Check a message with a shared secret

A valid MAC is a shared-key integrity check, not encryption or a public digital signature.

• HMAC is a hash-based message authentication code (MAC). A MAC is a check value, or tag, calculated with a secret key. A cryptographic hash compresses input bytes to a fixed-size digest; HMAC combines a hash with a shared key using the specific construction in §2. An unkeyed checksum does not serve the same purpose (§1).
• The sender and receiver use the same chosen hash and secret K. The sender sends message bytes and a tag; the receiver calculates the tag for the bytes it received and checks it. The drawing illustrates that use of a MAC, not a wire format or an application authorization decision (§§1,6).
• All numerical examples use public test keys. The main example is RFC 4231 §4.2: HMAC-SHA-256, key byte 0b repeated 20 times and the eight ASCII bytes "Hi There" (without a trailing NUL). Its full 32-byte tag in four hexadecimal groups is b0344c61d8db3853 5ca8afceaf0bf12b 881dc200c9833da7 26e9376c2e32cff7. A displayed tag prefix followed by … is only a preview, not an actual shortened tag.
• In the changed-message example, "Hi There!" is nine bytes but carries the original tag. Recomputing with the same key produces a different tag, so this chosen comparison fails before message acceptance. This is a computed example, not proof that all distinct messages have distinct tags.
• The message remains readable. Both key holders can generate valid tags; this is not a public digital signature or proof to a third party of which holder sent the message. HMAC itself supplies neither a packet format nor freshness, replay prevention, secret exchange or key storage. Algorithm and protocol security depend on their surrounding design (§§1,3,6).