L1 <--> L2 communication

In the following section, we will look at cross-chain communication, mixing L1 and L2 for composability and profits.

Objective

The goal is to setup a minimal-complexity mechanism, that will allow a base-layer (L1) and the Aztec Network (L2) to communicate arbitrary messages such that:

• L2 functions can call L1 functions.
• L1 functions can call L2 functions.
• The rollup-block size have a limited impact by the messages and their size.

High Level Overview

This document will contain communication abstractions that we use to support interaction between private functions, public functions and Layer 1 portal contracts.

Fundamental restrictions for Aztec:

• L1 and L2 have very different execution environments, stuff that is cheap on L1 is most often expensive on L2 and vice versa. As an example, keccak256 is cheap on L1, but very expensive on L2.
• L1 and L2 have causal ordering, simply meaning that we cannot execute something on L1 that depends on something happening on L2 and vice versa.
• Private function calls are fully "prepared" and proven by the user, which provides the kernel proof along with commitments and nullifiers to the sequencer.
• Public functions altering public state (updatable storage) must be executed at the current "head" of the chain, which only the sequencer can ensure, so these must be executed separately to the private functions.
• Private and public functions within Aztec are therefore ordered such that first private functions are executed, and then public. For a more detailed description of why, see above.
• There is an explicit 1:1 link from a L2 contract to an L1 contract, and only the messages between a pair is allowed. See Portal for more information.
• Messages are consumables, and can only be consumed by the recipient. See Message Boxes for more information.

With the aforementioned restrictions taken into account, cross-chain messages can be operated in a similar manner to when public functions must transmit information to private functions. In such a scenario, a "message" is created and conveyed to the recipient for future use. It is worth noting that any call made between different domains (private, public, cross-chain) is unilateral in nature. In other words, the caller is unaware of the outcome of the initiated call until told when some later rollup is executed (if at all). This can be regarded as message passing, providing us with a consistent mental model across all domains, which is convenient.

As an illustration, suppose a private function adds a cross-chain call. In such a case, the private function would not have knowledge of the result of the cross-chain call within the same rollup (since it has yet to be executed).

Similarly to the ordering of private and public functions, we can also reap the benefits of intentionally ordering messages between L1 and L2. When a message is sent from L1 to L2, it has been "emitted" by an action in the past (an L1 interaction), allowing us to add it to the list of consumables at the "beginning" of the block execution. This practical approach means that a message could be consumed in the same block it is included. In a sophisticated setup, rollup $n$ could send an L2 to L1 message that is then consumed on L1, and the response is added already in $n+1$. However, messages going from L2 to L1 will be added as they are emitted.

info

Because everything is unilateral and async, the application developer have to explicitly handle failure cases such that user can gracefully recover. Example where recovering is of utmost importance is token bridges, where it is very inconvenient if the locking of funds on one domain occur, but never the minting or unlocking on the other.

Components

Portal

A "portal" refers to the part of an application residing on L1, which is associated with a particular L2 address (the confidential part of the application). The link between them is established explicitly to reduce access control complexity. On public chains, access control information such as a whitelist in a mapping or similar data structure can simply be placed in public storage. However, this is not feasible for contracts in Aztec. Recall that public storage can only be accessed (up to date) by public functions which are called AFTER the private functions. This implies that access control values in public storage only work for public functions. One possible workaround is to store them in private data, but this is not always practical for generic token bridges and other similar use cases where the values must be publicly known to ensure that the system remains operational. Instead, we chose to use a hard link between the portal and the L2 address.

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Note, that we at no point require the "portal" to be a contract, it could be an EOA on L1.

Message Boxes

In a logical sense, a Message Box functions as a one-way message passing mechanism with two ends, one residing on each side of the divide, i.e., one component on L1 and another on L2. Essentially, these boxes are utilized to transmit messages between L1 and L2 via the rollup contract. The boxes can be envisaged as multi-sets that enable the same message to be inserted numerous times, a feature that is necessary to accommodate scenarios where, for instance, "deposit 10 eth to A" is required multiple times. The diagram below provides a detailed illustration of how one can perceive a message box in a logical context.

• Here, a sender will insert a message into the pending set, the specific constraints of the actions depend on the implementation domain, but for now, say that anyone can insert into the pending set.
• At some point, a rollup will be executed, in this step messages are "moved" from pending on Domain A, to ready on Domain B. Note that consuming the message is "pulling & deleting" (or nullifying). The action is atomic, so a message that is consumed from the pending set MUST be added to the ready set, or the state transition should fail. A further constraint on moving messages along the way, is that only messages where the sender and recipient pair exists in a leaf in the contracts tree are allowed!
• When the message have been added to the ready set, the recipient can consume the message as part of a function call.

Something that might seem weird when comparing to other cross-chain setups, is that we are "pulling" messages, and that the message don't need to be calldata for a function call. For Abritrum and the like, execution is happening FROM the "message bridge", which then calls the L1 contract. For us, you call the L1 contract, and it should then consume messages from the message box. Why? Privacy! When pushing, we would be needing a full calldata. Which for functions with private inputs is not really something we want as that calldata for L1 -> L2 transactions are committed to on L1, e.g., publicly sharing the inputs to a private function.

