A hash function is a one way algorithm that takes input of any size and produces a fixed length output called a hash or digest. The same input always yields the same digest, yet changing even a single bit produces a completely different result. That property makes hashing the standard way to verify integrity, since a matching hash means the data has not been altered in transit or storage.
A cryptographic hash needs three qualities: it should be preimage resistant, so you cannot work backward from a digest to the input, second preimage resistant, so you cannot find a different input matching a known one, and collision resistant, so no two inputs share a digest. SHA-256 and SHA-3 are the current workhorses, while MD5 and SHA-1 are broken and should never protect anything new. The classic exam trap is confusing hashing with encryption, since hashing is not reversible and uses no key, so it delivers integrity, not confidentiality.
How are hash functions tested on the CISSP exam?
Hashing is covered in Domain 3: Security Architecture and Engineering, which carries 13 percent of the exam. Know the required properties, the applications like password storage and signatures, and which algorithms are deprecated.