This computer science lesson is the fifth in a series about cryptocurrency and non-fungible tokens, also known as NFTs. The series examines some of the technologies underlying various cryptocurrencies such as Bitcoin, as well as some the ethical, cultural and legal implications of these digital currencies. In this particular lesson you will how Bitcoin transactions are validated before they are added to the blockchain. You will learn about the role of hashing algorithms, including SHA256 and RIPEMD160, when creating a new Bitcoin address for pay to public key hash P2PKH transactions, and the role of asymmetric key cryptography, also known as public private key cryptography, when creating a digital signature for a transaction. You will see how cryptographic techniques are used to verify the ownership of UTXOs and prevent double spending of a UTXO. In addition, you will learn about the role of the transaction pool, also known as the mempool, and the role of a database called the UTXO set, copies of which are maintained by nodes in the bitcoin network.