Casper FFG

Basics

• Casper FFG (the Friendly Finality Gadget)
• PoS mining consensus algorithm / finality gadget.
• “Casper is a security-deposit based economic consensus protocol. This means that nodes, so called “bonded validators”, have to place a security deposit (an action we call “bonding”) in order to serve the consensus by producing blocks. The protocol’s direct control of these security deposits is the primary way in which Casper affects the incentives of validators. Specifically, if a validator produces anything that Casper considers “invalid”, their deposit are forfeited along with the privilege of participating in the consensus process. The use of security deposits addresses the “nothing at stake” problem; that behaving badly is not expensive. There is something at stake, and bonded validators who misbehave in an objectively verifiable manner will lose it.”

51% Attack Defense

• Vitalik responding to 51% attacks in Casper FFG (10-2019):

"In the case of a finality-reversion attack, we already know that the attacker can get slashed and so the attack can be very costly (think: millions of ETH). Attacks that try to muddy the waters of which block came first, so as to make extra-protocol coordination on which chain to continue with after the attack, are even more costly. Censorship attacks are more challenging in that, absent social responses, they succeed and cause harm “by default”, though clients can implement features that reject chains that seem to censor too much, and such attacks can be defeated by extra-protocol consensus on a “minority soft fork” that cleverly uses the NO_SURROUND slashing condition to focus on a chain that the attacker cannot legally join.

In either case, a 51% attack would be a moment of joy for the community, as at the cost of a day or perhaps a week’s disruption, an amount of the attacker’s ETH would get burned that counteracts years of protocol-level issuance, increasing the value of ETH and hence making future attacks more expensive. Furthermore, this can be done via soft forks or fork choice rule features, with no need for hard forks that take the attacker’s coins out manually."

Competion

GRANDPA and Casper FFG

• From the Polkadot wiki (7-5-2021):

The two main differences between GRANDPA and Casper FFG are:

1. "in GRANDPA, different voters can cast votes simultaneously for blocks at different heights
2. GRANDPA only depends on finalized blocks to affect the fork-choice rule of the underlying block production mechanism"