SignalService

SignalService is a cross-chain signaling contract that enables secure message passing 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.