1What is the approximate target time for creating a new block in the Bitcoin blockchain?
Bitcoin blockchain
Easy
A.10 minutes
B.1 second
C.1 hour
D.1 minute
Correct Answer: 10 minutes
Explanation:
The Bitcoin protocol is designed to target an average block creation time of 10 minutes. This is maintained by an algorithm that adjusts the mining difficulty every 2016 blocks.
Incorrect! Try again.
2What is the very first block in the Bitcoin blockchain commonly called?
Bitcoin blockchain
Easy
A.The Prime Block
B.The Alpha Block
C.The Genesis Block
D.The Final Block
Correct Answer: The Genesis Block
Explanation:
The Genesis Block is the first block of a blockchain. In Bitcoin, it was mined by Satoshi Nakamoto and is hardcoded into the software.
Incorrect! Try again.
3Who is the pseudonymous creator or group of creators of Bitcoin?
Bitcoin blockchain
Easy
A.Hal Finney
B.Charles Hoskinson
C.Vitalik Buterin
D.Satoshi Nakamoto
Correct Answer: Satoshi Nakamoto
Explanation:
Satoshi Nakamoto is the name used by the presumed pseudonymous person or persons who developed Bitcoin, authored the Bitcoin white paper, and created its original reference implementation.
Incorrect! Try again.
4How are blocks in the Bitcoin blockchain cryptographically connected to form a chain?
Bitcoin blockchain
Easy
A.Each block contains a direct memory pointer to the previous one.
B.Each block contains the hash of the previous block.
C.Each block contains the private key of the previous block.
D.Each block is signed by a central server.
Correct Answer: Each block contains the hash of the previous block.
Explanation:
This linking mechanism creates an immutable chain. The hash of the previous block's header is included in the current block's header, ensuring that any change to a past block would invalidate all subsequent blocks.
Incorrect! Try again.
5In the context of digital currencies, what does the 'double-spending' problem refer to?
Challenges and solutions
Easy
A.Exchanging one cryptocurrency for another twice.
B.Paying twice the price for a single item.
C.A user successfully spending the same digital money more than once.
D.A bug that doubles a user's wallet balance.
Correct Answer: A user successfully spending the same digital money more than once.
Explanation:
Double-spending is a potential flaw where the same single digital token can be spent multiple times. Bitcoin solves this with its public ledger and a consensus mechanism like Proof of Work.
Incorrect! Try again.
6What is a '51% attack' on a Proof of Work blockchain like Bitcoin?
Challenges and solutions
Easy
A.An attack that steals 51% of all Bitcoins in circulation.
B.A situation where a single entity controls more than 50% of the network's mining power.
C.A social engineering attack targeting 51% of users.
D.A software bug that affects 51% of the network nodes.
Correct Answer: A situation where a single entity controls more than 50% of the network's mining power.
Explanation:
If an attacker controls the majority of the network's mining hash rate, they can potentially reverse their own transactions and prevent other transactions from being confirmed, enabling double-spending.
Incorrect! Try again.
7How does the Bitcoin network's difficulty adjustment feature work as a solution?
Challenges and solutions
Easy
A.It makes transactions more expensive.
B.It changes the block reward for miners.
C.It deletes old transactions from the blockchain to save space.
D.It aims to keep the block creation time at approximately 10 minutes.
Correct Answer: It aims to keep the block creation time at approximately 10 minutes.
Explanation:
The mining difficulty is adjusted every 2016 blocks (roughly two weeks) to account for changes in the total network hash rate, ensuring the average time to find a new block remains close to the 10-minute target.
Incorrect! Try again.
8What is the primary role of the Proof of Work (PoW) consensus algorithm in Bitcoin?
Proof of work
Easy
A.To create new Bitcoin out of thin air.
B.To speed up transaction processing to millions per second.
C.To encrypt user wallets.
D.To secure the network and achieve consensus on the valid transaction ledger.
Correct Answer: To secure the network and achieve consensus on the valid transaction ledger.
Explanation:
PoW makes it computationally expensive to add new blocks, which secures the blockchain from tampering and allows decentralized nodes to agree on a single, valid chain of transactions.
Incorrect! Try again.
9The process of performing the computations for Bitcoin's Proof of Work is commonly called what?
Proof of work
Easy
A.Staking
B.Validating
C.Mining
D.Forging
Correct Answer: Mining
Explanation:
Mining is the term for the process where network participants (miners) use specialized hardware to solve the cryptographic puzzle, validate transactions, and add new blocks to the blockchain.
Incorrect! Try again.
10What cryptographic hash algorithm is central to Bitcoin's Proof of Work?
Proof of work
Easy
A.SHA-256
B.MD5
C.SHA-1
D.Scrypt
Correct Answer: SHA-256
Explanation:
Bitcoin's Proof of Work algorithm uses the SHA-256 (Secure Hash Algorithm 256-bit) hashing function. Miners repeatedly hash the block header until the result is below a certain target value.
Incorrect! Try again.
11What do miners receive as a reward for successfully creating a new block in Bitcoin?
Proof of work
Easy
A.Newly created bitcoins (block reward) and transaction fees.
B.A certificate of appreciation.
C.Voting rights in the network's governance.
D.Interest on the bitcoins they hold.
Correct Answer: Newly created bitcoins (block reward) and transaction fees.
Explanation:
The miner who successfully solves the PoW puzzle and adds a valid block to the chain is rewarded with a block reward (newly minted bitcoins) and all the transaction fees from the transactions included in that block.
Incorrect! Try again.
12In a Proof of Stake (PoS) system, what is the primary factor that determines a user's chance of creating the next block?
Proof of stake
Easy
A.The amount of cryptocurrency they hold and are willing to 'stake'.
B.The number of transactions they have sent.
C.The age of their account on the network.
D.The speed of their computer.
Correct Answer: The amount of cryptocurrency they hold and are willing to 'stake'.
Explanation:
In PoS, validators are chosen to create new blocks based on the number of coins they hold and have locked up as collateral. The more coins staked, the higher the chance of being selected.
Incorrect! Try again.
13A major advantage of Proof of Stake (PoS) when compared to Proof of Work (PoW) is that it is significantly more...
Proof of stake
Easy
A.Energy-efficient
B.Decentralized
C.Time-tested and established
D.Complex to implement
Correct Answer: Energy-efficient
Explanation:
PoS does not require intensive computational work to secure the network. This drastically reduces the electricity consumption compared to PoW, making it a much more environmentally friendly consensus mechanism.
Incorrect! Try again.
14What is the process of creating new blocks often called in a Proof of Stake system?
Proof of stake
Easy
A.Forging or Minting
B.Mining or Hashing
C.Signing or Authorizing
D.Printing or Coding
Correct Answer: Forging or Minting
Explanation:
While 'mining' is the term associated with PoW, 'forging' or 'minting' are commonly used to describe the block creation process in a PoS system.
Incorrect! Try again.
15In Delegated Proof of Stake (DPoS), how are the block producers (validators) chosen?
Alternatives to Bitcoin consensus
Easy
A.They are the wealthiest coin holders.
