Provably-fair proof

const seed = "3535d9e1e08fab8b66a31e48f9ead297a92186c9067a30f72c0f6a302239c648";
const hashStr = "ceeded0baaea23af17f0684da4202c3d70f70b8a4b09766a6958b248cc994de3";
const clientSeed = "dac65d0bba488f884748987ea262d8d09e64aa7d4850ca1974a345b92fee0983";
const lottoId = "4d140c15-3cdd-432e-ae8f-aa84eb952c2d";
const sold = 66;
const entropy = "mempool.space:945561:000000000000000000005fc0717695a4ceee2033290ab2a847ed225e15ff172e";

async function run() {
  const enc = new TextEncoder();
  const seedBytes = enc.encode(seed);
  const hashed = await crypto.subtle.digest("SHA-256", seedBytes);
  const recomputed = [...new Uint8Array(hashed)]
    .map((b) => b.toString(16).padStart(2, "0")).join("");
  console.log("hashOk:", recomputed === hashStr);

  const key = await crypto.subtle.importKey(
    "raw", seedBytes, { name: "HMAC", hash: "SHA-256" }, false, ["sign"]
  );
  const message = entropy
    ? clientSeed + ":" + lottoId + ":" + entropy
    : clientSeed + ":" + lottoId;
  const sig = await crypto.subtle.sign("HMAC", key, enc.encode(message));
  const hex = [...new Uint8Array(sig)]
    .map((b) => b.toString(16).padStart(2, "0")).join("");
  const idx = Number(BigInt("0x" + hex.slice(0, 16)) % BigInt(sold));
  console.log("hmac:", hex);
  console.log("winnerIndex:", idx);
}
run();