Note that a distributed ledger can only function normally if it can achieve a final state (i.e., reaches consensus) within a specific time span (for example, in Bitcoin network it is controlled at 10-minute intervals). However, if nodes cease to work, a consensus will never be reached and the blockchain will cease to provide services.

Based on your understanding on public blockchain such as Bitcoin/Ethereum and private blockchain such as Corda/Hyperledger, examine and analyse the cost of attack for an attacker.