Automatic Safe Block number

[kevsul]

The Quasar should maintain a Safe Block number, and reject exits involving outputs that are included in later blocks. This is to prevent the Quasar losing funds in the event of a mass exit scenario - any output that gets included into a block later than the block that caused the byzantine event is at risk of not being able to exit.

The original Quasar construction proposed that this Safe Block number be updated periodically by the Quasar owner. However, this manual operation would cost gas.

An alternative is to consider the Safe Block number as being the last submitted plasma chain block number, minus a number of blocks as a buffer.

The buffer protects the Quasar from accepting exits from outputs that are included in or after a block that caused a byzantine event.

Note that this strategy requires that the Quasar owner trusts the plasma operator. A malicious operator could submit an invalid block stealing all the money in the vault and also submit buffer number of further blocks at the same time. Thus the Quasar thinks that the Safe Block is actually after the invalid block, which would allow a user to fast exit funds that the Quasar would not be able to subsequently standard exit.