When discussing consensus mechanisms for different cryptocurrencies, one issue that often causes arguments is a lack of understanding (and definition) of the security model that they provide for the historical data in the ledger. While each consensus model aims to prevent various theoretical attacks, it’s important to understand the goals for the model.
Every security model has two main parts: assumptions and guarantees. If the assumptions used as inputs hold true, then so should the guarantees that are output by the model.
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http://www.coindesk.com/bitcoins-security-model-deep-dive/
