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HQC (Hamming Quasi-Cyclic)

What

HQC is a code-based post-quantum key-encapsulation mechanism selected by NIST in March 2025 as the fifth PQC algorithm standardised (after ML-KEM, ML-DSA, SLH-DSA, Falcon). The NIST HQC standard is being drafted as of 2026. HQC's security reduces to the hardness of decoding random quasi-cyclic codes — a structurally different mathematical foundation from lattice- based primitives like ML-KEM and ML-DSA.

(Source: sources/2026-04-16-meta-post-quantum-cryptography-migration-at-meta-framework-lesson)

Why it matters: algorithmic-diversity hedge

The architectural role HQC plays is mathematical diversity against the risk that lattice-based PQC primitives (ML-KEM, ML-DSA) all have a shared cryptanalytic failure mode. Meta names this explicitly:

HQC has been recently selected by NIST for standardization. It is developed based on different math than ML-KEM, which is important if weaknesses are discovered in ML-KEM or its modular lattices approach, ensuring that an alternative method for PQC protection can still be deployed to protect organizations from SNDL attacks. (Source: sources/2026-04-16-meta-post-quantum-cryptography-migration-at-meta-framework-lesson)

This is defence in depth applied at the algorithm-family axis: if a general attack on the module-lattice class emerges, HQC is the already-deployed non-lattice alternative organisations can pivot to without a standards-adoption wait.

Meta cryptographers as co-authors

Meta cryptographers co-authored HQC (per NIST's 2025 announcement), and Meta also co-authored the related code-based candidates BIKE and Classical McEliece — all submitted through the NIST PQC process. This is Meta's direct contribution to the community-vetted standards leg of its three-external- dependencies migration model.

Sibling code-based candidates

  • BIKE — bit-flipping key encapsulation, also quasi-cyclic codes. Meta co-authored. Did not win NIST standardisation through the main competition.
  • Classical McEliece — the original 1978 code-based scheme; very large public keys (hundreds of KB) but extremely conservative security. Meta co-authored. Not NIST-standardised in the main competition but has an independent ISO standard.

Trade-offs vs ML-KEM

  • Public key size: HQC's public keys are considerably larger than ML-KEM's (tens of kB vs ~1 kB at comparable security levels).
  • Bandwidth: HQC ciphertexts are also larger.
  • Performance: slower than ML-KEM in typical benchmarks.
  • Diversity value: non-lattice mathematical foundation.

The trade-off is why ML-KEM is the default and HQC is the hedge — HQC is deployed when organisations either want the diversity insurance or when ML-KEM is later found to be weak.

Standardisation state (as of 2026-04)

  • NIST HQC standard: being drafted — not yet finalised.
  • NIST HQC selection: March 2025.
  • Library implementations: available in LibOQS; Meta contributes.

Seen in

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