create a list of attack scenarios on plasma chain

[johannbarbie]

objective is to rationalize the need of 2-phase prepare commit transactions on the child chain (or other mitigations, like the expiration time proposed by @davidknott in last plasma call).

assumptions: child chain works without any signed confirmations(as in MVP spec):

Deposit not credited Attack

• alice deposits funds into plasma contract
• Validator creates UTXO in next plasma block
• Validator censors all future transactions by Alice to spend the UTXO
• Alice initiates exit from plasma => all good?

Frustration Attack

• Alice sends tx with output to Bob
• Validator withholds tx
• Alice gets frustrated and initiated exit with value of tx
• Validator includes tx in next block and challenges Alice's exit

Block Withholding Attack

• Validator constructs a block with invalid txns (0 input, or wrong sigs)
• Validator submits block hash
• Aalidator starts exit of funds => no-one can challenge validator, because block data missing

Double Spend by Exit Attack

• Alice sends tx1 to bob
• Bob spends from tx1 with tx2
• Alice initiates an exit with value of tx1
• Bob challenges Alice's exit with tx2

Valueless UTXO Attack

• Alice deposits x into plasma
• Validator includes UTXO with x in next block
• Alice initiates an exit of x
• Alice waits 14 days and receives x back
• Alice starts using UTXO on childchain

[johannbarbie]

https://github.com/omisego/research/blob/master/plasma/plasma-mvp/specifications/no-confirmations.md