A cryptographic hash function takes an input of any size and produces a fixed-length string of characters called a hash or digest. SHA-256, for example, always outputs 256 bits regardless of input size. Three properties matter. First, it is deterministic, the same input always produces the same output. Second, it is one-way, you cannot reconstruct the input from the hash. Third, it is collision-resistant, finding two different inputs that produce the same hash is astronomically unlikely. Change a single character in the input and the output changes completely. In practice, hashes verify data integrity. Software publishers post hash values alongside downloads. Your computer recomputes the hash and compares. A mismatch means the file was altered. Password systems store hashes instead of plaintext, so breaches reveal no usable passwords. Blockchain networks use hashes to chain blocks together, creating an immutable ledger where altering one transaction would break every subsequent link.
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