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EC-Council CBP practice questions

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Question 1Blockchain fundamentals and cryptography

In a blockchain, what property of a cryptographic hash function ensures that even a one-bit change to the input produces a completely different output digest?

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

Explanation

The avalanche effect describes how a tiny change in the input, such as flipping one bit, yields an output that differs in roughly half its bits and appears uncorrelated. It is a diffusion property, distinct from the security goals of preimage and collision resistance. Determinism only means the same input always yields the same digest.

Question 2Blockchain fundamentals and cryptography

Which characteristic of a public blockchain most directly prevents a participant from spending the same unspent transaction output twice in separate blocks?

  1. Deterministic wallet address derivation
  2. Private key secrecy held by the sender
  3. Use of a Merkle root in each block header
  4. Global ledger replication with consensus ordering Correct answer.

Explanation

Double spending is prevented because all validating nodes maintain a replicated ledger and agree, through consensus, on a single ordering of transactions. Once a UTXO is consumed in a confirmed block, later blocks referencing it are rejected by honest nodes. Key secrecy and Merkle roots support integrity but do not by themselves establish a global spend order.

Question 3Blockchain fundamentals and cryptography

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

  1. The node's own private key
  2. A symmetric session key shared with the sender
  3. The sender's private key
  4. The sender's public key Correct answer.

Explanation

Digital signatures are asymmetric: the signer uses the private key and any verifier uses the corresponding public key. The public key can be recovered or supplied alongside the transaction, so no secret is needed to verify. Symmetric keys and the verifier's own key play no part in this check.

Question 4Blockchain fundamentals and cryptography

What is the primary purpose of including a nonce in a block header for a proof-of-work blockchain?

  1. To encrypt the block's transactions
  2. To vary the header input until the hash meets the target Correct answer.
  3. To reduce the size of the Merkle tree
  4. To identify the miner's public key

Explanation

Miners repeatedly change the nonce so that the hash of the header falls below the difficulty target. The nonce is the adjustable field that makes this trial-and-error search possible without altering the transactions. It provides no confidentiality and is not an identity field.

Question 5Blockchain fundamentals and cryptography

In a Merkle tree used by a block, what does the root hash commit to?

  1. Only the coinbase transaction
  2. The current difficulty target
  3. The previous block's header hash
  4. The entire set of transactions in the block Correct answer.

Explanation

Leaf hashes of each transaction are combined pairwise up the tree, so the root is a single digest that commits to every transaction in the block. Changing any transaction changes the root. The previous block hash and difficulty target are separate header fields.

Question 6Blockchain fundamentals and cryptography

Which statement correctly distinguishes a permissionless blockchain from a permissioned one?

  1. Permissioned chains always use proof of work for Sybil resistance
  2. Permissionless chains let anyone join and validate without prior approval Correct answer.
  3. Permissionless chains require a consortium to approve each validator
  4. Permissioned chains publish all membership rules as open source

Explanation

A permissionless blockchain allows any party to read, submit transactions, and participate in consensus without an identity gatekeeper. Permissioned chains restrict validator or participant membership through an authority. Open-source publication and consensus algorithm choice are independent of the permission model.

Question 7Blockchain fundamentals and cryptography

Which cryptographic primitive provides confidentiality for data stored off-chain before its reference hash is anchored on-chain?

  1. A cryptographic hash function
  2. A Merkle proof
  3. A digital signature scheme
  4. Symmetric encryption such as AES Correct answer.

Explanation

Confidentiality requires encryption, and symmetric ciphers such as AES are the standard tool for protecting data at rest. A hash or Merkle proof only provides integrity and tamper evidence, and a signature provides authenticity, not secrecy. Anchoring a hash on-chain proves integrity while the ciphertext stays private.

Question 8Blockchain fundamentals and cryptography

What does a blockchain node actually verify when it checks a transaction's digital signature?

  1. That the recipient address has previously received funds
  2. That the signature was produced by the private key matching the referenced public key Correct answer.
  3. That the sender's account balance is sufficient for the transfer
  4. That the transaction fee exceeds the current median fee

Explanation

Signature verification is a mathematical check that the signing key corresponds to the public key controlling the referenced output. Balance sufficiency is a separate state check, and fee or recipient history checks are policy or mempool concerns. Conflating these steps is a common conceptual error.

