ERC-7521: General Intents for Smart Contract Wallets

A generalized intent specification for smart contract wallets, allowing authorization of current and future intent structures at sign time

Metadata

Status: DraftStandards Track: ERCCreated: 2023-09-19

Authors

Stephen Monn (@pixelcircuits), Bikem Bengisu (@supiket)

Links

GitHub page Discussion on Ethereum Magicians

Abstract

A generalized intent specification entry point contract which enables support for a multitude of intent standards as they evolve over time. Instead of smart contract wallets having to constantly upgrade to provide support for new intent standards as they pop up, a single entry point contract is trusted to handle signature verification which then passes off the low level intent data handling and defining to other contracts specified by users at intent sign time. These signed messages, called a UserIntent, are gossipped around any host of mempool strategies for MEV searchers to look through and combine with their own UserIntent into an object called an IntentSolution. MEV searchers then package up an IntentSolution object they build into a transaction making a handleIntents call to a special contract. This transaction then goes through the typical MEV channels to eventually be included in a block.

Motivation

See also "ERC-4337: Account Abstraction via Entry Point Contract specification" and the links therein for historical work and motivation.

This proposal uses the same entry point contract idea to enable a single interface which smart contract wallets can support now to unlock future-proof access to an evolving intent landscape. It seeks to achieve the following goals:

Achieve the key goal of enabling intents for users: allow users to use smart contract wallets containing arbitrary verification logic to specify intent execution as described and handled by various other intent standard contracts.
Decentralization
- Allow any MEV searcher to participate in the process of solving signed intents
- Allow any developer to add their own intent standard definitions for users to opt-in to at sign time
Be forward thinking for future intent standard compatibility: Define an intent standard interface that gives future intent standard defining contracts access to as much information about the current handleIntents execution context as possible.
Keep gas costs down to a minimum: Include key intent handling logic, like intent segment execution order, into the entry point contract itself in order to optimize gas efficiency for the most common use cases.
Enable good user experience
- Avoid the need for smart contract wallet upgrades when a user wants to use a newly developed intent standard.
- Enable complex intent composition that only needs a single signature.

Specification

Users package up intents they want their wallet to participate in, in an ABI-encoded struct called a UserIntent:

Field	Type	Description
`sender`	`address`	The wallet making the intent
`intentData`	`bytes[]`	Data defined by the intent standard broken down into multiple segments for execution
`signature`	`bytes`	Data passed into the wallet along with the nonce during the verification step

The intentData parameter is an array of arbitrary bytes whose use is defined by an intent standard. Each item in this array is referred to as an intent segment. The first 32 bytes of each segment is used to specify the intent standard ID to which the segment data belongs. Users send UserIntent objects to any mempool strategy that works best for the intent standards being used. A specialized class of MEV searchers called solvers look for these intents and ways that they can be combined with other intents (including their own) to create an ABI-encoded struct called an IntentSolution:

Field	Type	Description
`timestamp`	`uint256`	The time at which intents should be evaluated
`intents`	`UserIntent[]`	List of intents to execute
`order`	`uint256[]`	Order of execution for the included intents

The solver then creates a solution transaction, which packages up an IntentSolution object into a single handleIntents call to a pre-published global entry point contract.

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

The core interface required for an intent standard to have is:

The core interface required for a wallet to have is:

Required entry point contract functionality

The entry point's handleIntents function must perform the following steps. It must make two loops, the verification loop and the execution loop.

In the verification loop, the handleIntents call must perform the following steps for each UserIntent:

Validate timestamp value on the IntentSolution by making sure it is within an acceptable range of block.timestamp or some time before it.
Call validateUserIntent on the wallet, passing in the UserIntent and the hash of the intent. The wallet should verify the intent's signature. If any validateUserIntent call fails, handleIntents must skip execution of at least that intent, and may revert entirely.

In the execution loop, the handleIntents call must perform the following steps for all segments on the intentData bytes array parameter on each UserIntent:

Call executeUserIntent on the intent standard, specified by the first 32 bytes of the intentData (the intent standard ID). This call passes in the entire IntentSolution as well as the current executionIndex (the number of times this function has already been called for any standard or intent before this), segmentIndex (index in the intentData array to execute for) and context data. The executeUserIntent function returns arbitrary bytes per intent which must be remembered and passed into the next executeUserIntent call for the same intent.

It's up to the intent standard to choose how to parse the intentData segment bytes and utilize the context data blob that persists across intent execution.

