API e especificação do recibo

curl -X POST https://proof.sapir861.com/api/v1/check \
  -H "content-type: application/json" \
  -d '{"text":"See Marbury v. Madison, 5 U.S. 137 (1803)."}'

import { readFileSync } from "node:fs";   // node verify.mjs receipt.json  (Node 20+)
const r = JSON.parse(readFileSync(process.argv[2] || "receipt.json", "utf8"));
const canon = o => o === null || typeof o !== "object" ? JSON.stringify(o) : Array.isArray(o) ? "[" + o.map(canon).join(",") + "]"
  : "{" + Object.keys(o).sort().filter(k => o[k] !== undefined).map(k => JSON.stringify(k) + ":" + canon(o[k])).join(",") + "}";
const msg = new TextEncoder().encode(canon(r.body));
const hex = Buffer.from(await crypto.subtle.digest("SHA-256", msg)).toString("hex");
const pub = (await (await fetch("https://proof.sapir861.com/.well-known/proofkit-pubkey")).json()).jwk;
const key = await crypto.subtle.importKey("jwk", pub, { name: "Ed25519" }, false, ["verify"]);
const ok = await crypto.subtle.verify({ name: "Ed25519" }, key, Buffer.from(r.sig, "base64url"), msg);
console.log(ok && hex === r.digest && pub.x === r.body.key.x ? "VALID" : "INVALID");

# pip install cryptography ; python verify.py receipt.json
import json, sys, base64, hashlib, urllib.request
from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PublicKey
r = json.load(open(sys.argv[1] if len(sys.argv) > 1 else "receipt.json", encoding="utf-8"))
b64 = lambda s: base64.urlsafe_b64decode(s + "=" * (-len(s) % 4))
msg = json.dumps(r["body"], sort_keys=True, separators=(",", ":"), ensure_ascii=False).encode()
pub = json.load(urllib.request.urlopen("https://proof.sapir861.com/.well-known/proofkit-pubkey"))["jwk"]
try:
    Ed25519PublicKey.from_public_bytes(b64(pub["x"])).verify(b64(r["sig"]), msg)
    ok = hashlib.sha256(msg).hexdigest() == r["digest"] and pub["x"] == r["body"]["key"]["x"]
except Exception: ok = False
print("VALID" if ok else "INVALID")

Sem instalar nada: abra https://proof.sapir861.com/pt/verify e cole o receipt.json.
A verificação corre localmente no navegador (WebCrypto Ed25519). Nada é enviado.