Qualification Round

This round has one purpose to check are you dedicated enough to challenge yourself later in the live competition. The online task is totally solvable for anybody who has the ambition and the will to solve it. So take some passion and start the work. The first limited number of participants who successfully finished the online round will win the right to take apart in the live round.


24th of March

Difficulty level

approx. 20 minutes to finish


There is no scoring, your solution is either "go" or "no go". Plus points for having cool stickers on your laptop.




The organizers of ChaosStack decided to venture into the crypto space, by creating their very own blockchain. They’ve built the chain, started sending transactions and mining blocks, but they would like to have a separate verification software too.

Technical details

Your task is to create a program that verifies the incoming chain, and in case of invalid blocks, return the first offending block’s correct hash (which was calculated by you). Your program will have to communicate with the ChaosStack grader system using REST API.

In order to get points you have to:

  1. Acquire a set of blocks from the API
  2. Check if the chain is valid; In case it isn’t, get the first offending block
  3. Then submit your solution to the grader (see Submit a solution in the API docs).