Hashfunktsioone

Hashfunktsioone, or hash functions, are mathematical mappings that assign to every input of arbitrary length a fixed-size output. They are deterministic: the same input yields the same output, and they are designed to be efficient to compute. A key feature is the avalanche effect: small changes to the input produce large, unpredictable changes in the output. In many applications, outputs are assumed to be uniformly distributed across the output space.

There are two broad categories: cryptographic hash functions and non-cryptographic hash functions. Cryptographic hash functions aim to be one-way and resistant to preimage and collision attacks. Their typical properties include preimage resistance, second-preimage resistance, and collision resistance. They are used for data integrity, digital signatures, and blockchains. Widely used cryptographic hashes include SHA-256 and SHA-3; MD5 and SHA-1 are now considered insecure due to discovered collisions.

Non-cryptographic hash functions are optimized for speed and distribution properties in data structures such as hash tables or checksums, where security is less important. Examples include MurmurHash, CityHash, and xxHash.

Passwords should not be hashed with general-purpose hash functions, because they are too fast. Passwords are better protected with salted, memory-hard hash functions such as bcrypt, scrypt, or Argon2, often with a work factor and unique salt per password.

Applications of hash functions encompass data integrity verification, content-addressable storage, deduplication, and the construction of data structures like Merkle trees in distributed systems.