Certified Blockchain Expert (CBE) — Questions and Answers

Question 1: In decentralized finance (DeFi), what is a 'flash loan' and why is it unique to blockchain?

A loan issued via SMS to underbanked populations
An uncollateralized loan that must be borrowed and repaid within a single transaction block (Correct answer)
A mortgage approved in under 24 hours
A government-backed loan processed instantly by banks

Correct answer: An uncollateralized loan that must be borrowed and repaid within a single transaction block

Flash loans exploit smart contract atomicity: if the loan isn't repaid within the same transaction, the entire operation reverts, eliminating default risk.

Question 2: What is the role of a cryptographic hash function's 'avalanche effect' in blockchain security?

It compresses transaction data to reduce block size
It speeds up block validation by batching hashes
It generates multiple hashes per block for redundancy
A small change in input produces a drastically different output hash, preventing predictability (Correct answer)

Correct answer: A small change in input produces a drastically different output hash, preventing predictability

The avalanche effect ensures that even a single-bit change in input results in a completely different hash, making tampering immediately detectable.

Question 3: For cryptocurrencies, which of the following challenges did Blockchain solve?

Anonymity (Correct answer)
Double Spending
Destination of currencies
None of the above

Correct answer: Anonymity

Explanation: <br> The fundamental difficulty is that the anonymity-provisioning mechanism must allow other members to verify updates to the blockchain. Most blockchain-based e-cash protocols' pseudonymous feature is insufficient for users to achieve identification and transaction privacy.

Question 4: Which consensus mechanism does Ethereum use after 'The Merge' in 2022?

Proof of Authority
Proof of Work
Proof of Stake (Correct answer)
Delegated Proof of Stake

Correct answer: Proof of Stake

After The Merge, Ethereum switched from Proof of Work to Proof of Stake, where validators stake 32 ETH to propose and attest blocks.

Question 5: What is 'front-running' in the context of blockchain transaction ordering?

Adding transactions to a block before verifying their signatures
Observing a pending transaction in the mempool and submitting a competing one with higher gas to be processed first (Correct answer)
Mining empty blocks to delay competitors' transactions
Broadcasting a transaction on multiple chains simultaneously

Correct answer: Observing a pending transaction in the mempool and submitting a competing one with higher gas to be processed first

Front-running exploits the public mempool by inserting a transaction with higher gas fees ahead of a target transaction to capture arbitrage or MEV profits.

Question 6: What is 'contract upgradeability' and which pattern is most commonly used to achieve it?

Delegating calls to a swappable logic contract via a persistent proxy; the proxy pattern (Correct answer)
Using Ethereum's built-in upgrade opcode; UPGRADECONTRACT
Storing code on IPFS and updating the hash; content-addressed storage
Redeploying a new contract and migrating all state; direct redeployment

Correct answer: Delegating calls to a swappable logic contract via a persistent proxy; the proxy pattern

The proxy pattern keeps a stable address and storage while delegating logic calls to an implementation contract that can be swapped, enabling upgrades without migrating state.

Question 7: What is a commitment scheme in cryptography, and how is it used in blockchain protocols?

A legal agreement between miners
A method to bind a value secretly, then reveal it later without being able to change it (Correct answer)
A way to compress the blockchain state
A technique to batch-sign multiple transactions

Correct answer: A method to bind a value secretly, then reveal it later without being able to change it

A commitment scheme allows a party to commit to a value (hiding it) and later open (reveal) it, with the guarantee it cannot be altered after committing.

Question 8: In the context of decentralized storage, how does blockchain enhance data resilience compared to centralized cloud storage?

By storing all files on a single high-performance server
By requiring government approval for file uploads
By distributing encrypted file shards across many nodes so no single point of failure exists (Correct answer)
By compressing files to zero bytes on-chain

Correct answer: By distributing encrypted file shards across many nodes so no single point of failure exists

Networks like Filecoin or Storj split and distribute encrypted data shards so the network remains functional even if many nodes go offline.

Question 9: Which blockchain network type requires participants to be vetted and approved before joining?

Decentralized blockchain
Open-source blockchain
Public blockchain
Permissioned blockchain (Correct answer)

Correct answer: Permissioned blockchain

Permissioned blockchains require identity verification and approval before a participant can join the network.

Question 10: What is stealth addressing in privacy-focused blockchains?

Using zero-knowledge proofs to hide wallet balances
Replacing wallet addresses with usernames to avoid tracking
Generating a unique one-time address for each transaction so the recipient's master address is never published on-chain (Correct answer)
Encrypting the sender's address using the miner's public key

Correct answer: Generating a unique one-time address for each transaction so the recipient's master address is never published on-chain

Stealth addresses allow senders to derive a fresh address for each transaction using the recipient's public key, ensuring payments cannot be linked to a known address.

Question 11: Which type of oracle attack manipulates price feeds to exploit DeFi smart contracts?

Selfish mining
Flash loan price manipulation (Correct answer)
Eclipse attack
Sybil attack

Correct answer: Flash loan price manipulation

Attackers use flash loans to temporarily skew on-chain price feeds, tricking smart contracts into executing trades at artificially inflated or deflated prices within a single block.

Question 12: What is the significance of the 'longest chain rule' (Nakamoto Consensus) in Bitcoin?

Validators vote on which chain is longest at each epoch
Nodes always follow the chain with the most cumulative proof-of-work as the valid chain (Correct answer)
Blocks with the most transactions are always prioritized
The chain that has been running the longest in time is considered valid

Correct answer: Nodes always follow the chain with the most cumulative proof-of-work as the valid chain

Nodes adopt the chain representing the greatest cumulative computational work, making it economically infeasible for attackers to rewrite history.

Question 13: In Ethereum, what is the purpose of the ABI (Application Binary Interface)?

To establish consensus between nodes
To define how to encode and decode function calls and data (Correct answer)
To store contract bytecode on-chain
To manage gas fee calculations

Correct answer: To define how to encode and decode function calls and data

The ABI defines the encoding scheme for function signatures and parameters, allowing external callers to interact with a deployed smart contract.

Question 14: What is 'phishing via fake token approval' and how does it compromise DeFi users?

Creating lookalike token contracts with similar names to steal swap fees
Tricking users into signing unlimited token approval transactions that grant malicious contracts full spending authority (Correct answer)
Sending fake airdrop tokens that execute malicious code when transferred to a DEX
Impersonating MetaMask pop-ups to capture seed phrases during approval flows

Correct answer: Tricking users into signing unlimited token approval transactions that grant malicious contracts full spending authority

Malicious sites prompt users to approve unlimited token allowances to attacker contracts, which can then drain approved tokens at any time without further user interaction.

