FHE x402: Encrypted Payment Protocol for the Agent Economy

April 5, 2026 · View on GitHub

v1.0.0 — March 2026

Table of Contents

  1. Abstract
  2. The Problem
  3. The Solution
  4. Architecture
  5. ERC Standards Integration
  6. Revenue Model
  7. Competitive Landscape
  8. Agent Integrations
  9. Roadmap
  10. Team
  11. The Ask

Abstract

FHE x402 is a token-centric privacy protocol for AI agent payments on Ethereum. Agents wrap USDC into encrypted cUSDC (an ERC-7984 confidential token), hold it directly in their wallets, and transfer it peer-to-peer with fully encrypted amounts. There is no intermediary contract holding funds — agents own their encrypted balances outright. Payment verification uses a thin nonce registry (X402PaymentVerifier) so servers can confirm that a payment occurred and meets a minimum price, without ever learning the actual amount transferred. The protocol integrates natively with the x402 HTTP payment standard, ERC-8004 agent identity, and ERC-8183 agentic commerce.

900+ tests. Deployed on Ethereum Sepolia. Two contracts. No trusted setup.

The Problem

"Crypto privacy is needed if you want to make API calls without compromising the information of your access patterns. Even with a local AI agent, you can learn a lot about what someone is doing if you see all of their search engine calls. [...] providers will demand an anti-DoS mechanism, and realistically payment per call. By default that will be credit card or some corposlop stablecoin thing — so we need crypto privacy."

— Vitalik Buterin, March 2026

AI Agents Have No Financial Privacy

The agent economy is growing rapidly:

  • $600M+ cumulative x402 payment volume across all chains (Q1 2026)
  • 122M+ x402 transactions processed
  • 24,000+ agents registered on ERC-8004
  • ZERO amount privacy: every payment amount is publicly visible on-chain

Why This Matters

  • Strategy Leakage: Competing agents can monitor each other's API spending and data purchases
  • MEV Extraction: Visible payment amounts enable front-running and sandwich attacks
  • Competitive Intelligence: Payment history reveals operational strategy
  • Price Discrimination: Servers can adjust prices based on observed spending patterns

The Privacy Gap

LayerStandardPrivacy
Identity & TrustERC-8004Public (by design)
Payment Protocolx402Public (amounts visible)
SettlementEthereum L1Public (transparent)

FHE x402 fills the missing privacy layer for payment amounts.

The Solution

Token-Centric Encrypted Payments

FHE x402 takes a fundamentally different approach from privacy mixers or shielded ledgers. Instead of locking funds in a shared contract, agents hold encrypted tokens directly in their own wallets:

ERC-7984 Confidential Token (cUSDC)

  • Agents wrap plaintext USDC into encrypted cUSDC
  • All cUSDC balances are stored as euint64 (FHE-encrypted 64-bit unsigned integers)
  • Agents hold cUSDC in their own wallets — no shared contract, no custodial risk
  • confidentialTransfer() moves encrypted amounts peer-to-peer with zero protocol fee
  • When done, agents unwrap cUSDC back to plaintext USDC

Silent Failure Pattern

  • FHE encrypted booleans cannot be branched on in Solidity
  • Insufficient balance transfers 0 instead of reverting
  • No information leaks through transaction success/failure

x402 Native Integration

  • Scheme: fhe-confidential-v1
  • Drop-in Express middleware: fhePaywall(config)
  • Auto-402 fetch: fheFetch(url, options)
  • Event-based verification (no ZK proofs needed)

ERC-8004 Complementary

  • Agent identity: PUBLIC (ERC-8004 registry)
  • Agent reputation: PUBLIC (ERC-8004 feedback)
  • Payment amounts: ENCRYPTED (FHE x402)

Agent Framework Integrations

  • Virtuals GAME Plugin: 5 GameFunctions — autonomous agent payments
  • OpenClaw Skill: 6 scripts — declarative skill for any OpenClaw agent
  • ElizaOS Plugin: 3 actions — example integration
  • npm SDK: fhe-x402-sdk — ready for any framework

Facilitator Server

  • x402-standard compatible: /verify, /info, /health endpoints
  • Dual event verification (ConfidentialTransfer + PaymentVerified on-chain)
  • API key authentication with constant-time comparison

How It Works

Agent wraps USDC → receives cUSDC in wallet
    |
Agent discovers API via ERC-8004 registry
    |
Agent requests API → receives HTTP 402
    |
Agent encrypts payment amount with fhevmjs
    |
cUSDC.confidentialTransfer(to, encryptedAmount, inputProof)
    |
verifier.recordPayment(payer, server, nonce, minPrice)
    |
  [payAndRecord() — single TX combining both steps]
    |
Server verifies ConfidentialTransfer + PaymentVerified events on-chain
    |
Agent receives API response → HTTP 200
    |
On-chain: encrypted balances change, amounts are HIDDEN
          sender/receiver addresses are PUBLIC (x402 requirement)

Privacy Model

WhatVisible?
Wrap amount (USDC to cUSDC)Yes (plaintext USDC enters encrypted domain)
Unwrap amount (cUSDC to USDC)Yes (plaintext USDC exits encrypted domain)
Transfer amount (cUSDC to cUSDC)No (FHE encrypted)
cUSDC balanceNo (FHE encrypted)
Sender addressYes (x402 requirement)
Recipient addressYes (x402 requirement)
Transaction occurredYes (event emitted)

Key difference from ZK-UTXO (PrivAgent): FHE x402 encrypts amounts but keeps participants public. This is a deliberate design choice — x402 requires sender/recipient to be known for payment verification. FHE provides amount privacy without the complexity of UTXO management, ZK circuits, or trusted setup ceremonies.

