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Security Fundamentals & Crypto · Intermediate

Initialization Vector (IV)

A random or unique value combined with a key to ensure the same plaintext encrypts to different ciphertext.

Definition

IVs prevent patterns and replay of identical ciphertext across messages encrypted with the same key. Requirements vary by mode: CBC needs unpredictable IVs, while CTR and GCM need unique nonces. IVs are usually sent in the clear with the ciphertext.

Example

WEP's 24-bit IV repeated quickly on busy networks, enabling key recovery.

Exam tip

IVs are not secret but must not repeat; WEP's IV reuse is the classic exam example.

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