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Ark Terminology

This section provides a glossary of terms and concepts used throughout the Ark protocol documentation. Understanding these terms is essential for grasping the intricacies of Ark's architecture and functionality.

Ark Operator​

The entity responsible for coordinating an instance of the Ark protocol, including batching, signing, and liquidity provision.

Commitment Transaction​

A Bitcoin transaction constructed and broadcast by an Ark Operator. It contains:

  • One or more Batch Outputs, which define sets of virtual claims (VTXOs)
  • One or more Connector Outputs, used to enforce atomic swaps or exits
  • Optional standard outputs (e.g., Collaborative Exits)

Batch Output​

A Taproot output in a Commitment Transaction that encapsulates a tree of virtual transactions. Each leaf represents a VTXO. The Batch Output is secured by a MuSig2 key and includes a fallback script path controlled by the Operator after expiry.

Batch Expiry​

The predefined timeout after which the Operator can reclaim the funds in a Batch Output. Ensures liquidity rotation and encourages timely batch swaps.

Collateral​

BTC locked in a Batch Output to back the virtual transaction tree. It ensures users can unroll their VTXOs unilaterally. After batch expiry, the Ark Operator can reclaim this collateral.

Connector​

A special output in a Commitment Transaction used for atomic swaps. It anchors Forfeit Transactions via Virtual Connectors, ensuring swaps only succeed if the Commitment Transaction is confirmed.

Virtual Connector​

An offchain representation of a Connector Output used in a Forfeit Transaction. It cryptographically links a VTXO forfeiture to a specific Commitment Transaction, enforcing atomicity and preventing partial execution.

VTXO (Virtual Transaction Output)​

A virtual output representing offchain BTC value within a Batch Output. VTXOs can be spent via:

  • Collaborative path: with the Operator, for fast offchain usage
  • Exit path: unilaterally via a timelocked onchain unroll

VTXO states:

  • Preconfirmed: Created by an Ark Transaction but not yet in a batch
  • Unconfirmed: Leaf of a batch not yet confirmed onchain
  • Confirmed: Part of a confirmed Commitment Transaction
  • Expired: The batch has expired; Operator regains control
  • Spent: Consumed in another transaction
  • Replaced: Invalidated due to batch conflict or double-spend

Preconfirmed​

A state in which a VTXO has been created via an Ark Transaction but not yet included in a confirmed Commitment Transaction. It is usable offchain but depends on the Operator’s honest behavior and is subject to equivocation risk.

Unrolling​

The act of broadcasting a chain of presigned virtual transactions to convert a VTXO into an onchain UTXO. This enables unilateral exit without Operator cooperation.

Virtual Transaction​

A fully valid, signed Bitcoin transaction that exists offchain under normal conditions. Used to model spending relationships in the virtual execution layer. Becomes effective onchain only during an exit.

Ark Transaction​

A virtual transaction that produces new VTXOs.

Batch Swap​

A mechanism to replace a VTXO with a new one in a fresh batch. It uses a Commitment Transaction and a virtual Connector to ensure atomicity and maintain custody guarantees. It is a form of atomic swap.

Atomic Swap​

A trust-minimized asset exchange (typically between a VTXO and another VTXO or UTXO) that either completes entirely or fails completely. Enforced by Connectors and Forfeit Transactions.

Collaborative Exit​

An onchain UTXO output provided in a Commitment Transaction in exchange for forfeiting a VTXO. This method allows coordinated withdrawal without unilateral unrolling.

Forfeit Transaction​

A special Ark Transaction that spends a user’s VTXO and sends its value to the Ark Operator. Used during batch swaps to conditionally exchange a VTXO for a new one or for a Collaborative Exit.

Boarding Address​

A Taproot address with an unspendable internal key and two script paths:

  1. A unilateral path (user-only, timelocked)
  2. A cooperative path (user + Operator)

Used for onboarding BTC into Ark. Once the boarding transaction is confirmed, the user can register the UTXO with the Operator to enter the system.

Native Exit​

The process of converting a VTXO into an onchain UTXO, either by:

  1. Unrolling: Broadcasting the presigned exit path
  2. Collaborative Exit: Receiving a standard UTXO in a Commitment Transaction in exchange for forfeiting the VTXO