The order of execution for UserIntent segments in the intentData array always follows the same order defined on the intentData parameter. However, the order of execution for segments between UserIntent objects can be specified by the order parameter of the IntentSolution object. For example, an order array of [1,1,0,1] would result in the second intent being executed twice (segments 1 and 2 on intent 2), then the first intent would be executed (segment 1 on intent 1), followed by the second intent being executed a third time (segment 3 on intent 2). If no ordering is specified in the solution, or all segments have not been processed for all intents after getting to the end of the order array, a default ordering will be used. This default ordering loops from the first intent to the last as many times as necessary until all intents have had all their segments executed. If the ordering calls for an intent to be executed after it's already been executed for all its segments, then the executeUserIntent call is simply skipped and execution across all intents continues.

Before accepting a UserIntent, solvers must use an RPC method to locally call the validateIntent function of the entry point, which verifies that the signature and data formatting is correct; see the Intent validation section below for details.

Registering new entry point intent standards

The entry point's registerIntentStandard function must allow for permissionless registration of new intent standard contracts. During the registration process, the entry point contract must verify the contract is meant to be registered by calling the isIntentStandardForEntryPoint function on the intent standard contract. This function passes in the entry point contract address which the intent standard can then verify and return true or false. If the intent standard contract returns true, then the entry point registers it and gives it a standard ID which is unique to the intent standard contract, entry point contract and chain ID.

Intent standard behavior executing an intent

The intent standard's executeUserIntent function is given access to a wide set of data, including the entire IntentSolution in order to allow it to be able to implement any kind of logic that may be seen as useful in the future. Each intent standard contract is expected to parse the UserIntent objects intentData parameter and use that to validate any constraints or perform any actions relevant to the standard. Intent standards can also take advantage of the context data it can return at the end of the executeUserIntent function. This data is kept by the entry point and passed in as a parameter to the executeUserIntent function the next time it is called for an event. This gives intent standards access to a persistent data store as other intents are executed in between others. One example of a use case for this is an intent standard that is looking for a change in state during intent execution (like releasing tokens and expecting to be given other tokens).

Smart contract wallet behavior executing an intent

The entry point does not expect anything from the smart contract wallets after validation and during intent execution. However, intent standards may wish for the smart contract wallet to perform some action during execution. The smart contract wallet generalizedIntentDelegateCall function must perform a delegate call with the given calldata at the calling intent standard. In order for the wallet to trust making the delegate call it must call the verifyExecutingIntentForStandard function on the entry point contract to verify both of the following:

The msg.sender for generalizedIntentDelegateCall on the wallet is the intent standard contract that the entry point is currently calling executeUserIntent on.
The smart contract wallet is the sender on the UserIntent that the entry point is currently calling executeUserIntent for.

Smart contract wallet behavior validating an intent

The entry point calls validateUserIntent for each intent on the smart contract wallet specified in the sender field of each UserIntent. This function provides the entire UserIntent object as well as the precomputed hash of the intent. The smart contract wallet is then expected to analyze this data to ensure it was actually sent from the specified sender. If the intent is not valid, the smart contract wallet should throw an error in the validateUserIntent function. It should be noted that validateUserIntent is restricted to view only. Any kind of updates to state for things like nonce management, should be done in an individual segment on the intent itself. This allows for maximum customization in the way users define their intents while enshrining only the minimum verification within the entry point needed to ensure intents cannot be forged.

The function validateUserIntent also has an optional address return value for the smart contract wallet to return if the validation failed but could have been validated by a signature aggregation contract earlier. In this case, the smart contract wallet would return the address of the trusted signature aggregation smart contract; see the Extension: signature aggregation section below for details. If there were no issues during validation, the smart contract wallet should just return address(0).

Solver intent validation

To validate a UserIntent, the solver makes a view call to validateIntent(intent) on the entry point. This function checks that the signature passes validation and that the segments on the intent are properly formatted. If the call reverts with any error, the solver should reject the UserIntent.

Simulation

Solvers are expected to handle simulation in typical MEV workflows. This most likely means dry running their solutions at the current block height to determine the outcome is as expected. Successful solutions can then be submitted as a bundle to block builders to be included in the next block.

Extensions

The entry point contract may enable additional functionality to reduce gas costs for common scenarios.

Extension: signature aggregation

We add the additional function handleIntentsAggregated to the entry point contract that allows an aggregated signature to be provided in place of verifying signatures for intents individually. Additionally, we introduce a new interface for a contract acting as the signature aggregator that handles all logic for aggregated signature verification.

The core interface required for the entry point to have is:

The handleIntentsAggregated function takes in a list of solutions, the address of the aggregation contract, a bitfield indicating which intents the aggregate signature represents (1 for included, 0 for excluded) and lastly, the aggregated signature itself. The entry point contract will call to the aggregator contract to verify the aggregated signature for the involved intents. Then, during normal validation, the entry point contract verifies that the smart contract wallets that sent the intents in the aggregated signature all return the address of the signature aggregator contract that was used; see the Smart contract wallet behavior validating an intent section above.

