Question 1

How does proof-of-work block mining work?

Accepted Answer

Proof-of-work requires a miner to find a nonce value such that SHA-256(previousHash + data + nonce) produces a hash starting with a required number of leading zeros (the difficulty). The difficulty determines average mining time: each leading zero approximately quadruples the expected nonces to check. Bitcoin's block time of ~10 minutes is maintained by adjusting difficulty every 2016 blocks to keep pace with the total network hash rate.

Question 2

Why is blockchain tamper-evident?

Accepted Answer

Each block contains the SHA-256 hash of the previous block in its header. If you modify any data in block N, its hash changes, invalidating block N+1's previousHash field (which no longer matches), cascading all the way to the chain tip. To tamper successfully, an attacker must re-mine every subsequent block faster than the honest network adds new blocks — requiring more than 50% of the total hash rate (a '51% attack').

Question 3

What is a Merkle tree in a blockchain?

Accepted Answer

A Merkle tree is a binary hash tree where each leaf node is a transaction hash, and each parent node is the hash of its two children. The root hash (Merkle root) in the block header efficiently summarises all transactions: changing any single transaction changes its leaf hash, a parent hash, and ultimately the Merkle root, invalidating the block. Merkle proofs allow lightweight clients to verify a transaction with O(log n) hashes instead of downloading the full block.

Question 4

What is the difference between Proof-of-Work and Proof-of-Stake?

Accepted Answer

Proof-of-Work (PoW) requires expending real computational energy to mine blocks; the longest chain wins consensus. It is energy-intensive (Bitcoin uses ~130 TWh/year) but battle-tested. Proof-of-Stake (PoS) selects validators proportional to their staked cryptocurrency; Ethereum's PoS uses ~0.01% of its former PoW energy. PoS is more energy-efficient but introduces different security trade-offs: 51% attacks require acquiring 51% of the staked currency rather than hash rate.

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