Abstract: Military operations are increasingly being conducted by ad hoc coalitions. The need to provide a communications and information system infrastructure for coalition members with connections to the command support systems of contingent members, particularly that of the lead nation, poses difficult information security issues. This paper discusses the heightened need to support need-to-know separation within a coalition environment compared with a single nation tactical deployment. Security architecture options for a coalition ommunications and information system are discussed and the enhancement of these architectures with public key infrastructure (PKI) per-message confidentiality services is proposed. The integration of public key infrastructure technology within a coalition deployment is explored, as is the extension of the confidentiality services offered by this technology to agencies external to the coalition deployment via public and rear-link networks.