About HMAC Verifier

Verify HMAC-SHA256 signatures with constant-time comparison analysis. Part of the DevTools Surf developer suite. Browse more tools in the Security / Crypto collection.

Use Cases

• Verify incoming webhook payloads from Stripe, GitHub, or Shopify before processing.
• Check if a received request signature matches a recomputed HMAC from the request body and shared secret.
• Debug signature mismatch issues by comparing computed vs. received HMACs side by side.
• Validate HMAC tokens generated by a partner API before trusting their data.

Tips

• Always use constant-time comparison for HMAC verification in production code — timing attacks can reveal the correct HMAC by measuring response time differences.
• Include the timestamp in the signed payload and reject HMAC-valid requests older than 5 minutes to prevent replay attacks.
• Log verification failures with the truncated input hash, not the secret key, for debugging without exposing credentials.

Fun Facts

• Stripe, GitHub, Shopify, and most major SaaS platforms use HMAC-SHA256 for webhook signature verification. The 2021 OWASP API Security report listed signature bypass as a top-5 API vulnerability.
• Timing attacks on HMAC comparison were first demonstrated practically in 2009 against web frameworks that used string equality (==) for signature checks — some frameworks remained vulnerable for years after disclosure.
• Node.js's crypto.timingSafeEqual() was added in v6.6.0 (2016) specifically to make constant-time comparison easy after timing attack vulnerabilities were reported in several popular npm authentication packages.

FAQ

Why does my HMAC not match even though the key is correct?

Common causes: the payload has trailing whitespace or a different encoding, the key is base64-encoded on one side and raw bytes on the other, or the signature is hex vs. base64 encoded differently.

What's a timing attack on HMAC?

If HMAC comparison exits early on the first mismatched byte, an attacker can measure response time to guess bytes one at a time. Constant-time comparison eliminates this by always comparing all bytes.

How do I prevent replay attacks with HMAC?

Include a timestamp in the signed payload and verify that the timestamp is within an acceptable window (typically 5-15 minutes). Reject requests with a valid HMAC but an expired timestamp.