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BTA CBSP practice questions

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Question 1Blockchain Security Fundamentals

In a public blockchain, what property makes it practically infeasible for a single participant to rewrite past confirmed transactions?

  1. Nodes delete old blocks after a fixed time window
  2. Transactions are encrypted with the sender's private key
  3. Each transaction requires approval from a central authority
  4. Historical blocks are protected by accumulated proof-of-work or stake Correct answer.

Explanation

Tamper resistance in a public chain comes from the cumulative cost of producing the chain of blocks. To replace a past block, an attacker must redo the work (or reacquire the stake) for that block and every block after it while outpacing honest participants. This economic barrier, not encryption of the ledger, is what makes rewriting history impractical.

Question 2Blockchain Security Fundamentals

A user wants to confirm that a signed transaction truly originated from the holder of a specific private key. Which cryptographic mechanism provides this assurance?

  1. A digital signature verified with the public key Correct answer.
  2. Splitting the key with Shamir's scheme
  3. Symmetric encryption of the payload
  4. Hashing the transaction twice

Explanation

Digital signatures bind a message to a private key, and anyone holding the corresponding public key can verify the signature. Public-key verification is what proves origin and integrity without revealing the secret. Symmetric encryption and hashing do not establish which key holder produced a message.

Question 3Blockchain Security Fundamentals

Which practice best protects a cryptocurrency wallet's private keys from malware on a general-purpose computer?

  1. Reusing the same password across exchange accounts
  2. Storing the seed phrase in a cloud note for backup
  3. Posting the public address alongside the seed phrase
  4. Keeping keys in a hardware wallet that signs offline Correct answer.

Explanation

A hardware wallet keeps the private key inside a secure element and signs transactions without exposing the secret to the host machine. Malware on a connected computer can read files and keystrokes but cannot extract a key that never leaves the device. Cloud storage, password reuse, and public disclosure all increase exposure.

Question 4Blockchain Security Fundamentals

What is the primary security purpose of a consensus mechanism such as proof of work in a blockchain network?

  1. Compressing block data to reduce storage costs
  2. Ensuring nodes agree on a single ordered history Correct answer.
  3. Encrypting peer-to-peer network traffic
  4. Assigning legal identities to validators

Explanation

Consensus rules determine which chain of blocks the network treats as canonical, so honest nodes converge on one ordered transaction history. This prevents conflicting spends from both being accepted. Compression, transport encryption, and identity registration are separate concerns.

Question 5Blockchain Security Fundamentals

An attacker broadcasts two conflicting transactions spending the same coins to different merchants almost simultaneously. What is this attack called?

  1. Double spending Correct answer.
  2. Sybil attack
  3. Replay attack
  4. Dusting attack

Explanation

Double spending is the attempt to spend the same digital coins more than once by releasing conflicting transactions. Consensus ordering ensures only one of them can be confirmed into the canonical chain. The other named attacks target different weaknesses such as signature reuse, identity flooding, or address tracking.

Question 6Blockchain Security Fundamentals

Which statement accurately describes the role of a hash function in linking blockchain blocks?

  1. Each block stores the full contents of its parent block
  2. Each block includes a copy of every prior signature
  3. Each block stores the public key of the previous miner
  4. Each block includes the hash of its parent block header Correct answer.

Explanation

A block header contains the hash of the preceding block header, forming a chain in which altering any ancestor changes all later hashes. This linkage is what lets nodes detect tampering cheaply. Blocks do not store full copies of ancestors or unrelated keys and signatures.

Question 7Blockchain Security Fundamentals

What does the term '51% attack' describe in a proof-of-work blockchain?

  1. Controlling a majority of hash rate to reorg recent blocks Correct answer.
  2. Persuading half the full nodes to change the rules
  3. Gaining a majority of tokens held in a foundation treasury
  4. Owning more than half of all wallet addresses

Explanation

An entity with more than half the network hash rate can mine blocks faster than the rest and can reorganize recent history to reverse its own transactions. It cannot forge signatures or spend other users' coins. The attack is about producing blocks, not about holdings, addresses, or node rule changes.

