Validators run as part of the Multisig ISM or the Economic Security Module, fulfilling the security layer of the Hyperlane protocol. If using a different ISM, validators are not required. They are responsible for attesting to messages by signing a merkle root (also referred to as a checkpoint) when new dispatches happen in the Mailbox. Signatures must be made available for anyone (typically the Relayer) to fetch and submit alongside the message to the destination chain. Unlike many other protocols, Hyperlane does not have an enshrined validator set. Anyone is free to run their own validator, as long as the receiver contract specifies a Multisig ISM that includes their validator. Validator checkpoints are public. This ensures the transport layer is permissionless, since anyone can fetch the signatures and call Mailbox.process() to deliver the message. The Relayer merely provides a convenience service for message senders. It’s important that the Validator only signs finalized checkpoints to avoid compromising the safety of the protocol. Signing messages that get reorged out leads to slashing in the economic security module. The Validator agent connects to a single chain to check for new messages. If validating multiple chains, multiple instances of this agent must be run. Hyperlane is developing the validator as a binary implemented in Rust that can be easily run by anyone. Operationally, validators need to run this binary and have access to an RPC provider or node for the chain that they are validating for. We hope that the majority of Hyperlane validators will come from each chain’s existing node operator community.
Want to run a validator? Follow the validators guide.

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In more detail

The Validator agent uses two key types: one for signing transactions on the blockchain it is validating, and another for signing checkpoints on the Ethereum-compatible ECDSA curve. Since checkpoints are public, when a Validator first starts operating it must announce its checkpoint location via an onchain announce transaction. This is the only transaction ever submitted by the Validator agent; the blockchain-specific key is otherwise not used at all. There is also the option of manually announcing a new Validator to avoid configuring the blockchain-specific key. Validators can be announced by anyone. For messages using a Multisig ISM, validators read the current merkle root by calling MerkleTreeHook.latestCheckpoint(). Validators use the MerkleTreeHook.latestCheckpoint() function to determine when new transactions need to be indexed. This polling mechanism ensures validators can start signing new messages without the need to backfill the entire tree.

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Multisig ISM Prerequisites

To validate messages from an origin chain, the Validator’s checkpoint signing key must be registered in the Multisig ISM specified by the destination chain recipient. This can only be done when the ISM contract is deployed, but this is easily done by sending a deploy(validatorAddresses, threshold) transaction on any implementation of StaticThresholdAddressSetFactory.

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AVS Operator Prerequisites

To run a Validator for the EigenLayer AVS and provide economic security, the Validator must register as an AVS operator. See more details in the AVS Operator Guide.

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Architecture

The validator is comprised of two main tasks: an Indexer for the origin chain, and a Checkpoint Submitter to publish signed merkle roots.

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The Indexer

The only event type indexed by the Validator is InsertedIntoTree from the Merkle Tree Hook contract that the Mailbox calls whenever a new message is dispatched. This happens over RPC, and the agent builds an in-memory merkle tree based on the merkle insertion events, which are also cached to a local RocksDB database for fast startup. Whenever a new message is dispatched, the Checkpoint Submitter generates a checkpoint from the in-memory tree and adds it to a queue for processing.

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The Checkpoint Submitter

The checkpoint submitter uses the ECDSA key to sign checkpoints and publish them to highly-available storage. The schema of a checkpoint is 
pub struct Checkpoint {
    /// The merkle tree hook address
    pub merkle_tree_hook_address: H256,
    /// The mailbox / merkle tree hook domain
    pub mailbox_domain: u32,
    /// The checkpointed root
    pub root: H256,
    /// The index of the checkpoint
    pub index: u32,
}
Checkpoint storage that is currently supported by the agent implementation is AWS S3 and Google Cloud Storage. The Azure Blob Storage contribution is still in progress.

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Liveness implications

Depending on the threshold configured in the Multisig ISM, Validator downtime can halt liveness of the protocol. The submitter prioritizes new checkpoints over old ones, so that the Relayer can fetch metadata for newer messages first, which are more likely to not have been delivered yet.

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Default ISM Validators

The number of validators depend on the Interchain Security Module (ISM) configuration. The Default ISM uses a specific validator set, you can view the Validator details such as threshold, operator and addresses here.