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⚠️ Draft Standards Track: ERC

ERC-5189: Account Abstraction via Endorsed Operations

An account abstraction proposal that avoids protocol changes while maintaining compatibility with existing smart contract wallets.

Authors Agustín Aguilar (@agusx1211), Philippe Castonguay (@phabc), Michael Standen (@ScreamingHawk)
Created 2022-06-29
Discussion Link https://ethereum-magicians.org/t/erc-account-abstraction-via-endorsed-operations/9799

Abstract

This ERC proposes a form of account abstraction (AA) that ensures compatibility with existing smart contract wallets and provides flexibility for alternative designs while avoiding introducing changes to the consensus layer. Instead of defining a strict structure for AA transactions, this proposal introduces the figure of endorser contracts. These smart contract instances are tasked with determining the quality of the submitted AA transactions, thus safely helping bundlers determine if a transaction should be kept in the mempool or not. Developers that intend to make their smart contract wallet compatible with this ERC must create and deploy an instance of an endorser or use an existing one compatible with their wallet.

Motivation

This account abstraction proposal aims to implement a generalized system for executing AA transactions while maintaining the following goals:

  • Achieve the primary goal of account abstraction: allow users to use smart contract wallets containing arbitrary verification and execution logic instead of EOAs as their primary account.
  • Decentralization:
    • Allow any bundler to participate in the process of including AA transactions.
    • Work with all activity happening over a public mempool without having to concentrate transactions on centralized relayers.
    • Define structures that help maintain a healthy mempool without risking its participants from getting flooded with invalid or malicious payloads.
    • Avoid trust assumptions between bundlers, developers, and wallets.
  • Support existing smart contract wallet implementations: Work with all the smart contract wallets already deployed and active while avoiding forcing each wallet instance to be manually upgraded.
  • Provide an unrestrictive framework: Smart contract wallets are very different in design, limitations, and capabilities from one another; the proposal is designed to accommodate almost all possible variations.
  • No overhead: Smart contract wallets already have a cost overhead compared to EOA alternatives, the proposal does not worsen the current situation.
  • Support other use cases:
    • Privacy-preserving applications.
    • Atomic multi-operations (similar to EIP-3074).
    • Payment of transaction fees using ERC-20 tokens.
    • Scheduled execution of smart contracts without any user input.
    • Applications that require a generalistic relayer.

Specification

To avoid Ethereum consensus changes, we do not attempt to create new transaction types for account-abstracted transactions. Instead, AA transactions are packed up in a struct called Operation, operations are structs composed by the following fields:

Field Type Description
entrypoint address Contract address that must be called with callData to execute the operation.
callData bytes Data that must be passed to the entrypoint call to execute the operation.
gasLimit uint64 Minimum gasLimit that must be passed when executing the operation.
feeToken address Contract address of the ERC-20 token used to repay the bundler. (address(0) for the native token).
endorser address Address of the endorser contract that should be used to validate the operation.
endorserCallData bytes Additional data that must be passed to the endorser when calling isOperationReady().
endorserGasLimit uint64 Amount of gas that should be passed to the endorser when validating the operation.
maxFeePerGas uint256 Max amount of basefee that the operation execution is expected to pay. (Similar to EIP-1559 max_fee_per_gas).
priorityFeePerGas uint256 Fixed amount of fees that the operation execution is expected to pay to the bundler. (Similar to EIP-1559 max_priority_fee_per_gas).
baseFeeScalingFactor uint256 Scaling factor to convert block.basefee into the feeToken unit.
baseFeeNormalizationFactor uint256 Normalization factor to convert block.basefee into the feeToken unit.
hasUntrustedContext bool If true, the operation may have untrusted code paths. These should be treated differently by the bundler (see untrusted environment).
chainId uint256 Chain ID of the network where the operation is intended to be executed.

These Operation objects can be sent to a dedicated operations mempool. A specialized class of actors called bundlers (either block producers running special-purpose code, or just users that can relay transactions to block producers) listen for operations on the mempool and execute these transactions.

Transactions are executed by calling the entrypoint with the provided callData. The entrypoint can be any contract, but most commonly it will be the wallet contract itself. Alternatively it can be an intermediary utility that deploys the wallet and then performs the transaction.

Endorser functionality

Mempool participants need to be able to able to filter “good operations” (operations that pay the bundler the defined fee) from “bad operations” (operations that either miss payment or revert altogether).

