Abstract. In this paper, we identify some issues in the interplay between practice and theory in cryptography, issues that have repeatedly appeared in different incarnations over the years. These issues are related to fundamental concepts in the field, e.g., to what extent we can prove that a system is secure and what theoretic results on security mean for practical applications. We argue that several such issues are often overlooked or misunderstood, and that it may be very productive if both theoreticians and practitioners think more consciously about these issues and act accordingly.