What is a nonce?
A nonce is a unique value used once to prevent replay attacks and duplicate requests. Learn where nonces are used and generate one free.
A nonce ("number used once") is a value that must be unique for each use. It stops an attacker from replaying a captured request and stops a client from accidentally processing the same operation twice.
Preventing replay
If a request includes a fresh nonce that the server records and rejects on reuse, capturing and re-sending that request later does nothing — the server has already seen the nonce. This is how APIs defend against replay attacks and how payment/idempotency systems avoid double-charging.
Nonces in the wild
You will find nonces in OAuth flows, cryptographic protocols (where a nonce must never repeat with the same key), Content-Security-Policy script tags, and idempotency keys on APIs. A nonce just needs to be unique; a random token generator is a simple way to produce one.
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