Minimal Viable Plasma

[DZack]

Worse yet, isn’t there an obvious attack vector there wherein someone can enter with 0.001 eth, try to exit with 1 eth, and thereby constantly grief

A random user couldn’t do this, because the attempted 1 eth exit would require a merkle proof of the corresponding utxo (which doesn’t actually exist). This griefing vector only works if you can get an invalid utxo included in the plasma block (and thus the Merkle root), so the operator would have to be in on it.

[MihailoBjelic]

There is a difference that makes the mass exit threat in plasma MVP worse (I think this may be clearer in retrospect): for a centralized channel hub, the malicious/ hacked hub operator can force each user to exit onto the main chain, but it requires the hub to broadcast a transaction for each user (or more specifically, each channel). Whereas in plasma MVP, just one transaction from the operator is enough to force all users to exit.

Good observation, but I think in reality it could be pretty much the same thing. If a centralized hub tries something malicious in one (or a few) channel(s) and the counterparty user is forced to close the channel, the news will spread (by observing the main chain or through off-chain channels) and people will start leaving the hub, i.e. closing their channels. Does that make sense?

[DZack]

Indeed, the panic-factor may ultimately lead to channel exits anyway. (Although I, for one, would defiantly keep my channel open with the hub until forced to settle; cryptoeconomics, dammit!!!)

[pepesza]

An other problem could be a double spend on the parent chain. If Alice deposits on the plasma chain and immediately sends the coins to Bob on the Plasma chain, but also double spends her deposit ether on the parent chain by sending it to Carol.
If the parent chain reorganizes, the finalized chain could end up with both the transactions to Bob and Carol, but without the deposit.

Operator, while submitting a block, adds a hash describing his knowledge about deposits in the contract. Contract computes his own summary of state of deposits and compares it with submitted one. If not equal, transaction submitting the block is rejected. This solution enables operator to spend deposits as soon as they appear, without waiting.

Gas cost of this solution is 200 + 42 (SLOAD, SHA3 on two words) gas per deposit made to the chain, paid by operator.