Cryptocurrency Blockchain and Consensus Mechanisms 3 — Questions and Answers
Question 1: What is a UTXO in the context of Bitcoin's transaction model?
- Unspent Transaction Output — a discrete chunk of bitcoin available to spend (Correct answer)
- Universal Token Exchange Object — a standard for cross-chain swaps
- User Transaction eXecution Order — the sequence in which the mempool processes txns
- Unsigned Transaction eXtension — a partially signed bitcoin transaction
Correct answer: Unspent Transaction Output — a discrete chunk of bitcoin available to spend
UTXOs are discrete outputs from previous transactions that can be used as inputs in new transactions, forming Bitcoin's accounting model.
Question 2: How does Delegated Proof of Stake (DPoS) differ from standard Proof of Stake?
- DPoS uses computational puzzles instead of coin weighting
- Token holders vote for a fixed set of delegates who produce blocks on their behalf (Correct answer)
- DPoS eliminates block rewards entirely and relies on transaction fees
- DPoS selects validators randomly without any voting mechanism
Correct answer: Token holders vote for a fixed set of delegates who produce blocks on their behalf
In DPoS, coin holders elect a small group of trusted delegates who are responsible for validating transactions and producing blocks.
Question 3: What is 'block finality' in the context of blockchain consensus?
- The maximum number of transactions a block can include
- The point at which a block is considered irreversible and cannot be reorganized (Correct answer)
- The time required to propagate a block to all network nodes
- The process of closing a block after the timestamp expires
Correct answer: The point at which a block is considered irreversible and cannot be reorganized
Finality means a confirmed block is permanently settled and cannot be undone, though its definition varies — probabilistic in PoW, deterministic in some PoS systems.
Question 4: In a blockchain, what is a 'chain reorganization' (reorg)?
- Reordering transactions within a mempool to maximize miner fees
- When a longer competing chain replaces the current canonical chain (Correct answer)
- Reshuffling validator slots in a proof-of-stake epoch
- Compressing old block data to free up node storage
Correct answer: When a longer competing chain replaces the current canonical chain
A reorg happens when nodes discover a competing chain with more accumulated work or weight, causing them to switch chains and orphan previously accepted blocks.
Question 5: What is the primary security assumption behind Nakamoto consensus (Bitcoin's PoW)?
- That at least two-thirds of validators are honest at all times
- That honest miners collectively control more than 50% of the total hash rate (Correct answer)
- That cryptographic hash functions will never be broken
- That all nodes are synchronized within one block time
Correct answer: That honest miners collectively control more than 50% of the total hash rate
Nakamoto consensus is secure as long as honest participants control the majority of hash power, making an attack prohibitively expensive.
Question 6: What distinguishes a 'permissioned' blockchain from a 'permissionless' one?
- Permissioned blockchains use PoW; permissionless blockchains use PoS
- Permissioned blockchains restrict who can join and validate; permissionless ones are open to anyone (Correct answer)
- Permissioned blockchains store data off-chain; permissionless store on-chain
- Permissioned blockchains have no native token; permissionless always have one
Correct answer: Permissioned blockchains restrict who can join and validate; permissionless ones are open to anyone
Permissioned blockchains require identity verification and approval to participate, making them suitable for enterprise environments, while permissionless blockchains like Bitcoin are open.
Question 7: What is the 'longest chain rule' in Bitcoin?
- The rule that blocks with more transactions are always preferred over shorter ones
- Nodes always adopt the chain with the greatest cumulative proof-of-work as the valid chain (Correct answer)
- The protocol rule that limits the maximum chain depth to prevent data bloat
- Miners must extend the chain continuously or lose their block reward
Correct answer: Nodes always adopt the chain with the greatest cumulative proof-of-work as the valid chain
Bitcoin nodes follow the chain representing the most accumulated computational work, which is typically the longest chain by block count.
Cryptocurrency Blockchain and Consensus Mechanisms 3
What is a UTXO in the context of Bitcoin's transaction model?