hashfunksjonen

hashfunksjonen refers to a function that maps input data of arbitrary length to a fixed-size string of characters or bits. Hash functions are designed to be fast to compute and to produce hash values that depend on the entire input, providing a compact representation of data.

Key properties include determinism (the same input yields the same hash), fixed output length, and non-reversibility (reconstructing the original input from the hash should be impractical). In cryptographic contexts, additional requirements apply: resilience to preimage attacks (hard to find an input with a given hash), second-preimage attacks (hard to find a different input with the same hash), and collision resistance (hard to find any two inputs with the same hash).

Non-cryptographic hash functions are used for quick data lookup, checksums, and data structures such as hash tables. Cryptographic hash functions emphasize resistance to deliberate manipulation and are designed so that small changes in the input produce large, seemingly random changes in the output, a property known as the avalanche effect.

Common algorithms include MD5, SHA-1, SHA-256, SHA-3, and BLAKE2. MD5 and SHA-1 are now considered broken for collision resistance and are discouraged in security-sensitive contexts; SHA-256 and SHA-3 are widely used as secure baselines. In password storage, hash functions are often combined with salting and key stretching to increase resistance to offline attacks.

Applications of hashfunksjonen span data integrity verification, digital signatures, password storage, content addressing in distributed systems, software package management, and the linking of data in blockchain technologies. Design choices balance speed, resource usage, and resistance to various attacks, with modern practice favoring strong cryptographic hashes or specialized password-hashing schemes.