🔑 Horcrux

Horcrux is a distributed signing tool that splits the validator's private key into multiple parts, ensuring its security. This approach prevents key compromise even if one part is accessed.

Preparing the Infrastructure

Horcrux requires at least two servers (three or more are recommended) to split the key. Setup involves:

Horcrux servers for distributed signing.

Validator node to interact with the blockchain network.

Ensure the following:

  1. SSH access and server setup.

  2. Install Horcrux on each server.

  3. Configure firewalls to allow communication between servers on the Horcrux ports.

  4. Time synchronization: Make sure all servers use NTP for synchronized time.

sudo apt install ntp  
sudo systemctl enable ntp  
sudo systemctl start ntp

Server Requirements for Horcrux-Based Validator Setup

Signer Nodes (Horcrux Co-signers)

Type: VPS (Virtual Private Server) Recommended Specifications:

  • CPU: 4 vCPU

  • RAM: 4 GB

  • Storage: 80 GB NVMe SSD

  • Network: 1 Gbps bandwidth

Required Open Ports:

  • 2222 — for communication between Horcrux co-signers

  • SSH (22) — for remote administration

  • Prometheus port (optional, if using monitoring)

Sentry Nodes (Validator Nodes or Observers)

Type: Bare-metal or VPS Recommended Specifications:

  • CPU: 4 vCPU

  • RAM: 16 GB

  • Storage: 250 GB SSD

  • Network: Stable and fast connection

Required Open Ports:

  • 1234 — private port for connection from Horcrux signers

  • 26656 — P2P port for communication with the blockchain network

Update system and install build tools

sudo apt -q update
sudo apt -qy install curl git jq lz4 build-essential
sudo apt -qy upgrade

Install GO

sudo rm -rf /usr/local/go
curl -Ls https://go.dev/dl/go1.23.6.linux-amd64.tar.gz | sudo tar -xzf - -C /usr/local
eval $(echo 'export PATH=$PATH:/usr/local/go/bin' | sudo tee /etc/profile.d/golang.sh)
eval $(echo 'export PATH=$PATH:$HOME/go/bin' | tee -a $HOME/.profile)
echo "export PATH=$PATH:/usr/local/go/bin:$HOME/go/bin" >> $HOME/.bash_profile
source $HOME/.bash_profile
go version

Installing Horcrux

Run the following commands on each co-signer node:

git clone https://github.com/strangelove-ventures/horcrux.git
cd horcrux
make install

After installation, verify it with:

horcrux version

Initialize Horcrux Configuration

Run the following command on one of the co-signer nodes to generate the shared configuration:

horcrux config init \
  --node "tcp://<VALIDATOR_IP>:1234" \
  --cosigner "tcp://<COSIGNER_1_IP>:2222" \
  --cosigner "tcp://<COSIGNER_2_IP>:2222" \
  --cosigner "tcp://<COSIGNER_3_IP>:2222" \
  --threshold 2 \
  --grpc-timeout 1000ms \
  --raft-timeout 1000ms

🔧 Note:

  • Replace <VALIDATOR_IP_X> with the actual IP addresses of your validator (or sentry) nodes that expose the priv_validator_laddr port (1234).

  • Replace <COSIGNER_X_IP> with the IPs of each co-signer node using port 2222.

  • The order of the --cosigner flags determines which shard (cosigner_1, cosigner_2, cosigner_3) goes to which server. Be consistent when distributing keys and configuration files.

Generate ECIES Keys (for Secure Communication Between Co-Signers)

Run this once to generate the ECIES encryption keys:

horcrux create-ecies-shards --shards 3

This will create the following directory structure:

./cosigner_1/
  ecies_keys.json
./cosigner_2/
  ecies_keys.json
./cosigner_3/
  ecies_keys.json

Fragment the Validator Private Key

⚠️ Copy the priv_validator_key.json file from your validator node into your working directory before running this command.

Then run:

horcrux create-ed25519-shards --chain-id <CHAIN_ID> --key-file ./priv_validator_key.json --threshold 2 --shards 3

This will update the structure:

./cosigner_1/
  <CHAIN_ID>_shard.json
  ecies_keys.json

./cosigner_2/
  <CHAIN_ID>_shard.json
  ecies_keys.json

./cosigner_3/
  <CHAIN_ID>_shard.json
  ecies_keys.json

✅ At this point, you should move priv_validator_key.json to a secure storage location — it is no longer needed in the signer environment.

Distribute Config Files to Co-Signer Nodes

Each co-signer node must have the following files placed in their respective ~/.horcrux/ directory:

~/.horcrux/
├── config.yaml
├── ecies_keys.json
├── <CHAIN_ID>_shard.json
└── state/
    └── <CHAIN_ID>_priv_validator_state.json

❗ Important: Ensure that each cosigner receives only their own files according to the order in which you ran horcrux config init and generated shards (i.e., cosigner_1, cosigner_2, cosigner_3).

Setting Up the Horcrux Service

Create a service file: On each server, create a file at /etc/systemd/system/horcrux.service:

sudo tee /etc/systemd/system/horcrux.service > /dev/null << EOF  
[Unit]  
Description=MPC Signer node  
After=network-online.target  
[Service]  
User=$USER  
WorkingDirectory=/home/$USER  
ExecStart=$(which horcrux) start  
Restart=on-failure  
RestartSec=3  
LimitNOFILE=4096  
[Install]  
WantedBy=multi-user.target  
EOF

Start and check the service:

sudo systemctl daemon-reload  
sudo systemctl enable horcrux.service
sudo systemctl start horcrux.service  
sudo systemctl status horcrux.service

Share Consensus State Between Signers

Each signer node must start from the same consensus state to avoid double-signing.

🛑 On the validator node:

sudo systemctl stop <node_service>

  1. Open the validator's consensus state file:

    cat .<NODE_HOME>/data/priv_validator_state.json

    Example output:

    {
  "height": "6513530",
  "round": 0,
  "step": 3,
  "signature": "IEOS7EJ8C6Z...",
  "signbytes": "6B080211BA83..."
}

  2. Create a stripped-down version of this file to be placed in each co-signer’s state directory:

    {
  "height": "6513530",
  "round": "0",
  "step": 3
}

  3. Copy this file to all co-signer nodes as:

    ~/.horcrux/state/<CHAIN_ID>_priv_validator_state.json

Configuring the Validator Node

  • Update the validator's configuration file: In the config.toml file of your validator node, specify the remote Horcrux address for signing:

priv_validator_laddr = "tcp://0.0.0.0:1234"

  • Restart the validator:

sudo systemctl restart <node_service>

Check Horcrux logs:

sudo journalctl -u horcrux -f

Monitoring and Testing

  • Check connectivity: Ensure the validator node successfully connects to the Horcrux servers.

  • Test signing: Verify transactions are signed correctly by creating a test transaction on the network.

  • Setup monitoring: Use tools like Prometheus and Grafana to monitor the validator and Horcrux servers.

Security Recommendations

  • Isolate Horcrux servers: Restrict access to only the validator node.

  • Keep software updated: Regularly update Horcrux and the operating systems.

  • Backup configuration: Store copies of the configuration and key shares in a secure location.

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