Quiz: Mining and Proof of Work

Multiple Choice

Question 1

What is the fundamental problem that Proof of Work solves in a permissionless network?

A) It prevents users from sending Bitcoin without sufficient funds. B) It provides Sybil resistance by making block production costly. C) It encrypts transactions so only the sender and receiver can read them. D) It ensures that all nodes have identical copies of the software.

Answer: B. Proof of Work provides Sybil resistance by requiring real-world energy expenditure to produce blocks. In a permissionless network where anyone can join, an attacker could create millions of fake identities to dominate a voting-based system. PoW makes this infeasible because each "vote" (hash attempt) requires computational work regardless of how many identities the attacker creates.

Question 2

Bitcoin uses SHA-256 applied twice (SHA-256d) rather than once. The primary cryptographic reason for double hashing is:

A) It doubles the security level from 128 bits to 256 bits. B) It protects against length extension attacks inherent in Merkle-Damgard constructions. C) It makes mining exactly twice as slow, ensuring 10-minute block times. D) It was required by U.S. export regulations on cryptographic software.

Answer: B. SHA-256 is based on the Merkle-Damgard construction, which is vulnerable to length extension attacks. Applying the hash twice (hashing the hash) eliminates this vulnerability because the intermediate hash is a fixed-length value. The double hashing does add computational cost, but this was not the design motivation — the difficulty target, not the hash function's speed, determines block time.

Question 3

The Bitcoin difficulty adjustment occurs every:

A) 100 blocks (approximately 17 hours) B) 1,000 blocks (approximately 7 days) C) 2,016 blocks (approximately 14 days) D) 210,000 blocks (approximately 4 years)

Answer: C. The difficulty adjusts every 2,016 blocks, which at the target rate of one block per 10 minutes equals approximately 14 days (2 weeks). The adjustment formula compares the actual time elapsed for the last 2,016 blocks against the expected time of 1,209,600 seconds, then scales the target proportionally.

Question 4

The difficulty adjustment is clamped to a factor of 4 in either direction. If 80% of the network hashrate suddenly went offline, approximately how many adjustment periods would it take for block times to return to 10 minutes?

A) 1 period (2 weeks at normal speed, but much longer in practice) B) 2-3 periods C) Approximately 4-5 periods D) It would never fully adjust

Answer: B. With 80% hashrate gone, blocks would take approximately 50 minutes on average. The first period would take approximately 70 days (5x the normal 14 days). The difficulty would then drop by the maximum factor of 4, leaving blocks at approximately 12.5-minute intervals. The second adjustment would bring it close to the target. So 2-3 adjustment periods, spanning roughly 3-4 months of real time.

Question 5

Which of the following is NOT something a 51% attacker can do?

A) Double-spend by reversing their own recent transactions B) Prevent specific transactions from being confirmed (censorship) C) Spend Bitcoin from addresses they do not control D) Orphan blocks mined by honest miners

Answer: C. A 51% attack controls block production, not private keys. The attacker can choose which transactions to include or exclude from blocks, can mine a private chain to replace the public one (enabling double-spends of their own transactions), and can orphan honest miners' blocks. However, they cannot create valid signatures for addresses they do not hold the private keys for — the cryptographic security of individual accounts is independent of mining.

Question 6

The Bitcoin Gold 51% attack of May 2018 was economically feasible primarily because:

A) Bitcoin Gold had a very high market capitalization, making the reward worthwhile. B) The Equihash algorithm was GPU-mineable, and hash power could be rented cheaply. C) The attacker compromised Bitmain's ASIC manufacturing facility. D) Bitcoin Gold did not use Proof of Work at all.

Answer: B. Bitcoin Gold used the Equihash algorithm, which was designed to be ASIC-resistant and remained GPU-mineable. This meant hash power could be rented on services like NiceHash at a fraction of the cost of purchasing dedicated hardware. The attack highlighted the vulnerability of PoW chains where hash power is commoditized and rentable.

Question 7

In the Eyal-Sirer selfish mining attack, a miner with less than 50% of the hashrate can earn disproportionate revenue by:

A) Mining only blocks with high transaction fees and ignoring low-fee blocks. B) Withholding found blocks and releasing them strategically to orphan honest miners' blocks. C) Submitting shares to a mining pool but discarding valid blocks (block withholding). D) Exploiting the difficulty adjustment to mine during low-difficulty periods.

Answer: B. Selfish mining works by withholding discovered blocks and continuing to mine privately. When the honest network finds a block, the selfish miner releases their withheld block, creating a competition. If the selfish miner has enough hashrate (approximately 25-33% depending on network connectivity), this strategy yields more revenue than honest mining because it causes honest miners to waste work on blocks that will be orphaned.