Question 8Blockchain Security Fundamentals

Why should a smart contract be audited before it holds significant value?

  1. Audits make the contract's bytecode smaller
  2. Audits guarantee the contract can never be exploited
  3. Audits can reveal logic flaws that let funds be drained Correct answer.
  4. Audits remove the need for any access controls

Explanation

Independent review looks for logic errors, missing checks, and unsafe external calls that could let an attacker take funds. Audits reduce risk but cannot prove absence of bugs, so they complement, rather than replace, careful design and controls.

Question 9Blockchain Security Fundamentals

A contract function transfers Ether to an arbitrary address before updating its internal balance record. Which vulnerability class does this pattern most directly create?

  1. Reentrancy Correct answer.
  2. Front-running
  3. Integer overflow
  4. Signature malleability

Explanation

When state is updated after an external call, the callee can re-enter the function and act on stale balances. The checks-effects-interactions ordering, where balances are updated before the transfer, is the standard mitigation. The other classes involve arithmetic, transaction ordering, or signature encoding rather than call ordering.

Question 10Blockchain Security Fundamentals

A developer uses block.timestamp to seed a lottery winner inside a smart contract. What is the main security concern?

  1. Miners or validators can nudge the value within limits Correct answer.
  2. Timestamps reveal the caller's private key
  3. Timestamps are stored as strings and cannot be compared
  4. Timestamps are always identical across blocks

Explanation

Block producers have some latitude in setting the timestamp, so a value derived from it can be influenced by whoever produces the block. On-chain values like timestamps and block hashes are therefore poor randomness sources for high-value outcomes. A secure design uses a verifiable randomness beacon or commit-reveal scheme.

Question 11Blockchain Security Fundamentals

Why is a flash loan particularly dangerous when combined with a protocol that prices assets using a spot reserve ratio?

  1. It permanently locks the borrowed funds in escrow
  2. It supplies large temporary capital to distort the price within one transaction Correct answer.
  3. It bypasses the need for a valid signature
  4. It forces the protocol to pay interest immediately

Explanation

Flash loans provide uncollateralized capital that must be repaid within the same transaction, so an attacker can move a pool's reserves, trade against a protocol that reads that spot price, and unwind before repayment. This is the classic oracle-manipulation pattern. The danger comes from temporary price distortion, not from interest, signatures, or escrow.

Question 12Blockchain Security Fundamentals

A lending protocol reads an asset price from a single decentralized exchange pool. Which change most directly reduces manipulation risk?

  1. Increasing the protocol's collateral ratio
  2. Lowering the liquidation threshold
  3. Shortening the interest accrual period
  4. Using a time-weighted average price from multiple sources Correct answer.

Explanation

A time-weighted average across several independent sources is far harder to move within a single transaction than a spot reserve ratio. Collateral and liquidation parameters affect solvency but do not fix a manipulable price feed. Interest accrual timing is unrelated to oracle integrity.

Question 13Blockchain Security Fundamentals

What is the defining characteristic of a Sybil attack in a peer-to-peer network?

  1. One attacker intercepts and alters traffic between two peers
  2. One attacker guesses a victim's private key by brute force
  3. One attacker withholds blocks from a subset of peers
  4. One attacker creates many pseudonymous identities to gain disproportionate influence Correct answer.

Explanation

A Sybil attack floods the network with identities the attacker controls, aiming to outvote or outnumber honest participants in reputation, discovery, or governance systems. Defenses include proof-of-work, proof-of-stake, and identity attestations that make identities costly. Interception, key guessing, and block withholding are distinct attack types.

Question 14Blockchain Security Fundamentals

A smart contract stores an owner address and lets anyone call a function that sets the owner. What is the most direct consequence?

  1. The contract's bytecode becomes immutable
  2. Any caller can seize administrative control Correct answer.
  3. Gas costs for all users increase
  4. The contract can no longer receive Ether

Explanation

Without an access check such as a modifier restricting the call to the current owner, the ownership transfer function is open to anyone. An attacker can take over privileged functions and drain or reconfigure the contract. Bytecode immutability, gas costs, and payable behavior are unaffected by this missing check.

