from": 0x0000000000000000000000000000000000000000,
    "to": [entry point address],
    "input": [simulatePaymasterValidation header] +
             abi_encode(serialize(op), gasUsedByPayForSelfOp),
    "gas": op.verificationGas - gasUsedByPayForSelfOp,
}

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While simulating, the client should make sure that:

Neither call's execution trace invokes any forbidden opcodes
The first call does not access mutable state of any contract except the wallet itself
The second call does not access mutable state of any contract except the paymaster itself

If a contract (i) does not contain the SELFDESTRUCT or DELEGATECALL opcode [except perhaps inside pushdata] and (ii) has nonempty code, mutable state of that contract is defined as storage slots (accessed with SLOAD or SSTORE). If a contract does not satisfy either of the above conditions, mutable state of that contract includes code and storage. Note that balance can not be read in any case because of the forbidden opcode restriction. Writing balance (via value-bearing calls) to any address is not restricted.

In practice, restrictions (2) and (3) basically mean that the only external accesses that the wallet and the paymaster can make are reading code of other contracts if their code is guaranteed to be immutable (eg. this is useful for calling or delegatecalling to libraries).

If any of the three conditions is violated, the client should reject the op. If both calls succeed (or, if op.paymaster == ZERO_ADDRESS and the first call succeeds) without violating the three conditions, the client should accept the op. On a bundler node, the storage keys accessed by both calls must be saved as the accessList of the UserOperation

# Forbidden opcodes

The forbidden opcodes are to be forbidden when depth > 2 (i.e. when it is the wallet, paymaster, or other contracts called by them that are being executed). They are: GASPRICE, GASLIMIT, DIFFICULTY, TIMESTAMP, BASEFEE, BLOCKHASH, NUMBER, SELFBALANCE, BALANCE, ORIGIN, GAS. They should only be forbidden during verification, not execution. These opcodes are forbidden because their outputs may differ between simulation and execution, so simulation of calls using these opcodes does not reliably tell what would happen if these calls are later done on-chain.

# Reputation scoring and throttling/banning for paymasters

Clients maintain two mappings with a value for each paymaster:

opsSeen: Map[Address, int]
opsIncluded: Map[Address, int]

When the client learns of a new paymaster, it sets opsSeen[paymaster] = 0 and opsIncluded[paymaster] = 0 .

The client sets opsSeen[paymaster] +=1 each time it adds an op with that paymaster to the UserOperationPool, and the client sets opsIncluded[paymaster] += 1 each time an op that was in the UserOperationPool is included on-chain.

Every hour, the client sets opsSeen[paymaster] -= opsSeen[paymaster] // 24 and opsIncluded[paymaster] -= opsIncluded[paymaster] // 24 for all paymasters (so both values are 24-hour exponential moving averages).

We define the status of a paymaster as follows:

OK, THROTTLED, BANNED = 0, 1, 2

def status(paymaster: Address,
           opsSeen: Map[Address, int],
           opsIncluded: Map[Address, int]):
    if paymaster not in opsSeen:
        return OK
    min_expected_included = opsSeen[paymaster] // MIN_INCLUSION_RATE_DENOMINATOR
    if min_expected_included + THROTTLING_SLACK >= opsIncluded[paymaster]:
        return OK
    elif min_expected_included + BAN_SLACK >= opsIncluded[paymaster]:
        return THROTTLED
    else:
        return BANNED

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Stated in simpler terms, we expect at least 1 / MIN_INCLUSION_RATE_DENOMINATOR of all ops seen on the network to get included. If a paymaster falls too far behind this minimum, the paymaster gets throttled (meaning, the client does not accept ops from that paymaster if there is already an op from that paymaster, and an op only stays in the pool for 10 blocks), If the paymaster falls even further behind, it gets banned. Throttling and banning naturally reverse over time because of the exponential-moving-average rule.

Non-bundling clients and bundlers should use different settings for the above params:

Param	Client setting	Bundler setting
`MIN_INCLUSION_RATE_DENOMINATOR`	100	10
`THROTTLING_SLACK`	10	10
`BAN_SLACK`	50	50

To help make sense of these params, note that a malicious paymaster can at most cause the network (only the p2p network, not the blockchain) to process BAN_SLACK * MIN_INCLUSION_RATE_DENOMINATOR / 24 non-paying ops per hour.

# RPC methods

eth_sendUserOperation

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "eth_sendUserOperation",
  "params": [
    {
      sender, // Address
      nonce, // uint256
      initCode, // bytes
      callData, //bytes
      callGas, // uint256
      verificationGas, // uint256
      maxFeePerGas, // uint256
      maxPriorityFeePerGas, // uint256
      paymaster, // address
      paymasterData, // bytes
      signature // bytes
    }
  ]
}

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eth_callUserOperation

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "eth_callUserOperation",
  "params": [
    {
      sender, // Address
      nonce, // uint256
      initCode, // bytes
      callData, //bytes
      callGas, // uint256
      verificationGas, // uint256
      maxFeePerGas, // uint256
      maxPriorityFeePerGas, // uint256
      paymaster, // address
      paymasterData, // bytes
      signature, // bytes
      blockNumber, // hex-encoded uint256
    }
  ]
}

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# Rationale

The main challenge with a purely smart contract wallet based account abstraction system is DoS safety: how can a miner including an operation make sure that will actually pay fees, without having to first execute the entire operation? Requiring the miner to execute the entire operation opens a DoS attack vector, as an attacker could easily send many operations that pretend to pay a fee but then revert at the last moment after a long execution. Similarly, to prevent attackers from cheaply clogging the mempool, nodes in the P2P network need to check if a operation will pay a fee before they are willing to forward it.

