This simple idea of linking blocks is what allows the blockchain to become immutable — a key requirement for creating a trustless system independent of any external influence and guaranteed to be free from data manipulation.
In addition to the transaction data in each block, the hashed output from the previous block is included. Since a strong hashing algorithm means no two inputs will ever have the same output, any attempt to change the contents of a previous block would produce a different output hash — which would then cascade through all subsequent blocks, making the tampering instantly detectable.
In this way, anyone can verify for themselves that the data on the blockchain is accurate, correct, and immutable. Genius, utter genius.
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