An efficient hierarchical key assignment scheme for access control based on one-way hash function is proposed, which allows the user in a security class to derive the keys of its subordinating classes, so that the superior class can access the information encrypted and owned by its subordinating classes. This scheme is also flexible to change the hierarchical relationship among security classes, for example, it is not difficult to add new security classes to the hierarchy, or to delete security classes from the hierarchy dynamically. By analysis and comparison, it is shown that some features, such as efficiency, extensibility, and security, are possessed by this scheme.