In this proposal, we expect wallets to have a validateUserOp method that takes as input a UserOperation, and verify the signature and pay the fee. This method is required to be almost-pure: it is only allowed to access the storage of the wallet itself, cannot use environment opcodes (eg. TIMESTAMP), and can only edit the storage of the wallet, and can also send out ETH (needed to pay the entry point). The method is gas-limited by the verificationGas of the UserOperation; nodes can choose to reject operations whose verificationGas is too high. These restrictions allow miners and network nodes to simulate the verification step locally, and be confident that the result will match the result when the operation actually gets included into a block.

The entry point-based approach allows for a clean separation between verification and execution, and keeps wallets' logic simple. The alternative would be to require wallets to follow a template where they first self-call to verify and then self-call to execute (so that the execution is sandboxed and cannot cause the fee payment to revert); template-based approaches were rejected due to being harder to implement, as existing code compilation and verification tooling is not designed around template verification.

# Paymasters

Paymasters facilitate transaction sponsorship, allowing third-party-designed mechanisms to pay for transactions. Many of these mechanisms could be done by having the paymaster wrap a UserOperation with their own, but there are some important fundamental limitations to that approach:

No possibility for "passive" paymasters (eg. that accept fees in some ERC-20 token at an exchange rate pulled from an on-chain DEX)
Paymasters run the risk of getting griefed, as users could send ops that appear to pay the paymaster but then change their behavior after a block

The paymaster scheme allows a contract to passively pay on users' behalf under arbitrary conditions. It even allows ERC-20 token paymasters to secure a guarantee that they would only need to pay if the user pays them: the paymaster contract can check that there is sufficient approved ERC-20 balance in the validatePaymasterUserOp method, and then extract it with transferFrom in the postOp call; if the op itself transfers out or de-approves too much of the ERC-20s, the inner postOp will fail and revert the execution and the outer postOp can extract payment (note that because of storage access restrictions the ERC-20 would need to be a wrapper defined within the paymaster itself).

# First-time wallet creation

It is an important design goal of this proposal to replicate the key property of EOAs that users do not need to perform some custom action or rely on an existing user to create their wallet; they can simply generate an address locally and immediately start accepting funds.

This is accomplished by having the entry point itself create wallets using CREATE2. The UserOperation struct has an initCode field; this field would be empty for all operations by a given wallet after the first, but the first operation would fill in the initCode. The entry point uses EIP-2470 deployer contract to create the wallet, and then performs the operation. The user can compute the address of their wallet by locally running the EIP 1014 CREATE2 address formula. The salt used is the nonce of the UserOperation. (The entry point contract has a utility method getSenderAddress() for that purpose)

# Entry point upgrading

Wallets are encouraged to be DELEGATECALL forwarding contracts for gas efficiency and to allow wallet upgradability. The wallet code is expected to hard-code the entry point into their code for gas efficiency. If a new entry point is introduced, whether to add new functionality, improve gas efficiency, or fix a critical security bug, users can self-call to replace their wallet's code address with a new code address containing code that points to a new entry point. During an upgrade process, it's expected that two mempools will run in parallel.

# Backwards Compatibility

This ERC does not change the consensus layer, so there are no backwards compatibility issues for Ethereum as a whole. Unfortunately it is not easily compatible with pre-ERC-4337 wallets, because those wallets do not have a validateUserOp function. If the wallet has a function for authorizing a trusted op submitter, then this could be fixed by creating an ERC-4337-compatible wallet that re-implements the verification logic as a wrapper and setting it to be the original wallet's trusted op submitter.

# Reference Implementation

See https://github.com/opengsn/account-abstraction/tree/main/contracts

# Security considerations

The entry point contract will need to be very heavily audited and formally verified, because it will serve as a central trust point for all ERC 4337 wallets. In total, this architecture reduces auditing and formal verification load for the ecosystem, because the amount of work that individual wallets have to do becomes much smaller (they need only verify the validateUserOp function and its "check signature, increment nonce and pay fees" logic) and check that other functions are msg.sender == ENTRY_POINT gated (perhaps also allowing msg.sender == self), but it is nevertheless the case that this is done precisely by concentrating security risk in the entry point contract that needs to be verified to be very robust.

Verification would need to cover two primary claims (not including claims needed to protect paymasters, and claims needed to establish p2p-level DoS resistance):

Safety against arbitrary hijacking: The entry point only calls a wallet generically if validateUserOp to that specific wallet has passed (and with op.calldata equal to the generic call's calldata)
Safety against fee draining: If the entry point calls validateUserOp and passes, it also must make the generic call with calldata equal to op.calldata