Cryptocurrencies are built on a distributed ledger called a blockchain. A blockchain is a tamper-proof, chronological record of transactions maintained by a network of computers rather than a central authority. The data is stored in a series of blocks where a new block is connected to the previous block. Every block is an integral part of the sequence and cannot be changed. If someone tries to change the data in the particular block, the entire arrangement will be rendered invalid. The sequence is 16 bits long, and extracting the whole series gives you no information, making cryptocurrencies almost impossible to hack successfully.