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- What is the agreement property in a consensus protocol?
a. All processes eventually terminate
b. All non-faulty processes decide on some value proposed by a process
c. All non-faulty processes decide on the same value
d. Every process’s decided value must be unique
Answer: c. All non-faulty processes decide on the same value (Wikipedia) - What is the validity (weak) property in consensus?
a. If all correct processes propose the same value v, then v must be the decision
b. The decided value must be the input of some (correct) process
c. Every process must decide exactly one value
d. All processes must decide simultaneously
Answer: b. The decided value must be the input of some (correct) process (Wikipedia) - Which failure model allows arbitrary, malicious behavior (sending conflicting or incorrect messages)?
a. Crash failure
b. Omission failure
c. Byzantine failure
d. Timing failure
Answer: c. Byzantine failure (Wikipedia) - In the classic result by Lamport, Shostak, Pease, to tolerate t Byzantine faults, one needs at least how many total processes n (without signatures)?
a. n > t
b. n > 2t
c. n > 3t
d. n > 4t
Answer: c. n > 3t (Wikipedia) - Which consensus algorithm is widely used in practical systems for crash fault tolerance (not Byzantine)?
a. PBFT
b. Paxos
c. Byzantine Agreement via garbled circuits
d. Gossip protocol
Answer: b. Paxos (Wikipedia) - What is the main difference between synchronous and asynchronous consensus models?
a. Whether messages are authenticated
b. Whether processes may crash
c. Whether there is a known bound on message delivery time
d. Whether validity is enforced
Answer: c. Whether there is a known bound on message delivery time (Wikipedia) - In a distributed system, what is the consensus number of a concurrent object?
a. The number of processes that can concurrently access it
b. The maximum number of processes that can reach consensus using that object
c. The complexity of consensus algorithms using that object
d. The number of rounds required
Answer: b. The maximum number of processes that can reach consensus using that object (Wikipedia) - Which of the following has infinite consensus number?
a. Atomic read/write registers
b. Test-and-set
c. Compare-and-swap
d. Queue with only enqueue operation
Answer: c. Compare-and-swap (Wikipedia) - What is resilient consensus in multi-agent systems under attack?
a. Consensus that fails when any agent acts maliciously
b. Consensus achieved despite presence of malicious or compromised agents
c. Consensus only under perfect network conditions
d. Consensus reached by majority voting only
Answer: b. Consensus achieved despite presence of malicious or compromised agents (PMC) - What is secure and privacy-preserving consensus as proposed by Ruan et al.?
a. Consensus ignoring privacy entirely
b. Consensus where agents exchange full state openly
c. Consensus using homomorphic cryptography to preserve privacy
d. Consensus using noise injection that always biases results
Answer: c. Consensus using homomorphic cryptography to preserve privacy (arXiv) - In consensus over networks, what is a weighted average consensus?
a. Each agent equally influences the final average
b. Some agents have higher weight and influence than others
c. Only maximum values are considered
d. Agents vote for binary consensus only
Answer: b. Some agents have higher weight and influence than others (arXiv) - Which property of consensus ensures termination?
a. Agreement
b. Integrity
c. Termination
d. Validity
Answer: c. Termination (i.e., every correct process eventually decides) (Wikipedia) - In a synchronous system with up to t fail-stop (crash) failures, consensus can be achieved with how many rounds?
a. t + 1
b. 2t + 1
c. t
d. 3t + 1
Answer: a. t + 1 (in many classic protocols) (cybersecurity.seas.wustl.edu) - What is a 51% attack in blockchain consensus context?
a. When 51% of network nodes crash
b. When an attacker controls over 51% of computational power and can dictate consensus
c. When 51% of nodes refuse to vote
d. When 51% of messages are delayed
Answer: b. When an attacker controls over 51% of computational power and can dictate consensus (Wikipedia) - Which consensus approach is used in Bitcoin?
a. Paxos
b. Proof-of-Work
c. PBFT
d. Raft
Answer: b. Proof-of-Work (Wikipedia) - What is the primary disadvantage of consensus protocols requiring full message exchange (all-to-all)?
a. They are insecure
b. High message complexity / communication overhead
c. They never terminate
d. They only work in asynchronous systems
Answer: b. High message complexity / communication overhead (cybersecurity.seas.wustl.edu) - In consensus literature, integrity is sometimes called which of the following?
