recently i was given 72 hours for a take home. they required me to keep the “challenge” confidential and assign the rights to the code to them. i’m wondering what you all think of that.

How generic was the challenge?

In the sense of would this be a problem that might reasonably come up in the course of working elsewhere ... and how comprised would you be had it done so given you'd seen it before and signed right to your solution elsewhere.

How much are they paying for you to do the work? How does that compare to your hourly-rate, or salary? If for free they're clearly taking advantage, so I'd assess based on payment.

That is a red flag and I would most likely pass on the job. It sounds like they want to potentially use whatever solution you come up with.