Webhook Signature Verifier

The sending service computes an HMAC (keyed hash) over the raw request body using a shared secret key. The result is sent as a header (e.g. X-Hub-Signature-256: sha256=abc123). Your server independently computes the same HMAC with the same secret and compares the two values. If they match, the payload is authentic and hasn't been tampered with.

Timing attacks can reveal the correct signature one character at a time by measuring how long a comparison takes. Standard string equality short-circuits on the first mismatch. Use `crypto.timingSafeEqual()` (Node.js) or `hmac.compare_digest()` (Python) which always compare all bytes in the same amount of time.

Always prefer HMAC-SHA256. SHA-1 is considered cryptographically weak and deprecated for new systems. It's still used by some older providers (GitHub's legacy X-Hub-Signature header uses SHA-1) but SHA-256 provides stronger collision resistance.

Many providers prefix the hex signature with the algorithm name: GitHub sends `sha256=<hex>`, Stripe sends `v1=<hex>`. This prefix is part of the header value, not the computed HMAC. When verifying, strip the prefix before comparing, or include it in your expected value calculation.

Yes — always verify webhook signatures in production. Skipping verification means any attacker who discovers your endpoint URL can send arbitrary fake events. The verification process is inexpensive and prevents spoofed events from triggering critical business logic.