Nonce

A nonce (number used once) gives freshness or uniqueness. In AEAD ciphers it makes encrypting the same plaintext twice produce different ciphertexts; it does not need to be secret or random, only unique per key. In protocols it proves a response is fresh (OIDC nonce, attestation challenges, request signing). In CSP it marks which inline scripts the server intended to run.

Requirements differ by use. AES-GCM needs uniqueness per key, and a single repeat is fatal. A CSP nonce must be unpredictable and regenerated on every response, or an attacker can reuse it. A replay-protection nonce must be remembered by the server for as long as the request could be accepted, which is why replay caches pair nonces with a timestamp window.

Where it goes wrong: random nonces that are too short, counters that reset after a restart or are shared by two writers, CSP nonces cached in a CDN page, and replay caches that expire entries before the timestamp window closes.