I think I’ve got a better and even easier solution: learn to read and write shorthand. It will take a bit of investment, but shouldn’t ultimately take too long – when I was a kid I found an instruction manual and started teaching myself. I lost interest and never finished, but it was pretty easy to pick up. The big advantage is that you can write quickly without going through the painstaking process of making a table and encoding/decoding things. Ultimately, it’s just a very obscure form of cursive. Also, shorthand is so rare nowadays that there’s little to no chance of your housemate being able to read it, or even knowing what it is – just tell him it’s sanskrit or something.
For anyone interested in codes, let me heartily endorse Simon Singh’s great “The Code Book.”
You could always try writing in AES-256. If it’s good enough to be a current US government standard, it should be good enough to keep your errands safe.
And you think writing in code isn’t going to set them off? I have to agree with **Czarcasm ** - write plaintext notes and keep them hidden on your person.
Presuming you aren’t, say, picking up shotguns or reminding yourself to hide bodies or planning a crime, why would you hide it? Why would a roommate care about normal errands? Why would it matter if they knew?
I know you mentioned you have been on some medications in a previous thread. Sometimes meds can make you nervous and suspicious of others. Some meds can even do this if you stop them too suddenly. Perhaps this is happening to you?
I would think that having bulletin boards full of scrambled letters will be pretty suspicious, and even with Carroll’s method, you can’t decipher it without sitting down and running it through a table. You can recreate the table from memory, but I highly doubt you can decipher on the fly.
Shorthand is a good idea, it’s somewhat standardized, designed to be read on the fly, and most people don’t know it.