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JOEL LIKES MEN In [[cryptography]], an '''initialization vector''' ('''IV''') or '''starting variable''' ('''SV''')<ref>ISO/IEC 10116:2006 ''Information technology — Security techniques — Modes of operation for an ''n''-bit block cipher''</ref> is a fixed-size input to a [[cryptographic primitive]] that is typically required to be [[random]] or [[pseudorandom]]. [[Randomization]] is crucial for [[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 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]]. Randomization is also required for other primitives, such as [[universal hash function]]s and [[message authentication code]]s based thereon.
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]] (''number used once''), and the primitives are described as ''stateful'' as opposed to ''randomized''. This is because the 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 uses a [[sequence number]] as a nonce.
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