Cryptocurrencies are secured by a process known as cryptography. This involves using mathematical techniques to secure the transactions and prevent any fraudulent activity. The most popular way of securing a cryptocurrency is through the use of a Proof-of-Work algorithm, which is used to validate blocks of transactions and prevent double spending. Additionally, many cryptocurrencies also use a consensus mechanism called Proof-of-Stake, which requires users to stake their coins in order to participate in the network and validate transactions.