Payment channels introduce a new and undesirable assumption that a party must remain on-line and synchronized with the blockchain at all times to defend against execution fork attacks. An execution fork can revert a payment channel’s history, potentially causing financial damage to a party that is innocent except for having crashed. To provide security even to parties that may go off-line for an extended period of time, watchtowers are under development, protocols that are enables such parties to delegate to a third party, called the custodian, to cancel execution forks on their behalf.