By instead pulling, we can have the "message" be something that is derived from the arguments instead. This way, a private function to perform second half of a deposit, could leak the "value" deposited and "who" made the deposit (as this is done on L1), but the new owner can be hidden on L2.

To support messages in both directions we logically require two of these message boxes (one in each direction), and then message passing between L1 and L2 is supported! However, due to the limitations of each domain, the message box for sending messages into the rollup and sending messages out are not fully symmetrical. In reality, the setup looks closer to the following:

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The L2 -> L1 pending messages set only exist logically, as it is practically unnecessary. For anything to happen to the L2 state (e.g., update the pending messages), the state will be updated on L1, meaning that we could just as well insert the messages directly into the ready set.

Rollup Contract

The rollup contract has a few very important responsibilities. The contract must keep track of the L2 rollup state root, perform state transitions and ensure that the data is available for anyone else to synchronise to the current state.

To ensure that state transitions are performed correctly, the contract will derive public inputs for the rollup circuit based on the input data, and then use a verifier contract to validate that inputs correctly transition the current state to the next. All data needed for the public inputs to the circuit must be from the rollup block, ensuring that the block is available. For a valid proof, the rollup state root is updated and it will emit an event to make it easy for anyone to find the data by event spotting.

As part of state transitions where cross-chain messages are included, the contract must "move" messages along the way, e.g., from "pending" to "ready".

Kernel Circuit

For L2 to L1 messages, the public inputs of a user-proof will contain a dynamic array of messages to be added, of size at most MAX_MESSAGESTACK_DEPTH, limited to ensure it is not impossible to include the transaction. The circuit must ensure, that all messages have a sender/recipient pair, and that those pairs exist in the contracts tree and that the sender is the L2 contract that actually emitted the message. For consuming L1 to L2 messages the circuit must create proper nullifiers.

Rollup Circuit

The rollup circuit must ensure that, provided two states $S$ and $S'$ and the rollup block $B$, applying $B$ to $S$ using the transition function must give us $S'$, e.g., $T(S, B) \mapsto S'$. If this is not the case, the constraints are not satisfied.

For the sake of cross-chain messages, this means inserting and nullifying L1 $\rightarrow$ L2 in the trees, and publish L2 $\rightarrow$ L1 messages on chain. These messages should only be inserted if the sender and recipient match an entry in the contracts leaf (as checked by the kernel).

Messages

While a message could theoretically be arbitrary long, we want to limit the cost of the insertion on L1 as much as possible. Therefore, we allow the users to send 32 bytes of "content" between L1 and L2. If 32 suffices, no packing required. If the 32 is too "small" for the message directly, the sender should simply pass along a sha256(content) instead of the content directly. In this manner, the message content can be "unpacked" on the receiving domain.

The message that is passed along, require the sender/recipient pair to be communicated as well (we need to know who should receive the message and be able to check). By having the pending messages be a contract on L1, we can ensure that the sender = msg.sender and let only content and recipient be provided by the caller. Summing up, we can use the structs seen below, and only store the commitment (sha256(LxLyCrossMsg)) on chain or in the trees, this way, we need only update a single storage slot per message.

struct L1Actor {
    address: actorAddress,
    uint256: chainid,
}

struct L2Actor {
    bytes32: actorAddress
}

struct L1L2CrossMsg {
    L1Actor: sender,
    L2Actor: recipient,
    bytes32: content,
}

struct L2L1CrossMsg {
    L2Actor: sender,
    L1Actor: recipient,
    bytes32: content,
}

Combined Architecture

The following diagram shows the overall architecture, combining our the earlier sections.

A note on L2 access control

Many applications rely on some form of access control to function well. USDC have a blacklist, where only parties not on the list should be able to transfer. And other systems such as Aave have limits such that only the pool contract is able to mint debt tokens and transfers held funds.

Access control like this cannot easily be enforced in the private domain, as reading is also nullifying(to ensure data is up to date). However, as it is possible to read historic public state, one can combine private and public functions to get the desired effect.

Say the public state holds a mapping(address user => bool blacklisted) and a value with the block number of the last update last_updated. The private functions can then use this public blacklist IF it also performs a public function call that reverts if the block number of the historic state is older than the last_updated. This means that updating the blacklist would make pending transactions fail, but allow a public blacklist to be used. Similar would work for the Aave example, where it is just a public value with the allowed caller contracts. Example of how this would be written is seen below. Note that because the isFresh is done in public, the user might not know when he is generating his proof whether it will be valid or not.

function transfer(
    secret address to,
    secret uint256 amount,
    secret HistoricState state
    ) secret returns(bool) {
    // Some spoonfeeding of proofs that check
    if (blacklisted[msg.sender] && isFresh(state.blockNumber)) {
        revert();
    }
}

function isFresh(pub uint256 blockNumber) public returns(bool){
    return blockNumber > last_updated;
}