A Verifiable Delay Function, or VDF, is a cryptographic primitive that takes a set amount of sequential time to compute, but whose result anyone can check quickly. Dan Boneh, Joseph Bonneau, Benedikt Bünz, and Ben Fisch defined it that way in 2018 (CRYPTO 2018 / IACR ePrint 2018/601): a VDF requires a specified number of sequential steps to evaluate, yet produces a unique output that can be efficiently and publicly verified.

The important part of that definition is the asymmetry. Evaluation has to take a specified number of sequential steps — work that should not speed up just because you have more machines. Verification should be fast, public, and unique: given the input and parameters, there is one correct output, and anyone can check a proof of it without rerunning the delay.

That combination matters when a system needs time to pass, and needs a proof that it actually passed.…