hey crypto nerds :boost_requested:

i'm trying to throw together a simple proof-of-work captcha with cryptographic challenges. you don't need to prove someone isn't a bot, you just need to waste their time so it's prohibitively expensive to spam you. however, if generating and verifying challenges takes the same amount of computation on the server as does solving them on the client, it's vulnerable to DoS attacks.

so, does there exist a crypto system i can use to generate and verify challenges cheaply, but which is tunably compute-hard for the solver? or am i asking for a holy grail type thing here