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A message authentication code (MAC) is a fixed-length value created from a message and a secret key shared by the sender and receiver. The sender sends the message with its MAC; the receiver uses the same key to verify the tag. A valid match helps show that the message was not altered and that it came from someone able to use the shared key.
How does a message authentication code work?
- Share and protect a secret key. The sender and receiver must have access to the same secret key.
- Create a tag. The sender runs the message and key through a MAC algorithm, producing a fixed-length tag.
- Send the message and tag. The tag accompanies the message; it does not conceal the message contents.
- Verify the tag. The receiver uses the shared key and the received message to verify the tag. If verification fails, the receiver rejects the message as unauthenticated or altered.
The key makes a valid tag difficult for someone without the key to predict for a new message, even if that person has observed tags for other messages, within the algorithm’s supported security level. NIST describes the role of MACs in its Message Authentication Codes project.
What does a MAC protect—and what does it not prove?
Integrity and data origin within a shared-key group
A successful check supports the conclusion that the message has not changed since a valid tag was created and that the tag was generated by someone able to use the shared key. This is data-origin authentication in the context of the parties that share that key.
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Not confidentiality or public proof of authorship
A MAC alone does not encrypt a message, so it does not hide the contents. It also cannot establish to an outside observer which particular key holder created a valid tag: any party with the shared key can generally generate one. For that reason, a MAC does not provide non-repudiation. A digital signature uses a private signing key and a corresponding public verification key, allowing verification without giving verifiers the ability to create signatures.
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How is a MAC different from a hash?
A hash function maps data to a digest without requiring a secret key. A hash can help detect differences if the expected digest is trusted separately, but a plain hash does not authenticate who supplied the message: an attacker who changes the message could also calculate a new hash. A MAC combines the message with a shared secret, making valid-tag generation dependent on access to that key.
What are HMAC, KMAC, and CMAC?
NIST identifies HMAC, KMAC, and CMAC as approved general-purpose MAC algorithm families. They use different underlying constructions, so selection should follow the protocol’s requirements and applicable standards rather than an assumption that one is universally faster or safer.
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| Family | Construction | Reference |
|---|---|---|
| HMAC | Uses a cryptographic hash function with a shared secret key. | NIST FIPS 198-1 |
| KMAC | A keyed hash based on KECCAK; includes KMAC128 and KMAC256 variants. | NIST SP 800-185 |
| CMAC | Based on a symmetric-key block cipher, such as AES. | NIST SP 800-38B |
NIST’s MAC project page also lists GMAC, an authentication-only specialization of GCM. It is another option in the authenticated-encryption family, not a reason to treat ordinary MACs as encryption.
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- Protect the key. A MAC depends on the shared secret remaining confidential. Anyone who obtains it can create valid tags.
- Use the protocol’s specified algorithm and parameters. The correct choice depends on the protocol and applicable standards; do not substitute an algorithm or parameters without checking those requirements.
- Use a vetted cryptographic implementation. Rely on the platform or library’s verification function rather than implementing MAC calculations yourself. Verification should use the implementation’s appropriate comparison method.
- Do not mistake a valid MAC for encryption. If message contents must be hidden, a MAC by itself is insufficient; use the cryptographic construction required by the protocol.
What is the current status of NIST’s MAC standards?
Standards status can change. NIST published FIPS 198-1, its HMAC specification, in July 2008. A NIST planning note dated June 23, 2025 proposed withdrawing it and moving the specification to SP 800-224; that note describes a proposal, not confirmation that the transition is complete.
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NIST SP 800-38B specifies CMAC. Its publication page records an original publication date of May 2005 and an update dated October 6, 2016. A planning note dated April 10, 2025 says NIST decided to revise the publication, but the page does not establish that a revised final version has appeared. Check the linked NIST pages for the latest publication status before relying on a specific standards edition.
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