This categorization is facilitated by the endorser; the endorser must be a deployed smart contract that implements the following interface:

interface Endorser {
  struct GlobalDependency {
    bool baseFee;
    bool blobBaseFee;
    bool chainId;
    bool coinBase;
    bool difficulty;
    bool gasLimit;
    bool number;
    bool timestamp;
    bool txOrigin;
    bool txGasPrice;
    uint256 maxBlockNumber;
    uint256 maxBlockTimestamp;
  }

  struct Constraint {
    bytes32 slot;
    bytes32 minValue;
    bytes32 maxValue;
  }

  struct Dependency {
    address addr;
    bool balance;
    bool code;
    bool nonce;
    bool allSlots;
    bytes32[] slots;
    Constraint[] constraints;
  }

  function isOperationReady(
    address _entrypoint,
    bytes calldata _data,
    bytes calldata _endorserCallData,
    uint256 _gasLimit,
    uint256 _maxFeePerGas,
    uint256 _maxPriorityFeePerGas,
    address _feeToken,
    uint256 _baseFeeScalingFactor,
    uint256 _baseFeeNormalizationFactor,
    bool _hasUntrustedContext
  ) external returns (
    bool readiness,
    GlobalDependency memory globalDependency,
    Dependency[] memory dependencies
  );
}

Endorsers SHOULD be registered in the EndorserRegistry with an amount of burned ETH. The amount of ETH to be burned is not specified in this proposal as mempool operators are free to set their own minimum thresholds. Mempool operators MAY accept operations from endorsers without any burned ETH, but they would increase their risk exposing themselves to denial of service attacks. Mempool operators MAY publish the minimum amount of burned ETH required for each endorser.

When the isOperationReady method is called, the endorser must return this information:

  • readiness: when returning true, it means the transaction MUST be executed correctly and the bundler MUST be paid the offered gas fees (even if the underlying intent of the operation fails).
  • globalDependency: a list of possible dependencies that don’t belong to a given address, defines if the execution of the transaction MAY be invalidated by a change on one of these global variables. maxBlockNumber and maxBlockTimestamp are used as global constraints.
  • dependencies: a comprehensive list of addresses and storage slots that must be monitored; any state change in these dependencies MUST trigger a re-evaluation of the operation’s readiness.

The information provided by the endorser helps the mempool operator maintain a pool of “good” AA transactions that behave correctly; but it only provides a soft guarantee that the transaction will be executed correctly. Bundlers must always simulate the result of the execution before including a transaction in a block.

If the result of a simulation fails and the endorser still returns readiness == true with the same dependencies, then the endorser can not be trusted and it MUST be banned by the mempool operator.

The dependency list serves as a shortcut for the bundler to know which operations are fully independent from each other. This shortcut is useful for (a) clearing the mempool from operations that are no longer valid, and (b) for bundlers to know which operations can be included in the same block.

For efficiency, additional information MAY be provided to the endorser with _endorserCallData. If used, the endorser MUST validate that the provided _endorserCallData is valid and relevant to the other values provided.

While the isOperationReady call is not a view function, calls to the endorser MUST NOT be submitted on chain. Calls to isOperationReady MUST only be made off-chain by the bundler.

Global Dependencies

Field Type Description
baseFee bool true if the block.basefee should be considered a dependency.
blobBaseFee bool true if the block.blockbasefee should be considered a dependency.
chainId bool true if the block.chainid should be considered a dependency.
coinbase bool true if the block.coinbase should be considered a dependency.
difficulty bool true if the block.difficulty should be considered a dependency.
gasLimit bool true if the block.gaslimit should be considered a dependency.
number bool true if the block.number should be considered a dependency.
timestamp bool true if the block.timestamp should be considered a dependency.
txOrigin bool true if the tx.origin should be considered a dependency.
txGasPrice bool true if the tx.gasprice should be considered a dependency.
maxBlockNumber uint256 The maximum value of block.number that readiness applies to.
maxBlockTimestamp uint256 The maximum value of block.timestamp that readiness applies to.

The endorser MAY use the maxNumber and maxTimestamp fields to limit the validity of the readiness result. This is useful for operations that are only valid for a certain period of time.

Note that all values are inclusive. If the endorser determines the validity of the operation is indefinite, the maxNumber and maxTimestamp fields should be set to type(uint256).max.

