The mandatory and most deployed authentication method used in the session initiaton protocol, the Digest Access Authentication method, is weak. Other, more secure authentication methods have emerged, but have seen little adoption yet. In this paper, support for using a generic authentication method, the generic security services API, is added to the session initiaton protocol. When using this method, the session initation protocol does not need to support nor implement other authentication methods, only use the provided API library. This enables the session initiation protocol to transparently support and use more secure authentication methods in a unified and generic way.