Also covering BitVM and Zero-Knowledge Contingent Payments (ZKCP)

Accountable Computing Contracts (ACC) are payments that the receiving party can spend if they verifiably run a specified function on a specified set of inputs. If the receiving party doesn’t run the function or doesn’t run it correctly, the paying party can reclaim the payment after a period of time.

For example, Alice claims she has a solution to a puzzle. Bob wants to buy the solution to the puzzle, but Alice is unwilling to give him a solution until she’s guaranteed to receive a payment. Bob is similarly unwilling to pay Alice until he’s sure the solution is correct. They decide to write a program that will return true if it verifies the solution is correct. Then Bob pays money to a transaction output that will allow Alice to claim the money if she provides a solution that was verified by the program. If the solution is incorrect, either Alice’s spend will be invalid or she will need to pay a penalty that is equal to or greater than the amount of the payment.

There have been several proposals and implementations of this idea for Bitcoin:

  • Zero-Knowledge Contingent Payments (ZKCPs) allow Alice to prove that she ran the program on her puzzle solution and that the solution has a particular hash digest. Bob can then create an HTLC that pays Alice if she discloses the preimage for that hash digest. If Alice doesn’t disclose it, Bob can reclaim his funds after the HTLC timelock expires.

  • BitVM allows Bob to deposit money into a contract that compactly references the program they’re using for verification. Alice can then provide the solution. If Bob is satisfied, he releases the money to Alice. If he fails to take action, Alice can claim the money after a period of time. If he isn’t satisfied, he can challenge Alice to prove that their program returns true when run on her solution, breaking the challenge into multiple steps that each require an onchain transaction. BitVM is available on Bitcoin today.

  • MATT works similar to BitVM, although it requires a soft fork. As a tradeoff, it can be much more efficient than BitVM due to changing the consensus rules to make this type of proving efficient.

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