A nonce is a number used only one time for security in cryptography or encryption algorithms. A nonce is an abbreviation for ‘number used once’ and is a type of random sequence or string of numbers or characters generated for a particular purpose. Nonces are commonly used in computer networks and cryptography to authenticate data transmissions and prevent potential data tampering or cyber attacks.
Nonces can also be used to enable replay protection which prevents an attacker from re-transmitting previously sent data in an attempt to break into an application or system. It does this by requiring the nonce to be changed each time a communication is sent.
The nonce could either be generated randomly or might be tied to information that the sender and receiver already possess such as a timestamp. Nonces are also commonly used in digital signatures with public keys to ensure that they are not counterfeited.
Nonces are also used in many blockchains such as Bitcoin, Ethereum, Litecoin and many more. In this context, a nonce is generated when a data block is added to a ledger and it must increase as each new data block is added sequentially. The purpose of a nonce in a blockchain is to ensure that no blocks are modified and also provides an inequality proof.
This tells the system that the hash function dictates that miners must spend their time and resources addressing the problem and cannot just select transactions from the past and try to reinsert them into the chain.