Question 9Blockchain fundamentals and cryptography

A 51% attacker on a proof-of-work chain wants to reverse a confirmed transaction. What must the attacker's hashrate advantage enable?

  1. Decrypting the victim's private key from the signature
  2. Building a longer alternative chain from a block before the target transaction Correct answer.
  3. Rewriting the difficulty adjustment formula
  4. Forcing honest nodes to accept an invalid Merkle root

Explanation

A majority-hash attacker can privately mine a competing chain that diverges before the target transaction and overtake the honest chain's accumulated work. Reorganization then replaces the original history. The attack does not break cryptography or alter consensus rules; it exploits the longest-chain fork-choice rule.

Question 10Blockchain fundamentals and cryptography

Why does a hash-based commitment scheme bind the committer to a value before it is revealed?

  1. The digest reveals the committed value to all parties
  2. The digest can be decrypted only with the committer's private key
  3. Finding a different value with the same digest is computationally infeasible Correct answer.
  4. The commitment includes a timestamp signed by a notary

Explanation

Binding relies on collision and second-preimage resistance: the committer cannot later produce a different value hashing to the same published digest. Hiding relies on preimage resistance so the value stays secret until revealed. No encryption key or notary is required for the basic scheme.

Question 11Blockchain fundamentals and cryptography

A wallet derives multiple addresses from a single seed phrase. Which mechanism makes this possible without storing each private key separately?

  1. Multi-signature threshold signing
  2. Hierarchical deterministic key derivation Correct answer.
  3. Shamir secret sharing of the seed
  4. Symmetric key wrapping with a password

Explanation

Hierarchical deterministic wallets derive a tree of child keys from one master seed using standardized derivation paths, so any address key can be regenerated on demand. The seed alone is backed up. Multi-signature and secret sharing address authorization and backup resilience, not deterministic derivation.

Question 12Blockchain fundamentals and cryptography

Which consensus family relies on a set of known validators exchanging signed votes and finalizing a block once a supermajority threshold is reached?

  1. Proof of elapsed time
  2. Byzantine fault-tolerant voting consensus Correct answer.
  3. Proof of storage
  4. Nakamoto-style proof of work

Explanation

BFT-style protocols such as PBFT and its blockchain variants finalize blocks through explicit pre-prepare, prepare, and commit votes among a known validator set. Finality is immediate once the threshold is met. Proof of work instead uses probabilistic longest-chain convergence without validator voting.

Question 13Blockchain fundamentals and cryptography

An organization wants transaction data visible only to a subset of consortium members while still being verifiable chain-wide. Which approach best fits this requirement?

  1. Remove the Merkle root so outsiders cannot verify inclusion
  2. Move all validation to a single trusted node
  3. Store encrypted payloads on-chain and share decryption keys only with authorized members Correct answer.
  4. Publish plaintext payloads and rely on member honesty

Explanation

Encrypting the payload keeps confidentiality for unauthorized members while the on-chain hash or ciphertext remains available for chain-wide integrity checks. Authorized members receive the keys out of band. Removing integrity structures or centralizing validation undermines the trust model rather than solving privacy.

Question 14Blockchain fundamentals and cryptography

What is the key distinction between a hard fork and a soft fork?

  1. A hard fork requires a token sale, while a soft fork does not
  2. A hard fork changes only the block size, while a soft fork changes only signatures
  3. A hard fork makes previously valid blocks invalid, while a soft fork tightens rules compatibly Correct answer.
  4. A hard fork is temporary, while a soft fork is permanent

Explanation

A hard fork introduces a rule change that old nodes consider invalid, so the network can split unless all nodes upgrade. A soft fork restricts validation rules so unupgraded nodes still accept the new blocks. The distinction is about backward compatibility, not funding, scope, or duration.

Question 15Blockchain fundamentals and cryptography

Why is a longer confirmation depth generally treated as stronger evidence that a transaction will not be reversed on a proof-of-work chain?

  1. Older transactions are removed from the UTXO set automatically
  2. Confirmations transfer the transaction to a new smart contract
  3. Each new block encrypts the transaction more strongly
  4. The cumulative work an attacker must match grows with each additional block Correct answer.

