"Admittedly, the security obtained by OTPW is not comparable with that of a challenge-response system in which the user has a PIN protected special calculator that generates the response. On the other hand, a piece of paper is much more portable, much more robust, and much cheaper than a special calculator. OTPW was designed for the large user base, for which an extra battery-powered device is inconvenient or not cost effective and who therefore still use normal Unix passwords everywhere."