Optimistic payout in BitVM implementations like Citrea?

When analyzing the operational structure of UTXO-based transactions, particularly in the context of deposits and withdrawals on a blockchain bridge, it is crucial to understand that the UTXO assigned for a withdrawal is not necessarily the same as the UTXO created during the user's initial deposit. Instead, the process follows a first-in-first-out (FIFO) system where each subsequent withdrawal is matched with a corresponding deposit in the order they were made, irrespective of the individual making the withdrawal. This ensures that a withdrawal today could be linked to a deposit from potentially a year ago.

To maintain consistent trust assumptions across all transactions, every deposit must be presigned by a predetermined set of signers. This approach negates the reliability on signers who are only active at the time of a user's deposit, which might seem intuitive but fails to enhance security or trust. The reason behind this fixed signer set is to ensure uniformity in the authentication process, so that even if a withdrawal taps into an older deposit, the security guarantee remains intact due to the endorsement by the original set of signers.

There was consideration for a more flexible design that would allow volunteers to partake in the signing process. However, this idea was discarded as it introduces complexity without actually solidifying trust guarantees. Specifically, such a system would be vulnerable to attacks where a MuSig2 signing ceremony could be easily disrupted, and there would be no feasible method for the rollup to verify the inclusion of all volunteer signers. This vulnerability underscores why sticking with a fixed set of trusted signers is pivotal for maintaining security integrity within the transaction framework of the bridge.