Is there a good candidate for an algorithm that would takes n days to break depending on how many bytes of the key are missing? I'm writing a story where you need to collect pieces (bytes) of a key and depending on how you have you get an head start in brute-forcing the message.
The message is around 200 chars long (so quite small). Count on computer resources be what would feasible for a non state-actor (bot nets etc), but still pretty resourceful. The key length / piece length does matter much, but should be viable for 6-8 "pieces"
The main problem I see is that time with brute forcing strong encryption is usually measured in years, or even million of years, and I need it more to be around 3 days per "piece"