Explanation

Reversing a transaction requires out-mining all work accumulated after it, and that required work grows with each subsequent block. Confirmation depth therefore maps to increasing cost for an attacker. Blocks do not add encryption, migrate transactions, or prune them by age.

Question 16Blockchain fundamentals and cryptography

In a typical blockchain structure, how is each block linked to its predecessor?

  1. The nonce is incremented from the previous block's nonce
  2. The block stores a copy of the previous block's transactions
  3. The header stores the hash of the previous block's header Correct answer.
  4. The coinbase output pays the previous miner

Explanation

Each block header includes the parent block's hash, forming a hash chain that makes altering any historical block change all subsequent hashes. Transactions are not copied between blocks, and nonces are independent search values. This linkage is what gives the ledger its tamper-evident structure.

Question 17Blockchain fundamentals and cryptography

A developer wants to prove that one specific transaction is included in a block without downloading every transaction. What should be provided?

  1. The complete UTXO set at that height
  2. The full block body plus all prior headers
  3. The private keys of the sending addresses
  4. The Merkle branch of sibling hashes from the leaf to the root Correct answer.

Explanation

A Merkle proof consists of the sibling hashes along the path from the target leaf to the root, letting a verifier recompute the root and compare it to the known header value. This is logarithmic in the number of transactions. Full block data, private keys, and the UTXO set are unnecessary for inclusion proof.

Question 18Blockchain fundamentals and cryptography

Which statement best explains why hash functions used in blockchain mining are designed to be preimage resistant?

  1. Nodes can compress the blockchain by reversing hashes
  2. Miners cannot shortcut the search by computing the input that yields a target hash Correct answer.
  3. Users can recover lost private keys from public keys
  4. Miners can decrypt other miners' blocks to claim rewards

Explanation

Preimage resistance means there is no practical way to invert a hash, so finding a nonce that meets the target requires brute-force trial and error. This is what converts electricity into provable work. Reversal would break mining, not enable compression or key recovery.

Question 19Blockchain fundamentals and cryptography

What security property does a multi-signature address add compared with a single-key address?

  1. The signing keys are derived from one shared seed phrase
  2. A spend requires authorization from a threshold of distinct keys Correct answer.
  3. The address can be reused without any privacy loss
  4. The transaction payload is encrypted for each cosigner

Explanation

Multi-signature schemes require m-of-n keys to authorize a spend, so compromising one key is insufficient. This reduces single points of failure for custody. It does not encrypt payloads, guarantee privacy on reuse, or require a shared seed.

Question 20Blockchain fundamentals and cryptography

Which cryptographic assumption underpins the security of ECDSA signatures used on many blockchains?

  1. The elliptic-curve discrete logarithm problem is computationally hard Correct answer.
  2. Finding hash collisions is computationally easy
  3. Integer factorization of large semiprimes is infeasible
  4. Symmetric keys can be exchanged over public channels

Explanation

ECDSA security rests on the difficulty of recovering a private scalar from a public point on an elliptic curve, the discrete logarithm problem. Factoring underlies RSA, not ECDSA, and easy collisions would undermine hash security. Symmetric key exchange is a separate problem addressed by key agreement protocols.

Question 21Blockchain fundamentals and cryptography

A node receives two blocks at the same height with valid proofs of work. Under the standard longest-chain rule, how is the tie resolved?

  1. Nodes merge the transactions of both blocks into one
  2. Nodes reject both blocks and wait for a vote by all miners
  3. Nodes build on whichever block they received first and converge when one branch gains more work Correct answer.
  4. Nodes select the block with the larger Merkle root

Explanation

Ties are broken temporarily by first-seen preference, and the network converges once one branch accumulates more work. Miners extend the branch they consider best, so the shorter branch is eventually abandoned. There is no merge of competing blocks at the same height.

Question 22Blockchain fundamentals and cryptography

An attacker withholds a block and releases it just after honest miners publish the next block, attempting to waste their work. Which property of the network does this selfish-mining strategy primarily exploit?

