> ## 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.

# Multisig ISM

The `MultisigISM` is one of the most commonly used ISM types. These ISMs verify that `m` of `n` [Validators](/docs/protocol/agents/validators) have attested to the validity of a particular interchain message.

## Multisig Module Types

There are two types of `MultisigISM` modules:

```solidity theme={null}
enum Types {
    ...
    MERKLE_ROOT_MULTISIG,
    MESSAGE_ID_MULTISIG,
    ...
}
```

### MerkleRootMultisigIsmMetadata

**Type: `Types.MERKLE_ROOT_MULTISIG`**

**Format of metadata:**

| **Component**                                | **Length (bytes)** | **Description**                                         |
| -------------------------------------------- | ------------------ | ------------------------------------------------------- |
| 1st component                                | 32                 | Origin merkle tree address                              |
| 2nd component                                | 4                  | Index of message ID in merkle tree                      |
| 3rd component                                | 32                 | Signed checkpoint message ID                            |
| 4th component                                | 1024               | Merkle proof                                            |
| 5th component                                | 4                  | Signed checkpoint index (computed from proof and index) |
| 6th component (len == threshold \* 65 bytes) | threshold \* 65    | Validator signatures                                    |

### MessageIdMultisigIsmMetadata

**Type: `Types.MESSAGE_ID_MULTISIG`**

**Format of metadata:**

| **Component**                                | **Length (bytes)** | **Description**            |
| -------------------------------------------- | ------------------ | -------------------------- |
| 1st component                                | 32                 | Origin merkle tree address |
| 2nd component                                | 32                 | Signed checkpoint root     |
| 3rd component                                | 4                  | Signed checkpoint index    |
| 4th component (len == threshold \* 65 bytes) | threshold \* 65    | Validator signatures       |

## Interface

`MultisigISMs` must implement the `IMultisigIsm` interface.

```solidity theme={null}
// SPDX-License-Identifier: MIT OR Apache-2.0
pragma solidity >=0.8.0;

import {IInterchainSecurityModule} from "../IInterchainSecurityModule.sol";

interface IMultisigIsm is IInterchainSecurityModule {
    /**
     * @notice Returns the set of validators responsible for verifying _message
     * and the number of signatures required
     * @dev Can change based on the content of _message
     * @param _message Hyperlane formatted interchain message
     * @return validators The array of validator addresses
     * @return threshold The number of validator signatures needed
     */
    function validatorsAndThreshold(
        bytes calldata _message
    ) external view returns (address[] memory validators, uint8 threshold);
}
```

## Configure

The hyperlane-monorepo contains [`MultisigISM` implementations](https://github.com/hyperlane-xyz/hyperlane-monorepo/tree/main/solidity/contracts/isms/multisig) (including a [legacy](https://github.com/hyperlane-xyz/hyperlane-monorepo/blob/main/solidity/contracts/isms/multisig/LegacyMultisigIsm.sol) version and more gas-efficient [versions](https://github.com/hyperlane-xyz/hyperlane-monorepo/blob/main/solidity/contracts/isms/multisig/StaticMultisigIsm.sol) deployable via factories) that application developers can deploy off-the-shelf, specifying their desired configuration.

To configure a `MultisigISM` instance:

1. Developers define the set of `n` Validators on each origin chain.
2. A threshold is set, specifying the number (`m`) of Validator signatures required to confirm a message.

Validator signatures are **not** specific to an ISM. In other words, developers can configure their `MultisigISM` to use **any** validator that's running on Hyperlane.

## Customize

For more specific use cases, developers can fork the abstract `MultisigISM` implementation provided in the Hyperlane monorepo. The primary customization involves implementing the `validatorsAndThreshold()` function.

Custom implementations allow developers to adjust the security model to meet the needs of their application. For example, a custom implementation could vary the number of signatures required, based on the content of the message being verified.