a. Agreement
b. Validity
c. Termination
d. Uniqueness
Answer: b. Validity / integrity in some formulations (i.e., the decision value was proposed by some correct process) (Wikipedia) - The Phase King algorithm tolerates Byzantine faults in what model?
a. Asynchronous message passing
b. Synchronous message passing
c. Shared memory
d. Broadcast with faults
Answer: b. Synchronous message passing (Wikipedia) - Which assumption allows consensus protocols to tolerate more faults by using digital signatures?
a. Process anonymity
b. Share memory only
c. Message authentication / signatures
d. Unbounded delays
Answer: c. Message authentication / signatures (written communication model) (Wikipedia) - In a partially synchronous system, which property is relaxed?
a. Agreement
b. Validity
c. Known bounds on message delivery always hold
d. Termination
Answer: c. Known bounds on message delivery always hold (only eventually) (cybersecurity.seas.wustl.edu) - What is the main goal of resilient consensus control in cyber-physical networks?
a. To achieve consensus ignoring malicious actions
b. To minimize consensus latency only
c. To maintain consensus performance despite attacks on communication or nodes
d. To require a central coordinator
Answer: c. To maintain consensus performance despite attacks on communication or nodes (PMC) - What is differential privacy in iterative consensus (Huang et al.)?
a. Agents publish full state always
b. Agents do not communicate at all
c. Agents add randomized noise to protect initial states while converging
d. Agents use deterministic encryption only
Answer: c. Agents add randomized noise to protect initial states while converging (arXiv) - Which of the following is an advantage of privacy-preserving consensus methods?
a. They are slower
b. They allow nodes to avoid revealing their private states
c. They reduce message complexity
d. They break agreement property
Answer: b. They allow nodes to avoid revealing their private states (arXiv) - What is secure average consensus?
a. Consensus ignoring security
b. Consensus of maximum values
c. Consensus where each agent ends up at the average of initial values securely
d. Consensus where only two nodes decide
Answer: c. Consensus where each agent ends up at the average of initial values securely (arXiv) - What challenge arises when malicious nodes manipulate consensus messages?
a. Increased memory usage only
b. Violation of validity, agreement, or both
c. Faster termination
d. Message complexity decreases
Answer: b. Violation of validity, agreement, or both (PMC) - The consensus assessments initiative questionnaire (CAIQ) in cybersecurity is used for what?
a. To enforce consensus in distributed algorithms
b. To evaluate cloud provider security controls
c. To design consensus protocols
d. To test network latency
Answer: b. To evaluate cloud provider security controls (UpGuard) - Which consensus protocol is optimized for permissioned blockchains and tolerates Byzantine faults?
a. Raft
b. PBFT (Practical Byzantine Fault Tolerance)
c. Single-leader Paxos
d. Proof-of-Work
Answer: b. PBFT (Practical Byzantine Fault Tolerance) (Wikipedia) - In consensus in distributed systems, message complexity refers to:
a. Number of messages each process sends
b. Complexity of message formats
c. Total number of messages exchanged among all nodes
d. Encryption complexity
Answer: c. Total number of messages exchanged among all nodes (cybersecurity.seas.wustl.edu) - What is fault tolerance threshold t in consensus protocols?
a. Number of nodes that must agree
b. Maximum number of faulty nodes that the protocol can handle
c. Number of rounds to reach consensus
d. Message delay bound
Answer: b. Maximum number of faulty nodes that the protocol can handle (Wikipedia) - In consensus design, network synchrony is one of three major factors alongside which two others?
a. Process speed and memory
b. Component faults and consensus protocol
c. Message authentication and encryption
d. Topology and latency
Answer: b. Component faults and consensus protocol (cybersecurity.seas.wustl.edu) - What is strong validity in some consensus definitions?
a. Decision must be one of the proposed values
b. If all correct processes propose the same v, they must decide v
c. Decision must be different from all proposals
d. Decision can be any arbitrary value
Answer: b. If all correct processes propose the same v, they must decide v (Wikipedia) - In consensus protocols, what is a leader (or coordinator) used for?
a. To break symmetry and propose a value / drive rounds
b. To crash first
c. To hide messages
d. To broadcast only to itself
Answer: a. To break symmetry and propose a value / drive rounds (cybersecurity.seas.wustl.edu) - What is the “written communication model” in consensus?
a. Messages have no signature
b. Only oral communication allowed
c. Messages carry full history / signatures, so receivers know original sender chain
d. No messages allowed
Answer: c. Messages carry full history / signatures, so receivers know original sender chain (Wikipedia) - Which consensus protocol is more efficient under crash failures in practice and easier to understand than Paxos?
