According to https://csrc.nist.gov/glossary/term/extendable_output_functions, eXtendable-Output Function (XOF) is a function on bit strings in which the output can be extended to any desired length. It's similar to MessageDigest but you can call digest() with a length argument for arbitrary times.

We should add public API for it as a new crypto primitive. It should contain methods like update(), squeeze(), reset(). The major difference from MessageDigest is that squeeze() can be called multiple times and it does not auto reset().