In this paper we propose a new proactive password checker, a program which prevents the choice of easy-to-guess passwords. The checker uses a decision tree which has been set up applying the Minimum Description LengthPrinciple and a Pessimistic Pruning Technique to refine its predictive power. Experimental results show a substantial improvement in performances of this checker with respect to previous proposals. Moreover, the system is user-friendly and can be adapted to a per-site policy by the system administrator.