Hashed

Hashed describes data that has been processed by a hash function to produce a digest or hash value. A hash function maps input of arbitrary length to an output of fixed length. In general, hashing aims to be deterministic, so the same input yields the same hash. In cryptographic contexts, a hash should be fast to compute but hard to invert and hard to find collisions, though no hash is perfectly collision-free. There are two main categories: cryptographic hashes used for security (such as SHA-256, SHA-3, and BLAKE2) and non-cryptographic hashes used for data structures or checksums (such as MurmurHash or CityHash). The outputs are often called digests or hashes.

Hashed data are used to verify integrity; you compare hashes to detect tampering. They are also used in password storage; passwords are not stored in plain text but as salted hashes. A unique salt is added to each password before hashing to thwart precomputation and rainbow tables. Password hashing may involve multiple iterations (key stretching) to slow attacks. In indexing and data retrieval, hash values can serve as keys in hash tables; in file systems, content-addressable storage uses content hashes to identify blocks.

Important limitations: a hash does not reveal the original input; however, hash values can sometimes be reversed via brute force or rainbow tables for weak hashes, especially when unsalted or using short digests. Salt and pepper, stronger algorithms, and proper iteration mitigate these risks. Common examples of current cryptographic hashes include SHA-256, SHA-3-256, and BLAKE2; older hashes like MD5 or SHA-1 are considered insecure for cryptographic use.