ch04-02-hello-account.md

Hello World! Account Contract

This section guides you through the creation of the simplest possible account contract, adhering to the SNIP-6 standard. The account contract will be the simplest implementation of an account contract, with the following features:

• Signature validation for transactions will be not enforced. In other words, every transaction will be considered valid no matter who signed it; there will be no pivate key.
• It will make a single call and not multicall in the execution phase.
• It will only implement the SNIP-6 standard which is the minimum to be considered an account contract.

We will deployed using starknet.py and use it to deploy other contracts.

Setting Up Your Project

For deploying an account contract to Starknet's testnet or mainnet, use Scarb version 2.3.1, which is compatible with the Sierra 1.3.0 target supported by both networks. For the latest information, review the Starknet Release Notes. As of November 2023, Scarb version 2.3.1 is the recommended choice.

To check your current Scarb version, run:

scarb --version

To install or update Scarb, refer to the Basic Installation instructions in Chapter 2, covering macOS and Linux environments:

curl --proto '=https' --tlsv1.2 -sSf https://docs.swmansion.com/scarb/install.sh | sh

Starting a New Scarb Project

Begin by creating a new project (more details in the Scarb subchapter in Chapter 2):

scarb new hello_account

Check the generated project structure:

$ tree .
.
└── hello_account
    ├── Scarb.toml
    └── src
        └── lib.cairo

By default, Scarb sets up for vanilla Cairo. Add Starknet capacities by editing Scarb.toml to include the starknet dependency:

[package]
name = "hello_account"
version = "0.1.0"

[dependencies]
starknet = ">=2.3.0"

[[target.starknet-contract]]
sierra = true
casm = true
casm-add-pythonic-hints = true

Replace the code in src/lib.cairo with the Hello World account contract:

use starknet::account::Call;

// IERC6 obtained from Open Zeppelin's cairo-contracts/src/account/interface.cairo
#[starknet::interface]
trait ISRC6<TState> {
    fn __execute__(self: @TState, calls: Array<Call>) -> Array<Span<felt252>>;
    fn __validate__(self: @TState, calls: Array<Call>) -> felt252;
    fn is_valid_signature(self: @TState, hash: felt252, signature: Array<felt252>) -> felt252;
}

#[starknet::contract]
mod HelloAccount {
    use starknet::VALIDATED;
    use starknet::account::Call;
    use starknet::get_caller_address;

    #[storage]
    struct Storage {} // Empty storage. No public key is stored.

    #[external(v0)]
    impl SRC6Impl of super::ISRC6<ContractState> {
        fn is_valid_signature(
            self: @ContractState, hash: felt252, signature: Array<felt252>
        ) -> felt252 {
            // No signature is required so any signature is valid.
            VALIDATED
        }
        fn __validate__(self: @ContractState, calls: Array<Call>) -> felt252 {
            let hash = 0;
            let mut signature: Array<felt252> = ArrayTrait::new();
            signature.append(0);
            self.is_valid_signature(hash, signature)
        }
        fn __execute__(self: @ContractState, calls: Array<Call>) -> Array<Span<felt252>> {
            let sender = get_caller_address();
            assert(sender.is_zero(), 'Account: invalid caller');
            let Call{to, selector, calldata } = calls.at(0);
            let _res = starknet::call_contract_syscall(*to, *selector, calldata.span()).unwrap();
            let mut res = ArrayTrait::new();
            res.append(_res);
            res
        }
    }
}

Compile your project to ensure the setup is correct:

scarb build

SNIP-6 Standard

To define an account contract, implement the ISRC6 trait:

#[starknet::interface]
trait ISRC6<TState> {
    fn __execute__(self: @TState, calls: Array<Call>) -> Array<Span<felt252>>;
    fn __validate__(self: @TState, calls: Array<Call>) -> felt252;
    fn is_valid_signature(self: @TState, hash: felt252, signature: Array<felt252>) -> felt252;
}

The __execute__ and __validate__ functions are designed for exclusive use by the Starknet protocol to enhance account security. Despite their public accessibility, only the Starknet protocol can invoke these functions, identified by using the zero address. In this minimal account contract we will not enforce this restriction, but we will do it in the next examples.

Validating Transactions

The is_valid_signature function is responsible for this validation, returning VALIDATED if the signature is valid. The VALIDATED constant is imported from the starknet module.

use starknet::VALIDATED;

Notice that the is_valid_signature function accepts all the transactions as valid. We are not storing a public key in the contract, so we cannot validate the signature. We will add this functionality in the next examples.

fn is_valid_signature(
            self: @ContractState, hash: felt252, signature: Array<felt252>
        ) -> felt252 {
            // No signature is required so any signature is valid.
            VALIDATED
        }

The __validate__ function calls the is_valid_signature function with a dummy hash and signature. The __validate__ function is called by the Starknet protocol to validate the transaction. If the transaction is not valid, the execution of the transaction is aborted.

fn __validate__(self: @ContractState, calls: Array<Call>) -> felt252 {
            let hash = 0;
            let mut signature: Array<felt252> = ArrayTrait::new();
            signature.append(0);
            self.is_valid_signature(hash, signature)
        }

In other words we have implemented a contract that accepts all the transactions as valid. We will add the signature validation in the next examples.

Executing Transactions

The __execute__ function is responsible for executing the transaction. In this minimal account contract we will only execute a single call. We will add the multicall functionality in the next examples.

fn __execute__(self: @ContractState, calls: Array<Call>) -> Array<Span<felt252>> {
            let Call{to, selector, calldata } = calls.at(0);
            let _res = starknet::call_contract_syscall(*to, *selector, calldata.span()).unwrap();
            let mut res = ArrayTrait::new();
            res.append(_res);
            res
        }

The __execute__ function calls the call_contract_syscall function from the starknet module. This function executes the call and returns the result. The call_contract_syscall function is a Starknet syscall, which means that it is executed by the Starknet protocol. The Starknet protocol is responsible for executing the call and returning the result. The Starknet protocol will also validate the call, so we do not need to validate the call in the __execute__ function.

Deploying the Contract

[TODO]