Provably-fair proof

const seed = "1f9f66b775884f8b51e00c9fc1adf05b389021b094b3342f9c050cb00f3f152a";
const hashStr = "005e2c45f7f29960a12492dc08fe5a3a82f714f2811afebd6de7439003fbc753";
const clientSeed = "347e9cb5b1c49d3ee0159f8890657210dfe72cd95ccf6cf060b0275bde91fc6b";
const lottoId = "4888b617-8c86-492a-958e-4b380f2f3869";
const sold = 50;
const entropy = "mempool.space:952751:00000000000000000000e7e516f4265b0d2842d3f3f4aa07c5d7d058fe98ecda";

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();