Blockchain Developer Core Blockchain Data Structures — Questions and Answers

Question 1: In a Merkle tree, what is stored at each leaf node?

The hash of an individual transaction (Correct answer)
The block header
A pointer to the previous block
The nonce value

Correct answer: The hash of an individual transaction

Leaf nodes contain the hashes of individual transactions, which are then combined upward to form the Merkle root.

Question 2: What property of cryptographic hash functions ensures a tiny input change produces a completely different output?

The avalanche effect (Correct answer)
Collision resistance
Determinism
Preimage resistance

Correct answer: The avalanche effect

The avalanche effect means a single-bit input change cascades into a drastically different hash output.

Question 3: How does a block reference the block before it in the chain?

By including the previous block's hash in its header (Correct answer)
By storing the previous block's full data
By using a sequential database ID
By timestamp comparison

Correct answer: By including the previous block's hash in its header

Each block header contains the hash of the previous block, creating an immutable cryptographic link.

Question 4: What is the primary benefit of a Merkle tree for lightweight (SPV) clients?

They can verify a transaction's inclusion without downloading the full block (Correct answer)
They can mine blocks faster
They eliminate the need for hashing
They compress all transactions into one byte

Correct answer: They can verify a transaction's inclusion without downloading the full block

Merkle proofs let SPV clients confirm a transaction belongs to a block using only a small branch of hashes.

Question 5: If a Merkle tree has an odd number of leaf nodes, how is this commonly handled in Bitcoin?

The last hash is duplicated to make a pair (Correct answer)
The tree is rejected
A zero hash is appended
The odd node becomes the root directly

Correct answer: The last hash is duplicated to make a pair

Bitcoin duplicates the final hash when a level has an odd count so it can be paired and hashed.

Question 6: What data structure is a blockchain fundamentally based on?

A hash-linked list of blocks (Correct answer)
A balanced binary search tree
A hash table
A doubly-linked circular queue

Correct answer: A hash-linked list of blocks

A blockchain is essentially a singly-linked list where each node links backward via cryptographic hashes.

Question 7: Why is a blockchain considered tamper-evident?

Altering one block changes its hash and breaks all subsequent links (Correct answer)
Blocks are encrypted with private keys
Miners manually inspect every block
Each block has a digital watermark

Correct answer: Altering one block changes its hash and breaks all subsequent links

Changing any block's data invalidates its hash, which cascades and invalidates every later block's reference.

Question 8: What does the Merkle root in a block header summarize?

All transactions in the block (Correct answer)
Only the coinbase transaction
The previous block's transactions
The miner's wallet balance

Correct answer: All transactions in the block

The Merkle root is a single hash representing every transaction included in that block.

Question 9: Which field is NOT typically part of a Bitcoin block header?

The full list of transactions (Correct answer)
The Merkle root
The nonce
The previous block hash

Correct answer: The full list of transactions

Transactions are stored in the block body, while the header holds only summary metadata like the Merkle root.

Question 10: What is a Patricia Merkle Trie used for in Ethereum?

Storing account state and enabling efficient state proofs (Correct answer)
Mining new blocks
Encrypting peer-to-peer messages
Compressing the genesis block

Correct answer: Storing account state and enabling efficient state proofs

Ethereum uses Patricia Merkle Tries to store and prove account state, storage, and transaction data efficiently.

Question 11: What makes a hash function suitable as a blockchain building block?

It is one-way and collision-resistant (Correct answer)
It is reversible on demand
It produces variable-length output
It depends on a secret key

Correct answer: It is one-way and collision-resistant

Blockchain hash functions must be one-way and collision-resistant so outputs cannot be forged or reversed.

Question 12: In a Merkle tree, how many hashes must an SPV client receive to prove inclusion in a tree of N transactions?

Approximately log2(N) (Correct answer)
Exactly N
Exactly N/2
Always one

Correct answer: Approximately log2(N)

A Merkle proof requires only about log2(N) sibling hashes to reconstruct the root path.

Question 13: What is the genesis block?

The first block in a blockchain with no predecessor (Correct answer)
The most recently mined block
A block containing only the Merkle root
An empty placeholder block

Correct answer: The first block in a blockchain with no predecessor

The genesis block is the very first block and is the only one without a reference to a previous block.

Question 14: Why are hashes preferred over plaintext for linking blocks?

Fixed-length output that uniquely fingerprints variable data (Correct answer)
They are human-readable
They can be decrypted to recover data
They shrink storage to zero

Correct answer: Fixed-length output that uniquely fingerprints variable data

Hashes give a compact, fixed-size fingerprint of arbitrary data, ideal for compact and verifiable links.

Question 15: What happens to the Merkle root if a single transaction in the block is modified?

The Merkle root changes completely (Correct answer)
Only that leaf hash changes but the root stays the same
Nothing, the root is independent
The root reverts to zero

Correct answer: The Merkle root changes completely

Because the root is derived from all leaves, modifying any transaction propagates a new root value.

Question 16: Which hash algorithm does Bitcoin primarily use for block hashing?

SHA-256 (Correct answer)
MD5
Keccak-256
Blake2b

Correct answer: SHA-256

Bitcoin uses double SHA-256 for block and transaction hashing.

Question 17: What is the role of the nonce in a block header?

A value miners adjust to find a valid proof-of-work hash (Correct answer)
A counter of total transactions
A timestamp of block creation
The miner's account identifier

Correct answer: A value miners adjust to find a valid proof-of-work hash

The nonce is varied during mining to produce a header hash meeting the difficulty target.

Question 18: Compared to a list, why is a Merkle tree more efficient for verifying data integrity?

It allows verifying a subset without rehashing all data (Correct answer)
It stores data unhashed
It removes the need for any hashing
It uses less than one hash total

Correct answer: It allows verifying a subset without rehashing all data

A Merkle tree enables verifying individual items with a short proof rather than rehashing the entire dataset.

Question 19: What is an 'uncle' (ommer) block reference in some blockchain data models?

A valid block not on the main chain but referenced for partial reward (Correct answer)
A duplicate of the genesis block
A corrupted block header
A block with no transactions

Correct answer: A valid block not on the main chain but referenced for partial reward

Uncle/ommer blocks are valid stale blocks referenced by the main chain, historically rewarding their miners.

Question 20: What guarantees immutability in a hash-linked chain of blocks?

Each block's hash depends on its content and the prior block's hash (Correct answer)
A central authority signs each block
Blocks are stored read-only on disk
Transactions are encrypted twice

Correct answer: Each block's hash depends on its content and the prior block's hash

Content-dependent hashing chained backward means any change breaks every subsequent link, enforcing immutability.

Question 21: How are transactions ordered within a block's Merkle tree construction?

In a defined, deterministic order so the root is reproducible (Correct answer)
Randomly each time
Alphabetically by sender address
By transaction fee descending only

Correct answer: In a defined, deterministic order so the root is reproducible

Transactions follow a deterministic order so any node rebuilding the tree computes the identical Merkle root.

Blockchain Developer Core Blockchain Data Structures

In a Merkle tree, what is stored at each leaf node?

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