Nonce
Number used once — in mining and for uniqueness of Ethereum transactions.
Nonce (Number used ONCE) is a term with two different uses in the crypto context.
1. Mining nonce (Bitcoin):
•Miners search for a nonce value that, combined with block data, gives a hash below target threshold
•They change nonce from 0 to 4 billion trying every combination
•When found → valid block, get reward
•Brute force in milliseconds — 100+ exahash/s on global network
2. Account nonce (Ethereum):
•Every Ethereum address has a nonce that increases with each transaction
•Nonce 0 = first transaction, nonce 1 = second...
•Prevents replay attacks — same transaction can't be broadcast twice
•Transactions must be processed in order (nonce 5 can't come before nonce 4)
Practical nonce problem:
•"Stuck transaction" — waiting because previous transaction with lower gas price isn't going through
•Solution: send new transaction with SAME nonce and higher gas price
•MetaMask Advanced Settings → Customize transaction nonce