What property makes Proof of Stake easier to apply in permissioned blockchains?

Question

There are a number of efforts driving towards the transition from the current proof-of-work towards proof-of-stake (more info here). Meanwhile proof-of-stake is already in use in some permissioned blockchains (including Ethereum adaptations such as Eris).

What makes this transition feasible now for permissioned blockchains, but still work-in-progress for public blockchains?

Answer 1

In a permissioned/private context, the validators have an additional layer of trust. Namely, that they have access to the blockchain in the first place. Bank A is unlikely to 51% attack Bank B, when everyone who has access (read: Bank B's lawyers) can see what they did. Ergo, there's little reason to additionally incentivize good behavior/disincentivize bad behavior. Similarly, the economics of new blocks (inflation? no inflation but mandatory fees?) are generally irrelevant, because validators have their own reasons.

In a public blockchain, there's no such trust. If validator 0x45cdf112... attacks the network, there's a good chance that no legal trail to its masters can be found. The best that the protocol can do is threaten and bribe validators into behaving, which immediately opens a whole field of game theoretic questions. Additionally, since validators may be validators solely for profit, economic questions become important.

Answer 2

There are a few PoS implementation already:

NXT: nxt.org Peercoin: https://peercoin.net A few more are listed at: https://en.wikipedia.org/wiki/Proof-of-stake

A PoS of stake implementation has not been developed that has as strong byzantine fault tolerance guarantees as PoW. Even for Caspar Vitalik has used the term "weak subjectivity" to describe it's security guarantees in contrast to Bitcoin's "objectivity".

All publicly known PoS implementations have some caveat that weakens them or don't properly defend against some attack, maybe they

• Are on a permissioned blockchain and not truly decentralized
• Require a trusted set of federation nodes to prevent failure
• Allow validators to game the system and mine blocks that will let them be validators perpetually
• Don't defend against long range attacks (rewriting the blockchain over a long period of time)
• some other attack

Ethereum has a higher market cap than any of the other currencies implementing PoS so Vlad/Vitalik want to make sure Caspar is correct.