Question 15Blockchain Security Fundamentals

Which scenario best illustrates a phishing risk specific to blockchain users?

  1. A malicious site requests a wallet signature that authorizes a token transfer Correct answer.
  2. A validator goes offline during its assigned slot
  3. A miner publishes a block with an invalid merkle root
  4. A node rejects a transaction for insufficient fees

Explanation

Wallet-signing prompts can encode approvals that grant spenders access to tokens, and users who sign without reading the payload hand over control. This is a common drainer pattern. Invalid merkle roots, fee rejection, and offline validators are protocol-level events, not user-targeted deception.

Question 16Blockchain Security Fundamentals

Why does a replay attack matter when a signed transaction valid on one chain is also accepted on a forked chain?

  1. It lets an observer reuse the same signature to move funds on the other chain Correct answer.
  2. It causes the original chain to lose its consensus rules
  3. It reveals the sender's private key to the observer
  4. It doubles the block reward for miners on both chains

Explanation

If both chains share the same transaction format and chain identifier, a valid signature on one is also valid on the other, so an observer can rebroadcast it there. Including a chain identifier in the signed payload, as EIP-155 does for Ethereum, prevents this. The attack does not change rewards, alter consensus, or expose keys.

Question 17Blockchain Security Fundamentals

A contract's withdrawal loop iterates over an unbounded array of recipients. What security risk does this create?

  1. The loop makes the contract's storage immutable
  2. The loop encrypts the recipient addresses
  3. The loop disables event logging
  4. The loop can exceed the block gas limit and block withdrawals Correct answer.

Explanation

Every iteration consumes gas, so if the array grows large enough the transaction can no longer fit in a block and the function reverts for everyone. This denial-of-service pattern is fixed by pull-payment designs where each recipient withdraws individually. Storage mutability, encryption, and logging are unrelated to loop bounds.

Question 18Blockchain Security Fundamentals

What is the purpose of a multi-signature wallet in an organization's treasury?

  1. To hide transaction amounts from the public ledger
  2. To let a single administrator move funds faster
  3. To automatically convert holdings into a stablecoin
  4. To require approval from several keys before a transaction executes Correct answer.

Explanation

A multisignature wallet enforces an m-of-n threshold, so no single compromised key can move treasury funds. This reduces single points of failure in key management. It does not speed up single-admin actions, conceal amounts on a transparent chain, or perform automatic conversions.

Question 19Blockchain Security Fundamentals

An attacker sends a valid transaction with an extremely low gas price that is unlikely to be mined, aiming to fill peers' mempools. Which defense most directly addresses this?

  1. Disabling peer discovery
  2. Removing transaction signatures
  3. Applying per-peer mempool limits and fee thresholds Correct answer.
  4. Increasing the block size limit

Explanation

Mempool flooding works by consuming limited node memory with transactions that will never confirm. Nodes mitigate it by capping the number of transactions per peer and evicting entries below a dynamic fee floor. Larger blocks, removing signatures, and disabling discovery do not target the resource being exhausted.

Question 20Blockchain Security Fundamentals

Which property of a cryptographic hash makes it infeasible to find two different inputs producing the same output?

  1. Collision resistance Correct answer.
  2. Preimage resistance
  3. Avalanche effect
  4. Determinism

Explanation

Collision resistance is the property that finding any two distinct inputs with the same digest is computationally infeasible. Preimage resistance concerns reversing a given digest to an input, while determinism and the avalanche effect describe output behavior rather than collision difficulty.

Question 21Blockchain Security Fundamentals

A protocol's governance token allows a holder to vote immediately after borrowing it through a flash loan. Which design flaw does this expose?

  1. Proposals require a minimum deposit
  2. Votes are recorded in an event log
  3. Voting power is measured at the moment of the call Correct answer.
  4. Voting power is snapshotted at a past block

Explanation

If voting weight is read from current balances, an attacker can borrow a large amount, vote, and repay within one transaction. Snapshotting balances at a block before the proposal was created prevents this, because borrowed tokens do not count. Minimum deposits and event logging do not by themselves stop the borrow-vote-repay pattern.