  1. The propagation delay between miners and the longest-chain reward rule Correct answer.
  2. The confidentiality of encrypted mempool transactions
  3. The collision resistance of the block hash function
  4. The threshold required by multi-signature wallets

Explanation

Selfish mining exploits information asymmetry caused by propagation latency, letting the attacker publish a longer private chain at a strategic moment so honest miners waste effort on an orphaned block. It does not attack hash collision resistance or wallet thresholds. The advantage grows with the attacker's share of hashrate and network delay.

Question 23Blockchain fundamentals and cryptography

Why can a zero-knowledge proof let a verifier accept that a private witness satisfies a statement without learning the witness?

  1. The proof relies on the verifier trusting the prover's identity
  2. The verifier decrypts the witness after the proof is accepted
  3. The proof is complete, sound, and reveals nothing beyond the statement's truth Correct answer.
  4. The prover publishes the witness hashed with a random salt

Explanation

A zero-knowledge proof satisfies completeness (true statements are provable), soundness (false statements cannot be proven), and zero knowledge (no information about the witness leaks). The verifier never obtains the witness. Hashing a witness or trusting the prover's identity does not provide these guarantees.

Question 24Blockchain fundamentals and cryptography

A protocol uses a hash function with a 256-bit output. Why does a birthday attack require roughly 2^128 operations rather than 2^256?

  1. The output space shrinks by half for each block mined
  2. Preimage attacks always require half the output length in bits
  3. Collisions among any pair of inputs appear after about the square root of the output space Correct answer.
  4. Hash outputs are truncated to 128 bits during verification

Explanation

The birthday bound shows that among roughly 2^(n/2) random inputs, a collision is likely in an n-bit output space because the number of pairs grows quadratically. This is why 256-bit hashes are considered to offer about 128 bits of collision resistance. Preimage resistance remains near 2^256.

Question 25Blockchain fundamentals and cryptography

A smart contract verifies a Merkle root stored in a block header and accepts a user-supplied proof. What must the contract recompute to confirm the leaf is included?

  1. Decrypt the leaf with the contract's private key and compare plaintext
  2. Check the leaf against the UTXO set at the block height
  3. Hash the leaf with each supplied sibling in the correct order up to the stored root Correct answer.
  4. Sum all sibling hashes and compare with the block nonce

Explanation

The contract must hash the candidate leaf with each sibling hash in the documented left/right order, producing a candidate root that must equal the trusted root. Any mismatch means the proof is invalid. Decryption, summation, or UTXO lookups are not part of Merkle verification.

Question 26Blockchain fundamentals and cryptography

In blockchain terminology, what does a cryptographic hash function's avalanche effect describe?

  1. Hashing a private key twice yields the corresponding public key
  2. The digest length grows proportionally with the input length
  3. Repeated hashing of the same input always returns the same digest
  4. A small change in the input produces an output that appears uncorrelated with the original output Correct answer.

Explanation

The avalanche effect means a one-bit change in the input should flip roughly half the output bits, so outputs look unrelated. This property is what makes block headers sensitive to any tampering with transaction data. Determinism, fixed output length, and key derivation are separate properties and are not what the avalanche effect names.

Question 27Blockchain fundamentals and cryptography

Which property of a cryptographic hash function prevents a miner from finding a different block body that produces an already-published block hash?

  1. Deterministic output
  2. Collision resistance Correct answer.
  3. Fixed digest length
  4. Input padding scheme

Explanation

Collision resistance means it is computationally infeasible to find two distinct inputs with the same digest. If a miner could find such a collision, an existing block hash could be reused for a different body. Determinism, fixed length, and padding are implementation features that do not by themselves prevent this substitution.

Question 28Blockchain fundamentals and cryptography

What role does a private key play in an asymmetric key pair used by a blockchain wallet?

  1. It authorizes spending by producing signatures that anyone with the public key can verify Correct answer.
  2. It is shared with validators so they can confirm the account balance
  3. It replaces the hash function when computing the block header
  4. It encrypts the public key so that the address can be regenerated

Explanation

The private key is the secret that signs transactions, and the matching public key lets any verifier check that signature. The private key must never be shared with validators or anyone else. Public-key derivation and block hashing do not depend on distributing the private key.

Question 29Blockchain fundamentals and cryptography

Which statement describes a public key in an asymmetric cryptography scheme used for blockchain signatures?

