This paper examines methods for providing PKI interoperability among units of a coalition of armed forces drawn from different nations. The area in question is tactical identity management, for the purposes of confidentiality, integrity and non-repudiation in such a dynamic coalition. The interoperating applications under consideration range from email and other forms of store-and-forward messaging to TLS and IPSEC-protected real-time communications. Six
interoperability architectures are examined with advantages and disadvantages of each described in the paper.