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ASPs are always-on servers that provide liquidity to the network, similar to how Lightning service providers work.


Any user that onboards the Ark or receives a payment inside an Ark.


Ark has a UTXO set that lives off the chain. These utxos are referred to as virtual UTXOs or VTXOs in short.


Boarding the Ark​

When a User sends funds to the Ark and gets VTXOs in return.

Unilateral exit​

When a user decides to withdraw his funds from the Ark to mainchain, without asking the ASP for permission.


Periodic transaction crafted by the ASP that hits mainchain and creates new VTXOs.



  • In an optimistic scenario, transactions marked with a * should never hit onchain.
  • All time periods used on timelocks (5s, 24h, 4w, 1y) are arbitrary: any ASP can use different values.


  • A: Alice pubkey
  • ASP: ASP pubkey
  • cov**(script): covenant that forces the spending transaction to have a mandatory first output with the script
  • and(A,B): both conditions needed to unlock
  • or(A,B): only one condition needed to unlock
  • and(A, or(S, after(24h): (Alice and Server) or (Alice after 24h)


  • A Virtual UTXO: it looks like an UTXO but it should never hit onchain, thus a Virtual UTXO
Boarding or Round transaction(Alice + ASP) or (Alice after 24h)

Boarding transaction​

  • Alice funds an output that can be accepted as a VTXO in a future round
  • A covenant forces the creation of an output with the same script as VTXO. No need for interactivity after funding it, anyone can spend.
  • ASP can unlock after a timeout ie. 4 weeks
  • Alice is required to be online to maintain access to funds: after the timeout, ASP becomes the only owner funds
Alice’s UTXO(ASP after 4w) or cov((Alice + ASP) or (Alice after 24h))

Forfeit transaction​

  • Insurance for the ASP, in case Alice tries to double spend her VTXO after spending it inside Ark
  • Before the ASP funds Bob’s VTXO in the next Round transaction, he must receive this transaction signed by Alice
  • Uses a connector from the next Round transaction to achieve atomicity
VTXO spending Alice + ASPASP
Connector from next Round transaction

Round transaction​

  • Funded by the ASP, creates VTXOs
  • Has at least two outputs:
    • A shared output with a VTXOs tree
    • A connectors output with a connectors tree
  • A new transaction is broadcasted every 5 seconds
ASP UTXOShared output
Connectors output

Shared output​

  • Represents a tree of transactions
  • In an optimistic scenario, this tree is never revealed
  • Each leaf on this tree represents a VTXO
  • Tree can have a radix higher than 2 (ex: radix 4)