Question 22Blockchain Security Fundamentals

In a proof-of-stake network, what does the 'nothing at stake' problem refer to?

  1. Validators must run full nodes to receive rewards
  2. Validators can vote on every competing fork because doing so costs them no capital Correct answer.
  3. Validators lose their stake if they double-sign
  4. Validators cannot withdraw their staked funds for a fixed period

Explanation

Without penalties, a validator could sign blocks on multiple competing chains at no marginal cost and profit regardless of which chain wins. Modern proof-of-stake protocols impose slashing conditions that destroy stake for equivocation, making this behavior costly. Lockup periods, node requirements, and double-sign penalties are separate design features.

Question 23Blockchain Security Fundamentals

A contract performs an external call to an untrusted address and then checks a return value using a low-level call without verifying success. What is the most serious result?

  1. The call silently fails while the contract continues as if it succeeded Correct answer.
  2. The called address is forced to pay gas
  3. The transaction is automatically reverted by the EVM
  4. The compiler refuses to deploy the contract

Explanation

Low-level calls return a success flag rather than reverting automatically, so ignoring that flag lets execution continue after a failed call. State may then be updated as though the transfer or interaction occurred, creating an inconsistency attackers can exploit. The compiler does not block deployment, the callee does not pay gas, and the EVM does not revert on its own.

Question 24Blockchain Security Fundamentals

Why does an eclipse attack pose a serious threat even without compromising consensus rules?

  1. It lets an attacker isolate a node and feed it a false view of the chain and mempool Correct answer.
  2. It forces all miners to adopt a new consensus algorithm
  3. It allows the attacker to forge signatures on the victim's behalf
  4. It reduces the total supply of the network's native asset

Explanation

By occupying all of a victim's peer slots with attacker-controlled nodes, an eclipse attack cuts the victim off from honest peers. The victim may then accept a stale or fabricated view of balances and transactions, enabling double-spend acceptance or wasted mining. It does not forge signatures, change supply, or alter consensus rules.

Question 25Blockchain Security Fundamentals

A cross-chain bridge holds locked assets on a source chain and mints representations on a destination chain. Which weakness most commonly leads to catastrophic losses?

  1. The bridge token has a fixed maximum supply
  2. The bridge's validator set or key custody is compromised or too centralized Correct answer.
  3. The destination chain uses a different block time
  4. The source chain charges higher transaction fees

Explanation

Bridges concentrate trust in the parties or contracts that attest to source-chain deposits and authorize minting. If those keys or the validator threshold are compromised, an attacker can mint unbacked representations and drain the locked collateral. Block times, fee levels, and supply caps do not create that single point of failure.

Question 26Blockchain Security Fundamentals

In a blockchain network, what does a node's peer discovery mechanism primarily accomplish?

  1. It locates and connects to other nodes to exchange blocks and transactions Correct answer.
  2. It encrypts private keys before wallet storage
  3. It computes the mining reward schedule
  4. It reaches consensus on the canonical chain tip

Explanation

Peer discovery is the process by which a node learns the network addresses of other participants and establishes connections so it can gossip transactions and synchronize blocks. It is a networking function and does not itself decide consensus or custody of keys.

Question 27Blockchain Security Fundamentals

A user wants to verify that a downloaded wallet binary has not been altered. Which practice most directly provides that assurance?

  1. Renaming the file to a trusted-looking name
  2. Disabling automatic updates after installation
  3. Comparing the file hash against the publisher's signed release checksum Correct answer.
  4. Running the binary inside a virtual machine

Explanation

A cryptographic hash of the downloaded file, compared against a checksum signed by the publisher, reveals any tampering because even a single changed byte produces a different digest. The signature on the checksum authenticates the source of the expected value.

Question 28Blockchain Security Fundamentals

What is the core security property provided by a hardware wallet?

  1. It anonymizes transactions on the public ledger
  2. It guarantees low transaction fees
  3. It keeps signing keys inside a dedicated device so they never enter the general-purpose host Correct answer.
  4. It backs up the seed phrase to a cloud service automatically

Explanation

A hardware wallet isolates private key material in a tamper-resistant environment and performs signing internally, so a compromised computer never sees the keys. This limits exposure to malware on the connected host.

