Every public electricity supply in the UK has a unique MPAN number, the last digit of this is the check digit. I tried to impress my dad with this knowledge, and he gamely showed interest. 'You mean a check digit like on the end of credit card numbers?', he asked. I confirmed it was the same idea, but calculated in a different way. 'So', he went on, 'you don't want to be telling people how that works do you!' (careless talk costs lives etc). Excellent! This was an opportunity for me to show off even more.
I wore my tongue to a stump talking about 'security through obscurity' and Kerckhoff's principle. He then said sarcastically, 'So I should provide a map of my house to burglars, should I?'. To which, in a triumphant know-all manner, I brandished Peter Swire's Disclosure Matrix!