HMAC Generator

How to use HMAC Generator

1. Pick the algorithm the other system uses — SHA-256 unless its documentation says otherwise.
2. Enter the shared secret key exactly, with no added whitespace.
3. Paste the exact message being signed — the raw body, not a reformatted copy.
4. Choose hex or Base64 to match the format you are comparing against, then copy the result.
5. Computed with the Web Crypto API, the browser’s own cryptographic implementation.
6. Hex output is lowercase; some providers publish uppercase, so compare case-insensitively.
7. The key and message stay in your browser — but treat any secret pasted into any web page as worth rotating afterwards if it is a production key.

What HMAC adds that a plain hash does not

A hash proves a message has not changed. It does not prove who sent it, because anyone can hash anything. HMAC mixes a secret key into the hash in a specific two-pass construction, so only someone holding the key can produce a valid digest for a message — and anyone holding the key can verify one. That is why every webhook provider signs its payloads this way: the signature in the header proves the request came from them and arrived intact.

Why the construction matters, not just the ingredients

The obvious approach — hashing the key followed by the message — is broken for the SHA-1 and SHA-2 families, because their internal structure lets an attacker append data to a message and compute a valid digest for the longer version without knowing the key. That is the length-extension attack. HMAC’s two nested hashes with padded key variants close it. The practical lesson is to use HMAC rather than inventing a scheme, even a plausible one.

Debugging a signature mismatch

When a webhook signature will not verify, it is almost never the algorithm. In order of likelihood: you are hashing the parsed and re-serialised body instead of the exact raw bytes received; the output encoding differs (hex against Base64, or uppercase against lowercase hex); the key has trailing whitespace from a copy-paste or a .env file; or the provider signs a constructed string — timestamp plus a dot plus body — rather than the body alone. Reproduce it here with a known payload before you change any code.

Choosing the algorithm, and comparing safely

SHA-256 is the default nearly everywhere and the right choice for new work. HMAC-SHA1 is still in wide use and, unusually, is not considered broken — HMAC’s security does not rest on the collision resistance that SHA-1 lost — but there is no reason to choose it now. Whichever you use, compare signatures with a constant-time comparison in production code: a normal string comparison returns early on the first differing byte, which leaks enough timing information to forge a signature byte by byte.