What is a long-range attack in proof-of-stake?

A long-range attack is a security vulnerability in proof-of-stake (PoS) systems where an attacker uses old private keys to rewrite the blockchain's history from far in the past. In PoS, the right to create blocks comes from holding stake rather than from ongoing computational work. If someone once held a large stake and later sold it, their old private keys can potentially be used to produce an alternative chain history — even years after the fact — because the original stake creation cost nothing to maintain. For a beginner, this means PoS history is not automatically locked in the way proof-of-work history is, since rewriting a PoW chain requires re-doing all the original computational work.

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