Week 14: Building a Multi-Chain ERC20 Token Bridge

This week was all about building in action—the kind of week where ideas turn into working systems. We rolled up our sleeves to architect a cross-chain ERC20 token bridge, enabling seamless transfers between Base and Sepolia testnets. The challenge? Ensuring secure, trust-minimized asset movement while handling blockchain finality. Here’s how we tackled it.

Blockchains are siloed by design, but real-world use cases demand interoperability. Our goal was to Let users move ERC20 tokens from Base to Sepolia (and vice versa).

Avoid centralized custodians—make it permissionless and event-driven.

Handle chain finality delays safely.

Architecture Breakdown

Smart Contracts (Both Chains)

Token Contracts: Deployed identical ERC20 tokens on both Base and Sepolia (with mint/burn logic).

Bridge Contracts:

Lock/Burn: On the source chain, users lock tokens (or burn them for mint-burn bridges).

Mint/Release: On the destination chain, the bridge mints/releases tokens upon proof verification.

Backend Relayer

Event Listening: Polled source chain for TokenLocked or TokenBurned events.

Finality Handling: Waited for enough block confirmations to prevent reorg attacks.

Proof Submission: Relayed transaction proofs to the destination chain’s bridge contract.

User Flow

User locks 100 tokens on Base → Relayer detects event → After finality, relayer triggers minting of 100 tokens on Sepolia.

Key Challenges & Lessons
Finality Risks: Sepolia and Base have different finality times—had to account for delays.

Relayer Reliability: Designed fail-safes to prevent double-spending if the relayer crashed mid-process.

Cost Optimization: Batch submissions to reduce gas fees for cross-chain transactions.

Why This Matters
Bridges are the highways of Web3. By building one, we:
✔️ Understood the security pitfalls (e.g., reorgs, relay trust).
✔️ Learned to coordinate contracts across chains.
✔️ Saw how event-driven backends power decentralized systems.