Question 29Blockchain Security Fundamentals

Which statement best describes a permissionless blockchain's trust model?

  1. All transactions are reversible by a support team
  2. Security derives from economic incentives and cryptographic verification rather than identity checks Correct answer.
  3. Participants must be vetted by a central authority before validating blocks
  4. Only the founding organization may propose new blocks

Explanation

Open networks rely on cryptographic verification plus economic incentives such as staking or mining costs to discourage misbehavior, instead of screening participants by identity. Anyone may join and validate under the published rules.

Question 30Blockchain Security Fundamentals

An attacker obtains a user's public address and transaction history from a block explorer. Which privacy risk does this most directly create?

  1. Linkage of the user's activity into a profile that supports targeted phishing or extortion Correct answer.
  2. Immediate theft of funds from the address
  3. Corruption of the address's private key
  4. Loss of consensus participation by the user's node

Explanation

Public ledgers expose the full history tied to an address, so an observer can cluster activity and infer identity or holdings, which enables social engineering and coercion. Knowing an address alone does not permit spending its funds.

Question 31Blockchain Security Fundamentals

A team wants to reduce the impact of a single compromised administrator key that can pause a protocol. Which control best addresses this?

  1. Increasing the pause function's gas limit
  2. Adding an event log entry whenever the pause is triggered
  3. Publishing the administrator address in the documentation
  4. Requiring a threshold of independently held keys to authorize the pause Correct answer.

Explanation

Splitting authority across multiple independent key holders with a threshold requirement means a single stolen key cannot act alone. This removes the single point of failure that a lone administrator key represents.

Question 32Blockchain Security Fundamentals

Why is storing a seed phrase as a plaintext note in a cloud-synced folder considered poor practice?

  1. It slows down transaction signing
  2. Cloud providers charge extra for text files
  3. Text files cannot be read after a device restart
  4. The phrase becomes exposed to account compromise, provider staff, and synchronization to other devices Correct answer.

Explanation

A seed phrase is the master secret from which all keys are derived, so any copy in a synced account inherits every weakness of that account, including credential theft and broad replication. Offline, access-controlled storage limits the number of places the secret exists.

Question 33Blockchain Security Fundamentals

A contract's access control modifier checks that the caller equals a stored address, but the initializer function that sets that address has no guard. What is the most direct consequence?

  1. The contract's events stop being emitted
  2. The contract's bytecode is deleted from the chain
  3. Gas costs for all users double permanently
  4. An attacker can claim ownership by calling the initializer first Correct answer.

Explanation

An unguarded initializer can be invoked by anyone, so whoever calls it first sets the privileged address and gains the protected capabilities. This is why initialization must be restricted to a trusted deployer or performed atomically at deployment.

Question 34Blockchain Security Fundamentals

A protocol's on-chain price feed reports a value that has not updated for several hours while the market moves sharply. Which risk control most directly limits damage from using that stale value?

  1. Rounding the price to fewer decimals
  2. Emitting an event whenever the price is read
  3. Caching the price in contract storage indefinitely
  4. Checking the feed's update timestamp and rejecting prices older than a defined threshold Correct answer.

Explanation

A freshness check against a maximum acceptable age causes the protocol to halt or fall back when the oracle stops updating, preventing trades against outdated values. This directly bounds exposure to stale data.

Question 35Blockchain Security Fundamentals

What distinguishes a hot wallet from a cold wallet in operational security terms?

  1. Hot wallets keep keys on an internet-connected system, while cold wallets keep keys offline Correct answer.
  2. Hot wallets are always multi-signature
  3. Cold wallets cannot sign transactions
  4. Hot wallets use different cryptographic algorithms

Explanation

The distinction is connectivity: hot wallets hold keys on networked devices for convenience, while cold wallets keep keys on systems isolated from the internet. The tradeoff is convenience against exposure to remote compromise.

Question 36Blockchain Security Fundamentals

A user signs a message that authorizes a token allowance for a third-party contract without reading the details. Which threat does this behavior most directly enable?

