Core Blockchain Data Structures Flashcards

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

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.

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

Answer: The avalanche effect

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

How does a block reference the block before it in the chain?

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.

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

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.

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

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.

What data structure is a blockchain fundamentally based on?

Answer: A hash-linked list of blocks

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

Why is a blockchain considered tamper-evident?

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.

What does the Merkle root in a block header summarize?

Answer: All transactions in the block

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

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

Answer: The full list of transactions

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

What is a Patricia Merkle Trie used for in Ethereum?

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.

What makes a hash function suitable as a blockchain building block?

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.

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

Answer: Approximately log2(N)

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

What is the genesis block?

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.

Why are hashes preferred over plaintext for linking blocks?

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.

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

Answer: The Merkle root changes completely

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

Which hash algorithm does Bitcoin primarily use for block hashing?

Answer: SHA-256

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

What is the role of the nonce in a block header?

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.

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

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.

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

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.

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

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.