Blockhchain is feverish with ZK (Zero Knowledge) - a technology that allows to do something that seems impossible: prove a statement to be true without revealing anything else about it: proving your age or nationality without giving your data up confirming you have enough funds for a transaction ensuring that a photo is cropped from an authentic photo taken by a particular camera compressing the results of binary execution in a simple confirmation string. All of this is now possible. Even Google is integrating ZK tech in Google Wallet! However, making it possible is only the first step, the second is making it secure. Unfortunately, there are lots of challenges due to the complexity of Zero Knowledge Protocols. One of the most efficient ways we traditionally test programs is fuzzing, but zero knowledge circuits don't have branching which fuzzing relies on. So what can we do? In this talk Innokentii will explore how to apply the fuzzing techniques to Zero Knowledge circuits.