Formula: hash = SHA(message) → fixed-length hex digest
SHA (Secure Hash Algorithm) produces a fixed-size fingerprint from any input. SHA-1 outputs 160 bits (40 hex chars), SHA-256 outputs 256 bits (64 hex chars), and SHA-512 outputs 512 bits (128 hex chars). Hashes are one-way — you cannot reverse them to recover the original input. They're used for data integrity verification, password storage, and digital signatures.