🔐HMAC

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Also known as: Hash-based Message Authentication Code

A message authentication code built from a cryptographic hash function and a secret key, providing integrity and authenticity.

DESCRIPTION

HMAC wraps a cryptographic hash function (e.g., SHA-256) in a keyed construction using inner and outer paddings to produce a MAC. It maintains security even if the underlying hash has certain weaknesses, provided the compression function remains sound. HMAC is widely deployed in TLS, OAuth-style API signing, and secure storage. Key separation and correct parameter choices are essential. Its simplicity, provable security bounds, and broad support make HMAC a default integrity mechanism.

ALTERNATIVE DEFINITIONS

IETF RFC 2104

HMAC is a mechanism for message authentication using cryptographic hash functions.

Canonical definition describing HMAC as a keyed-hash mechanism.

Source

NIST SP 800-107 Revision 1

HMAC is a specific construction for calculating a message authentication code (MAC) involving a cryptographic hash function in combination with a secret key.

Formal NIST recommendation highlighting construction specifics.

Source

RELATED TERMS

MAC SHA-256 SHA-3 Keyed Hash Password-Based Key Derivation Function 2 TLS API Authentication Key Derivation Function

COMPETING TERMS

CMAC GMAC

KNOWLEDGE AREAS

Data Security Software Engineering Testing & Validation

TARGET AUDIENCE

Cybersecurity Engineer Security Architect Software Developer MLOps Engineer IT Auditor

EXTERNAL SOURCES

• Wikipedia - HMAC
• IETF RFC 4868 - HMAC-SHA2 for IPsec specification
• IETF - HMAC test cases and conformance
• OAuth - API signing with HMAC in OAuth 1.0

TOP QUESTIONS

Why is HMAC widely used?

It is simple, efficient, widely standardized, and has strong security proofs under standard hash assumptions.

Category: Relevancy

Which hash functions are recommended for HMAC?

Use SHA-256, SHA-384, SHA-512, or SHA-3 variants depending on performance and footprint needs.

Category: Recommendation

Does HMAC protect against replay attacks?

Only if you include nonces, timestamps, or sequence numbers in the data covered by HMAC.

Category: Risk

How long should HMAC keys be?

At least as long as the hash output; 256 bits is common for SHA-256 HMAC.

Category: Requirements

Is HMAC affected by length extension attacks?

No. HMAC’s construction prevents classic Merkle–Damgård length extension vulnerabilities.

Category: Security

Can I truncate HMAC tags?

Yes, but ensure sufficient tag length (e.g., 128 bits) for the threat model and collision resistance.

Category: Best practices

How is HMAC used in API authentication?

Clients sign canonical request data with HMAC; servers recompute and verify to authenticate and ensure integrity.

Category: Example

Should I reuse keys between HMAC and encryption?

No. Derive separate keys via a KDF for MAC and encryption to avoid cross-protocol risks.

Category: Mistakes

How to validate HMAC implementations?

Use official test vectors (e.g., RFC 4231) and constant-time comparisons for tag verification.

Category: Testing