B.They are appointed by the network's founding developers.
C.They are chosen randomly by the protocol.
D.They are elected by other coin holders through a voting system.
Correct Answer: They are elected by other coin holders through a voting system.
Explanation:
DPoS is a variation of PoS where users stake their coins to vote for a limited number of 'delegates' or 'witnesses'. These elected delegates are then responsible for producing and validating blocks.
Incorrect! Try again.
16Which type of consensus algorithm is generally best suited for private or permissioned blockchains where participants are known?
PBFT and similar algorithms are designed for systems with a known, limited number of validators. They provide high transaction throughput and fast finality, making them ideal for trusted, permissioned environments.
Incorrect! Try again.
17Proof of Authority (PoA) is a consensus mechanism where block validators are chosen based on their...
Alternatives to Bitcoin consensus
Easy
A.Established reputation and identity
B.Amount of staked coins
C.Age on the network
D.Computational power
Correct Answer: Established reputation and identity
Explanation:
In PoA, validators do not stake coins but instead stake their real-world reputation. Blocks are created by pre-approved participants (authorities) who are trusted entities, making the system highly efficient but centralized.
Incorrect! Try again.
18What is the name of the built-in, stack-based scripting language of Bitcoin?
Bitcoin scripting language and their use
Easy
A.C++
B.Script
C.Solidity
D.Javascript
Correct Answer: Script
Explanation:
Bitcoin has a simple, stack-based scripting language officially known as Script. It is used to define the conditions that must be met to spend transaction outputs.
Incorrect! Try again.
19Why is Bitcoin's scripting language intentionally designed to be non-Turing-complete?
Bitcoin scripting language and their use
Easy
A.To reduce the potential for bugs and prevent infinite loops.
B.To make it easier for new programmers to learn.
C.To make it faster than other languages.
D.To allow for more complex smart contracts, like those on Ethereum.
Correct Answer: To reduce the potential for bugs and prevent infinite loops.
Explanation:
By purposely omitting features like complex loops, the language ensures that scripts will always terminate and have predictable execution costs. This greatly enhances network security by limiting the attack surface for malicious scripts.
Incorrect! Try again.
20A multi-signature (multisig) wallet, which requires M-of-N signatures to spend funds, is a feature enabled by what component of Bitcoin?
Bitcoin scripting language and their use
Easy
A.The Proof of Work algorithm
B.The Bitcoin scripting language
C.The peer-to-peer network structure
D.The fixed supply of 21 million coins
Correct Answer: The Bitcoin scripting language
Explanation:
The Bitcoin Script allows for the creation of complex spending conditions beyond a simple signature check. A multisig transaction's script specifies that multiple private keys must provide signatures before the funds can be moved.
Incorrect! Try again.
21A user sends 1 BTC from an address that holds a single UTXO of 3 BTC to a new address. Assuming a 0.001 BTC transaction fee, how is the UTXO set updated after this transaction is confirmed in a block?
Bitcoin blockchain
Medium
A.The 3 BTC UTXO is consumed, and three new UTXOs are created: 1 BTC for the recipient, 1.999 BTC for change, and 0.001 BTC for the miner.
B.The original 3 BTC UTXO is updated to a value of 1.999 BTC, and a new 1 BTC UTXO is created for the recipient.
C.The 3 BTC UTXO is consumed, and two new UTXOs are created: one for 1 BTC (to the recipient) and one for 1.999 BTC (as change to the sender).
D.The 3 BTC UTXO is consumed, and one new UTXO of 2.999 BTC is created for the sender's change address.
Correct Answer: The 3 BTC UTXO is consumed, and two new UTXOs are created: one for 1 BTC (to the recipient) and one for 1.999 BTC (as change to the sender).
Explanation:
The UTXO model is not like a bank account balance where a value is debited. UTXOs are indivisible and are fully consumed in a transaction. The transaction creates new UTXOs from the inputs: one for the recipient (1 BTC) and one for the change back to the sender (3 - 1 - 0.001 = 1.999 BTC). The fee is implicitly claimed by the miner and does not become a new UTXO.
Incorrect! Try again.
22What is the primary security and efficiency advantage of using a Merkle root in the Bitcoin block header instead of including all individual transaction hashes directly?
Bitcoin blockchain
Medium
A.It enables Simplified Payment Verification (SPV) clients to verify a transaction's inclusion without downloading the entire block.
B.It allows for transaction hashes to be compressed, saving space in the block.
C.It provides a way to order transactions chronologically within the block header.
D.It makes the block header's hash easier to calculate for miners.
Correct Answer: It enables Simplified Payment Verification (SPV) clients to verify a transaction's inclusion without downloading the entire block.
Explanation:
The Merkle root is a single hash representing all transactions in a block. An SPV or "light" client can confirm a transaction is included by downloading only the block headers and the "Merkle path" (a small number of intermediate hashes) for that specific transaction. This is far more efficient than downloading every transaction in the entire block.
Incorrect! Try again.
23The Bitcoin network difficulty is designed to adjust every 2016 blocks to target an average block time of 10 minutes. If, due to a massive increase in hash rate, the last 2016 blocks were mined in only 7 days, what would the network's response be?
Proof of work
Medium
A.The block reward would be halved to slow down miners.
B.The network would temporarily halt to allow for manual adjustment.
C.The difficulty would decrease by approximately 50%.
D.The difficulty would increase by approximately 100%.
Correct Answer: The difficulty would increase by approximately 100%.
Explanation:
The target time for 2016 blocks is 2016 blocks 10 minutes/block = 20160 minutes, which is exactly 14 days. If the blocks were found in 7 days, the network was running twice as fast as intended. To bring the block time back to the 10-minute target, the difficulty must be doubled (increased by 100%). The formula is `New Difficulty = Old Difficulty (Expected Time / Actual Time), which isOld Difficulty (14 days / 7 days) = Old Difficulty 2`.
Incorrect! Try again.
24How did the Segregated Witness (SegWit) upgrade primarily address the issue of third-party transaction malleability in Bitcoin?
Challenges and solutions
Medium
A.By encrypting the digital signature so it cannot be altered.
B.By requiring multiple signatures for every transaction, making alteration more difficult.
C.By moving the signature data (the "witness") from the main transaction body to a separate data structure, so it no longer affects the transaction ID (txid).
D.By increasing the network fee for any transaction that has been altered.
Correct Answer: By moving the signature data (the "witness") from the main transaction body to a separate data structure, so it no longer affects the transaction ID (txid).
Explanation:
Transaction malleability occurs when a third party alters the signature of a transaction without invalidating it, which changes the transaction ID (txid). SegWit solves this by moving the signature (witness) data out of the part of the transaction that is hashed to calculate the txid. Since the txid no longer depends on the witness data, the txid cannot be changed by manipulating the signature.
Incorrect! Try again.
25What is the "nothing-at-stake" problem in Proof of Stake consensus mechanisms, and why is it generally not an issue in Proof of Work?
Proof of stake
Medium
A.Staked coins can be lost due to network errors, while PoW mining equipment is a physical asset.
