What is Blockchain?
A blockchain is a chain of blocks that contain information.
It was originally intended to time stamp digital documents so it is not possible to back date them or to tamper with them, almost like a notary.
Blockchain was used to create bitcoin.
Why is it unique?
Blockchain is a distributed open ledger. Once data has been recorded in a blockchain it becomes very difficult to change it.
Each block in a blockchain contains the data, the hash of the block and the has of the previous block.
The data stored inside a block depends on the type of blockchain.
The bitcoin blockchain for example stores the sender, receiver and the amount of coins.
What makes Blockchain secure?
A block also has a hash. A hash is like a unique number, kind of like a fingerprint to a block. If you change anything inside the block the hash will change meaning it has been tampered.
Because the block has the hash of the previous block, this makes it very secure as all blocks have some sort of relationship due to each one of them containing the hash of the block above them.
This is the case for all blocks aside from the very first block: the genesis block.
If you tamper with the second block, the has will change and this will then mean the next block will no longer have a valid hash of the previous block. This will make all blocks invalid in the chain.
Hashes will not prevent tampering though as you can use a fast computer to calculate the right hashes for all the blocks. This will make them valid again.
Proof of Work
Blockchains have what is called proof of work. This mechanism slows down the creation of new blocks. In bitcoins case it takes 10 minuites to calculate the required proof of work and add a new block to the chain. This means if you tamper with 1 block you will need to calculate the proof of work for all other blocks.
Hacking the Blockchain
Aside from the hash and proof of work blockchain also uses something else to make it secure.
This is distribution. Blockchain is distributed as it uses a peer to peer network to manage the chain rather than a central chain. When someone joins the blockchain network they get a full copy of the blockchain which is verified by a all nodes(each user essentially).
When a new block is created that block is sent to everyone on the network and verified by their nodes then added to the blockchain. If the nodes don't agree that the block is valid then it will be rejected.
So to hack or tamper into the blockchain you will need to tamper with all the blocks, redo the proof of work for all the blocks and take control of more than 50% of the nodes.
Blockchain is getting better with things such as smart contracts.