Bitcoin

You can download it and run it on your computer. Go on, try it. When you run the program, it will connect to other computers who are also running this program, and they will start sharing a file with you. This file is called the blockchain, and it is basically a big list of transactions. When a new transaction enters the network, it gets relayed from computer to computer until everyone has a copy of the transaction. At roughly 10 minute intervals, a random computer (node) on the network will add the latest transactions they have received on to the blockchain, and share the updates with everyone else on the network. As a result, the Bitcoin program creates a large network of computers that communicate with each other to share a file and update it with new transactions. What problem does Bitcoin solve? It was possible to relay transactions across a network of computers before Bitcoin. However, the problem is that you can insert conflicting transactions in to a network of computers. For example, you could create two separate transactions that spend the same digital coin, and send both of these transactions in to the network at the same time. This is known as a "double-spend". So if you're creating an electronic payment system without a central point of control, you have the problem of figuring out which of these transactions came "first", and this is a difficult thing to do when you have a network of computers all acting independently. Some computers will receive the green transaction first, and some computers will receive the red transaction first. Who's to decide which one came "first" and should be the only one written to the file? Bitcoin solves this problem by forcing nodes to keep all the transactions they receive in memory before writing them to a file. Then, at 10-minute intervals, a random node on the network will add the transactions from their memory on to the file. This updated file is then shared with the network, and nodes will accept the transactions in the updated file as "correct", removing any conflicting transactions from their memory. As a result, no double-spend transactions will ever be written to the file, and all nodes can update their files in agreement with one another. The process of adding transactions on to the file is called mining, and it is basically a network-wide competition that cannot be controlled by a single node on the network. How does mining work? To start with, each node stores the latest transactions they have received in their memory pool, which is just temporary memory on their computer. Any node can then try and mine the transactions from their memory pool on to the file (the blockchain). To do this, a node will gather the transactions from its memory pool in to a container called a block, and then use processing power to try and add this block of transactions on to the blockchain. So where does this processing power come in? Well, to add this block to the blockchain, you must feed your block of transactions in to something called a hash function. A hash function is basically a mini computer program that will take in any amount of data, scramble it, and spit out a completely random (yet unique) number. For your block to be successfully added on to the blockchain, this number (the block hash) must be below the target, which is a threshold number that everyone on the network agrees upon. If your resulting block hash is not below the target, you can make a small adjustment to the data inside the block and put it through the hash function again. This will produce a completely different number that will hopefully be below the target. If not, you adjust the block and try again. So in summary, the process of mining uses processing power to perform hash calculations as fast as you can to try and be the first computer on the network to get a block hash below the target. If you're successful, you can add your block of transactions on to the blockchain and share it with the rest of the network. NOTE: Although it is still possible for anyone to try and mine blocks, it is no longer competitive to do so on a home computer. There is now specialized hardware that has been designed to perform hash calculations as fast (and as efficiently) as possible, which means that mining is now mostly performed by those with access to specialized hardware and cheap electricity. Where do bitcoins come from? As an incentive to use processing power to try and add new blocks of transactions on to the blockchain, each new block makes available a fixed amount of bitcoins that did not previously exist. Therefore, if you are able to successfully mine a block, you are able to "send" yourself these new bitcoins as a reward for your effort. This reward of new bitcoins is called the block reward, and is the reason why the process is called "mining". Why is the file called the "blockchain"? As we have seen, transactions are not added to the file individually - they are collected together and added in blocks. Each of these new blocks builds on top of an existing one, and so the file is made up of a chain of blocks; hence, blockchain. Furthermore, every node on the network will always adopt the longest chain of blocks they receive as the "official" version of the blockchain. This means that miners will always try to build on top of the "tip" of the longest known chain of blocks, as any blocks that are not part of the longest chain will not be considered valid by other nodes. Therefore, if someone wanted to rewrite the history of transactions, they would need to rebuild a longer chain of blocks to create a new longest chain for other nodes to adopt. However, to achieve this, a single miner would need to have more computer processing power than the rest of the network combined. As a result, the combined effort of the network makes it difficult for any individual to "outrun" the network and rewrite the blockchain. How do transactions work? You can think of the blockchain as being a storage facility for safe deposit boxes, which we call outputs. These outputs are just containers that hold various amounts of bitcoin. When you make a bitcoin transaction, you select some outputs and unlock them, then create new outputs and put new locks on them. So when you "send" someone bitcoins, you are actually placing an amount of bitcoins in to a new safe deposit box, and putting a lock on it that only the person you are "sending" the bitcoins to can unlock. For example, if I wanted to send you some bitcoins, I would select some outputs from the blockchain that I can unlock, and create a new output from them that only you can unlock. Also, if I didn't want to send you all of the bitcoins that I had unlocked, I would create an extra output as my "change" and lock it to myself. Moving forward, if you want to send your bitcoins to someone else, you would repeat the process of selecting existing outputs (that you can unlock) and creating new outputs from them. As a result, bitcoin transactions form a graph-like structure, where the movement of bitcoins is connected by a series of transactions. Lastly, when a transaction gets mined on to the blockchain, the outputs that were used up (spent) in the transaction cannot be used in another transaction, and the newly created outputs will be available to be moved on in a future transaction. How do you own bitcoins? To be able to "receive" bitcoins, you need to have your own set of keys. This set of keys is like your account number and password, except in Bitcoin they're called your public key and your private key. For example, if I wanted to send you some bitcoins, you would first need to give me your public key. When I create the transaction, I would place your public key inside the lock on the output (the safe deposit box). You would then use your private key to unlock this output when you want to send the bitcoins on to someone else. So where can you get a public and private key? Well, with the help of cryptography you can actually generate them yourself. In short, your private key is just a large random number, and your public key is a number calculated from this private key. But the clever part is; you can give your public key to someone else, but they cannot work out the private key from it. Now, when you want to unlock bitcoins that are assigned to your public key, you use your private key to create what's called a digital signature. This digital signature proves that you are the owner of the public key (and therefore can unlock the bitcoins), without having to reveal your private key. This digital signature is also only valid for the transaction it was created for, so it cannot be used to unlock other bitcoins locked to the same public key. This system is known as "Public Key Cryptography", and has been available since 19781. Bitcoin makes use of this system to allow anyone to create keys for sending and receiving bitcoins securely, without the need of a central authority to issue accounts and passwords. Putting it all together.