> ## 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.

# Create Your Own ISM

Developers can specify or override the default ISM by implementing the `ISpecifiesInterchainSecurityModule` interface in their application.

<Note>
  If no ISM is specified, or if the specified ISM is the null address, the
  default ISM configured on the destination chain's Mailbox will be used.
</Note>

## ISM Interface

ISMs must implement the `InterchainSecurityModule` interface. This implementation can be configured, composed, and customized according to the needs of their application.

Specifically, this interface must be implemented in the same smart contract that implements `handle()`.

The interface consists of two functions: `verify` and `moduleType`.

<Accordion title="InterchainSecurityModule Interface">
  ```solidity Solidity theme={null}
  // SPDX-License-Identifier: MIT OR Apache-2.0
  pragma solidity >=0.8.0;

  enum ModuleType {
  UNUSED,
  ROUTING,
  AGGREGATION,
  LEGACY_MULTISIG,
  MERKLE_ROOT_MULTISIG,
  MESSAGE_ID_MULTISIG,
  NULL, // used with relayer carrying no metadata
  CCIP_READ,
  ARB_L2_TO_L1,
  POLYGON_POS,
  OP_STACK
  }

  interface InterchainSecurityModule {
  /\*\*
  _ @notice Returns an enum that represents the type of security model
  _ encoded by this ISM.
  _ @dev Relayers infer how to fetch and format metadata.
  _/
  function moduleType() external view returns (uint8);

      /**
       * @notice Defines a security model responsible for verifying interchain
       * messages based on the provided metadata.
       * @param _metadata Off-chain metadata provided by a relayer, specific to
       * the security model encoded by the module (e.g. validator signatures)
       * @param _message Hyperlane encoded interchain message
       * @return True if the message was verified
       */
      function verify(
          bytes calldata _metadata,
          bytes calldata _message
      ) external returns (bool);

  }

  ```
</Accordion>

## Verify

```solidity theme={null}
/**
 * @notice Defines a security model responsible for verifying interchain
 * messages based on the provided metadata.
 * @param _metadata Off-chain metadata provided by a relayer, specific to
 * the security model encoded by the module (e.g. validator signatures)
 * @param _message Hyperlane encoded interchain message
 * @return True if the message was verified
 */
function verify(
    bytes calldata _metadata,
    bytes calldata _message
) external returns (bool);
```

The primary function that ISMs must implement is `verify()`. The [Mailbox](/docs/protocol/core/mailbox) will call `IInterchainSecurityModule.verify()` before delivering a message to its recipient. If `verify()` reverts or returns `false`, the message will not be delivered.

### Parameters

* `_metadata`: consists of arbitrary bytes provided by the [Relayer](/docs/protocol/agents/relayer). Typically, these bytes are specific to the ISM. For example, the `_metadata` for a [Multisig ISM](/docs/protocol/ISM/standard-ISMs/multisig-ISM) must include validator signatures.

* `_message`: consists of the Hyperlane message being verified. ISMs can use this to inspect details about the message being verified. For example, a [Multisig ISM](/docs/protocol/ISM/standard-ISMs/multisig-ISM) could change validator sets based on the origin chain of the message.

<Warning>
  See the
  [`Message.sol`](https://github.com/hyperlane-xyz/hyperlane-monorepo/blob/main/solidity/contracts/libs/Message.sol)
  library for more information on the format of the Hyperlane message passed to
  `verify()`.
</Warning>

## Module Type

The secondary function that ISMs must implement is `moduleType()`.

```solidity theme={null}
function moduleType() external view returns (uint8);
```

This is used to signal to the [Relayer](/docs/protocol/agents/relayer) what to include in `_metadata`. ISMs **must** return one of the supported module types.

```solidity theme={null}
enum ModuleType {
    UNUSED,
    ROUTING,
    AGGREGATION,
    LEGACY_MULTISIG,
    MERKLE_ROOT_MULTISIG,
    MESSAGE_ID_MULTISIG,
    NULL, // used with relayer carrying no metadata
    CCIP_READ,
    ARB_L2_TO_L1,
    POLYGON_POS,
    OP_STACK
}
```

<Note>
  * All ISM contracts must implement the ISM interface, which requires `moduleType` to be defined. This type is branched on by the Relayer in order to determine the required metadata for that ISM. For more information on module types and their metadata formats, see [Protocol](/docs/protocol/ISM/modular-security#core-concepts).

  * For more information on the available module types and their respective metadata, please visit the ISM docs outlined in this section, e.g. [Multisig ISM](/docs/protocol/ISM/standard-ISMs/multisig-ISM).
</Note>