B.In the event of a fork, a PoS validator can cheaply vote for both chains to maximize potential rewards, whereas a PoW miner must split their expensive hash power.
C.PoS validators risk nothing if they propose an invalid block, unlike PoW miners who lose electricity costs.
D.Validators have no incentive to validate transactions, whereas PoW miners are paid block rewards.
Correct Answer: In the event of a fork, a PoS validator can cheaply vote for both chains to maximize potential rewards, whereas a PoW miner must split their expensive hash power.
Explanation:
The "nothing-at-stake" problem arises because creating a block in PoS has a very low marginal cost. If a fork occurs, a rational validator has an economic incentive to build on both chains to maximize their chances of earning rewards. In PoW, a miner must dedicate their computational resources and associated energy cost to only one chain, making it economically irrational to support multiple forks simultaneously.
Incorrect! Try again.
26A Pay-to-Script-Hash (P2SH) transaction output is locked with a hash of a redeem script. What must the spender provide in the input's scriptSig to successfully unlock these funds?
Bitcoin scripting language and their use
Medium
A.A new script hash that matches the original one.
B.The full, unhashed redeem script and the data (e.g., signatures) that make that redeem script evaluate to true.
C.The private key used to generate the address, which the network then uses to generate the redeem script.
D.Just the digital signature corresponding to the public key.
Correct Answer: The full, unhashed redeem script and the data (e.g., signatures) that make that redeem script evaluate to true.
Explanation:
Unlocking a P2SH output is a two-step validation process. First, the spender provides the full redeem script, which the network hashes to ensure it matches the script hash in the output's scriptPubKey. Second, the spender provides the necessary signatures or other data required by the redeem script itself. The network then executes this redeem script with the provided data to finally validate the transaction.
Incorrect! Try again.
27How does Delegated Proof of Stake (DPoS) attempt to solve the scalability and performance issues seen in Proof of Work, and what is the primary trade-off?
Alternatives to Bitcoin consensus
Medium
A.It requires all coin holders to participate in block validation, which improves security but slows down the network.
B.It randomly selects a new block producer every 10 minutes from the entire pool of stakers, improving decentralization.
C.It uses a small, fixed number of elected block producers, allowing for faster block confirmation, at the cost of increased centralization.
D.It replaces mining with a lottery system, reducing energy consumption but making the network vulnerable to Sybil attacks.
Correct Answer: It uses a small, fixed number of elected block producers, allowing for faster block confirmation, at the cost of increased centralization.
Explanation:
In DPoS, coin holders vote for a limited number of "delegates" or "witnesses" who are responsible for producing blocks. Because the number of block producers is small and known (e.g., 21), consensus can be reached very quickly. This significantly increases transaction throughput. The main trade-off is that power is concentrated in the hands of these few elected entities, making the system less decentralized than Bitcoin's PoW.
Incorrect! Try again.
28A malicious actor gains control of 51% of the Bitcoin network's hash rate. Which of the following attacks could they successfully execute?
Challenges and solutions
Medium
A.Spend their own bitcoins, wait for confirmation, and then create a fork to reverse that specific transaction, allowing them to double-spend the coins.
B.Steal bitcoins from an address for which they do not possess the private key.
C.Reverse any transaction on the blockchain, including one from several years ago.
D.Create new bitcoins out of thin air, exceeding the 21 million supply limit.
Correct Answer: Spend their own bitcoins, wait for confirmation, and then create a fork to reverse that specific transaction, allowing them to double-spend the coins.
Explanation:
A 51% attack gives the attacker control over the ordering of transactions and the ability to mine a new, longer version of the blockchain. They could broadcast a transaction (e.g., paying an exchange), let it confirm, and then use their majority hash power to mine a new chain that excludes that transaction. This new, longer chain would become the accepted one, effectively reversing their own payment. They cannot, however, change consensus rules (like the coin supply limit) or forge signatures to steal funds from others.
Incorrect! Try again.
29In the context of Bitcoin's PoW, what does it mean for the hash function (SHA-256) to be "puzzle-friendly"?
Proof of work
Medium
A.The function is easy to compute in one direction but extremely difficult to reverse.
B.Given a specific output range y, it is infeasible to find an input x such that H(x) falls in y by any method more efficient than random guessing.
C.The output of the hash function is always a fixed length, regardless of the input size.
D.A small change in the input x results in a completely different, unpredictable output hash.
Correct Answer: Given a specific output range y, it is infeasible to find an input x such that H(x) falls in y by any method more efficient than random guessing.
Explanation:
The "puzzle-friendly" property is crucial for PoW. It means there's no shortcut to solving the mining puzzle (finding a hash below the target). The only way a miner can find a valid hash is by repeatedly trying different nonces (inputs) until they get lucky. This makes the mining process a fair competition based purely on computational power.
Incorrect! Try again.
30What is the primary reason for Bitcoin's scripting language (Script) being intentionally designed as non-Turing complete?
Bitcoin scripting language and their use
Medium
A.To reduce the computational power required by miners to run the scripts.
B.To prevent infinite loops and ensure transaction validation is always predictable and finite, protecting the network from denial-of-service attacks.
C.To make the language easier for new developers to learn.
D.To comply with international financial regulations that limit the complexity of smart contracts.
Correct Answer: To prevent infinite loops and ensure transaction validation is always predictable and finite, protecting the network from denial-of-service attacks.
Explanation:
A Turing complete language allows for complex computations, including loops of indeterminate length. If Bitcoin's Script were Turing complete, a malicious actor could create a transaction with a script that runs forever. When nodes tried to validate this transaction, their systems would get stuck, effectively launching a denial-of-service (DoS) attack. By intentionally limiting the language (e.g., no complex loops), every script is guaranteed to terminate in a predictable amount of time, making validation secure.
Incorrect! Try again.
31In a Proof of Stake system that implements "slashing", what is the main purpose of this mechanism?
Proof of stake
Medium
A.To reduce the total supply of the cryptocurrency over time.
B.To randomly select which validator's stake will be used for the next block reward.
C.To reward validators who have the longest uptime.
D.To create a strong economic disincentive for validators who act maliciously, such as by double-signing blocks.
Correct Answer: To create a strong economic disincentive for validators who act maliciously, such as by double-signing blocks.
Explanation:
Slashing is a punitive measure designed to enforce honest behavior from validators. If a validator is caught performing a malicious act, such as attesting to two different blocks at the same height (double-signing), the protocol automatically destroys a portion of their staked coins. This creates a significant financial penalty, making attacks costly and irrational.
Incorrect! Try again.
32What is the fundamental principle behind how the Lightning Network aims to solve Bitcoin's scalability problem?
Challenges and solutions
Medium
A.By compressing transaction data on the main blockchain using a new algorithm like ZK-SNARKs.
B.By increasing Bitcoin's block size from 1MB to 8MB to accommodate more transactions.
C.By reducing the block time from 10 minutes to 1 minute, thereby increasing on-chain throughput.
D.By creating an off-chain, peer-to-peer layer of payment channels where multiple transactions can occur without being immediately broadcast to the main blockchain.
