In reply to Omar Santos.

Hmm...I don't think I was clear. I didn't mean to suggest that the 'noise' cookie should actually be used for anything. It just needs to exist, with a random length each time. That ought to be enough to mask the subtle length changes that the attacker is looking for.

With enough guesses, you could analyse the average length change for each inserted string, and still extract the secret(s). But it would blow out the attack time from 30 seconds to hours, and in the real world, that means it will often fail. The victim will finish watching the decoy lolcats video and close the website long before the attack completes.

As far as I can tell, that would be pretty simple to implement, and the amount of extra traffic caused by the cookies would be far less than the savings you'd get from being able to retain compression. You'd want a decent PRNG seed, of course.