BA-11 · LESSON 07

Ark and shared UTXOs

KEY DEFINITIONS

Terms used in this lesson

UTXO: Unspent transaction output: one discrete chunk of bitcoin that can be used as a transaction input.
VTXO: Virtual transaction output: an off-chain claim connected to a version-specific shared Bitcoin transaction structure and unilateral-exit path.
Transaction ID: The identifier produced by hashing a Bitcoin transaction’s non-witness serialization.

Emerging protocols explore coordinated UTXO ownership and efficient exits with new availability assumptions.

55 min Difficulty 3/5 Not started

VISUAL MODEL

See the whole mechanism before going step by step

Use the diagrams to orient yourself. Then the guided explanation below teaches every part in depth.

4 explanatory views

CONCEPT-SPECIFIC MAP · state machineOne valid action transforms a known stateSelect any state to inspect its exact role.

RELATIONSHIP MAPOrdered mechanism flow

01
BEFOREKnown state
Identify the facts and objects that exist before the action.
02
ACTIONArk and shared UTXOs
The Ark and shared UTXOs lesson isolates the event that proposes a change.
03
RULEDeterministic check
The enforcing rule accepts or rejects the proposed transition.
04
AFTERSuccessor state
Accepted work creates a new state without silently changing the prior facts.
05
FAILUREState unchanged
A failed condition must not produce a partially accepted successor.

Rule shown: A valid transition names the prior state, action, enforcing rule, and successor state.

STAGE 1 OF 5BEFORE: Known state

Identify the facts and objects that exist before the action.

Governing relationship: A valid transition names the prior state, action, enforcing rule, and successor state. Complete path: 1. BEFORE, Known state. Identify the facts and objects that exist before the action. 2. ACTION, Ark and shared UTXOs. The Ark and shared UTXOs lesson isolates the event that proposes a change. 3. RULE, Deterministic check. The enforcing rule accepts or rejects the proposed transition. 4. AFTER, Successor state. Accepted work creates a new state without silently changing the prior facts. 5. FAILURE, State unchanged. A failed condition must not produce a partially accepted successor.

1. State transitionWhat exists, what changes, and what results

BEFORE

User A has a 150,000-sat VTXO in a separate teaching graph with three exit transactions, relative delays [0, 2, 1], a 1,500-sat fee per step, current height 200, and expiry 2…

MECHANISM

Users and an Ark service provider coordinate funding or refresh transactions that commit shared ownership structures on Bitcoin.

AFTER

Shared UTXOs improve batching, but version-pinned exit data, delays, expiry, and fee access remain central assumptions.

Text equivalent: Starting from User A has a 150,000-sat VTXO in a separate teaching graph with three exit transactions, relative delays [0, 2, 1], a 1,500-sat fee per step, current height 200, and expiry 210. The mechanism begins when Users and an Ark service provider coordinate funding or refresh transactions that commit shared ownership structures on Bitcoin. The resulting lesson is Shared UTXOs improve batching, but version-pinned exit data, delays, expiry, and fee access remain central assumptions.

2. Mechanism pathSelect a stage to inspect its job

STAGE 1 OF 5

Enter a shared round

Users and an Ark service provider coordinate funding or refresh transactions that commit shared ownership structures on Bitcoin.

Complete path: 1. Enter a shared round: Users and an Ark service provider coordinate funding or refresh transactions that commit shared ownership structures on Bitcoin. 2. Receive virtual outputs: A VTXO is an off-chain enforceable claim connected to a pre-signed transaction tree rather than an immediately separate confirmed UTXO. 3. Transfer or refresh: Users can exchange claims or join later rounds, with service availability, expiry, and liquidity rules determined by the version. 4. Exit cooperatively: A responsive service can batch settlement or refresh many users efficiently. 5. Exercise unilateral paths: If service coordination fails, a user broadcasts a sequence of pre-signed transactions and waits through defined delays, paying on-chain fees and relying on data availability.

3. Rule and failure boundaryWhat accepts the result and where it can stop

ACCEPTED PATH

The pinned graph requires every retained dependent transaction, valid node signature, exact parent txid, relative delay, expiry headroom, and fee-funded positive successor value.

CHANGED CONDITION

Provider outage plus lost data, a corrupted node signature, exhausted fees, or a final height after expiry makes the unilateral path unavailable; zero-delay dependencies may still share one block.

Important distinction: Shared-UTXO protocols give every user an ordinary confirmed UTXO at all times. Users often hold enforceable virtual claims whose on-chain realization requires coordinated refresh or a multi-step unilateral exit.

Text equivalent: The accepted path is enforced by The pinned graph requires every retained dependent transaction, valid node signature, exact parent txid, relative delay, expiry headroom, and fee-funded positive successor value. If the required condition changes, Provider outage plus lost data, a corrupted node signature, exhausted fees, or a final height after expiry makes the unilateral path unavailable; zero-delay dependencies may still share one block.

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Learn the idea

Ark is a developing family of Bitcoin protocol designs, not one frozen transaction format. A virtual transaction output, or VTXO, is an off-chain claim connected to pre-signed Bitcoin transactions. A service provider coordinates rounds and liquidity, while a user's unilateral exit depends on the exact version's retained transaction data, expiry rules, dependency order, and fees. This lesson's lab is explicitly pinned to Ark shared-output concept model v2, captured 2026-08-05. Its three-step example uses relative delays [0, 1, 1]. Zero-delay dependent transactions may share a block; dependency order alone does not require one block per step. The model teaches common boundaries without claiming to reproduce one canonical deployed Ark release.

GUIDED EXPLANATION1/5 ideas inspected

Enter a shared round

Users and an Ark service provider coordinate funding or refresh transactions that commit shared ownership structures on Bitcoin.

The worked example follows the explanation

Inspect every idea above to open it.

Important distinction“Shared-UTXO protocols give every user an ordinary confirmed UTXO at all times.”

Users often hold enforceable virtual claims whose on-chain realization requires coordinated refresh or a multi-step unilateral exit.

FINISH LEARNING FIRST

The questions unlock after every required learning activity

Inspect every guided idea, open the worked example, rebuild its mechanism, and complete the deterministic lesson tool. Your progress is saved automatically.