Correct Answer: By creating an off-chain, peer-to-peer layer of payment channels where multiple transactions can occur without being immediately broadcast to the main blockchain.
Explanation:
The Lightning Network is a Layer 2 solution. It allows users to open payment channels with each other. Transactions within these channels happen off-chain, are nearly instant, and have very low fees. Only the opening and closing transactions of a channel need to be recorded on the main Bitcoin blockchain, which vastly reduces the load on the network and allows for a high volume of small payments.
Incorrect! Try again.
33The block reward in Bitcoin is the primary incentive for miners. How do transaction fees contribute to the long-term security model of the Bitcoin network?
Proof of work
Medium
A.Transaction fees are paid directly to full nodes for validating transactions, separate from the miners' block reward.
B.Transaction fees are designed to eventually become the primary incentive for miners as the block reward subsidy diminishes to zero through periodic halvings.
C.Transaction fees are burned to reduce the total supply of Bitcoin.
D.Transaction fees are used to speed up the difficulty adjustment process.
Correct Answer: Transaction fees are designed to eventually become the primary incentive for miners as the block reward subsidy diminishes to zero through periodic halvings.
Explanation:
The Bitcoin protocol includes a "halving" event approximately every four years, which cuts the new bitcoin subsidy in half. Over time, this subsidy will trend towards zero. For the network to remain secure, miners must continue to be incentivized to expend energy. Transaction fees, paid by users for block inclusion, are intended to take over as the main source of revenue for miners, ensuring the security budget of the network remains substantial.
Incorrect! Try again.
34What is a key characteristic of consensus mechanisms like Practical Byzantine Fault Tolerance (PBFT) that makes them suitable for permissioned blockchains but less so for public ones like Bitcoin?
Alternatives to Bitcoin consensus
Medium
A.PBFT allows anyone to become a validator without permission, which is insecure in a private setting.
B.PBFT consumes significantly more energy than Proof of Work, making it too expensive for public networks.
C.PBFT is probabilistic and never achieves absolute finality, making it unsuitable for enterprise use.
D.PBFT relies on a known, and often small, set of validators, providing transaction finality but requiring participants to be identified.
Correct Answer: PBFT relies on a known, and often small, set of validators, providing transaction finality but requiring participants to be identified.
Explanation:
PBFT algorithms are designed for systems where the participants (validators) are known and their total number is fixed. The consensus process involves multiple rounds of voting, which is efficient with a small number of validators but does not scale to the large, dynamic, and anonymous set of participants in a public network like Bitcoin. Its major advantage in a permissioned setting is fast performance and deterministic finality (transactions are irreversible once confirmed).
Incorrect! Try again.
35What is the role of the nLockTime parameter in a Bitcoin transaction?
Bitcoin blockchain
Medium
A.It specifies the maximum amount of time a miner has to include the transaction in a block.
B.It indicates the exact time the transaction was created, which is used to resolve transaction ordering conflicts.
C.It defines a future time (as a block height or Unix timestamp) before which the transaction is invalid and cannot be included in a block.
D.It is a nonce used by the sender to prevent transaction replay attacks on other blockchains.
Correct Answer: It defines a future time (as a block height or Unix timestamp) before which the transaction is invalid and cannot be included in a block.
Explanation:
nLockTime is a field that specifies the earliest time a transaction can be legally mined. If the value is less than 500 million, it's interpreted as a block height; otherwise, it's a Unix timestamp. This feature enables use cases like time-locked payments or creating payment channels where transactions are only valid after a certain period, forming a crucial component for more complex protocols like the Lightning Network.
Incorrect! Try again.
36A standard Pay-to-Public-Key-Hash (P2PKH) transaction script is validated by concatenating the scriptSig and scriptPubKey and executing them. For a valid transaction, what should be the final state of the stack after execution?
Bitcoin scripting language and their use
Medium
A.The stack should contain the number 0.
B.The stack should be empty.
C.The stack should contain the public key of the spender.
D.The stack should contain a single value that evaluates to TRUE (a non-zero number).
Correct Answer: The stack should contain a single value that evaluates to TRUE (a non-zero number).
Explanation:
The validation process executes the scriptSig (pushing the signature and public key to the stack) followed by the scriptPubKey (which contains opcodes). The final opcode, OP_CHECKSIG, consumes the public key and signature, compares them, and pushes a single TRUE (represented as 1) or FALSE (0) value back. For the transaction to be valid, the script must complete with a single non-zero value left on top of the stack.
Incorrect! Try again.
37Which of the following describes the concept of "probabilistic finality" in Nakamoto Consensus (Proof of Work) as used by Bitcoin?
Alternatives to Bitcoin consensus
Medium
A.Transactions have a 50/50 chance of being reversed until they are at least 10 blocks deep.
B.Finality is determined by a vote among a committee of nodes, and the result is probabilistic.
C.A transaction is considered absolutely final the moment it is included in one block.
D.The probability of a confirmed transaction being reversed decreases exponentially as more blocks are added on top of it, but never reaches absolute certainty.
Correct Answer: The probability of a confirmed transaction being reversed decreases exponentially as more blocks are added on top of it, but never reaches absolute certainty.
Explanation:
In Bitcoin's PoW consensus, a transaction is never 100% final in a mathematical sense. The security of a transaction is based on the amount of cumulative work built on top of the block containing it. An attacker wanting to reverse a transaction would have to re-mine that block and all subsequent blocks faster than the rest of the network. The cost to do this grows with each new block added. Therefore, the likelihood of a reversal becomes vanishingly small as more blocks (confirmations) are added, leading to "probabilistic finality".
Incorrect! Try again.
38The "selfish mining" attack is a strategy where a miner finds a block but keeps it private, attempting to mine a second block on top of their secret one. What is the primary goal of this strategy?
Challenges and solutions
Medium
A.To earn a disproportionately larger share of mining rewards than their hash rate would normally entitle them to, by selectively orphaning blocks from honest miners.
B.To steal bitcoins directly from other users' wallets.
C.To reduce the overall network difficulty, making it easier to mine future blocks.
D.To shut down the entire Bitcoin network by creating an endless chain of orphaned blocks.
Correct Answer: To earn a disproportionately larger share of mining rewards than their hash rate would normally entitle them to, by selectively orphaning blocks from honest miners.
Explanation:
In selfish mining, a miner with a secret chain can get a head start. If the public chain finds a competing block, the selfish miner can release their longer private chain, which causes the honest miner's block to be orphaned and the selfish miner to win the reward. By strategically releasing their privately mined blocks, they can cause honest miners to waste effort on a chain that will ultimately be abandoned, thus increasing the selfish miner's relative share of total block rewards.
Incorrect! Try again.
39A miner is trying to find a valid block hash. The current target requires the block hash to start with 10 leading zeros. If the network difficulty increases significantly, what would be the most likely new target?
Proof of work
Medium
A.A target requiring the hash to start with 12 leading zeros.
B.A target requiring the hash to start with 8 leading zeros.
C.A target requiring the hash to be an even number.
D.A target requiring the hash to contain the previous block's nonce.
Correct Answer: A target requiring the hash to start with 12 leading zeros.
