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A message authentication code (MAC) is a fixed-length cryptographic tag made from a message and a secret key shared by the sender and receiver. The receiver uses that key to check the tag: a mismatch means the message or tag is not valid. This provides integrity and authentication within the group that knows the key, but it does not hide the message or prove to outsiders which key-holder created it.
How a message authentication code works
A MAC system has two inputs: the message and a secret key. The sender runs them through a MAC algorithm to produce a tag, then sends the message and tag. The receiver uses the same key and algorithm to verify the received pair. If verification fails, the receiver rejects it as unauthenticated or altered.
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- Share and protect a key. The sender and receiver must have access to the same secret key through a secure key-management process.
- Generate a tag. The sender computes a MAC over the message using the agreed algorithm and key.
- Send the pair. The message and its tag travel together; the MAC does not encrypt the message.
- Verify before trusting. The receiver checks the tag with the same key and accepts the message only if verification succeeds.
A secure MAC is designed to make it computationally infeasible for someone without the key to predict a valid tag for a new message, even after observing tags for other messages, within the algorithm’s supported security level. Use a vetted cryptographic implementation and the parameters required by the relevant protocol.
What a MAC proves—and what it does not
A valid tag provides evidence that the message has not changed since a party with access to the shared key generated the tag. It authenticates the message’s origin only in that shared-key context: every party holding the key can generally create valid tags. Consequently, a MAC alone cannot prove to an outside observer which participant authored a message, and it does not provide non-repudiation.
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- Integrity: a change to the message should cause verification to fail.
- Shared-key data-origin authentication: a valid tag indicates that someone able to use the shared key generated it.
- Not confidentiality: the message remains readable unless it is separately encrypted.
- Not public authorship proof: the verifier also possesses the key and could generate a tag.
MAC vs. hash vs. digital signature
These mechanisms address related but different needs. A hash is unkeyed, a MAC uses a shared secret, and a digital signature uses a private signing key with a corresponding public verification key.
| Mechanism | Key arrangement | What verification establishes |
|---|---|---|
| Cryptographic hash | No secret key is required. | It produces a digest for data. A digest alone does not authenticate who supplied the data; the expected digest must be trusted separately. |
| MAC | Generating and verifying parties share a secret key. | Integrity and data-origin authentication among parties able to use that key. |
| Digital signature | The signer uses a private key; others can verify with a public key. | Can support public verification, unlike a shared-key MAC. |
Choose according to who must verify the message and what the evidence needs to show. If only parties sharing a secret need to check integrity and origin, a MAC may fit. If independent parties need to verify a signer without possessing the signing secret, a digital-signature scheme addresses a different requirement.
Common MAC algorithm families
NIST identifies HMAC, KMAC, and CMAC as approved general-purpose MAC algorithms. They use different underlying constructions, so follow the algorithm and parameters specified by the protocol rather than treating the families as interchangeable.
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|---|---|---|
| HMAC | A cryptographic hash function used with a shared secret key. | FIPS 198-1 |
| KMAC | A keyed hash based on KECCAK; variants include KMAC128 and KMAC256. | SP 800-185 |
| CMAC | A MAC based on a symmetric-key block cipher, such as AES. | SP 800-38B |
HMAC
HMAC combines a hash function with a secret key. NIST FIPS 198-1, published in July 2008, specifies HMAC. NIST’s June 23, 2025 planning note proposed withdrawing FIPS 198-1 and moving the specification to SP 800-224; that note describes a proposal, not confirmation that the transition is complete. See NIST’s FIPS 198-1 publication page for status.
KMAC
KMAC is based on KECCAK and is specified in NIST SP 800-185, which defines KMAC128 and KMAC256. Consult the standard and the protocol’s requirements for the appropriate variant and parameters.
CMAC
CMAC uses a symmetric-key block cipher construction. NIST SP 800-38B specifies CMAC; its publication page lists May 2005 as the original publication date and October 6, 2016 as the update date. On April 10, 2025, NIST noted that it had decided to revise the publication. The page does not establish that a revised final version has appeared: NIST SP 800-38B.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.MACs and authenticated encryption
A MAC by itself protects integrity and supports shared-key authentication; it does not encrypt content. Some authenticated-encryption constructions can be specialized for authentication-only use. NIST calls the authentication-only specialization of GCM GMAC. See NIST’s Message Authentication Codes project for its overview and references.
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What to check when choosing or implementing a MAC
- Use the MAC family and parameters required by the applicable protocol and current standards.
- Use a vetted cryptographic library rather than designing a construction yourself.
- Keep the secret key protected and restrict access to parties that need to generate or verify tags.
- Use the implementation’s verification function to check received tags before acting on the message.
- Check current standard and validation status: NIST’s MAC project page links to references and Cryptographic Algorithm Validation Program resources.
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