From a computer science perspective, a blockchain is simply a data structure used to store data.  There are many other data structures available, such as databases (rows,columns), comma separated lists (csv files), text files, etc.  However, the blockchain has some unique properties, which I will explain below.

The easiest way to imagine a blockchain is to think of it like a book.  Just like a book has numerous pages, a blockchain has numerous blocks in linear order.  In a book, each page has the contents (i.e. the story) and additional information in the header, such as the page number.  Similarly, each block has the contents (i.e. the individual payment transactions) and additional information in the header to point to the previous block.  However, this is where things get really interesting, let me explain.  Each block has a unique fingerprint, which is generated based on the contents of the block (i.e. payment transactions) and the fingerprint of the previous block.  The implication is that if anyone tried to remove, change or replace one of the blocks in the chain, then the fingerprint of the following block would no longer correctly reference the previous block.  The new fingerprint of the changed block would no longer link up to the next block in the sequence.  The chain would be broken, and the intrusion would be detected. 

The only way to change a block, is to also change all of the blocks that follow it.  The further in the past a block is located, the harder it is to change.  Fortunately, because changing or adding new blocks is a very complex and expensive process, regenerating multiple blocks is simply not a feasible option.  Hence the blockchain is considered to be immutable or tamper proof.  In a future article I will discuss how blocks are added to the blockchain, and the complexity involved.  



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