Bitcoin Could Embrace Quantum‑Safe Upgrades While Lightning Privacy Remains Exposed

Even if Bitcoin eventually adopts post-quantum protections for on‑chain outputs, five components of the Lightning Network’s off‑chain payment system still require separate upgrades, according to a research preprint published on September 12.

The study introduces PQLN, a hybrid post‑quantum extension built as a rust‑lightning prototype, which secures gossip traffic, encrypted peer connections, payment invoices and reusable payment offers, as well as onion routing packets that hide routing details.

Lightning currently relies on secp256k1‑based ECDSA, Schnorr signatures, or ECDH. PQLN preserves these classical mechanisms while adding ML‑DSA signatures to gossip, invoices and offers, and incorporates hybrid ML‑KEM key exchange for transport, payment onion routes, and BOLT 12 privacy paths.

The authors created a feature‑gated rust‑lightning fork and a modified ldk‑sample node. Their 12‑scenario matrix demonstrated that post‑quantum and vanilla binaries derived from the same rust‑lightning base can coexist; depending on route and configuration, mixed setups either employ classical protection or fail to close.

This compatibility does not guarantee network‑wide readiness. The codebase is presented as a research artifact, and the study postponed testing against other Lightning implementations.

Results show that communication overhead dominates performance. During static synchronization of the full network graph, the default ML‑DSA‑44 and ML‑KEM‑768 configuration increased gossip downloads by a factor of 10.2 and stored graph data by 8.8 times compared with the vanilla Lightning implementation.

A more modest Falcon‑derived FN‑DSA configuration lowered the download factor to 4.2, but NIST still classifies that scheme as a work in progress rather than a final standard.

Private payment routing also incurs bandwidth penalties. A fixed 21.8 KB post‑quantum ciphertext list adds 19–53 ms per hop on a simulated 10 Mbit/s link and 160–187 ms per hop at 1 Mbit/s.

Conversely, ML‑DSA‑44 signing averaged 327 µs on the authors’ 16‑core AMD Ryzen Threadripper PRO 3955WX workstation equipped with 64 GB RAM; this hardware‑specific measurement should not be interpreted as end‑to‑end payment latency.

PQLN does not address Bitcoin‑rooted risks. It does not protect on‑chain keys that secure funding, commitment, HTLC, or penalty transactions, so safeguarding channel funds still requires Bitcoin‑layer post‑quantum changes.

The preprint assumes a future quantum adversary; it does not report any present‑day quantum compromise of Bitcoin or Lightning. The prototype indicates that Lightning’s off‑chain defenses can be evaluated independently of a Bitcoin upgrade, though it does not confirm network readiness for migration. The key operational question remains how much additional gossip, storage, and payment traffic node operators can tolerate.

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