leetcode/api/src/links.ts

/**
 * One-tap email links: `GET /log?p=704&r=pass&d=2026-08-31&sig=<hmac>`.
 * The sig is HMAC-SHA256 over `p|r|d` with LINK_KEY (WebCrypto, hex).
 * Verification is constant-time; links older than 3 days are rejected.
 */

async function hmacKey(secret: string): Promise<CryptoKey> {
  return crypto.subtle.importKey(
    "raw",
    new TextEncoder().encode(secret),
    { name: "HMAC", hash: "SHA-256" },
    false,
    ["sign", "verify"],
  );
}

export async function signLink(
  key: string,
  p: number,
  r: string,
  d: string,
): Promise<string> {
  const mac = await crypto.subtle.sign(
    "HMAC",
    await hmacKey(key),
    new TextEncoder().encode(`${p}|${r}|${d}`),
  );
  return [...new Uint8Array(mac)].map((b) => b.toString(16).padStart(2, "0")).join("");
}

export async function verifyLink(
  key: string,
  p: number,
  r: string,
  d: string,
  sig: string,
): Promise<boolean> {
  if (!/^[0-9a-f]{64}$/.test(sig)) return false;
  const bytes = new Uint8Array(32);
  for (let i = 0; i < 32; i++) bytes[i] = Number.parseInt(sig.slice(i * 2, i * 2 + 2), 16);
  // crypto.subtle.verify is constant-time; never compare hex strings directly.
  return crypto.subtle.verify(
    "HMAC",
    await hmacKey(key),
    bytes,
    new TextEncoder().encode(`${p}|${r}|${d}`),
  );
}

/** Constant-time equality for webhook signatures and admin bearer keys. */
export async function timingSafeEqual(a: string, b: string): Promise<boolean> {
  // HMAC both sides with a random key: unequal-length inputs and content
  // differences are equally invisible to timing.
  const key = await crypto.subtle.generateKey({ name: "HMAC", hash: "SHA-256" }, false, [
    "sign",
  ]);
  const enc = new TextEncoder();
  const [ma, mb] = await Promise.all([
    crypto.subtle.sign("HMAC", key, enc.encode(a)),
    crypto.subtle.sign("HMAC", key, enc.encode(b)),
  ]);
  const va = new Uint8Array(ma);
  const vb = new Uint8Array(mb);
  let diff = 0;
  for (let i = 0; i < va.length; i++) diff |= va[i]! ^ vb[i]!;
  return diff === 0;
}

/** GitHub webhook `X-Hub-Signature-256: sha256=<hex>` verification. */
export async function verifyWebhook(
  secret: string,
  body: ArrayBuffer,
  header: string | null,
): Promise<boolean> {
  if (!header?.startsWith("sha256=")) return false;
  const mac = await crypto.subtle.sign("HMAC", await hmacKey(secret), body);
  const expected = [...new Uint8Array(mac)].map((b) => b.toString(16).padStart(2, "0")).join("");
  return timingSafeEqual(header.slice(7), expected);
}