Ark (Protocol)

Ark is an experimental Bitcoin layer-2 protocol designed for low-cost off-chain payments without the channel setup that Lightning requires. First proposed by developer Burak Keceli in 2023, it lets many users share a single on-chain output and transact off-chain through a coordinator, settling to the base chain only when needed. Several implementations have reached public beta, but the protocol remains young and should be treated as experimental.

Virtual UTXOs and the service provider

Ark's core primitive is the virtual transaction output (VTXO): an off-chain claim on Bitcoin value backed by a shared on-chain output controlled by an Ark Service Provider (ASP). The ASP acts as a coordinator and liquidity provider, batching user payments into rounds (sometimes called pools) at short intervals. A VTXO is a leaf in a presigned transaction tree whose root is the shared UTXO, so it is not its own output on the blockchain. VTXOs are short-lived, typically expiring after a few weeks, after which value is refreshed into new VTXOs.

Custody and unilateral exit

Each VTXO is backed by presigned transactions that let a user reclaim funds on-chain unilaterally if the ASP goes offline or misbehaves. The ASP coordinates rounds but is designed not to take custody of user funds. The trade-offs being studied include liquidity costs, VTXO expiry mechanics, and the reliance on a coordinator, all of which differ from Lightning's payment-channel model.

Ark is one of several scaling approaches building above Bitcoin's base layer. To compare it with channel-based and federated designs, see our entries on Statechain and Taproot Assets.