Securing Excel Doc

Today I almost sent an excel doc containing sensitive financial information to the wrong person

Fortunately I caught myself before hitting the send button but it definitely got me thinking about protecting certain workbooks.

Anyone know of a better alternative to the 'password protection' method offered by Excel? I haven't tried personally, but I'm sure such passwords could be brute forced especially since excel doesn't limit the number of failed attempts.

I did some searching to see if it was possible to use public/private keys but couldn't find anything.

Thanks
James