Is blockchain secure?

   Once a block is added to the end of the blockchain, it is very difficult to go back and change the content of the block.    Because each block contains its own hash and the hash of the block before it.    Hash codes are created by a mathematical function (hash function).

Once a block is added to the blockchain, it becomes extremely difficult to alter its contents. This is because each block contains a cryptographic hash of the previous block's header, creating a chain of blocks that are linked together
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