Three things make cryptocurrencies unique: decentralisation, scalability and security. Firstly, cryptos can function without a central governing body. They can also scale enough to challenge fiat currencies. And finally, they employ one of the highest forms of security available to humankind: cryptographic encryption. In this article, we will learn about a concept at the heart of blockchain security — the nonce —what it is and how it is used in crypto. The word nonce is short for ‘number used only once.’ It may sound like a simple concept, but its use cases range from small transactions to space stations. In finance and cryptography, a nonce refers to a randomly generated number, and it is used to verify transactions or perform security checks. For example, the captchas we often find on websites are nonces (albeit with letters included) as they are used just once. In a much simpler case, even the OTPs sent to verify transactions are nonces that have the singular use of verifying something for a limited period of time.