A mechanism that uses a shared key to detect tampering with a message.
Short for Hash-based Message Authentication Code. A mechanism that combines a shared key with a hash function, letting a message's integrity and authenticity be verified at the same time. A plain hash function might look like it detects tampering too, but anyone without the key can compute the same hash, so it really doesn't; folding in a shared key is what lets HMAC also authenticate the sender, which is its real value.
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