  1. It must be kept secret because it can be used to forge signatures
  2. It is identical to the private key but encoded in hexadecimal
  3. It can be derived from the private key and is safe to distribute for verification Correct answer.
  4. It is generated only after the first transaction is broadcast

Explanation

A public key is mathematically derived from the private key and is meant to be shared so others can verify signatures. Knowing the public key does not let an attacker produce valid signatures. It is not the same value as the private key, and key pairs are generated before any transaction.

Question 30Blockchain fundamentals and cryptography

In a proof-of-work blockchain, what is the nonce field in a block header used for?

  1. It identifies the software version of the node
  2. It is varied so the header hash satisfies the network's difficulty target Correct answer.
  3. It stores the timestamp of the previous block
  4. It records the total fees collected by the miner

Explanation

Miners iterate the nonce to find a header hash below the difficulty target. The nonce is a tunable field, not a fee ledger, timestamp, or version marker. Other header fields carry those data, and the nonce's only purpose is to change the hash input.

Question 31Blockchain fundamentals and cryptography

What distinguishes a public blockchain from a private blockchain?

  1. A public chain allows anyone to read and submit transactions without an invitation Correct answer.
  2. A public chain always uses proof of work rather than any other consensus
  3. A private chain must be operated by a single government authority
  4. A private chain cannot use cryptographic hashes for block linkage

Explanation

Public blockchains are open to any participant for reading and transacting, while private chains restrict participation. Consensus mechanism and block linkage are independent of that openness. A private chain may be run by a consortium rather than a single government.

Question 32Blockchain fundamentals and cryptography

In a typical blockchain, what does the block header primarily contain?

  1. A copy of the entire state of all accounts
  2. The full contents of every transaction in the block
  3. Metadata such as the previous block hash, Merkle root, timestamp, and difficulty Correct answer.
  4. The private keys of the validators who signed the block

Explanation

The header holds compact metadata that commits to the block's contents through the Merkle root and links to the prior block via its hash. Full transactions live in the block body, not the header. Validator private keys and full account state are never placed in the header.

Question 33Blockchain fundamentals and cryptography

Which statement best describes a Merkle tree in a blockchain block?

  1. It hashes pairs of transaction hashes upward until a single root hash remains Correct answer.
  2. It orders transactions by fee before they are executed
  3. It stores the public keys of every node that relayed the block
  4. It encrypts each transaction with the previous transaction's key

Explanation

A Merkle tree combines transaction hashes pairwise up to a single root, which the block header commits to. This structure enables compact inclusion proofs. It does not encrypt transactions, sort them by fee, or record relaying nodes.

Question 34Blockchain fundamentals and cryptography

Which algorithm is a widely used elliptic-curve signature scheme for authorizing blockchain transactions?

  1. HMAC-SHA512
  2. SHA-256
  3. ECDSA Correct answer.
  4. AES-256

Explanation

ECDSA is an elliptic-curve digital signature algorithm commonly used to sign transactions. AES is a symmetric cipher, SHA-256 is a hash function, and HMAC-SHA512 is a message authentication construction. None of those three produces the asymmetric signatures a blockchain verifies.

Question 35Blockchain fundamentals and cryptography

A developer wants to prove that a specific transaction is included in a block without downloading every transaction. Which structure should the proof rely on?

  1. The public key of the miner who produced the block
  2. The block's nonce and difficulty target
  3. A list of every transaction hash in the block
  4. A Merkle branch from the transaction hash up to the block's Merkle root Correct answer.

Explanation

A Merkle branch provides the sibling hashes needed to recompute the root from one leaf, so inclusion can be checked compactly. Supplying every transaction hash defeats the purpose of a lightweight proof. The nonce, difficulty, and miner key do not establish that a particular transaction is in the block.

Question 36Blockchain fundamentals and cryptography

A user signs a transaction with a private key, and a node verifies the signature using the corresponding public key. What does a successful verification establish?

  1. The signature was created by the holder of the private key and the message was not altered Correct answer.
  2. The transaction is guaranteed to be valid under all consensus rules
  3. The transaction will be included in the next block
  4. The sender's account balance is sufficient to cover the transfer

Explanation

Signature verification binds the message to the private key holder and detects any change to the signed data. It does not check balances, enforce every consensus rule, or promise inclusion. Those are separate validation and mining steps.

