Method, apparatus and systems are disclosed that may be implemented in a Wireless Transmit/Receive Unit (WTRU) for unicast communication with another WTRU. The method may include sending, by the WTRU to the other WTRU via a PC5 interface, a communication request indicating first security information for the WTRU and receiving, by the WTRU from the other WTRU via the PC5 interface, information which is integrity protected indicating second security information for the other WTRU. The method may also include deriving, by the WTRU from the indicated second security information, a first set of security keys for communication via a first layer using the U-plane and a second set of security keys for communication via a second layer using the C-plane; and sending, by the WTRU to the other WTRU via the PC5 interface, a message that is at least integrity protected using the derived first set of keys.