A method comprising:- providing a first and a second user equipment, each user equipment comprising a mobile device (110 111 305 310) configured for wireless connectivity to a wireless communication system comprising a wireless network of interconnected network elements, the first and second user equipment configured to communicate over a communication path of said network elements, characterized in that, the method further comprises the steps of:- further configuring each user equipment for mutual device-to-device, D2D, communication in the absence of said wireless connectivity, said D2D communication facilitating the first and second user equipment to communicate directly with each other over an air interface (135 335) between the first and second user equipment, no network elements being included in the D2D communication path; decrypting, at the first user equipment (305; 310), information received in a first frame over the air interface (135 335) during device-to-device, D2D, communication with the second user equipment (310; 305), wherein the information is decrypted using a cryptographic function that is applied to a security key (KEY_1; KEY_2; KEY_3), receiving a reference frame from the wireless network, at a first time, namely a reference time (T_REF), by the first user equipment, storing a reference frame number associated with the reference frame, by the first user equipment, and subsequently, determining the first frame number of the first frame, based on the reference frame number stored by the first user equipment, the corresponding reference time (T_REF) stored by the first user equipment, and a second time, namely a current time (T_1), and configuring the cryptographic function as applied to both the security key (KEY_1; KEY_2; KEY_3) and the determined first frame number of the first frame. providing a first and a second user equipment, each user equipment comprising a mobile device (110 111 305 310) configured for wireless connectivity to a wireless communication system comprising a wireless network of interconnected network elements, the first and second user equipment configured to communicate over a communication path of said network elements, characterized in that, the method further comprises the steps of:- further configuring each user equipment for mutual device-to-device, D2D, communication in the absence of said wireless connectivity, said D2D communication facilitating the first and second user equipment to communicate directly with each other over an air interface (135 335) between the first and second user equipment, no network elements being included in the D2D communication path; decrypting, at the first user equipment (305; 310), information received in a first frame over the air interface (135 335) during device-to-device, D2D, communication with the second user equipment (310; 305), wherein the information is decrypted using a cryptographic function that is applied to a security key (KEY_1; KEY_2; KEY_3), receiving a reference frame from the wireless network, at a first time, namely a reference time (T_REF), by the first user equipment, storing a reference frame number associated with the reference frame, by the first user equipment, and subsequently, determining the first frame number of the first frame, based on the reference frame number stored by the first user equipment, the corresponding reference time (T_REF) stored by the first user equipment, and a second time, namely a current time (T_1), and configuring the cryptographic function as applied to both the security key (KEY_1; KEY_2; KEY_3) and the determined first frame number of the first frame. further configuring each user equipment for mutual device-to-device, D2D, communication in the absence of said wireless connectivity, said D2D communication facilitating the first and second user equipment to communicate directly with each other over an air interface (135 335) between the first and second user equipment, no network elements being included in the D2D communication path; decrypting, at the first user equipment (305; 310), information received in a first frame over the air interface (135 335) during device-to-device, D2D, communication with the second user equipment (310; 305), wherein the information is decrypted using a cryptographic function that is applied to a security key (KEY_1; KEY_2; KEY_3), receiving a reference frame from the wireless network, at a first time, namely a reference time (T_REF), by the first user equipment, storing a reference frame number associated with the reference frame, by the first user equipment, and subsequently, determining the first frame number of the first frame, based on the reference frame number stored by the first user equipment, the corresponding reference time (T_REF) stored by the first user equipment, and a second time, namely a current time (T_1), and configuring the cryptographic function as applied to both the security key (KEY_1; KEY_2; KEY_3) and the determined first frame number of the first frame.