# Smart Contracts

For more details about any of these contracts, please refer to the source code for the [contracts](https://github.com/succinctlabs/telepathy-contracts) and [circuits](https://github.com/succinctlabs/telepathy-circuits) respectively.

## Light Client

The [Light Client contract](https://github.com/succinctlabs/telepathy-contracts/blob/main/src/lightclient/LightClient.sol) is responsible for keeping up-to-date with the state of Ethereum. In particular, it keeps track of an append-only list of Ethereum block headers based on the verification that >2/3rds of the sync committee has provided signatures.

To add a new block header to a Light Client, one must generate a zkSNARK proof that validates the latter claim and pass the proof as an argument to the [step](https://github.com/succinctlabs/telepathy-contracts/blob/fe9566b7837487dfb6d39e708a2c0bfb7c52ceaf/src/lightclient/LightClient.sol#L94) function:

```solidity
step(LightClientStep memory update)
```

where an [`update`](https://github.com/succinctlabs/telepathy-contracts/blob/fe9566b7837487dfb6d39e708a2c0bfb7c52ceaf/src/lightclient/LightClient.sol#L15) contains all the necessary information for validation:

```solidity
struct LightClientStep {
    uint256 attestedSlot;
    uint256 finalizedSlot;
    uint256 participation;
    bytes32 finalizedHeaderRoot;
    bytes32 executionStateRoot;
    Groth16Proof proof;
}
```

This allows the Light Client to check the following conditions:

* \>2/3rds participation signatures from the current sync committee
* A valid finality proof for the `finalizedHeaderRoot`
* A valid execution state root proof against the `finalizedHeaderRoot`

If those conditions all pass, the current head of the Light Client is set to the `finalizedSlot` with the `finalizedHeaderRoot` stored on-chain.

## Router

The [Router contract](https://github.com/succinctlabs/telepathy-contracts/tree/main/src/amb) is how developers use Telepathy Protocol to achieve cross-chain communication between their contracts. It is responsible for handling both the sending and receiving of messages.

#### Sending

When the Router is used on the source chain to send messages, the entry point is the Router's [`send`](https://github.com/succinctlabs/telepathy-contracts/blob/main/src/amb/TargetAMB.sol#L104) function:

```solidity
send(uint32 destinationChainId, bytes32 destinationAddress, bytes calldata data)
```

The `destinationChainId` and `destinationAddress` parameters are used to specify the chain and the address of the destination contract. The `data` parameter is used to specify the message that the destination contract receives. The `send` function emits a `SentMessage` event, which is used for validation on the destination chain.

#### Receiving

When on the destination chain, the Router contract is responsible for relaying the message by using the light client to verify the message was actually sent on the source chain, and then calling `handleTelepathy` with the correct arguments. This happens when [`executeMessageFromLog`](https://github.com/succinctlabs/telepathy-contracts/blob/main/src/amb/TargetAMB.sol#L104) function is called:

```solidity
executeMessageFromLog(
    bytes calldata srcSlotTxSlotPack,
    bytes calldata messageBytes,
    bytes32[] calldata receiptsRootProof,
    bytes32 receiptsRoot,
    bytes[] calldata receiptProof,
    bytes memory txIndexRLPEncoded,
    uint256 logIndex
)
```

An off-chain actor, known as a [Relayer](https://docs.telepathy.xyz/actors#relayer), will call this function with the required information and Merkle proofs that are used to validate the message against the stored header root in the `LightClient`. Once validated, the message is relayed to the specified`destinationAddress` via the [`handleTelepathy`](https://github.com/succinctlabs/telepathy-contracts/blob/fe9566b7837487dfb6d39e708a2c0bfb7c52ceaf/src/amb/interfaces/ITelepathy.sol#L67)function:

```solidity
handleTelepathy(uint32 sourceChainId, address sourceAddress, bytes memory data)
```

A destination contract must implement this function to receive the message.