Provably-fair proof

const seed = "254279f7f6df205f33e670056fbbed52e9a6ea3219731379411bb3d05e2ebc6f";
const hashStr = "d95b17f1833302d4eca9e0415757f2a7f99de64a722dce50fb5836f971da63a4";
const clientSeed = "9e808f419deebc7b13e6e0047c7e39708094cd18b13c50599c4b799abba9ce72";
const lottoId = "76024d01-b272-4ec9-a84f-ab3252955765";
const sold = 50;
const entropy = null;

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();