Nonce

Definition

A nonce is a value used within a protocol to distinguish operations or vary an input, but its exact role depends on context. In Bitcoin mining, the block header includes a 32-bit nonce field that miners can change while searching for a hash satisfying the target. Exhausting that field does not exhaust every possible candidate, because miners can also change other permitted data, such as information in the coinbase transaction that affects the Merkle root. In account-based systems such as Ethereum, an account transaction nonce instead helps order transactions and prevent repeated execution of the same authorized operation. Cryptographic signature schemes use another kind of nonce with strict security requirements. These meanings are related by the use of a distinguishing value, but they are not interchangeable. A miner's header nonce is not a wallet password, an account balance, or a measure of mining difficulty.