Question 15: What security risk does 'tx.origin' pose compared to 'msg.sender' in Solidity?

tx.origin exposes validator addresses making them targets for slashing
tx.origin is deprecated and causes contract compilation failures
tx.origin reveals the original EOA, making phishing contracts able to pass authorization checks (Correct answer)
tx.origin leaks private key data to other contracts in the call chain

Correct answer: tx.origin reveals the original EOA, making phishing contracts able to pass authorization checks

Using tx.origin for authentication allows malicious intermediary contracts to impersonate the original caller, bypassing security checks.

Question 16: Which Ethereum scaling solution uses fraud proofs and requires a challenge period for withdrawals?

Plasma Chain
Optimistic Rollup (Correct answer)
State Channel
ZK-Rollup

Correct answer: Optimistic Rollup

Optimistic Rollups assume transactions are valid by default and rely on fraud proofs during a 7-day challenge window before finality on L1.

Question 17: Which blockchain characteristic makes it most suitable for a government-issued digital currency (CBDC)?

Anonymous and decentralized
Permissioned with central bank control (Correct answer)
Governed by a miner consortium
Fully public and permissionless

Correct answer: Permissioned with central bank control

CBDCs require central bank oversight and KYC compliance, so a permissioned network with controlled access is most appropriate.

Question 18: Which feature is most associated with permissionless blockchains?

Identity verification required to join
Transactions are kept confidential from non-members
Anyone can participate in transaction validation (Correct answer)
A central administrator controls the nodes

Correct answer: Anyone can participate in transaction validation

Permissionless blockchains allow anyone to join, submit transactions, and participate in consensus without needing approval.

Question 19: In the context of Proof of Work (PoW) blockchains, what does the term 'probabilistic finality' mean?

A transaction is considered final as soon as it is included in a block.
There is a 50/50 chance that any given transaction will be reversed.
The network's validators vote to finalize each block, making it irreversible.
The certainty of a transaction being irreversible increases as more blocks are added after it, but never reaches 100%. (Correct answer)

Correct answer: The certainty of a transaction being irreversible increases as more blocks are added after it, but never reaches 100%.

Probabilistic finality, a characteristic of Nakamoto consensus (used in PoW), means that the likelihood of a transaction being reversed decreases as more blocks are built on top of the block containing it. While the probability of a reversal becomes extremely low after several confirmations (e.g., 6 blocks in Bitcoin), it never becomes an absolute certainty. This is in contrast to mechanisms like pBFT which offer absolute or deterministic finality.

Question 20: Which of the following best describes a stablecoin?

A cryptocurrency with low transaction volume
A blockchain token that cannot be transferred
A token whose value is pegged to a stable asset like USD or gold (Correct answer)
A token exclusively used for paying network fees

Correct answer: A token whose value is pegged to a stable asset like USD or gold

A stablecoin is a cryptocurrency designed to maintain a stable value by pegging to a reference asset such as the US dollar or a commodity.

Question 21: Which concept describes bundling multiple smart contract interactions into one atomic transaction?

Batch minting
Multi-sig approval
Multicall (Correct answer)
Delegate call

Correct answer: Multicall

Multicall allows a single transaction to execute multiple contract calls atomically, saving gas and ensuring all-or-nothing execution.

Question 22: What is 'front-running' in the context of smart contracts and blockchain?

Forking a blockchain to roll back an unfavorable transaction result
Submitting the same transaction twice to guarantee at least one confirmation
Inserting a transaction ahead of a pending one after observing it in the mempool to profit (Correct answer)
Deploying a contract before a competitor's identical contract to claim priority

Correct answer: Inserting a transaction ahead of a pending one after observing it in the mempool to profit

Front-runners monitor the mempool, then pay a higher gas price to have their transaction mined first, exploiting information about a pending trade.

Question 23: What role does a blockchain explorer serve for end users?

It stores private keys securely for hardware wallets
It mines new blocks on behalf of users
It provides a searchable interface to view transactions, blocks, and wallet addresses on the blockchain (Correct answer)
It converts cryptocurrency between different blockchains

Correct answer: It provides a searchable interface to view transactions, blocks, and wallet addresses on the blockchain

Blockchain explorers like Etherscan or Blockchain.info let users look up transaction status, wallet balances, and block details in real time.

Question 24: A Layer-2 blockchain solution primarily aims to solve which problem of the base (Layer-1) blockchain?

Scalability and transaction throughput limitations (Correct answer)
Inability to store large files
Absence of native tokens
Lack of smart contract support

Correct answer: Scalability and transaction throughput limitations

Layer-2 solutions process transactions off the main chain to increase throughput and reduce fees while inheriting Layer-1 security.

Question 25: What does the ERC-20 standard primarily define?

A common interface for fungible tokens on Ethereum (Correct answer)
Rules for non-fungible tokens on Ethereum
The Ethereum consensus mechanism specification
A standard for cross-chain atomic swaps

Correct answer: A common interface for fungible tokens on Ethereum

ERC-20 defines a standard interface (functions like transfer, approve, allowance) for fungible tokens so wallets and exchanges can interact with any compliant token uniformly.

Question 26: What does 'gas limit' represent in an Ethereum transaction?

The maximum amount of computational work the sender authorizes (Correct answer)
The maximum ETH price the sender will pay per unit of gas
The block size limit enforced by miners
The total ETH transferred to the recipient

Correct answer: The maximum amount of computational work the sender authorizes

The gas limit is the maximum units of computation the sender permits for the transaction; unused gas is refunded and exhausted gas causes a revert.

Question 27: Which of the following is a primary characteristic of the Delegated Proof of Stake (DPoS) consensus mechanism?

All token holders directly participate in validating blocks.
Miners compete to solve complex computational puzzles.
Validators are chosen based on the age of their staked coins.
Token holders vote to elect a limited number of block producers. (Correct answer)

Correct answer: Token holders vote to elect a limited number of block producers.

In Delegated Proof of Stake (DPoS), token holders use their stake to vote for a limited number of 'delegates' or 'witnesses'. These elected officials are then responsible for validating transactions and producing blocks. This system is designed to be more efficient and faster than traditional Proof of Stake.

Question 28: A DeFi protocol's smart contract has a function that (1) checks a user's balance, (2) sends them their requested funds, and then (3) updates their balance. An attacker exploits this by creating a malicious contract that, upon receiving funds, immediately calls the withdrawal function again before the original call can update the balance. This vulnerability is known as a:

Timestamp Dependence Attack
Denial of Service Attack
Reentrancy Attack (Correct answer)
Integer Overflow Attack

Correct answer: Reentrancy Attack

A Reentrancy Attack occurs when an external call is made to an untrusted contract before the original function resolves its state changes. The malicious contract can then 're-enter' the original function, repeatedly executing a part of it (like a withdrawal) before the state (like the user's balance) is updated. The famous DAO hack of 2016 was a reentrancy attack.

Question 29: How does a 'BGP hijacking' attack threaten Bitcoin mining pools?

