While there is a great deal of sophistication in modern cryptology, simple (and simplistic) explanations of cryptography remain useful and perhaps necessary. Many of the explanations are informal; others are embodied in formal methods, particularly in formal methods for the analysis of security protocols. This note (intended to accompany a talk at the Crypto 2000 conference) describes some of those explanations. It focuses on simple models of attacks, pointing to partial justifications of these models.