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This article was co-authored by David Philipson.
In ERC-4337 Gas Estimation we discussed how gas works in ERC-4337 and our method for gas estimation. With this method, we should be able to provide users with accurate values for submitting their user operations. Alas, its not always that simple.
Let’s dive into a few other problems that complicate ERC-4337 gas estimation.
In most smart contract account implementations the signature field is computed off-chain by hashing a user operation and signing that hash using some signature scheme (ECDSA being the most popular). This signature is verified onchain by the account contract during the verification phase.
This signature must be computed after gas is estimated, as those fields are included in the hash.
However, there are portions of the gas estimation step that require the signature field to be populated: preVerificationGas and verificationGasLimit .
In the section on preVerificationGas estimation in part one of this series, we discussed three calculations that the bundler uses to account for unmetered gas.
Notice that steps 2 and 3 (calculating the user ops's share of the calldata gas cost and the user op's share of any execution gas overhead) both directly depend on the length and content of a user operation, including the signature field.
Almost all useful smart contract account implementations will require some signature verification. This gas needs to be estimated! This estimation process cannot revert or else verification gas cannot be implemented.
How do we choose what signature to use?
To solve these problems, many ERC-4337 bundlers use a “Dummy Signature” that must be provided by the caller of eth_estimateUserOperationGas based on the specific account type.
This dummy signature has a few constraints:
A dummy signature value for SimpleAccount can be found in our documentation for the eth_estimateUserOperationGas method.
This dummy signature has the following properties:
Account implementors need to ensure they write their validation functions such that supplying a dummy signature is possible.
The same requirements above apply to the paymasterAndData field during gas estimation: for any paymaster implementation that relies on the gas fields’ values (i.e. for computing a signature), we must solve the same problem of supplying a dummy value.
This dummy paymasterAndData has a few constraints:
For example, a sponsoring paymaster that relies on the verification of a sponsorship signature over the user operation hash will need to supply a dummy paymasterAndData value during estimation with similar properties as outlined above:
Paymaster implementors need to insure they write their validation functions such that supplying a dummy paymasterAndData is possible.
The estimation for verificationGasLimit must take into account the gas cost of any token transfers during the validation phase. Many user ops involve a transfer of tokens during the validation to pay up front for gas, with a refund at the end of the operation for any gas that wasn’t consumed.
There are two cases:
If maxFeePerGas is set to a non-zero value, the transfer during validation will revert if the account doesn’t hold enough tokens to pay the fee. This means that users must fund their accounts prior to attempting to estimate gas.
A desired workflow that isn’t possible now:
A possible solution here is to tell the user to leave maxFeePerGas unset, or set to zero, when calling eth_estimateUserOperationGas. However, this will set the gas cost to zero and thus a zero value for whatever token transfer needs to occur. In many account/paymaster types a zero gas cost will skip the call to transfer. This leads to a gas underestimation when a zero maxFeePerGas is used.
Another problem has to do with using a non-zero maxFeePerGas and attempting the binary search estimation approach defined in "Attempt 3: Binary Search," from part 1 of this series.
The first step in that approach is to ensure that the operation is possible by using a maximum gas value. The sender may not have enough assets to pay for gas at that maximum value and the bundler doesn’t always have enough information to determine the balance of the fee token (i.e. the ERC-20 paymaster case) to determine the correct upper bound.
The approach taken by Rundler for eth_estimateUserOperationGas is as follows:
Bullet #1 is a deviation from the ERC-4337 spec which states “gas limits (and prices) parameters are optional, but are used if specified”
For example, if using an ERC-20 paymaster that does a transfer during the validation phase always add a static 75K gas to the verificationGasLimit return value from eth_estimateUserOperationGas. This requires clients to be aware of the paymaster contract they’re calling into and have prior knowledge of underestimated gas costs due to transfers.
The Alchemy aa-sdk allows paymaster middleware to modify this gas limit accordingly.
Dummy values and the token transfer problem impacts developers building smart contract accounts, paymasters, and account clients.
Smart contract account developers need to ensure that a dummy signature value is always able to be determined client-side based on how the account validation function is going to run. This signature must not cause a revert and must result in a maximum gas estimate.
Paymaster developers must ensure that a dummy paymasterAndData value is always able to be determined based on how the paymaster validation function is going to run. This value must not cause a revert and must result in a maximum gas estimate.
Alchemy simplifies this for users in its sponsoring paymaster by combining the gas estimation process and the simulation process via alchemy_requestGasAndPaymasterAndData
Account client developers who are integrating with a paymaster that performs transfers during the validation phase (or any other logic based on the fee value), and are using Alchemy’s endpoints for estimation, need to add any potential gas associated with the transfer to the verificationGasLimit returned by estimation.
Stay tuned for a later post outlining more ERC-4337 gas estimation complications!
The next articles in this 4-part series explore the challenges estimating gas on layer 2 blockchains like Optimism and Arbitrum and signature aggregators.
The series concludes with a walkthrough of the user operation fee estimation process. If you missed part one, learn how ERC-4337 gas estimation works!