incrediBILL - 6:06 pm on Oct 21, 2011 (gmt 0)
Controlling access to the decryption key through WMT would allow Google to enforce terms and conditions on webmasters wishing to use the QS.
Using that theory there wouldn't be any abuse in AdSense yet there is, MFAs and worse all over the place, not to mention click fraud. Google bans them as they go along and more just pop up in their place.
Likewise, AdWords also has fraudulent ads in it, so Google doesn't do as good of a job policing as one would hope.
That's why it won't work, you can't trust the webmaster to maintain privacy.
Plus, how do you prove a webmaster is breaking the privacy?