"Strengthening Passwords"
Martin Abadi, T. Mark A. Lomas, and Roger Needham
Note #1997-033, September 4, 1997 (with minor revisions on December 16, 1997)

Despite their notorious vulnerability, traditional passwords remain 
important for security. In this paper we describe a method for 
strengthening passwords. Our method does not require users to memorize 
or to write down long passwords, and does not rely on smart-cards or 
other auxiliary hardware. The main cost of our method is that it lengthens 
the process of checking a password.