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CS Professional · Artificial Intelligence, Data Analytics and Cyber Security - Laws and Practice · Internet and Other Technologies

A public blockchain network is validated using Proof of Work. An attacker controlling a majority of total computing power secretly builds a longer alternative chain to reverse a payment already confirmed. Which term describes this attack and why is it feasible only with majority control?

This is a 51% attack. Because Proof of Work nodes accept the longest valid chain, an attacker holding majority computing power can outpace honest miners, publish a longer alternate chain, and reverse confirmed transactions, enabling double spending. Smaller power shares cannot reliably do this.

  1. APhishing attack; because it needs users' passwords
  2. B51% attack; because the network accepts the longest valid chain, so majority hash power can outpace honest minersCorrect
  3. CDenial-of-service attack; because it floods the network with requests
  4. DSybil attack; because it needs a majority of wallets to be encrypted

Explanation

In Proof of Work, nodes follow the longest valid chain. An attacker with over half the hash power can eventually produce a longer private chain and release it, enabling double spending. Phishing and DoS are different mechanisms, and Sybil relies on fake identities rather than hash power.

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