Blockchain Developer Core Blockchain Data Structures Questions and Answers

Question 1

What is the primary advantage of organizing transactions within a block into a Merkle tree instead of a simple list?

Accepted Answer

It enables efficient and secure verification of a transaction's inclusion without needing the entire block's transaction data.

Answer

The Merkle tree structure allows a verifier (like a light client) to confirm a transaction is part of a block by only needing the transaction's hash, the Merkle root (from the block header), and the 'Merkle proof' (a small number of intermediate hashes). This is far more efficient than downloading and hashing all transactions in the block.

Question 2

A block header in a blockchain contains several key pieces of data to summarize the block. Which of the following is typically found in the block's *body* rather than its header?

Accepted Answer

The complete list of individual transactions

Answer

The block header contains metadata and a summary of the block, including the previous block hash, the Merkle root, the nonce, and a timestamp. The full list of transactions, which can be very large and variable in size, constitutes the block body.

Question 3

An attacker attempts to alter the amount of a single transaction within a previously confirmed block (e.g., Block #500) on a public blockchain. What is the most direct and immediate cryptographic consequence of this action?

Accepted Answer

The Merkle root stored in the header of Block #500 will no longer be valid.

Answer

Modifying even a single bit of a transaction will change its hash. This change will cascade up the Merkle tree, resulting in a new Merkle root. The new Merkle root will not match the one originally stored in the header of Block #500, thus cryptographically invalidating the block's integrity and breaking the chain's hash link to Block #501.

Question 4

In an account-based blockchain like Ethereum, what is the primary purpose of the 'State Trie' (also known as the World State Trie)?

Accepted Answer

To provide a data structure that maps every account address to its corresponding state, including its balance, nonce, and storage.

Answer

The State Trie is a key-value mapping where keys are account addresses and values contain the account's state (nonce, balance, codeHash, and storageRoot). The root hash of this entire trie is stored in the block header, providing a cryptographic commitment to the entire state of the blockchain at that specific block.

Question 5

The integrity of the chain of blocks relies on cryptographic hash functions. Which property of these functions is most critical for preventing an attacker from finding two different sets of block data that result in the exact same block hash?

Accepted Answer

Collision Resistance

Answer

Collision resistance is the property that makes it computationally infeasible to find any two different inputs (x and y) such that H(x) = H(y). In the context of a blockchain, this prevents an attacker from creating a fraudulent block with different data that has the same hash as a legitimate block, which would be necessary to alter the chain without being detected.

Blockchain Practice Test

Blockchain Practice Test

Blockchain Developer Core Blockchain Data Structures Questions and Answers