NonXOF

NonXOF is a term used in some cryptographic discussions to refer to primitives that produce a fixed-length output and do not provide extendable-output capabilities. It is not a standard label in formal literature; more commonly these functions are described as fixed-output hash functions, MACs, or PRFs. The term nonXOF is mainly used to contrast with XOFs, which allow outputs of arbitrary length.

The distinction between XOFs and nonXOFs lies in output length. XOFs, such as SHAKE, are sponge-based and can be squeezed to any chosen length within security constraints. NonXOFs produce a predetermined number of output bits for each input, typically defined by the function’s design, for example 256 bits for SHA-256.

Examples of nonXOF constructions include fixed-output hash functions like SHA-256 and other SHA-3 variants that define a specific digest length, as well as MACs and PRFs built from these hashes. Legacy hashes such as MD5 are generally considered insecure and discouraged. By contrast, XOF examples include SHAKE-128 and SHAKE-256, which can emit variable-length outputs.

Applications and security considerations: NonXOFs are used for data integrity, digital signatures, and message authentication where a fixed output length is sufficient. Security properties of these functions focus on preimage resistance, second preimage resistance, and collision resistance. When a variable-length output is required, XOFs are the appropriate choice, but they require careful domain separation and output-length management.

Notes: The label “nonXOF” is informal and not widely standardized. For any specific function, refer to its official specification to understand exact output length, security properties, and usage guidelines.