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[[File:KDF chain.png|300px|thumb|right|Example of a Key Derivation Function chain as used in the [[Signal Protocol]]. The output of one KDF function is the input to the next KDF function in the chain.]]
In [[cryptography]], a '''key derivation function''' ('''KDF''') is a cryptographic algorithm that derives one or more [[key (cryptography)|secret key]]s from a secret value such as a master key, a [[password]], or a [[passphrase]] using a [[pseudorandom function]] (which typically uses a [[cryptographic hash function]] or [[block cipher]]).<ref>{{cite book|author=Bezzi, Michele|chapter=Data privacy |editor=Camenisch, Jan|title=Privacy and Identity Management for Life|publisher=Springer|year=2011|isbn=9783642203176|pages=185–186|chapter-url=https://books.google.com/books?id=vYxzh3C6OPUC&pg=PA185|display-authors=etal|display-editors=etal}}</ref>{{Ref RFC|8018}}<ref>{{cite web|author=Chen, Lily|title=NIST SP 800-108: Recommendation for Key Derivation Using Pseudorandom Functions|publisher = NIST|date = October 2009|url=https://csrc.nist.gov/publications/detail/sp/800-108/final }}</ref> KDFs can be used to stretch keys into longer keys or to obtain keys of a required format, such as converting a group element that is the result of a [[Diffie–Hellman key exchange]] into a symmetric key for use with [[Advanced Encryption Standard|AES]]. [[HMAC|Keyed cryptographic hash functions]] are popular examples of pseudorandom functions used for key derivation.<ref>{{cite book|author=Zdziarski, Jonathan|title=Hacking and Securing IOS Applications: Stealing Data, Hijacking Software, and How to Prevent It| publisher=O'Reilly Media|year=2012|isbn=9781449318741|pages=252–253|url=https://books.google.com/books?id=2D50GNA1ULsC&pg=PA252}}</ref>
==History==
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