Using BGP routing to prioritize attacker blocks over honest miner blocks
Poisoning DNS cache entries for mining pool hostnames via BGP communities
Rerouting internet traffic to intercept pool communications, allowing attackers to steal hashrate or delay block propagation (Correct answer)
Exploiting the Bitcoin Gossip Protocol to inject false peer announcements

Correct answer: Rerouting internet traffic to intercept pool communications, allowing attackers to steal hashrate or delay block propagation

BGP hijacking can redirect mining pool traffic through attacker-controlled routers, enabling hashrate theft, block withholding, or network partitioning.

Question 30: The Practical Byzantine Fault Tolerance (PBFT) consensus mechanism is known for providing high performance and low-latency transaction finality. What is its primary limitation, making it less suitable for large, open, permissionless blockchains?

Its performance degrades significantly as the number of nodes increases due to high communication overhead. (Correct answer)
High energy consumption due to complex computations.
The requirement for all nodes to stake a large amount of cryptocurrency.
Slow block confirmation times, often taking several minutes.

Correct answer: Its performance degrades significantly as the number of nodes increases due to high communication overhead.

Practical Byzantine Fault Tolerance (PBFT) requires a high volume of messages to be sent between nodes to reach consensus. The communication complexity is O(n^2), meaning the number of messages grows exponentially with the number of nodes (n). This makes it inefficient and unscalable for large, public networks like Bitcoin or Ethereum but very effective for smaller, permissioned systems where the number of validators is known and limited.

Question 31: A consortium of private companies wants to establish a blockchain for supply chain management. They require high transaction throughput, low latency, and a mechanism where block producers are known and trusted entities. Which consensus mechanism is most suitable for this scenario?

Proof of Authority (PoA) (Correct answer)
Proof of Stake (PoS)
Proof of Burn (PoB)
Proof of Work (PoW)

Correct answer: Proof of Authority (PoA)

Proof of Authority (PoA) is ideal for private or consortium blockchains where participants are known and trusted. In PoA, validators are pre-approved entities who stake their reputation, not computational power or coins. This results in high throughput and efficiency, which is perfect for enterprise use cases like supply chain management where performance and accountability are key.

Question 32: In a Proof of Stake (PoS) consensus mechanism, what is the primary role of a 'validator'?

To solve complex mathematical puzzles faster than other participants.
To vote for delegates who will then be responsible for securing the network.
To propose and verify new blocks of transactions after being selected based on their economic stake. (Correct answer)
To provide storage space on their hard drives to secure the network.

Correct answer: To propose and verify new blocks of transactions after being selected based on their economic stake.

In a PoS system, validators are responsible for the core functions of maintaining the blockchain. They are chosen to create new blocks and validate transactions based on the amount of cryptocurrency they have staked as collateral. Unlike PoW miners, they do not compete by solving puzzles. Their stake acts as an incentive to act honestly, as malicious behavior can result in the loss of their staked funds (a process known as 'slashing').

Question 33: What is the Ethereum Virtual Machine (EVM)?

A hardware device for storing ETH securely
A cloud server that hosts the Ethereum network
A virtual machine for running Ethereum node software
A Turing-complete runtime environment that executes smart contract bytecode (Correct answer)

Correct answer: A Turing-complete runtime environment that executes smart contract bytecode

The EVM is a sandboxed, stack-based virtual machine embedded in every Ethereum node that executes smart contract code deterministically.

Question 34: In asymmetric cryptography, if Alice wants to send Bob a confidential message, which key does she use to encrypt it?

Alice's private key
Bob's private key
Alice's public key
Bob's public key (Correct answer)

Correct answer: Bob's public key

Alice encrypts with Bob's public key so that only Bob, holding the corresponding private key, can decrypt the message.

Question 35: What is the purpose of transaction fees in a blockchain network?

To pay for the electricity used in verification
To replace the block reward after all coins are mined
To incentivize miners/validators to include transactions in blocks and prevent spam (Correct answer)
To fund the blockchain foundation's development

Correct answer: To incentivize miners/validators to include transactions in blocks and prevent spam

Transaction fees reward miners or validators for processing transactions and also deter spam by making mass frivolous transactions costly.

Question 36: In Hyperledger Fabric, what is the role of the 'orderer' node?

To validate node permissions on the network
To establish consensus by ordering transactions into blocks (Correct answer)
To execute chaincode (smart contracts)
To store user identity certificates

Correct answer: To establish consensus by ordering transactions into blocks

The orderer service in Hyperledger Fabric is responsible for collecting transactions, ordering them, and packaging them into blocks for distribution.

Question 37: What is a 'Soulbound Token' (SBT) as proposed by Vitalik Buterin?

A non-transferable NFT permanently linked to a specific wallet as a form of digital identity or credential (Correct answer)
A token that gains value automatically over time through built-in inflation mechanics
A governance token that grants voting rights proportional to holding duration
A token that burns itself after one use to prevent double-spending

Correct answer: A non-transferable NFT permanently linked to a specific wallet as a form of digital identity or credential

SBTs are non-transferable tokens that represent credentials, achievements, or memberships tied to an individual's on-chain identity, similar to a digital resume.

Question 38: A smart contract is designed to trigger an insurance payout for a flight delay. To function correctly, it needs to know the official landing time of a specific flight. How does a smart contract on a deterministic blockchain typically obtain this type of real-world information?

By using a built-in time function within the EVM to access real-world clocks.
Through a trusted external data feed service known as a blockchain oracle. (Correct answer)
By allowing users to vote on the landing time after the fact.
By directly querying a public flight tracking API from within the contract code.

Correct answer: Through a trusted external data feed service known as a blockchain oracle.

Blockchains and smart contracts cannot directly access off-chain data because it would break the deterministic nature required for consensus. A blockchain oracle is a necessary third-party service that acts as a bridge, fetching and verifying real-world data (like flight times or asset prices) and relaying it to the smart contract in a secure and reliable way.

Question 39: What is a 'flash loan' in DeFi?

A loan that charges very low interest rates
A loan processed instantly with no KYC
An uncollateralized loan that must be borrowed and repaid within the same transaction (Correct answer)
A loan backed by NFT collateral

Correct answer: An uncollateralized loan that must be borrowed and repaid within the same transaction

Flash loans are uncollateralized DeFi loans that must be borrowed and repaid within a single blockchain transaction, or the entire transaction reverts.

Question 40: In a threshold signature scheme (e.g., t-of-n multisig via Shamir's Secret Sharing), what must be true to reconstruct the secret?

Shares must be combined in a specific order
Any t shares out of n are sufficient (Correct answer)
All n shares must be combined
The dealer must participate in reconstruction

Correct answer: Any t shares out of n are sufficient

Shamir's Secret Sharing is designed so that any t (threshold) shares are sufficient to reconstruct the secret, while fewer than t reveal nothing.

Question 41: A major criticism of the Proof of Work (PoW) consensus mechanism is its high energy consumption. Which of the following consensus mechanisms was primarily designed as a more energy-efficient alternative by replacing computational work with an economic stake?