a. Raft
b. PBFT
c. Phase King
d. Two-phase commit
Answer: a. Raft (Wikipedia) - Under Byzantine failures (without signatures), if t = 1, what is the minimum n required?
a. 2
b. 3
c. 4
d. 5
Answer: c. 4 (n > 3t, so n ≥ 4) (Wikipedia) - What is a blockchain consensus finality?
a. That blocks can always be reversed
b. That once decided, a block cannot be changed
c. That consensus is always instantaneous
d. That nodes don’t vote
Answer: b. That once decided, a block cannot be changed (or very unlikely) - In consensus, state machine replication is a use case for what?
a. Caching
b. Achieving consistency across replicas via consensus
c. Encryption only
d. Load balancing
Answer: b. Achieving consistency across replicas via consensus (Wikipedia) - What is Sybil attack threat in consensus systems?
a. Nodes crash randomly
b. An attacker creates many fake identities to influence consensus
c. Message delay attack only
d. Node speed variation
Answer: b. An attacker creates many fake identities to influence consensus (Wikipedia) - What is quorum in consensus systems?
a. Half of nodes plus one (or threshold) needed to decide
b. All nodes always required
c. Only one node is enough
d. Number of messages exchanged
Answer: a. Half of nodes plus one (or threshold) needed to decide - In resilient consensus, what is the role of a confidence parameter (Ballotta & Yemini)?
a. To ignore trust information
b. To weigh trust observations and adjust resilience
c. To slow down consensus always
d. To break agreement
Answer: b. To weigh trust observations and adjust resilience in presence of malicious agents (arXiv) - What is Atomic Broadcast in distributed systems?
a. Broadcast that never delivers messages
b. Broadcast ensuring that messages are delivered in the same total order to all correct nodes
c. Broadcast only to a subset of nodes
d. Broadcast without ordering guarantees
Answer: b. Broadcast ensuring messages are delivered in same total order to all correct nodes (used in consensus) (Wikipedia) - In consensus for multi-valued data, one can reduce to a series of binary consensus instances. What is this approach called?
a. Single consensus
b. Multi-Paxos
c. Reduction approach
d. State machine replication
Answer: b. Multi-Paxos (often implemented via repeated single-value or binary consensus) (Wikipedia) - What is the “weak validity” property (as opposed to strong validity)?
a. Decision must match the majority input
b. Decision must be the input of some correct process (not necessarily specific)
c. Decision may be arbitrary
d. All proposals must be equal
Answer: b. Decision must be the input of some correct process (not necessarily all) (Wikipedia) - Which type of attacker in consensus only passively eavesdrops but does not alter messages?
a. Byzantine (active) attacker
b. Crash attacker
c. Passive attacker
d. Omission attacker
Answer: c. Passive attacker (steals state without altering) - In consensus under privacy constraints, what is a drawback of using noise injection?
a. It increases accuracy
b. It may bias the result or slow convergence
c. It reduces security
d. It breaks termination always
Answer: b. It may bias the result or slow convergence (trade-off between privacy and accuracy) (arXiv) - What is attack detection in resilient consensus context?
a. Ignoring attacks
b. Identifying malicious behavior and excluding from consensus
c. Preventing consensus entirely
d. Crashing nodes intentionally
Answer: b. Identifying malicious behavior and excluding from consensus (PMC) - Which consensus model assumes that after some unknown time, the system becomes synchronous?
a. Fully synchronous
b. Partially synchronous
c. Asynchronous
d. Semi-synchronous
Answer: b. Partially synchronous - In consensus, what is Byzantine fault tolerance (BFT)?
a. Tolerance only to crash faults
b. Ability to reach agreement despite Byzantine failures
c. Requiring a central coordinator
d. No fault tolerance
Answer: b. Ability to reach agreement despite Byzantine failures (Wikipedia) - What is scalability challenge in consensus systems?
a. They scale perfectly
b. As number of nodes grows, message overhead and latency increase steeply
c. More nodes reduce complexity
d. Consensus stops working
Answer: b. As number of nodes grows, message overhead and latency increase steeply - What is a fallback consensus method when consensus cannot be reached (e.g. due to blocks)?
a. Abort the system
b. Randomized tie-breaking, or fallback protocols
c. Force decision by leader only
d. Drop all processes
Answer: b. Randomized tie-breaking or fallback protocols to resolve deadlock situations
