bcrypt is a password-hashing function designed by Niels Provos and David Mazières. It is based on the Blowfish cipher and presented at USENIX in 1999. Besides incorporating a salt to protect against rainbow table attacks, bcrypt is an adaptive function: over time, the iteration count can be increased to make it slower, so it remains resistant to brute-force search attacks even with increasing computation power.
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usenix.org →playanimportantroleinthevastmajorityofuser- authenticationsystems. Thispaperdiscusseswaysofbuildingsystemsin whichpasswordsecuritykeepsupwithhardware speeds.Wepresenttwoalgorithmswithadaptable cost eksblow sh,ablockcipherwithapurposefully expensivekeyschedule,andbcrypt,arelatedhash function.Failingamajorbreakthroughincomplex- itytheory,thesealgorithmsshouldallowpassword- basedsystemstoadapttohardwareimprovements andremainsecure20yearsintothefuture. Therestofthepaperisorganizedasfollows.In Section2,wediscussrelatedworkonpasswordsecu- rity.InSection3,weexplaintherequirementsfora goodpasswordscheme.Section4presentseksblow- sh,a64-bitblockcipherthatletsuserstunethe costofthekeyschedule.Section5introducesthe variable-costbcryptpasswordhashingfunctionand describesourimplementationintheOpenBSDop- eratingsystem.Finally,Section6comparesbcrypt totwowidely-usedpasswordhashingfunctions. 2RelatedWork Passwordguessingattackscanbecategorizedby theamountofinteractiontheyrequirewithanau- thenticationsystem.Inon-lineattacks,theperpe- tratormustmakeuseofanauthenticationsystem tocheckeachguessofapassword.Ino -lineat- tacks,anattackerobtainsinformation suchasa passwordhash thatallowshimtocheckpassword guessesonhisown,withnofurtheraccesstothe system.On-lineattacksaregenerallyconsiderably slowerthano -lineones.Systemscandetecton- lineattacksfairlyeasilyanddefendagainstthemby slowingtherateofpasswordchecking.Incontrast, onceanattackerhasobtainedpasswordveri cation information,theonlyprotectionasystemhasfrom o -lineattacksisthecomputationalcostofchecking potentialpasswords. Techniquesformitigatingthethreatofo -linepass- wordguessinggenerallyaspiretooneoftwogoals limitingasystem& 39;ssusceptibilitytoo -lineattacks orincreasingtheircomputationalcost.Asasimple exampleoftheformer,manymodernUNIXsystems nowkeeppasswordhashessecretfromusers,stor- ingtheminaread-protectedshadowpassword le ratherthaninthestandardopenlyreadableone. Muchoftheworkonpreventingo -linepassword attackshascenteredaroundcommunicationover insecurenetworks.Ifcryptographicprotocolsrely onuser-chosenpasswordsaskeys,theymayopen themselvesuptoo -lineguessingattacks.Gong et.al.[7]suggestseveralprotocoldesigntricksto thwartpasswordguessingbynetworkattackers.Un- fortunately,theirmostinterestingproposalsrequire encryptionalgorithmswithunusualanddicultto achieveproperties. Severalpeoplehavedesignedsecurepasswordpro- tocolsthatletusersauthenticatethemselvesover insecurenetworkswithouttheneedtorememberor certifypublickeys.BellovinandMerritt[2,3] rst proposedtheidea,givingseveralconcreteproto- colsputativelyresistanttoo -lineguessingattacks. Patel[11]latercryptanalyzedthoseprotocols,but peoplehavesincecontinueddevelopingandre ning othersinthesamevein.Morerecentproposalssuch asSRP[16]showpromiseofbeingsecure. Ofcourse,evenasecurepasswordprotocolrequires someservercapableofvalidatinguserswithcorrect passwords.Anattackerwhoobtainsthatserver& 39;s secretstatecanmountano -lineguessingattack. Becausesecurepasswordprotocolsrequirepublic keycryptography[8],theydohaveatunablekey lengthparameter.However,thisparameterpri- marilycontrolsthedicultyofmountingo -line attackswithoutaserver& 39;ssecretstate;itonlyin- directlya ectsthecostofano -lineattackgiven thatstate.Tuningkeylengthtopreservepassword guessingcostswouldhaveotherunintendedconse- quences,forinstanceincreasingmessagesizesand costingserversunnecessarycomputation.Bycom- biningaschemelikeSRPwiththebcryptalgorithm presentedinthispaper,however,onecanvarythe costofguessingpasswordsindependentlyfrommost otherpropertiesoftheprotocol. Whateverprogressoccursinpreventingo -lineat- tacks,onecanneverrulethemoutentirely.Infact, thedecisiontohaveanopenlyreadablepassword lewasnotanoversightonthepartoftheUNIX systemdesigners[9].Rather,itwasareactionto thedicultyofkeepingthepassword lesecretin previoussystems,andtotherealizationthatasup- posedlysecretpassword lewouldneedtoresist o -lineguessinganyway.Thisrealizationremains equallytruetoday.Asidefromtheobviousissues
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bcrypt is a password-hashing function designed by Niels Provos and David Mazières. It is based on the Blowfish cipher and presented at USENIX in 1999. Besides incorporating a salt to protect against rainbow table attacks, bcrypt is an adaptive function: over time, the iteration count can be increased to make it slower, so it remains resistant to brute-force search attacks even with increasing computation power.
The bcrypt function is the default password hash algorithm for OpenBSD, and was the default for some Linux distributions such as SUSE Linux.
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Discovered by embedding cosine similarity (sentence-transformers MiniLM, 384-dim).