he way the blockchain is designed prevents Bitcoins from being spent again after they've already been used. The blockchain also makes it very difficult to create new Bitcoins out of thin air. In conjunction with the cryptographic measures outlined above, the Bitcoin blockchain is based on a consensus proof-of-work model. This means that not only do mining computers have to provide answers to the aforementioned encrypted equations, but in order for data to be added to the blockchain, a majority of the mining computers on the network have to agree that the computations were correct. To overcome this, an attacker would effectively have to hack the majority of computers on the network (to which there are hundreds across the world) at the same time while providing mathematical proof of work. This makes Bitcoin incredibly hard to hack. Security is arguably the cryptocurrency's greatest asset. To date, Bitcoin has not been hacked.
Some exchanges that act as an online meeting grounds for buyers and sellers of cryptocurrency have been hacked, however. The Mt. Gox attack is the most notable one. In 2014, 850,000 Bitcoins were stolen from the Japan-based exchange. Users can mitigate their risk by taking their assets off of these exchanges and storing them in cryptocurrency wallets.
