What is Blockchain Technology? How does a blockchain work?
Blockchains are
incredibly popular nowadays. But what is a blockchain?
How do they work, what
problems do they solve and how can they be used?
Like the name indicates,
a blockchain is a chain of blocks that contains information.
This technique was
originally described in 1991 by a group of researchers and was originally intended
to timestamp digital documents so that it’s not possible to backdate them or to
tamper with them. Almost like a notary.
However it went by
mostly unused until it was adapted by Satoshi Nakamoto in 2009 to create the
digital cryptocurrency Bitcoin. A blockchain is a distributed ledger that is
completely open to anyone.
They have an interesting
property: once some data has been recorded inside a blockchain, it becomes very
difficult to change it. So how does that work?
Well, let’s take a
closer look at a block. Each block contains some data, the hash of the block
and the hash of the previous block. The data that is stored inside a block
depends on the type of blockchain. The Bitcoin blockchain for example stores
the details about a transaction in here, such as the sender, receiver and
amount of coins. A block also has a hash. You can compare a hash to a
fingerprint. It identifies a block and all of its contents and it's always
unique, just as a fingerprint. Once a block is created, it’s hash is being
calculated. Changing something inside the block will cause the hash to change. 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
we have a chain of 3 blocks. As you can see, each block has a hash and the hash
of the previous block. So block number 3 points to block number 2 and number 2
points to number 1. Now the first block is a bit special, it cannot point to previous
blocks because it's the first one. We call this the genesis block. Now let's
say that you tamper with the second block. This causes the hash of the block to
change as well. In turn that will make block 3 and all following blocks invalid
because they no longer store a valid hash of the previous block. So changing a
single block will make all 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 Bitcoins 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'll need to recalculate the proof-of-work for all the
following 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 send 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
successfully tamper with a blockchain you'll 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
tampered block become accepted by everyone else. This is almost impossible to
do! Blockchains are also constantly evolving. One of the more recent
developments is the creation of smart contracts. These contracts are simple
programs that are stored on the blockchain and can be used to automatically
exchange coins based on certain conditions. More on smart contracts in a later
video. The creation of blockchain technology peaked a lot of people’s interest.
Soon, others realized that the technology could be used for other things like
storing medical records, creating a digital notary or even collecting taxes. So
now you know what a blockchain is, how it works on basic level and what
problems it solves.
Want to learn how you
can implement a simple blockchain with Javascript?
Then checkout this video
her and as always: thank you very much for watching.
