Slasher

• A protocol suggested (1-2014) in Ethereum suggested that would allow users to "punish" a cheater who forges on top of more than one blockchain branch.
• From Wikipedia:

"This proposal assumes that one must double-sign to create a fork and that one can be punished for creating a fork while not having stake. However, Slasher was not adopted; Ethereum developers concluded proof of stake is "non-trivial" (10-2014), opting instead to adopt a proof-of-work algorithm named Ethash."

• From Vitalik's post:

"Essentially, by explicitly punishing double-signing, Slasher in a lot of ways, although not all, makes proof of stake act like a sort of simulated proof of work. An important incidental benefit of Slasher is the non-revert property. In proof of work, sometimes after one node mines one block some other node will immediately mine two blocks, and so some nodes will need to revert back one block upon seeing the longer chain. Here, every block requires two thirds of the signers to ratify it, and a signer cannot ratify two blocks at the same height without losing their gains in both chains, so assuming no malfeasance the blockchain will never revert. From the point of view of a decentralized application developer, this is a very desirable property as it means that “time” only moves in one direction, just like in a server-based environment.

However, Slasher is still vulnerable to one particular class of attack: long-range attacks. Instead of trying to start a fork from ten blocks behind the current head, suppose that an attacker tries to start a fork starting from ten thousand blocks behind, or even the genesis block - all that matters is that the depth of the fork must be greater than the duration of the reward lockup. At that point, because users’ funds are unlocked and they can move them to a new address to escape punishment, users have no disincentive against signing on both chains. In fact, we may even expect to see a black market of people selling their old private keys, culminating with an attacker single-handedly acquiring access to the keys that controlled over 50% of the currency supply at some point in history.

One approach to solving the long-range double-signing problem is transactions-as-proof-of-stake, an alternative PoS solution that does not have an incentive to double-sign because it’s the transactions that vote, and there is no reward for sending a transaction (in fact there’s a cost, and the reward is outside the network); however, this does nothing to stop the black key market problem. To properly deal with that issue, we will need to relax a hidden assumption."