## Crafting an Unsigned PackedUserOperation for ERC-4337 This lesson focuses on a crucial preliminary step in interacting with ERC-4337 Account Abstraction: generating an *unsigned* `PackedUserOperation` struct. This forms the foundation for creating a fully signed operation, which is essential for testing functions like `validateUserOp` within a smart contract account. **Context and Motivation** Previously, you might have tested the `execute` function of a smart contract account, such as `MinimalAccount.sol`. The next logical step is to test the `validateUserOp` function. To do this effectively, we need to construct a valid `PackedUserOperation` struct, its corresponding hash (`userOpHash`), and potentially any `missingAccountFunds`. A complete `PackedUserOperation` requires a signature from the account owner. Instead of embedding the complex logic for creating and signing this operation directly within our test files (e.g., `MinimalAccountTest.t.sol`), we'll build this functionality within a Foundry script file (`SendPackedUserOp.s.sol`). This approach offers several advantages: 1. **Reusability:** A script for sending user operations will likely be necessary for interacting with the account on testnets or mainnet. 2. **Testability:** The logic developed in the script, especially for generating the operation data, can be imported and utilized within Foundry tests. 3. **Focused Testing:** This allows us to test the signature generation process itself within our test environment. **The `PackedUserOperation` Struct** The central data structure we're working with is `PackedUserOperation`. It's typically imported from an account abstraction library (e.g., `lib/account-abstraction/contracts/interfaces/PackedUserOperation.sol`). For this lesson, the relevant fields are: ```solidity struct PackedUserOperation { address sender; // The account sending the operation. uint256 nonce; // Anti-replay protection nonce. bytes initCode; // Code to deploy the account if it doesn't exist (ignored for now). bytes callData; // The actual transaction data (function call) to execute. bytes32 accountGasLimits; // Packed verificationGasLimit and callGasLimit. uint256 preVerificationGas; // Gas limit for pre-verification steps. bytes32 gasFees; // Packed maxFeePerGas and maxPriorityFeePerGas. bytes paymasterAndData; // Data for paymaster interaction (ignored for now). bytes signature; // The signature validating the operation (left blank for unsigned). } ``` **Building the `SendPackedUserOp.s.sol` Script** We'll create a new Foundry script file named `SendPackedUserOp.s.sol` to house our logic. 1. **Basic Script Structure:** The script begins with the standard SPDX license, pragma version, and necessary imports. We import `Script` from `forge-std` and the `PackedUserOperation` struct. ```solidity // SPDX-License-Identifier: MIT pragma solidity 0.8.24; import {Script} from "forge-std/Script.sol"; import {PackedUserOperation} from "lib/account-abstraction/contracts/interfaces/PackedUserOperation.sol"; contract SendPackedUserOp is Script { function run() public {} // Entry point for script execution, empty for now // Helper functions will be defined below } ``` 2. **Main Function: `generateSignedUserOperation` (Initial Version)** Our primary goal is to create a function that will eventually return a *signed* `PackedUserOperation`. For now, it will lay the groundwork and prepare the unsigned version. ```solidity function generatedSignedUserOperation(bytes memory callData, address sender) public // Not 'view' or 'pure' as it will use vm cheatcodes for nonce and later for signing returns (PackedUserOperation memory) { // Step 1: Generate the unsigned UserOperation data uint256 nonce = vm.getNonce(sender); // Fetch current nonce for the sender PackedUserOperation memory unsignedUserOp = _generateUnsignedUserOperation( callData, sender, nonce ); // Step 2: Sign the UserOperation (to be implemented later) // For now, we'll return the unsigned version return unsignedUserOp; } ``` In this function, we use the Foundry cheatcode `vm.getNonce(sender)` to retrieve the current nonce for the `sender` address. This is crucial for replay protection. 