In a blockchain, what ensures tamper detection across the chain?

• A. Independent encryption of each block
• B. Public-key encryption of all blocks
• C. Cryptographic hashes linking blocks
• D. Centralized authentication servers

Answer: (C)

Cryptographic hashes linking blocks provide tamper detection across the chain. Each block header includes the hash of the previous block, so any change to a block alters its hash and breaks the chain’s linkage. When nodes verify the chain, they rehash blocks and ensure each link points to the correct predecessor; a modified block would cascade through the chain and be immediately detectable. In networks using proof-of-work, tampering would also require redoing work for all following blocks, making the attack practically infeasible. Within a block, a Merkle root summarizes all transactions, allowing efficient verification that specific transactions are included, but the essential tamper-detection mechanism across blocks is the hash pointers that chain blocks together. Encryption alone protects confidentiality or authenticity in other contexts, and centralized authentication servers reintroduce a single point of failure, not the tamper-evident structure that distributed ledgers rely on.