ed as part of the Starlight research program. program is investigating methods for military-relevant information security es which are genuinely cost effective. The e Link is a retrofittable device which fits to 1 off the shelf workstations and PCs which are to classified networks. The Link allows ess to low or unclassijied networks, such as et, in a manner which is accreditable via the ing users to have windows at 1. I troduction + e Interactive Link forms part of a suite of products developed as part of the Starlight research program. research program is investigating methods for military-relevant information security s which are genuinely cost ][4][51[61[9][ 111. Much previous research in tems has not yielded the anticipated results. this research has focused on methods for igh assurance trusted computing bases to secure ][8][15][16]. The surance levels, and ability and state of t of the infosec experience of the 1980's, the earch program focuses on methods forachieving security capabilities in a retrofittable, incremental manner. That is, we forsook the traditional view of "having to build security in at the beginning" for information systems such as those epitomised by workstations, and subscribed to an approach which focused on building security from a network view where any component which was complex but had a short product lifecycle (e.g. workstations) was deemed to be untrusted. In essence, we concentrated our effort in configuration methods for networks, and the construction of limited functionality, high assurance devices which could be retrofitted to untrusted components in the network to achieve the desired results. Further details on this philosophy are available elsewhere[2]. This paper focuses on how Starlight achieves one particular security capability, i.e. the Interactive Link. The term Interactive Link refers to a solution for achieving a major desire of many military commanders. That is, to be able to retrofit a cost effective device to commercial off the shelf workstations and PC's which are connected to classified networks and yet still retain interactive access to low or unclassified networks, such as the Internet, in a manner which is accreditable via the authorising government agencies. By accreditable, we refer to methods whereby subversion of any application or operating system on any machine attached to the high network will not enable misuse or leakage of information in an unauthorised manner. As such, the reader can deduce that any known method, or indeed concept, based on the firewall (smart packet filtering) paradigm to achieve this interactive link is considered insufficient and indeed insecure. However, we also wished that any solution devised could compete reasonably well on a cost basis with firewall type solutions. The current implementation of the Interactive Link is to be evaluated to an ITSEC assurance level of E5 although the design is such that it can reach E6 with surprisingly little additional effort on the part of the evaluating facility.The following sections discus...

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