Question 1

What is HMAC?

Accepted Answer

HMAC (Hash-based Message Authentication Code) is a mechanism for verifying both the integrity and authenticity of a message. It combines a cryptographic hash function (such as SHA-256) with a secret key to produce a fixed-size signature. Unlike a plain hash, an HMAC cannot be forged without knowing the secret key.

Question 2

What is the difference between HMAC and a regular hash?

Accepted Answer

A regular hash (e.g., SHA-256) depends only on the input data — anyone with the same data gets the same hash. HMAC adds a secret key, so only parties who know the key can produce or verify the correct HMAC. This makes HMAC suitable for authentication, while plain hashes are only suitable for integrity checks where the hash itself is trusted.

Question 3

What HMAC algorithm should I use?

Accepted Answer

HMAC-SHA256 is the most widely used and recommended choice for new applications. It is used in JWT signatures (HS256), AWS Signature Version 4, webhook verification, and many API authentication schemes. HMAC-SHA512 provides a larger output for higher-security requirements. Avoid HMAC-MD5 and HMAC-SHA1 for new implementations.

Question 4

What is HMAC used for in APIs?

Accepted Answer

HMAC is commonly used in API authentication to verify that requests come from an authorized client and haven't been tampered with in transit. The client signs the request body or specific headers with a shared secret key; the server recomputes the HMAC and compares. This pattern is used by Stripe webhooks, GitHub webhooks, AWS API authentication, and many payment gateways.

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