  1. A consensus-level chain reorganization
  2. Unauthorized spending of the approved tokens by that contract Correct answer.
  3. Corruption of the user's seed phrase
  4. A denial-of-service attack on the user's node

Explanation

Approval messages grant spending rights, so signing one blindly hands the approved party the ability to move those tokens up to the allowance. Careless approval signing is a common route to asset drain.

Question 37Blockchain Security Fundamentals

Why does client diversity matter for the security of a proof-of-stake network?

  1. It increases the total staked amount
  2. A bug in one dominant client implementation could disrupt a large share of validators at once Correct answer.
  3. It removes the need for slashing conditions
  4. It guarantees lower block times

Explanation

When most validators run the same software, a single implementation flaw can cause simultaneous failures across a large fraction of the validator set, threatening liveness or finality. Spreading stake across independent clients reduces correlated failure risk.

Question 38Blockchain Security Fundamentals

A governance proposal would let a small group upgrade a protocol's contracts with no delay. Which change most directly reduces the risk of a rushed malicious upgrade?

  1. Enforcing a timelock during which users can exit or review the change Correct answer.
  2. Removing event emission from the upgrade function
  3. Allowing the upgrade to be executed by a single signature
  4. Reducing the quorum required to pass proposals

Explanation

A timelock inserts a mandatory waiting period between approval and execution, giving users time to inspect the change and withdraw funds if it looks hostile. It converts a sudden takeover into an observable, interruptible process.

Question 39Blockchain Security Fundamentals

An auditor finds that a contract's privileged role can be transferred to any address without confirmation, and the role can mint unbounded tokens. Which combination of weaknesses is present?

  1. Signature malleability and nonce reuse
  2. Reentrancy and integer overflow
  3. Front-running and timestamp dependence
  4. Unrestricted privilege transfer and unbounded minting authority Correct answer.

Explanation

The two findings are independent: the role can be handed to an arbitrary party, and that role carries supply-inflating power, so a single transfer can hand an attacker unlimited issuance. Together they turn an access-control gap into a direct inflation risk.

Question 40Blockchain Security Fundamentals

A protocol accepts user-supplied calldata and forwards it to an arbitrary target contract while holding token approvals. Which design change most directly limits the damage an attacker can cause?

  1. Compressing the calldata before forwarding
  2. Increasing the slippage tolerance on swaps
  3. Restricting the callable target set and allowed function selectors to an explicit allowlist Correct answer.
  4. Emitting an event before each forwarded call

Explanation

An allowlist of destinations and selectors constrains what the forwarding logic can be tricked into invoking, so arbitrary calls that would move approved tokens are rejected. This closes the path that turns user input into unauthorized transfers.

Question 41Blockchain Architecture and Cryptography

Which cryptographic property primarily prevents an attacker from changing a signed transaction without detection?

  1. Anonymity through address rotation
  2. Integrity protection through hash-based digital signatures Correct answer.
  3. Availability through redundant peer replication
  4. Confidentiality through symmetric key wrapping

Explanation

Digital signatures bind a transaction's content to a private key, so any alteration of the payload changes the hash and invalidates the signature. Peers can therefore reject tampered data deterministically. Confidentiality, availability, and anonymity are separate goals that signatures do not by themselves provide.

Question 42Blockchain Architecture and Cryptography

A hash function used in a blockchain must make it computationally infeasible to find any input mapping to a given output. Which property is this?

  1. Second-preimage resistance
  2. Preimage resistance Correct answer.
  3. Collision resistance
  4. Determinism

Explanation

Preimage resistance means that given a digest, an attacker cannot feasibly invert the function to recover an input producing it. Collision resistance concerns finding any two distinct inputs with equal digests, and second-preimage resistance concerns finding a different input matching a specific known input. Determinism merely guarantees identical output for identical input.

Question 43Blockchain Architecture and Cryptography

In a Merkle tree of transaction hashes, what does the root hash commit to?

  1. The previous block's difficulty target
  2. Only the largest transaction in the block
  3. The complete ordered set of transactions in the block Correct answer.
  4. The block producer's public key

Explanation

Each leaf is a transaction hash and each parent combines its children, so the root depends on every transaction and their order. Changing any leaf changes the root, which is how blocks commit to their contents. The root says nothing about producer identity or difficulty.

