A hash function takes input of any size and produces a fixed-length output called a hash, digest or fingerprint. The same input always gives the same hash, but even a one-character change produces a completely different result. A good cryptographic hash is one-way, meaning you cannot work backward from the hash to the input, and collision-resistant, meaning it is infeasible to find two different inputs with the same hash. Hashing protects integrity, not confidentiality: it does not hide data, it lets you detect whether data has changed. It is also the foundation of how systems should store passwords.
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