A cryptocurrency is a medium of exchange using cryptography to secure the transactions and to control the creation of new units. Cryptocurrencies are a subset of alternative currencies or specifically of digital currencies. The first decentralized cryptocurrency to be created was bitcoin in 2009. Since then, numerous cryptocurrencies have been created. A feature that is typical in cryptocurrency is decentralized control, distinct from a centralized electronic money system such as PayPal. Another common feature is that transactions are publicly recorded in a ledger. An example is bitcoin, where all transactions are recorded in the block chain.