Explanation:
In Bitcoin's PoW, finding a valid block means finding a hash that is numerically less than the current target value. A lower target value is harder to find and thus represents a higher difficulty. Requiring more leading zeros in the hash's hexadecimal representation corresponds to a much smaller numerical target. Therefore, an increase in difficulty means the target becomes smaller, which is commonly described as requiring more leading zeros.
Incorrect! Try again.
40In the context of the Unspent Transaction Output (UTXO) model, what primarily prevents a user from spending the same digital coin twice in two different unconfirmed transactions that are broadcast simultaneously?
Bitcoin blockchain
Medium
A.The user's private key changes after each transaction is broadcast, invalidating the second transaction's signature.
B.Miners will typically only accept the first of the two conflicting transactions they see into their memory pool, and will reject the second as an invalid double-spend attempt.
C.Each coin is time-stamped, and only the earliest transaction is considered valid by the network.
D.The Bitcoin protocol limits each address to one broadcast transaction per minute.
Correct Answer: Miners will typically only accept the first of the two conflicting transactions they see into their memory pool, and will reject the second as an invalid double-spend attempt.
Explanation:
While the blockchain's confirmation ultimately prevents double-spending, the network's first line of defense is the mempool policy of individual nodes and miners. When a node receives a transaction spending a specific UTXO, it adds it to its memory pool. If it then receives a second, conflicting transaction spending the same UTXO, it will identify it as a double-spend attempt and reject it. Whichever of the two transactions gets mined into a block first becomes canonical, invalidating the other permanently.
Incorrect! Try again.
41A P2SH address funds a redeem script with the following logic: OP_IF <2 pubkeyA pubkeyB pubkeyC 3 OP_CHECKMULTISIG> OP_ELSE <NOW + 90 days> OP_CHECKLOCKTIMEVERIFY OP_DROP <pubkeyD> OP_CHECKSIG OP_ENDIF. What is the most precise description of the spending conditions for the funds locked in this script?
Bitcoin scripting language and their use
Hard
A.A 2-of-3 multisignature from keys A, B, or C can spend the funds immediately, OR a single signature from key D can spend the funds, but only in a transaction with an nLockTime set to at least 90 days in the future.
B.A 2-of-3 multisignature from keys A, B, and C is required, AND the transaction must be signed by key D after a 90-day delay.
C.The funds can only be spent by a 2-of-3 multisignature from A, B, and C if a non-zero value is pushed to the stack, otherwise key D can spend them immediately.
D.The script is invalid because OP_CHECKLOCKTIMEVERIFY cannot be combined with OP_CHECKMULTISIG in the same redeem script.
Correct Answer: A 2-of-3 multisignature from keys A, B, or C can spend the funds immediately, OR a single signature from key D can spend the funds, but only in a transaction with an nLockTime set to at least 90 days in the future.
Explanation:
This script uses OP_IF/OP_ELSE/OP_ENDIF to create two distinct spending paths. To trigger the OP_IF branch, the spender must push a non-zero value (e.g., OP_1) onto the stack. The script then executes the 2-of-3 OP_CHECKMULTISIG. To trigger the OP_ELSE branch, the spender pushes a zero value (OP_0). This path first checks if the spending transaction's nLockTime is greater than or equal to the specified timestamp using OP_CHECKLOCKTIMEVERIFY. If it is, OP_DROP removes the timestamp from the stack, and the script proceeds to require a single signature from key D via OP_CHECKSIG.
Incorrect! Try again.
42A mining pool contributes 15% of the total network hashrate. The network difficulty is stable, resulting in an average block time of 10 minutes for the entire network. What is the approximate probability that this specific pool will find at least one block within the next 60 minutes?
Proof of work
Hard
A.Approximately 15%
B.Exactly 60%
C.Approximately 59.4%
D.Exactly 90%
Correct Answer: Approximately 59.4%
Explanation:
Block discovery follows a Poisson process. The average time for the entire network to find a block is 10 minutes, so the network-wide rate is 0.1 blocks/minute. The pool's rate is blocks/minute. The probability of an event occurring at least once in an interval is given by . For a 60-minute period, the calculation is: . This is approximately 59.4%.
Incorrect! Try again.
43In the context of Bitcoin's privacy, what is the primary limitation of using a new address for every transaction (a core principle of wallet hygiene) in preventing deanonymization via transaction graph analysis?
Challenges and solutions
Hard
A.It actually increases the on-chain footprint, making it easier for analysts to identify large entities.
B.It fails to break the 'common-input-ownership' heuristic, where multiple inputs to a single transaction are assumed to belong to the same entity.
C.It is computationally expensive and not supported by most modern wallet software.
D.It does not protect against analysis of transaction timing and value, which can still link activities.
Correct Answer: It fails to break the 'common-input-ownership' heuristic, where multiple inputs to a single transaction are assumed to belong to the same entity.
Explanation:
While using new addresses for receiving payments and for change is a crucial privacy practice, its effectiveness is limited when a user needs to construct a transaction that requires consolidating multiple UTXOs. When a transaction has multiple inputs, chain analysis tools operate on the strong assumption that all those input addresses are controlled by the same wallet or entity. This allows them to link previously separate addresses and activity clusters together, effectively merging the user's pseudonymous identities. Techniques like CoinJoin are specifically designed to break this heuristic.
Incorrect! Try again.
44Before the activation of Segregated Witness (SegWit), third-party transaction malleability was a significant issue. How could a third party (e.g., a node or miner) alter a transaction's hash (txid) without invalidating the ECDSA signature itself, and what was the main consequence for chained transactions?
Bitcoin blockchain
Hard
A.By adding extra data to the coinbase transaction, which changed the Merkle root and thus the txid of every transaction in the block.
B.By using a different but cryptographically valid serialization for the signature (e.g., changing S to -S mod N), which changed the txid and broke the input reference for any unconfirmed child transaction.
C.By altering the scriptPubKey, which is not covered by the signature, thus changing the txid and redirecting the funds.
D.By changing the nLockTime field of the transaction, which is not signed, causing the txid to change and the transaction to be delayed.
Correct Answer: By using a different but cryptographically valid serialization for the signature (e.g., changing S to -S mod N), which changed the txid and broke the input reference for any unconfirmed child transaction.
Explanation:
The txid is the double-SHA256 hash of the entire transaction data, including the signature script. The ECDSA signature scheme has properties that allow for multiple valid encodings of the same signature. The most famous example is that if (r, s) is a valid signature, so is (r, -s mod N), where N is the order of the curve. A third party could take a transaction from the mempool, change the signature encoding, and produce a transaction with a new txid that was still valid and spent the same inputs to the same outputs. The main problem this caused was for unconfirmed chained transactions. If a child transaction was created to spend an output from the parent, it would reference the parent's original txid. If the parent was malleated before confirmation, its txid would change, invalidating the child transaction.
Incorrect! Try again.
45The 'Nothing-at-Stake' problem in early Proof-of-Stake designs describes a scenario where validators are economically incentivized to validate on all competing forks, preventing consensus. How do modern PoS systems like Ethereum 2.0 primarily solve this problem?
