@veil_/x402-pqc

Quantum-resistant x402 payment protocol. Drop-in for AI agents and developers who need post-quantum payment headers without trusting ECDSA.

x402-pqc replaces ECDSA with ML-DSA-65 for payment header signing and adds ML-KEM-768 for optional payment channel encryption. It is a superset of the x402 spec — x402-pqc headers contain all the same semantic fields, just quantum-resistant.

Two modes

| Mode | Handshake | Per-payment | Header size | Best for | |------|-----------|-------------|-------------|----------| | Base | — | ML-DSA-65 sign (≈3.3 KB) | ~14 KB (hex) | One-off payments, auditable | | Session | ML-DSA-65 × 2 + ML-KEM-768 | HMAC-SHA256 (~350 B) | < 500 B | High-frequency agent payments |

The session handshake (one-time cost) establishes a shared 32-byte key via ML-KEM-768. All subsequent payments in that session use HMAC-SHA256 — symmetric throughput comparable to plain HTTP headers.

Performance vs ECDSA (from x402-pqc spec)

| | ECDSA | x402-pqc base | x402-pqc session | |---|---|---|---| | Key size | 32 B | 1952 B (DSA pk) | session key 32 B | | Signature size | 64 B | 3309 B | 32 B (HMAC) | | Sign latency | ~0.1 ms | ~5 ms | <0.1 ms | | Quantum-safe | ✗ | ✓ | ✓ |

Install

npm install @veil_/x402-pqc

Quick start — agent payment in 5 lines

import { generatePQCKeypair } from '@veil_/pqc-wallet';
import { createX402PQCHeader, verifyX402PQCHeader } from '@veil_/x402-pqc';

const keypair = generatePQCKeypair();
const header = createX402PQCHeader({ amount: '0.01', recipient: '0xABCD…', network: 'base' }, keypair);
const result = verifyX402PQCHeader(header);
// result.valid === true, result.payer === keypair.address

Session mode — high-frequency payments

import { generatePQCKeypair } from '@veil_/pqc-wallet';
import { createSession, confirmSession, deriveSessionKey, createSessionPayment, verifySessionPayment } from '@veil_/x402-pqc';

// --- handshake (done once) ---
const payer  = generatePQCKeypair();
const payee  = generatePQCKeypair();

const sessionOpen   = createSession(payer, payee.publicKey.kem);
const confirmation  = confirmSession(sessionOpen, payee);
const sessionKey    = deriveSessionKey(sessionOpen, confirmation, payer.encapsulationKey);

// --- per-payment (HMAC only, < 500 B) ---
const header = createSessionPayment({ amount: '0.001', recipient: payee.address, network: 'base' }, sessionKey, sessionOpen.sessionId);
const result = verifySessionPayment(header, sessionKey);
// result.valid === true

Optional payload encryption

import { generatePQCKeypair } from '@veil_/pqc-wallet';
import { encryptPaymentMetadata, decryptPaymentMetadata } from '@veil_/x402-pqc';

const payee = generatePQCKeypair();
const enc = encryptPaymentMetadata({ invoiceId: 'inv-001', tags: ['api'] }, payee.publicKey.kem);
const dec = decryptPaymentMetadata(enc, payee.encapsulationKey);

Replay prevention

verifyX402PQCHeader checks the timestamp is within ±300 seconds of now.

For session payments, verifySessionPayment maintains an internal NonceStore to reject replays. For base headers you should deploy your own persistent store:

import { NonceStore } from '@veil_/x402-pqc';

const store = new NonceStore(600); // 10-minute TTL
// Before accepting a payment:
if (!store.checkAndStoreNonce(nonce)) throw new Error('replay detected');

Production note: NonceStore is in-memory. Multi-instance deployments must use a shared persistent store (Redis, Postgres) keyed by nonce.

Spec

Full specification: x402-pqc spec v0.1.0