[SR-Users] authentication for client applications

Hi, I am working on a project where a custom sip client will be integrated into a suite of applications to provide voip. The sip client will be working with Kamailio. The goal is to ensure that the client is authorized for communication with kamailio before allowing any calls to be made. Conventional username/password authentication for individual users will also be used once the client has been authenticated. Currently other applications in the suite use a digital signature in the http headers when communicating with server processes. If the signature is validated by the server process then the applications identity is validated and communication with the server process is allowed. Is it possible to include a public key and digital signature in the register events and have kamailio perform the transformation to validate the client's identity? If so which module provides such functionality? Has something like this been implemented in the past? Thanks for any input. ttyl,Dave