> ## Documentation Index
> Fetch the complete documentation index at: https://docs.hyperlane.xyz/llms.txt
> Use this file to discover all available pages before exploring further.

# Polygon PoS Hook

Polygon PoS has their own interface for message passing between Ethereum and Polygon called the [`fx portal`](https://github.com/0xPolygon/fx-portal). Messages passed via this interface benefit from the security of the Polygon [state sync mechanism](https://docs.polygon.technology/pos/architecture/bor/state-sync/).

To provide this security as an option in Hyperlane, we created a Hook and ISM combo that can be transparently configured to reuse the fx-portal interface.

See the [addresses](/docs/reference/addresses/deployments/mailbox) page for Hook and ISM addresses.

## How It Works

Polygon FX-Portal provide FxChild (FxChild.sol) and FxRoot (FxRoot.sol) as the main contracts on which the bridge works. It calls and passes data to user-defined methods on another chain without needing a mapping.

The FxChild/FxRoot contract are provided and maintained by the polygon team. You can find the address of this contract in Polygon's [Fx-Portal repo](https://github.com/0xPolygon/fx-portal).
The PolygonPosHook sends payloads with FxRoot. Validators will pick up the message and pass it to the other chain. You can find detailed state sync mechanism in [documentation](https://docs.polygon.technology/pos/how-to/bridging/l1-l2-communication/state-transfer/) `StateReceiver` at `0x0000000000000000000000000000000000001001` is allowed to call `onStateReceive` in FxChild contract. FxChild calls `processMessageFromRoot` in abstract `CrossChainEnabledPolygonChild` on which PolygonPosISM implements. `CrossChainEnabledPolygonChild` is maintained by [OpenZeppelin](https://docs.openzeppelin.com/contracts/4.x/api/crosschain#)

```mermaid theme={null}
flowchart TB
    subgraph Origin Ethereum
      Sender
      M_O[(Mailbox)]
      Hook[PolygonPosHook]
      Eth[(FxRoot)]

      Sender -- "dispatch(...)" --> M_O
      M_O -- "postDispatch(message)" --> Hook
      Hook -- "sendMessage(messageId)" --> Eth
    end

    M_O -. "relay" .-> M_D
    Eth -. "validator node" .-> Polygon

    subgraph Destination Polygon PoS
      Recipient
      M_D[(Mailbox)]
      ISM{PolygonPosISM}
      Polygon[(FxChild)]

      M_D -- "verify(..., message)" --> ISM
      M_D -- "handle(...)" --> Recipient
      ISM -. "interchainSecurityModule()" .- Recipient

      Polygon -- "verifyMessageId(messageId)" --> ISM
    end

    style Eth fill: #ff0402
    style Polygon fill: #ff0402
```