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In [[computer science]], a '''one-way function''' is a [[function (mathematics)|function]] that is easy to compute on every input, but hard to [[Inverse function|invert]] given the [[image (mathematics)|image]] of a random input. Here, "easy" and "hard" are to be understood in the sense of [[computational complexity theory]], specifically the theory of [[polynomial time]] problems. This has nothing to do with whether the function is [[One-to-one function|one-to-one]]; finding any one input with the desired image is considered a successful inversion. (See {{slink||Theoretical definition}}, below.)
The existence of such one-way functions is still an open [[conjecture]]. Their existence would prove that the [[complexity classes]] [[P = NP problem|P and NP are not equal]], thus resolving the foremost unsolved question of theoretical computer science.<ref name=Goldreich>[[Oded Goldreich]] (2001). Foundations of Cryptography: Volume 1, Basic Tools ([http://www.wisdom.weizmann.ac.il/~oded/PSBookFrag/part2N.ps draft available] from author's site). Cambridge University Press. {{isbn|0-521-79172-3}}. See also [http://www.wisdom.weizmann.ac.il/~oded/foc-book.html wisdom.weizmann.ac.il].</ref>{{rp|ex. 2.2, page 70}} The converse is not known to be true, i.e. the existence of a proof that P ≠ NP would not directly imply the existence of one-way functions.<ref>[[Shafi Goldwasser|Goldwasser, S.]] and [[Mihir Bellare|Bellare, M.]] [http://cseweb.ucsd.edu/~mihir/papers/gb.html "Lecture Notes on Cryptography"] {{Webarchive|url=https://web.archive.org/web/20120421084751/http://cseweb.ucsd.edu/~mihir/papers/gb.html |date=2012-04-21 }}. Summer course on cryptography, MIT, 1996–2001.</ref>
In applied contexts, the terms "easy" and "hard" are usually interpreted relative to some specific computing entity; typically "cheap enough for the legitimate users" and "prohibitively expensive for any [[Black hat hacking|malicious agents]]".{{citation needed|date=September 2023}} One-way functions, in this sense, are fundamental tools for [[cryptography]], [[personal identification]], [[authentication]], and other [[data security]] applications. While the existence of one-way functions in this sense is also an open conjecture, there are several candidates that have withstood decades of intense scrutiny. Some of them are essential ingredients of most [[telecommunications]], [[e-commerce]], and [[Online banking|e-banking]] systems around the world.
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