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 VAX11VAX is a trademark of Digital Equipment Corporation. Virtual Machine Monitor, as described in [4] and [5].

The covert storage channels found in disk I/O optimization schemes are studied. The authors examine the source of the problems in the context of various disk architectures, propose several classes of generic solutions and conclude with recommendations for future storage-system architectures. The work was done as part of the covert channel analysis for Digital's VAX security kernel.>

Covert channels have traditionally been categorized as either storage channels or timing channels. The author 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 R.A. Kemmerer's (1982) shared resource matrix methodology has the potential to detect all channels that have storage characteristics, 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.>

Various optimizations have been suggested for reducing formal flow violations produced by a standard information flow security analysis for the certification of programs or specifications of computer systems. These optimizations are specific to particular source language constructions and rely on the analysis tool recognizing these constructions through syntactic processing in order that the optimization(s) might be applied. A new optimization for eliminating formal flow violations, suggested by the semantics of the text under analysis, is presented. It is shown that the syntactic optimizations mentioned above can be viewed as special cases of the generalized optimization.>