Long (private) short urls

[cryptofuture]

I think 5 or 6 character UID is good for url shortener. But actually there is always brute-force possibility. And something like checkbook for 15 symbols UID will be great. This will be still shorter then some really long urls, but should give brute-force protection from future attacks.
Urls will be short by default, but if user want more strong urls, this option should safe the day.
Any comments?

[cryptofuture]

I have working prototype now. Need tweak some thing, before pushing to github.

[cryptofuture]

Done. Called it "Strong URL".