A hard-core predicate is a concept from cryptography, particularly in the context of cryptographic primitives like pseudorandom generators and one-way functions. It refers to a function or value that is difficult to compute when given only limited information about a related hard problem, typically the output of a one-way function.
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A hard-core predicate is an efficiently computable bit of an input to a one-way function that cannot be predicted from the function output with nonnegligible advantage over random guessing. For a one-way function , padding it to gives the inner-product predicate . This is the Goldreich–Levin construction. It provides a way to mask a bit using an otherwise publicly revealed one-way function value.