Blockchain Fundamentals & Architecture Flashcards

In a Merkle Patricia Trie (MPT) as used in Ethereum, what is the primary purpose of the 'extension node' distinct from a 'branch node'?

Answer: To compress shared path prefixes that have only a single child, reducing trie depth

Extension nodes in the Merkle Patricia Trie exist specifically to optimize path compression. When a sequence of nibbles leads to only a single descendant, an extension node collapses that shared prefix into a single node rather than creating a chain of branch nodes with 15 empty slots each. Branch nodes handle the 16-way branching (0–f nibbles). This distinction is what makes the MPT space-efficient compared to a naive radix trie.

A blockchain network uses a BFT consensus mechanism with 100 validators. An adversary controls 32 validators. Which of the following statements is most accurate regarding the network's safety and liveness under classical BFT assumptions?

Answer: Safety is guaranteed but liveness may be violated because 32 > ⌊(100−1)/3⌋

Classical BFT (e.g., PBFT) tolerates up to f faulty nodes in a network of n = 3f + 1, meaning f = ⌊(n−1)/3⌋. For n=100, f = ⌊99/3⌋ = 33. The adversary controls 32 < 33, so safety (no two honest nodes finalize conflicting blocks) is still guaranteed. However, liveness — the guarantee that progress continues — can be disrupted by fewer than f+1 faulty nodes in some BFT variants, as adversaries can stall proposal rounds or refuse to participate, meaning 32 adversarial validators can potentially halt finalization without violating safety.

Which cryptographic property is specifically violated if a blockchain's hash function allows an attacker to find two different Merkle tree leaf sets that produce the same root hash, without requiring a full preimage?

Answer: Collision resistance

Collision resistance is the property that it is computationally infeasible to find any two distinct inputs x and y such that H(x) = H(y). If an attacker can find two different leaf sets that produce the same Merkle root, they have found a collision in the underlying hash function. Second-preimage resistance would apply if the attacker were given a specific existing leaf set and tried to find a different one with the same root. Preimage resistance concerns recovering input from output. The scenario described — freely finding any two colliding inputs — is specifically a collision resistance violation.

In the context of Ethereum's EIP-1559 fee mechanism, what happens to the base fee when a block is exactly 50% full (at the target gas limit)?

Answer: The base fee remains unchanged because the block hit the target utilization exactly

Under EIP-1559, Ethereum targets 50% block fullness (the 'target gas used' is half the maximum block gas limit). The base fee adjustment algorithm increases the base fee by up to 12.5% when blocks are above target and decreases it by up to 12.5% when blocks are below target. When a block is exactly at the 50% target, the adjustment factor is zero — the base fee remains exactly the same for the next block. This equilibrium behavior is fundamental to the mechanism's price discovery design.

A sidechain bridge uses a 'two-way peg' where locked assets on the mainchain are represented by wrapped tokens on the sidechain. If the sidechain's consensus mechanism is compromised by a 51% attack, which of the following best describes the worst-case impact on mainchain asset holders who used the bridge?

Answer: Attackers can unlock mainchain assets without burning wrapped tokens by forging fraudulent withdrawal proofs accepted by the bridge contract

In a federated or SPV two-way peg bridge, the mainchain bridge contract often accepts withdrawal proofs based on sidechain block headers or federation signatures. If a 51% attacker controls sidechain consensus, they can produce fraudulent block headers or reorg the sidechain to forge valid-looking withdrawal proofs — claiming that wrapped tokens were burned when they weren't. The bridge contract on the mainchain, if it relies on sidechain state proofs, would release the locked mainchain assets to the attacker. This is a well-documented risk vector distinguishing sidechains from rollups with on-chain validity proofs.

In a UTXO-based blockchain, a transaction has 3 inputs totaling 5.0 BTC and sends 4.97 BTC to a recipient. No explicit fee output is declared. What is the miner's fee, and why?

Answer: 0.03 BTC — miners implicitly collect all unspent input value not allocated to outputs

In the UTXO model, a transaction is valid as long as the sum of output values does not exceed the sum of input values. The difference — inputs minus outputs — is not explicitly assigned but is implicitly claimable by the miner who includes the transaction in a block, typically via their coinbase transaction. Here, 5.0 BTC input − 4.97 BTC output = 0.03 BTC, which becomes the miner fee. No special output or opcode is required to designate this; the protocol enforces that unallocated UTXO value goes to the block producer.