Dependencies

Field Type Description
addr address Contract address of the dependencies entry. (Only one entry per address should be allowed).
balance bool true if the balance of addr should be considered a dependency of the operation.
code bool true if the code of addr should be considered a dependency of the operation.
nonce bool true if the nonce of addr should be considered a dependency of the operation.
allSlots bool true if all storage slots of addr should be considered a dependency of the operation.
slots bytes32[] List of all storage slots of addr that should be considered dependencies of operation.
constraints Constraint[] List of storage slots of addr that have a range of specific values as dependencies.

The endorser does not need to include all accessed storage slots on the dependencies list, it only needs to include storage slots that after a change may also result in a change of readiness.

Note that allSlots, constraints and slots are mutually exclusive. If allSlots is set to true, then constraints and slots MUST be an empty array. If a slot is listed in constraints, it must not be listed in slots. The endorser should prefer to use constraints over slots, and slots over allSlots whenever possible.

E.g. A wallet may pay fees using funds stored as WETH. During isOperationReady(), the endorser contract may call the balanceOf method of the WETH contract to determine if the wallet has enough WETH balance. Even though the ETH balance of the WETH contract and the code of the WETH contract are being accessed, the endorser only cares about the user’s WETH balance for this operation and hence does not include these as dependencies.

Constraints

Field Type Description
slot bytes32 Storage slot of addr that has a range of specific values as dependencies.
minValue bytes32 Minimum value (inclusive) of slot that readiness applies to.
maxValue bytes32 Maximum value (inclusive) of slot that readiness applies to.

The endorser can use the minValue and maxValue fields to limit the validity of the readiness result. This allows the endorser to fully validate an operation, even when this operation depends on storage values that are not directly accessible by the endorser.

When an exact value is required, minValue and maxValue should be set to the same value.

Misbehavior detection

The endorser contracts may behave maliciously or erratically in the following ways:

  • (1) It may consider an operation ready, but when the operation is executed it transfers less than the agreed-upon fees to the bundler.
  • (2) It may consider an operation ready, but when the operation is executed the top-level call fails.
  • (3) It may change the status from ready to not-ready while none of the dependencies register any change.

The bundler must always discard and re-evaluate the readiness status after a change on any of the dependencies of the operation, meaning that only operations considered ready are candidates for constructing the next block.

If, when simulating the final inclusion of the operation, the bundler discovers that it does not result in correct payment (either because the transaction fails, or transferred amount is below the defined fee), then it should proceed to ban the endorser for one of the following reasons:

  1. The endorser returns isOperationReady() == true even though the operation is not healthy to be included in a block.
  2. The operation changed readiness status from true to false while all dependencies remained unchanged.

After an endorser is banned, the mempool operator should drop all operations related to such endorser.

Untrusted environment

In some scenarios, the endorser may not be able to fully validate the operation but may be able to infer that a given code path should be safe. In these cases, the endorser can mark a section of the operation as untrusted. Any storage slots (balance, code, nonce, or specific slots) accessed in this untrusted context should be automatically considered as dependencies.

interface Endorser {
  event UntrustedStarted();
  event UntrustedEnded();
}

The endorser can use the UntrustedStarted and UntrustedEnded events to signal the start and end of an untrusted context. The bundler should listen to these events and extend the dependencies list accordingly.

Only the top-level endorser can signal an untrusted context; any other events with the same signature but emitted by a different contract should be ignored.

If multiple events are emitted, the bundler should count the number of UntrustedStarted and UntrustedEnded events and only consider the untrusted context as ended when the number of UntrustedEnded events is equal to the number of UntrustedStarted events.

Untrusted contexts can be opened and closed multiple times and can be nested.

If hasUntrustedContext is set to false, the bundler should ignore any UntrustedStarted and UntrustedEnded events.

Automatic dependency graph construction

All code executed within the untrusted environment must be monitored. If the code executes any of the following opcodes, the dependency graph must be extended accordingly.

Opcode Dependency
BALANCE dependencies[addr].balance = true
ORIGIN global.txOrigin = true
CODESIZE None
CODECOPY None
GASPRICE global.txGasPrice = true
EXTCODESIZE dependencies[addr].code = true
EXTCODECOPY dependencies[addr].code = true
EXTCODEHASH dependencies[addr].code = true
COINBASE global.coinbase = true
TIMESTAMP global.timestamp = true
NUMBER global.number = true
DIFFICULTY global.difficulty = true
PREVRANDAO global.difficulty = true
CHAINID global.chainid = true
SELFBALANCE dependencies[self].balance = true
BASEFEE global.basefee = true
SLOAD dependencies[addr].slots[slot] = true
CREATE dependencies[addr].nonce = true
CREATE2 dependencies[contract].code = true

Notice that untrusted environments generate a lot of dependencies and may generate many false positives. This may lead to numerous re-evaluations and thus to the operation being dropped from the mempool.