Question 37Blockchain fundamentals and cryptography

Why does a blockchain use a chained hash of previous block headers rather than simply storing blocks in a numbered list?

  1. Altering an old block changes its hash and invalidates every later header that references it Correct answer.
  2. It allows nodes to delete old blocks without affecting new ones
  3. It reduces the size of each transaction signature
  4. It lets miners skip proof of work for later blocks

Explanation

Because each header includes the prior header's hash, any change to an old block changes its hash and breaks the chain of references. That makes history tamper-evident. Chaining does not shrink signatures, permit skipping work, or allow arbitrary deletion without consequences.

Question 38Blockchain fundamentals and cryptography

A consortium wants every member to be able to verify the ledger but wants only approved organizations to produce blocks. Which design fits best?

  1. A centralized database with no cryptographic signatures
  2. A permissionless proof-of-work chain open to all miners
  3. A public chain with anonymous mining pools
  4. A permissioned blockchain with a known validator set Correct answer.

Explanation

Restricting block production to approved organizations is the defining feature of a permissioned chain with a known validator set. A permissionless chain lets anyone mine, which conflicts with the approval requirement. A plain centralized database also removes the shared verifiable ledger the consortium wants.

Question 39Blockchain fundamentals and cryptography

A transaction output is locked to a script requiring two of three specified public keys to sign. What does this arrangement provide?

  1. The funds are encrypted so only the blockchain can read them
  2. The output is automatically refunded if not spent within a day
  3. The output can be spent by any one of the three keys alone
  4. Spending requires cooperation of a threshold of key holders, reducing single-key risk Correct answer.

Explanation

A two-of-three script is a threshold or multisignature condition, so at least two key holders must approve a spend. This removes the single point of failure of one key. It does not allow any single key to spend, encrypt balances, or impose automatic time refunds.

Question 40Blockchain fundamentals and cryptography

A hash function used in a blockchain produces a 160-bit digest. Roughly how many hash evaluations does a generic collision attack require?

  1. About 2^160
  2. About 160
  3. About 2^320
  4. About 2^80 Correct answer.

Explanation

By the birthday bound, finding a collision in an n-bit hash takes about 2^(n/2) work, which is 2^80 for 160 bits. A preimage search would need about 2^160, and 2^320 or 160 are not meaningful bounds here. The square-root reduction is why digest length must be chosen with collision resistance in mind.

Question 41Blockchain fundamentals and cryptography

What is the purpose of a coinbase transaction in a proof-of-work block?

  1. It stores the private key of the miner
  2. It pays the block reward and collected fees to the miner Correct answer.
  3. It reverses the previous block's transactions
  4. It records the votes of validators on the next block

Explanation

The coinbase transaction is the first transaction in a block and creates the reward plus fees for the miner. It does not undo prior transactions or expose private keys. Validator voting belongs to other consensus designs, not to a proof-of-work coinbase.

Question 42Blockchain fundamentals and cryptography

A node receives a block whose header hash meets the difficulty target but whose Merkle root does not match the transactions in the body. What should the node do?

  1. Accept the block but mark the transactions as unconfirmed
  2. Forward the block without validation to peers
  3. Reject the block because the header does not commit to the supplied transactions Correct answer.
  4. Accept the block and repair the Merkle root locally

Explanation

The Merkle root in the header must match the transactions in the body, otherwise the header does not commit to them. A valid proof of work alone is insufficient for block validity. Nodes reject such a block rather than repairing or propagating it.

Question 43Blockchain fundamentals and cryptography

In proof-of-stake systems, what does a validator typically put at risk to discourage misbehavior?

  1. Staked tokens that can be slashed if the validator signs conflicting blocks Correct answer.
  2. The private keys of all delegators
  3. The physical mining hardware it operates
  4. The block reward of the previous epoch only

Explanation

Proof-of-stake validators bond stake that can be slashed for provable faults such as equivocation. This economic penalty substitutes for the resource cost in proof of work. Delegator keys and mining hardware are not the mechanism, and penalties can exceed one epoch's reward.

Question 44Blockchain fundamentals and cryptography

Why does a blockchain typically hash transaction data instead of embedding raw transaction bytes directly into the block header?