Question 8

As of 2026, Bitcoin's estimated annual electricity consumption is approximately:

A) 15-17 TWh (comparable to a small European country like Estonia) B) 150-170 TWh (comparable to Poland or Egypt) C) 1,500-1,700 TWh (comparable to the entire European Union) D) 15,000 TWh (comparable to total global electricity production)

Answer: B. Bitcoin's annualized electricity consumption is estimated at 150-170 TWh per year according to the Cambridge Bitcoin Electricity Consumption Index, representing approximately 0.6-0.7% of global electricity production. This is comparable to the total electricity consumption of Poland or Egypt.

Question 9

The "stranded energy" argument in defense of Bitcoin mining refers to:

A) The energy stored in Bitcoin itself, which can be converted back to electricity. B) Mining operations that co-locate with energy sources (like flared gas) that would otherwise be wasted. C) The idea that Bitcoin miners only operate when electricity is cheap, reducing grid stress. D) A legal argument that miners should not pay for electricity used for financial purposes.

Answer: B. The stranded energy argument holds that some Bitcoin mining operations specifically target energy that would otherwise be wasted — such as natural gas that is flared at oil wells because pipeline infrastructure is unavailable, or renewable energy that is curtailed because the grid cannot absorb it. Companies like Crusoe Energy place mining containers at flare sites, converting waste gas to electricity.

Question 10

Which of the following best describes the relationship between mining pools and network security?

A) Mining pools increase security because they consolidate hashrate under professional management. B) Mining pools decrease security because the pool operator controls all members' hash power and can direct it maliciously. C) Mining pools introduce centralization risk, but individual miners can leave pools instantly, creating a self-correcting mechanism. D) Mining pools have no effect on security because they do not change the total network hashrate.

Answer: C. Mining pools create centralization risk because the pool operator constructs block templates and directs members' hash power. However, individual miners voluntarily join pools and can leave at any time. When GHash.io briefly exceeded 50% in 2014, miners voluntarily redistributed. This self-correction is real but imperfect — it requires miners to be attentive and willing to act, and the response is not instantaneous.

Short Answer

Question 11

Explain why the 32-bit nonce field in the block header is insufficient for modern mining, and describe two methods miners use to expand the search space.

Model Answer: The 32-bit nonce provides only 2^32 (approximately 4.3 billion) possible values. A modern ASIC can exhaust this space in under a second, far too quickly at current difficulty levels. Miners expand the search space by: (1) modifying the extra nonce field in the coinbase transaction, which changes the Merkle root and thus the block header, creating a new nonce space; and (2) adjusting the block header timestamp within the protocol's acceptable range, which also changes the header and creates fresh nonce space.

Question 12

A miner has 100 TH/s of hashrate in a network with 500 EH/s total hashrate. The block reward is 3.125 BTC, average fees are 0.3 BTC per block, and BTC is priced at $80,000. Calculate the miner's expected daily revenue in USD.

Model Answer: - Miner's fraction of network: 100 TH/s / 500,000,000 TH/s = 2 x 10^-7 - Blocks per day: 144 - Revenue per block: (3.125 + 0.3) x $80,000 = $274,000 - Expected daily revenue: 2 x 10^-7 x 144 x $274,000 = $7.89 per day

Question 13

Describe three reasons why a 51% attack against Bitcoin is considered economically irrational even if technically possible.

Model Answer: (1) The capital cost is enormous — acquiring 51% of hashrate would require billions of dollars in ASIC hardware that cannot be procured quickly. (2) An attacker with majority hashrate earns more by mining honestly than by attacking; the attack has negative expected value once the likely BTC price crash is considered. (3) The attack would be detected immediately, allowing exchanges to halt deposits, the community to mobilize countermeasures, and the attacker's window of profitable exploitation to be extremely narrow.

Question 14

Explain the difference between Pay-Per-Share (PPS) and Pay-Per-Last-N-Shares (PPLNS) pool reward schemes. Which is better for a miner who pool-hops frequently, and why?

Model Answer: In PPS, the pool pays a fixed rate per valid share regardless of whether the pool finds blocks. The pool absorbs variance. In PPLNS, rewards are distributed based on shares submitted during a window of the N most recent shares before a block is found. PPS is better for pool-hoppers because they receive guaranteed payment for every share submitted, regardless of timing. PPLNS penalizes hoppers because their shares may not fall within the reward window if they leave before a block is found.

Question 15

Present the strongest argument for and against Bitcoin's energy consumption in two sentences each.

Model Answer: For: Some mining operations consume energy that would otherwise be wasted (flared gas, curtailed renewables), converting waste energy into network security while reducing methane emissions and improving renewable energy economics. Against: Regardless of energy source, Bitcoin's 150+ TWh annual consumption represents a significant and growing claim on global energy resources, and the renewable energy consumed by miners has an opportunity cost — it could otherwise displace fossil fuels on the grid.