> For the complete documentation index, see [llms.txt](https://black-sheep-finance.gitbook.io/black-sheep-finance-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://black-sheep-finance.gitbook.io/black-sheep-finance-docs/webhooks/signatures.md).

# Signature verification

Every webhook is signed with **HMAC-SHA256** using your endpoint's signing secret. Verify before trusting the payload.

## Headers

| Header                   | Value                                      |
| ------------------------ | ------------------------------------------ |
| `X-BlackSheep-Signature` | `sha256=<hex digest>`                      |
| `X-BlackSheep-Event`     | event name, e.g. `approval.pending`        |
| `X-BlackSheep-Delivery`  | unique delivery UUID (for log correlation) |

## Algorithm

```
hex( HMAC_SHA256( secret, raw_request_body ) )
```

The body is the **raw bytes** of the JSON payload — verify before `JSON.parse`. Don't re-serialize.

## Node.js

```ts
import { createHmac, timingSafeEqual } from "crypto";

export function verifyBlackSheep(rawBody: string, header: string, secret: string) {
  const provided = header.replace(/^sha256=/, "");
  const expected = createHmac("sha256", secret).update(rawBody).digest("hex");
  const a = Buffer.from(provided, "hex");
  const b = Buffer.from(expected, "hex");
  return a.length === b.length && timingSafeEqual(a, b);
}
```

## Cloudflare Workers / edge runtimes

```ts
export async function verifyBlackSheep(rawBody: string, header: string, secret: string) {
  const key = await crypto.subtle.importKey(
    "raw", new TextEncoder().encode(secret),
    { name: "HMAC", hash: "SHA-256" }, false, ["sign"],
  );
  const sig = await crypto.subtle.sign("HMAC", key, new TextEncoder().encode(rawBody));
  const expected = [...new Uint8Array(sig)].map(b => b.toString(16).padStart(2, "0")).join("");
  const provided = header.replace(/^sha256=/, "");
  // constant-time compare
  if (provided.length !== expected.length) return false;
  let diff = 0;
  for (let i = 0; i < expected.length; i++) diff |= provided.charCodeAt(i) ^ expected.charCodeAt(i);
  return diff === 0;
}
```

## Express example

```ts
import express from "express";
import { verifyBlackSheep } from "./verify";

const app = express();

app.post(
  "/hooks/blacksheep",
  express.raw({ type: "application/json" }), // preserve raw body
  (req, res) => {
    const sig = req.header("X-BlackSheep-Signature") ?? "";
    if (!verifyBlackSheep(req.body.toString("utf8"), sig, process.env.BS_WEBHOOK_SECRET!)) {
      return res.status(401).send("invalid signature");
    }
    const payload = JSON.parse(req.body.toString("utf8"));
    handleEvent(payload);
    res.status(200).send("ok");
  },
);
```

## What to reject

* Missing signature header → 401.
* Mismatched signature → 401.
* Timestamp older than 5 minutes (replay protection) → 401.
* Duplicate `event_id` you've already processed → 200 + skip (at-least-once delivery).


---

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