Nonce

A Nonce is a sequential number attached to a blockchain transaction from a specific account. It ensures the network processes transactions in the correct order. It also prevents replay attacks, where an attacker resends a valid transaction. Also called a transaction nonce or sequence number.

The nonce works by starting at zero for each new account. The sender increments it by one for every outgoing transaction. Blockchain nodes verify that the transaction's nonce matches the account's current nonce. They reject transactions with incorrect nonces. For example, if an account's nonce is 5, the next transaction must use 6. This creates an ordered queue.

Nonces matter for security and reliability. They stop double-spending by enforcing sequence. Without them, attackers could replay transactions across blocks or chains. They also enable smart contract logic that depends on transaction order.

Key characteristics include uniqueness per account, strict incrementation, and protocol enforcement. Some chains like Ethereum reset nonces on failures, but most require exact matches. Miners use a separate nonce for proof-of-work blocks, distinct from transaction nonces.