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.

The IV size depends on the cryptographic primitive used; for block ciphers it is generally the cipher's block-size. In encryption schemes, the unpredictable part of the IV has at best the same size as the key to compensate for time/memory/data tradeoff attacks.<ref>{{cite journal |author = Alex Biryukov |title = Some Thoughts on Time-Memory-Data Tradeoffs |journal = IACR ePrint Archive |year = 2005 |url = http://eprint.iacr.org/2005/207 }}</ref><ref>{{cite journal |author1 = Jin Hong |1author2 = Palash Sarkar |title = Rediscovery of Time Memory Tradeoffs |journal = IACR ePrint Archive |year = 2005 |url = http://eprint.iacr.org/2005/090 }}</ref><ref>{{cite conference

| last1 = Biryukov | first1 = Alex

| last2 = Mukhopadhyay | first1first2 = Sourav

| lastlast3 = 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

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| 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 |authorauthor2 = Joseph Lano |author3 = Bart Preneel |title = Comments on the Rediscovery of Time/Memory/Data Trade-off Algorithm |institution = ECRYPT Stream Cipher Project |number = 140 |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 [[RC1RC4]] 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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== 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>

{{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 }}

</ref>

== See also ==