The 3GPP proposes security architecture for Interworking WLAN-3GPP. However, it presents inefficiencies in the authentication procedure, which bring a greater consumption of radio resources, processing on the devices and delayed authentication. Due to previous reasons, we propose a security middleware that reduces the steps in the authentication of the WLAN-3GPP interworking and ensures the SIP signaling between the user equipment and the first entry point to the IMS network (P-CSCF).