I would use something like this to measure water usage in combination with an presence sensor in the shower stall. Water flowing + someone present in shower stall = someone taking a shower.

I don’t understand how you expect the verification of a physical book to work. You’d have to scan/OCR the entire book to verify the signature.

Alternatively you could do a hash/signature per page to make verification of a single page easier.

Many public keys chain-linked with one master key - are just for example

Have a look at Merkle trees.

Microwaved coffee…. I just threw up in my mouth a little.

fzf fuzzy finder. Great tool to quickly find those files you were looking for.