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Security Fundamentals & Crypto · Intermediate

Nonce

A 'number used once' that ensures uniqueness in a cryptographic operation to prevent replay and keystream reuse.

Definition

Nonces appear in authenticated encryption modes, stream ciphers, challenge-response authentication, and protocols such as Kerberos and TLS. They may be random or counters, but must not repeat for the same key. Reuse can expose plaintext or allow forgery.

Example

A server sends a random nonce in a challenge; the client returns an HMAC of it to prove key possession without replay.

Exam tip

Nonces defeat replay attacks; reusing a GCM nonce is a critical failure.

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