3. **Helper Function: `_generateUnsignedUserOperation`** To maintain clean and organized code, we'll create an internal helper function dedicated to populating the `PackedUserOperation` struct *without* the signature. ```solidity function _generateUnsignedUserOperation(bytes memory callData, address sender, uint256 nonce) internal pure // This function doesn't read state or use cheatcodes returns (PackedUserOperation memory) { // Example gas parameters (these may need tuning) uint128 verificationGasLimit = 16777216; uint128 callGasLimit = verificationGasLimit; // Often different in practice uint128 maxPriorityFeePerGas = 256; uint128 maxFeePerGas = maxPriorityFeePerGas; // Simplification for example // Pack accountGasLimits: (verificationGasLimit << 128) | callGasLimit bytes32 accountGasLimits = bytes32( (uint256(verificationGasLimit) << 128) | uint256(callGasLimit) ); // Pack gasFees: (maxFeePerGas << 128) | maxPriorityFeePerGas bytes32 gasFees = bytes32( (uint256(maxFeePerGas) << 128) | uint256(maxPriorityFeePerGas) ); return PackedUserOperation({ sender: sender, nonce: nonce, initCode: hex"", // Empty for existing accounts callData: callData, accountGasLimits: accountGasLimits, preVerificationGas: verificationGasLimit, // Often related to verificationGasLimit gasFees: gasFees, paymasterAndData: hex"", // Empty if not using a paymaster signature: hex"" // Crucially, the signature is blank for an unsigned operation }); } ``` **Key aspects of `_generateUnsignedUserOperation`:** * **Gas Parameters:** The `verificationGasLimit`, `callGasLimit`, `maxPriorityFeePerGas`, and `maxFeePerGas` are provided as example values. In a real-world scenario, these would require careful calculation or dynamic retrieval based on network conditions and operation complexity. * **Packing Gas Limits and Fees:** The `accountGasLimits` field (a `bytes32`) packs two `uint128` values: `verificationGasLimit` and `callGasLimit`. This is achieved by left-shifting the `verificationGasLimit` by 128 bits and then performing a bitwise OR operation with `callGasLimit`. A similar process is used for `gasFees`, packing `maxFeePerGas` and `maxPriorityFeePerGas`. Both `uint128` values are first cast to `uint256` before bitwise operations. * **Ignored Fields:** * `initCode`: This field is left empty (`hex""`) because we are assuming the smart contract account (`sender`) already exists. It's only used when deploying a new account via a user operation. * `paymasterAndData`: This is also left empty, assuming no paymaster is involved in sponsoring gas fees for this operation. * **Signature Field:** The `signature` field is explicitly set to `hex""`. This is the defining characteristic of an *unsigned* `PackedUserOperation`. By calling `_generateUnsignedUserOperation` from `generatedSignedUserOperation`, we now have a structured way to create the data payload of a user operation before it's signed. **Key Concepts Recap** * **ERC-4337 & Account Abstraction:** The overarching framework that necessitates `PackedUserOperation` for user-initiated actions. * **`PackedUserOperation`:** The core data structure that bundles a user's intent, replacing traditional transactions for smart contract accounts. * **Unsigned Operation:** The `PackedUserOperation` struct populated with all necessary data *except* the cryptographic signature. * **Foundry Scripts (`.s.sol`):** Used for deployment, interaction, and complex setup logic that can be reused in tests. * **Foundry Cheatcodes:** Tools like `vm.getNonce()` allow scripts and tests to interact with and query the blockchain virtual machine's state. * **Bit Shifting and Packing:** A technique to efficiently store multiple smaller data values within a single larger field (e.g., two `uint128` values in one `bytes32`). * **Helper Functions:** A good practice for modularizing code, making it more readable and maintainable. **Important Considerations** * **Gas Parameterization:** The hardcoded gas limits and fee values (`verificationGasLimit`, `callGasLimit`, `maxFeePerGas`, `maxPriorityFeePerGas`) are illustrative. In practice, these are critical and complex. Incorrect values can lead to operation failure. They often need to be estimated or determined dynamically. * **`initCode` Usage:** Only populate `initCode` if the user operation is intended to deploy a new smart contract account. For operations targeting existing accounts, it must be empty. * **Paymaster Logic:** The `paymasterAndData` field is only relevant when employing a paymaster to cover gas costs. * **Separation of Concerns:** Generating the unsigned data payload separately from the signing process is a clean architectural choice. **Next Steps** With the `_generateUnsignedUserOperation` helper in place and `generatedSignedUserOperation` set up to produce the unsigned struct, the next logical step (to be covered subsequently) involves: 1. Calculating the `userOpHash` of the `unsignedUserOp`. 2. Signing this hash using the `sender`'s private key (likely using Foundry cheatcodes like `vm.sign` or by managing private keys within the script). 3. Populating the `signature` field of the `PackedUserOperation` struct with this cryptographic signature. 4. Returning the fully populated, *signed* `PackedUserOperation`. This will complete the `generatedSignedUserOperation` function, enabling us to generate operations ready for validation by the `validateUserOp` function of our smart contract account.