Security key derivation in dual connectivity

A communication system including a user equipment and a master base station,

• the user equipment comprising:
• a receiver, which, in operation, receives an incremented freshness counter value from the master base station, the incremented freshness counter value being set in response to a change request of a secondary security key from a secondary base station when a value of a COUNT exceeds a threshold value, wherein the secondary security key is a security key for the secondary base station;
• and
• circuitry, which, in operation,
• derives an updated secondary security key using the incremented freshness counter value and a currently active security key of the user equipment, corresponding to a currently active security key of the master base station, without having to refresh the currently active security key of the user equipment;
• and
• uses the derived updated secondary security key in a communication link with the secondary base station;
• and
• the master base station comprising:
• a transmitter, which, in operation, transmits the incremented freshness counter value to the user equipment.