SignalService

SignalService is a cross-chain signaling contract that enables secure message passing in Taiko Alethia. It allows applications to send signals, synchronize chain data, and verify signals using Merkle proofs. The contract ensures that cross-chain messages are securely persisted and verifiable without introducing additional trust assumptions.

Features

Cross-chain message synchronization: Facilitates Merkle-proof-based message verification.
State root and signal caching: Stores and verifies state roots and signal roots for cross-layer communication.
Efficient storage: Uses slot-based storage for optimized signal retrieval.
Authorization mechanism: Controls who can sync chain data to prevent unauthorized updates.

Contract Methods

`sendSignal`

Stores a signal in contract storage for cross-chain message passing.

Parameter	Type	Description
`_signal`	`bytes32`	The signal (message) to store.

Returns:

Return Value	Type	Description
`slot_`	`bytes32`	The storage slot where the signal is stored.

`syncChainData`

Synchronizes state roots and signal roots from other chains.

Parameter	Type	Description
`_chainId`	`uint64`	Identifier of the source chain.
`_kind`	`bytes32`	Type of data being synced (e.g., state root, signal root).
`_blockId`	`uint64`	Block ID from which the data is sourced.
`_chainData`	`bytes32`	Data to be stored as the signal.

Returns:

Return Value	Type	Description
`signal_`	`bytes32`	The signal corresponding to the stored chain data.

`proveSignalReceived`

Verifies that a signal has been received on the target chain using Merkle proofs.

Parameter	Type	Description
`_chainId`	`uint64`	Source chain identifier.
`_app`	`address`	Address that sent the signal.
`_signal`	`bytes32`	The signal being verified.
`_proof`	`bytes`	Merkle proof that the signal was persisted on the source chain.

Returns:

Return Value	Type	Description
`numCacheOps_`	`uint256`	The number of newly cached items.

Mechanism:

Uses _verifySignalReceived() to validate the proof.
Caches state roots and signal roots for future verification.

`verifySignalReceived`

Read-only version of proveSignalReceived, verifying without modifying state.

Parameter	Type	Description
`_chainId`	`uint64`	Source chain identifier.
`_app`	`address`	Address that sent the signal.
`_signal`	`bytes32`	The signal being verified.
`_proof`	`bytes`	Merkle proof that the signal was persisted on the source chain.

Technical Note:

Uses _verifySignalReceived() for validation.
Does not cache or modify contract state.

`isSignalSent`

Checks whether a given signal has been sent.

Parameter	Type	Description
`_app`	`address`	Address that sent the signal.
`_signal`	`bytes32`	The signal to check.

Returns:

Return Value	Type	Description
`bool`	`bool`	Returns `true` if the signal has been sent, otherwise `false`.

`isChainDataSynced`

Checks if a given chain data has been successfully synced.

Parameter	Type	Description
`_chainId`	`uint64`	Source chain identifier.
`_kind`	`bytes32`	Data type identifier.
`_blockId`	`uint64`	Block ID from which the data is sourced.
`_chainData`	`bytes32`	Data to check for synchronization.

Returns:

Return Value	Type	Description
`bool`	`bool`	Returns `true` if the data is synced, otherwise `false`.

`getSyncedChainData`

Fetches the latest synchronized chain data.

Parameter	Type	Description
`_chainId`	`uint64`	Source chain identifier.
`_kind`	`bytes32`	Data type identifier.
`_blockId`	`uint64`	Block ID (if `0`, retrieves the most recent).

Returns:

Return Value	Type	Description
`blockId_`	`uint64`	Block ID of the synced data.
`chainData_`	`bytes32`	Synchronized data from the source chain.

`signalForChainData`

Derives a unique signal for given chain data.

Parameter	Type	Description
`_chainId`	`uint64`	Source chain identifier.
`_kind`	`bytes32`	Data type identifier.
`_blockId`	`uint64`	Block ID from which the data is sourced.

Returns:

Return Value	Type	Description
`signal_`	`bytes32`	Unique signal hash representing the chain data.

`getSignalSlot`

Computes the storage slot where a signal is stored.

Parameter	Type	Description
`_chainId`	`uint64`	Chain ID of the signal.
`_app`	`address`	Address that initiated the signal.
`_signal`	`bytes32`	Signal message to retrieve.

Returns:

Return Value	Type	Description
`slot_`	`bytes32`	Storage slot where the signal is stored.

Events

`SignalSent`

Emitted when a signal is sent.

Parameter	Type	Description
`app`	`address`	Address that initiated the signal.
`signal`	`bytes32`	The signal (message) that was sent.
`slot`	`bytes32`	Storage slot of the signal.
`value`	`bytes32`	The signal’s value.

`ChainDataSynced`

Emitted when state or signal roots are synced from another chain.

Parameter	Type	Description
`chainId`	`uint64`	Source chain identifier.
`blockId`	`uint64`	Block ID associated with the data.
`kind`	`bytes32`	Data type identifier (state root or signal root).
`data`	`bytes32`	The synchronized chain data.
`signal`	`bytes32`	Signal associated with the chain data.

`Authorized`

Emitted when an address is authorized or deauthorized for syncing chain data.

Parameter	Type	Description
`addr`	`address`	The address being authorized or deauthorized.
`authorized`	`bool`	`true` if authorized, `false` otherwise.

State Variables

Variable	Type	Description
`topBlockId`	`mapping(uint64 => mapping(bytes32 => uint64))`	Tracks the highest synced block ID for each chain and data type.
`isAuthorized`	`mapping(address => bool)`	Stores addresses authorized to sync chain data.