Abstract
The hierarchical cryptographic key assignment is used to assign cryptographic keys to a set of partially ordered classes so that the user in a higher class can derive the cryptographic key for users in a lower class. However, the existing secure schemes for the cryptographic key assignment in a hierarchy do not consider the situation where a user may be employed for only a period of time. If a user resigned from his position and he premeditatedly eavesdrops on data transmissions, then he can also decrypt some data to obtain useful messages. Thus, all messages are likely to be compromised throughout the system. In this paper, we propose a new cryptographic key assignment scheme in which the cryptographic keys are generated from the identity number of users. Our aim is to minimize the potential damage over a public network. Therefore, as a user who has resigned from his class premeditatedly eavesdrops on later messages, he cannot decrypt the message with his old keys. Moreover, in the proposed method, the key generation and key derivation are quite simple, and the number of the public/secret parameters for each authenticated user is fixed which differs from most previously proposed schemes.