So in other words: hashes are very useful when you want to detect changes to blocks. If the fingerprint of a block changes, it no longer is the same block. The third element inside each block is the hash of the previous block.

This effectively creates a chain of blocks and it’s this technique that makes a blockchain so secure.
 

Let’s take an example here: here we have a chain of 3 blocks. As you can see, each block has a hash and the hash of the previous block. So the block no.3 points to block no.2 and block no.2 points to block no.1.

Now the first block is a bit special, it cannot point to previous blocks because it’s the first one. We call this block the genesis block.

Now let’s say that you tamper with the second block. This cause the hash of the block to change as well.  
In turn that will make block 3 and all following blocks invalid because they longer store a valid hash of the previous block.
So changing in a single block will make following blocks invalid. But using hashes is not enough to prevent tampering. Computers these days are very fast and can calculate hundreds of thousands of hashes per second.

You could effectively tamper with a block and recalculate all the hashes of other blocks to make your blockchain valid again. So to mitigate this, blockchains have something called proof-of-work. It’s a mechanism that slows down the creation of new blocks.

In bitcoin’s case: it takes about 10 minutes to calculate the required proof-of-work and add a new block to the chain.

This mechanism makes it very hard to tamper with the blocks because if you tamper with 1 block, you will need to recalculate the proof-of-work for all blocks. So the security of a blockchain comes from its creative use of hashing and the proof-of-work mechanism. But there is one more way that blockchains secure themselves and that’s by being distributed. Instead of using a central entity to manage the chain, blockchains use a peer-to-peer network and anyone is allowed to join.

When someone joins this network, he gets the full copy of the blockchain. The node can use this to verify that everything is still in order.

Now let’s see what happens when someone creates a new block. That new block is sent to everyone on the network. Each node then verifies the block to make sure that it hasn’t been tampered with. If everything checks out each node adds this block to their own blockchain.

All the nodes in this network create consensus. They agree about what blocks are valid and which aren’t. Blocks that are tampered with will be rejected by other nodes in the network. So to the successfully tamper with a blockchain you will need to tamper with all blocks on the chain, redo the proof-of-work for each block and take control of more than 50% of the peer-to-peer network.

Only then will your tamper block become accepted by everyone else. So this is almost impossible to do! Blockchains are also constantly evolving.
 

One of the more recent developments is the creation of the smart contract. These contracts are simple programs that are stored on the blockchain and can be used to automatically exchange coins based on certain conditions.
 

The creation of blockchain technology peaked a lot of people’s interest. Soon others realize that the technology could be used for other things like storing medical records, E-notary, collecting taxes.