# exhibit402: Proof of Publication

Two independent observers on different networks capture your URL at the same frozen instant; matching captures are CERTIFIED, the evidence is stored permanently, and an ERC-721 certificate carrying every observer hash is minted to the wallet that paid.

## What it does
POST /capture with JSON {url, mode} to permanently certify what a public web
page served. Two independent standardized browsers (Google Cloud and Fly.io,
different networks, same metro) capture the URL at one shared frozen
timestamp. Byte-identical captures are CERTIFIED; disagreement is WITNESSED
(pixel agreement percentage + diff map, a deliverable). You receive the
canonical screenshot and a signed manifest stored permanently on Arweave
(ar:// + IPFS CIDv1 + SHA-256), an RFC 3161 timestamp token from an
independent authority, and an ERC-721 certificate on Base minted to the
wallet that paid.

## When to use
- Prove to your principal that work was completed: certify the resulting page.
- Freeze a resolution source, terms page, price, or job posting before you
  rely on it or before a dispute over what it said.
- Document a page you expect to change or disappear (scam sites, edited
  announcements, delisted products).
- Confirm another party's certificate is genuine: GET /verify for $0.02.

## Pricing (x402, USDC on Base, eip155:8453)
- $1.00 mode "fullpage" (default, whole page)
- $0.50 mode "viewport" (first 1280x800 screen)
Response is HTTP 202 with a claim URL; poll GET /claim/{job_id} (free) every
~15s. Fulfillment normally takes 30-120 seconds. Rejected captures (oversize,
unreachable, content-screened) refund automatically minus $0.10; service
faults refund in full.

## Endpoints
- POST /capture: paid, starts a certification job
- GET /verify?id={token|arweave-tx|job}: paid ($0.02), re-verifies any
  certificate: re-downloads the stored bytes, recomputes every SHA-256, and
  checks them against the manifest and the on-chain record
- GET /claim/{job_id}: free, resolves to the certificate or a refund
- GET /openapi.json: full API spec and policy
- GET /system-description.md: versioned description of the automated process
- GET /evidence: free guide: preserving a web page as evidence
- GET /.well-known/x402: x402 resource listing

## Evidence-grade design
Fixed clock, pinned browser build, per-observer serving evidence (redirect
chain, response headers, TLS chain, DNS answers, DOM), hash-based
identification, RFC 3161 timestamp over the capture digest, versioned system
description referenced by SHA-256 from every manifest.

Contact: dylan.caponi@gmail.com
Base URL: https://exhibit402.dylan-caponi.workers.dev