Proof of stake
Hard
A.By using a Delegated Proof of Stake (DPoS) model where only a small number of trusted validators can create blocks.
B.By requiring validators to solve a small proof-of-work puzzle for each block they produce.
C.By introducing 'slashing' conditions, where a validator's stake is destroyed if they are caught signing blocks for multiple competing chains at the same height.
D.By increasing block rewards to a level where the incentive to be honest on one chain outweighs the potential rewards from all other chains combined.
Correct Answer: By introducing 'slashing' conditions, where a validator's stake is destroyed if they are caught signing blocks for multiple competing chains at the same height.
Explanation:
Modern PoS protocols solve the Nothing-at-Stake problem by creating a significant economic disincentive for equivocating (voting on multiple forks). This is achieved through slashing. If a validator signs two different blocks at the same block height or makes contradictory attestations, another user can submit proof of this malicious behavior to the chain. The protocol then automatically punishes the offending validator by 'slashing' a portion of their staked capital, which is permanently destroyed. This makes it economically irrational to vote on multiple chains, as the potential loss from slashing far outweighs any potential reward from being on the 'winning' fork.
Incorrect! Try again.
46A Pay-to-Witness-Script-Hash (P2WSH) output is created for a 2-of-2 multisig script. How is this transaction's output script (scriptPubKey) represented on the Bitcoin blockchain, and what data must the spender provide in the transaction's witness field?
Bitcoin scripting language and their use
Hard
A.The scriptPubKey is OP_HASH160 <20-byte-hash-of-script> OP_EQUAL. The spender provides the two signatures and the full redeem script in the scriptSig.
B.The scriptPubKey is empty. The spender provides the two signatures, the two public keys, and the script hash in the witness field.
C.The scriptPubKey contains the full script: <2 pubkeyA pubkeyB 2 OP_CHECKMULTISIG>. The spender provides only the two signatures in the witness field.
D.The scriptPubKey is OP_0 <32-byte-SHA256-hash-of-script>. The spender provides the two signatures followed by the full serialized redeem script in the witness field.
Correct Answer: The scriptPubKey is OP_0 <32-byte-SHA256-hash-of-script>. The spender provides the two signatures followed by the full serialized redeem script in the witness field.
Explanation:
SegWit introduced new output types. For P2WSH (the SegWit equivalent of P2SH), the scriptPubKey is a version byte (OP_0) followed by the 32-byte SHA256 hash of the redeem script. This is different from legacy P2SH which uses OP_HASH160 and a 20-byte hash. To spend this output, the transaction's scriptSig must be empty. The spending logic is provided entirely in the witness field, which must contain the signatures required by the script (in this case, two signatures) and, finally, the full redeem script itself. The node verifies by first hashing the provided redeem script to ensure it matches the 32-byte hash in the scriptPubKey, and then executing the redeem script with the provided signatures.
Incorrect! Try again.
47How does the concept of 'finality' in Bitcoin's Nakamoto Consensus (Proof-of-Work) fundamentally differ from finality in a classical consensus algorithm like Practical Byzantine Fault Tolerance (PBFT)?
Alternatives to Bitcoin consensus
Hard
A.Bitcoin provides probabilistic finality that grows stronger with each confirmation, whereas PBFT provides absolute (deterministic) finality once a round of voting is complete, assuming less than 1/3 of nodes are malicious.
B.Both provide probabilistic finality, but Bitcoin's is based on economic incentives while PBFT's is based on cryptographic signatures.
C.Bitcoin provides absolute finality after 6 confirmations, while PBFT finality is always probabilistic.
D.Finality in Bitcoin is achieved when a block is broadcast, while in PBFT it is achieved only after a supermajority of nodes have mined a subsequent block.
Correct Answer: Bitcoin provides probabilistic finality that grows stronger with each confirmation, whereas PBFT provides absolute (deterministic) finality once a round of voting is complete, assuming less than 1/3 of nodes are malicious.
Explanation:
This is a core distinction. In Bitcoin, a transaction is never 100% final. With each new block added on top (a 'confirmation'), the probability of a reorganization deep enough to reverse the transaction decreases exponentially, approaching (but never reaching) zero. This is probabilistic finality. In PBFT, consensus is reached through a multi-stage voting process among a known set of validators. Once a supermajority (2/3 + 1) of validators agree on a block, it is considered final, and the protocol guarantees it will not be reverted unless more than 1/3 of the validators are Byzantine (malicious or faulty). This is absolute or deterministic finality within the model's assumptions.
Incorrect! Try again.
48The Bitcoin difficulty adjustment algorithm retargets every 2016 blocks. If a sudden and sustained 40% decrease in network hashrate occurs immediately following a difficulty adjustment, what will be the approximate average block time during the next 2016-block epoch?
Proof of work
Hard
A.6 minutes
B.16.67 minutes
C.10 minutes
D.14 minutes
Correct Answer: 16.67 minutes
Explanation:
The difficulty is fixed for the 2016-block period. A 40% decrease means the new hashrate is 60% (or 0.6) of the old hashrate. Since block time is inversely proportional to hashrate, the new average block time will be the old time divided by the new hashrate factor. New Time = (Old Time) / (New Hashrate Multiplier) = 10 minutes / 0.6 = 16.67 minutes. The difficulty will only adjust downwards to correct this after this slow 2016-block period has completed.
Incorrect! Try again.
49In the Bitcoin Lightning Network, what is the primary role of the to_self_delay parameter in the script of a commitment transaction output, and why is it crucial for the security of a cheating party's funds?
Challenges and solutions
Hard
A.It defines the maximum amount of time a payment (HTLC) can be in transit before it is automatically cancelled.
B.It is a network-wide constant that prevents transactions from being confirmed too quickly, reducing the risk of chain re-organizations.
C.It forces the cheating party to wait for a specified period before they can claim their own funds from a broadcasted old state, giving the other party a window to broadcast a penalty transaction.
D.It sets the expiration date for the entire payment channel, after which all funds are returned to their original owners.
Correct Answer: It forces the cheating party to wait for a specified period before they can claim their own funds from a broadcasted old state, giving the other party a window to broadcast a penalty transaction.
Explanation:
When a party broadcasts a commitment transaction (especially a revoked, old one), the output paying back to themselves is not immediately spendable. It is locked by a script containing a relative timelock (e.g., OP_CHECKSEQUENCEVERIFY). This to_self_delay (e.g., 144 blocks) means that even if they broadcast an old state to cheat, they cannot access their own part of the funds until that delay has passed. Crucially, this output also has a second spending path: it can be spent immediately by the other party if they possess the revocation key for that specific state. This delay period is therefore the 'justice window' where the honest party (or their watchtower) can detect the fraud and sweep all the funds in the channel as a penalty.
Incorrect! Try again.
50A Bitcoin block header contains a Merkle Root, which is a hash of all transactions in the block. If an attacker could find a SHA-256 collision (i.e., two different transaction datasets T1 and T2 where HASH256(T1) = HASH256(T2)), how could they exploit this to defraud a user?
