Ark

Users in one round jointly create a transaction tree anchored by a root transaction onchain. Their claim is represented by a branch ending in a virtual UTXO, or VTXO. [2]

An out-of-round payment transfers a VTXO through a presigned transaction cosigned by the operator. Until it is refreshed in a later round, the receiver relies on neither the sender nor an earlier sender in the chain colluding with the operator to double-spend the payment. [3]

A unilateral exit means publishing the relevant branch of the transaction tree down to the user's own onchain output. It is not free: the user must retain the keys and path data, watch the timing and pay base-layer fees. [1]

VTXOs expire. Before expiry they must be renewed in a new round or withdrawn unilaterally; otherwise the operator gains the ability to sweep the underlying bitcoin through a predetermined branch and the user loses a unilaterally enforceable claim. [5]

The operator coordinates rounds, cosigns ordinary transfers and provides availability. Before expiry it should not be able to redirect a properly managed VTXO, but it can censor new operations, stop responding and force the user into a slower or more expensive exit. [5]

Ark can be built without a consensus change by using presigned pseudo-covenants and key deletion. Covenants such as CTV can reduce coordination and data overhead and enable more efficient trees for more users. [4]

Ark is not a Lightning channel: it uses rounds, shared trees and expiring VTXOs. The two approaches can work together, for example for entering Lightning or for channel factories. [1]

Practical implementations and their models are still evolving. Before use, verify the specific exit path, the VTXO data backup, expiry period, renewal, fees, network status and the exact assumptions about the operator. [1]