Seriously, the easier way to do this is just make a password protected zip file. Boom. Done. No extra mucking around with random bytes and whatnot. You want to share? Give the password to your friends.

Perfect being the enemy of good enough, etc.

Also its a bit unfair. Given this attack, design a perfect defense. OK here's an easy one. Oh OK well that works, but they'll just try a different attack. Well yeah, but that wasn't the initial challenge provided.

Also precomputed rainbow tables aren't so funny when you have gig wide columns instead of hash wide columns. For that alone its an entertaining idea.