Consensus Algorithm Security Flashcards

What is a 'finality reversion' attack in the context of PoS blockchains, and what condition makes it theoretically possible?

Answer: An attacker reverses a finalized block by accumulating more than 1/3 of stake to prevent supermajority agreement on the next checkpoint

Reversing finality in Casper-style PoS requires the attacker to equivocate with enough stake to break the 2/3 supermajority, which is detectable and subject to severe slashing penalties.

Which consensus security property ensures that if two honest nodes both finalize a block, those blocks must be the same block?

Answer: Safety (agreement)

Safety (also called agreement or consistency) guarantees that no two correct nodes ever commit different values at the same block height.

In the Ethereum Merge's PoS design, what happens to a validator that goes offline for an extended period during non-emergency conditions?

Answer: The validator incurs inactivity leak penalties that gradually reduce its effective balance

Inactivity leaks drain inactive validators' balances quadratically over time, incentivizing return to participation and enabling the network to recover finality if too many validators go offline.

An eclipse attack targeting a consensus node aims primarily to accomplish what?

Answer: Isolate the node from honest peers so the attacker controls all information it receives about the blockchain state

In an eclipse attack, the attacker monopolizes all of the victim node's peer connections, feeding it false chain data and enabling double-spend or consensus manipulation against that node.

Hybrid consensus models that combine PoW and PoS (such as early Decred) are designed to raise the cost of which specific attack?

Answer: 51% attacks, by requiring an adversary to dominate both hash rate and stake simultaneously

Hybrid PoW+PoS forces an attacker to acquire majority hash rate AND majority stake simultaneously, making a 51% attack significantly more expensive.

What distinguishes a 'bribery attack' from a traditional 51% attack on a PoS network?

Answer: A bribery attack temporarily purchases votes from existing validators rather than acquiring majority stake outright

In a bribery attack, the adversary incentivizes existing validators to vote for a specific fork by paying them more than their honest staking reward, bypassing the need to own a majority of stake.

The 'CAP theorem' as applied to blockchain consensus states that during a network partition, a system must choose between which two properties?

Answer: Consistency (safety) and Availability (liveness)

During a network partition, a consensus system must sacrifice either consistency (all nodes see the same data) or availability (the system keeps processing requests), as defined by the CAP theorem.