hashes

In computing, a hash is the result of a function that maps input data of arbitrary size to a fixed-size output, commonly called a hash value or digest. A good hash function is deterministic, fast to compute, and designed so that small changes in the input produce large, unpredictable changes in the output (the avalanche effect). Hashes are widely used for data integrity, indexing, and content addressing.

Hash functions come in two main varieties: cryptographic hashes and non-cryptographic hashes. Cryptographic hashes, such as SHA-256, SHA-3, and BLAKE3, aim to be preimage and collision resistant, making it hard to recover inputs or find two inputs that yield the same hash. Non-cryptographic hashes, such as MurmurHash or CityHash, are optimized for speed and uniform distribution but are not secure against tampering and should not be used for security purposes.

Common applications include verifying data integrity (checksums), creating digital signatures and message authentication codes, and storing passwords with salt and key-stretching algorithms (for example bcrypt, scrypt, Argon2). They are also used in content-addressable storage and version control where the hash identifies content, as in Git’s use of hash digests. In data structures, hash functions underpin hash tables and, in distributed systems, consistent hashing.

Hash collisions—two distinct inputs producing the same hash—are possible by the pigeonhole principle. Security relies on the collision resistance of the chosen hash; many older hashes (MD5, SHA-1) have demonstrated feasible collisions and are no longer considered secure for cryptographic purposes. No hash function is immune to all attacks, but a well-designed cryptographic hash remains resistant to practical preimage and collision attacks for its intended lifetime.