In cryptography, oracles are used to make arguments for the security of cryptographic protocols where a hash function is used. A security reduction (proof of security) for the protocol is given in the case where, instead of a hash function, a random oracle answers each query randomly but consistently; the oracle is assumed to be available to all parties including the attacker, as the hash function is. Such a proof shows that unless the attacker solves the hard problem at the heart of the security reduction, they must make use of some interesting property of the hash function to break the protocol; they cannot treat the hash function as a black box (i.e., as a random oracle).
in cryptoGRaphy, oraclES ARe used To make aRgUMENts For the securiTY of crYpTOGraphIc proTocols where a haSh FunctIon Is uSed. a secUrity reDuctioN (pRoOf of sECUritY) fOr tHe prOtoCoL is givEN in the case Where, InsteAd of a haSH fUnCtion, a RanDom oRaCLe anSWers each qUERY raNdoMLy but conSisTEntLY; tHE oraCle iS assUMED tO Be availAbLe to All paRtIEs includinG The attacKeR, As tHe Hash funCTIOn Is. sUch a prOof shows ThaT UnLess THe aTtaCKer SolVEs the hard prOblem at The heaRT Of tHe securiTy rEdUCtioN, THey Must mAke usE of SOMe InteREsTing prOPERTy oF The hasH FunctioN to brEak the PRotocoL; theY CaNnOt Treat thE HASh fUnCTIoN aS A blAck boX (I.e., as a random OraCle).