L1-L2 Messaging

A crucial feature of a Layer 2 is its ability to interact with Layer 1.

Starknet has its own L1-L2 messaging system, which is different from its consensus mechanism and the submission of state updates on L1. Messaging is a way for smart-contracts on L1 to interact with smart-contracts on L2 (or the other way around), allowing us to do "cross-chain" transactions. For example, we can do some computations on one chain and use the result of this computation on the other chain.

Bridges on Starknet all use L1-L2 messaging. Let's say that you want to bridge tokens from Ethereum to Starknet. You will simply have to deposit your tokens into the L1 bridge contract, which will automatically trigger the minting of the same token on L2. Another good use case for L1-L2 messaging would be DeFi pooling.

On Starknet, it's important to note that the messaging system is asynchronous and asymmetric.

Asynchronous: this means that in your contract code (being Solidity or Cairo), you can't await the result of the message being sent on the other chain within your contract code execution.
Asymmetric: sending a message from Ethereum to Starknet (L1->L2) is fully automated by the Starknet sequencer, which means that the message is being automatically delivered to the target contract on L2. However, when sending a message from Starknet to Ethereum (L2->L1), only the hash of the message is sent to L1 by the Starknet sequencer. You must then consume the message manually via a transaction on L1.

Let's dive into the details.

The StarknetMessaging Contract

The crucial component of the L1-L2 Messaging system is the StarknetCore contract. It is a set of Solidity contracts deployed on Ethereum that allows Starknet to function properly. One of the contracts of StarknetCore is called StarknetMessaging and it is the contract responsible for passing messages between Starknet and Ethereum. StarknetMessaging follows an interface with functions allowing to send messages to L2, receiving messages on L1 from L2 and canceling messages.

interface IStarknetMessaging is IStarknetMessagingEvents {

    function sendMessageToL2(
        uint256 toAddress,
        uint256 selector,
        uint256[] calldata payload
    ) external returns (bytes32);

    function consumeMessageFromL2(uint256 fromAddress, uint256[] calldata payload)
        external
        returns (bytes32);

    function startL1ToL2MessageCancellation(
        uint256 toAddress,
        uint256 selector,
        uint256[] calldata payload,
        uint256 nonce
    ) external;

    function cancelL1ToL2Message(
        uint256 toAddress,
        uint256 selector,
        uint256[] calldata payload,
        uint256 nonce
    ) external;
}

Starknet messaging contract interface

In the case of L1->L2 messages, the Starknet sequencer is constantly listening to the logs emitted by the StarknetMessaging contract on Ethereum. Once a message is detected in a log, the sequencer prepares and executes an L1HandlerTransaction to call the function on the target L2 contract. This takes up to 1-2 minutes to be done (few seconds for ethereum block to be mined, and then the sequencer must build and execute the transaction).

L2->L1 messages are prepared by contract's execution on L2 and are part of the block produced. When the sequencer produces a block, it sends the hash of each message prepared by the contract's execution to the StarknetCore contract on L1, where they can then be consumed once the block they belong to is proven and verified on Ethereum (which for now is around 3-4 hours).

Sending Messages from Ethereum to Starknet

If you want to send messages from Ethereum to Starknet, your Solidity contracts must call the sendMessageToL2 function of the StarknetMessaging contract. To receive these messages on Starknet, you will need to annotate functions that can be called from L1 with the #[l1_handler] attribute.

Let's take a simple contract taken from this tutorial where we want to send a message to Starknet. The _snMessaging is a state variable already initialized with the address of the StarknetMessaging contract. You can check all Starknet contract and sequencer addresses here.

// Sends a message on Starknet with a single felt.
function sendMessageFelt(
    uint256 contractAddress,
    uint256 selector,
    uint256 myFelt
)
    external
    payable
{
    // We "serialize" here the felt into a payload, which is an array of uint256.
    uint256[] memory payload = new uint256[](1);
    payload[0] = myFelt;

    // msg.value must always be >= 20_000 wei.
    _snMessaging.sendMessageToL2{value: msg.value}(
        contractAddress,
        selector,
        payload
    );
}

The function sends a message with a single felt value to the StarknetMessaging contract. Be aware that your Cairo contract will only understand felt252 data type, so if you want to send more complex data, you must ensure that the data serialization into the uint256 array follows the Cairo serialization scheme.

It's important to note that we have {value: msg.value}. In fact, the minimum value we have to send here is 20k wei, due to the fact that the StarknetMessaging contract will register the hash of our message in the storage of Ethereum.

In addition to those 20k wei, since the L1HandlerTransaction executed by the sequencer is not tied to any account (the message originates from L1), you must also ensure that you pay enough fees on L1 for your message to be deserialized and processed on L2.

The fees of the L1HandlerTransaction are computed in a regular manner as it would be done for an Invoke transaction. For this, you can profile the gas consumption using starkli or snforge to estimate the cost of your message execution.

