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Also known as: Galois Message Authentication Code
An authentication-only variant of Galois/Counter Mode providing message integrity without encryption.
DESCRIPTION
GMAC is a message authentication code derived from GCM, using polynomial hashing over GF(2^128) to compute an authentication tag. It provides integrity and authenticity for messages without confidentiality, making it suitable for scenarios where encryption is handled elsewhere or not required. GMAC requires a unique nonce per key to avoid catastrophic security failures. It is standardized by NIST and commonly implemented alongside AES.
ALTERNATIVE DEFINITIONS
NIST
GMAC is an authentication-only variant of GCM.
Explicitly identifies GMAC’s relation to GCM and its authentication-only property.
Wikipedia
GMAC is an authentication-only mode of operation that provides a message authentication code using the GCM construction.
Frames GMAC within the family of block cipher modes and GCM.
RELATED TERMS
AES-GCM Rijndael MAC Authenticated Encryption Nonce Authentication Tag NIST SP 800-38D CTR Mode
COMPETING TERMS
KNOWLEDGE AREAS
Data Security Data Privacy & Compliance Infrastructure
TARGET AUDIENCE
Security Architect Cybersecurity Engineer Chief Information Security Officer Software Developer Data Privacy Officer
EXTERNAL SOURCES
- Wikipedia - Galois/Counter Mode
- NIST - Official GCM/GMAC specification
- IETF - AEAD interfaces and algorithms
- MDN Web Docs - Overview of cryptographic primitives
TOP QUESTIONS
When should GMAC be used instead of GCM?
Use GMAC when only authenticity and integrity are required, and confidentiality is handled elsewhere or not needed.
Category: Requirements
What is the biggest risk when using GMAC?
Reusing nonces with the same key severely compromises security. Enforce unique nonces and strong key management.
Category: Risk
How does GMAC compare to HMAC?
GMAC relies on block cipher operations and GCM structure; HMAC uses hash functions. Choice depends on hardware acceleration and ecosystem.
Category: Alternatives
What standards define GMAC?
NIST SP 800-38D defines GCM and GMAC; various RFCs reference AEAD constructions compatible with GMAC.
Category: Definition
Is GMAC suitable for streaming messages?
Yes, but maintain nonce uniqueness and consider segment authentication if partial verification is needed.
Category: Requirements
What performance characteristics does GMAC have?
GMAC benefits from AES-NI and carry-less multiplication instructions, providing high throughput for integrity checks.
Category: Example
How is the GMAC tag verified?
The receiver recomputes the tag using the shared key and nonce, comparing it with the transmitted tag.
Category: Definition
Can GMAC protect metadata as well as payload?
Yes, additional authenticated data (AAD) can be included to bind headers or context into the tag.
Category: Requirements
What is a common mistake with GMAC implementations?
Inadequate nonce management or mixing keys across environments. Use robust key rotation and deterministic nonce schemes.
Category: Mistakes