Has to be easy enough for an old laptop to prove in under 5 seconds but hard enough that (if this becomes popular) someone with a mining rig can’t be cracking m

Has to be easy enough for an old laptop to prove in under 5 seconds but hard enough that (if this becomes popular) someone with a mining rig can’t be cracking millions of these a day. Also the idea ain’t so new!: http://www.hashcash.org/hashcash.pdf http://www.hashcash.org/hashcash.pdf