NonXOFs

NonXOFs, or non-extendable-output functions, are cryptographic hash functions that produce a fixed-length digest for any given input. They are contrasted with extendable-output functions (XOFs), such as SHAKE, which can generate outputs of arbitrary length by requesting more output bits. The term “NonXOF” is used to differentiate fixed-output hash functions from those designed to yield variable-length streams.

These functions are designed to be deterministic and to exhibit properties such as preimage resistance and collision resistance under standard assumptions. Their security relies on the design of the underlying compression function (in Merkle–Damgård constructions) or on the sponge parameters in sponge-based designs. NonXOFs typically have standardized output lengths, such as 128, 160, 224, 256, or 512 bits, chosen to fit particular security goals and applications.

Typical examples include MD5, SHA-1, SHA-256, and SHA-3 variants that produce fixed-length digests (for example, SHA3-256). Older functions like MD5 and SHA-1 have fallen out of favor for new designs due to vulnerabilities, while SHA-256 and SHA3-256 remain widely used in practice. Since their outputs are fixed in length, NonXOFs are well suited for conventional data integrity checks and digital signature schemes, where a predictable digest size simplifies protocol design.

Limitations of NonXOFs include the inability to tailor output length on demand, a contrast to XOFs that can produce long or short outputs as needed. In contexts requiring flexible output or streaming, XOFs or other constructions may be preferred.

See also: Extendable-output function, hash function, SHA-2, SHA-3, SHAKE.