Lets say I've met someone on a website using a computer at the local library and would like to arrange a meeting with this person for tea and crumpets or go to the local milk bar, but we need to keep the location of the meeting a secret or the PM will get a bit pissy that he wasn't invited and send over his droogs for a bit of ultraviolence and give my mum the old in out in out again. Are there any public key encryption schemes that could be done by hand or with a normal calculator without the additional processing power of a smartyphone, would allow us to exchange 4-16 little bytes of information, and can't be easily cracked?