Even if Bitcoin eventually adopts post-quantum protections for on-chain outputs, the Lightning Network's off-chain payment system would require separate upgrades to defend against quantum threats, according to a September 12 research preprint.
Researchers proposed PQLN, a hybrid post-quantum extension tested through a rust-lightning research prototype. The system targets five key functions: gossip distribution of node and channel information, encrypted peer connections, signed invoices and reusable payment offers, and onion packets that conceal routing details.
Current Limitations and Bandwidth Trade-offs
Lightning currently relies on secp256k1-based ECDSA, Schnorr signatures, and ECDH for these functions. PQLN maintains classical mechanisms while adding ML-DSA signatures for gossip, invoices, and offers, plus hybrid ML-KEM key exchange for transport and payment onions.
Testing revealed significant operational challenges. With default ML-DSA-44 and ML-KEM-768 settings, full network gossip downloads increased 10.2 times compared to standard Lightning, while stored graph data grew 8.8 times. A smaller FN-DSA configuration reduced the download increase to 4.2 times, though that scheme remains under development rather than a finalized standard.
Private payment routing introduced additional latency. Post-quantum ciphertext lists added 19 to 53 milliseconds per hop on a 10 Mbit/s connection and 160 to 187 milliseconds per hop at 1 Mbit/s speeds.
Unresolved On-Chain Security
PQLN does not address Bitcoin-layer vulnerabilities. The prototype leaves unprotected the on-chain keys securing funding, commitment, and HTLC outputs, along with penalty transactions. Safeguarding Lightning channel funds would still require separate post-quantum protections at Bitcoin's base layer.
The research indicates Lightning's off-chain defenses can be tested independently of Bitcoin upgrades, but the primary operational question for node operators remains whether networks could absorb the increased gossip, storage, and payment traffic required by post-quantum schemes.
