Editing Hash collision (section)

{{Short description|Hash function phenomenon}}
[[File:Hash table 4 1 1 0 0 1 0 LL.svg|thumb|254x254px|John Smith and Sandra Dee share the same hash value of 02, causing a hash collision.]]
In [[computer science]], a '''hash collision''' or '''hash clash'''<ref>{{Citation|last=Thomas|first=Cormen|title=Introduction to Algorithms |date=2009|pages=253|publisher=MIT Press|isbn=978-0-262-03384-8}}</ref>  is when two distinct pieces of data in a [[hash table]] share the same hash value. The hash value in this case is derived from a [[hash function]] which takes a data input and returns a fixed length of bits.<ref>{{Citation|last=Stapko|first=Timothy|title=Embedded Security|date=2008|url=http://dx.doi.org/10.1016/b978-075068215-2.50006-9|work=Practical Embedded Security|pages=83–114|publisher=Elsevier|doi=10.1016/b978-075068215-2.50006-9|isbn=9780750682152|access-date=2021-12-08}}</ref>

Although hash algorithms, especially cryptographic hash algorithms, have been created with the intent of being [[Collision resistance|collision resistant]], they can still sometimes map different data to the same hash (by virtue of the [[pigeonhole principle]]). Malicious users can take advantage of this to mimic, access, or alter data.<ref>{{cite web|last1=Schneier|first1=Bruce|author-link1=Bruce Schneier|title=Cryptanalysis of MD5 and SHA: Time for a New Standard|url=https://www.schneier.com/essays/archives/2004/08/cryptanalysis_of_md5.html|url-status=dead|archive-url=https://web.archive.org/web/20160316114109/https://www.schneier.com/essays/archives/2004/08/cryptanalysis_of_md5.html|archive-date=2016-03-16|access-date=2016-04-20|website=Computerworld|quote=Much more than encryption algorithms, one-way hash functions are the workhorses of modern cryptography.}}</ref>

Due to the possible negative applications of hash collisions in [[data management]] and [[computer security]] (in particular, [[cryptographic hash function]]s), collision avoidance has become an important topic in computer security.