Hash

A hash is a function that maps data of arbitrary length to a fixed-size string of characters, commonly called a hash value or digest. In computing, hash functions are used to enable fast data lookup with hash tables, to detect changes, and, in many cases, to support security.

A hash function should be deterministic, producing the same output for the same input, and it should have a fixed output length. Non-cryptographic hashes emphasize speed and distribution; cryptographic hashes add resistance to preimage and collision attacks. Common properties include being fast to compute, having uniform output distribution, and resisting easy inversion.

Cryptographic hash functions are designed to satisfy stronger properties such as preimage resistance, second-preimage resistance, and collision resistance. These properties underpin uses in digital signatures, message authentication, and data integrity guarantees. Examples of hash algorithms include MD5, SHA-1, SHA-256, SHA-3, and BLAKE3; MD5 and SHA-1 are now considered insecure for collision resistance and are generally avoided in security contexts.

Uses range from data integrity checksums and password hashing (often with salts and key-stretching algorithms like bcrypt, scrypt, or Argon2) to digital signatures, blockchain, and various authentication mechanisms. In data structures, a hash function maps keys to bucket indices in a hash table, with collisions resolved by chaining or open addressing.

In practice, cryptographic hashes are designed to be deterministic, collision-resistant, and resistant to efficient preimage attacks, while non-cryptographic hashes prioritize speed and practical distribution for general-purpose data handling.