Introducing security mechanisms into distributed simulation infrastructures like HLA and DIS offers the Defence and wider user community flexibility to deploy and execute simulations on networks with varying levels of classification. The SecProxy architecture, currently under development, aims to provide an extensive security system for use in a variety of distributed simulation infrastructures, notably HLA and DIS. The SecProxy architecture builds upon previously defined security requirements concerning: multi-level security; access control; and service authentication (establishing presence of authorised software agents). This paper also discusses more common security concerns such as sniffing, denial of service and spoofing attacks and how they are addressed by SecProxy. The architecture is shown to be suitable for transparent implementation in legacy simulations; thereby allowing legacy HLA federations and DIS simulations to be executed on non-private networks.