What is Hash function?
A cryptographic hash function is a one-way mathematical function that maps an input of arbitrary size to a fixed-length output called a hash, digest, or checksum. The function is deterministic (same input always produces the same output), fast to compute, and practically impossible to reverse — you cannot reconstruct the input from the output.
A secure cryptographic hash function has three key properties. Preimage resistance means that given a hash value, it is computationally infeasible to find any input that produces it. Second preimage resistance means that given an input and its hash, it is infeasible to find a different input with the same hash. Collision resistance means it is infeasible to find any two different inputs that produce the same hash.
The most widely used hash functions today are the SHA-2 family (SHA-256, SHA-384, SHA-512) and SHA-3. MD5 and SHA-1 are deprecated for security use because practical collision attacks exist, but they still appear in non-security contexts such as file deduplication and legacy checksums.
Hash functions serve many purposes in computing. Password storage: instead of storing passwords in plain text, systems store the hash (ideally salted and with a slow function like bcrypt or Argon2) so that even if the database is stolen, passwords are not immediately compromised. Data integrity: checksums let you verify that a downloaded file was not corrupted or tampered with. Digital signatures: a signature is computed over the hash of a document, not the document itself, for efficiency. Content-addressing: Git identifies every commit, tree, and blob by its SHA-1 hash (migrating to SHA-256). Bitcoin uses double-SHA-256 to link blocks in the chain.
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