In Ethereum, every transaction has a nonce. The nonce is the number of transactions sent from a given address.
In English, a nonce is a number that can only be used once. In cryptography, a nonce is a one-time code selected in a random or pseudo-random manner that is used to securely transmit a main password, preventing replay attacks.
Each time you send a transaction, the nonce value increases by 1. There are rules about what transactions are considered valid transactions, and the nonce is used to enforce some of these rules. Specifically:
Transactions must be in order. You cannot have a transaction with a nonce of
1mined before one with a nonce of0.No skipping! You cannot have a transaction with a nonce of
2mined if you have not already sent transactions with a nonce of1and0.
Why does it matter?
This value prevents double-spending, as the nonce will always specify the order of transactions. If a double-spend does occur, it’s typically due to the following process:
A transaction is sent to one party.
They wait for it to register.
Something is collected from this first transaction.
Another transaction is quickly sent with a high gas price.
The second transaction is mined first, therefore invalidating the first transaction.
This is why exchanges wait for you to have a certain number of confirmations before allowing you to trade freshly-deposited funds.
When using the Ethereum blockchain, the above is not possible.
In Ethereum, this method of “double-spending” is not possible because each transaction has a nonce included with it. Even if you attempt to do the above, it will not work as the second transaction (nonce of 3) cannot be mined before the first transaction (nonce of 2).
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