Security and privacy of the Lightning Network

Lightning is private by design: balances aren't gossiped, and onions hide senders and receivers. But privacy here is a matter of degree, and a curious or hostile node has tricks.

Probing

Alice can send Dina a payment with a random payment hash. It can never settle, because nobody knows the preimage. Look at how it fails:

• If it reaches Dina, she says "I don't know this payment" (incorrect_payment_details).
• If a channel on the way is too small, a node says so earlier.

So each failure answers a yes-or-no question: can this amount get through? Ask enough questions and you learn a channel's balance. That is probing.

⚡Probe a route's balance

Python

NodeAlice · LNDBob · c-lightningChan · c-lightningDina · LNDNo node

from jifunze_lab import probe # probe(sats) sends a payment with a fake hash from Alice to Dina. It can never succeed, # but the error tells us whether the amount could have reached Dina (True) or not (False). print("10,000 sats:", probe(10_000)) print("5,000,000 sats:", probe(5_000_000)) low, high = 0, 1_000_000 # TODO: binary-search between low and high (steps of 10,000 sats are fine) # and print the largest amount that can reach Dina, as: max_sats: N

▶ RunCheckReset lessonRestore starter code

These exercises need a laptop with the lab running. You can read the lesson on your phone.

Your binary search should land within 20,000 sats of the route's real limit, without ever paying a sat.

Other attacks

• Channel jamming. An attacker sends many payments to themselves and never settles them, filling channels' HTLC slots (483 per side) or locking up liquidity until the time locks expire. Proposed defences include upfront fees and reputation for peers.
• Fee and timelock manipulation. An attacker can try to make an HTLC expire before it can be claimed on-chain when fees are high. Anchor outputs let nodes raise fees on their closing transactions.
• Privacy leaks. Your node's public channels, aliases and on-chain funding outputs can be linked to you. Private channels, onion services (Tor) and careful coin selection help.
• Eclipse attacks. A node fed only by attacker-controlled peers can be shown a false view of the chain. Connect to several honest Bitcoin nodes.