Block-level dependencies are specially sensitive as they will be shared with a large number of operations.

It is recommended to use untrusted environments only when necessary, like when an endorser needs to validate a nested signature to a wallet that is not under its control.

Fee payment

The operation is expected to pay at least gasUsed * effectiveGasPrice to tx.origin, the gasUsed includes both the execution cost of the operation and the cost of its calldata.

The payment is always made in the feeToken.

The maxFeePerGas and priorityFeePerGas are expressed in the unit of the feeToken.

The effective gas price is calculated as follows:

effectiveGasPrice = Min(maxFeePerGas, ((block.baseFee * _baseFeeScalingFactor) / _baseFeeNormalizationFactor) + priorityFeePerGas)

block.baseFee is the base fee of the block in the native token unit, and _baseFeeScalingFactor and _baseFeeNormalizationFactor are used to convert the base fee into the feeToken unit.

Operation identification

Operations can be identified by their operation hash, which is calculated as a CIDv1 multihash of a raw file, containing the canonical JSON representation of the operation. This hash is never used on-chain, but it serves as a unique pointer to the operation that can be shared across systems.

The operation MAY be pinned on the IPFS network; this would allow other participants to retrieve the content of the operation after the operation has been removed from the mempool. This pinning is not mandatory, and it may be performed by the mempool operator or by the wallet itself if visibility of the operation is desired.

Bundler behavior upon receiving an operation

Bundlers can add their own rules for how to ensure the successful relaying of AA transactions and for getting paid for relaying these transactions. However, we propose here a baseline specification that should be sufficient.

When a bundler receives an operation, it SHOULD perform these sanity checks:

  • The endorserGasLimit is sufficiently low (<= MAX_ENDORSER_GAS).
  • The endorser (i) is registered and has enough burn (>= MIN_ENDORSER_BURN), and (ii) it has not been internally flagged as banned.
  • The gasLimit is at least the cost of a CALL with a non-zero value.
  • The feeToken is address(0) or a known ERC-20 token that the bundler is willing to accept.
  • The maxFeePerGas and priorityPerGas are above a configurable minimum value the bundler is willing to accept.
  • If another operation exists in the mempool with the exact same dependency set AND the same endorser address, the maxFeePerGas and priorityFeePerGas of the newly received operation MUST be 12% higher than the one on the mempool to replace it. (Similar with how EOA with same nonce work)

If the operation passes these checks, then the bundler MUST call isOperationReady() on the endorser. If the endorser considers the operation ready, and the constraints are within bounds, then the client MUST add the operation to the mempool. Otherwise, the operation MUST be dropped.

The endorser result SHOULD be invalidated and its readiness SHOULD be re-evaluated if any of the values of the provided dependencies change. If the operation readiness changes to false, the operation MUST be discarded.

Before including the operation in a block, a last simulation MUST be performed, this time without calling the endorser, but by constructing the block and probing the result. All transactions in the block listed before the operation must be simulated and the endorser must be queried again there for readiness in-case some dependencies changed.

If the operation fails during simulation, the endorser must be banned because (i) it returned a bad readiness state or (ii) it changed the operation readiness independently from the dependencies.

Additional events that must invalidate the readiness are:

  • A transaction or operation modifies the same storage slots (as the dependencies) is queued before the given operation.

Optional rules

Mempool clients could implement additional rules to further protect against maliciously constructed transactions.

  • Limit the size of accepted dependencies to MAX_OPERATION_DEPENDENCIES, dropping operations that cross the boundary.
  • Limit the number of times an operation may trigger a re-evaluation to MAX_OPERATION_REEVALS, dropping operations that cross the boundary.
  • Limit the number of operations in the mempool that depend on the same dependency slots.

If these rules are widely adopted, wallet developers should keep usage of dependencies to the lowest possible levels.

Evaluation

To evaluate an operation, the bundler must call the isOperationReady() method, with a gasLimit above or equal to endorserGasLimit.

If the call fails, or the endorser returns readiness == false, then the operation must be dropped from the mempool.

If the call succeeds and returns readiness == true, then the operation can be kept in the mempool and used when constructing a block.

After operation inclusion

There is no limit in-place that defines that an operation can only be executed once.

