Verify webhook HMAC signatures

import { createHmac, timingSafeEqual } from "node:crypto";

type VerifyInput = {
  /** The raw request body, exactly as received */
  payload: string | Buffer;
  /** Header in the form "t=1700000000,v1=<hex signature>" */
  header: string | null;
  secret: string;
  /** Reject signatures older than this to block replays */
  toleranceSec?: number;
};

function sign(secret: string, timestamp: number, payload: string | Buffer): Buffer {
  return createHmac("sha256", secret).update(`${timestamp}.`).update(payload).digest();
}

export function verifyWebhookSignature({
  payload,
  header,
  secret,
  toleranceSec = 300,
}: VerifyInput): boolean {
  if (!header) return false;

  const parts = new Map(
    header.split(",").map((part) => {
      const [key, ...rest] = part.trim().split("=");
      return [key, rest.join("=")] as const;
    }),
  );

  const timestamp = Number(parts.get("t"));
  const signature = parts.get("v1");
  if (!Number.isInteger(timestamp) || !signature) return false;

  const age = Math.abs(Date.now() / 1000 - timestamp);
  if (age > toleranceSec) return false;

  const expected = sign(secret, timestamp, payload);
  const received = Buffer.from(signature, "hex");

  // timingSafeEqual throws on length mismatch, so check that first
  return received.length === expected.length && timingSafeEqual(received, expected);
}

/** Produce a header for tests or for webhooks you send yourself */
export function signWebhook(secret: string, payload: string): string {
  const timestamp = Math.floor(Date.now() / 1000);
  return `t=${timestamp},v1=${sign(secret, timestamp, payload).toString("hex")}`;
}