Delegated Proof of Stake (DPoS)
Proof of Stake (PoS) (Correct answer)
Proof of Authority (PoA)
Proof of Burn (PoB)

Correct answer: Proof of Stake (PoS)

Proof of Stake (PoS) was developed to address the high energy consumption of PoW. Instead of requiring miners to perform energy-intensive computations, PoS selects validators to create new blocks based on the number of coins they hold and are willing to 'stake' as collateral. This method is significantly more energy-efficient.

Question 42: What is the 'block reward halving' event in Bitcoin, and what is its economic purpose?

A community vote to reduce miner income during bear markets
A periodic reduction of the Bitcoin mining reward by 50%, controlling the issuance rate toward a fixed supply cap (Correct answer)
An automatic adjustment triggered when Bitcoin's price doubles
A scheduled event that doubles transaction fees to fund network security

Correct answer: A periodic reduction of the Bitcoin mining reward by 50%, controlling the issuance rate toward a fixed supply cap

Every ~210,000 blocks (~4 years), Bitcoin's block reward halves, slowing new coin issuance and enforcing the 21 million BTC supply cap.

Question 43: What vulnerability did the 2016 DAO hack on Ethereum exploit?

A backdoor in the Ethereum compiler that generated incorrect bytecode
Integer overflow allowing an attacker to claim more tokens than deposited
Reentrancy: the withdraw function called an external contract before updating the internal balance (Correct answer)
A private key compromise of the DAO's multisig wallet administrators

Correct answer: Reentrancy: the withdraw function called an external contract before updating the internal balance

The DAO attacker repeatedly called the withdraw function before the balance was updated, draining ~3.6 million ETH through recursive external calls.

Question 44: What is 'miner extractable value' (MEV) and why is it a security concern?

The computational value miners extract from solving hash puzzles during PoW
Profits miners capture by reordering, inserting, or censoring transactions, creating unfair advantages and potential instability (Correct answer)
Fees extracted from smart contracts during automated liquidations
Revenue validators earn from staking rewards beyond the base issuance rate

Correct answer: Profits miners capture by reordering, inserting, or censoring transactions, creating unfair advantages and potential instability

MEV allows block producers to exploit their ordering power to profit at users' expense, leading to network congestion, unfairness, and potential consensus instability.

Question 45: What is an 'oracle' in the context of smart contract architecture?

A cryptographic algorithm for key generation
A node that has full governance rights over a blockchain
A service that provides off-chain real-world data to on-chain smart contracts (Correct answer)
A type of wallet for storing NFTs

Correct answer: A service that provides off-chain real-world data to on-chain smart contracts

Oracles bridge the gap between blockchains and the external world by feeding real-world data (prices, weather, sports results) to smart contracts that cannot access it natively.

Question 46: In Hierarchical Deterministic (HD) wallets, what cryptographic function derives child keys from a parent key?

RSA key derivation
HMAC-SHA512 (Correct answer)
ECDSA signing
AES-CBC encryption

Correct answer: HMAC-SHA512

HD wallets use HMAC-SHA512 with the parent key and chain code to derive child private and public keys deterministically (BIP-32).

Question 47: Which consensus mechanism requires participants to demonstrate their commitment to the network by permanently destroying a certain amount of cryptocurrency?

Proof of Elapsed Time
Proof of Stake (PoS)
Proof of Capacity
Proof of Burn (PoB) (Correct answer)

Correct answer: Proof of Burn (PoB)

Proof of Burn (PoB) is a consensus mechanism where participants, often called miners or validators, intentionally send cryptocurrency to a verifiably unspendable address. This act of 'burning' coins serves as an economic sacrifice, demonstrating a long-term commitment to the network and granting the right to validate transactions and mine new blocks.

Question 48: What is a 'flash loan attack' and which property makes flash loans uniquely risky?

Flooding the network with rapid small loans to exhaust liquidity pools
Quickly withdrawing staked ETH before a slashing event is recorded
Borrowing large uncollateralized sums within one transaction to manipulate prices and repay, possible because atomicity guarantees repayment (Correct answer)
Borrowing tokens across chains using bridges faster than price oracles update

Correct answer: Borrowing large uncollateralized sums within one transaction to manipulate prices and repay, possible because atomicity guarantees repayment

Flash loans are uncollateralized because they must be repaid in the same transaction; attackers use the temporary capital to manipulate DeFi protocols profitably.

Question 49: In post-quantum cryptography, why are current blockchain signature schemes (ECDSA, BLS) considered at risk?

Quantum computers eliminate the need for hashing entirely
Grover's algorithm reduces hash output size to zero
Shor's algorithm on a sufficiently large quantum computer can solve the elliptic curve discrete logarithm problem, breaking ECDSA and BLS (Correct answer)
Quantum computers can factor large primes faster, breaking SHA-256 directly

Correct answer: Shor's algorithm on a sufficiently large quantum computer can solve the elliptic curve discrete logarithm problem, breaking ECDSA and BLS

Shor's algorithm efficiently solves the discrete logarithm problem, which underpins the security of both ECDSA and BLS signatures used in blockchain.

Question 50: Which consensus attack requires controlling more than one-third of stake in a Proof-of-Stake system?

Finality delay or safety failure attacks requiring ≥1/3 of stake to prevent or break consensus (Correct answer)
Sybil attacks requiring 33 separate validator identities
Double-spend attacks requiring exactly 34% of the network stake
Long-range attacks requiring 33% historical validator keys

Correct answer: Finality delay or safety failure attacks requiring ≥1/3 of stake to prevent or break consensus

In BFT-based PoS systems like Ethereum's Casper, controlling ≥1/3 of stake lets an attacker prevent finality or, with ≥2/3, revert finalized blocks.

Question 51: Which of the following can be used to protect anonymity in blockchain applications?

Centralized application hosting
End-to-end encryption (Correct answer)
Blockchain-based data cubes
Cryptographic hashing

Correct answer: End-to-end encryption

Explanation: <br> The goal of end-to-end encryption is to prevent data from being read or secretly manipulated by anybody other than the sender and recipient (s). The sender encrypts the messages, but the third party has no way of decrypting them and stores them encrypted. The encrypted data is retrieved and decrypted by the recipient.

Question 52: A malicious actor gains control of 52% of the total hashing power on a Proof of Work (PoW) network. Which of the following actions can this actor successfully perform?

Change the fundamental consensus rules of the blockchain, such as the block reward amount.
Prevent specific new transactions from being confirmed and potentially reverse their own recent transactions. (Correct answer)
Create new coins out of thin air, exceeding the protocol's supply limit.
Steal cryptocurrency directly from another user's wallet.

Correct answer: Prevent specific new transactions from being confirmed and potentially reverse their own recent transactions.

