An Analysis of Covert Timing Channels

Covert channels have traditionally been categorized as either storage channels or timing channels [8, 9]. This paper questions this categorization, and discusses channels that cannot be clearly identified as either storage or timing channels, but have aspects of both.A new model of timing channels is presented, which allows for channels that have characteristics of both storage channels and timing channels, and a method is given for constructing all channels in a computer system that have timing channel characteristics. Since Kemmerer's shared resource matrix methodology [7] has the potential to detect all channels that have storage characteristics, it is argued that the two methods jointly have the capability to construct all channels in a computer system.The approach to timing channels provides a justification of specific mechanisms for reducing their bandwidth which were employed in the VAX Virtual Machine Monitor, as described in [4] and [5].