A cryptocurrency (or crypto currency) is a medium of exchange using cryptography to secure the transactions and to control the creation of new units.[1] Cryptocurrencies are a subset of alternative-currencies or specifically of Digital-currencies. The first cryptocurrency to be created was Bitcoin in 2009.[dubious – discuss] 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.