how many private keys can you get on one?
i dunno. it would depend on how much redundancy you felt was adequate.
an LP holds as much music as a CD though - right? so a few hundred meg, less the data cost of analog>digital translation.
edit:
and it would be much cheaper to store everything on paper and scan it instead, MUCH cheaper.
yes, it's analog and has 0 bytes - but an analog signal which is designed to be converted to digital can carry information. as i pointed out - USB turntables are interesting, and do precisely that. the trick is to design a way to convert a digital piece of information to an analog signal that can easily be converted back. i've imagined playing a Word Doc using one's favorite CD player...