The core interface required for an aggregator to have is:

The validateSignatures function serves as the main function for the entry point contract to call to verify an aggregated signature. The aggregateSignatures function can be used by solvers off-chain to calculate the aggregated signature if they do not already have optimized custom code to perform the aggregation.

Extension: embedded intent standards

We extend the entry point logic to include the logic of several identified common intent standards. These standards are registered with their own standard ID at entry point contract creation time. The functions validateUserIntent and executeUserIntent for these standards are included as part of the entry point contracts code in order to reduce external calls and save gas.

Extension: handle multi

We add the additional function handleIntentsMulti(IntentSolution[] calldata solutions) to the entry point contract. This allows multiple solutions to be executed in a single transaction to enable gas saving in intents that touch similar areas of storage.

Extension: nonce management

We add the functions getNonce(address sender, uint256 key) and setNonce(uint256 key, uint256 nonce) to the entry point contract. These functions allow nonce data to be stored in the entry point contracts storage. Nonces are stored at a per sender level and are available to be read by anyone. However, the entry point contract enforces that nonces can only be set for a user by a currently executing intent standard and only for the sender on the intent currently being executed.

Extension: data blobs

We enable the entry point contract to skip the validation of UserIntent objects with either a sender field of address(0) or an empty intentData field (rather than fail validation). Similarly, they are skipped during execution. The intentData field or sender field is then free to be treated as a way to inject any arbitrary data into intent execution. This data could be useful in solving an intent that has an intent standard which requires some secret to be known and proven to it, or an intent whose behavior can change according to what other intents are around it. For example, an intent standard that signals a smart contract wallet to transfer some tokens to the sender of the intent that is next in line for the execution process.

Rationale

The main challenge with a generalized intent standard is being able to adapt to the evolving world of intents. Users need to have a way to express their intents in a seamless way without having to make constant updates to their smart contract wallets.

In this proposal, we expect wallets to have a validateUserIntent function that takes as input a UserIntent, and verifies the signature. A trusted entry point contract uses this function to validate the signature and forwards the intent handling logic to the intent standard contracts specified in the first 32 bytes of each segment in the intentData array field on the UserIntent. The wallet is then expected to have a generalizedIntentDelegateCall function that allows it to perform intent related actions from the intent standard contracts, using the verifyExecutingIntentForStandard function on the entry point for security.

The entry point based approach allows for a clean separation between verification and intent execution, and prevents wallets from having to constantly update to support the latest intent standard composition that a user wants to use. The alternative would involve developers of new intent standards having to convince wallet software developers to support their new intent standards. This proposal moves the core definition of an intent into the hands of users at signing time.

Solvers

Solvers facilitate the fulfillment of a user's intent in search of their own MEV. They also act as the transaction originator for executing intents on-chain, including having to front any gas fees, removing that burden from the typical user.

Solvers will rely on gossiping networks and solution algorithms that are to be determined by the nature of the intents themselves and the individual intent standards being used.

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 intent standard contracts will also have to be re-registered to the new entry point.

Intent standard upgrading

Because intent standards are not hardcoded into the wallet, users do not need to perform any operation to use any newly registered intent standards. A user can simply sign an intent with the new intent standard.

Backwards Compatibility

This ERC does not change the consensus layer, so there are no backwards compatibility issues for Ethereum as a whole. There is a little more difficulty when trying to integrate with existing smart contract wallets. If the wallet already has support for ERC-4337, then implementing a validateUserIntent function should be very similar to the validateUserOp function, but would require an upgrade by the user.

Reference Implementation

See https://github.com/essential-contributions/ERC-7521

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-7521 supporting 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 validateUserIntent function and its "check signature" logic) and gate any calls to generalizedIntentDelegateCall by checking with the entry point using the verifyExecutingIntentForStandard function. The concentrated security risk in the entry point contract, however, needs to be verified to be very robust since it is so highly concentrated.

Verification would need to cover one primary claim (not including claims needed to protect solvers, and intent standard related infrastructure):

Safety against arbitrary hijacking: The entry point only returns true for verifyExecutingIntentForStandard when it has successfully validated the signature of the UserIntent and is currently in the middle of calling executeUserIntent on the standard specified in the intentData field of a UserIntent which also has the same sender as the msg.sender wallet calling the function.

Additional heavy auditing and formal verification will also need to be done for any intent standard contracts a user decides to interact with.