A 51% attack on a PoW network gives the attacker control over the block creation process. This allows them to censor transactions by refusing to include them in their blocks and to perform a 'double-spend' by creating a secret longer chain where they reverse their own transactions and then broadcasting it to the network. However, they cannot steal funds from addresses they don't own the private keys to, change the core protocol rules, or create coins outside of the established block reward system.

Question 53: A blockchain network aims to be highly energy-efficient and reduce the hardware barrier to entry for participation. However, a major criticism is that it could lead to a 'rich get richer' scenario where the wealthiest participants have the most influence. Which consensus mechanism does this describe?

Practical Byzantine Fault Tolerance (PBFT)
Proof of Work (PoW)
Proof of Burn (PoB)
Proof of Stake (PoS) (Correct answer)

Correct answer: Proof of Stake (PoS)

Proof of Stake (PoS) selects validators based on the number of tokens they hold and are willing to 'stake'. This is highly energy-efficient compared to PoW. However, a common criticism is that individuals with more tokens (the 'richest') have a higher chance of being selected to validate blocks and earn rewards, potentially leading to wealth concentration and centralization of power.

Question 54: Which of the following best describes a zero-knowledge proof in a blockchain context?

A technique for compressing transaction data
A method to encrypt block headers
A consensus mechanism for lightweight nodes
Proving knowledge of information without revealing the information itself (Correct answer)

Correct answer: Proving knowledge of information without revealing the information itself

A zero-knowledge proof lets a prover convince a verifier that a statement is true without disclosing any underlying secret data.

Question 55: The secret, the function, the key, and the _____ are the essential components of any cryptographic system.

Hash (Correct answer)
Token
Cipher
None of the above

Correct answer: Hash

Explanation: <br> When discussing blockchain technology, the word hashing or (hash) is commonly used. Hashing is the process of transforming and generating any length of input data into a fixed-length string using a specific algorithm. The Bitcoin hash algorithm is SHA-256, which stands for Secure Hashing Algorithm 256 bits. Because the original data cannot be decrypted, this approach is a one-way cryptographic function.

Question 56: What is a 'replay attack' in the context of blockchain transactions, and which cryptographic element prevents it?

Reusing hash values; prevented by SHA-256
Broadcasting a valid signed transaction on another chain; prevented by chain ID in signatures (Correct answer)
Reusing old blocks; prevented by Merkle roots
Duplicating wallet keys; prevented by ECC

Correct answer: Broadcasting a valid signed transaction on another chain; prevented by chain ID in signatures

A replay attack reuses a signed transaction on a different chain; including the chain ID in the signed data (EIP-155) binds the signature to one specific chain.

Question 57: What does 'APY' represent in DeFi yield products?

Annual Percentage Yield, which includes the effect of compounding interest (Correct answer)
Automated Payment Yield from staking rewards
Asset Price Yield, the token price appreciation over a year
Average Protocol Yield

Correct answer: Annual Percentage Yield, which includes the effect of compounding interest

APY (Annual Percentage Yield) represents the real rate of return on an investment over one year, including the compounding of interest or rewards.

Question 58: A key difference between Proof of Work (PoW) and Proof of Stake (PoS) lies in how they achieve Sybil resistance. How does PoS primarily prevent a Sybil attack?

By requiring validators to stake their real-world identity and reputation.
By electing a small, trusted group of validators to secure the network.
By making computational power prohibitively expensive for an attacker.
By making it economically irrational for an attacker to acquire a majority of the network's staked assets. (Correct answer)

Correct answer: By making it economically irrational for an attacker to acquire a majority of the network's staked assets.

In Proof of Stake (PoS), validators must lock up a significant amount of the network's native cryptocurrency as collateral. To launch a successful 51% attack, an attacker would need to acquire and stake more than half of the total value of all staked coins. This would be incredibly expensive, and any malicious action could lead to the loss of their staked funds ('slashing'), making such an attack economically unfeasible and irrational.

Question 59: What is the 'long-range attack' threat specific to Proof-of-Stake blockchains?

An attacker gradually increases network latency to isolate validators
An attacker mines blocks over long distances using satellite relays
An attacker sustains a 51% attack for an extended period
An attacker uses old private keys from past validators to rewrite blockchain history from a distant point (Correct answer)

Correct answer: An attacker uses old private keys from past validators to rewrite blockchain history from a distant point

Long-range attacks use old validator keys (when stake was cheaper) to create an alternative chain from far in the past, which PoS systems mitigate with checkpointing.

Question 60: What is a 'time lock' contract, and why do DeFi protocols use them?

A contract that enforces a mandatory delay between proposing and executing governance changes (Correct answer)
A contract that batches hourly transactions to save gas
A contract that locks tokens based on the current ETH price
A contract that pays staking rewards only at fixed calendar dates

Correct answer: A contract that enforces a mandatory delay between proposing and executing governance changes

Time locks impose a waiting period (e.g., 48 hours) on admin actions, giving users time to react and exit before potentially harmful changes take effect.

Question 61: What is a Merkle tree used for in blockchain?

Efficiently verifying the integrity of transaction sets (Correct answer)
Generating new wallet addresses
Signing blocks with miner identity
Encrypting user private keys

Correct answer: Efficiently verifying the integrity of transaction sets

A Merkle tree hashes pairs of transactions recursively, producing a single root hash that lets nodes verify any transaction without downloading the full block.

Question 62: In multi-signature (multisig) wallets, which cryptographic scheme is commonly used to allow M-of-N signers to authorize a transaction without revealing individual private keys using threshold signatures?

RSA secret sharing
Shamir's Secret Sharing combined with threshold ECDSA (Correct answer)
AES key splitting
HMAC key aggregation

Correct answer: Shamir's Secret Sharing combined with threshold ECDSA

Threshold ECDSA, often built on Shamir's Secret Sharing, lets M of N parties collaboratively sign without any single party exposing their private key share.

Question 63: In a Directed Acyclic Graph (DAG) based blockchain architecture like IOTA, how are transactions confirmed?

By each new transaction validating two previous transactions, creating a web-like structure instead of a chain (Correct answer)
By smart contracts running consensus algorithms automatically
By a single committee of elected validators per epoch
By miners competing to solve hash puzzles in linear blocks

Correct answer: By each new transaction validating two previous transactions, creating a web-like structure instead of a chain

In DAG architectures like IOTA's Tangle, users must validate two prior transactions when submitting their own, creating a mesh of confirmations rather than a linear chain of blocks.

Question 64: How does blockchain support carbon credit markets in the US?

It mines credits using fossil fuels
It creates transparent, auditable records of carbon offset issuance and retirement to prevent double-counting (Correct answer)
It burns carbon physically to generate tokens
It replaces renewable energy infrastructure

Correct answer: It creates transparent, auditable records of carbon offset issuance and retirement to prevent double-counting

On-chain carbon credit registries prevent the same offset from being sold twice by maintaining a single authoritative retirement record.

