Provably-fair proof

const seed = "59a333470c93da10576fa3aa6249704ab30e9273495d44ce57670110c0ce631b";
const hashStr = "64d52210459ca52b852b4a94c4ce2534d89066bbe217353229d4f1b507d245fb";
const clientSeed = "bc12c403ec6033f816f94b0e28dff0c8a0e298fbeb45a7350a85041d11a4aede";
const lottoId = "1c44903a-2c46-4745-9891-0bda998d898e";
const sold = 70;
const entropy = "mempool.space:951909:00000000000000000000efc192f4efd050945f391e9cde8845fc4139ce29b9f8";

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();