eXtendable-Output Functions (XOFs)
Our existing hashing functions are split into two main categories. The first is the cryptography hashes, such as MD5, SHA-1, SHA-256 and SHA-3 [here]. These are typically fast methods and which can take any amount of data as an input and produce a fixed hash output — a fingerprint of the data. The other type of hash is the non-cryptographic hash and these tend to be even faster than the cryptographic hashes, but have a lesser security strength. Typical non-cryptographic methods are SipHash, xxHash and FarmHash [here]. These hashes are typically used for indexing such as with hash tables and Bloom filters. But, there’s another type of permutated output, and these are defined as eXtendable-Output Functions (XOFs).
XOFs differ from hashing functions in that they can produce any length of output — from one byte to an infinitely long byte stream. They can thus be used in applications where we need something larger or smaller than the 256-bit (16 bytes) or 512-bit (32 bytes) output value that our cryptographic hashing methods produce. For cryptographic methods, most modern applications now use SHA-256 (256-bit hash), SHA-512 (512-bit hash) or SHA-3 (256-bit).
In designing a modern hashing the focus is typically on a fixed hash output, but designers also implement a variable-length output for XOF. Overall, the main XOF methods are SHAKE128, SHAKE256, SHAKE128…
