The present invention is directed to a system for providing a trusted environment for untrusted computing systems. The system may include a HAC subsystem managing shared resources and a trusted bus switch for controlling a COTS processor to access the shared resources. The shared resources such as memory and several I/O resources reside on the trusted side of the trusted bus switch. Alternatively, the system may include a SCM as an add-on module to an untrusted host environment. Only authenticated applications including COTS OS execute on the SCM while untrusted applications execute on the untrusted host environment. The SCM may control secure resource access from the untrusted host through a plug-in module interface. All secure resources may be maintained on the trusted side of the plug-in module interface.