Umbra

Confidential payment authorization for autonomous agents, built on Zama's FHEVM. Balances, spend limits, frequency limits, and transfer amounts stay encrypted end to end. The only thing that ever goes on-chain in plaintext is whether a payment was approved or declined.

Built for the Zama Developer Program, Season 3, Builder Track.

The problem

Autonomous agents that pay each other today do it on public ledgers. Every balance, every fee, every spend limit is visible to every competitor and counterparty, forever. An agent's entire pricing strategy and treasury leak the moment its first transaction lands on chain.

What Umbra does

Umbra is a private payment gate that sits between agents. An agent deposits funds into an encrypted vault, sets an encrypted per-service spend limit and frequency cap, and from then on every payment request is authorized entirely on ciphertext. No plaintext amount is ever submitted, computed on, or stored on-chain.

1. ENCRYPT    →  caller's wallet encrypts the amount client-side via Zama Relayer SDK
2. AUTHORIZE  →  FHE.le(amount, balance)
                 FHE.le(amount, spendLimit)
                 FHE.lt(paymentCount, maxCount)   // frequency check
                 FHE.and across all three
3. SETTLE     →  FHE.select resolves transfer to zero if any check fails
                 balances update on ciphertext
                 only an approved/declined flag is ever made public

Live on Sepolia

Contract	Address
AgentVault	0x2CcBF6614159924337A2281d537869ff2c0d0f66
SpendPolicy	0x959c14a9b51C4257695Db187273F6f6B9D5CC772
PaymentGate	0x9b3480B39a07B091574bccA6F044c03f50dD460C

Features

Confidential balances — agent vault holds euint64 balances that are never exposed. Only the agent owner, holding the right EIP-712 signature, can decrypt their own balance via the Zama relayer's user-decryption endpoint.

Encrypted spend limits — per-service spend caps stored as euint64. PaymentGate runs FHE.le(amount, limit) before settling, entirely on ciphertext. Neither the limit value nor the payment amount is ever visible.

Frequency limiting — per-service payment caps within a rolling 7,200- block window (~24h on Sepolia). The count stays encrypted; only the window start block is public. FHE.lt(count, maxCount) adds a third authorization layer on top of balance and spend checks.

Encrypted spend analytics — AgentVault tracks _totalSpent and _totalReceived as encrypted running totals, updated inside settlePayment. Owners can decrypt all four values (balance, limit, spent, received) in a single EIP-712 user-decryption request from the dashboard.

Wallet auto-detect — agentsOf(address) returns all agent IDs registered to a wallet. The dashboard and try-it page call this on connect and skip the registration step if agents are already found.

Wallet ownership proof — before any decryption, the user signs a personal_sign message binding their wallet to a specific agent ID and timestamp. Verified client-side via viem's recoverMessageAddress.

Repo structure

umbra-contracts/   Solidity contracts, tests, deploy scripts (Hardhat + FHEVM)
umbra-app/         Next.js app: landing page, live dashboard, try-it widget

Each package has its own README:

• umbra-contracts/README.md
• umbra-app/README.md

Try it

The live dashboard reads real Sepolia events directly from the deployed contracts, no backend, no indexer. The try-it widget walks through the full 6-step flow:

1. Register agents (skipped automatically if wallet already has one)
2. Encrypt & deposit
3. Set spend limit
4. Set frequency limit
5. Grant PaymentGate FHE access
6. Send confidential payment

Every input is encrypted client-side by the Zama Relayer SDK before it touches the chain.

Test results

AgentVault  (8/8)   PaymentGate  (11/11)
────────────────    ──────────────────────
✔ registration      ✔ approved within limit
✔ deposit           ✔ declined over limit
✔ withdrawal        ✔ declined over balance
✔ safe over-bal     ✔ totalSpent on approve
✔ access control    ✔ totalReceived on approve
✔ agentsOf          ✔ totalSpent on decline
✔ totalSpent init   ✔ freq approve
✔ totalRcvd init    ✔ freq decline at cap
                    ✔ no freq limit set
                    ✔ blocksUntilReset=0 before
                    ✔ blocksUntilReset>0 after

19/19 passing, FHEVM mock mode.

Built on

@fhevm/solidity@0.11.1 · @zama-fhe/relayer-sdk@0.4.3 · ERC-7984 · ERC-8004 · x402 · Next.js 16 · wagmi v2 · viem

Roadmap

• Agent-initiated invoices (InvoiceRegistry.sol, standalone contract)
• Agent Health Score (0–100 combining spend discipline, frequency compliance, encryption status)
• Agent management actions (modify limits, revoke access, pause agent)
• Privacy mode toggle for the dashboard

Author

Built by Abu Olumi.