The bundler MUST NOT drop an operation after successfully including such operation in a block, the operation must remain in the mempool and a last isOperationReady() call must be performed.

If the endorser still returns readiness == true (after inclusion) then the operation SHOULD be treated as any other healthy operation, and thus it MAY be kept in the mempool.

Endorser registry

The endorser registry serves as a place to register the burn of each endorser, anyone can increase the burn of any endorser by calling the addBurn() function.

All burn is effectively locked forever; slashing can’t be reliably proved on-chain without protocol alterations, so it remains a virtual event on which mempool operators will ignore the deposited ETH.

Implementation

(EXAMPLE)

// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.15;

contract EndorserRegistry {
  event Burned(
      address indexed _endorser,
      address indexed _sender,
      uint256 _new,
      uint256 _total
  );

  mapping(address => uint256) public burn;

  function addBurn(address _endorser) external payable returns (uint256) {
    uint256 total = burn[_endorser] + msg.value;
    burn[_endorser] = total;

    emit Burned(_endorser, msg.sender, msg.value, total);

    return total;
  }
}

Rationale

Griefing protection

The main challenge with a purely smart contract wallet-based account abstraction system is DoS safety: how can a bundler that includes an operation make sure it will be paid without executing the entire operation?

Bundlers could execute the entire operation to determine if it is healthy or not, but this operation may be expensive and complex for the following reasons:

  • The bundler does not have a way to simulate the transaction with a reduced amount of gas; it has to use the whole gasLimit, exposing itself to a higher level of griefing.
  • The bundler does not have a way to know if a change to the state will affect the operation or not, and thus it has to re-evaluate the operation after every single change.
  • The bundler does not have a way to know if a change to the state will invalidate a large portion of the mempool.

In this proposal, we add the endorser as a tool for the bundlers to validate arbitrary operations in a controlled manner, without the bundler having to know any of the inner workings of such operation.

In effect, we move the responsibility from the wallet to the wallet developer; the developer must code, deploy and burn ETH for the endorser; this is a nearly ideal scenario because developers know how their wallet operations work, and thus they can build tools to evaluate these operations efficiently.

Additionally, the specification is kept as simple as possible as enforcing a highly structured behavior and schema for smart contract wallet transactions may stagnate the adoption of more innovative types of wallets and the adoption of a shared standard among them.

Burned ETH

Anyone can deploy a endorser contract and wallet clients are the one providing which endorser contract should be used for the given transaction. Instead of having each bundler rely on an off-chain registry that they need to maintain, the endorser registry can be called to see if the requested endorser contract is present and how much ETH was burned for it. Bundlers can then decide a minimum treshshold for how much ETH burnt is required for an endorser contract to be accepted. Bundlers are also free to support endorsers contract that are not part of the registry or are part of it but have no ETH burned associated.

Minimum overhead

Since the validation of an AA transactions is done off-chain by the bundler rather than at execution time, there is no additional gas fee overhead for executing transactions. The bundler bears the risk rather than all users having to pay for that security.

Differences with alternative proposals

  1. This proposal does not require monitoring for forbidden opcodes or storage access boundaries. Wallets have complete freedom to use any EVM capabilities during validation and execution.
  2. This proposal does not specify any replay protection logic since all existing smart contract wallets already have their own, and designs can vary among them. Nonces can be communicated to the bundler using a dependency.
  3. This proposal does not specify a pre-deployment logic because it can be handled directly by the entrypoint.
  4. This proposal does not require wallets to accept execution transactions from a trusted entrypoint contract, reducing overhead and allowing existing wallets to be compatible with the proposal.
  5. This proposal does not distinguish between execution and signature payloads, this distinction remains implementation-specific.

Backwards Compatibility

This ERC does not change he consensus layer, nor does impose changes on existing smart contract wallets, so there are no backwards compatibility issues.

Security Considerations

This ERC does not make changes to on-chain interactions. Endorsers are explicitly for off-chain validations.

Bundlers are responsible for managing their own security and for ensuring that they are paid for the transactions they include in blocks.

Copyright and related rights waived via CC0.

Citation

Please cite this document as:

Agustín Aguilar (@agusx1211), Philippe Castonguay (@phabc), Michael Standen (@ScreamingHawk), "ERC-5189: Account Abstraction via Endorsed Operations [DRAFT]," Ethereum Improvement Proposals, no. 5189, June 2022. [Online serial]. Available: https://eips.ethereum.org/EIPS/eip-5189.