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🧪 Exbrid

Exbrid is an experiment exploring an alternate approach to message passing between Ethereum and Polkadot by embedding a Polkadot light client (Smoldot) directly inside an Ethereum node (Reth) using its plugin system ExEx.

Instead of relying on separate off-chain relayers, this prototype shows how finalized Ethereum block information can be captured from within the Ethereum node itself and submitted directly to a Polkadot-based Substrate chain (like Paseo) using Subxt, with Smoldot as the embedded client.

Initial Research

❗ Disclaimer: This is an early-stage prototype for research and exploration purposes only. It is not intended for production or secure cross-chain message passing.

The following questions are yet to be answered before it can be used safely:

  • How will the Substrate/Polkadot runtime know that the message containing the finalized hash is actually correct?
  • Can we get around the fact that two sides need to run an onchain light client of one another?
  • Maybe other security or trust assumptions that we must validate. TBD.

🧭 Project Overview

  • Runs a full Ethereum node using Reth with a custom ExEx extension.
  • Captures finalized Ethereum block numbers and hashes.
  • Uses Smoldot to run a lightweight Polkadot client embedded in the same process.
  • Uses Subxt to submit system.remark_with_event transactions to a Substrate-based chain with the finalized Ethereum block info.

This allows Ethereum to directly push information to Polkadot with no external services or full Polkadot node dependencies.

📁 Structure

.
├── src/
│   └── main.rs                 # Reth ExEx plugin + Subxt integration
├── paseo_metadata.scale        # Subxt metadata for Paseo chain
├── paseo.raw.json              # Chain spec for Smoldot light client
├── Cargo.toml
├── README.md

⚙️ Requirements

  • Rust >= 1.73
  • Subxt CLI to generate metadata
  • protoc if compiling Smoldot from source
  • cast (optional) to send test Ethereum transactions
  • OpenSSL (for generating JWT secrets when working with Holesky)

🔬 Running the Experiment Locally (Dev Mode)

🧹 Optional Cleanup (Reset Dev Chain)

pkill -f reth
rm -rf "/Users/<username>/Library/Application Support/reth/dev"
rm -rf "/Users/<username>/Library/Caches/reth"

▶️ Start the Node in Dev Mode

cargo run -- node --dev --http

This will:

  • Start a Reth dev node
  • Capture finalized Ethereum blocks
  • Run Smoldot light client to connect to Paseo
  • Submit finalized block data to Polkadot via remark_with_event

⛽ Trigger Block Production (in Dev Mode)

Dev mode only produces blocks when new transactions are submitted.

Use cast to send a dummy transaction and trigger block production:

cast send \
  --private-key 0xac0974bec39a17e36ba4a6b4d238ff944bacb478cbed5efcae784d7bf4f2ff80 \
  --value 0 \
  0x000000000000000000000000000000000000dEaD \
  --rpc-url http://localhost:8545

Important: Only submit this transaction after you see this log in your terminal:

✅ Smoldot is ready

This indicates that the light client is fully synced and ready to submit transactions.

🔄 End-to-End Flow Summary

Ethereum (Reth Node)
    │
    └───▶ ExEx captures finalized block
               │
               └───▶ Broadcasts (block_number, block_hash)
                            │
                            ▼
                 Subxt + Smoldot light client
                            │
                            └───▶ Submit remark to Polkadot chain

🧪 Holesky Testnet Setup (Optional)

This project can also be tested on Ethereum Holesky testnet instead of --dev mode.

1. Generate JWT Secret

openssl rand -hex 32 > /tmp/jwt/jwt.hex

2. Start Reth on Holesky

cargo run -- node \
  --chain holesky \
  --authrpc.jwtsecret /tmp/jwt/jwt.hex \
  --http

3. Start Lighthouse for Beacon Node Sync

lighthouse bn \
  --network holesky \
  --purge-db \
  --datadir /tmp/lighthouse-holesky \
  --execution-endpoint http://localhost:8551 \
  --execution-jwt /tmp/jwt/jwt.hex \
  --checkpoint-sync-url https://holesky.beaconstate.info/

This setup allows finalized blocks from Holesky to be captured and relayed to a Substrate chain.

🛠 Setup Notes

📥 Download Paseo Chain Spec

curl -o paseo.raw.json https://paseo-r2.zondax.ch/chain-specs/paseo.raw.json

⚙️ Generate Subxt Metadata for Paseo

subxt codegen --url https://rpc.paseo.nodestake.top --output paseo_metadata.scale

🔐 Key Management (Experimental Only)

The current prototype uses a hardcoded test mnemonic to sign transactions:

let phrase = Mnemonic::parse("girl run radar student point expect segment process ocean ability artwork ahead").unwrap();

⚠️ This is insecure and must be replaced with proper key management (e.g., remote signer, vault integration) if the approach is ever extended toward production usage.

👨‍🔬 Purpose

This project is not intended to be a fully-featured bridge, but rather a proof-of-concept and a technical experiment that demonstrates:

  • The feasibility of embedding a Polkadot light client into Ethereum
  • Relaying finalized data across chains without external services
  • Leveraging Reth’s ExEx system as a programmable Ethereum environment