Password Entropy and Password Quality

Passwords are the first line of defense for many computerized systems. The quality of these passwords decides the security strength of these systems. Many studies advocate using password entropy as an indicator for password quality where lower entropy suggests a weaker or less secure password. However, a closer examination of this literature shows that password entropy is very loosely defined. In this paper, we first discuss the calculation of password entropy and explain why it is an inadequate indicator of password quality. We then establish a password quality assessment scheme: password quality indicator (PQI). The PQI of a password is a pair (D, L), where D is the Levenshtein's editing distance of the password in relation to a dictionary of words and common mnemonics, and L is the effective password length. Finally, we propose to use PQI to prescribe the characteristics of good quality passwords.