Because that&#x27;s still an unsolved research problem.<p>Rogaway mentions it, in some detail, in his recent paper:<p><a href="https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=10655418" rel="nofollow">https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=10655418</a>