Question 65: What does the 'immutable' keyword do in Solidity?

Marks a function that can never be overridden by child contracts
Allows a variable to be set once at construction and then permanently read-only (Correct answer)
Prevents the variable from being read outside the contract
Makes a mapping that cannot be deleted after deployment

Correct answer: Allows a variable to be set once at construction and then permanently read-only

'immutable' variables are written once in the constructor and then inlined into the bytecode, making reads cheaper than storage reads while still being configurable per deployment.

Question 66: A user wants to send an encrypted message to another user on a blockchain-based platform. The sender uses the receiver's public key to encrypt the message. Which key must the receiver use to decrypt the message?

The sender's public key
The sender's private key
A shared symmetric key
The receiver's private key (Correct answer)

Correct answer: The receiver's private key

This scenario describes asymmetric encryption, also known as public-key cryptography. Data encrypted with a public key can only be decrypted by the corresponding private key. Therefore, the receiver must use their own secret private key to decrypt the message that was encrypted with their publicly available key.

Question 67: A developer deploys a smart contract but soon discovers a critical bug. Due to the immutability of the blockchain, the contract code cannot be changed. Which design pattern allows developers to update contract logic while preserving the contract's address and state?

The Proxy Pattern (Correct answer)
The Factory Pattern
The Singleton Pattern
The Observer Pattern

Correct answer: The Proxy Pattern

The Proxy Pattern is the most common method for creating upgradeable smart contracts. It works by separating the contract's logic and its data storage into two separate contracts: an implementation contract (logic) and a proxy contract (storage). Users interact with the proxy contract, which delegates calls to the implementation contract. To upgrade, the proxy is simply pointed to a new implementation contract, allowing the logic to be updated while the state and address remain unchanged.

Question 68: Which technique do attackers use in a 'sandwich attack' on decentralized exchanges?

Intercept messages between two nodes and alter transaction data
Create two fraudulent blocks surrounding a valid block to trigger a reorg
Insert malicious code between two legitimate smart contract calls
Place a buy order before and a sell order after a victim's large trade to profit from price impact (Correct answer)

Correct answer: Place a buy order before and a sell order after a victim's large trade to profit from price impact

A sandwich attack front-runs a victim's trade to push the price up, then back-runs it to sell at a profit, exploiting the victim's price slippage.

Question 69: Which vulnerability allowed the 2016 DAO hack, draining ~$60M worth of ETH?

Timestamp manipulation
Reentrancy attack (Correct answer)
Integer overflow
Front-running exploit

Correct answer: Reentrancy attack

The DAO hack exploited a reentrancy vulnerability where the attacker's fallback function recursively called withdraw() before the balance was updated.

Question 70: What does 'ABI encoding' accomplish when you call a smart contract function externally?

It translates Solidity source code into EVM opcodes at runtime
It serializes the function selector and arguments into bytes the EVM can parse (Correct answer)
It encrypts the function call so only the contract owner can read it
It compresses the transaction payload to reduce gas costs automatically

Correct answer: It serializes the function selector and arguments into bytes the EVM can parse

ABI encoding converts a function name and its typed arguments into a standardized byte array: the first 4 bytes are the function selector, followed by encoded parameters.

Question 71: An attacker wants to isolate a single, specific node from the rest of the blockchain network. They do this by monopolizing all of the target node's incoming and outgoing peer connections with nodes they control. This allows the attacker to feed the victim false transaction data and a counterfeit view of the blockchain. Which network-level attack is being described?

Eclipse Attack (Correct answer)
Denial-of-Service Attack
Forking Attack
Sybil Attack

Correct answer: Eclipse Attack

An eclipse attack is a targeted network-level attack where a malicious actor takes control of all of a victim node's peer connections, effectively isolating it from the honest network. This 'eclipses' the node's view of the real blockchain, making it vulnerable to deception, such as accepting double-spent transactions.

Question 72: A smart contract function is designed to transfer a user's remaining token balance. The code is written as `balances[user] -= amount;`. If a user with a balance of 100 tokens calls this function with an `amount` of 101, and the contract is running on an older, unprotected version of Solidity, the user's balance could wrap around to a very large number. This vulnerability is known as:

Reentrancy
Integer Underflow (Correct answer)
Timestamp Dependence
Oracle Manipulation

Correct answer: Integer Underflow

Integer underflow occurs when an arithmetic operation results in a value smaller than the minimum value the data type can hold, causing it to wrap around to the maximum possible value. In this case, subtracting 101 from 100 would result in -1, which for an unsigned integer wraps around to the largest possible value, potentially allowing the user to claim an enormous balance. Modern Solidity versions (0.8.0+) have built-in checks to prevent this.

Question 73: Which lending protocol introduced the concept of 'aTokens' that accrue interest in real time?

Compound
Uniswap
Curve Finance
Aave (Correct answer)

Correct answer: Aave

Aave introduced aTokens (e.g., aDAI, aUSDC) that are minted on deposit and accrue interest in real time directly in the holder's wallet.

Question 74: A consortium of international banks plans to create a private blockchain for interbank settlements. They require high transaction throughput, low latency, and the ability to function even if some participating nodes become malicious or faulty. The participants are all known and permissioned entities. Which consensus mechanism would be most suitable for this scenario?

Proof of Work (PoW)
Practical Byzantine Fault Tolerance (pBFT) (Correct answer)
Delegated Proof of Stake (DPoS)
Proof of Stake (PoS)

Correct answer: Practical Byzantine Fault Tolerance (pBFT)

Practical Byzantine Fault Tolerance (pBFT) is well-suited for private or consortium blockchains where participants are known. It provides high throughput, low-latency finality, and is designed to tolerate a certain number of malicious (Byzantine) nodes (up to one-third). Since the banking consortium consists of permissioned entities, pBFT's efficiency and fault tolerance make it an ideal choice.

Question 75: What is 'finality' in blockchain terminology?

The last block ever added to a chain
The point at which a transaction is considered irreversible and permanently recorded (Correct answer)
The final audit of a smart contract
The closing of a payment channel

Correct answer: The point at which a transaction is considered irreversible and permanently recorded

Finality means a transaction can no longer be reversed, altered, or cancelled, giving participants certainty that the record is permanent.

Question 76: What is the purpose of BIP-39 mnemonic seed phrases in blockchain wallets from a cryptographic standpoint?

They encode a random entropy value as human-readable words, which deterministically derive wallet keys via PBKDF2 (Correct answer)
They hash private keys into 12 or 24 shorter keys
They store public keys as readable text for sharing
They encrypt private keys using AES-256

Correct answer: They encode a random entropy value as human-readable words, which deterministically derive wallet keys via PBKDF2

BIP-39 converts a random entropy value into a word list and uses PBKDF2-HMAC-SHA512 with the words as input to derive a master seed for HD wallet key generation.

Question 77: Which architectural pattern does Cosmos use to achieve blockchain interoperability?

