A Hash Function is a one-way mathematical algorithm that converts input data of any size into a fixed-size output (hash value or digest). The same input always produces the same hash, but even tiny changes produce completely different outputs. Hashing verifies data integrity—if the hash matches, the data hasn’t been altered.
Cryptographic hash functions must be collision-resistant (hard to find two inputs with the same hash), pre-image resistant (can’t determine input from hash), and second pre-image resistant (can’t find different input with same hash as known input). Common algorithms include SHA-256, SHA-3, and the deprecated MD5 and SHA-1.
CISSP Relevance
Domain 3 (Security Architecture and Engineering) covers hashing extensively. Understand the difference between hashing and encryption (hashing is one-way), hash function properties, and applications including password storage, digital signatures, and integrity verification. Know which algorithms are current versus deprecated.
NIST hash standards are in FIPS 180-4 Secure Hash Standard.
Related terms: Digital Signature, Encryption