Ahmet Kirk fits post quantum signatures into Lightning

Off-chain Bitcoin payments face a quantum threat long before the base layer settlement network. Adversaries can record encrypted gossip and onion routing traffic today, storing the data until quantum hardware matures enough to decrypt private channel states.

To neutralize this threat, developer Ahmet Kirk and co-author Abdullah built the Post-Quantum Lightning Network prototype in Rust Lightning. Their 11,000-line implementation integrates NIST-standardized MLDSA-44 signatures while maintaining full backward compatibility with existing classical nodes. The design preserves standard 1,300-byte onion packets by routing hybrid post-quantum secrets alongside Hash Time-Locked Contract messages in a fixed 20-slot update list rather than stuffing ciphertexts directly into the onion.

"The quantum threat to Bitcoin is closer off-chain than on-chain."

- Ahmet Kirk, Bitcoin Optech

That backward compatibility carries a heavy bandwidth penalty. On Bitcoin Optech, researchers noted that nodes exchanging MLDSA-44 signatures consume ten times more gossip bandwidth than standard nodes. Switching to unstandardized Falcon signatures drops that overhead to fourfold. Bolt 11 invoice signatures also require splitting across four tag fields to bypass Rust Lightning's 639-byte message limit, though MLDSA signing itself adds just 0.33 milliseconds of computation overhead.

While second-layer developers refine post-quantum defenses, the underlying base layer infrastructure is preparing its own overhaul. Bitcoin Core contributor Jan B outlined the community testing push for version 32.0, which replaces the legacy libevent library with a custom HTTP server enforcing strict RFC rules. Downstream applications like hardware wallets, block explorers, and Lightning daemons must validate their setups before the October launch to prevent RPC formatting breaks.

"Automated software tests cannot replicate a node running in someone's closet."

- Jan B, Bitcoin Optech

The rush to harden off-chain software extends beyond theoretical future threats. Core Lightning released emergency update 26.06.8 after discovering critical bugs in channel state machines. Malicious peers could trick victim nodes into treating forced channel closes as cooperative shutdowns by issuing decoy requests before broadcasting revoked commitments. By duping the victim node, attackers effectively blocked it from executing penalty transactions to recover stolen funds.

Core Lightning also patched severe vulnerabilities in channel splices and dual-funding negotiations. Without strict bounds, malicious peers could propose astronomical fee rates or trigger database overflows, trapping victim nodes in infinite crash loops upon restart. Developers introduced PR 9507 to cap peer-proposed fee rates at 4,000 satoshis per vbyte, illustrating how off-chain state machine complexity constantly creates new attack vectors.

Other major implementations are scrambling to patch related edge cases. LND updated its Atomic Multipath Payments logic to prevent total invoice cancellations when a single path drops, while LDK added Replace-By-Fee bumping for stuck splices. Off-chain developers are demonstrating that while base-layer consensus changes move at a crawl, second-layer security can iterate rapidly to stay ahead of adversaries.