Architecture

Protocol Stack

+------------------------------------------+
|  Agent Frameworks                        |
|  Virtuals GAME · OpenClaw · ElizaOS      |
+------------------------------------------+
|  ERC-8183: Agentic Commerce              |
|  (Job escrow + completion payments)      |
+------------------------------------------+
|  ERC-8004: Identity + Reputation         |
|  (Agent discovery & trust)               |
+------------------------------------------+
|  FHE x402: Amount Privacy Layer          |
|  (Encrypted balances + silent failure)   |
+------------------------------------------+
|  x402: Payment Protocol                  |
|  (HTTP 402 → pay → 200)                 |
+------------------------------------------+
|  Ethereum (Sepolia / Mainnet)            |
+------------------------------------------+

Core Components

  1. ConfidentialUSDC — ERC-7984 confidential token wrapper. Wraps plaintext USDC into encrypted cUSDC. Agents hold cUSDC directly in their wallets. Fee-free peer-to-peer transfers. 0.1% fee on wrap and unwrap only. 2-step async unwrap via KMS.
  2. X402PaymentVerifier — Thin nonce registry. Records payment nonces with minPrice for server-side price verification. Permissionless (any agent can record).
  3. TypeScript SDK — Payment handler, Express middleware, auto-402 fetch wrapper, facilitator server, ERC-8004 helpers, ERC-8183 escrow helpers.
  4. Agent Plugins — Virtuals GAME (5 functions, 30 tests), OpenClaw (6 scripts, 25 tests), ElizaOS (3 actions).
  5. Frontend Demo — React app for wrap/transfer/unwrap.

Smart Contracts (Sepolia — Live)

ContractAddress
MockUSDC0xc89e913676B034f8b38E49f7508803d1cDEC9F4f
ConfidentialUSDC0xE944754aa70d4924dc5d8E57774CDf21Df5e592D
X402PaymentVerifier0x4503A7aee235aBD10e6064BBa8E14235fdF041f4
Treasury0xF505e2E71df58D7244189072008f25f6b6aaE5ae

All contracts verified on Etherscan.

ERC Standards Integration

FHE x402 is built on three complementary ERC standards:

ERC-7984: Confidential Token Standard

The foundation. ConfidentialUSDC implements the ERC-7984 interface with ERC7984ERC20Wrapper for USDC wrapping. Provides confidentialBalanceOf, confidentialTransfer, setOperator, and standard wrap/unwrap lifecycle.

ERC-8004: Agent Identity & Reputation

Agents register their FHE x402 payment capabilities in the ERC-8004 registry. The SDK provides fhePaymentMethod() for registration and fhePaymentProof() for proof-of-payment feedback. Identity is public by design — only payment amounts are encrypted.

ERC-8183: Agentic Commerce Protocol

Job escrow for multi-step agent workflows. A client agent posts a job with encrypted escrow, a provider agent completes the work, and funds are released on completion. 1% completion fee funds protocol development. Enables complex agent-to-agent commerce beyond simple API payments.

Revenue Model

3-Tier Fee Structure

Fee TypeAmountTriggerRecipient
Wrap/Unwrap feemax(0.1%, $0.01)Entering or exiting encrypted domainTreasury
Escrow completion fee (ERC-8183)1%Job completed and escrow releasedTreasury
Facilitator SaaS$0.01-0.05/TX or monthly subscriptionPayment verification serviceFacilitator operator

Why Transfers Are Free

Agent-to-agent confidentialTransfer() carries zero protocol fee. This is intentional — fee-free transfers maximize adoption and network effects. Revenue comes from the entry/exit points (wrap/unwrap) and from higher-value escrow completions (ERC-8183).

Unit Economics

AmountWrap FeeTransfer FeeUnwrap FeeTotal Round-Trip
1 USDC0.0100.010.02
10 USDC0.0100.010.02
100 USDC0.1000.100.20
1000 USDC1.0001.002.00

Growth Scenarios

Conservative (2026):

  • 3% of Ethereum x402 volume = ~$15M
  • ~3M wrap/unwrap events at $0.01 min fee = ~$30K protocol revenue
    • ERC-8183 escrow completions

Optimistic (2027):

  • 7% of x402 volume with L2 deployment = ~$500M
  • ~100M events = ~$1M protocol revenue
    • facilitator SaaS subscriptions

