Bridge security example

[JoshOrndorff]

Add a concrete example to the bridge lesson.

Imagine two sovereigns S, the source, and D, the destination. They are both Proof of Stake Deterministic finality (with something like Grandpa or Tendermint).

Imagine a high-value payment (maybe like 10% of the source chain's liquid supply) is sent across the bridge to the destination. On the destination chain, the funds never appear. WTF happened?

On explanation. The value of the payment exceeded the economic security of the source chain. An attacker who wanted to freeze those funds bribed the validators of the source chain. The bribed validators colluded to violate the protocol and finalized two conflicting chains: one with the payment, another without. Of course they were slashed for that on the source chain, but that is just the cost of doing business. The bribe value exceeded the cost of the slash.