HMAC Generator

Why a key changes everything

A plain hash proves only that the data has not changed — anyone can recompute it, so anyone who can edit the data can also fix the hash. HMAC folds a secret key into the compression function, so producing a valid tag requires the key. That is what turns a checksum into an authentication code: it answers “did this come from someone who holds the key, and is it intact?”

Structurally HMAC is H(key ⊕ opad ‖ H(key ⊕ ipad ‖ message)). The two nested hashes and the padding constants exist so that the construction stays secure even when the underlying hash has weaknesses that would break a naive H(key ‖ message) — the length-extension problem. In practice you never implement it by hand; every platform ships it.

Where you will meet it

• JWT with HS256 — the signature is an HMAC over base64url(header) + "." + base64url(payload) with the shared secret as the key.
• Webhook verification — Stripe, GitHub, Slack and friends sign the raw request body and send the result in a header.
• AWS Signature Version 4 — a chain of HMACs that derives a signing key from your secret access key.
• Cookie and form signing — frameworks sign a value so a client cannot tamper with it unnoticed.
• PBKDF2, HKDF, TOTP — all built on HMAC as the primitive.

Comparing a tag safely

Compare the computed tag against the received one with a constant-time function, not ==. A byte-by-byte comparison that returns early leaks how many leading bytes matched, which is enough to recover a valid tag one byte at a time. Use crypto.timingSafeEqual() in Node, hmac.compare_digest() in Python, or hash_equals() in PHP.

FAQ