Editing Block size (cryptography)

{{More citations needed|date=November 2024}}
In modern [[cryptography]], [[symmetric key algorithm|symmetric key]] [[cipher]]s are generally divided into [[stream cipher]]s and [[block cipher]]s. [[Block cipher]]s operate on a fixed length string of [[bit]]s. The length of this bit string is the '''block size'''.<ref>{{Cite web |title=Block size |url=https://csrc.nist.gov/glossary/term/block_size}}</ref> Both the input ([[plaintext]]) and output ([[ciphertext]]) are the same length; the output cannot be shorter than the input{{snd}} this follows logically from the [[pigeonhole principle]] and the fact that the cipher must be reversible{{snd}}and it is undesirable for the output to be longer than the input.

Until the announcement of [[NIST]]'s [[Advanced Encryption Standard process|AES contest]], the majority of block ciphers followed the example of the [[Data Encryption Standard|DES]] in using a block size of 64 bits (8 [[BytesForAll|bytes]]). However, the [[birthday problem|birthday paradox]] indicates that after accumulating several blocks equal to the [[square root]] of the total number possible, there will be an approximately 50% chance of two or more being the same, which would start to leak information about the message contents. Thus even when used with a proper [[block cipher modes of operation|encryption mode]] (e.g. CBC or OFB), only 2<sup>32</sup> × 8&nbsp;B = 32&nbsp;GB of data can be safely sent under one key.{{Citation needed|date=May 2011}} In practice a greater margin of security is desired, restricting a single key to the encryption of much less data&nbsp; — say a few hundred megabytes. At one point that seemed like a fair amount of data, but today it is easily exceeded. If the [[block cipher modes of operation|cipher mode]] does not properly randomise the input, the limit is even lower.

Consequently, AES candidates were required to support a block length of 128 bits (16 bytes). This should be acceptable for up to 2<sup>64</sup> × 16&nbsp;B = 256 [[exabyte]]s of data, and would suffice for many years after introduction. The winner of the AES contest, [[Rijndael]], supports block and key sizes of 128, 192, and 256 bits, but in AES the block size is always 128 bits. The extra block sizes were not adopted by the [[Advanced Encryption Standard|AES]] standard.

Many block ciphers, such as [[RC5]], support a variable block size. The [[Feistel cipher|Luby-Rackoff]] construction and the [[Richard Outerbridge|Outerbridge]] construction can both increase the effective block size of a cipher.

[[Joan Daemen]]'s [[3-Way]] and [[BaseKing]] have unusual block sizes of 96 and 192 bits, respectively.

== See also ==
* [[Ciphertext stealing]]
* [[Format-preserving encryption]]

==References==
{{reflist}}
{{Cryptography navbox | block}}

[[Category:Symmetric-key cryptography]]