A single relay chain with parachain slots
Merged mining across all chains
Cross-chain atomic swaps only
The Inter-Blockchain Communication (IBC) protocol connecting sovereign chains via hubs (Correct answer)

Correct answer: The Inter-Blockchain Communication (IBC) protocol connecting sovereign chains via hubs

Cosmos uses the Inter-Blockchain Communication (IBC) protocol to allow independent blockchains (zones) to transfer tokens and data through hub blockchains.

Question 78: A developer is choosing a cryptographic hash algorithm for a new blockchain. Which of the following is NOT a critical property of a secure hash function for this purpose?

Pre-image resistance
Reversibility (Correct answer)
Deterministic output
Collision resistance

Correct answer: Reversibility

Reversibility, the ability to determine the original input from its hash output, is the opposite of what is required for a secure cryptographic hash function. Hash functions must be one-way functions. Pre-image resistance (one-way property), collision resistance (uniqueness), and deterministic output (same input always produces the same output) are all essential properties for ensuring the integrity and security of a blockchain.

Question 79: Which cryptographic weakness made early Bitcoin brain wallets (password-derived keys) vulnerable to theft?

Bitcoin's elliptic curve allowed recovery of the private key from two signed messages with the same nonce
Brain wallets leaked passwords through timing side-channels in the key derivation function
Low-entropy passwords could be brute-forced offline since the public address alone revealed which hash function to attack (Correct answer)
SHA-256 collisions allowed attackers to find alternative passwords producing the same private key

Correct answer: Low-entropy passwords could be brute-forced offline since the public address alone revealed which hash function to attack

Brain wallets hash a password to derive a private key; weak passwords are trivially brute-forced against all known Bitcoin addresses using GPU clusters.

Question 80: What is 'contract self-destruct' (selfdestruct) and why is it considered dangerous?

A method to upgrade contract logic; dangerous because it bypasses audits
A pattern to pause a contract; dangerous because it freezes user funds
A function to delete contract code and send ETH to a target; dangerous because it can break dependent contracts (Correct answer)
A way to burn tokens; dangerous because it is irreversible

Correct answer: A function to delete contract code and send ETH to a target; dangerous because it can break dependent contracts

selfdestruct removes contract bytecode from state and forces ETH to a recipient, which can break contracts expecting to interact with the now-deleted address.

Question 81: What is a 'bridge exploit' and why are cross-chain bridges high-value targets?

Exploiting latency between blockchains to double-spend assets in transit
Using bridge protocols to launder stolen funds across multiple blockchains
Attacking smart contracts that hold large reserves of locked assets to mint unbacked tokens on the destination chain (Correct answer)
Compromising bridge validators by bribing them to approve fraudulent transfers

Correct answer: Attacking smart contracts that hold large reserves of locked assets to mint unbacked tokens on the destination chain

Bridges lock assets on one chain and mint wrapped tokens on another; exploiting the lock contract allows attackers to mint tokens without depositing real assets, as seen in the Ronin and Wormhole hacks.

Question 82: In a Merkle Patricia Trie (used in Ethereum's state), what cryptographic guarantee does the root hash provide?

It compresses the state to a fixed 32-byte encryption key
It encrypts all account balances against unauthorized access
It signs all transactions with the validator's private key
It commits to the entire state so any modification to any account changes the root hash (Correct answer)

Correct answer: It commits to the entire state so any modification to any account changes the root hash

The Merkle Patricia Trie root hash is a cryptographic commitment to the full state; altering any account balance or contract storage changes the root, making tampering detectable.

Question 83: Which of the following describes a key difference between Proof of Stake (PoS) and Delegated Proof of Stake (DPoS)?

PoS relies on computational puzzles, while DPoS relies on staking.
PoS networks are immune to 51% attacks, while DPoS networks are not.
In DPoS, token holders vote for a limited number of delegates to validate blocks, whereas in PoS, any user meeting the stake requirement can potentially be a validator. (Correct answer)
DPoS is significantly less energy-efficient than PoS.

Correct answer: In DPoS, token holders vote for a limited number of delegates to validate blocks, whereas in PoS, any user meeting the stake requirement can potentially be a validator.

The primary distinction between PoS and DPoS is the selection process for block validators. In a typical PoS system, any participant who stakes the required amount of cryptocurrency has a chance to be selected to create a new block. In DPoS, token holders use their stake to vote for a smaller, fixed number of 'delegates' or 'witnesses' who are then responsible for block production and validation. This system is often considered more efficient and scalable but can lead to a higher degree of centralization.

Question 84: A smart contract has a `withdraw` function where the code first sends Ether to the user's address and then updates the user's balance in the contract's storage. Which classic smart contract vulnerability does this specific order of operations create?

Integer Overflow
Denial of Service (DoS)
Timestamp Dependence
Reentrancy Attack (Correct answer)

Correct answer: Reentrancy Attack

This sequence creates a reentrancy vulnerability. A malicious contract can exploit this by implementing a fallback function that recursively calls the `withdraw` function *after* receiving the Ether but *before* the original call updates the balance. This allows the attacker to drain the contract's funds. The secure pattern is Checks-Effects-Interactions: update the balance first, then send the Ether.

Question 85: What is the key difference between Proof of Work used on a public blockchain and Proof of Authority used on a private blockchain?

PoA relies on trusted validators while PoW relies on computational competition (Correct answer)
PoW requires identity verification for miners
PoW is faster and cheaper than PoA
PoA requires more energy than PoW

Correct answer: PoA relies on trusted validators while PoW relies on computational competition

PoA uses pre-approved trusted validators making it efficient for private networks, while PoW uses energy-intensive competition open to anyone.

Question 86: What does 'access control misconfiguration' most commonly allow in smart contract exploits?

Users bypassing gas limits on computationally expensive functions
Unauthorized users calling privileged functions like mint, pause, or upgrade due to missing onlyOwner checks (Correct answer)
External contracts reading private state variables in Solidity
Contracts accessing memory outside their allocated storage slots

Correct answer: Unauthorized users calling privileged functions like mint, pause, or upgrade due to missing onlyOwner checks

Missing or incorrect modifiers (e.g., onlyOwner) allow arbitrary users to call administrative functions, enabling minting, fund draining, or contract upgrades.

Question 87: What is 'signature malleability' and which Bitcoin transaction issue did it enable?

Altering a signature's encoding without invalidating it, allowing third parties to change transaction IDs before confirmation (Correct answer)
Extending signature length to exceed block size limits and cause transaction rejection
Forging digital signatures by exploiting weak elliptic curve parameters
Reusing the same signature across different transactions on separate chains

Correct answer: Altering a signature's encoding without invalidating it, allowing third parties to change transaction IDs before confirmation

Signature malleability let attackers tweak ECDSA signatures to produce valid but different transaction IDs (txids), complicating transaction tracking and was central to the MtGox issues.

