Checkpoint transactions protect Arkade operators against double-spend attempts from users.
Once the server detects a double-spend attempt, it reacts in one of two ways, depending on the transaction state:

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Warding Off Griefing Attacks 

Checkpoint transactions are a core component of the Arkade protocol to ward off potential griefing attacks. They allow the operator to broadcast said transaction when detecting fraudulent user behaviour. A malicious user could chain multiple offchain payments to itself, ultimately batch swap the resulting VTXO or exit the Ark entirely, but then broadcast a unilateral exit path from a prior VTXO of the transaction chain. This attack would force the operator to publish all prior Arkade transactions up to the batch output that anchors them or the unilateral exit, potentially resulting in significant costs.
Checkpoint transactions allow the server to only broadcast one transaction to defend itself in case of a griefing attack.
*Checkpoint transactions mitigate this attack. They are special transactions that serve as intermediate states in offchain transaction processing.
For a deep dive into checkpoint txs, visit Annex A of our Ark Litepaper.

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Checkpoint Transaction Structure 

A checkpoint transaction is essentially a self-send of the user, but removing the exit script path from the VTXO script leaf, transferring it to the server. It uses a Taproot script with two paths:
  • A+S (collaborative)
  • S+CSV (server after timeout)
checkpoint-tx.png Checkpoints are signed by both the user and the operator before the related VTXO becomes eligible for a batch swap or a unilateral exit. 
In the offchain environment no forfeit transaction is signed, but only once a VTXO is batch swapped. Until then, transactions operate under a preconfirmation trust model.

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Potential Attack Scenarios 

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User broadcasts only part of tx chain

One type of attack against the operator could be that a user broadcasts only a part of an offchain transaction chain, (orange) forcing the operator to broadcast all transactions of that chain (violet), resulting in high onchain cost: tx-chain-attack-nocp.png With checkpoint transactions, it suffices for the operator to broadcast one single onchain transaction, reducing the defense cost significantly: tx-chain-attack-cp.png

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User broadcasts only 1 input of a 2-input tx

Another way to attack the server would be if a user only broadcasts one input of a two-input offchain transaction, forcing the operator to broadcast the second branch of that DAG, resulting in high onchain cost: 2-input-attack-nocp.png With checkpoint transactions, again, it suffices for the operator to broadcast one single onchain transaction, reducing the defense cost significantly: 2-input-attack-cp.png Overall, checkpoint transactions allow the operator to claim a VTXO, unless the VTXO holder posts the Arkade transaction following this exact checkpoint transaction.
Checkpoint transactions provide the operator with a low‑cost mechanism to mitigate attacks, such as partial or selective broadcasting of offchain transaction chains.