ISSUE #3: Sybil-Resistant Fee-Bump Relayer & Gas Sponsorship Service for Zero-XLM Onboarding

[amankoli09]

Title

[Backend/Relayer] Zero-XLM Fee-Bump Transaction Relayer with Cryptographic Rate-Limiting & Device Attestation

Description

To provide an Apple-like frictionless onboarding flow, new users must be able to create a LynxX Smart Wallet and interact with Soroban contracts without purchasing XLM first. An off-chain backend Relayer service is required to sponsor transaction fees by wrapping user-signed envelopes inside Stellar Fee-Bump Transactions (FeeBumpTransaction) while protecting the relayer fee pool from Sybil DDoS draining.

Domain / Component

• Component: Backend Relayer Service / Enclave API
• Layer: Off-Chain Infrastructure & Gas Sponsorship

Complexity & Priority

• Complexity: Hard (Sybil resistance, cryptographic attestation, concurrency)
• Priority: P1 (Essential for UX Adoption)

Estimated Timeframe

• 2 Weeks (Backend architecture, rate-limiting middleware, and Stellar SDK relayer integration)

Technical Requirements

1. Create a backend API endpoint /api/v1/relayer/sponsor that accepts a user-signed Soroban transaction XDR.
2. Validate that the inner transaction invokes an allowed LynxX contract (fund, escrow, factory, or account creation).
3. Wrap the transaction inside a FeeBumpTransaction signed by the Relayer's treasury account.
4. Integrate rate-limiting via IP/Device Attestation tokens (Apple App Attest / WebAuthn Device Integrity) and enforce a daily XLM sponsorship cap per user.

Acceptance Criteria

• New accounts with 0 XLM balance can successfully execute contract calls via the /sponsor endpoint.
• Relayer automatically rejects requests exceeding 5 sponsored transactions per device per 24 hours.
• Relayer rejects any inner transaction attempting to transfer arbitrary XLM away from the relayer or unauthorized contracts.
• Transaction submission returns the final confirmed transaction hash within 3 seconds on testnet.

Dependencies & Prerequisites

• Node.js / Rust relayer server with access to a funded Stellar Testnet/Mainnet treasury keypair.
• @stellar/stellar-sdk v15+.

[Bolajiomo99]

"Hi team, I'd like to take this on. I have strong experience building high-throughput Web3 relays and anti-Sybil infrastructure. I can implement the /api/v1/relayer/sponsor endpoint using @stellar/stellar-sdk to securely validate inner transactions and wrap them in a FeeBumpTransaction signed by the treasury key. I'll integrate Device Attestation token verification paired with strict IP rules to enforce the 5-tx daily limit, apply deep validation to block unauthorized contract interactions, and optimize submission pipelines for sub-3-second responses. Ready to deliver a production-grade sponsorship layer.