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The POST /v1/sponsor endpoint is DarkPool’s gas onramp. It uses Sui’s native sponsored-transaction protocol with no Enoki Portal allowlist, no third-party dependency, and the ability to sponsor any package. Both zkLogin and wallet users sign trades without holding any SUI.

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Why we shipped it

The user had configured Allowed Addresses in the Enoki Portal with the package IDs (wrong field; that section gates sender addresses, not Move targets), so every sponsorship attempt 403’d. Asking the user to redo the Portal config was both fragile and a third-party dependency. Self-sponsorship sidesteps it entirely. Enoki sponsorship stays as Strategy 1 for zk users. The user paid for the credits and we shouldn’t waste them when the Portal is correctly configured.

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Wire protocol

Self-sponsorship wire protocol. Step 1: Frontend builds a PTB. Step 2: Frontend POSTs to slash v1 slash sponsor with txKindBytes and sender. Backend runs Transaction.fromKind, setSender(user), setGasOwner(sponsor), setGasBudget(cap), then sponsor.sign(). Backend returns txBytes and sponsorSig. Step 3: User signs the same bytes (wallet via useSignTransaction; zk via EnokiKeypair.signTransaction). Step 4: Frontend calls suiClient.executeTransaction with the bytes and the array of sponsor and user signatures. Sponsor pays gas.

Frontend builds a PTB-kind, backend signs as gas owner, user co-signs the same bytes, then either side submits. Chain enforces sender = user.

Chain enforces sender == user (their signature is mandatory). Gas comes from the sponsor.

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Abuse bounds

Three caps prevent a hostile PTB: Redis keys sponsor:user:<addr>:<yyyy-mm-dd> and sponsor:global:<yyyy-mm-dd> use INCRBYFLOAT with EXPIRE-to-end-of-day. Worst case: one bad actor at the per-user cap drains 0.5 SUI/day, the global cap drains 5 SUI/day total. Both fit a hackathon budget.
PTB-command inspection (reject tx.gas references in TransferObjects and SplitCoins) is a v0.4 hardening follow-up.

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Frontend integration

packages/frontend/src/lib/wallet.ts is the three-strategy signer:
  1. Self-sponsor first. Works for both wallet and zk users. Sponsors any package.
  2. Enoki sponsorship. Zk users only, on SponsorUnavailableError.
  3. Direct execution. User pays gas (last resort).
All 11 sign call sites flow through useTxSigner() which encapsulates the cascade.

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GET /v1/sponsor

The UI uses this to decide whether to show the “Gas sponsored” badge.