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Security Fundamentals & Crypto · Advanced

Hamming Quasi-Cyclic (HQC)

Code-based key encapsulation mechanism selected by NIST in March 2025 as a backup to ML-KEM.

Definition

HQC relies on the hardness of decoding problems in error-correcting codes, providing mathematical diversity from lattice-based ML-KEM. It has larger keys and ciphertexts than ML-KEM. NIST is standardizing it so a non-lattice alternative is available if lattice cryptanalysis advances.

Example

An organization's crypto-agility plan lists HQC as a fallback KEM if ML-KEM is weakened.

Exam tip

HQC is a backup KEM, not a signature scheme, and not a replacement for ML-KEM in normal use.

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