The signature of the sendMessageToL2 is:

function sendMessageToL2(
        uint256 toAddress,
        uint256 selector,
        uint256[] calldata payload
    ) external override returns (bytes32);

The parameters are as follows:

toAddress: The contract address on L2 that will be called.
selector: The selector of the function of this contract at toAddress. This selector (function) must have the #[l1_handler] attribute to be callable.
payload: The payload is always an array of felt252 (which are represented by uint256 in Solidity). For this reason we've inserted the input myFelt into the array. This is why we need to insert the input data into an array.

On the Starknet side, to receive this message, we have:

    #[l1_handler]
    fn msg_handler_felt(ref self: ContractState, from_address: felt252, my_felt: felt252) {
        assert(from_address == self.allowed_message_sender.read(), 'Invalid message sender');

        // You can now use the data, automatically deserialized from the message payload.
        assert(my_felt == 123, 'Invalid value');
    }

We need to add the #[l1_handler] attribute to our function. L1 handlers are special functions that can only be executed by an L1HandlerTransaction. There is nothing particular to do to receive transactions from L1, as the message is relayed by the sequencer automatically. In your #[l1_handler] functions, it is important to verify the sender of the L1 message to ensure that our contract can only receive messages from a trusted L1 contract.

Sending Messages from Starknet to Ethereum

When sending messages from Starknet to Ethereum, you will have to use the send_message_to_l1 syscall in your Cairo contracts. This syscall allows you to send messages to the StarknetMessaging contract on L1. Unlike L1->L2 messages, L2->L1 messages must be consumed manually, which means that you will need your Solidity contract to explicitly call the consumeMessageFromL2 function of the StarknetMessaging contract in order to consume the message.

To send a message from L2 to L1, what we would do on Starknet is:

        fn send_message_felt(ref self: ContractState, to_address: EthAddress, my_felt: felt252) {
            // Note here, we "serialize" my_felt, as the payload must be
            // a `Span<felt252>`.
            syscalls::send_message_to_l1_syscall(to_address.into(), array![my_felt].span())
                .unwrap();
        }

We simply build the payload and pass it, along with the L1 contract address, to the syscall function.

On L1, the important part is to build the same payload sent by the L2. Then in your Solidity contract, you can call consumeMessageFromL2 by passing the L2 contract address and the payload. Please be aware that the L2 contract address expected by the consumeMessageFromL2 is the address of the contract that sends the message on the L2 by calling send_message_to_l1_syscall.

function consumeMessageFelt(
    uint256 fromAddress,
    uint256[] calldata payload
)
    external
{
    let messageHash = _snMessaging.consumeMessageFromL2(fromAddress, payload);

    // You can use the message hash if you want here.

    // We expect the payload to contain only a felt252 value (which is a uint256 in Solidity).
    require(payload.length == 1, "Invalid payload");

    uint256 my_felt = payload[0];

    // From here, you can safely use `my_felt` as the message has been verified by StarknetMessaging.
    require(my_felt > 0, "Invalid value");
}

As you can see, in this context we don't have to verify which contract from L2 is sending the message (as we do on the L2 to verify which contract from L1 is sending the message). But we are actually using the consumeMessageFromL2 of the StarknetCore contract to validate the inputs (the contract address on L2 and the payload) to ensure we are only consuming valid messages.

Note: The consumeMessageFromL2 function of the StarknetCore contract is expected to be called from a Solidity contract, and not directly on the StarknetCore contract. The reason for that is because the StarknetCore contract is using msg.sender to actually compute the hash of the message. And this msg.sender must correspond to the to_address field that is given to the function send_message_to_l1_syscall that is called on Starknet.

Cairo Serde

Before sending messages between L1 and L2, you must remember that Starknet contracts, written in Cairo, can only understand serialized data. And serialized data is always an array of felt252. In Solidity we have uint256 type, and felt252 is approximately 4 bits smaller than uint256. So we have to pay attention to the values contained in the payload of the messages we are sending. If, on L1, we build a message with values above the maximum felt252, the message will be stuck and never consumed on L2.

So for instance, an actual uint256 value in Cairo is represented by a struct like:

struct u256 {
    low: u128,
    high: u128,
}

which will be serialized as TWO felts, one for low, and one for high. This means that to send only one u256 to Cairo, you'll need to send a payload from L1 with TWO values.

uint256[] memory payload = new uint256[](2);
// Let's send the value 1 as a u256 in cairo: low = 1, high = 0.
payload[0] = 1;
payload[1] = 0;

If you want to learn more about the messaging mechanism, you can visit the Starknet documentation.

You can also find a detailed guide here to test the messaging system locally.

The Cairo Programming Language

L1-L2 Messaging

The StarknetMessaging Contract

Sending Messages from Ethereum to Starknet

Sending Messages from Starknet to Ethereum

Cairo Serde