Why Smart Contracts are NOT feasible on Plasma

[sg]

As per this PlasmaMVP exit discussion, out-of-thin-air-exit causes mass exit.

Exiting invalid tx in omg mvp?

The contract doesn’t prevent validators from including this type of transaction. However, once a transaction like that is included, everyone on the Plasma MVP chain needs to exit within a specified period of time (~1 week). Basically it doesn’t matter if we do the check or not. The contract can check that the transaction is valid (inputs >= outputs), but the validators can just spend the invalid UTXO and create a transaction that appears to be valid.

Let’s say in the EVM Plasma context, contract-deposited-or-locked UTXO might be vulnerable to this attack coz the withdrawal Tx would update the timestamp of this UTXO.

Now the options are only two

1. Contract exits the locked UTXO (only if the contract has that spec)
2. Withdraw deposit and exit (This UTXO’s exit will be inferior priority than out-of-thin-air-exit Tx so that Rootchain deposited fund would be damaged)

This is also one of another difficulty of EVM Plasma.
“UTXO history’s full scanning before exit” could be mitigation though, this construction must answer some questions which are “Who will do that?”, “Is it fairly decentralized?”, and “Is the gaming probability enough mitigated?”