Question 44Blockchain Architecture and Cryptography

What distinguishes a public key from a private key in an asymmetric key pair?

  1. The public key expires periodically, while the private key is permanent
  2. The public key is used only for hashing, while the private key is used only for encryption
  3. The public key is derived from the private key by hashing, so it cannot be reversed
  4. The public key may be shared freely, while the private key must remain secret Correct answer.

Explanation

Asymmetric cryptography relies on publishing one key while keeping its mate confidential. The public key verifies signatures or encrypts to the holder, and the private key signs or decrypts. The security model collapses if the private key is disclosed.

Question 45Blockchain Architecture and Cryptography

What is the main purpose of a consensus algorithm such as proof of work in a public blockchain?

  1. To compress block data before propagation
  2. To encrypt peer-to-peer gossip traffic
  3. To let mutually distrusting nodes agree on a single ordered history Correct answer.
  4. To assign legal identity to each validator

Explanation

Consensus rules determine which chain of blocks the network treats as canonical despite faulty or adversarial participants. They provide agreement on ordering and finality without a central authority. Compression, transport encryption, and identity registration are handled by other layers.

Question 46Blockchain Architecture and Cryptography

How does a typical blockchain achieve immutability of past records?

  1. Each block header includes the hash of its predecessor, chaining blocks together Correct answer.
  2. Each node stores a notarized copy of the ledger with a timestamping authority
  3. Validators vote to freeze each block permanently after one confirmation
  4. Transactions are encrypted with a key that is destroyed after inclusion

Explanation

Because each header commits to the prior header's hash, altering an old block changes every subsequent hash. An attacker would have to redo that work and outpace the honest chain. This chaining, not external notarization or key destruction, produces tamper evidence.

Question 47Blockchain Architecture and Cryptography

Which structure allows a light client to verify that a transaction is included in a block without downloading every transaction?

  1. A Merkle proof path from the transaction hash to the root Correct answer.
  2. A full copy of the block body
  3. A zero-knowledge range proof over balances
  4. A list of validator signatures on the block

Explanation

A Merkle proof supplies the sibling hashes along the path from the leaf to the root, letting the client recompute the root. Comparing that root to the one in a trusted block header confirms inclusion. The client never needs the full block body.

Question 48Blockchain Architecture and Cryptography

Why do blockchain systems commonly use a fixed-size digest such as 256 bits rather than storing full transaction data in headers?

  1. Fixed digests allow the original data to be reconstructed without storage
  2. Digests encrypt the block body so nodes cannot read it
  3. Shorter values are inherently collision-free regardless of algorithm
  4. A compact fixed-size commitment keeps headers small while still binding to the data Correct answer.

Explanation

Headers must stay small for efficient propagation and light-client verification, so they carry a fixed-length commitment instead of the full payload. The digest still changes if the underlying data changes. Digests are one-way and do not permit reconstruction or encryption.

Question 49Blockchain Architecture and Cryptography

A user signs a transaction with an ECDSA private key. Which key does a node use to check the signature?

  1. A symmetric session key negotiated with the user
  2. The block producer's private key
  3. The user's corresponding public key Correct answer.
  4. The node's own private key

Explanation

Signature verification requires the signer's public key, which is mathematically linked to the private key that produced the signature. The verifier recomputes values from the signature, message hash, and public key. No secret material from the node or producer is involved.

Question 50Blockchain Architecture and Cryptography

A developer wants to prove a single account balance is included in a large state tree. Which approach is most appropriate?

  1. Sign the balance with the account's private key and broadcast it
  2. Encrypt the balance with the state root as the key
  3. Provide the sibling hashes forming a Merkle path to the published state root Correct answer.
  4. Publish the entire state tree so peers can inspect it

Explanation

Inclusion proofs work by revealing only the path from the leaf to the root, which lets a verifier recompute the root and compare it to a trusted value. This is compact and does not require exposing unrelated state. Signatures prove authorship, and encryption does not prove membership.