noise-pq: add Noise_XXhfs_25519+XWing_ChaChaPoly_SHA256 spec (Stage 1 Working Draft)

[paschal533]

Summary

This PR adds a new protocol spec for Noise_XXhfs_25519+XWing_ChaChaPoly_SHA256, a post-quantum hybrid extension of the classical Noise XX handshake used in libp2p.

The spec is filed at Stage 1 (Working Draft) per the libp2p spec lifecycle.

• Specifies the e1 and ekem1 HFS token extensions from the Noise HFS draft, applied to the existing Noise XX pattern
• Uses X-Wing (ML-KEM-768 + X25519) as the KEM primitive, following draft-connolly-cfrg-xwing-kem-06
• Covers wire format (message A 1248 B, B 1232 B, C 64 B), token ordering requirements, cipher state split, ML-KEM implicit rejection semantics, and interoperability requirements
• Includes a test vector schema matching the reference implementation

Motivation

NIST finalized ML-KEM (FIPS 203) in August 2024. Harvest-now-decrypt-later attacks make forward secrecy the most urgent deployment target: an adversary recording today's traffic can break it retroactively once a quantum computer exists. The XXhfs pattern adds quantum-safe forward secrecy with no changes to the classical authentication layer, giving operators a safe migration path.

Reference Implementations

Both implementations are open PRs against their respective libraries:

• JavaScript: paschal533/js-libp2p-noise feat: add post-quantum hybrid Noise handshake ChainSafe/js-libp2p-noise#665 (Noise XX + HFS extension, test vectors, benchmarks)
• Python: libp2p/py-libp2p PR #1310 feat(security): Noise XXhfs post-quantum handshake for py-libp2p (research/WIP) py-libp2p#1310 (PatternXXhfs, TransportPQ, liboqs C backend)

Cross-language handshake interoperability has been validated against the published test vectors.

Test plan

• Review spec against Noise HFS extension spec for correctness
• Review token ordering (encryptAndHash before mixKey in ekem1)
• Verify wire format sizes match the reference implementation
• Check lifecycle header format matches other specs in this repo
• Identify interest group members (need 3+ for Stage 2)