Bitcoin blockchain
Hard
A.They could create a block that is valid for both T1 and T2, show the user a block explorer confirming T1 (a payment to the user), but broadcast the block containing T2 (a payment to the attacker) to the network.
B.They could use the collision to create a single transaction that appears to pay two different addresses with the same funds.
C.They could mine a block much faster because finding the collision reduces the nonce search space.
D.This is a purely theoretical attack because the double-SHA256 (HASH256) used for txids is fundamentally immune to collision attacks, unlike single-SHA256.
Correct Answer: They could create a block that is valid for both T1 and T2, show the user a block explorer confirming T1 (a payment to the user), but broadcast the block containing T2 (a payment to the attacker) to the network.
Explanation:
The Merkle root in the block header commits to the set of transaction hashes (txids). If an attacker finds a collision where two different transactions T1 and T2 result in the same txid, they can build a Merkle tree that is valid for both. They could get the user to agree to a payment in T1. The attacker would then mine a block containing T2 but with the same Merkle root. The block header's hash and proof-of-work would be valid. They could then privately show the user evidence of the block containing T1 (which has a valid Merkle proof against the block header) while broadcasting the block containing T2 to the rest of the network, effectively stealing the funds. While computationally infeasible today, this demonstrates the critical importance of collision resistance in the hash function used for Merkle trees.
Incorrect! Try again.
51A 'long-range attack' is a specific vulnerability in Proof-of-Stake systems. It is most effective against which type of network participant?
Proof of stake
Hard
A.Existing, fully-synced validators who have been online for a long time.
B.Exchanges and large custodians who process a high volume of transactions.
C.New nodes that are syncing the blockchain for the first time or nodes that have been offline for an extended period.
D.Miners who are switching from a Proof-of-Work chain to a Proof-of-Stake chain.
Correct Answer: New nodes that are syncing the blockchain for the first time or nodes that have been offline for an extended period.
Explanation:
A long-range attack involves an attacker using old private keys (from a time when they controlled a large amount of stake) to create a long, alternative history from a point deep in the past. Because creating PoS blocks is cheap, they can generate this chain quickly. This attack is not effective against existing online nodes, as they have already accepted the legitimate chain and won't re-organize past a certain point. However, a brand new node syncing from genesis has no prior knowledge of the 'correct' chain. It might see the attacker's long, valid-looking (but fake) chain and accept it as the truth. This is why modern PoS protocols rely on social consensus and weak subjectivity checkpoints to provide new nodes with a trusted recent state to sync from, mitigating this attack.
Incorrect! Try again.
52The Taproot upgrade introduces Pay-to-Taproot (P2TR) outputs. In the case of a 'script-path spend', where one of the complex spending conditions from the Merkle tree is used, what must be included in the witness data in addition to the inputs required by the script itself?
Bitcoin scripting language and their use
Hard
A.The entire Merkle tree, containing all possible spending scripts.
B.The full public key of every potential participant in all possible script paths.
C.A signature from the 'internal key' proving that the key-path spend was not possible.
D.The specific script being executed, along with a Merkle proof (the branch of hashes) connecting that script's hash to the Merkle root that is committed to in the Taproot output.
Correct Answer: The specific script being executed, along with a Merkle proof (the branch of hashes) connecting that script's hash to the Merkle root that is committed to in the Taproot output.
Explanation:
A P2TR output's address commits to a tweaked public key, where the tweak is derived from an internal public key and the Merkle root of the possible spending scripts. For a cooperative 'key-path' spend, only a signature is needed. However, for a non-cooperative 'script-path' spend, the spender must prove that their desired spending condition was one of the ones originally committed to. To do this, they provide in the witness: 1) The inputs for the script they are using (e.g., signatures, preimages). 2) The full body of that script. 3) The Merkle proof, which consists of the sibling hashes needed to re-calculate the Merkle root. The verifying node uses the provided script and Merkle proof to recalculate the root, checks that it matches the commitment in the UTXO, and then executes the script.
Incorrect! Try again.
53In a 'selfish mining' attack, a mining pool with hashrate keeps its discovered blocks private. What is the primary game-theoretic reason this strategy can be more profitable than honest mining for ?
Proof of work
Hard
A.The selfish miner earns higher transaction fees by exclusively including high-fee transactions in their secret blocks.
B.By strategically releasing their blocks, they can create forks and orphan blocks found by the honest majority, causing the honest miners' effective hashrate to be wasted and increasing the selfish miner's relative share of rewards.
C.The selfish miner can perform double-spends in every secret block they mine, which is more profitable than the block reward.
D.By keeping blocks secret, the selfish miner solves them at a lower difficulty, as the public network difficulty has not yet adjusted to their hashrate.
Correct Answer: By strategically releasing their blocks, they can create forks and orphan blocks found by the honest majority, causing the honest miners' effective hashrate to be wasted and increasing the selfish miner's relative share of rewards.
Explanation:
The core of selfish mining profitability is not about earning more from a single block, but about increasing the miner's long-term share of total rewards beyond their share of hashrate (). When the selfish miner has a secret chain and the public chain finds a block, the selfish miner broadcasts their competing block. The network is now split. The selfish miner mines on their chain, while honest miners are split or mine on the public chain. If the selfish miner finds the next block, their chain is now longer and the honest block gets orphaned. This means the honest miners' work was wasted. By successfully orphaning blocks from the honest network, the selfish miner reduces the competition's revenue, thereby increasing their own proportional revenue above their direct hashrate contribution.
Incorrect! Try again.
54The coinbase transaction in a Bitcoin block has a unique rule: its outputs cannot be spent for 100 blocks. What is the primary security rationale behind this 'coinbase maturity' rule?
Bitcoin blockchain
Hard
A.To act as a mandatory staking period, forcing miners to have a long-term interest in the network's health.
B.To give the network time to verify the miner's identity and ensure they are not a malicious actor.
C.To mitigate the economic instability of a deep chain reorganization; it ensures that a miner cannot spend a block reward from a block that is later orphaned.
D.To prevent inflation by time-locking newly created bitcoin, ensuring a stable money supply.
Correct Answer: To mitigate the economic instability of a deep chain reorganization; it ensures that a miner cannot spend a block reward from a block that is later orphaned.
Explanation:
If a miner could spend their block reward immediately, and then a chain reorganization occurred that orphaned their block, the original coinbase transaction would become invalid. Consequently, any transaction that spent those coins would also become invalid, as would any transactions that spent those, and so on. This could cause a cascade of transaction invalidations across the network, creating chaos and economic loss. By requiring a 100-block wait, the rule ensures that a block is very deeply buried in the chain and thus extremely unlikely to ever be orphaned before its reward becomes spendable, preserving the stability of the ledger.
Incorrect! Try again.
55Delegated Proof of Stake (DPoS) systems, like EOS, achieve high throughput by limiting block production to a small, elected set of 'witnesses' or 'block producers'. What is the most significant centralization risk inherent in this design?
Alternatives to Bitcoin consensus
Hard
A.The voting process is susceptible to low voter turnout, leading to a system that does not reflect the will of the majority of token holders.
B.The high cost of server hardware required to be a block producer centralizes power among the wealthy.
