Initialization vector: Difference between revisions

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{{Short description|Input to a cryptographic primitive}}

In [[cryptography]], an '''initialization vector''' ('''IV''') or '''starting variable'''<ref>ISO/IEC 10116:2006 ''Information technology — Security techniques — Modes of operation for an ''n''-bit block cipher''</ref> is an input to a [[cryptographic primitive]] being used to provide the initial state. The IV is typically required to be [[random]] or [[pseudorandom]], but sometimes an IV only needs to be unpredictable or unique. [[Randomization]] is crucial for some [[encryption]] schemes to achieve [[semantic security]], a property whereby repeated usage of the scheme under the same [[cryptographic key|key]] does not allow an attacker to infer relationships between (potentially similar) segments of the encrypted message. For [[block cipher]]s, the use of an IV is described by the [[Block cipher mode of operation|modes of operation]].

Some cryptographic primitives require the IV only to be non-repeating, and the required randomness is derived internally. In this case, the IV is commonly called a [[cryptographic nonce|nonce]] (a number used only once), and the primitives (e.g. [[Block_cipher_mode_of_operation#CBC|CBC]]) are considered ''stateful'' rather than ''randomized''. This is because an IV need not be explicitly forwarded to a recipient but may be derived from a common state updated at both sender and receiver side. (In practice, a short nonce is still transmitted along with the message to consider message loss.) An example of stateful encryption schemes is the [[counter mode]] of operation, which has a [[sequence number]] for a nonce.