HQC — Code-Based KEM (NIST PQC4 Backup)
Hamming Quasi-Cyclic key encapsulation mechanism, the NIST PQC Round 4 family-disjoint backup KEM. Reserved precompile 0x012208 per LP-0120.
Provenance
Originally LP-4600, renumbered 2026-05-18 to consolidate all cryptographic primitives into the 4xxx range with family sub-ranges. The 4600 slot is now the ML-KEM family root (was LP-4318); HQC moved to 4660 within the PQ-KEM sub-range (4600-4699) where 4660-4699 holds code-based KEMs. The 4600 historical slot for HQC is documented in CRYPTO-MIGRATION-2026-05-18.md.
Abstract
HQC (Hamming Quasi-Cyclic) is the IND-CCA2-secure code-based key encapsulation mechanism selected in NIST PQC Round 4 as the family-disjoint backup to lattice-based ML-KEM (FIPS 203, LP-4600). This LP reserves the LP-4660 identifier for the HQC algorithm family, covering the HQC-128 / HQC-192 / HQC-256 parameter tiers (Lux network default: HQC-128 with 4,481 B ciphertext and ~150 µs verification, per LP-0120 §precompile 0x012208). HQC's security relies on the hardness of decoding random quasi-cyclic codes (Syndrome Decoding problem); a break of HQC does not imply a break of ML-KEM (different mathematical hardness assumption), making the pair an effective hedge against single-family cryptanalytic breakthroughs. HQC is consumed by the X-Wing+ KEM combiner (LP-4930) and is reserved for direct use in PQ-TLS 1.3 backup contexts. Full parameter tables, test vectors, and EVM gas-cost-model to be specified in a subsequent revision.
Reference implementation
- Primitive:
luxfi/crypto/kem/hqc(planned) - Hash backend: SHAKE256
- Precompile address:
0x012208(reserved per LP-0120)
Copyright
Copyright and related rights waived via CC0.