C.DPoS is more vulnerable to 51% attacks than PoW because it is cheaper to buy votes than to acquire equivalent hashrate.
D.The small number of elected producers can form cartels to censor transactions, extract rent (e.g., through MEV), or collude to maintain their elected positions, reducing the system's permissionlessness and censorship resistance.
Correct Answer: The small number of elected producers can form cartels to censor transactions, extract rent (e.g., through MEV), or collude to maintain their elected positions, reducing the system's permissionlessness and censorship resistance.
Explanation:
The core trade-off of DPoS is sacrificing decentralization for performance. By having a small, fixed set of known block producers (e.g., 21), the system becomes highly efficient. However, this creates a de facto oligopoly. These producers can communicate off-chain and collude. They might agree to vote for each other to stay in power, blacklist transactions from certain users (censorship), or alter the protocol for their own benefit. This introduces a level of trust in a small group that is antithetical to the goals of maximum decentralization seen in systems like Bitcoin.
Incorrect! Try again.
56From a privacy and efficiency standpoint, what is the primary advantage of transaction 'batching' as practiced by large exchanges?
Challenges and solutions
Hard
A.It perfectly anonymizes the sender by mixing their inputs with those of many other users, similar to a CoinJoin.
B.It obscures the total amount of funds held by the exchange by breaking down large UTXOs into thousands of smaller ones.
C.It reduces the total blockchain space required per payment by combining many logical payments into one transaction, thus lowering the aggregate transaction fees for the exchange.
D.It increases the number of confirmations required for a payment to be considered final, enhancing security.
Correct Answer: It reduces the total blockchain space required per payment by combining many logical payments into one transaction, thus lowering the aggregate transaction fees for the exchange.
Explanation:
A Bitcoin transaction has a certain base size (for version, locktime, etc.) and then a size per input and per output. If an exchange processed 100 withdrawals as 100 separate transactions, they would pay the base size cost 100 times. By batching, they create a single transaction with potentially many inputs and 100 outputs. This transaction only has the base size cost once. While the total size is large, the average size per withdrawal is significantly smaller than it would be otherwise. This leads to a dramatic reduction in total transaction fees paid to miners, making it a highly efficient practice. While it has privacy implications (linking all outputs to the exchange), its main driver is economic efficiency.
Incorrect! Try again.
57The Bitcoin network difficulty target is a 256-bit number . A valid block hash must satisfy the condition . The maximum possible hash value is . What is the precise expected number of hashes required to find one valid block?
Proof of work
Hard
A.
B.
C.
D.Difficulty, where Difficulty =
Correct Answer: Difficulty, where Difficulty =
Explanation:
The probability of a single hash being valid is the ratio of the number of successful outcomes to the total number of possible outcomes. The number of successful outcomes is (from 0 to T-1), and the total number of outcomes is (or M+1). So, . The expected number of trials for one success is the reciprocal of the probability, . The official Bitcoin 'difficulty' metric is a relative measure defined as the ratio of the easiest possible target (, also called the difficulty_1_target) to the current target (). So, Difficulty = . The expected number of hashes is not just but is directly related to this difficulty metric, which scales this expectation. Option C is the most precise definition used in practice.
Incorrect! Try again.
58Consider this Bitcoin script: OP_SIZE <32> OP_EQUALVERIFY OP_SHA256 <expected_hash> OP_EQUAL. What is this script demanding from the spender in order to unlock the funds?
Bitcoin scripting language and their use
Hard
A.Two numbers that when added together equal 32, and their hash matches <expected_hash>.
B.Any piece of data that is exactly 32 bytes long.
C.A piece of data that, when hashed with SHA256, results in <expected_hash>, and the data itself must be exactly 32 bytes long.
D.A digital signature that is exactly 32 bytes long and matches the public key hash.
Correct Answer: A piece of data that, when hashed with SHA256, results in <expected_hash>, and the data itself must be exactly 32 bytes long.
Explanation:
This is a hash puzzle with a size constraint. Let's trace the stack with the spender's input <data>: 1. The <data> is pushed to the stack. 2. OP_SIZE pushes the size of <data> (in bytes) onto the stack. 3. <32> is pushed. Stack is [<data>, size_of_data, 32]. 4. OP_EQUALVERIFY checks if size_of_data equals 32. If not, the script fails. If it does, both values are consumed. Stack is [<data>]. 5. OP_SHA256 hashes the top item, replacing it with its hash. Stack is [hash(data)]. 6. <expected_hash> is pushed. 7. OP_EQUAL compares the two hashes. The script succeeds only if they match. This is a common pattern for revealing a pre-image to a hash (like in HTLCs), but with the added constraint that the pre-image must be exactly 32 bytes.
Incorrect! Try again.
59A user creates a transaction with an nLockTime set to a future block height (e.g., 800,000) and all its input nSequence numbers set to 0xFFFFFFFF. What is the status of this transaction?
Bitcoin blockchain
Hard
A.The transaction is valid but can only be included in block 800,000 and no other block.
B.The transaction is valid and can be included in any block immediately, as the nSequence value disables the nLockTime feature for this transaction.
C.The transaction is invalid and will be rejected by all nodes.
D.The transaction is held in the mempool by nodes but will not be included in a block until block 800,000 is mined.
Correct Answer: The transaction is valid and can be included in any block immediately, as the nSequence value disables the nLockTime feature for this transaction.
Explanation:
This question tests a nuanced interaction between two fields. The nLockTime field sets an absolute time-lock for a transaction. However, this lock is only enabled if at least one of the transaction's inputs has an nSequence number less than the maximum value of 0xFFFFFFFF. If all inputs have their nSequence number set to 0xFFFFFFFF, the nLockTime logic is completely disabled for that transaction. Therefore, despite the future nLockTime value, the transaction is considered final and can be included in a block immediately.
Incorrect! Try again.
60In a Proof-of-Stake system using a Verifiable Random Function (VRF) for leader election, what is the primary security advantage this method provides over a publicly predictable algorithm (e.g., one based on the hash of the last block)?
Proof of stake
Hard
A.It is significantly more computationally efficient, reducing the carbon footprint of the network.
B.It prevents validators from manipulating the block data to increase their own chances of being selected in the next round.
C.It ensures that stakers with more stake have a proportionally higher chance of being selected.
D.It allows a validator to prove they were chosen legitimately without revealing in advance who the next validator would be, thus mitigating targeted DDoS attacks.
Correct Answer: It allows a validator to prove they were chosen legitimately without revealing in advance who the next validator would be, thus mitigating targeted DDoS attacks.
Explanation:
A major challenge in PoS is preventing attacks on the next block producer. If the selection algorithm is publicly predictable, an attacker knows exactly which node to target with a DDoS attack to stall the network. A VRF solves this by being non-predictable. Each validator can privately use their secret key and a public seed to compute a random output. Only they know if their output 'wins' the slot. No one else can predict the winner in advance. When the winner produces the block, they publish a proof along with it. Anyone can then use the validator's public key to verify the proof, confirming they were chosen legitimately. This provides verifiability without predictability.