TapThatX

Problem Statement

TapThat X is an NFC chip-powered blockchain automation and execution protocol that allows users to execute any smart contract transaction with a simple tap of their phone—no wallet popups, no friction.The blockchain user experience today is broken. Every transaction requires multiple MetaMask popups—approving actions, confirming gas, switching networks—which causes significant drop-off. Complex DeFi operations, like bridging assets or rebalancing positions, can require 5–7 popups and several minutes of manual clicks.Our solution brings the instant “tap-to-pay” experience of credit cards to blockchain operations, making even complex transactions seamless.Setup Phase (One-Time)Users register their HaLo NFC chips (from Arx Research) to their wallet on-chain by proving ownership via an EIP-712 signature.Users pre-configure smart contract actions, e.g., “Send $10 USDC to Alice” or “Rebalance my Aave position.”Execution Phase (Every Tap)User taps their phone near the chip; the chip signs an authorization message.A gasless relay API submits the transaction on the user’s behalf.Smart contracts verify chip ownership, signature freshness, and nonce to prevent replay attacks.The pre-configured action executes automatically on the target contract.Why this is powerfulThe protocol’s extension architecture unlocks capabilities beyond simple token transfers:ERC20 Transfers – Tap-to-pay token transfers with a single action.Dual-Chain ETH Bridge – One tap unwraps WETH and bridges it to Base Sepolia and OP Sepolia simultaneously—ideal for developers refilling testnet gas.Aave Position Rebalancing – One tap optimizes your Aave health factor using flash loans—a sequence impossible to execute manually, as withdrawing collateral to buy debt tokens would normally trigger liquidation.TapThat X completely removes wallet UX friction. Users tap their phone, the chip cryptographically authorizes the action, and complex DeFi operations execute atomically in ~3 seconds. From simple payments to strategies impossible to perform manually, TapThat X delivers credit card-level UX for blockchain.

Solution

We designed TapThat X as a modular five-contract protocol deployed on Base Sepolia and Optimism Sepolia:TapThatXRegistry – Manages chip-to-owner mappings. It leverages EIP-712 signatures with a chain-agnostic domain separator (no chainId), enabling chips to be registered once and used across multiple chains.TapThatXConfiguration – Stores pre-configured actions for each (owner, chip) pair, including target contract, encoded calldata, ETH value, and descriptions.TapThatXExecutor – The main entry point called by the relay. It fetches the action configuration and delegates execution to the Protocol while maintaining security checks.TapThatXProtocol – The validation engine. It uses the TapThatXAuth library to recover the chip’s address from EIP-712 signatures, validates ownership via the Registry, checks nonces (preventing replay attacks), enforces a 5-minute expiration window, and executes the authorized calls.TapThatXAuth – A lightweight library for secure EIP-712 signature recovery and timestamp validation.Extension Contracts (Plug-In Architecture)TapThatXBridgeETHViaWETH – Pulls the user’s pre-approved WETH, unwraps it to native ETH, and calls both L1StandardBridge contracts (Optimism + Base) to bridge simultaneously. This hack was born from a hackathon constraint: we kept running out of testnet gas across chains, so bridging in one atomic operation was necessary.TapThatXAaveRebalancer – Tackles the tricky problem of rebalancing Aave positions without owning the debt tokens. It executes flash loans in an atomic sequence: borrow debt → repay → withdraw collateral safely → swap collateral via Uniswap V2 → repay flash loan → return any excess. This sequence cannot be done manually through standard wallet UIs, proving the power of automation.Frontend StackNext.js 15 + TypeScript – Fast, modular frontend.@arx-research/libhalo – Web NFC API integration for secure HaLo chip signing.Wagmi + Viem – Handles Ethereum interactions and wallet connections (RainbowKit).Gasless Relay API – A Next.js API route that uses Viem to submit transactions on behalf of users, eliminating the need for them to hold gas on every chain.Notable Hacks & InnovationsChain-Agnostic Domain Separator – By removing the chainId from the EIP-712 domain in the Registry, a chip registered on one chain instantly works across any EVM-compatible network.Recursive BigInt Serialization for HaLo Chips – libhalo requires stringified JSON, while Viem uses BigInt for precision. We built a recursive serializer in the useHaloChip hook to convert BigInt → string before signing, and then revert to BigInt for contract calls.Flash Loan Sequence Reversal – Traditional manual approach (withdraw → swap → repay) is impossible due to liquidation risk. We reverse the flow: borrow debt → repay first → health factor improves → withdraw collateral safely. This sequence cannot be executed via any standard wallet interface, showcasing the unique automation TapThat X provides.