Question 88: A development team needs to build a decentralized application (dApp) that can transfer a specific token from the Ethereum network to the Avalanche network without using a centralized intermediary. Which type of technology is specifically designed to facilitate this kind of cross-chain asset transfer and communication?

A blockchain oracle
A Layer-2 optimistic rollup
An interoperability protocol or cross-chain bridge (Correct answer)
A state channel

Correct answer: An interoperability protocol or cross-chain bridge

Interoperability protocols, commonly implemented as cross-chain bridges, are designed to enable communication and asset transfer between two or more independent blockchain networks. They work by 'locking' an asset on the source chain and 'minting' a representative, wrapped asset on the destination chain, or by using other mechanisms to facilitate secure communication, effectively bridging the two ecosystems.

Question 89: What is the principal advantage of tokenizing an illiquid asset, such as a large commercial real estate building, on a blockchain?

It eliminates the need for property insurance and local government registration.
It enables fractional ownership and improves liquidity by allowing the asset to be traded as smaller, digital shares on secondary markets. (Correct answer)
It automates building maintenance and security through the use of smart contracts.
It guarantees a higher return on investment compared to non-tokenized real estate assets.

Correct answer: It enables fractional ownership and improves liquidity by allowing the asset to be traded as smaller, digital shares on secondary markets.

Tokenization converts ownership rights in an asset into digital tokens on a blockchain. For a high-value, illiquid asset like a building, this allows for fractional ownership, where many investors can buy small shares (tokens). These tokens can then be traded more easily on digital asset exchanges, significantly increasing the asset's liquidity and making it accessible to a wider pool of investors.

Question 90: What is a 'price oracle manipulation' attack in DeFi?

Bribing oracle node operators to submit false data feeds
Artificially moving an asset's spot price on a DEX used as a price reference to exploit lending protocols (Correct answer)
Exploiting round-trip latency between on-chain and off-chain price sources
Spoofing DNS entries to redirect oracle API calls to attacker servers

Correct answer: Artificially moving an asset's spot price on a DEX used as a price reference to exploit lending protocols

Attackers manipulate the spot price of a token on a DEX (often via flash loans) that a lending protocol uses as an oracle, enabling under-collateralized borrowing.

Question 91: Which attack does the 'avalanche effect' in hash functions help defend against?

Length-extension attacks
Differential cryptanalysis and pattern detection (Correct answer)
Replay attacks
Sybil attacks

Correct answer: Differential cryptanalysis and pattern detection

The avalanche effect ensures a single-bit change in input causes roughly half the output bits to flip, making differential analysis impractical.

Question 92: How does 'griefing' differ from theft in smart contract attacks?

Griefing only affects gas costs while theft targets token balances
Griefing corrupts contract storage permanently unlike theft which is reversible
Griefing steals funds but returns them anonymously to avoid detection
Griefing aims to cause financial loss or inconvenience to others without direct profit to the attacker (Correct answer)

Correct answer: Griefing aims to cause financial loss or inconvenience to others without direct profit to the attacker

Griefing attacks cause harm (wasted gas, locked contracts, failed transactions) as the goal itself rather than as a means to steal funds.

Question 93: An attacker creates thousands of pseudonymous nodes on a peer-to-peer blockchain network. The goal is to gain a disproportionately large influence, potentially overpowering honest nodes in consensus or disrupting network routing. What is this type of attack called?

Sybil Attack (Correct answer)
Eclipse Attack
51% Attack
Dusting Attack

Correct answer: Sybil Attack

A Sybil attack is characterized by a single entity creating multiple fake identities (Sybil nodes) to undermine the authority or reputation system of a network. By controlling a large number of these nodes, an attacker can out-vote honest participants or disrupt network operations.

Question 94: In SNARKs (Succinct Non-interactive Arguments of Knowledge), what is the 'trusted setup' and why is it controversial?

A one-time parameter generation where secret 'toxic waste' must be destroyed to ensure soundness (Correct answer)
The deployment of a new smart contract to mainnet
The handshake between two nodes establishing a TLS session
A mining pool initialization ceremony

Correct answer: A one-time parameter generation where secret 'toxic waste' must be destroyed to ensure soundness

The trusted setup generates public parameters; if the secret randomness ('toxic waste') is not destroyed, it could allow forging false proofs.

Question 95: What is the primary role of a 'validator node' in a Proof of Stake network?

To propose and attest to new blocks by staking cryptocurrency as collateral (Correct answer)
To execute smart contracts exclusively
To mine new blocks using computational power
To store the full transaction history only

Correct answer: To propose and attest to new blocks by staking cryptocurrency as collateral

Validators in PoS networks lock up (stake) cryptocurrency as collateral and are chosen to propose or vote on new blocks.

Question 96: What does the nonce field in a block header primarily serve in Proof of Work?

It is a number miners adjust to find a valid hash below the difficulty target (Correct answer)
It records the transaction count
It identifies the consensus algorithm version
It stores the sender's address

Correct answer: It is a number miners adjust to find a valid hash below the difficulty target

Miners increment the nonce repeatedly until the resulting block hash meets the required difficulty target.

Question 97: What is the primary risk of using weak random number generation in a blockchain smart contract?

Higher gas costs for random operations
Predictable outcomes that attackers can exploit before transactions confirm (Correct answer)
Slower block confirmation times
Incompatibility with other smart contracts

Correct answer: Predictable outcomes that attackers can exploit before transactions confirm

Weak randomness (e.g., using block hash or timestamp) is predictable by miners, allowing them to manipulate lottery or gambling contract outcomes.

Question 98: What distinguishes a consortium blockchain from a private blockchain?

Consortium blockchains are governed by multiple organizations (Correct answer)
Consortium blockchains are open to the public
Private blockchains are faster
Private blockchains use more nodes

Correct answer: Consortium blockchains are governed by multiple organizations

A consortium blockchain is governed by a group of organizations, while a private blockchain is controlled by a single entity.

Question 99: What does 'trustless' mean in the context of blockchain systems?

The network operates without any security protocols
Users are anonymous and their identities are never verified
Transactions are processed without any validation
Participants do not need to trust each other because the protocol enforces rules cryptographically (Correct answer)

Correct answer: Participants do not need to trust each other because the protocol enforces rules cryptographically

Trustless means the system's rules are enforced by math and code rather than requiring participants to trust each other or a central authority.

Question 100: Which of the following methods for generating pseudo-random numbers for smart contracts is recommended?

External oracle (Correct answer)
Block hashes
Private functions
Private variables

Correct answer: External oracle

The correct answer: <br> External oracle

Certified Blockchain Expert (CBE)

The Certified Blockchain Expert (CBE) certification by Blockchain Council validates comprehensive knowledge of blockchain technology including architecture, consensus mechanisms, cryptography, smart contracts, dApps, and security — targeting developers, architects, and professionals seeking to demonstrate blockchain expertise.

100

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