Crypto is a special case problem. I have a pretty good level of education (formal and personal) but I would still avoid rolling my own solution for anything non-trivial.

This is not so much because I expect to make a mistake, but more because of the impossibility of getting testing (and therefore maturity) that matches an established open source library.

For Java Script one possibility is http://code.google.com/p/clipperz/wiki/CryptoLibrary (AGPL), I'd be interested to hear of others.