1. A secure communication method for allowing a mobile host to communicate with a correspondent host over a Virtual Private Network via a Security Gateway (SG), the method comprising the steps of: (1) negotiating one or more Security Associations (SAs) between the mobile host and a correspondent host of a Virtual Private Network (VPN); (2) subsequently initiating a communication between the mobile host and the SG and sending an authentication certificate to the SG, the certificate containing at least the identity of a SA which will be used for subsequent communication between the mobile host and the correspondent host; (3) sending data packets from the mobile host to the correspondent host using the identified SA, via the SG; and (4) wherein said data packets are forwarded by the SG to the correspondent host only if they are authenticated by the SG.