ERC-7679: UserOperation Builder

Abstract

Different ERC-4337 smart account implementations encode their signature, nonce, and calldata differently. This makes it difficult for DApps, wallets, and smart account toolings to integrate with smart accounts without integrating with account-specific SDKs, which introduces vendor lock-in and hurts smart account adoption.

We propose a standard way for smart account implementations to put their account-specific encoding logic on-chain. It can be achieved by implementing methods that accept the raw signature, nonce, or calldata (along with the context) as an input, and output them properly formatted, so the smart account can consume them while validating and executing the User Operation.

Motivation

At the moment, to build a ERC-4337 UserOperation (UserOp for short) for a smart account requires detailed knowledge of how the smart account implementation works, since each implementation is free to encode its nonce, calldata, and signature differently.

As a simple example, one account might use an execution function called executeFoo, whereas another account might use an execution function called executeBar. This will result in the calldata being different between the two accounts, even if they are executing the same call.

Therefore, someone who wants to send a UserOp for a given smart account needs to:

• Figure out which smart account implementation the account is using.
• Correctly encode signature/nonce/calldata given the smart account implementation, or use an account-specific SDK that knows how to do that.

In practice, this means that most DApps, wallets, and AA toolings today are tied to a specific smart account implementation, resulting in fragmentation and vendor lock-in.

Specification

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in RFC 2119.

UserOp builder

To conform to this standard, a smart account implementation MUST provide a “UserOp builder” contract that implements the IUserOperationBuilder interface, as defined below:

Using the UserOp builder

To build a UserOp using the UserOp builder, the building party SHOULD proceed as follows:

1. Obtain the address of UserOpBuilder and a context from the account owner. The context is an opaque bytes array from the perspective of the building party. The UserOpBuilder implementation may need the context in order to properly figure out the UserOp fields. See Rationale for more info.
2. Execute a multicall (batched eth_calls) of getNonce and getCallData with the context and executions. The building party will now have obtained the nonce and calldata.
3. Fill out a UserOp with the data obtained previously. Gas values can be set randomly or very low. This userOp will be used to obtain a dummy signature for gas estimations. Sign the hash of userOp. (See Rationale for what a dummy signature is. See Security Considerations for the details on dummy signature security).
4. Call (via eth_call) formatSignature with the UserOp and context to obtain a UserOp with a properly formatted dummy signature. This userOp can now be used for gas estimation.
5. In the UserOp, change the existing gas values to those obtained from a proper gas estimation. This UserOp must be valid except for the signature field. Sign the hash of the UserOp and place the signature in the UserOp.signature field.
6. Call (via eth_call) formatSignature with the UserOp and context to obtain a completely valid UserOp.
   1. Note that a UserOp has a lot more fields than nonce, callData, and signature, but how the building party obtains the other fields is outside of the scope of this document, since only these three fields are heavily dependent on the smart account implementation.

At this point, the building party has a completely valid UserOp that they can then submit to a bundler or do whatever it likes with it.

Using the UserOp builder when the account hasn’t been deployed

To provide the accurate data to the building party, the UserOpBuilder will in most cases have to call the account. If the account has yet to be deployed, which means that the building party is looking to send the very first UserOp for this account, then the building party MAY modify the flow above as follows:

• In addition to the UserOpBuilder address and the context, the building party also obtains the factory and factoryData as defined in ERC-4337.
• When calling one of the view functions on the UserOp builder, the building party may use eth_call to deploy the CounterfactualCall contract, which is going to deploy the account and call UserOpBuilder (see below).
• When filling out the UserOp, the building party includes factory and factoryData.

The CounterfactualCall contract SHOULD:

• Deploy the account using factory and factoryData provided by the building party.
• Revert if the deployment has not succeeded.
• If the account has been deployed succesfully, call UserOpBuilder and return the data returned by UserOpBuilder to the building party.

See Reference Implementation section for more details on the CounterfactualCall contract.

Rationale

Context

The context is an array of bytes that encodes whatever data the UserOp builder needs in order to correctly determine the nonce, calldata, and signature. Presumably, the context is constructed by the account owner, with the help of a wallet software.

Here we outline one possible use of context: delegation. Say the account owner wants to delegate a transaction to be executed by the building party. The account owner could encode a signature of the public key of the building party inside the context. Let’s call this signature from the account owner the authorization.

Then, when the building party fills out the UserOp, it would fill the signature field with a signature generated by its own private key. When it calls getSignature on the UserOp builder, the UserOp builder would extract the authorization from the context and concatenates it with the building party’s signature. The smart account would presumably be implemented such that it would recover the building party’s public key from the signature, and check that the public key was in fact signed off by the authorization. If the check succeeds, the smart account would execute the UserOp, thus allowing the building party to execute a UserOp on the user’s behalf.

Dummy signature

The “dummy signature” refers to the signature used in a UserOp sent to a bundler for estimating gas (via eth_estimateUserOperationGas). A dummy signature is needed because, at the time the bundler estimates gas, a valid signature does not exist yet, since the valid signature itself depends on the gas values of the UserOp, creating a circular dependency. To break the circular dependency, a dummy signature is used.

However, the dummy signature is not just a fixed value that any smart account can use. The dummy signature must be constructed such that it would cause the UserOp to use about as much gas as a real signature would. Therefore, the dummy signature varies based on the specific validation logic that the smart account uses to validate the UserOp, making it dependent on the smart account implementation.

Backwards Compatibility

This ERC is intended to be backwards compatible with all ERC-4337 smart accounts as of EntryPoint 0.7.

For smart accounts deployed against EntryPoint 0.6, the IUserOperationBuilder interface needs to be modified such that the PackedUserOperation struct is replaced with the corresponding struct in EntryPoint 0.6.

Reference Implementation

Counterfactual call contract

The counterfactual call contract is inspired by ERC-6492, which devised a mechanism to execute isValidSignature (see ERC-1271) against a pre-deployed (counterfactual) contract.

Here’s an example of calling this contract using the ethers and viem libraries:

Security Considerations

Dummy Signature security

Since the properly formatted dummy signature is going to be publicly disclosed, in theory it can be intercepted and used by the man in the middle. Risks and potential harm of this is very low though as the dummy signature will be effectively unusable after the final UserOp is submitted (as both UserOps use the same nonce). However, to mitigate even this small issue, it is recommended that the UserOp which hash is going to be signed to obtain an un-foirmatted dummy signature (step 3 above) is filled with very low gas values.

Copyright

Copyright and related rights waived via CC0.