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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]] (
The
| last1 = Biryukov | first1 = Alex
| last2 = Mukhopadhyay | first2 = Sourav
| last3 = Sarkar | first3 = Palash
| editor1-last = Preneel | editor1-first = Bart
| editor2-last = Tavares | editor2-first = Stafford E.
| contribution = Improved Time-Memory Trade-Offs with Multiple Data
| doi = 10.1007/11693383_8
| pages = 110–127
| publisher = Springer
| series = Lecture Notes in Computer Science
| title = Selected Areas in Cryptography, 12th International Workshop, SAC 2005, Kingston, ON, Canada, August 11-12, 2005, Revised Selected Papers
| volume = 3897
| year = 2005| doi-access = free
| isbn = 978-3-540-33108-7
}}</ref><ref name="ECRYPT">{{cite tech report |author1 = Christophe De Cannière |author2 = Joseph Lano |author3 = Bart Preneel |title = Comments on the Rediscovery of Time/Memory/Data Trade-off Algorithm |institution = ECRYPT Stream Cipher Project |number = 40 |year = 2005 |url = http://www.ecrypt.eu.org/stream/papersdir/040.pdf }}</ref> When the IV is chosen at random, the probability of collisions due to the [[birthday problem]] must be taken into account. Traditional stream ciphers such as [[RC4]] do not support an explicit IV as input, and a custom solution for incorporating an IV into the cipher's key or internal state is needed. Some designs realized in practice are known to be insecure; the [[Wired Equivalent Privacy|WEP]] protocol is a notable example, and is prone to related-IV attacks.
==Motivation==
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A [[block cipher]] is one of the most basic [[cryptographic primitive|primitive]]s in cryptography, and frequently used for data [[encryption]]. However, by itself, it can only be used to encode a data block of a predefined size, called the [[block size (cryptography)|block size]]. For example, a single invocation of the [[Advanced Encryption Standard|AES]] algorithm transforms a 128-bit [[plaintext]] block into a [[ciphertext]] block of 128 bits in size. The [[cryptographic key|key]], which is given as one input to the cipher, defines the mapping between plaintext and ciphertext. If data of arbitrary length is to be encrypted, a simple strategy is to split the data into blocks each matching the cipher's block size, and encrypt each block separately using the same key. This method is not secure as equal plaintext blocks get transformed into equal ciphertexts, and a third party observing the encrypted data may easily determine its content even when not knowing the encryption key.
To hide patterns in encrypted data while avoiding the re-issuing of a new key after each block cipher invocation, a method is needed to [[randomization|randomize]] the input data. In 1980, the [[National Institute of Standards and Technology|NIST]] published a national standard document designated [[Federal Information Processing Standard]] (FIPS) PUB 81, which specified four so-called [[Block cipher mode of operation|block cipher modes of operation]], each describing a different solution for encrypting a set of input blocks. The first mode implements the simple strategy described above, and was specified as the [[electronic codebook]] (ECB) mode. In contrast, each of the other modes describe a process where ciphertext from one block encryption step gets intermixed with the data from the next encryption step. To initiate this process, an additional input value is required to be mixed with the first block, and which is referred to as an ''initialization vector''. For example, the [[cipher-block chaining]] (CBC) mode requires an unpredictable value, of size equal to the cipher's block size, as additional input
==Properties==
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== WEP IV ==
The [[802.11]] [[encryption]] [[algorithm]] called WEP (short for [[Wired Equivalent Privacy]]) used a short, 24-bit IV, leading to reused IVs with the same key, which led to it being easily cracked.<ref name="Intercepting_Mobile_Comm_Nik_Ian_Dav">{{cite
== SSL 2.0 IV ==
In [[Block cipher mode of operation#Cipher_block_chaining_(CBC)|cipher-block chaining mode]] (CBC mode), the IV need not be secret, but must be unpredictable (In particular, for any given plaintext, it must not be possible to predict the IV that will be associated to the plaintext in advance of the generation of the IV.) at encryption time. Additionally for the [[Block cipher mode of operation#OFB|output feedback mode]] (OFB mode), the IV must be unique.<ref>{{citation |author = Morris Dworkin |title = NIST Recommendation for Block Cipher Modes of Operation; Chapters 6.2 and 6.4 |date = 2001 |url = https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-38a.pdf }}</ref> In particular, the (previously) common practice of re-using the last ciphertext block of a message as the IV for the next message is insecure (for example, this method was used by SSL 2.0). If an attacker knows the IV (or the previous block of ciphertext) before he specifies the next plaintext, he can check his guess about plaintext of some block that was encrypted with the same key before. This is known as the TLS CBC IV attack, also called the [[Transport Layer Security#BEAST attack|BEAST attack]].<ref>{{citation |author = B. Moeller |title = Security of CBC Ciphersuites in SSL/TLS: Problems and Countermeasures |date = May 20, 2004 |url = http://www.openssl.org/~bodo/tls-cbc.txt |access-date = September 1, 2014 |archive-date = June 30, 2012 |archive-url = https://web.archive.org/web/20120630143111/http://www.openssl.org/~bodo/tls-cbc.txt |url-status = dead }}</ref>
== See also ==
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* [[Block cipher mode of operation|Block cipher modes of operation]]
* [[CipherSaber]] (RC4 with IV)
== References ==
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== Further reading ==
* {{cite book |first = B. |last = Schneier |
* {{cite book |
{{Cryptography navbox | block | stream}}
[[Category:Block cipher modes of operation]]
[[Category:Cryptography]]
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