Competitive Landscape

FeatureFHE x402PrivAgent (ZK)Mind Network x402zFhenix402Zaiffer
Ethereum L1YesNo (Base)No (Mind L2)Yes (Fhenix L2)Yes (Zama)
x402 nativeYesYesYesPartialNo
ERC-8004YesYesNoNoNo
ERC-8183 escrowYesNoNoNoNo
Amount privacyYes (FHE)Yes (ZK)Yes (FHE)Yes (FHE)Yes (FHE)
Sender privacyNoYesNoNoNo
No trusted setupYesNo (Groth16)YesYesYes
Silent failureYesNoNoNoYes
Token-centric (no shared contract)YesNo (shielded UTXO)NoPartialNo
Agent integrations3 frameworks2 frameworks000
Tests900+282UnknownUnknownUnknown

FHE x402 vs PrivAgent: Complementary approaches. FHE x402 provides amount privacy with simpler architecture (no circuits, no trusted setup, token-centric). PrivAgent provides full privacy (amounts + participants) with ZK-UTXO complexity.

FHE x402 vs Mind Network x402z / Fhenix402: All use FHE, but FHE x402 is the only one with ERC-8004 identity integration, ERC-8183 escrow, agent framework plugins, and a token-centric design where agents hold funds directly.

FHE x402 vs Zaiffer: Both use Zama's fhEVM. Zaiffer focuses on DeFi (encrypted AMM), while FHE x402 focuses on the agent payment layer with x402 integration.

Agent Integrations

Virtuals GAME Plugin (30 tests)

5 GameFunctions for autonomous agent payments:

  • fhe_wrap — Wrap USDC into encrypted cUSDC
  • fhe_pay — Encrypted transfer + nonce recording
  • fhe_unwrap — Request unwrap (cUSDC to USDC)
  • fhe_balance — Check USDC balance
  • fhe_info — Wallet and contract info

OpenClaw Skill (25 tests)

6 CLI scripts for declarative agent integration:

  • wrap.ts, pay.ts, unwrap.ts, balance.ts, info.ts, _wallet.ts (shared config)

ElizaOS Plugin (Example)

3 actions: FHE_PAY, FHE_BALANCE, FHE_WRAP. See examples/eliza-plugin/ for complete integration.

Any Framework (npm SDK)

import { fheFetch } from "fhe-x402-sdk";

const response = await fheFetch("https://api.example.com/data", {
  tokenAddress: "0xE944754aa70d4924dc5d8E57774CDf21Df5e592D",
  verifierAddress: "0x4503A7aee235aBD10e6064BBa8E14235fdF041f4",
  rpcUrl: "https://sepolia.infura.io/v3/...",
  signer: wallet,
  fhevmInstance: fhevm,
});

Roadmap

PhaseStatusDeliverables
v1.0.0CompleteToken-centric architecture. ConfidentialUSDC (ERC-7984) + X402PaymentVerifier. Single-TX payment (payAndRecord). Batch prepayment (recordBatchPayment). TypeScript SDK. Agent plugins. 900+ tests. Sepolia deployment.
v1.1In ProgressERC-8183 Agentic Commerce: job escrow, 1% completion fee. ERC-8004 full integration: identity + reputation + feedback.
v1.2PlannedERC-8126 risk scoring. Encrypted reputation (FHE + ERC-8004). Multi-token factory (cWETH, cDAI).
v2.0PlannedCross-chain L2 (Base, Arbitrum). Mainnet deployment with professional audit.

Team

Himess — Founder & Developer

  • 5+ years crypto/blockchain development
  • 80+ merged PRs: reth, revm, Base, Optimism, Miden VM, Celestia
  • Zama Developer Program participant — FHEVM Bootcamp curriculum (328 tests, 20 modules)
  • Arc x Lablab AI Hackathon Winner — ArcPay SDK
  • PrivAgent — ZK privacy protocol on Base (282 tests, V4.4)
  • MixVM — Cross-chain privacy bridge (CCTP V2 + LayerZero)

The Ask

What we've built (v1.0.0 — complete):

  • ConfidentialUSDC: ERC-7984 token wrapper with encrypted balances, fee-free transfers, 0.1% wrap/unwrap fee
  • X402PaymentVerifier: nonce registry with minPrice verification
  • Single-TX payment (payAndRecord) and batch prepayment (recordBatchPayment)
  • TypeScript SDK: payment handler, paywall middleware, auto-402 fetch, facilitator server
  • Agent integrations: Virtuals GAME (5 functions, 30 tests), OpenClaw (6 scripts, 25 tests), ElizaOS (3 actions)
  • ERC-8004 integration helpers
  • React frontend demo
  • 900+ tests, internally reviewed, deployed on Ethereum Sepolia

What we're building next:

  1. ERC-8183 Agentic Commerce — job escrow for multi-step agent workflows (1% completion fee)
  2. ERC-8004 full integration — identity + reputation + encrypted feedback
  3. Multi-token factory — wrap any ERC-20 as a confidential ERC-7984 token
  4. Cross-chain L2 deployment — Base, Arbitrum (when Zama coprocessor supports L2)
  5. Ethereum mainnet deployment with professional security audit

FHE x402 is licensed under the Business Source License 1.1. Converts to GPL-2.0 on March 1, 2030.