Lightning Improvement Package (LDK), software program used to construct Bitcoin Lightning purposes, has a privateness repair that blocks a documented probe linking an unannounced channel to the Bitcoin output backing it. The probe exploited how LDK forwarded onion messages, turning a guessed channel identifier into affirmation of a connection between two suspected nodes.
Bitcoin Optech highlighted the change on October 30. An unannounced channel stays out of the general public community map, however that alone didn’t forestall LDK’s message-forwarding lookup from confirming its funding hyperlink.
An actual short-channel ID, or SCID, works like blockchain coordinates. The Lightning specification defines it utilizing the block peak, the transaction’s place inside that block and the output’s place inside that transaction. Collectively, these establish the actual Bitcoin output funding a channel.
An attacker can derive a candidate ID from an output’s location with out but realizing whether or not it belongs to a channel between the suspected nodes. The documented probe provides that lacking affirmation. It requires the attacker to connect with each endpoints and assemble an onion-message path by way of them that returns to the attacker.
Earlier than the repair, LDK’s channel lookup may resolve an actual SCID even for an unannounced channel as soon as its funding confirmed. If the message returned efficiently, the attacker may affirm that the guessed output and the channel between these nodes matched.
The place the probe stops
A returned message confirms the candidate funding-output hyperlink beneath the documented probe circumstances.
LDK rejects the unannounced channel’s actual SCID except it’s the outbound alias. Legitimate aliases stay usable.
LDK blocks the unannounced channel’s real-ID lookup; legitimate aliases stay usable.
Requires attacker connections to each suspected endpoints and a candidate actual SCID. Rejection excepts a price equal to the outbound alias. The merge doesn’t set up launch inclusion or deployed pockets protection.
The demonstrated leak subsequently joins two items of data: a Bitcoin funding output and the Lightning nodes utilizing it.
What the repair adjustments
LDK now refuses to resolve an unannounced channel’s actual SCID for onion-message forwarding. Legitimate aliases stay usable as various identifiers, so a sender can deal with the channel with out utilizing its actual blockchain coordinates. The change closes this lookup route whereas preserving forwarding that makes use of an allowed alias.
The change is described within the pull request. Underneath BOLT 4’s onion-routing guidelines, an onion-message SCID should correspond to an introduced identifier or an area alias.
The repair’s inclusion in put in pockets variations stays unverified. Customers want affirmation from their pockets supplier earlier than assuming put in software program has this safety.