  1. Hashing guarantees transactions are sorted by timestamp
  2. A fixed-size root commits to all transactions while keeping the header compact and easy to compare Correct answer.
  3. Hashing removes the need for digital signatures on transactions
  4. Hashing encrypts the transactions so nodes cannot read them

Explanation

The Merkle root is a fixed-size commitment to all transactions, so the header stays compact and can be compared and relayed cheaply. Hashing is not encryption and does not replace signatures. It also does not impose any ordering on transactions.

Question 45Blockchain fundamentals and cryptography

A company wants to prove at a later date that a specific document existed on a given day without revealing the document. What should it publish on-chain?

  1. The full document text in a transaction output
  2. The document's encryption key in plain text
  3. A signed statement listing the document's author only
  4. A hash commitment to the document, with the document revealed later to verify Correct answer.

Explanation

Publishing a hash commitment anchors the document's content to a point in time while keeping it secret. Later, revealing the document lets anyone recompute the hash and confirm the match. Publishing the document or its key defeats confidentiality, and an author-only statement does not commit to content.

Question 46Blockchain fundamentals and cryptography

What does the difficulty target represent in a proof-of-work blockchain?

  1. The minimum fee a miner must pay to include a transaction
  2. A threshold that the block header hash must fall below to be accepted Correct answer.
  3. The number of confirmations required before a spend is final
  4. The maximum number of transactions allowed in a block

Explanation

The difficulty target sets the numeric threshold the header hash must be less than, controlling how much work is expected per block. Block size, fee minimums, and confirmation counts are separate parameters. Difficulty is periodically adjusted to keep block intervals stable.

Question 47Blockchain fundamentals and cryptography

A protocol adjusts its proof-of-work difficulty so that blocks arrive at a stable average interval. What happens if total network hashrate doubles while the target interval stays the same?

  1. Difficulty must decrease to keep miners profitable
  2. The Merkle root must be recomputed for every block
  3. The block reward must double automatically
  4. Difficulty must increase so blocks are not found too quickly Correct answer.

Explanation

With twice the hashrate, the same difficulty would yield blocks about twice as fast, so the protocol raises difficulty to restore the target interval. Rewards and Merkle roots are unrelated to this adjustment. Difficulty retargeting is the mechanism that keeps issuance predictable.

Question 48Blockchain fundamentals and cryptography

An attacker wants to find a second input producing the same 256-bit hash as a known block header without controlling the original input. Which attack is this, and why is it impractical?

  1. A length-extension attack, impractical because headers are fixed length
  2. A chosen-prefix attack, impractical because nonces are random
  3. A collision attack, impractical because it needs about 2^128 work
  4. A second-preimage attack, impractical because it needs about 2^256 work Correct answer.

Explanation

Finding a different input with the same hash as a given target is a second-preimage attack, costing about 2^256 for a 256-bit digest. Collision attacks are cheaper at about 2^128 but do not target a fixed hash. Length extension and chosen-prefix techniques do not apply to this fixed-target scenario.

Question 49Blockchain fundamentals and cryptography

A light client stores only block headers and wants to check a payment. What does it rely on to trust that the transaction is in a block without holding full block data?

  1. The miner's IP address and reputation
  2. The number of peers that relayed the transaction
  3. The transaction's digital signature alone
  4. A Merkle proof verified against the root in a header whose proof of work it has checked Correct answer.

Explanation

A light client validates the header's proof of work and then checks a Merkle proof against the header's root, which ties the transaction to that block. A signature authorizes a spend but does not prove inclusion. Peer addresses and relay counts are not cryptographic evidence.

Question 50Blockchain fundamentals and cryptography

A blockchain uses a hash function vulnerable to length-extension attacks for commitments of the form H(secret || value). Why is this construction unsafe?

  1. The hash output becomes reversible once the value is known
  2. The commitment can be reused across different blockchains
  3. An attacker knowing H(secret || value) can compute H(secret || value || padding || extra) without the secret Correct answer.
  4. The commitment reveals the secret through the digest length

Explanation

Length extension lets someone who knows a Merkle-Damgard digest append data and compute a valid digest for the longer message without knowing the secret. That breaks the binding property of the naive secret-prefix commitment. Reversibility and cross-chain reuse are not the issue here.