Verify signatures

Every request carries Baiyar-Signature: t=<unix seconds>,v1=<hex>. The v1 value is the HMAC-SHA256, with your endpoint secret as the key, of the string <t>.<raw request body>. While a secret is being rotated there are two v1 values: accept the request if any of them matches. Also reject a t that is more than 5 minutes from your clock, so a captured request cannot be replayed later.

Verify against the raw body, byte for byte, before you parse it.

Node.js

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

export function verifyBaiyarSignature(header, rawBody, secret, toleranceSeconds = 300) {
  const parts = header.split(",").map((part) => part.trim());
  const timestamp = Number(parts.find((part) => part.startsWith("t="))?.slice(2));
  if (!Number.isInteger(timestamp)) return false;
  if (Math.abs(Date.now() / 1000 - timestamp) > toleranceSeconds) return false;

  const expected = createHmac("sha256", secret).update(`${timestamp}.${rawBody}`).digest();
  return parts
    .filter((part) => part.startsWith("v1="))
    .some((part) => {
      const given = Buffer.from(part.slice(3), "hex");
      return given.length === expected.length && timingSafeEqual(given, expected);
    });
}

// Express: keep the raw body for this route.
app.post("/baiyar/webhook", express.raw({ type: "application/json" }), (req, res) => {
  const ok = verifyBaiyarSignature(
    req.get("Baiyar-Signature") ?? "",
    req.body.toString("utf8"),
    process.env.BAIYAR_WEBHOOK_SECRET,
  );
  if (!ok) return res.sendStatus(400);
  const event = JSON.parse(req.body.toString("utf8"));
  // Handle event.type, then answer quickly.
  res.sendStatus(200);
});

Python

import hashlib
import hmac
import time


def verify_baiyar_signature(header: str, raw_body: bytes, secret: str, tolerance_seconds: int = 300) -> bool:
    parts = [part.strip() for part in header.split(",")]
    timestamps = [part[2:] for part in parts if part.startswith("t=")]
    if not timestamps or not timestamps[0].isdigit():
        return False
    timestamp = int(timestamps[0])
    if abs(time.time() - timestamp) > tolerance_seconds:
        return False

    signed = f"{timestamp}.".encode() + raw_body
    expected = hmac.new(secret.encode(), signed, hashlib.sha256).hexdigest()
    return any(
        hmac.compare_digest(part[3:], expected) for part in parts if part.startswith("v1=")
    )


# Flask
@app.post("/baiyar/webhook")
def baiyar_webhook():
    raw = request.get_data()  # the raw bytes, before any parsing
    if not verify_baiyar_signature(request.headers.get("Baiyar-Signature", ""), raw, os.environ["BAIYAR_WEBHOOK_SECRET"]):
        return "", 400
    event = request.get_json()
    return "", 200